would normally indicate .htaccess or another name. (In Netscape servers, this file name is called .nsconfig, and uses a different syntax from .htaccess.)
Whether or Not to Use .htaccess .htaccess is often used in settings where a group network administrator wants to control who views or changes the contents of the directories that relate to his or her groups or users. In these settings, it is not practical or advisable to give the administrator primary access to all of the HTTPd server’s functions, and all of its other directories and configuration files. Having the local-level control provided by .htaccess files allows more flexibility for the administrator to create and change directory access controls, as needed. Some disadvantages to using .htaccess files have been noted: If an organization has several hundred .htaccess files on several hundred directories, each granting or denying user access to their own contents, it is more difficult for the company’s network administrators to prepare a global access or authentication strategy and keep up with changes. Also, .htaccess files can be overwritten very easily, causing
problems for users who once could access a directory’s contents, but now cannot. Finally, .htaccess files are more likely to be opened or retrieved by unauthorized users.
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htm htm is sometimes used as a short form of the file name suffix for an HTML file. For example, the file for our definition of computer might be named ‘‘computer.htm’’ instead of ‘‘computer.html’’. The main advantage is that it’s one character shorter. The disadvantage is that it’s not quite as easy to recognize as an HTML file. Prior to wide-spread use of Windows 95 and later systems, there was another reason to prefer the three-character suffix rather than the fourcharacter. Certain operating systems, such as Windows 3.1 and OS/2 used the original DOS operating system naming conventions, in which names were limited to eight characters and suffixes to three characters. So it was safer to create file names and suffixes that met this ‘‘8.3’’ limitation. (For example, if you created an HTML page coding hypertext links to your other pages based on a four-character suffix (‘‘.html’’) system and then moved that page to a Web server that only allowed a three-character suffix, your file name suffixes would all be shortened and your links would no longer work. And if you created links with the threecharacter suffix and moved it to a server that always required four-character suffixes for HTML files, you would be in trouble again.) Today, the operating systems most people are likely to create HTML pages on and serve them from are all operating systems (such as Windows 95/98/NT, Mac OS, and UNIXbased systems) that support longer file names and suffixes. So if you were building a new Web site, you would most likely use the four-character html suffix for clarity (unless you preferred the brevity of the three-character suffix) because you could know that you could safely use either suffix—as long as the suffixes in your links and your file names matched. However, if you have an existing Web site that started out using the three-character suffix, you may prefer to leave everything as it is. If you do move to a server that requires a four-character suffix for server consistency, you could do a search-and-replace at that time.
HTML HTML (Hypertext Markup Language) is the set of markup symbols or codes inserted in a file intended for display on a World Wide Web browser page. The markup tells the Web browser how to display a Web page’s words and images for the user. Each individual markup code is referred to as an element (but many people also refer to it as a tag). Some elements come in pairs that indicate when some display effect is to begin and when it is to end.
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HTML is a formal Recommendation by the World Wide Web Consortium (W3C) and is generally adhered to by the major browsers, Microsoft’s Internet Explorer and Netscape’s Navigator, which also provide some additional nonstandard codes. The current version of HTML is HTML 4.0. However, both Internet Explorer and Netscape implement some features differently and provide non-standard extensions. Web developers using the more advanced features of HTML 4 may have to design pages for both browsers and send out the appropriate version to a user. Significant features in HTML 4 are sometimes described in general as dynamic HTML. What is sometimes referred to as HTML 5 is an extensible form of HTML called Extensible Hypertext Markup Language (XHTML).
HTML 4.0 HTML 4.0 was the final version of the Hypertext Markup Language (HTML) before the Extensible Markup Language (XHTML) and remains the set of markup on which most large Web sites today are based. Like all HTML levels, HTML 4.0 was the official ‘‘recommendation’’ of the World Wide Web Consortium (W3C), the group that suggests industry standards for the Web.
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The cascading style sheet, the ability to control Web page content at multiple levels
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In practice, the two leading browsers, Netscape and Internet Explorer, support HTML 4.0 somewhat differently or offer non-standard approaches. These require Web developers that use more advanced features to create pages for each browser and send the appropriate pages to the user.
HTML comment You can include a comment in an HTML document. A comment will not be displayed and will help you remember why you coded something a particular way. A comment might look like this:
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See ‘‘Hypertext Transfer Protocol’’
HTTP 1.1 HTTP (Hypertext Transfer Protocol), the World Wide Web application protocol that runs on top of the Internet’s TCP/IP suite of protocols, now exists in a newer version, HTTP 1.1, that promises to bring Web pages a little faster to your browser and reduce some of the Web’s enormous traffic. Developed by a committee of the Internet Engineering Task Force (IETF) that includes the Web’s chief creator Tim Berners-Lee, HTTP 1.1 exists as a ‘‘proposal,’’ but in fact, most major Web servers and browser clients are at some stage of supporting it. Here’s a summary of how HTTP 1.1 makes information flow faster: L
Among new features introduced in HTML 4.0 were:
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Hint: Be sure to include two hyphens after the exclamation point and two hyphens before the greater-than sign. Not all browsers are picky about this, but some may be.
Instead of opening and closing a connection for each application request, HTTP 1.1 provides a persistent connection that allows multiple requests to be batched or pipelined to an output buffer. The underlying Transmission Control Protocol layer can put multiple requests (and responses to requests) into one TCP segment that gets forwarded to the Internet Protocol layer for packet transmission. Because the number of connection and disconnection requests for a sequence of ‘‘get a file’’ requests is reduced, fewer packets need to flow across the Internet. Since requests are pipelined, TCP segments are more efficient. The overall result is less Internet traffic and faster performance for the user.
Persistent connection is similar to Netscape’s HTTP 1.0 extension called KeepAlive, but provides better handling of requests that go through proxy servers. L
When a browser supporting HTTP 1.1 indicates it can decompress HTML files, a server will compress them for transport across the Internet, providing a substantial aggregate savings in the amount of data that has to be transmitted. (Image files are already in a compressed format so this improvement applies only to HTML and other non-image data types.)
In addition to persistent connections and other performance improvements, HTTP 1.1 also provides the ability to have multiple domain names share the same Internet address (IP address). This will simplify processing for Web servers that host a number of Web sites in what is sometimes called virtual hosting.
HTTPD See ‘‘Hypertext Transfer Protocol daemon’’
hue, saturation, and brightness
HTTPS HTTPS (Secure Hypertext Transfer Protocol) is a Web protocol developed by Netscape and built into its browser that encrypts and decrypts user page requests as well as the pages that are returned by the Web server. HTTPS is really just the use of Netscape’s Secure Socket Layer (SSL) as a sublayer under its regular HTTP application layering. (HTTPS uses port 443 instead of HTTP port 80 in its interactions with the lower layer, TCP/IP.) SSL uses a 40-bit key size for the RC4 stream encryption algorithm, which is considered an adequate degree of encryption for commercial exchange. Suppose you use a Netscape browser to visit a Web site to view their online catalog. When you’re ready to order, you will be given a Web page order form with a Uniform Resource Locator (URL) that starts with https://. When you click ‘‘Send,’’ to send the page back to the catalog retailer, your browser’s HTTPS layer will encrypt it. The acknowledgement you receive from the server will also travel in encrypted form, arrive with an https:// URL, and be decrypted for you by your browser’s HTTPS sublayer. HTTPS and SSL support the use of X.509 digital certificates from the server so that, if necessary, a user can authenticate the sender. SSL is an open, nonproprietary protocol that Netscape has proposed as a standard to the World Wide Consortium (W3C). HTTPS is not to be confused with SHTTP, a security-enhanced version of HTTP developed and proposed as a standard by EIT.
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2) As a network product, a hub may include a group of modem cards for dial-in users, a gateway card for connections to a local area network (for example, an Ethernet or a token ring), and a connection to a line (the main line in this example).
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hue, saturation, and brightness Hue, saturation, and brightness are aspects of color in the red, green, and blue (RGB) scheme. These terms are most often used in reference to the color of each pixel in a cathode ray tube (CRT) display. All possible colors can be specified according to hue, saturation, and brightness (also called brilliance), just as colors can be represented in terms of the R, G, and B components. Most sources of visible light contain energy over a band of wavelengths. Hue is the wavelength within the visible-light spectrum at which the energy output from a source is greatest. This is shown as the peak of the curves in the accompanying graph of intensity versus wavelength. In this example, all three colors have the same hue, with a wavelength slightly longer than 500 nanometers, in the yellow-green portion of the spectrum.
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Also see stackable hub. In general, a hub is the central part of a wheel where the spokes come together. The term is familiar to frequent fliers who travel through airport ‘‘hubs’’ to make connecting flights from one point to another. In data communications, a hub is a place of convergence where data arrives from one or more directions and is forwarded out in one or more other directions. A hub usually includes a switch of some kind. (And a product that is called a ‘‘switch’’ could usually be considered a hub as well.) The distinction seems to be that the hub is the place where data comes together and the switch is what determines how and where data is forwarded from the place where data comes together. Regarded in its switching aspects, a hub can also include a router. 1) In describing network topologies, a hub topology consists of a backbone (main circuit) to which a number of outgoing lines can be attached (‘‘dropped’’), each providing one or more connection port for device to attach to. For Internet users not connected to a local area network, this is the general topology used by your access provider. Other common network topologies are the bus network and the ring network. (Either of these could possibly feed into a hub network, using a bridge.)
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Saturation is an expression for the relative bandwidth of the visible output from a light source. In the diagram, the saturation is represented by the steepness of the slopes of the curves. Here, the outside curve represents a color having low saturation, the middle curve represents a color having greater saturation, and the inside curve represents a color with fairly high saturation. As saturation increases, colors appear more ‘‘pure.’’ As saturation decreases, colors appear more ‘‘washed-out.’’
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Whatis?Com’s Encyclopedia of Technology Terms Brightness is a relative expression of the intensity of the energy output of a visible light source. It can be expressed as a total energy value (different for each of the curves in the diagram), or as the amplitude at the wavelength where the intensity is greatest (identical for all three curves). In the RGB color model, the amplitudes of red, green, and blue for a particular color can each range from 0 to 100 percent of full brilliance. These levels are represented by the range of decimal numbers from 0 to 255, or hexadecimal numbers from 00 to FF.
human factors In industry, human factors (also known as ergonomics) is the study of how humans behave physically and psychologically in relation to particular environments, products, or services. Many large manufacturing companies have a Human Factors department or hire a consulting firm to study how any major new product will be accepted by the users that it is designed for. A human factors specialist typically has an advanced academic degree in Psychology or has special training. The term usability is now sometimes used as an alternative to human factors, although human factors is really a larger area of study, including responses that are unrelated to usability, such as reactions and preferences in relation to visual and other sensory stimuli.
human-computer interaction See ‘‘HCI’’
humanware Humanware is hardware and software that emphasizes user capability and empowerment and the design of the user interface. The process of building humanware generally consists of these steps: 1. Define users (age, mindset, environmental context, previous product experience and expectations, and so forth) and what they really want to do 2. Identify tasks they will need to do or capabilities they will want 3. Specify usability objectives (if possible, these should be measurable, such as how long to do something or how many mouse clicks to get to a specified task point) for each task or capability 4. Build a prototype of the user interface (it can be a paper or simulated prototype if time is short) 5. Test and verify or correct the prototype 6. Provide the prototype and usability objectives to the program designers and coders
In a typical human factors or usability study, a group of hired or volunteer test subjects that represent future end users is given tasks to do with a working prototype or early version of a product. Typically, a test subject is observed while doing a task, asked to react verbally as any problems occur or as observations are made, and interviewed after each test. (Test subjects have to be assured that it is the product—and not their own skills or capabilities—that is being tested.) Usually, the test is videotaped for later study. In some cases, successive corrections to the product are made during the course of testing.
7. Test the code against the prototype and objectives and, if necessary, redesign or recode the software
Human factors study can focus on general human behavior in relation to technology (such as studies of how people react to various type styles and sizes), on a generic type of product (such as wearable computers), on specific environment or product designs as a whole, or on some specific design aspects of a particular environment or product. Depending on objectives, the result of human factors study can include suggestions on how to redesign the object of study or a general guideline for designing such an object.
Hungarian notation
In addition to relatively formal human factors study, human factors can be said to be underway any time a designer thinks about the effects of the design on the end user, and, in fact, much corrective design work goes on without formal testing. A relatively new object of study is the design of a Web site, either as a general problem or in order to build a specific Web site that meets user needs.
8. Test the product with users or valid test subjects and revise as necessary 9. Get feedback from users and continually improve the product Philips Research uses the term for both software and hardware that is specially designed to interact with users, including its speech synthesis and speech recognition microchips.
In programming, Hungarian notation is a set of conventions for naming data objects in which a programmer adds a meaningful prefix of one or several characters to the object’s name to identify what type of object it is. The conventions suggest, among other things, that programmers choose prefixes that are suggestive of the type of object named and are easy to remember. Consistency in naming is very important. In one programmer’s version of Hungarian notation, a form might have the prefix ‘‘frm’’ and, if the form name is ‘‘Switchboard,’’ the extended name would be ‘‘frmSwitchboard.’’ This makes it clear that the reference is to a form called ‘‘Switchboard,’’ and not to some other type of object with the same name, such as a label (which might be called ‘‘lblSwitchboard’’). The conventions were developed to define naming procedures that would help programmers create prefixes that would be easy to remember and would
Hyper-G make objects easy to identify, both for themselves and for other programmers that might work with their code in the future. Dr. Charles Simonyi developed the convention at Microsoft, where it is used in the Windows source code. Microsoft says it’s known as Hungarian notation because the prefixes make the names seem to be non-English and also because Simonyi is from Hungary. Since Microsoft uses it extensively, many programmers outside Microsoft use it. Charles Petzold’s use of Hungarian notation in his book, ‘‘Programming Windows,’’ also has influenced many Windows programmers.
hybrid fiber coaxial network A hybrid fiber coaxial (HFC) network is a telecommunication technology in which optical fiber cable and coaxial cable are used in different portions of a network to carry broadband content (such as video, data, and voice). Using HFC, a local CATV company installs fiber optic cable from the cable head-end (distribution center) to serving nodes located close to business and residential users and from these nodes uses coaxial cable to individual businesses and homes. An advantage of HFC is that some of the characteristics of fiber optic cable (high bandwidth and low noise and interference susceptibility) can be brought close to the user without having to replace the existing coaxial cable that is installed all the way to the home and business. Both cable TV and telephone companies are using HFC in new and upgraded networks and, in some cases, sharing the same infrastructure to carry both video and voice conversations in the same system. Scientific Atlanta lists four reasons why cable TV and telephone companies are upgrading facilities to HFC: 1) The use of fiber optic cable for the backbone paths allows more data to be carried than coaxial cable alone. 2) The higher bandwidth supports reverse paths for interactive data flowing back from the user. 3) That portion of the infrastructure with fiber optic cable is more reliable than coaxial cable. Reliability is perceived as more important in an interactive environment. 4) Fiber optic cable is more efficient for interconnecting cable TV or phone companies that are consolidating with geographically adjacent companies.
hybrid online analytical processing Hybrid online analytical processing (HOLAP) is a combination of relational OLAP (ROLAP) and multidimensional OLAP (usually referred to simply as OLAP). HOLAP was developed to combine the greater data capacity of ROLAP with the superior processing capability of OLAP.
HOLAP can use varying combinations of ROLAP and OLAP technology. Typically it stores data in a both a relational database (RDB) and a multidimensional database (MDDB) and uses whichever one is best suited to the type of processing desired. The databases are used to store data in the most functional way. For data-heavy processing, the data is more efficiently stored in a RDB, while for speculative processing, the data is more effectively stored in an MDDB. HOLAP users can choose to store the results of queries to the MDDB to save the effort of looking for the same data over and over which saves time. Although this technique—called ‘‘materializing cells’’—improves performance, it takes a toll on storage. The user has to strike a balance between performance and storage demand to get the most out of HOLAP. Nevertheless, because it offers the best features of both OLAP and ROLAP, HOLAP is increasingly preferred.
Hydra See ‘‘Terminal Server product’’
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hyper 1) As an adjective, hyper is slang for ‘‘keyed up’’ or ‘‘overwrought.’’ 2) Hyper is also a prefix, from the Greek hyper-, meaning over, above, or excessive, used in such terms as hyperbole (exaggeration in describing something) and hypertext (text that extends to point to or include other text). 3) In science, also as a prefix, hyper- is something that exists in a space of four or more dimensions, such as a hyperplane.
HyperCard HyperCard is an early (1986) Macintosh application that enables users to author hypertext pages, called cards, without any programming knowledge. A set of cards, called a stack, can be linked together to allow readers to navigate through the stack in a non-linear manner by clicking on buttons or objects associated with an action script.
Hyper-G Hyper-G is a publishing system with hypertext features more advanced than those available with the Hypertext Transfer Protocol and today’s Web browser. Hyper-G was inspired in part by the ideas of Ted Nelson, who coined the term ‘‘hypertext,’’ and was developed in Graz, Austria, by a team of researchers at the Institute for Information Processing and Computer Supported New Media (IICM) and the Institute for HyperMedia Systems (IHM) of Joanneum Research. Assistance was provided by developers elsewhere.
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You can create link types (for example, for a single link, you might have one type called ‘‘definition’’ and another link type called ‘‘picture’’).
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One link type can be an annotation that an editor or a reviewer can create or modify. Thus, a number of collaborating reviewers or writers can share each others comments, made visible by the links they create.
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Links can overlap each other. For example, ‘‘clientserver’’ could contain a link for ‘‘client,’’ for ‘‘server,’’ and another for the term ‘‘client-server’’ as a whole.
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The structure of a document can be viewed graphically and even in 3-D.
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The author can control who can see a document, who can see a link, who can edit the document, and who can edit the links.
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You can have bi-directional links between HTML pages (or other hypertext documents). That is, you can know for any page what other HTML pages (including those at other Web sites) link to the page. The latter kind of link is known as a backlink.
Hyper-G is available in a commercial version called HyperWave. HyperWave consists of a server and clients that run on Windows and UNIX platforms. A version for Macintosh is in preparation. The most advanced client is the UNIX version; it can be run on Linux systems. HyperWave is compatible with HTML. Among other applications, HyperWave has ideal features for online peer or technical reviews and collaborative writing. Among the early users of Hyper-G or derivative versions were Oxford University Press and the European Space Agency.
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Hypermedia, a term derived from hypertext, extends the notion of the hypertext link to include links among any set of multimedia objects, including sound, motion video, and virtual reality. It can also connote a higher level of user/ network interactivity than the interactivity already implicit in hypertext.
hyperspace Hyperspace is a term that describes the total number of individual locations and all of their interconnections in a hypertext environment. It also connotes a perceived sense of this potentially confusing totality. If you’ve ever started out somewhere on the World Wide Web and gradually lost the sense that you know where you are going or where you are, you are in the condition known as ‘‘lost in hyperspace.’’ (Studies, however, show that the ‘‘lost in hyperspace’’ problem is relatively low on users’ lists of ‘‘things to improve.’’) Hyperspace can be considered the hypertextual or architectural aspect of cyberspace, a term that has even richer connotations.
hypertext Hypertext is the organization of information units into connected associations that a user can choose to make. An instance of such an association is called a link or hypertext link. (And the highlighted word ‘‘link’’ in the previous sentence is an example of a hypertext link.) Hypertext was the main concept that led to the invention of the World Wide Web, which is, after all, nothing more (or less) than an enormous amount of information content connected by an enormous number of hypertext links. The term was first used by Ted Nelson in describing his Xanadu system. Also see hypermedia.
Hypertext Transfer Protocol daemon On the Web, each server has an HTTPD or Hypertext Transfer Protocol daemon that waits in attendance for requests to come in from the rest of the Web. A daemon is a program that is ‘‘an attendant power or spirit’’ (Webster’s). It sits waiting for requests to come in and then forwards them to other processes as appropriate.
Hypertext Transfer Protocol The Hypertext Transfer Protocol (HTTP) is the set of rules for exchanging files (text, graphic images, sound, video, and other multimedia files) on the World Wide Web. Relative to the TCP/IP suite of protocols (which are the basis for information exchange on the Internet), HTTP is an application protocol. Essential concepts that are part of HTTP include (as its name implies) the idea that files can contain references to other files whose selection will elicit additional transfer requests. Any Web server machine contains, in addition to the HTML and other files it can serve, an HTTP daemon, a program that is designed to wait for HTTP requests and handle them when they arrive. Your Web browser is an HTTP client, sending requests to server machines. When the browser user
Hz enters file requests by either ‘‘opening’’ a Web file (typing in a Uniform Resource Locator) or clicking on a hypertext link, the browser builds an HTTP request and sends it to the Internet Protocol address indicated by the URL. The HTTP daemon in the destination server machine receives the request and, after any necessary processing, the requested file is returned. The latest version of HTTP is HTTP 1.1.
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i.LINK i.LINK is the Sony Corporation implementation of a standard, IEEE 1394, High Performance Serial Bus, for connecting devices to your personal computer. i.LINK provides a single plug-and-socket connection on which up to 63 devices can be attached with data transfer speeds up to 400 Mbps (megabits per second). The standard describes a serial bus or pathway between one or more peripheral devices and your computer’s microprocessor. Many peripheral devices now come equipped to meet IEEE 1394. i.LINK, Apple’s FireWire, and other IEEE 1394 implementations provide: L
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A very high-speed rate of data transfer that will accommodate multimedia applications (100 and 200 megabits per second today; with much higher rates later)
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A simple common plug-in serial connector on the back of your computer and on many different types of peripheral devices
In time, IEEE 1394 implementations are expected to replace and consolidate today’s serial and parallel interfaces, including Centronics parallel, RS-232C, and Small Computer System Interface (SCSI). The first products to be introduced with FireWire include digital cameras, digital video discs (DVDs), digital video tapes, digital camcorders, and music systems. Because IEEE 1394 is a peer-to-peer interface, one camcorder can dub to another without being plugged into a computer. With a computer equipped with the socket and bus capability, any device (for example, a video camera) can be plugged in while the computer is running.
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There are two levels of interface in IEEE 1394, one for the backplane bus within the computer and another for the point-to-point interface between device and computer on the serial cable. A simple bridge connects the two environments. The backplane bus supports 12.5, 25, or 50 megabits per second data transfer. The cable interface supports 100, 200, or 400 megabits per second. Each of these interfaces can handle any of the possible data rates and change from one to another as needed.
The serial bus functions as though devices were in slots within the computer sharing a common memory space. A 64-bit device address allows a great deal of flexibility in configuring devices in chains and trees from a single socket. IEEE 1394 provides two types of data transfer: asynchronous and isochronous. Asynchronous is for traditional load-andstore applications where data transfer can be initiated and an application interrupted as a given length of data arrives in a buffer. Isochronous data transfer ensures that data flows at a pre-set rate so that an application can handle it in a timed way. For multimedia applications, this kind of data transfer reduces the need for buffering and helps ensure a continuous presentation for the viewer. The 1394 standard requires that a device be within 4.5 meters of the bus socket. Up to 16 devices can be connected in a single chain, each with the 4.5 meter maximum (before signal attenuation begins to occur) so theoretically you could have a device as far away as 72 meters from the computer. Another new approach to connecting devices, the Universal Serial Bus (USB), provides the same ‘‘hot plug’’ capability as the 1394 standard. It’s a less expensive technology but data transfer is limited to 12 Mbps (million bits per second). Small Computer System Interface offers a high data transfer rate (up to 40 megabytes per second) but requires address preassignment and a device terminator on the last device in a chain. FireWire can work with the latest internal computer bus standard, Peripheral Component Interconnect (PCI), but higher data transfer rates may require special design considerations to minimize undesired buffering for transfer rate mismatches.
I18N See ‘‘internationalization’’
I2C bus The I2C (Inter-IC) bus is a bi-directional two-wire serial bus that provides a communication link between integrated circuits (ICs). Phillips introduced the I2C bus 20 years ago for mass-produced items such as televisions, VCRs, and audio equipment. Today, I2C is the de-facto solution for embedded applications. There are three data transfer speeds for the I2C bus: standard, fast-mode, and high-speed mode. Standard is 100 Kbps. Fast-mode is 400 Kbps, and high-speed mode supports speeds up to 3.4 Mbps. All are backward compatible. The I2C bus supports 7-bit and 10-bit address space devices and devices that operate under different voltages.
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I2C See ‘‘I2C bus’’
IA-64 IA-64 is a 64-bit processor architecture developed at Intel that is based on Explicitly Parallel Instruction Computing (EPIC) and designed as the foundation for Intel’s line of microprocessors through 2005. The Itanium is the first in Intel’s line of IA-64 processors.
IAB The IAB (Internet Architecture Board) is the Internet Society overseer of the technical evolution of the Internet. The IAB supervises the Internet Engineering Task Force (IETF), which oversees the evolution of TCP/IP, and the Internet Research Task Force (IRTF), which works on network technology.
IANA IANA (Internet Assigned Numbers Authority) is the organization under the Internet Architecture Board (IAB) of the Internet Society that, under a contract from the U.S. government, has overseen the allocation of Internet Protocol addresses to Internet service providers (ISPs). IANA also has had responsibility for the registry for any ‘‘unique parameters and protocol values’’ for Internet operation. These include port numbers, character sets, and MIME media access types. Partly because the Internet is now a global network, the U.S. government has withdrawn its oversight of the Internet, previously contracted out to IANA, and lent its support to a newly-formed organization with global, non-government representation, the Internet Corporation for Assigned Names and Numbers (ICANN). ICANN has now assumed responsibility for the tasks formerly performed by IANA.
IAP An IAP (Internet access provider) is a company that provides individuals and other ISP companies access to the Internet. An IAP is a type of Internet service provider (ISP). (The other main service that an ISP provides is Web site building and virtual hosting.) An IAP has the equipment and the telecommunication line access required to have POP on the Internet for the geographic area served. The larger IAPs have their own high-speed leased lines so that they are less dependent on the telecommunication providers and can provide better service to their customers. Among the largest national and regional ISPs are AT&T WorldNet, IBM Global Network, MCI, Netcom, UUNet, and PSINet.
IAPs also include regional providers such as New England’s NEARNet and the San Francisco Bay area BARNet. They also include thousands of local providers. In addition, Internet users can also get access through online service providers (OSP) such as America Online.
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The larger IAPs interconnect with each other through MAE (ISP switching centers run by MCI WorldCom) or similar centers. The arrangements they make to exchange traffic are known as peering agreements. There are several very comprehensive lists of IAPs (usually identified as ISPs) available to Web users.
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An iButton is a microchip similar to those used in a smart card but housed in a round stainless steel button of 17.35mm 6 3.1mm—5.89mm in size (depending on the function). The iButton was invented and is still manufactured exclusively by Dallas Semiconductor mainly for applications in harsh and demanding environments. Like a smart card, an iButton does not have an internal power source. It requires connection to a reader (known as a Blue Dot Receptor) in order to be supplied with power and to receive input and send output. Unlike some smart cards, there are currently no contactless iButtons: They require physical contact with a reader to function. There are iButtons that measure temperature (for food storage and transport); have an electronic ID (for physical access to buildings); and store e-cash (for purchases both in stores and via the web). For e-commerce applications, the iButton can support JavaCard 2.0/OpenCard standards in addition to proprietary software. iButtons have an advantage over conventional smart cards in term of durability and longevity. The stainless steel casing gives iButton a far greater ability to survive in a range of temperatures—all versions are functional from -40 C to +70 C—and in a much harsher range of environments (such as exposure to salt water and long term exposure to physical impacts) than the plastic smart card. For e-commerce and personal ID usage, iButtons can be mounted on a range of personal accessories: Watch, ring, key chain, or dog tag. As of early 2000, Dallas Semiconductor had shipped over 27 million iButtons around the world. This figure is below that of smart cards because of a larger installed user base for smart cards, the comparatively high cost of iButtons, the fact that iButtons have a long life, and because Dallas Semiconductor has not licensed the patents for external manufacture. Thus far, the major successes for iButton have been in Turkey as an e-purse for the mass transit system; in Argentina and Brazil for parking meters; and in the U.S. as Blue Mailbox attachments that improve postal efficiency.
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See ‘‘integrated circuit’’
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ICANN ICANN (Internet Corporation for Assigned Names and Numbers) is the private (non-government) non-profit corporation with responsibility for IP address space allocation, protocol parameter assignment, domain name system management, and root server system management functions, the services previously performed by the Internet Assigned Numbers Authority (IANA). (ICANN is usually pronounced EYE-can, as in ‘‘I can at least try to manage the Internet.’’) Initial members of the ICANN board were chosen by the late Dr. Jonathon Postel, who headed IANA. IANA derived its authority under a contract from the U.S. government which financed the original research network, Advanced Research Projects Agency Network, from which the Internet grew. The need to internationalize the governing of the Internet (among other concerns) led the U.S. government to recommend the origin of ICANN as a global, governmentindependent entity to manage the systems and protocols that keep the Internet going. The U.S. government is essentially turning over control of the Internet to ICANN although domain name registration performed by Network Solutions, Inc. will continue to be under U.S. government contract for a limited time. ICANN has a board of nineteen Directors, nine At-Large Directors, nine to be nominated by Supporting Organizations, and the President/CEO (ex officio). The nine At-Large Directors of the Initial Board are serving oneyear terms and will be succeeded by At-Large Directors elected by an at-large membership organization.
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Since its beginning, ICANN has had to deal with controversial issues (such as what new top-level domain names should be permitted and whether alternative root server systems should be allowed).
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different setting, the page may have unneeded space on the right size of the display window, but this is a risk that the designer chooses to take. A jello page is one in which the primary content is centered on the page. It also has a fixed-width column, but it is always centered in any size window and at any display resolution. Its center alignment eliminates the excessive right margin seen in an ‘‘ice’’ page (when viewed on a large monitor). But it can have excessive margins on both sides of the centered content. A liquid page, according to Glenn Davis of Project Cool, is (to paraphrase slightly): ...‘‘a Web page that will reflow to fit no matter what size window you pour it into.’’ Unlike the ice and jello approaches, a liquid page or content area leaves no blank margin on the right or the left. However, unless special care is taken, elements in a liquid page can sometimes have too much or too little space between them. More sophisticated designers sometimes use a combination of tables and tables-within-tables to define and control layout space and element placement in the various resolutions and windows sizes users may choose to use. To create liquid content spaces or pages, you may be able to determine the window size and dynamically adjust your content layout to meet each user’s requirements.
ICM file In Windows 95 and possibly other operating systems, an ICM (Image Color Matching) file contains a color system profile for a particular application or device such as a color printer or scanner. This profile is matched to a common profile that in turn can be mapped to other device’s individual profiles. This allows the color system used by one computer device to match the colors used by other applications and devices on the same or other computer platforms. A profile commonly adopted by some programs and devices is the Kodak Professional Color System profile. An ICM file usually carries the suffix ‘‘.icm‘‘. Microsoft, Apple, and Adobe are among the manufacturers who adhere to the ICC’s color profiling system.
For ICE, see Information and Content Exchange.
ICMP
Ice, jello, and liquid are related terms describing three approaches to controlling content placement on a Web page. Because the browser user can control and change both screen resolution and window size, the Web page designer is challenged to design a page that will achieve its intended effect in spite of user resizing.
ICMP (Internet Control Message Protocol) is a message control and error-reporting protocol between a host server and a gateway to the Internet. ICMP uses Internet Protocol (IP) datagrams, but the messages are processed by the IP software and are not directly apparent to the application user.
An ice page is one in which the primary content has a fixed width in pixels and assumes a left margin alignment. Such a page is designed to display optimally on one particular display resolution setting and window size and either specifies or assumes that size. If the resolution is set to a
icon 1) In a computer’s graphical user interface (GUI), an icon (pronounced EYE-kahn) is an image that represents an application, a capability, or some other concept or specific
IDL entity with meaning for the user. An icon is usually selectable but can also be a nonselectable image such as a company’s logo.
Integrated Drive Electronics (EIDE). IDE gets its name because the disk drive controller is built into the logic board in the disk drive.
2) On a Web page, an icon is often a graphical image that represents the topic or information category of another Web page. Frequently, the icon is a hypertext link to that page. Typically, icons are gathered in one or two places on a page, either as separate graphic files or as a single image map.
IDE was adopted as a standard by American National Standards Institute (ANSI) in November 1990. The ANSI name for IDE is Advanced Technology Attachment (ATA). The IDE (ATA) standard is one of several related standards maintained by the T10 Committee.
3) Icon is also a lexical programming language, commonly thought to be an evolution of the SNOBOL programming language.
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ICP
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An ICP (Internet commerce provider) is a company that sells software and services that enable a merchant with a Web site to build an online store on the merchant’s own site or on the provider’s site. Products and services can be prepackaged or customized or some combination. Typically, a pre-packaged ICP product lets a merchant set up an online store using one of several design templates, arranges for secure credit card transactions, and provides order tracking. An ICP may also provide services related to order fulfillment, such as shipping and customer relations.
iDEN (Integrated Digital Enhanced Network) is a wireless technology from Motorola combining the capabilities of a digital cellular telephone, two-way radio, alphanumeric pager, and data/fax modem in a single network. iDEN operates in the 800 MHz, 900MHz, and 1.5 GHz bands and is based on time division multiple access (TDMA) and GSM architecture. It uses Motorola’s Vector Sum Excited Linear Predictors (VSELP) vocoder for voice compression and QAM modulation to deliver 64 Kbps over a 25 KHz channel.
Some Internet access providers (IAP) and Web design shops also act as ICPs, offering customized services or serving as a vendor of packaged products.
ICQ ICQ (‘‘I Seek You’’) is a program you can download that will let you know when friends and contacts are also online on the Internet. ICQ allows you to page them, chat with them, and initiate and participate in PC-to-PC calls, PC-to-phone and phone-to-phone calling cards calls. Like AOL’s Instant Messenger (AIM), in order to use ICQ, both parties must have downloaded the program. ICQ enables you to send messages, files (single, multiple or whole directories), and URLs directly to your friends. In addition, you can initiate an IRC-style chat session or voice and video-voice connection and play games with other ICQ members that you are in touch with. Your contact is signaled of an incoming event in real time and has immediate access to it.
IDE IDE (Integrated Drive Electronics) is a standard electronic interface used between a computer motherboard’s data paths or bus and the computer’s disk storage devices. The IDE interface is based on the IBM PC Industry Standard Architecture (ISA) 16-bit bus standard, but it is also used in computers that use other bus standards. Most computers sold today use an enhanced version of IDE called Enhanced
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See ‘‘International Data Encryption Algorithm’’
iDEN is designed to give the mobile user quick access to information without having to carry around several devices. In early 2001, a new iDEN handset is scheduled for release that will tackle added business needs like logistics management and alerts. Currently, iDEN systems work in more than a dozen countries.
IDF See ‘‘interface device’’
IDL 1) IDL (interface definition language) is a generic term for a language that lets a program or object written in one language communicate with another program written in an unknown language. In distributed object technology, it’s important that new objects be able to be sent to any platform environment and discover how to run in that environment. An Object Request Broker (ORB) is an example of a program that would use an interface definition language to ‘‘broker’’ communication between one object program and another one. An interface definition language works by requiring that a program’s interfaces be described in a stub or slight extension of the program that is compiled into it. The stubs in each program are used by a broker program to allow them to communicate. 2) IDL (Interactive Data Language) is a language for creating visualizations based on scientific or other data.
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3) IDL (interactive distance learning) is a general term for learning that takes place through remote telecommunication and that allows students to participate from a distance. Television has been used for many years for non-interactive distance learning. Teleconference classes are becoming more common where higher bandwidth and such technologies as ISDN and satellite communication permit. The World Wide Web, with or without multimedia, offers new possibilities.
idoru An idoru is a virtual (computer-created) media star. If real pop music stars sometimes seem to become the conception of their producers’ dreams, perhaps it would be simpler to start from scratch and design your own media star to perform (think of a fully-developed hologram singing and dancing before tens of thousands of insane fans in Central Park), be interviewed by Rolling Stone, and even evolve and grow older! The idea is explored in the novel, Idoru, by science-fiction writer William Gibson.
IDSL is a system in which digital data is transmitted at 128 Kbps on a regular copper telephone line (twisted pair) from a user to a destination using digital (rather than analog or voice) transmission, bypassing the telephone company’s central office equipment that handles analog signals. IDSL uses the Integrated Services Digital Network (ISDN) Basic Rate Interface in ISDN transmission code.
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The differences between IDSL and ISDN are:
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IEEE 1394, High Performance Serial Bus, is an electronics standard for connecting devices to your personal computer. IEEE 1394 provides a single plug-and-socket connection on which up to 63 devices can be attached with data transfer speeds up to 400 Mbps (megabits per second). The standard describes a serial bus or pathway between one or more peripheral devices and your computer’s microprocessor. Many peripheral devices now come equipped to meet IEEE 1394. Two popular implementations of IEEE 1394 are Apple’s FireWire and Sony’s i.LINK. IEEE 1394 implementations provide: L
A simple common plug-in serial connector on the back of your computer and on many different types of peripheral devices
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A thin serial cable rather than the thicker parallel cable you now use to your printer, for example
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A very high-speed rate of data transfer that will accommodate multimedia applications (100 and 200 megabits per second today; with much higher rates later)
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Hot-plug and plug and play capability without disrupting your computer
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The ability to chain devices together in a number of different ways without terminators or complicated setup requirements
See ‘‘International Electrotechnical Commission’’
In time, IEEE 1394 implementations are expected to replace and consolidate today’s serial and parallel interfaces, including Centronics parallel, RS-232C, and Small Computer System Interface (SCSI). The first products to be introduced with FireWire include digital cameras, digital video discs (DVDs), digital video tapes, digital camcorders, and music systems. Because IEEE 1394 is a peer-to-peer interface, one camcorder can dub to another without being plugged into a computer. With a computer equipped with the socket and bus capability, any device (for example, a video camera) can be plugged in while the computer is running.
IEEE
Briefly How It Works
The IEEE (Institute of Electrical and Electronics Engineers) describes itself as ‘‘the world’s largest technical professional society—promoting the development and application of electrotechnology and allied sciences for the benefit of humanity, the advancement of the profession, and the wellbeing of our members.’’
There are two levels of interface in IEEE 1394, one for the backplane bus within the computer and another for the point-to-point interface between device and computer on the serial cable. A simple bridge connects the two environments. The backplane bus supports 12.5, 25, or 50 megabits per second data transfer. The cable interface supports 100, 200, or 400 megabits per second. Each of these interfaces can handle any of the possible data rates and change from one to another as needed.
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ISDN passes through the phone company’s central office voice network; IDSL bypasses it by plugging into a special router at the phone company end
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ISDN requires call setup; IDSL is a dedicated service
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ISDN may involve per-call fees; IDSL may be billed at a flat rate with no usage charges
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IDSL is a technology developed by Ascend Communications (now part of Lucent Technologies). IDSL is an expedient approach that allows use of existing ISDN card technology for data-only use.
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The IEEE fosters the development of standards that often become national and international standards. The organization publishes a number of journals, has many local chapters, and several large societies in special areas, such as the IEEE Computer Society.
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IIOP The serial bus functions as though devices were in slots within the computer sharing a common memory space. A 64-bit device address allows a great deal of flexibility in configuring devices in chains and trees from a single socket. IEEE 1394 provides two types of data transfer: asynchronous and isochronous. Asynchronous is for traditional load-andstore applications where data transfer can be initiated and an application interrupted as a given length of data arrives in a buffer. Isochronous data transfer ensures that data flows at a pre-set rate so that an application can handle it in a timed way. For multimedia applications, this kind of data transfer reduces the need for buffering and helps ensure a continuous presentation for the viewer. The 1394 standard requires that a device be within 4.5 meters of the bus socket. Up to 16 devices can be connected in a single chain, each with the 4.5 meter maximum (before signal attenuation begins to occur) so theoretically you could have a device as far away as 72 meters from the computer. Another new approach to connecting devices, the Universal Serial Bus (USB), provides the same ‘‘hot plug’’ capability as the 1394 standard. It’s a less expensive technology but data transfer is limited to 12 Mbps (million bits per second). Small Computer System Interface offers a high data transfer rate (up to 40 megabytes per second) but requires address preassignment and a device terminator on the last device in a chain. FireWire can work with the latest internal computer bus standard, Peripheral Component Interconnect (PCI), but higher data transfer rates may require special design considerations to minimize undesired buffering for transfer rate mismatches.
IETF The IETF (Internet Engineering Task Force) is the body that defines standard Internet operating protocols such as TCP/ IP. The IETF is supervised by the Internet Society’s Internet Architecture Board (IAB). IETF members are drawn from the Internet Society’s individual and organization membership. Standards are expressed in the form of Requests for Comments (RFCs).
IGMP See ‘‘Internet Group Management Protocol’’
IGP An IGP (Interior Gateway Protocol) is a protocol for exchanging routing information between gateways (hosts with routers) within an autonomous network (for example, a system of corporate local area networks). The routing information can then be used by the Internet Protocol (IP) or other network protocols to specify how to route transmissions.
There are two commonly used IGPs: the Routing Information Protocol (RIP) and the Open Shortest Path First (OSPF) protocol. Also see the Exterior Gateway Protocol (EGP).
IIOP IIOP (Internet Inter-ORB Protocol) is a protocol that makes it possible for distributed programs written in different programming languages to communicate over the Internet. IIOP is a critical part of a strategic industry standard, the Common Object Request Broker Architecture (CORBA). Using CORBA’s IIOP and related protocols, a company can write programs that will be able to communicate with their own or other company’s existing or future programs wherever they are located and without having to understand anything about the program other than its service and a name. CORBA and IIOP compete with a similar strategy from Microsoft called the Distributed Component Object Model (DCOM). (Microsoft and the Object Management Group, sponsors of CORBA, have agreed to develop software bridges between the two models so that programs designed for CORBA can communicate with programs designed for DCOM.) CORBA and IIOP assume the client/server model of computing in which a client program always makes requests and a server program waits to receive requests from clients. When writing a program, you use an interface called the General Inter-ORB Protocol (GIOP). The GIOP is implemented in specialized mappings for one or more network transport layers. Undoubtedly, the most important specialized mapping of GIOP is IIOP, which passes requests or receives replies through the Internet’s transport layer using the Transmission Control Protocol (TCP). Other possible transport layers would include IBM’s Systems Network Architecture (SNA) and Novell’s IPX. For a client to make a request of a program somewhere in a network, it must have an address for the program. This address is known as the Interoperable Object Reference (IOR). Using IIOP, part of the address is based on the server’s port number and IP address. In the client’s computer, a table can be created to map IORs to proxy names that are easier to use. The GIOP lets the program make a connection with an IOR and then send requests to it (and lets servers send replies). A Common Data Representation (CDR) provides a way to encode and decode data so that it can be exchanged in a standard way. A somewhat similar protocol, the Remote Method Invocation (RMI), was developed by Sun Microsystems to serve its cross-platform framework for the Java programming language. Sun has provided a way so that programming that uses the RMI can be mapped to IIOP.
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IIS (Internet Information Server) is a group of Internet servers (including a Web or Hypertext Transfer Protocol server and a File Transfer Protocol server) with additional capabilities for Microsoft’s Windows NT and Windows 2000 Server operating systems. IIS is Microsoft’s entry to compete in the Internet server market that is also addressed by Apache, Sun Microsystems, O’Reilly, and others. With IIS, Microsoft includes a set of programs for building and administering Web sites, a search engine, and support for writing Web-based applications that access databases. Microsoft points out that IIS is tightly integrated with the Windows NT and 2000 Servers in a number of ways, resulting in faster Web page serving.
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A typical company that buys IIS can create pages for Web sites using Microsoft’s Front Page product (with its WYSIWYG user interface). Web developers can use Microsoft’s Active Server Page (ASP) technology, which means that applications—including ActiveX controls—can be imbedded in Web pages that modify the content sent back to users. Developers can also write programs that filter requests and get the correct Web pages for different users by using Microsoft’s Internet Server Application Program Interface (ISAPI) interface. ASPs and ISAPI programs run more efficiently than common gateway interface (CGI) and server-side include (SSI) programs, two current technologies. (However, there are comparable interfaces on other platforms.) Microsoft includes special capabilities for server administrators designed to appeal to Internet service providers (ISPs). It includes a single window (or ‘‘console’’) from which all services and users can be administered. It’s designed to be easy to add components as snap-ins that you didn’t initially install. The administrative windows can be customized for access by individual customers. IIS works closely with the Microsoft Transaction Server to access databases and provide control at the transaction level. It also works with Microsoft’s Netshow in the delivery of streaming audio and video, delayed or live.
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An ILEC (incumbent local exchange carrier) is a telephone company in the U.S. that was providing local service when the Telecommunications Act of 1996 was enacted. ILECs include the former Bell operating companies (BOCs) which were grouped into holding companies known collectively as the regional Bell operating companies (RBOCs) when the Bell System was broken up by a 1983 consent decree. ILECs are in contradistinction to CLECs (competitive local exchange carriers). A ‘‘local exchange’’ is the local ‘‘central office’’ of an LEC. Lines from homes and businesses terminate at a local exchange. Local exchanges connect to other local exchanges
within a local access and transport area (LATA) or to interexchange carriers (IXCs) such as long-distance carriers AT&T, MCI, and Sprint.
iLINK See ‘‘i.LINK’’
illegal operation An illegal operation, a term often seen in error messages, is a command to the operating system or an instruction to the computer processor that specifies an operation that is unknown to the operating system or processor and that therefore can’t be performed. The result is that the operating system or processor usually terminates the program that made the ‘‘illegal’’ request. If that program was an application that you were using, you may find that data you were entering was lost or, in some cases, can be recovered from a backed-up copy that the system made for you. A computer’s processor knows what operation to perform as the result of an operation code (or opcode) within the instruction. Failing to recognize an opcode, the processor will return information to the operating system which in turn will usually terminate the application program that sent the bad instruction and issue a message to the user about the illegal operation. An illegal operation may mean a bug in the application program (or occasionally in the operating system) or it may be the result of a combination of unusual circumstances that the program and operating system could or did not anticipate. In the latter case, the problem may not be repeated or may be intermittent. The usual practice if you get an illegal operation message is to continue working if the application is still running, or to restart the program if it has been terminated, and, if the problem recurs, to reboot the operating system and try again. If you continue to experience illegal operation errors with an application, report the problem to the software vendor.
ILOVEYOU virus The ILOVEYOU virus comes in an e-mail note with ‘‘I LOVE YOU’’ in the subject line and contains an attachment that, when opened, results in the message being re-sent to everyone in the recipient’s Microsoft Outlook address book and, perhaps more seriously, the loss of every JPEG, MP3, and certain other files on the recipient’s hard disk. Because Microsoft Outlook is widely installed as the e-mail handler in corporate networks, the ILOVEYOU virus can spread rapidly from user to user within a corporation. On May 4, 2000, the virus spread so quickly that e-mail had to be shut down in a number of major enterprises such as the Ford Motor Company. The virus reached an estimated 45 million users in a single day.
image map The attachment in the ILOVEYOU virus is a VBScript program that, when opened (for example, by double-clicking on it with your mouse), finds the recipient’s Outlook address book and re-sends the note to everyone in it. It then overwrites (and thus destroys) all files of the following file types: JPEG, MP3, VPOS, JS, JSE, CSS, WSH, SCT and HTA. Users who don’t have a backup copy will have lost these files. (In March 1999, a virus named Melissa virus also replicated itself by using Outlook address books, but was less harmful in destroying user files.) The ILOVEYOU virus also resets the recipient’s Internet Explorer start page in a way that may cause further trouble, resets certain Windows registry settings, and also acts to spread itself through Internet Relay Chat (Internet Relay Chat). One of the first steps companies used to ward off the ILOVEYOU virus was to screen out notes with ILOVEYOU in the subject line. However, hackers quickly introduced copycat variations with subject lines variously identifying ‘‘JOKE’’ and ‘‘Mother’s Day!’’ as the content, but containing the same or similar VBScript code. At least 12 variations have been identified. The most sinister mutation is undoubtedly the one with the subject line containing ‘‘VIRUS ALERT!!!’’ Posing as a virus fix from Symantec, the note starts out with ‘‘Dear Symantec Customer.’’ The attachment (which should not be opened) is ‘‘protect.vbs.’’ Companies and users are advised to get or update anti-virus software that can help screen for the virus and remove it for users whose systems have been infected. Users are always advised never to open an e-mail attachment without screening it with anti-virus software or knowing exactly who sent it and what it is.
IM See ‘‘instant messaging’’
iMac Now offered in multiple designer colors, the iMac is a lowcost version of Apple Computer’s Macintosh. The iMac was designed to attract people who have never owned a personal computer and also to win back former Mac users who have moved to personal computers. Released in mid-August 1998, the initial version of the iMac featured a sleeklymolded designer-colored translucent case with a built-in 15inch display, a fast 233 MHz processor, and the Mac OS operating system. Traditionally, somewhat higher in price than Intel-based PCs, a low-priced Mac symbolized Apple’s determination to compete in and increase its share of the personal computer market. The iMac’s designers have omitted a floppy disk drive with the idea that few users will miss it. Apple is advertising the iMac as easy to set up and ideal for simple applications and surfing the Web (the ‘‘i’’ in its name is said to stand for ‘‘Internet’’).
image Also see imaging. An image is a visual representation of something. In information technology, the term has several usages: 1) An image is a picture that has been created or copied and stored in electronic form. An image can be described in terms of vector graphics or raster graphics. An image stored in raster form is sometimes called a bitmap. An image map is a file containing information that associates different locations on a specified image with hypertext links.
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2) An image is a section of random access memory (RAM) that has been copied to another memory or storage location.
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Image compression is minimizing the size in bytes of a graphics file without degrading the quality of the image to an unaccceptable level. The reduction in file size allows more images to be stored in a given amount of disk or memory space. It also reduces the time required for images to be sent over the Internet or downloaded from Web pages. There are several different ways in which image files can be compressed. For Internet use, the two most common compressed graphic image formats are the JPEG format and the GIF format. The JPEG method is more often used for photographs, while the GIF method is commonly used for line art and other images in which geometric shapes are relatively simple. Other techniques for image compression include the use of fractals and wavelets. These methods have not gained widespread acceptance for use on the Internet as of this writing. However, both methods offer promise because they offer higher compression ratios than the JPEG or GIF methods for some types of images. Another new method that may in time replace the GIF format is the PNG format. A text file or program can be compressed without the introduction of errors, but only up to a certain extent. This is called lossless compression. Beyond this point, errors are introduced. In text and program files, it is crucial that compression be lossless because a single error can seriously damage the meaning of a text file, or cause a program not to run. In image compression, a small loss in quality is usually not noticeable. There is no ‘‘critical point’’ up to which compression works perfectly, but beyond which it becomes impossible. When there is some tolerance for loss, the compression factor can be greater than it can when there is no loss tolerance. For this reason, graphic images can be compressed more than text files or programs.
image map In Web page development, an image map is a graphic image defined so that a user can click on different areas of the image and be linked to different destinations. You make an
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Whatis?Com’s Encyclopedia of Technology Terms image map by defining each of the sensitive areas in terms of their x and y coordinates (that is, a certain horizontal distance and a certain vertical distance from the left-hand corner of the image). With each set of coordinates, you specify a Uniform Resource Locator or Web address that will be linked to when the user clicks on that area. The X and Y coordinates are expressed in pixels either in a separate file called a map file or in the same HTML file that contains the link to the image map. Popular tools like MapEdit provide a graphical interface for creating an image map (so that you don’t have to figure out the X and Y coordinate numbers yourself).
imaging Imaging is the capture, storage, manipulation, and display of images. In document imaging, the emphasis is on capturing, storing, and retrieving information from the images (which are often mainly images of text). In graphical imaging, the emphasis is on the manipulation of created images in order to achieve special effects through rotating, stretching, blurring, resizing, twirling, and other changes to the original image.
IMAP See ‘‘Internet Message Access Protocol’’
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Originally, the map file had to be sent to the server. Now the creator can place the map information either at the server or at the client (a ‘‘client-side map’’).
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Image maps are used widely on many Web sites as a more adventuresome form of main menu.
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imaginary number
Like FYI (for your information), IMHO (in my humble opinion) is an abbreviation for a phrase sometimes used in online chatting and e-mail. Variations include IMNSHO (in my not so humble opinion).
IMHO
For similar terms, see chat abbreviations.
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An imaginary number is a quantity of the form ix, where x is a real number and i is the positive square root of -1. The term ‘‘imaginary’’ probably originated from the fact that there is no real number z that satisfies the equation z2 = 1. But imaginary numbers are no less ‘‘real’’ than real numbers. The quantity i is called the unit imaginary number. In engineering, it is denoted j, and is known as the j operator.
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The unit imaginary number has some intriguing properties. For example:
i-Mode is the packet-based service for mobile phones offered by Japan’s leader in wireless technology, NTT DoCoMo. Unlike most of the key players in the wireless arena, i-Mode eschews the Wireless Application Protocol (WAP) and uses a simplified version of HTML, Compact Wireless Markup Language (CWML) instead of WAP’s Wireless Markup Language (WML). NTT DoCoMo has said that eventually it will support WAP and WML, but the company has not said exactly when this will happen.
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(-i)2 = -1 but -i is different from i i3 = i2i = (-1)i = -i
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i4 = i2i2 = (-1)(-1) = 1 i5 = i3i2 = (i3)(-1) = (-i)(-1) = i
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in = i(n-4) when n is a natural number larger than 4
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As i is raised to higher natural-number powers, the resultant cycles through four values: i, -1, -i, and 1 in that order. No real number behaves like that!
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The set I of imaginary numbers consists of the set of all possible products iw, where w is an element of the set R of real numbers. Therefore, the sets I and R are in one-to-one correspondence. The sum v + iw of a real number v and an imaginary number iw forms a complex number. The set C of all complex numbers corresponds one-to-one with the set R ? R of all ordered pairs of real numbers. The set C also corresponds one-to-one with the points on a geometric plane. Imaginary and complex numbers are used in engineering, particularly in electronics. Real numbers denote electrical resistance, imaginary numbers denote reactance, and complex numbers denote impedance.
IMing See ‘‘instant messaging’’
i-Mode
First introduced in 1999, i-Mode was the world’s first smart phone for Web browsing. The i-Mode wireless data service offers color and video over many phones. Its mobile computing service enables users to do telephone banking, make airline reservations, conduct stock transactions, send and receive e-mail, and have access to the Internet. As of early 2000, i-Mode had an estimated 5.6 million users.
impedance Impedance, denoted Z, is an expression of the opposition that an electronic component, circuit, or system offers to alternating and/or direct electric current. Impedance is a vector (two-dimensional)quantity consisting of two independent scalar (one-dimensional) phenomena: resistance and reactance. Resistance, denoted R, is a measure of the extent to which a substance opposes the movement of electrons among its atoms. The more easily the atoms give up and/or accept
impression electrons, the lower the resistance, which is expressed in positive real number ohms. Resistance is observed with alternating current (AC) and also with direct current (DC). Examples of materials with low resistance, known as electrical conductors, include copper, silver, and gold. Highresistance substances are called insulators or dielectrics, and include materials such as polyethylene, mica, and glass. A material with an intermediate levels of resistance is classified as a semiconductor. Examples are silicon, germanium, and gallium arsenide. Reactance, denoted X, is an expression of the extent to which an electronic component, circuit, or system stores and releases energy as the current and voltage fluctuate with each AC cycle. Reactance is expressed in imaginary number ohms. It is observed for AC, but not for DC. When AC passes through a component that contains reactance, energy might be stored and released in the form of a magnetic field, in which case the reactance is inductive (denoted +jXL); or energy might be stored and released in the form of an electric field, in which case the reactance is capacitive (denoted -jXC). Reactance is conventionally multiplied by the positive square root of -1, which is the unit imaginary number called the j operator, to express Z as a complex number of the form R + jXL (when the net reactance is inductive) or R7jXC (when the net reactance is capacitive).
depicted on the negative imaginary axis, moving downward. As an example, a complex impedance consisting of 4 ohms of resistance and +j5 ohms of inductive reactance is denoted as a vector from the origin to the point on the plane corresponding to 4 + j5.
+j 6 (4 + j 5) +j 4
+j 2
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In series circuits, resistances and reactances add together independently. Suppose a resistance of 100.00 ohms is connected in a series circuit with an inductance of 10.000 mH. At 4.0000 MHz, the complex impedance is:
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ZRL = R + jXL = 100.00 + j251.33
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If a capacitor of 0.0010000 mF is put in place of the inductor, the resulting complex impedance at 4.0000 MHz is:
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ZRC = R - jXC = 100.007j39.789 If all three components are connected in series, then the reactances add, yielding a complex impedance of: ZRLC = 100 + j251.337j39.789 = 100 + j211.5 This is the equivalent of a 100-ohm resistor in series with an inductor having +j211.5 ohms of reactance. At 4.0000 MHz, this reactance is presented by an inductance of 8.415 mH, as determined by plugging the numbers into the formula for inductive reactance and working backwards. (See the definition of for this formula, and for the corresponding formula for capacitive reactance.) Parallel RLC circuits are more complicated to analyze than are series circuits. To calculate the effects of capacitive and inductive reactance in parallel, the quantities are converted to inductive susceptance and capacitive susceptance. Susceptance is the reciprocal of reactance. Susceptance combines with conductance, which is the reciprocal of resistance, to form complex admittance, which is the reciprocal of complex impedance. Entire volumes have been devoted to the theoretical and practical aspects of resistance, conductance, reactance, susceptance, impedance, and admittance. An intermediate electronics text or reference book is recommended for further study.
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impression +j 2
+j 4
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The illustration shows a coordinate plane modified to denote complex-number impedances. Resistance appears on the horizontal axis, moving toward the right. (The left-hand half of this coordinate plane is not normally used because negative resistances are not encountered in common practice.) Inductive reactance appears on the positive imaginary axis, moving upward. Capacitive reactance is
In Web advertising, the term impression is sometimes used as a synonym for view, as in ad view. Online publishers offer and their customers buy advertising measured in terms of ad views or impressions. Since a single Web page can contain multiple ads (depending on its design), a site usually registers more ad views per unit of time than Web pages per unit of time. (Hits is the term for any requested file, including each of a page’s images. Although hits are of interest for traffic measurement purposes, they have no significance for advertisers.) Page or ad impressions are logged in a log that is maintained by the site server. Programs like Web Trends read the log, abstract meaning from it, and generate a report about site usage. Other programs, such as Central Ad, can keep track of all ad impressions that have been sent and how many of these were clicked on by users.
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IMT-2000 direct spread
incumbent local exchange carrier
A
See ‘‘WCDMA’’
See ‘‘ILEC’’
in the wild
Indefeasible Right of Use
According to noted computer virus expert Paul Ducklin, in order for a virus to be considered in the wild, ‘‘it must be spreading as a result of normal day-to-day operations on and between the computers of unsuspecting users.’’ Although there are an estimated 47,000 computer viruses, fewer than 600 are said to be circulating outside of laboratories and research facilities—hence, in the wild. Experts say these wild viruses pose the most significant threat to computers. Wild viruses typically contain a damaging payload and the ability to wipe out all computer files, sometimes even damaging a computer’s BIOS.
In telecommunications, Indefeasible Right of Use (IRU) is the effective long-term lease (temporary ownership) of a portion of the capacity of an international cable. IRUs are specified in terms of a certain number of channels of a given bandwidth. IRU is granted by the company or consortium of companies that built the (usually optical fiber) cable. Some IRU legal agreements forbid resale of the capacity ownership. For at least one major international cable owner, an IRU ownership period is granted for 25 years.
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The Wild List Organization International has compiled a cumulative list of viruses considered to be in the wild. New viral strains are added as they are reported and can be verified. Recent examples of computer viruses in the wild include Melissa, CIH, and W.32 Navidad (the Kriz virus).
in-band signaling
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In the public switched telephone network, (PSTN), in-band signaling is the exchange of signaling (call control) information on the same channel that the telephone call itself is using. Today, most long-distance communication uses out-of-band signaling as specified in various Signaling System 7 (SS7) standards.
An IRU gives a large-scale Internet service provider (ISP) the ability to assure its own customers of international service on a long-term basis.
Indeo Indeo is a compression algorithm developed by Intel and used by Microsoft’s Video for Windows. Other digital video compression algorithms include MPEG and PLV.
independent software vendor See ‘‘ISV’’
Indexed Sequential Access Method
incubator
See ‘‘ISAM’’
In the business world, an incubator is an enterprise that is set up to provide office space, equipment, and sometimes mentoring assistance and capital to new businesses that are just getting started. (The term is familiar in poultry farming, where an incubator is a specially-equipped home for baby chickens.) Business incubators are set up by universities, non-profit groups, and increasingly by venture capitalists, especially for new Web businesses.
inductance
On the Web, the pace of growth and competition, especially of retail and business-to-business (B2B) Web sites, is fostering software packages for the creation of ‘‘out-of-thebox’’ Web sites. For more ambitious Web sites and technology product development, venture capitalists are discovering that the incubation idea provides an organized way to finance and monitor the progress of its fledgling ‘‘chicks.’’ In early 2000, the number of incubators in the U.S. was approaching 1,000, with the largest located in major cities and many university-sponsored incubators scattered throughout the country. There are also incubators in the UK and elsewhere.
See ‘‘inductor’’
inductor An inductor is a passive electronic component that stores energy in the form of a magnetic field. In its simplest form, an inductor consists of a wire loop or coil. The inductance is directly proportional to the number of turns in the coil. Inductance also depends on the radius of the coil and on the type of material around which the coil is wound. For a given coil radius and number of turns, air cores result in the least inductance. Dielectric materials such as wood, glass, and plastic are essentially the same as air for the purposes of inductor winding. Ferromagnetic substances such as iron, laminated iron, and powdered iron increase the inductance obtainable with a coil having a given number of turns. In some cases, this increase is on the order of thousands of times. The shape of the core is also significant. Toroidal (donut-shaped) cores provide more inductance, for a given core material and number of turns, than solenoidal (rod-shaped) cores.
infomediary The standard unit of inductance is the henry, abbreviated H. This is a large unit. More common units are the microhenry, abbreviated mH (1 mH =10-6H) and the millihenry, abbreviated mH (1 mH =10-3 H). Occasionally, the nanohenry (nH) is used (1 nH = 10-9 H). It is difficult to fabricate inductors onto integrated circuit (IC) chips. Fortunately, resistors can be substituted for inductors in most microcircuit applications. In some cases, inductance can be simulated by simple electronic circuits using transistors, resistors, and capacitors fabricated onto IC chips. Inductors are used with capacitors in various wireless communications applications. An inductor connected in series or parallel with a capacitor can provide discrimination against unwanted signals. Large inductors are used in the power supplies of electronic equipment of all types, including computers and their peripherals. In these systems, the inductors help to smooth out the rectified utility AC, providing pure, battery-like DC.
industrial strength In information technology, industrial strength is the ability of a hardware or software product or a system to work capably and dependably in the operational world of business. Prior to the advent of today’s information technology revolution, the term was used to describe such products as cleansers, lubricants, and stepladders. In the world of mainframe computers, IBM’s flagship operating system, OS/390 (and its predecessor versions of MVS), has been described as having industrial strength.
Industry Standard Architecture See ‘‘ISA’’
InfiniBand InfiniBand is an architecture and specification for data flow between processors and I/O devices that promises greater bandwidth and almost unlimited expandability in tomorrow’s computer systems. In the next few years, InfiniBand is expected to gradually replace the existing Peripheral Component Interconnect (PCI) shared-bus approach used in most of today’s personal computers and servers. Offering throughput of up to 2.5 gigabytes per second and support for up to 64,000 addressable devices, the architecture also promises increased reliability, better sharing of data between clustered processors, and built-in security. InfiniBand is the result of merging two competing designs, Future I/O, developed by Compaq, IBM, and Hewlett-Packard, with Next Generation I/O, developed by Intel, Microsoft, and Sun Microsystems. For a short time before the group came up with a new name, InfiniBand was called System I/O.
Existing PCs and server systems are hampered with an inflexible and relatively slow internal data flow system, including today’s 64-bit, 66 MHz PCI bus. As the amount of data coming into and flowing between components in the computer increases, the existing bus system becomes a bottleneck. InfiniBand provides a revolutionary new approach. Instead of sending data in parallel (typically 32 bits at a time, but in some computers 64 bits) across the backplane bus (data path), InfiniBand specifies a serial (bitat-a-time) bus. Fewer pins and other electrical connections are required, saving manufacturing cost and improving reliability. The serial bus can carry multiple channels of data at the same time in a multiplexing signal. InfiniBand also supports multiple memory areas, each of which can addressed by both processors and storage devices. Unlike the present I/O subsystem in a computer, InfiniBand seems like a full-fledged network. The InfiniBand Trade Organization describes the new bus as an I/O network and views the bus itself as a switch since control information will determine the route a given message follows in getting to its destination address. In fact, InfiniBand uses Internet Protocol Version 6 (IPv6) with its 128-bit address, allowing an almost limitless amount of device expansion. With InfiniBand, data is transmitted in packets that together form a communication called a message. A message can be a remote direct memory access (RDMA) read or write operation, a channel send or receive message, a transactionbased operation (that can be reversed), or a multicast transmission. Like the channel model many mainframe users are familiar with, all transmission begins or ends with a channel adapter. Each processor (your PC or a data center server, for example) has what is called a host channel adapter (HCA) and each peripheral device has a target channel adapter (TCA). These adapters can potentially exchange information that ensures security or work with a given Quality of Service level.
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infomediary An infomediary is a Web site that provides specialized information on behalf of producers of goods and services and their potential customers. The term is a composite of information and intermediary. The advent of the Web has made possible quick 24-hour access to information databases that previously were not available. Gathering these information aggregates and adding services to them is now the business of companies like the Thomas Register of Manufacturers that can bring a base of information from the print medium to the Web. Perhaps more importantly as infomediaries are Web sites that are gathering, organizing, and linking to the new information and services that is being added to the Web.
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Whatis?Com’s Encyclopedia of Technology Terms Infomediaries can be divided into those intended for consumers and those intended for businesses. Any consumer e-commerce site that provides information as well as an order form could be classed as an infomediary. However, the term is more frequently used to refer to sites that offer information for businesses about suppliers and other businesses.
infomercial On television, an infomercial is a short or regular-length television program that combines information presentation with an integrated suggestion to buy a particular product or service. Exercise equipment and correspondence courses are two products sometimes sold through infomercials. In the print media, the equivalent to an infomercial is the multi-page ‘‘advertising supplement’’ that is formatted to look like the news part of the medium and contains real information content. The Internet analog of the infomercial appears to be what advertisers generally refer to as ‘‘rich media,’’ including interactive banner ads that sometimes include Flash animations. Similar neologisms include infotainment and edutainment.
infonesia Infonesia is an inability to remember where you saw or heard an item of information. The condition is usually temporary but can recur frequently. It is more common in ‘‘information societies’’, with their flows of e-mail, seasonal catalogs, personal digital assistants, and trial magazine subscriptions. Possibly the most pernicious form of infonesia is internesia, the inability to remember which Web site you saw an item of information on. A person who is racking their brain to remember the information source could properly be termed an infonesiac, although this usage is not common.
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Information is stimuli that has meaning in some context for its receiver. Some (if not all) kinds of information can be converted into data and passed on to another receiver. Relative to the computer, we can say that: Information is made into data, put into the computer where it is stored and processed as data, and then put out as data in some form that can be perceived as information. Knowledge is information that is readily accessible to its user (for example, either in one’s brain or in a nearby dictionary).
Information and Content Exchange Information and Content Exchange (ICE) is an XML-based standard protocol for electronic business-to-business (B2B) asset management. ICE defines an architecture and a common language that can be used as a means of automating Web content syndication (information sharing and reuse between Web sites) for publishing and ecommerce uses. Members of the ICE Authoring Group (which includes representatives from Adobe Systems, Microsoft, Sun Microsystems, Vignette, and National Semiconductor) are committed to the further development of ICE as an open standard. ICE version 1.1 was released in June 2000. ICE enables the automation of data supplying, exchanging, updating, and controlling without requiring the supplier to manually package content, or to maintain knowledge about the structure of recipient Web sites. ICE specifies creation of a trust relationship based on the Open Profiling System (OPS). Use of the ICE protocol enables data sharing between servers, so that, for example, syndicated portions of a Web page can be automatically updated when the source is updated, or new content can be automatically entered and integrated with existing content on a Web page. Each protocol message between servers consists of a valid (conforming to namespace criteria) XML document. XML tags are used to format data to be processed by the servers involved in the transaction. ICE is intended to make it possible for almost every aspect of B2B asset exchange to be automated. Use of the protocol allows information service providers to specify content, customize it for specific recipients, schedule its delivery, and maintain it. Nevertheless, the success of content syndication is still reliant on the development of a standard metadata vocabulary, considered to be another essential element. The Publishing Requirements for Industry Standard Metadata (PRISM) working group (an organization made up of content providers and vendors) is collaborating to develop such a standard.
information architecture 1) In technical writing, information architecture is the set of ideas about how all information in a given context should be treated philosophically and, in a general way, how it should be organized. In the context of a company making computer products, an information architect might say that ‘‘All of our product information should serve customer needs as expressed by tasks that they have to do with our products,’’ and would then develop a framework for organizing all existing and future product information in modules related to customer tasks. The results of this thinking would be expressed in an information architecture document.
information technology For product information in the example just discussed, the information architecture document would become the foundation for information planning, which focuses on using the architecture for a specific division, set of products, or individual product. In addition to information architecture and information planning, information design focuses more narrowly on activities that support the architecture and planning, such as style guidelines, graphic design motifs, page design, and the information aspects of industrial design (labels, knobs, and other physical aspects of the user interface).
2. Information planning, which focuses on all aspects required to prepare and support the information of a specific set of products, single product, or event over the product life or other time span. This generally includes understanding the product or event goals, studying the audience and their needs, considering possible information media, defining specific information ‘‘units’’ (books, chapters, Web pages, visualizations, and so forth), specifying the people who will work on them, what the schedule is, and how this work will relate to the work of others. The output is an information plan.
2) In Web site design, information architecture has a meaning similar to (1), but focused somewhat more narrowly on Web content as building blocks to be fit into a site’s visual design and navigation scheme.
3. Information design, which focuses more narrowly on the information itself in one or more information units, and may encompass the information aspects of industrial design (labels, knobs, and the physical interface), information content design, page design, Web site design, illustration design, typography decisions, and so forth. Information design can be applied to a single work, such as a city map, or to a corporation’s entire set of customer information. The output may be part of an information plan, a separate information design document, or simply the designed object or set of objects.
In technical writing in general and in Web site design in particular, information architecture is related to and generally supports the overlapping areas of content management, content distribution or syndication, and electronic publishing. Information architecture is also related to the treatment of information modules or building blocks as ‘‘objects’’ that can be described in one or a set of data definitions, typically using the Extensible Markup Language (XML). For example, an encyclopedic topic could be treated as a building block that could be distributed electronically using an XML definition that described each of the items in the topic (title, main content, author, date of authorship, and so forth).
information design Information design is the detailed planning of specific information that is to be provided to a particular audience to meet specific objectives. The information designer may or may not have available (or may create) an information architecture that defines the overall pattern or structure that is imposed on the information design and an information plan that defines information units and how they are to be completed. The output of an information design is sometimes expressed in written instructions, plans, sketches, drawings, or formal specifications. However, on very small projects, information design is likely to be much less formal. Information design can be distinguished from information architecture and information planning. In one view, there are three hierarchical levels of activity: 1. Information architecture, which is the general set of ideas about how all information in a given context should be organized. For example, one might say that ‘‘All of our product information should serve customer needs as expressed by tasks they have to do with our products,’’ and then develop a pattern that organizes all product information in modules related to customer tasks. The output is an information architecture document.
Information design ideas can often be tested in a usability laboratory by observing surrogate users trying to use the designed information and getting their feedback. The practice of information design invites questions into how people learn or prefer to learn and how they use information. It also raises questions about how to design information for different cultural and other contextual differences in the audience. Since information is now commonly delivered using electronic media with new possibilities for user interaction and as product designers have become more aware of the importance of usability, a new term, interaction design, has arisen as a corollary of information design. A number of universities now offer courses or degree programs centered on information design.
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Information Technology Information Sharing and Analysis Center
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See ‘‘IT-ISAC’’
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Information Technology Infrastructure Library
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See ‘‘ITIL’’
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information technology
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infosurfing
infranet
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Infosurfing is using the Internet and World Wide Web so that you get maximum information in the shortest amount of time, which for many people means favoring textual content over images. Infosurfing is practiced by librarians, professional researchers, journalists, people addicted to news, and almost anyone that gets impatient with multimedia or likes the focus of just plain text.
Infranet is a term used to refer to the infrastructure of a network, especially the Internet. For the Internet, the infranet is that portion of the public telephone network on which data packets are exchanged using Internet protocols. It can also be said to include private network infrastructures that use the Internet set of protocols, TCP/IP, whether or not these interconnect with the public Internet. The term was used to describe the major switching hardware and technologies, such as frame relay and asynchronous transfer mode, in an April 1999 article, ‘‘The Next Net,’’ in Wired magazine. John Chambers, head of Cisco Systems, is credited with coining the term.
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The simplest way to infosurf is to turn the pictures (images) off, using your browser option. Many sites specify an alternate text string in the HTML image tag that provides a brief textual descripion of each image in the space allocated for it. Otherwise, you’ll get a broken image icon from your browser. Turning off graphics is best when you’re familiar with most sites and know what you’re missing. Some infosurfers also turn off Java and JavaScript, turning them back on only when wanted. Once you get the idea of turning things off, some infosurfers also turn off cookie though cookies don’t in themselves impede the downloading of text. If you have the latest levels of Netscape Communicator and Microsoft Internet Explorer, you can specify that cookies be disabled entirely or that you be prompted about whether to accept one. If you infosurf for certain kinds of information, you’ll obviously want to bookmark the sites that specialize in what you’re interested in. You may want to subscribe to e-mail newsletters such as those at ZDNet, Women’s Wire, or whatis.com. These newsletters summarize stories you can choose to link to or not.
O
Other techniques include: opening the browser several times and looking in one window while downloading to another and increasing the size of your browser cache. If you’re constantly checking the latest news, you might set your start page at a news site. If you like to learn new words every day, you might start with whatis.
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Infotainment, combining information with entertainment, is a fairly recent neologism for a television program, Web site feature, or other presentation that combines information with entertainment. Most of today’s popular fact-based television shows, such as those on the Animal Planet channel, could be classed as infotainment. Serious information publishing sites such as those of TechTarget.com (including whatis.com) provide a limited amount of infotainment in the form of user polls, quizes, and contests. Also see infomercial and edutainment.
infrared radiation Infrared radiation (IR) or the term infrared alone refers to energy in the region of the electromagnetic radiation spectrum at wavelengths longer than those of visible light, but shorter than those of radio waves. Correspondingly, the frequencies of IR are higher than those of microwaves, but lower than those of visible light. Scientists divide the IR spectrum into three regions. The wavelengths are specified in microns (symbolized m, where 1 m = 10-6 meter) or in nanometers (abbreviated nm, where 1 nm = 10-9 meter = 0.001 m). The near IR band contains energy in the range of wavelengths closest to the visible, from approximately 0.750 to 1.300 m (750 to 1300 nm). The intermediate IR band (also called the middle IR band) consists of energy in the range 1.300 to 3.000 m (1300 to 3000 nm). The far IR band extends from 2.000 to 14.000 m (3000 nm to 1.4000 x 104 nm). Infrared is used in a variety of wireless communications, monitoring, and control applications. A few of the applications include home-entertainment remote-control boxes, wireless local area networks, links between notebook computers and desktop computers, cordless modems, intrusion detectors, motion detectors, and fire sensors.
infrastructure In information technology and on the Internet, infrastructure is the physical hardware used to interconnect computers and users. Infrastructure includes the transmission media, including telephone lines, cable television lines, and satellites and antennas, and also the routers, aggregators, repeaters, and other devices that control transmission paths. Infrastructure also includes the software used to send, receive, and manage the signals that are transmitted. In some usages, infrastructure refers to interconnecting hardware and software and not to computers and other devices that are interconnected. However, to some information technology users, infrastructure is viewed as everything that supports the flow and processing of information.
InstallShield Infrastructure companies play a significant part in evolving the Internet, both in terms of where the interrconnections are placed and made accessible and in terms of how much information can be carried how quickly.
ingress Ingress (pronounced IHN-grehs, from Latin ingressus or stepping into) is the act of entering something. For example, in telecommunications, an ingress router is a router through which a data packet enters a network from another network.
inheritance In object-oriented programming, inheritance is the concept that when a class of objects is defined, any subclass that is defined can inherit the definitions of one or more general classes. This means for the programmer that an object in a subclass need not carry its own definition of data and methods that are generic to the class (or classes) of which it is a part. This not only speeds up program development; it also ensures an inherent validity to the defined subclass object (what works and is consistent about the class will also work for the subclass).
initial public offering See ‘‘IPO’’
initialization Initialization is the process of locating and using the defined values for variable data that is used by a computer program. For example, an operating system or application program is installed with defaults or user-specified values that determine certain aspects of how the system or program is to function. Typically, these values are stored in initialization files (in Windows, these can be identified as files with an INI suffix). When the operating system or an application program is first loaded into memory, a part of the program performs initialization—that is, it looks in the initialization files, finds definite values to substitute for variable values, and acts accordingly. For example, the desktop appearance and application programs that are to be started along with the operating system are identified and loaded. The process of the user specifying initialization values is sometimes called configuration.
inode In a UNIX-based operating system, an inode is a computerstored description of an individual file in a UNIX file system.
iNotes See ‘‘Shimmer’’
Input/Output Supervisor See ‘‘IOS’’
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insect robot See ‘‘robot’’
InstallAnywhere InstallAnywhere is a program that can used by software developers to package a product written in Java so that it can be installed on any major operating system. As its name indicates, a big advantage of InstallAnywhere is that the developer does not have to package different distribution versions of a product for different operating systems. InstallAnywhere refers to this idea as a ‘‘universal installer.’’ The idea is possible because programs written in Java can be compiled into Java bytecode that is independent of operating system differences. Since major operating systems today come with a Java, one InstallAnywhere-packaged Java-based product will install on any of the systems. InstallAnywhere is intended to be easy for developers to use. Customized installers will deploy a package from the Web or CD-ROM. An InstallAnywhere-packaged Java-based product will install on Windows 95/98/NT, Solaris, Linux, AIX, HP-UX, IRIX, OS/2 and any other Java-enabled platform.
Installfest An Installfest is a special occasion when computer users get together to help each other install new programming, usually the Linux operating system and related programs, often together with experts and the resources to download programming from the Internet. The Installfest idea was originated by Linux enthusiasts and has been well supported by Linux distributors, which include Red Hat, S. U. S. E., Slackware, Debian, and Caldera. Typically, an Installfest is sponsored by a local computer club. The Installfest idea has caught on, and, at some computer club meetings, the Installfest is an ongoing part of each regular meeting.
InstallShield InstallShield is a product used by software developers to package software so that users can install it easily and safely. (It can also be used to uninstall a product.) InstallShield provides the developer with templates and prebuilt interface objects that make it easy to package the software. Users who receive software packaged with InstallShield are assured that it will be installed quickly and without affecting other applications or system characteristics.
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instant messaging See also IRC (Internet Relay Chat) and ICQ (I Seek You). Instant messaging (sometimes called IM or IMing) is the ability to easily see whether a chosen friend or co-worker is connected to the Internet and, if they are, to exchange messages with them. Instant messaging differs from ordinary e-mail in the immediacy of the message exchange and also makes a continued exchange simpler than sending e-mail back and forth. Most exchanges are text-only. However, some services allow attachments. AOL first popularized instant messaging and, with its large membership, is likely to be the most popular service for some time to come. AOL’s Instant Messenger can be used by AOL members but there is no requirement to be connected to the Internet through AOL. An Open Source alternative to AOL’s instant messaging is called Jabber. In order for IMing to work, both users (who must subscribe to the service) must be online at the same time, and the intended recipient must be willing to accept instant messages. (It is possible to set your software to reject messages.) An attempt to send an IM to someone who is not online, or who is not willing to accept IMs, will result in notification that the transmission cannot be completed. If the online software is set to accept IMs, it alerts the recipient with a distinctive sound, a window that indicates that an IM has arrived and allowing the recipient to accept or reject it, or a window containing the incoming message. Under most conditions, IMing is truly ‘‘instant.’’ Even during peak Internet usage periods, the delay is rarely more than a second or two. It is possible for two people to have a real-time online ‘‘conversation’’ by IMing each other back and forth.
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Also see automatic language translation. Instant translation is the translation from one language to another of Web pages, e-mail, and online chat text by a computer program so that the translated results appear almost instantly. Instant translation services are gaining in popularity because they help people who work for global businesses communicate and enable e-commerce sites to appeal to customers in other countries. Because much of the material on the Web is written in English and the majority of the world speaks other languages, most instant translation is from English to another language. IDC estimates that the machine translation industry will grow to $378 million by 2003. Most instant translation tools store frequently-encountered words, sentences, and phrases in a database that is accessed when a new document is translated. Until recently, vendors like Lernout & Hauspie, Systran, and Transparent Language dominated the instant translation space. However, IBM is entering the arena with the WebSphere Translation Server,
targeting Internet service providers and other companies that run their own servers. The IBM product creates and distributes Web content globally without the need for special Web pages or separate infrastructure. WebSphere Translation Server translates material from English to French, Spanish, German, Italian, Chinese, Japanese, and Korean but does not convert material into English. Some companies are especially cautious when using instant translation software, having discovered that even small translation errors can result in big business problems.
instantiate An instance is a particular realization of an abstraction or template such as a class of objects or a computer process. To instantiate is to create such an instance by, for example, defining one particular variation of object within a class, giving it a name, and locating it in some physical place. 1) In object-oriented programming, some writers say that you instantiate a class to create an object, a concrete instance of the class. The object is an executable file that you can run in a computer. 2) In the object-oriented programming language, Java, the object that you instantiate from a class is, confusingly enough, called a class instead of an object. In other words, using Java, you instantiate a class to create a specific class that is also an executable file you can run in a computer. 3) In approaches to data modeling and programming prior to object-oriented programming, one usage of instantiate was to make a real (data-filled) object from an abstract object as you would do by creating an entry in a table (which, when empty, can be thought of as a kind of class template for the objects to be filled in).
Institute of Electrical and Electronics Engineers See ‘‘IEEE’’
instruction An instruction is an order given to a computer processor by a computer program. At the lowest level, each instruction is a sequence of 0s and 1s that describes a physical operation the computer is to perform (such as ‘‘Add’’) and, depending on the particular instruction type, the specification of special storage areas called ‘‘registers’’ that may contain data that to be used in carrying out the instruction or the location in computer memory of data that is to be used in carrying out the instruction, or a direct or indirect reference to the location in memory of the data. In a computer’s assembler language, each language statement generally corresponds to a single processor instruction. In higher-level languages, a language statement generally results (after program compilation) in multiple processor instructions.
Integrated Drive Electronics In assembler language, a ‘‘macro instruction’’ is one that, during processing by the assembler program, expands to become multiple instructions (based on a previously coded ‘‘macro definition’’).
int ’’int’’ is one of the top-level domain names that can be used when choosing a domain name. This domain name is intended for sites related to international treaties or containing international databases. (For this reason, the domain name is not widely used.) Along with the secondlevel domain name, the top-level domain name is required in Web and e-mail addresses. For more information, see gTLD (generic top-level domain name).
integer An integer (pronounced IN-tuh-jer) is a whole number (not a fractional number) that can be positive, negative, or zero. Examples of integers are: 75, 1, 5, 8, 97, and 3,043. Examples of numbers that are not integers are: 71.43, 1 3/4, 3.14, .09, and 5,643.1. The set of integers, denoted Z, is formally defined as follows: Z = {..., 73, 72, 71, 0, 1, 2, 3, ...} In mathematical equations, unknown or unspecified integers are represented by lowercase, italicized letters from the ‘‘late middle’’ of the alphabet. The most common are p, q, r, and s. The set Z is a denumerable set. Denumerability refers to the fact that, even though there might be an infinite number of elements in a set, those elements can be denoted by a list that implies the identity of every element in the set. For example, it is intuitive from the list {..., 73, 72, 71, 0, 1, 2, 3, ...} that 356,804,251 and 767,332 are integers, but 356,804,251.5, 767,332.89, 74/3, and 0.232323 ... are not. The elements of Z can be paired off one-to-one with the elements of N, the set of natural numbers, with no elements being left out of either set. Let N = {1, 2, 3, ...}. Then the pairing can proceed in this way: Z
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In infinite sets, the existence of a one-to-one correspondence is the litmus test for determining cardinality, or size. The set of natural numbers and the set of rational numbers have the same cardinality as Z. However, the sets of real numbers, imaginary numbers, and complex numbers have cardinality larger than that of Z.
integrated circuit An integrated circuit (IC), sometimes called a chip or microchip, is a semiconductor wafer on which thousands or millions of tiny resistors, capacitors, and transistors are fabricated. An IC can function as an amplifier, oscillator, timer, counter, computer memory, or microprocessor. A particular IC is categorized as either linear (analog) or digital, depending on its intended application. Linear ICs have continuously variable output (theoretically capable of attaining an infinite number of states) that depends on the input signal level. As the term implies, the output signal level is a linear function of the input signal level. Ideally, when the instantaneous output is graphed against the instantaneous input, the plot appears as a straight line. Linear ICs are used as audio-frequency (AF) and radio-frequency (RF) amplifiers. The operational amplifier (op amp) is a common device in these applications. Digital ICs operate at only a few defined levels or states, rather than over a continuous range of signal amplitudes. These devices are used in computers, computer networks, modems, and frequency counters. Logic gates are the fundamental building blocks of digital ICs that work with binary data, that is, signals that have only two different states, called low (logic 0) and high (logic 1).
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integrated development environment
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(The abbreviation IDE also stands for Integrated Drive Electronics.)
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An integrated development environment (IDE) is a programming environment that has been packaged as an application program, typically consisting of a code editor, a compiler, a debugger, and a graphical user interface (GUI) builder. The IDE may be a standalone application or may be included as part of one or more existing and compatible applications. The BASIC programming language, for example, can be used within Microsoft Office applications, which makes it possible to write a WordBasic program within the Microsoft Word application. IDEs provide a userfriendly framework for many modern programming languages, such as Visual Basic, Java, and PowerBuilder. IDEs for developing HTML applications are among the most commonly used. For example, many people designing Web sites today use an IDE (such as HomeSite, DreamWeaver, or FrontPage) for Web site development that automates many of the tasks involved.
Integrated Digital Enhanced Network See ‘‘iDEN’’
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Integrated Services Digital Network
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See ‘‘ISDN’’
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integration Integration (from the Latin integer, meaning whole or entire) generally means combining parts so that they work together or form a whole. In information technology, there are several common usages: 1) Integration during product development is a process in which separately produced components or subsystems are combined and problems in their interactions are addressed. 2) Integration is an activity by companies that specialize in bringing different manufacturers’ products together into a smoothly working system. 3) In marketing usage, products or components said to be integrated appear to meet one or more of the following conditions: A) They share a common purpose or set of objectives. (This is the loosest form of integration.) B) They all observe the same standard or set of standard protocols or they share a mediating capability, such the Object Request Broker (ORB) in the Common Object Request Broker Architecture (CORBA). C) They were all designed together at the same time with a unifying purpose and/or architecture. (They may be sold as piece-parts but they were designed with the same larger objectives and/or architecture.) D) They share some of the same programming code. E) They share some special knowledge of code (such as a lower-level program interface) that may or may not be publicly available. (If not publicly available, companies have been known to sue to make it available in order to make competition fair.) In describing its Internet Information Manager (IIS), Microsoft says that it is ‘‘tightly integrated,’’ apparently meaning that it meets conditions (A), (B), to some extent (C), possibly a bit of (D), and, if any, an unstated amount of (E).
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Integrity, in terms of data and network security, is the assurance that information can only be accessed or modified by those authorized to do so. Measures taken to ensure integrity include controlling the physical environment of networked terminals and servers, restricting access to data, and maintaining rigorous authentication practices. Data integrity can also be threatened by environmental hazards, such as heat, dust, and electrical surges. Practices followed to protect data integrity in the physical environment include: Making servers accessible only to network administrators, keeping transmission media (such as cables and connectors) covered and protected to ensure
that they cannot be tapped, and protecting hardware and storage media from power surges, electrostatic discharges, and magnetism. Network administration measures to ensure data integrity include: maintaining current authorization levels for all users, documenting system administration procedures, parameters, and maintenance activities, and creating disaster recovery plans for occurrences such as power outages, server failure, and virus attacks.
intellectual capital Intellectual capital is knowledge that can be exploited for some money-making or other useful purpose. The term combines the idea of the intellect or brain-power with the economic concept of capital, the saving of entitled benefits so that they can be invested in producing more goods and services. Intellectual capital can include the skills and knowledge that a company has developed about how to make its goods or services; individual employees or groups of employees whose knowledge is deemed critical to a company’s continued success; and its aggregation of documents about processes, customers, research results, and other information that might have value for a competitor that is not common knowledge.
intelligent agent On the Internet, an intelligent agent (or simply an agent) is a program that gathers information or performs some other service without your immediate presence and on some regular schedule. Typically, an agent program, using parameters you have provided, searches all or some part of the Internet, gathers information you’re interested in, and presents it to you on a daily or other periodic basis. An agent is sometimes called a bot (short for robot). Other agents have been developed that personalize information on a Web site based on registration information and usage analysis. Other types of agents include specific site watchers that tell you when the site has been updated or look for other events and analyst agents that not only gather but organize and interpret information for you. The practice or technology of having information brought to you by an agent is sometimes referred to as push technology.
Intelligent Network Intelligent Network (IN) is a telephone network architecture originated by Bell Communications Research (Bellcore) in which the service logic for a call is located separately from the switching facilities, allowing services to be added or changed without having to redesign switching equipment. According to Bell Atlantic, IN is a ‘‘service-specific’’ architecture. That is, a certain portion of a dialed phone number, such as 800 or 900, triggers a request for a specific service. A later version of IN called Advanced Intelligent
interactivity Network (AIN) introduces the idea of a ‘‘serviceindependent’’ architecture in which a given part of a telephone number can interpreted differently by different services depending on factors such as time of day, caller identity, and type of call. AIN makes it easy to add new services without having to install new phone equipment. Bellcore called its network IN/1. It included this model: L
The customer’s telephone
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The switching system (starting with the switch a call is handled by first, usually at a telephone company central office (CO)
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A database called a service control point (SCP) that defines the possible services and their logic A service management system (SMS)
For more information, see AIN.
intelligent peripheral interface See ‘‘IPI’’
Interactive System Productivity Facility See ‘‘ISPF’’
interactive TV Interactive TV means allowing the viewer to interact with the television set in ways other than simply controlling the channel and the volume and handling videotapes. Typical interactive TV uses are selecting a video film to view from a central bank of films, playing games, voting or providing other immediate feedback through the television connection, banking from home, and shopping from home. Interactive TV involves adding a special ‘‘set-top unit’’ to the existing television set. In addition, other installation and infrastructure arrangements are required, depending on the particular approach. Most services involve offering special programming, news, and home shopping and a number offer video-on-demand and home banking. Since interactive TV still requires a considerable investment by both the service provider and the consumer and because interactive applications are still being explored, it is somewhat difficult to predict how soon it will become widely deployed.
Interactive Voice Response Interactive Voice Response (IVR) is a software application that accepts a combination of voice telephone input and touch-tone keypad selection and provides appropriate responses in the form of voice, fax, callback, e-mail and perhaps other media. IVR is usually part of a larger application that includes database access. Common IVR applications include: L
Bank and stock account balances and transfers
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Surveys and polls
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Call center forwarding
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Simple order entry transactions
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Selective information lookup (movie schedules, etc.)
An IVR application provides pre-recorded voice responses for appropriate situations, keypad signal logic, access to relevant data, and potentially the ability to record voice input for later handling. Using computer telephony integration (CTI), IVR applications can hand off a call to a human being who can view data related to the caller at a display.
interactivity In computers, interactivity is the sensory dialog that occurs between a human being (or possibly another live creature) and a computer program. (Programs that run without immediate user involvement are not interactive; they’re usually called batch or background programs.) Games are usually thought of as fostering a great amount of interactivity. However, order entry applications and many other business applications are also interactive, but in a more constrained way (offering fewer options for user interaction). On the World Wide Web, you not only interact with the browser (the Web application program) but also with the pages that the browser brings to you. Hypertext or the word and picture links you can connect to are the most common form of interactivity when using the Web (which can be thought of as a giant, interconnected application progam). In addition to hypertext, the Web (and many non-Web applications in any computer system) offer other possibilities for interactivity. Any kind of user input, including typing commands or clicking the mouse, is a form of input. Displayed images and text, printouts, motion video sequences, and sounds are output forms of interactivity.
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The earliest form of interaction with computers was indirect and consisted of submitting commands on punched cards and letting the computer read them and perform the commands. Later computer systems were designed so that average people (not just programmers) could interact immediately with computers, telling them what programs to run and then interacting with those programs, such as word processors (then called ‘‘editors’’), drawing programs, and other interactive programs. The first interactive humancomputer interfaces tended to be input text sequences called ‘‘commands’’ (as in ‘‘DOS commands’’) and terse one-line responses from the system.
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In the late 1970’s, the first graphical user interfaces (GUI) emerged from the Xerox PARC Lab, found their way into the Apple Mcintosh personal computer, and then into Microsoft’s Windows operating systems and thus into almost all personal computers available today. GUIs inherently promoted interactivity because they offered the user more interaction options.
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interexchange carrier
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An interexchange carrier (IXC) is a telephone company that provides connections between local exchanges in different geographic areas. IXCs provide interLATA service as described in the Telecommunications Act of 1996. They’re commonly referred to as ‘‘long-distance carriers.’’ IXCs include AT&T, MCI, Sprint, and others.
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The personal computer (desktop or notebook, abundant memory and external storage, keyboard, large display)
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Smart Phone (digital cellular phone with little or no computing, accepts Wireless Markup Language-coded pages from Web servers)
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Smart Pager (also known as a two-way pager, has the ability to respond by pressing buttons or by using a tiny keyboard)
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Standard Telephone (voice and key input interface, no computer capability)
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Hybrid (often a combined phone and handheld computer)
interface As a noun, an interface is either:
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the fly for different kinds of interface devices, defines six generic classes of interface devices that characterize today’s technology:
1. A user interface, consisting of the set of dials, knobs, operating system commands, graphical display formats, and other devices provided by a computer or a program to allow the user to communicate and use the computer or program. A graphical user interface (GUI) provides its user a more or less ‘‘picture-oriented’’ way to interact with technology. A GUI is usually a more satisfying or user-friendly interface to a computer system. 2. A programming interface, consisting of the set of statements, functions, options, and other ways of expressing program instructions and data provided by a program or language for a programmer to use. 3. The physical and logical arrangement supporting the attachment of any device to a connector or to another device. As a verb, to interface means to communicate with another person or object. With hardware equipment, to interface means making an appropriate physical connection so that two pieces of equipment can communicate or work together effectively.
interface device An interface device (IDF) is a hardware component or system of components that allows a human being to interact with a computer, a telephone system, or other electronic information system. The term is often encountered in the mobile communication industry where designers are challenged to build the proper combination of portability, capability, and ease of use into the interface device. The overall set of characteristics provided by an interface device is often referred to as the user interface (and, for computers—at least, in more academic discussions—the human-computer interface or HCI). Today’s desktop and notebook computers have what has come to be called a graphical user interface (GUI) to distinguish it from earlier, more limited interfaces such as the command line interface (CLI). An interface device generally must include some form or forms of output interface, such as a display screen or audio signals, and some form or forms of input interface, such as buttons to push, a keyboard, a voice receiver, or a handwriting tablet. Harmonia, a company that makes a program for developing Web pages that can be formatted on
If you are creating content for a human receiver, you need at least a general understanding of the type or types of interface devices that your audience may be using. The Extensible Markup Language (XML) is designed to allow Web or other content to be created independently of the type of interface device that may be used. An individual style sheet designed for specific interface device types can be created using the Extensible Stylesheet Language (XSL). Then, when a user requests a page, that user’s interface device type can be recognized and the appropriate style sheet used to format the page that is sent. Harmonia has developed a User Interface Markup Language (UIML) that facilitates this process.
Inter-IC bus See ‘‘I2C bus’’
Interior Gateway Protocol See ‘‘IGP’’
interlaced display An interlaced display is a cathode-ray tube (CRT) display in which the lines are scanned alternately in two interwoven rasterized lines. In a CRT display, there are several hundred horizontal lines in a frame (full screen). These lines are scanned from left to right, and from top to bottom. The refresh rate (number of frames scanned per second) varies, but it is normally between 60 and 100 hertz. Refresh rates slower than 60 Hz produce distracting screen flicker, which can cause headaches and eye fatigue.
International Standard Recording Code Most CRT computer monitors scan each line in turn from top to bottom at the lowest resolution levels (640 6 480 and 800 6 600 pixel). However, at the higher resolutions, such as 1024 6 768 or 1200 6 800, the frame is sometimes scanned in interlaced fashion: first the odd-numbered lines, and then the even-numbered lines. This allows for a lower refresh rate without producing flicker. With text and fixed graphics displays, this scheme can work well. However, with animated graphics—especially images that move or change form rapidly—interlacing can produce a fluttering effect at least as irritating as screen flicker. For serious animated-graphics work and video editing, a non- interlaced CRT display is recommended. The refresh rate should be as high as the system will allow, ideally 70 Hz or more.
interlaced GIF An interlaced GIF (Graphics Interchange Format) is a GIF image that seems to arrive on your display like an image coming through a slowly-opening Venetian blind. A fuzzy outline of an image is gradually replaced by seven successive waves of bit streams that fill in the missing lines until the image arrives at its full resolution. Among the advantages for the viewer using 14.4 Kbps and 28.8 Kbps modems are that the wait time for an image seems less and the viewer can sometimes get enough information about the image to decide to click on it or move elsewhere. For users with faster connections, there is little difference in effect between an interlaced GIF and a non-interlaced GIF.
intermediate distribution frame IDF also stands for interface device. An intermediate distribution frame (IDF) is a free-standing or wall-mounted rack for managing and interconnecting the telecommunications cable between end user devices and a main distribution frame (MDF). For example, an IDF might be located on each floor of a multi-floor building routing the cabling down the walls to an MDF on the first floor. The MDF would contain cabling that would interconnect to the phone company or to other buildings.
Intermediate System-to-Intermediate System protocol See ‘‘IS-IS’’
international callback See ‘‘callback’’
International Data Encryption Algorithm IDEA (International Data Encryption Algorithm) is an encryption algorithm developed at ETH in Zurich, Switzerland. It uses a block cipher with a 128-bit key, and is generally considered to be very secure. It is considered
among the best publicly known algorithms. In the several years that it has been in use, no practical attacks on it have been published despite of a number of attempts to find some. IDEA is patented in the United States and in most of the European countries. The patent is held by Ascom-Tech. Non-commercial use of IDEA is free. Commercial licenses can be obtained by contacting Ascom-Tech.
International Electrotechnical Commission On the Internet, ‘‘IEC’’ usually means the International Electrotechnical Commission. In telephony, IEC may also refer to an ‘‘interexchange carrier’’ (a long-distance carrier).
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International Organization for Standardization
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See ‘‘ISO’’
H International Standard Book Number See ‘‘ISBN’’
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International Standard Recording Code
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The International Standard Recording Code (ISRC) is an international code used to identify sound and audio-visual recordings on compact discs (CDs), music videos, and other media, primarily to ensure royalty payments. ISRC was developed because of the need to reliably identify all recordings, especially digital recordings. ISRC was developed by the International Organization for Standardization (ISO) in 1986 and is also known as ISO 3901. The ISRC code is contained in the subcode of each track or recording. The ISRC code is made up of letters and numbers and is 12 characters in length. The code consists of characters for the country, registrant, year of reference, and designation code separated by dashes. An example of an ISRC code is GB-Z03-01-53900.
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The country code identifies the country of residence of the registrant of the recording such as the producer or owner. It consists of two letters that are allocated to each country by the ISO. For example, FR is the code for France. The registrant code identifies the producer of the recording. It consists of three characters, which contain both letters and numbers. The year of reference is the code that identifies the year in which the ISRC code was assigned to that recording. The code is the last two digits of the year. For 2001, the year of reference code is 01. The final code is the designation code. This code must be five digits in length and is assigned by the producer or owner of the recording.
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An ISRC code may not be reused. For example, if a recording is remixed or the playing time changes, a new ISRC code must be assigned. The same ISRC code may be used if a
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Whatis?Com’s Encyclopedia of Technology Terms recording is sold or recompiled without any editing. The owner of the recording, copyright organizations, broadcasting organizations, and libraries use ISRC codes. The ISO Technical Committee (TC) in charge of ISRC is ISO/ TC 46.
International System of Units The International System of Units (abbreviated ‘‘SI’’ from the French version of the name) is a scientific method of expressing the magnitudes or quantities of seven important natural phenomena. This system was formerly called the meter-kilogram-second (MKS) system. All SI units can be expressed in terms of standard multiple or fractional quantities, as well as directly. Multiple and fractional SI units are defined by prefix multipliers according to powers of 10 ranging from 10-24 to 1024. Meter The meter (abbreviation, m) is the SI unit of displacement or length. One meter is the distance traveled by a ray of electromagnetic (EM) energy through a vacuum in 1 /299,792,458 (3.33564095 6 10-9) second. The meter was originally defined as one ten-millionth (0.0000001 or 10-7) of the distance, as measured over the earth’s surface in a great circle passing through Paris, France, from the geographic north pole to the equator. Kilogram The kilogram (abbreviation, kg) is the SI unit of mass. It is defined as the mass of a particular international prototype made of platinum-iridium and kept at the International Bureau of Weights and Measures. It was originally defined as the mass of one liter (10-3 cubic meter) of pure water. Second The second (abbreviation, s or sec) is the SI unit of time. One second is the time that elapses during 9.192631770 6 109 cycles of the radiation produced by the transition between two levels of Cesium 133. It is also the time required for an EM field to propagate 299,792,458 (2.99792458 6 108) meters through a vacuum. Kelvin The Kelvin (abbreviation K), also called the degree Kelvin (abbreviation, oK), is the SI unit of temperature. One Kelvin is 1/273.16 (3.6609 6 10-3) of the thermodynamic temperature of the triple point of pure water (H2O). Ampere The ampere (abbreviation, A) is the SI unit of electric current. One ampere is the current that would produce a force of 0.0000002 (2 6 10-7) newton between two straight, parallel, perfectly conducting wires having infinite length and zero diameter, separated by one meter in a vacuum. One ampere represents 6.24 6 1018 unit electric charge carriers, such as electrons, passing a specified fixed point in one second.
Candela The candela (abbreviation, cd) is the SI unit of luminous intensity. It is the electromagnetic radiation, in a specified direction, that has an intensity of 1/683 (1.46 6 10-3) watt per steradian at a frequency of 540 terahertz (5.40 6 1014 hertz). Mole The mole (abbreviation, mol) is the SI unit of material quantity. One mole is the number of atoms in 0.012 kilogram of the most common isotope of elemental carbon (C-12). This is approximately 6.022169 6 1023, and is also called the Avogadro constant. Also see prefix multipliers.
internationalisation See ‘‘internationalization’’
internationalization Internationalization (sometimes shortened to ‘‘I18N, meaning ‘‘I—eighteen letters -N’’) is the process of planning and implementing products and services so that they can easily be adapted to specific local languages and cultures, a process called localization. The internationalization process is sometimes called translation or localization enablement. Enablement can include: L
Allowing space in user interfaces (for example, hardware labels, help pages, and online menus) for translation into languages that require more characters
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Developing with products (such as Web editors or authoring tools) that can support international character sets (Unicode)
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Creating print or Web site graphic images so that their text labels can be translated inexpensively
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Using written examples that have global meaning
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For software, ensuring data space so that messages can be translated from languages with single-byte character codes (such as English) into languages requiring multiple-byte character codes (such as Japanese Kanji)
internesia Internesia is an inability to remember which Web site or other Internet facility you saw an item of information on. It is an important form of the more general condition known as infonesia, which is the inability to remember where you saw an item of information from any source (newspapers, television, and so forth). Infojunkies are especially susceptible to internesia. A person who can never remember a Web site is an internesiac. Bookmarks may help, but many internesiacs bookmark so many sites, they can’t find anything on their bookmark list or remember the right search word.
Internet Group Management Protocol
Internet
Internet access provider
The Internet, sometimes called simply ‘‘the Net,’’ is a worldwide system of computer networks—a network of networks in which users at any one computer can, if they have permission, get information from any other computer (and sometimes talk directly to users at other computers). It was conceived by the Advanced Research Projects Agency (ARPA) of the U.S. government in 1969 and was first known as the ARPANet. The original aim was to create a network that would allow users of a research computer at one university to be able to ‘‘talk to’’ research computers at other universities. A side benefit of ARPANet’s design was that, because messages could be routed or rerouted in more than one direction, the network could continue to function even if parts of it were destroyed in the event of a military attack or other disaster.
See ‘‘IAP’’
Today, the Internet is a public, cooperative, and selfsustaining facility accessible to hundreds of millions of people worldwide. Physically, the Internet uses a portion of the total resources of the currently existing public telecommunication networks. Technically, what distinguishes the Internet is its use of a set of protocols called TCP/ IP (for Transmission Control Protocol/Internet Protocol). Two recent adaptations of Internet technology, the intranet and the extranet, also make use of the TCP/IP protocol. For many Internet users, electronic mail (e-mail) has practically replaced the Postal Service for short written transactions. Electronic mail is the most widely used application on the Net. You can also carry on live ‘‘conversations’’ with other computer users, using Internet Relay Chat (IRC). More recently, Internet telephony hardware and software allows real-time voice conversations. The most widely used part of the Internet is the World Wide Web (often abbreviated ‘‘WWW’’ or called ‘‘the Web’’). Its outstanding feature is hypertext, a method of instant crossreferencing. In most Web sites, certain words or phrases appear in text of a different color than the rest; often this text is also underlined. When you select one of these words or phrases, you will be transferred to the site or page that is relevant to this word or phrase. Sometimes there are buttons, images, or portions of images that are ‘‘clickable.’’ If you move the pointer over a spot on a Web site and the pointer changes into a hand, this indicates that you can click and be transferred to another site. Using the Web, you have access to millions of pages of information. Web browsing is done with a Web browser, the most popular of which are Microsoft Internet Explorer and Netscape Navigator. The appearance of a particular Web site may vary slightly depending on the browser you use. Also, later versions of a particular browser are able to render more ‘‘bells and whistles’’ such as animation, virtual reality, sound, and music files, than earlier versions.
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Internet Advertising Bureau The IAB (Internet Advertising Bureau) is an organization that fosters the growth of advertising on the Internet. Membership is available to corporations engaged in selling, measuring, monitoring, or producing advertising on the Internet. The IAB recommends a set of standard-size banner ads for World Wide Web pages.
Internet Architecture Board See ‘‘IAB’’
Internet Assigned Numbers Authority See ‘‘IANA’’
Internet commerce provider See ‘‘ICP’’
Internet Control Message Protocol See ‘‘ICMP’’
Internet Corporation for Assigned Names and Numbers See ‘‘ICANN’’
Internet Engineering Task Force See ‘‘IETF’’
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Internet Explorer Administration Kit Internet Explorer Administration Kit (IEAK) is a product from Microsoft that enables an organization to centrally manage the customizing, deployment, and maintenance of Microsoft’s Web browser, Internet Explorer, for users on different computer platforms. There is no royalty on the deployed copies. Using IEAK, a company can set up every user’s browser the same way. Microsoft offers a step-by-step process for using the kit.
Internet Explorer
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See ‘‘Microsoft Internet Explorer’’
T Internet Group Management Protocol The Internet Group Management Protocol (IGMP) is an Internet protocol that provides a way for an Internet computer to report its multicast group membership to adjacent routers. Multicasting allows one computer on the Internet to send content to multiple other computers that
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have identified themselves as interested in receiving the originating computer’s content. Multicasting can be used for such applications as updating the address books of mobile computer users in the field, sending out company newsletters to a distribution list, and ‘‘broadcasting’’ highbandwidth programs of streaming media to an audience that has ‘‘tuned in’’ by setting up a multicast group membership. Using the Open Systems Interconnection (OSI) communication model, IGMP is part of the Network layer. IGMP is formally described in the Internet Engineering Task Force (IETF) Request for Comments (RFC) 2236.
Internet Information Server
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See ‘‘IIS’’
Internet Inter-ORB Protocol See ‘‘IIOP’’
Internet maps
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Since the Internet began developing about 25 years ago, there has always been some kind of map showing what it looks like. Maps, even partial ones, of today’s enormous Internet, whether of its physical wiring, its information content, or some other aspect, challenge both creator and user. Yet a map, even a complicated one, has a way of bounding chaos and giving us a ready-packaged mental construct. Some time ago, we discovered Martin Dodge’s remarkable collection of Internet maps at the Centre for Advanced Spatial Analysis, University College London. His Atlas of Cyberspaces (http://www.cybergeography.org/ atlas/atlas.html) contains separate collections for maps of: L
ISP and Internet backbone networks
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Three-dimensional information spaces
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Information landscapes or visualizations
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Cyberspace using geographic metaphors
The Atlas of Cyberspaces also includes examples of Web site maps and several other map categories.
A less sophisticated protocol is Post Office Protocol 3 (POP3). With POP3, your mail is saved for you in your mail box on the server. When you read your mail, all of it is immediately downloaded to your computer and, unless you request it, no longer maintained on the server. IMAP can be thought of as a remote file server. POP can be thought of as a ‘‘store-and-forward’’ service. POP and IMAP deal with the receiving of e-mail from your local server and are not to be confused with Simple Mail Transfer Protocol (SMTP), a protocol for transferring e-mail between points on the Internet. You send e-mail with SMTP and a mail handler receives it on your recipient’s behalf. Then the mail is read using POP or IMAP.
Internet Open Trading Protocol Internet Open Trading Protocol (IOTP) is a set of standards that makes all electronic purchase transactions consistent for customers, merchants, and other involved parties, regardless of payment system. IOTP accommodates a wide range of payment systems such as Secure Electronic Transaction, digital cash, e-checks, and debit cards. Payment system data is encapsulated within IOTP messages. IOTP is designed to handle a transaction that involves a number of different parties: the customer, merchant, credit checker and certifier, bank, and delivery handler. IOTP uses the Extensible Markup Language (XML) to define data that encompasses everything that may be needed in a transaction. In the ‘‘real world,’’ you can negotiate certain factors when considering a purchase. These include choosing who you conduct business with, whether it will be conducted in person or by phone, the method of payment, the provision of a payment receipt, and the delivery of the product. In the virtual world, IOTP is designed to ensure that all of these factors are successfully and securely performed. Companies contributing to the development of IOTP include Hewlett Packard, IBM, JCP, MasterCard International, Smart Card Integrations, Sun Microsystems, and Wells Fargo Bank.
Internet presence provider and promoter See ‘‘IPPP’’
Internet Message Access Protocol Internet Message Access Protocol (IMAP) is a standard protocol for accessing e-mail from your local server. IMAP (the latest version is IMAP4) is a client/server protocol in which e-mail is received and held for you by your Internet server. You (or your e-mail client) can view just the heading and the sender of the letter and then decide whether to download the mail. You can also create and manipulate folders or mailboxes on the server, delete messages, or search for certain parts or an entire note. IMAP requires continual access to the server during the time that you are working with your mail.
Internet presence provider See ‘‘IPP’’
Internet problems Here are: L
Three rules-of-thumb for dealing with Internet problems
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A table showing the most common codes and messages you’re likely to see on your Web browser (HTTP), when accessing Usenet, using e-mail, or using the FTP protocol to upload or download files
Internet problems A Table Showing the Most Common Codes and Messages Error message
What it means
What you can do
400—Bad request
You entered a URL that is not accepted as correct. Possibly the server being contacted doesn’t recognize the document you are asking for or it may no longer exist. It is also possible that it is correct but you aren’t authorized to access it. The site being accessed is protected and you either entered the incorrect password or the host does not want users coming from your domain to enter. Some sites will only allow specific domains to access the site. For example, some U.S. government sites (with the .gov top-level domain name) will only permit users coming from .gov or .edu (educational sites) to gain access. Or some educational sites only allow access from other educational sites. You are not permitted to access this document. It is either password-protected or the host does not want your domain to enter. The host server can’t find the HTML document at the URL you’ve entered. Either you have mistyped the URL, the document no longer exists, or you’ve been given the wrong URL.
Check the URL to see if it’s correctly spelled and that every letter is in the right (upperor lower-) case. Check colons, numbers of slashes, and tildes.
401—Unauthorized
403—Forbidden
404—Not found
550—xxxxxxxxx is not a known user
Bad file request
Cannot add form submission result to bookmark list Connection refused by host Failed DNS lookup
File contains no data
Using e-mail, you sent a note to an e-mail address that isn’t recognized at the mail server you’ve specified. The e-mail note got all the way to the mail server (so that part’s right), but you may have entered the user part of the e-mail address incorrectly or the user may no longer have an account on that server. The form you’re trying to access is not supported by your browser or there is an error in the form.
This error message occurs when you’ve submitted a search request and then try to save the result as a bookmark. Even hough it may look like a valid address, the resulting page does not have a URL that can be reused so it can’t be added to your bookmark list. You’re not permitted to access this document either because it’s password-protected or the host doesn’t permit your domain to enter. The URL you requested could not be translated into a valid Internet address by the domain name system. This is either a result of a mistake in the system or you could have mistakenly entered an incorrect URL. The site you accessed doesn’t ontain any Web pages on it. It’s possible that you’re trying to access the site while the page is being uploaded.
If you are sure you have access to the site, try typing the password again. If you don’t have a password or your domain does not have access to the site, you won’t be able to access it.
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See above.
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If your URL ends in ‘‘.htm’’, try ‘‘.html’’ (and vice versa). To find out whether the site hosting the document is still active, delete everything after the last slash in the URL. If this doesn’t work, try deleting the slash and adding .htm or .html to the end. If the site is still running, try and find the page from another location on the site that you can reach. Make sure you entered the correct e-mail user name. If you suspect it’s an old address or was mistyped in a note sent to you, try looking up a new address using the person’s name. Try e-mail address finding tools.
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Try using a different browser. If the problem persists or another browser is not available, send an e-mail note to the site’s Webmaster. Use the Save As command to save the page to your hard drive. Then you can open the saved HTML page in your browser and add that to your bookmark list.
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If you have access to the site and you know the password, try again, making sure you type it correctly and with the correct case. Mistakes in DNS lookups are a common occurrence. Simply clicking the Reload button may correct the error. If the error persists, you may have entered an incorrect host name. Try typing the URL again. Try waiting a minute and clicking on the Reload button. Make sure that the URL is typed in correctly. If the problem persists, try again later.
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Error message
What it means
What you can do
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Helper application not found
You’re trying to load a file of a type that your browser doesn’t recognize. Most browsers can be enabled to view or load file types that they don’t otherwise recognize. These files can be sound files, movie clips, graphics, ZIP archive files, or any other type of file you are trying to download.
Host unavailable
The server computer that hosts this site could not be reached. The server may be offline or down for maintenance. The server you’re trying to reach is not connected, or you have lost your own connection. You may have also entered the URL incorrectly.
The dialog box that carries this message will tell you the file type that is missing. You can then follow your browser’s instructions for assigning a viewer for each file format you wish to view online. It’s often better to have certain file formats not assigned to helper applications for Internet browsers. When files are not assigned to helper applications, the dialog box you receive has a ‘‘Save to Disk’’ option. This can often be useful when you want to download files, such as ZIP archive files, to save them instead of extracting them right away. Try clicking on the Reload button. If it still doesn’t work, try again later.
B C D E F G Host unknown
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Network connection was refused by the server
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NNTP server error
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Permission denied
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Too many connections—try again later Too many users
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Unable to locate host
Some servers have limits on the number of people or request they can handle at a time. If you get this message, the server is probably too busy to handle one more user. When the server doesn’t create its own message to handle this error, this generic message is displayed instead. This error occurs when you are trying to log on to a Usenet newsgroup. An NNTP server is part of your Internet service provider’s software, so if this error occurs the software may be malfunctioning or the newsgroup does not exist. This error occurs when you are connected with an FTP site and you are either uploading or downloading. Sometimes the site administrator doesn’t want you to upload to the site, download a certain file, or access a certain directory. The site may also be too busy. The limit to the number of people who can use the site at one time has been exceeded. You’ll have to wait your turn. To avoid slow uploading and downloading rates, site administrators set a maximum number of users that can access the site at one time. Your request is one too many. The server is either down for maintenance or you may have lost your connection.
Click the Reload button and try again. Occasionally, packets don’t arrive on the Internet and a second try will work. Next, make sure you are connected to the Internet. If you have lost your connection, simply reconnect and try the URL again. If you still receive the error, check the URL for typos and case-sensitivity. Try clicking the Reload button until you connect. Or try again later.
Make sure that the URL is typed correctly. If you still receive the error, wait a while and try again. If the problem still persists, contact your access provider and make them aware of the problem. Check that you are using the correct procedure to upload or download the file. Then keep trying until you succeed. If you still receive the error and know that you’re able to upload or download to that site, contact the Webmaster by phone or e-mail. Keep clicking the Reload button until you connect or try again later when there is likely to be less traffic. Keep trying until you connect or try again later when there is less Internet traffic. Check to see whether the site has mirrors, and try one of those. Click the Reload button; sometimes there is an intermittent problem on the Internet. Then make sure you’re connected to the Internet. If you have lost your connection, simply reconnect and try the URL again. If you still receive the error, check the URL for typos and case- sensitivity.
Internet Protocol Error message
What it means
Unable to locate the server
The server indicated in the URL you typed or clicked from either no longer exists or was entered incorrectly.
Viewer not found
You can’t log on as an anonymous user
What you can do
Check the URL to see if everything is correct. Make sure that everything is correctly spelled and in the correct case (some servers are case-sensitive). You can also check with the source of the URL to verify that it is correct. Your browser doesn’t recognize files of this When this dialog box appears, search for a type. Most browsers can be extended to be able helper application that will open files of this to view or load file types that are not otherwise format. Then follow your browser’s recognized by browsers, such as movie or sound instructions for assigning this viewer to files. open files of that format online. This is another FTP error message. Most browsers Keep trying to access the FTP site until you submit ‘‘anonymous’’ as your user ID and your succeed. Sometimes the FTP server is too email address as your password. The FTP site you busy. However, if you don’t have a are connecting to may not allow ‘‘anonymous’’ browser that allows anonymous FTP access. This error will also occur when the FTP access or the site is password-protected, server is currently at its limit of ‘‘anonymous’’ enter your user ID and password manually users. Another possibility is that your browser using FTP software such as WS_FTP or doesn’t support anonymous FTP access (for CuteFTP. example, America Online’s browser).
Three Rules-of-Thumb for Dealing with Internet Problems
Internet Protocol
1. If you get a message saying the domain name server (DNS) can’t find your page and you’re sure you’ve typed it in correctly or clicked on a valid link, try it again—TWO more times! (Sometimes packets don’t get there!)
The Internet Protocol (IP) is the method or protocol by which data is sent from one computer to another on the Internet. Each computer (known as a host) on the Internet has at least one IP address that uniquely identifies it from all other computers on the Internet. When you send or receive data (for example, an e-mail note or a Web page), the message gets divided into little chunks called packets. Each of these packets contains both the sender’s Internet address and the receiver’s address. Any packet is sent first to a gateway computer that understands a small part of the Internet. The gateway computer reads the destination address and forwards the packet to an adjacent gateway that in turn reads the destination address and so forth across the Internet until one gateway recognizes the packet as belonging to a computer within its immediate neighborhood or domain. That gateway then forwards the packet directly to the computer whose address is specified.
2. If you get a ‘‘Not found’’ message, the page may be temporarily missing because of miscoding at the target site. Try it again tomorrow...or try the home page for the site and send e-mail asking them to restore the page. 3. Be aware that sometimes a page you’ve visited recently may be coming from your cache (or the cache on a proxy server within your company). To get the ‘‘fresh’’ version of the page, click on ‘‘Reload’’ in your tool bar.
Because a message is divided into a number of packets, each packet can, if necessary, be sent by a different route across the Internet. Packets can arrive in a different order than the order they were sent in. The Internet Protocol just delivers them. It’s up to another protocol, the Transmission Control Protocol (TCP) to put them back in the right order.
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Whatis?Com’s Encyclopedia of Technology Terms IP is a connectionless protocol, which means that there is no continuing connection between the end points that are communicating. Each packet that travels through the Internet is treated as an independent unit of data without any relation to any other unit of data. (The reason the packets do get put in the right order is because of TCP, the connection-oriented protocol that keeps track of the packet sequence in a message.) In the Open Systems Interconnection (OSI) communication model, IP is in layer 3, the Networking Layer. The most widely used version of IP today is Internet Protocol Version 4 (IPv4). However, IP Version 6 (IPv6) is also beginning to be supported. IPv6 provides for much longer addresses and therefore for the possibility of many more Internet users and devices. IPv6 includes the capabilities of IPv4 and any server that can support IPv6 packets can also support IPv4 packets.
Internet Protocol Security See ‘‘IPsec’’
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Internet Relay Chat
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For terms frequently used in online keyboard chatting, see chat abbreviations.
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Internet Relay Chat (IRC) is a system for chatting that involves a set of rules and conventions and client/server software. On the Web, certain sites such as Talk City or IRC networks such as the Undernet provide servers and help you download an IRC client to your PC. Talk City also offers an IRC client applet that it downloads for you as part of their home page so that you can start chatting right away. You can start a chat group (called a channel) or join an existing one. There is a protocol for discovering existing chat groups and their members. Depending on the type of network, nicknames can be reserved (registered) or just used during the session. Some channels encourage you to register a nickname that you always use and even offer space for a personal profile, picture, and personal home page link. Popular ongoing IRC channels are #hottub and #riskybus. A number of channels are set up and conducted in foreign languages. The most common IRC networks are IRCnet (mostly European), EFnet (mostly North American), Undernet, and Dalnet. Popular IRC clients include mIRC for Windows, IRCle for Mac OS, and irc2 (the original client) for UNIX-base operating systems. The IRC protocol uses Transmission Control Protocol (you can IRC via a Telnet client), usually on port 6667.
Internet Research Task Force The IRTF is an activity of the of the Internet Society. The Internet Research Task Force (IRTF) works on network technology. You can find out more about the IRTF at the Internet Society’s home page.
Internet Server Application Program Interface See ‘‘ISAPI’’
Internet service provider See ‘‘ISP’’
Internet Society The Internet Society is an international non-profit organization that acts as a guide and conscience for the workings of the Internet. It was founded in 1992 and is based on Reston, Virginia. The Internet Society supports the Internet Architecture Board (IAB), which supervises technical and other issues. Among the IAB’s activities is the Internet Engineering Task Force (IETF), which oversees the evolution of TCP/IP.
Internet time Internet time is the concept that events occur at a faster rate on the Internet than elsewhere or that, everywhere, the Internet is affecting the pace of change. Andy Grove, CEO of Intel, is quoted as saying, ‘‘The world now runs on Internet time.’’
Internet traffic Several major Internet backbone providers have Web sites that post hourly or more frequent statistics on how fast data is travelling on their backbone lines and the percentage of data packet that are being lost. Our table summarizes these sites, the information they provide, and how often they are updated. This Internet traffic statistics site...
Gives you this information
AT&T Internet Network Status http://ipnetwork.
Backbone delay (latency) or packet loss between specified U.S. cities (both pages updated every 30 minutes)
bgtmo.ip.att.net/delay. html or http:// ipnetwork.bgtmo.ip.att. net/loss.html
internetworking This Internet traffic statistics site...
Gives you this information
Cable and Wireless Global Internet Backbone
Backbone delay (latency) and packet loss by global region (updated every 10 minutes) Monthly statistics for North America, Transatlantic, and Europe
http://sla.cw.net
UUNET http://www.uunet.com/ lang.en/customers/sla/ latency.html
Internet voting See ‘‘e-voting’’
Internet2 Internet2 is a collaboration among more than 100 U.S. universities to develop networking and advanced applications for learning and research. Since much teaching, learning, and collaborative research may require real-time multimedia and high-bandwidth interconnection, a major aspect of Internet2 is adding sufficient network infrastructure to support such applications. But Internet2 also intends to investigate and develop new ways to use the Internet and the Internet2 infrastructure for its educational purposes. Although Internet2 is not envisioned as a future replacement for the Internet, its organizers hope to share their developments with other networks, including the Internet. Internet2 will include and further develop the National Science Foundation’s very high-speed Backbone Network Service (vBNS) that currently interconnects research supercomputer centers in the U.S. The involved institutions plan to continue using the existing Internet for ‘‘ordinary’’ services such as e-mail, personal Web access, and newsgroups. Internet2 collaborators plan to use Quality-of-Service (QoS) tools so that participants can reserve and use bandwidth for special events or in certain time periods. Here are some possibilities envisioned by Internet2: Distributed learning modules: Conceptually, teachers and students can be share materials in cyberspace with students learning in a self-directed manner under the supervision of an educational system or teacher. Internet2 foresees tools that would make it easy to create what they call LearningWare, using existing methodologies. Internet2 may also help realize the Instructional Management System (IMS), a standard process for using the Internet in developing and delivering learning packages and tracking outcomes. One can think of the IMS as a more structured way to exploit the potential learning materials on the World Wide Web.
New ways to envision and retrieve information: In the future, today’s text-oriented models of information structure could be replaced by interactive pictures of information structure (compare a textual taxonomy or table of contents with illustrations of interlinked and explodable animal forms, for example). With Internet2’s high-bandwidth connections, experiments in such information visualization will be possible; new ideas can be tried out. In environments where up-to-date information is valuable, information can be pushed to users at their request. Virtual environment sharing: Sometimes called teleimmersion, participants in teleconferences could share the perception that everyone was in the same physical place, possibly with virtual (but somewhat real) models of shared work objects such as architectural models or multimedia storyboards. You would be able to see yourself with others in a far-away conference room, talking and perhaps manipulating objects in the room. Virtual laboratory: A virtual laboratory would allow scientists in a number of different physical locations, each with unique expertise, computing resources, and/or data to collaborate efficiently not simply at a meeting but in an ongoing way. Effectively, such a project would extend and pool resources while engendering orderly communication and progress toward shared goals. For example, a group of astronomers and computer scientists at the supercomputing centers in the U.S. are attempting to share experiments and knowledge about the origin of the universe. Shared visualizations of alternative possibilities could conceivably suggest additional or refined alternatives. Virtual laboratories are also envisioned for the design and manufacturing of complex systems such as airplanes and for studying and forecasting weather patterns.
Internetwork Packet Exchange See ‘‘IPX’’
internetworking Internetworking is a term used by Cisco, BBN, and other providers of network products and services as a comprehensive term for all the concepts, technologies, and generic devices that allow people and their computers to communicate across different kinds of networks. For example, someone at a computer on a token ring local area network may want to communicate with someone at a computer on an Ethernet local area network in another country using a wide area network interconnection. The common internetwork protocols, routing tables, and related network devices required to achieve this communication constitute internetworking. The standard reference model for internetworking is Open Systems Interconnection (OSI), which could also be used as a model for intranetworking as well. OSI enables any technology to be related to another technology because each
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Compatibility is a related term. A product is compatible with a standard but interoperable with other products that meet the same standard (or achieve interoperability through a broker).
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can be related to the standard communication model. OSI provides a layering approach to the problem of exchanging data across a network or a network of networks so that the problem can be broken down into easier-to-understand components and so that boundaries between components can be more easily determined.
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InterNIC
An interpreted program, sometimes called a script, is a program whose instructions are actually a logically sequenced series of operating system commands, handled one at a time by a command interpreter. In turn, the command interpreter requests services from the operating system. The writer of the interpreted program need not be concerned by low-level storage management considerations. On the other hand, an interpreted program can not be as efficient as a compiled program, which has been processed by a language compiler. A language compiler converts source statements into something close to the strings of 0’s and 1’s that a processor ultimately is given to work on. Because this work is already done before a compiled program is run, it runs much more quickly.
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Unitl recently, InterNIC (Internet Network Information Center), a cooperative activity between the U.S. government and Network Solutions, Inc., was the organization responsible for registering and maintaining the com, net, and org top-level domain names on the World Wide Web. The actual registration was performed by Network Solutions, Inc. As a result of a new U. S. Government Statement of Policy (known as ‘‘the white paper’’) in October 1998, competition will be introduced in domain name registration for these top-level domains and a new, nonprofit global organization, the Internet Corporation of Assigned Names and Numbers (ICANN), has been designated to conduct the registrar accreditation process. ICANN has initially designated five new registrar companies—in addition to Network Solutions—for a twomonth test period. After that period, additional registrars are expected to be accredited. In addition to the com, net, and org domain name, geographically identified domains in the U.S. such as those for cities and towns are administered by US Domain Name Registration Services. There are also registries for Canada, Mexico, Europe, Asia Pacific countries, and other countries.
interoperability Interoperability (pronounced IHN-tuhr-AHP-uhr-uh-BIHLih-tee) is the ability of a system or a product to work with other systems or products without special effort on the part of the customer. Interoperability becomes a quality of increasing importance for information technology products as the concept that ‘‘The network is the computer’’ becomes a reality. For this reason, the term is widely used in product marketing descriptions.
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An interpreted program in a language such as Perl is much easier to write than a program written for a compiler. Interpreted programs are ideal for small tasks and for ‘‘gluing together’’ a succession of compiled programs that are run from the interpreted program.
interprocess communication Interprocess communication (IPC) is a set of programming interfaces that allow a programmer to create and manage individual program processes that can run concurrently in an operating system. This allows a program to handle many user requests at the same time. Since even a single user request may result in multiple processes running in the operating system on the user’s behalf, the processes need to communicate with each other. The IPC interfaces make this possible. Each IPC method has its own advantages and limitations so it is not unusual for a single program to use all of the IPC methods. IPC methods include: L
Pipes and named pipes
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Message queueing
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Semaphores
By adhering to published interface standards
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Shared memory
By making use of a ‘‘broker’’ of services that can convert one product’s interface into another product’s interface ‘‘on the fly’’
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Products achieve interoperability with other products using either or both of two approaches: L
interpreted
A good example of the first approach is the set of standards that have been developed for the World Wide Web. These standards include TCP/IP, Hypertext Transfer Protocol, and HTML. The second kind of interoperability approach is exemplified by the Common Object Request Broker Architecture (CORBA) and its Object Request Broker (ORB).
interrupt An interrupt is a signal from a device attached to a computer or from a program within the computer that causes the main program that operates the computer (the operating system) to stop and figure out what to do next. Almost all personal (or larger) computers today are interrupt-driven—that is, they start down the list of computer instructions in one program (perhaps an application such as a word processor) and keep running the instructions until either (A) they can’t go any
intrusion detection further or (B) an interrupt signal is sensed. After the interrupt signal is sensed, the computer either resumes running the program it was running or begins running another program.
Internet. The main purpose of an intranet is to share company information and computing resources among employees. An intranet can also be used to facilitate working in groups and for teleconferences.
Basically, a single computer can perform only one computer instruction at a time. But, because it can be interrupted, it can take turns in which programs or sets of instructions that it performs. This is known as multitasking. It allows the user to do a number of different things at the same time. The computer simply takes turns managing the programs that the user effectively starts. Of course, the computer operates at speeds that make it seem as though all of the user’s tasks are being performed at the same time. (The computer’s operating system is good at using little pauses in operations and user think time to work on other programs.)
An intranet uses TCP/IP, HTTP, and other Internet protocols and in general looks like a private version of the Internet. With tunneling, companies can send private messages through the public network, using the public network with special encryption/decryption and other security safeguards to connect one part of their intranet to another.
An operating system usually has some code that is called an interrupt handler. The interrupt handler prioritizes the interrupts and saves them in a queue if more than one is waiting to be handled. The operating system has another little program, sometimes called a scheduler, that figures out which program to give control to next. In general, there are hardware interrupts and software interrupts. A hardware interrupt occurs, for example, when an I/O operation is completed such as reading some data into the computer from a tape drive. A software interrupt occurs when an application program terminates or requests certain services from the operating system. In a personal computer, a hardware interrupt request (IRQ) has a value associated with it that associates it with a particular device.
Typically, larger enterprises allow users within their intranet to access the public Internet through firewall servers that have the ability to screen messages in both directions so that company security is maintained. When part of an intranet is made accessible to customers, partners, suppliers, or others outside the company, that part becomes part of an extranet.
interstitial An interstitial (something ‘‘in between’’) is a page that is inserted in the normal flow of editorial content structure on a Web site for the purpose of advertising or promotion. It can be more or less intrusive and the reaction of viewers usually depends on how welcome or entertaining the message is. An interstitial is usually designed to move automatically to the page the user requested after allowing enough time for the message to register or the ad(s) to be read.
intrabody signaling See ‘‘personal area network’’
intranet An intranet is a private network that is contained within an enterprise. It may consist of many interlinked local area networks and also use leased lines in the wide area network. Typically, an intranet includes connections through one or more gateway computers to the outside
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Intrusion detection (ID) is a type of security management system for computers and networks. An ID system gathers and analyzes information from various areas within a computer or a network to identify possible security breaches, which include both intrusions (attacks from outside the organization) and misuse (attacks from within the organization). ID uses vulnerability assessment (sometimes refered to as scanning), which is a technology developed to assess the security of a computer system or network.
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Intrusion detection functions include: L
Monitoring and analyzing both user and system activities
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Analyzing system configurations and vulnerabilities
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Assessing system and file integrity
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Ability to recognize patterns typical of attacks
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Analysis of abnormal activity patterns
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Tracking user policy violations
interrupt request See ‘‘IRQ’’
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ID systems are being developed in response to the increasing number of attacks on major sites and networks, including those of the Pentagon, the White House, NATO, and the U.S. Defense Department. The safeguarding of security is becoming increasingly difficult, because the possible technologies of attack are becoming ever more sophisticated; at the same time, less technical ability is required for the novice attacker, because proven past methods are easily accessed through the Web. Typically, an ID system follows a two-step process. The first procedures are host-based and are considered the passive component, these include: inspection of the system’s configuration files to detect inadvisable settings; inspection of the password files to detect inadvisable passwords; and inspection of other system areas to detect policy violations. The second procedures are network-based and are
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Whatis?Com’s Encyclopedia of Technology Terms considered the active component: mechanisms are set in place to reenact known methods of attack and to record system responses.
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In 1998, ICSA.net, a leading security assurance organization, formed the Intrusion Detection Systems Consortium (IDSC) as an open forum for ID product developers with the aim of disseminating information to the end user and developing industry standards.
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inverse multiplexing
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Inverse multiplexing speeds up data transmission by dividing a data stream into multiple concurrent streams that are transmitted at the same time across separate channels (such as a T-1 or E-1 lines) and are then reconstructed at the other end back into the original data stream. Just the reverse of ordinary multiplexing, which combines multiple signals into a single signal, inverse multiplexing is a technique commonly used where data in a high-speed local area network (LAN) flows back and forth into a wide area network (WAN) across the ‘‘bottleneck’’ of a slower line such as a T-1 (1.544 Mbps). By using multiple T-1 lines, the data stream can be load-balanced across all of the lines at the same time. As a general rule, inverse multiplexing across up to 8 T-1 lines (or E-1 lines in Europe and elsewhere) is said to be less expensive than the cost of renting a T-3 (45 Mbps) line (or E-3 line in Europe and elsewhere).
where pi is the ratio of a sphere’s circumference to its diameter, and is approximately equal to 3.14159. If the radius of a sphere is multiplied by some factor n, then the surface area increases by a factor of n2. This decreases the power per unit area by a factor of n2. Another way of saying this is that the power per unit area becomes n-2 times as great. Increasing the diameter of a sphere from 10 to 100 meters makes it 10 times as large in diameter, and gives it 100 times the surface area. This cuts the light power per unit area from a lamp at the sphere’s center by a factor of 100. The power landing on, say, one square centimeter of the sphere’s interior becomes 1/100, or 0.01 times, as great, if the sphere’s diameter grows from 10 to 100 meters. The law applies only as long as the point source is at the center of the sphere, so the rays from the source strike the sphere’s surface at right angles. Also see meter, square meter, and International System of Units (SI).
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Inverse multiplexing is also sometimes used in combination with frame relay and asynchronous transfer mode (ATM) transmission.
I/O (input/output), pronounced ‘‘eye-oh,’’ describes any operation, program, or device that transfers data to or from a computer. Typical I/O devices are printers, hard disks, keyboards, and mouses. In fact, some devices are basically input-only devices (keyboards and mouses); others are primarily output-only devices (printers); and others provide both input and output of data (hard disks, diskettes, writable CD-ROMs).
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Inverse multiplexing is sometimes called inverse muxing or imuxing.
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The inverse-square law is a principle that expresses the way radiant energy propagates through space. The rule states that the power intensity per unit area from a point source, if the rays strike the surface at a right angle, varies inversely according to the square of the distance from the source. Imagine a 40-watt lamp at the center of a spherical enclosure. The total power striking the surface of the sphere is 40 watts, no matter what the size of the sphere. Some people find this counter-intuitive, but it becomes obvious when we consider that all the radiated power (no more and no less) from the source must strike an enclosure of any size that completely surrounds the source. Thus, the power striking the interior of a sphere 10 meters across is the same (40 watts in this hypothetical example) as the power striking the interior of a sphere 100 meters across, 100 kilometers across, or 100,000 kilometers across. The power per unit area, however, does depend on the size of the sphere.
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The surface area A (in meters squared) of a sphere having radius r (in meters) is given by:
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In the Windows 95 operating system, IO.SYS is an executable binary file (in other words, a program) that, when the operating system is booted (started), processes instructions that tell the operating system how the computer is set up. In Windows 95, IO.SYS replaced the older MSDOS system files, IO.SYS and MSDOS.SYS. The newer IO.SYS contains all the information needed to start your computer and eliminates the need for the CONFIG.SYS and autoexec.bat files because the values in CONFIG.SYS and AUTOEXEC.BAT are in IO.SYS instead. Both of these files still come with Windows for older applications and drivers that need them. In computers using DOS or Windows 3.x, the IO.SYS is loaded with the MSDOS.SYS file when the computer is booted. MSDOS.SYS is a text file that determines whether DOS or Windows is loaded. Because IO.SYS is a sequence of executable code rather than readable text, it is not editable like MSDOS.SYS, CONFIG.SYS, and AUTOEXEC.BAT. To change default values that come with IO.SYS, you must change the CONFIG.SYS file. In loading IO.SYS, the system picks up any changed values that are present in the CONFIG.SYS file.
IP address
IOS IOS (Input/Output Supervisor) is a Microsoft Windows program that supervises the interaction between file system requests and events and input/output device drivers.
IP See ‘‘Internet Protocol’’
IP address This definition is based on Internet Protocol Version 4. See Internet Protocol Version 6 (IPv6) for a description of the newer 128-bit IP address. Note that the system of IP address classes described here, while forming the basis for IP address assignment, is generally bypassed today by use of Classless Inter-Domain Routing (CIDR) addressing. In the most widely installed level of the Internet Protocol (IP) today, an IP address is a 32-bit number that identifies each sender or receiver of information that is sent in packets across the Internet. When you request an HTML page or send e-mail, the Internet Protocol part of TCP/IP includes your IP address in the message (actually, in each of the packets if more than one is required) and sends it to the IP address that is obtained by looking up the domain name in the Uniform Resource Locator you requested or in the email address you’re sending a note to. At the other end, the recipient can see the IP address of the Web page requestor or the e-mail sender and can respond by sending another message using the IP address it received. An IP address has two parts: the identifier of a particular network on the Internet and an identifier of the particular device (which can be a server or a workstation) within that network. On the Internet itself—that is, between the router that move packets from one point to another along the route—only the network part of the address is looked at.
The Network Part of the IP Address The Internet is really the interconnection of many individual networks (it’s sometimes referred to as an internetwork). So the Internet Protocol (IP) is basically the set of rules for one network communicating with any other (or occasionally, for broadcast messages, all other networks). Each network must know its own address on the Internet and that of any other networks with which it communicates. To be part of the Internet, an organization needs an Internet network number, which it can request from the Network Information Center (NIC). This unique network number is included in any packet sent out of the network onto the Internet.
The Local or Host Part of the IP Address In addition to the network address or number, information is needed about which specific machine or host in a network is sending or receiving a message. So the IP address needs both
the unique network number and a host number (which is unique within the network). (The host number is sometimes called a local or machine address.) Part of the local address can identify a subnetwork or subnet address, which makes it easier for a network that is divided into several physical subnetworks (for example, several different local area networks) to handle many devices.
IP Address Classes and Their Formats Since networks vary in size, there are four different address formats or classes to consider when applying to NIC for a network number: L
Class A addresses are for large networks with many devices.
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Class B addresses are for medium-sized networks.
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Class C addresses are for small networks (fewer than 256 devices).
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Class D addresses are multicast addresses.
The first few bits of each IP address indicate which of the address class formats it is using. The address structures look like this: Network Local address (7 bits) (24 bits) Class B 10 Network Local address (14 bits) (16 bits) Class C 110 Network Local address (21 bits) (8 bits) Class D 1110 Multicast address (28 bits) The IP address is usually expressed as four decimal numbers, each representing eight bits, separated by periods. This is sometimes known as the dot address and, more technically, as dotted quad notation. For Class A IP addresses, the numbers would represent ‘‘network.local.local.local’’; for a Class C IP address, they would represent ‘‘network.network.network.local’’. The number version of the IP address can (and usually is) represented by a name or series of names called the domain name. Class A
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The Internet’s explosive growth makes it likely that, without some new architecture, the number of possible network addresses using the scheme above would soon be used up (at least, for Class C network addresses). However, a new IP version, IPv6, expands the size of the IP address to 128 bits, which will accommodate a large growth in the number of network addresses. For hosts still using IPv4, the use of subnets in the host or local part of the IP address will help reduce new applications for network numbers. In addition, most sites on today’s mostly IPv4 Internet have gotten around the Class C network address limitation by using the Classless Inter-Domain Routing (CIDR) scheme for address notation.
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Relationship of the IP Address to the Physical Address
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The machine or physical address used within an organization’s local area networks may be different than the Internet’s IP address. The most typical example is the 48-bit Ethernet address. TCP/IP includes a facility called the Address Resolution Protocol (ARP) that lets the administrator create a table that maps IP addresses to physical addresses. The table is known as the ARP cache.
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Static versus Dynamic IP Addresses
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The discussion above assumes that IP addresses are assigned on a static basis. In fact, many IP addresses are assigned dynamically from a pool. Many corporate networks and online services economize on the number of IP addresses they use by sharing a pool of IP addresses among a large number of users. If you’re an America Online user, for example, your IP address will vary from one logon session to the next because AOL is assigning it to you from a pool that is much smaller than AOL’s base of subscribers.
IP telephony IP telephony (Internet Protocol telephony) is a general term for the technologies that use the Internet Protocol’s packetswitched connections to exchange voice, fax, and other forms of information that have traditionally been carried over the dedicated circuit-switched connections of the public switched telephone network (PSTN). Using the Internet, calls travel as packets of data on shared lines, avoiding the tolls of the PSTN. The challenge in IP telephony is to deliver the voice, fax, or video packets in a dependable flow to the user. Much of IP telephony focuses on that challenge.
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ANSI/ISO 9318-3 Intelligent Peripheral Interface— Device Generic Command Set for Magnetic and Optical Disk Drives
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ANSI/ISO 9318-4 Intelligent Peripheral Interface— Device Generic Command Set for Magnetic Tape Drives
IPL IPL (initial program load) is a mainframe term for the loading of the operating system into the computer’s main memory. A mainframe operating system (such as OS/390) contains many megabytes of code that is customized by each installation, requiring some time to load the code into the memory. On a personal computer, booting or re-booting (restarting) is the equivalent to IPLing (the term is also used as a verb). In earlier operating systems, when you added devices to the hardware system, you had to stop the system, change the configuration file, and then ‘‘re-IPL,’’ an activity that meant the system would be unavailable for some period of time. Today’s systems provide dynamic reconfiguration so that the system can keep running.
Currently, unlike traditional phone service, IP telephony service is relatively unregulated by government. In the U.S., the Federal Communications Commission (FCC) regulates phone-to-phone connections, but says they do not plan to regulate connections between a phone user and an IP telephony service provider.
See ‘‘IPO’’
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IPC See ‘‘interprocess communication’’
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The latest version of IPI is IPI-3, which supports redundant array of independent disks. The official IPI standards are:
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IPI (Intelligent Peripheral Interface) is a high-bandwidth interface between a computer and a hard disk or a tape device. Devices using IPI can transfer data between the hard drive and RAM in the range between 3 and 25 megabytes per second.
IP telephony service providers include or soon will include local telephone companies, long distance providers such as AT&T, cable TV companies, Internet service providers (ISPs), and fixed service wireless operators. IP telephony services also affect vendors of traditional handheld devices.
VoIP is an organized effort to standardize IP telephony. IP telephony is an important part of the convergence of computers, telephones, and television into a single integrated information environment. Also see another general term, computer-telephony integration (CTI), which describes technologies for using computers to manage telephone calls.
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See ‘‘IPv6’’
IPO IPP An Internet presence provider (IPP) is a company that provides the disk space, high-speed Internet connection, and possibly the Web site design and other services for companies, organizations, or individuals to have a visible presence (meaning Web site) on the Internet. Using an IPP means that the owner of the Web site doesn’t need to have the files for it served from the owner’s computer. Although one can maintain a Web site from a smaller computer if site traffic is very low, most moderate- to high-traffic Web sites require a relatively expensive, higher-speed connection to the Internet. For this reason alone, most individuals and organizations put their sites on a server at an Internet presence provider.
IPX Many Internet service providers (ISPs) act as both Internet access provider (IAP) and Internet presence provider, although some provide only one or the other service. An IPP sometimes offers design and production services for a Web site as a package or on a consulting basis. Some Internet presence providers use the term hosting or virtual hosting for the ‘‘housing and maintenance’’ aspects of presence providing. Typical service packages at different prices include assistance with domain name registration, a specific amount of hard disk space for Web pages, a number of email addresses, the arrangement of server application or database access from Web pages, e-commerce shopping page setup, a specific amount of allowable Web data traffic, and assistance in setting up and maintaining the site. Some presence providers let you put your own server machine (or lease one to you) at their location and they manage it for you. This is called co-location.
IPPP An IPPP (Internet presence provider and promoter) is a company that helps an enterprise create a Web site, arrange for hosting (housing, maintaining, and providing Internet access) for the Web site, and promote an audience for it. Many Internet service providers (ISPs) are also IPPPs, but some ISPs simply offer users access to the Internet.
IPsec IPsec (Internet Protocol Security) is a developing standard for security at the network or packet processing layer of network communication. Earlier security approaches have inserted security at the Application layer of the communications model. IPsec will be especially useful for implementing virtual private networks and for remote user access through dial-up connection to private networks. A big advantage of IPsec is that security arrangements can be handled without requiring changes to individual user computers. Cisco has been a leader in proposing IPsec as a standard (or combination of standards and technologies) and has included support for it in its network routers. IPsec provides two choices of security service: Authentication Header (AH), which essentially allows authentication of the sender of data, and Encapsulating Security Payload (ESP), which supports both authentication of the sender and encryption of data as well. The specific information associated with each of these services is inserted into the packet in a header that follows the IP packet header. Separate key protocols can be selected, such as the ISAKMP/ Oakley protocol. Officially spelled IPsec by the IETF, the term often appears as IPSec and IPSEC.
IPv6 IPv6 (Internet Protocol Version 6) is the latest level of the Internet Protocol (IP) and is now included as part of IP support in many products including the major computer operating systems. IPv6 has also been called ‘‘IPng’’ (IP Next Generation). Formally, IPv6 is a set of specifications from the Internet Engineering Task Force (IETF). IPv6 was designed as an evolutionary set of improvements to the current IP Version 4. Network hosts and intermediate nodes with either IPv4 or IPv6 can handle packets formatted for either level of the Internet Protocol. Users and service providers can update to IPv6 independently without having to coordinate with each other. The most obvious improvement in IPv6 over the IPv4 is that IP addresses are lengthened from 32 bits to 128 bits. This extension anticipates considerable future growth of the Internet and provides relief for what was perceived as an impending shortage of network addresses. IPv6 describes rules for three types of addressing: unicast (one host to one other host), anycast (one host to the nearest of multiple hosts), and multicast (one host to multiple hosts). Additional advantages of IPv6 are: L
Options are specified in an extension to the header that is examined only at the destination, thus speeding up overall network performance.
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The introduction of an ‘‘anycast’’ address provides the possibility of sending a message to the nearest of several possible gateway hosts with the idea that any one of them can manage the forwarding of the packet to others. Anycast messages can be used to update routing tables along the line.
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Packets can be identified as belonging to a particular ‘‘flow’’ so that packets that are part of a multimedia presentation that needs to arrive in ‘‘real time’’ can be provided a higher quality-of-service relative to other customers. The IPv6 header now includes extensions that allow a packet to specify a mechanism for authenticating its origin, for ensuring data integrity, and for ensuring privacy.
IPX IPX (Internetwork Packet Exchange) is a networking protocol from Novell that interconnects networks that use Novell’s NetWare clients and servers. IPX is a datagram or packet protocol. IPX works at the Network layer of communication protocols and is connectionless (that is, it doesn’t require that a connection be maintained during an exchange of packets as, for example, a regular voice phone call does).
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Packet acknowledgment is managed by another Novell protocol, the Sequenced Packet Exchange (SPX). Other related Novell NetWare protocols are: the Routing Information Protocol (RIP), the Service Advertising Protocol (SAP), and the NetWare Link Services Protocol (NLSP).
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See ‘‘infrared radiation’’
IRC See ‘‘Internet Relay Chat’’
IRQ
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An IRQ (interrupt request) value is an assigned location where the computer can expect a particular device to interrupt it when the device sends the computer signals about its operation. For example, when a printer has finished printing, it sends an interrupt signal to the computer. The signal momentarily interrupts the computer so that it can decide what processing to do next. Since multiple signals to the computer on the same interrupt line might not be understood by the computer, a unique value must be specified for each device and its path to the computer. Prior to Plug-and Play (PnP) devices, users often had to set IRQ values manually (or be aware of them) when adding a new device to a computer. If you add a device that does not support Pnp, the manufacturer will hopefully provide explicit directions on how to assign IRQ values for it. If you don’t know what IRQ value to specify, you’ll probably save time by calling the technical support phone number for the device manufacturer and asking.
irrational number An irrational number is a real number that cannot be reduced to any ratio between an integer p and a natural number q. The union of the set of irrational numbers and the set of rational numbers forms the set of real numbers. In mathematical expressions, unknown or unspecified irrationals are usually represented by u through z. Irrational numbers are primarily of interest to theoreticians. Abstract mathematics has potentially far-reaching applications in communications and computer science, especially in data encryption and security. Examples of irrational numbers are 21/2 (the square root of 2), 31/3 (the cube root of 3), the circular ratio pi, and the natural logarithm base e. The quantities 21/2 and 31/3 are examples of algebraic numbers. Pi and e are examples of special irrationals known as a transcendental numbers. The decimal expansion of an irrational number is always nonterminating (it never ends) and nonrepeating (the digits display no repetitive pattern).
If x and z are irrationals such that x z, then there always exists an irrational y such that x y z. The set of irrationals is ‘‘dense’’ like the set Q of rationals. But theoretically, the set of irrationals is ‘‘more dense.’’ Unlike Q, the set of irrationals is nondenumerable. There are more nonterminating, nonrepeating decimals than is possible to list, even by implication. To prove this, suppose there is an implied list of all the nonterminating, nonrepeating decimal numbers between 0 and 1. Every such number consists of a zero followed by a decimal point, followed by an infinite sequence of digits from the set {0, 1, 2, 3, 4, 5, 6, 7, 8, 9}. Suppose the elements of the list are denoted x1, x2, x3, ... and the digits in the numbers are denoted aii. The list can be written like this: x1 x2 x3 x4 x5 x6 ...
= = = = = =
0.a11a12a13a14a15a16 0.a21a22a23a24a25a26 0.a31a32a33a34a35a36 0.a41a42a43a44a45a46 0.a51a52a53a54a55a56 0.a61a62a63a64a65a66
... ... ... ... ... ...
Even though we don’t know the actual values of any of the digits, it is easy to imagine a number between 0 and 1 that can’t be in this list. Think of a number y of the following form: y = 0.b11b22b33b44b55b66 ... such that no bii in y is equal to the corresponding aii in the list. The resulting number y is nonterminating and nonrepeating, is between 0 and 1, but is not equal to any xi in the list, because there is always at least one digit that does not match. The non-denumerability of the set of irrational numbers has far-reaching implications. Perhaps most bizarre is the notion that ‘‘not all infinities are created equal.’’ Although the set of rationals and the set of irrationals are both infinite, the set of irrationals is larger in a demonstrable way.
IRTF See ‘‘Internet Research Task Force’’
IRU See ‘‘Indefeasible Right of Use’’
IS 1) IS (information system) is the collection of technical and human resources that provide the storage, computing, distribution, and communication for the information required by all or some part of an enterprise. A special form of IS is a management information system (MIS), which provides information for managing an enterprise. 2) IS (information services) is a common name for an organization within an enterprise that is responsible for its data processing and information system or systems.
ISDN
ISA
Publishers, retailers, libraries, and readers use the ISBN as a way to specify a particular book without confusion.
See ‘‘ISA’’
The principles and procedures for international standard book numbering are contained in the International Organization for Standardization’s Recommendation 2108.
ISAM ISAM (Indexed Sequential Access Method) is a file management system developed at IBM that allows records to be accessed either sequentially (in the order they were entered) or randomly (with an index). Each index defines a different ordering of the records. An employee database may have several indexes, based on the information being sought. For example, a name index may order employees alphabetically by last name, while a department index may order employees by their department. A key is specified in each index. For an alphabetical index of employee names, the last name field would be the key. ISAM was developed prior to VSAM (Virtual Storage Access Method) and relational databases.
ISAPI ISAPI (Internet Server Application Program Interface) is a set of Windows program calls that let you write a Web server application that will run faster than a common gateway interface (CGI) application. A disadvantage of a CGI application (or ‘‘executable file,’’ as it is sometimes called) is that each time it is run, it runs as a separate process with its own address space, resulting in extra instructions that have to be performed, especially if many instances of it are running on behalf of users. Using ISAPI, you create a dynamic link library (DLL) application file that can run as part of the Hypertext Transport Protocol (HTTP) application’s process and address space. The DLL files are loaded into the computer when HTTP is started and remain there as long as they are needed; they don’t have to be located and read into storage as frequently as a CGI application. Existing CGI applications can be converted into ISAPI application DLLs without having to rewrite their logic. However, they do need to be written to be thread-safe so that a single instance of the DLL can serve multiple users. A special kind of ISAPI DLL is called an ISAPI filter, which can be designated to receive control for every HTTP request. You can create an ISAPI filter for encryption or decryption, for logging, for request screening, or for other purposes.
ISBN The ISBN (International Standard Book Number) is a unique number assigned to a book title by its publisher for tracking and ordering purposes. An example is: ISBN 0-385-49531-5 which is the ISBN for Simon Singh’s The Code Book, a book about crypography.
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iSCSI iSCSI is Internet SCSI (Small Computer System Interface), a new Internet Protocol (IP)-based storage networking standard for linking data storage facilities, developed by the Internet Engineering Task Force (IETF). By carrying SCSI commands over IP networks, iSCSI is used to facilitate data transfers over intranets and to manage storage over long distances. The iSCSI protocol is among the key technologies expected to help bring about rapid development of the storage area network (SAN) market, by increasing the capabilities and performance of storage data transmission. Because of the ubiquity of IP networks, iSCSI can be used to transmit data over local area networks (LANs), wide area networks (WANs), or the Internet and can enable locationindependent data storage and retrieval.
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How iSCSI Works: When an end user or application sends a request, the operating system generates the appropriate SCSI commands and data request, which then go through encapsulation and, if necessary, encryption procedures. A packet header is added before the resulting IP packets are transmitted over an Ethernet connection. When a packet is received, it is decrypted (if it was encrypted before transmission), and disassembled, separating the SCSI commands and request. The SCSI commands are sent on to the SCSI controller, and from there to the SCSI storage device. Because iSCSI is bidirectional, the protocol can also be used to return data in response to the original request. iSCSI is one of two main approaches to storage data transmission over IP networks; the other method, Fibre Channel over IP (FC/IP), translates Fibre Channel control codes and data into IP packets for transmission between geographically distant Fibre Channel SANs. FC/IP (also known as Fibre Channel tunneling or storage tunneling) can only be used in conjunction with Fibre Channel technology; in comparison, iSCSI can run over existing Ethernet networks. A number of vendors, including Cisco, IBM, and Nishan have introduced iSCSI-based products (such as switches and routers). Both iSCSI and FC/IP are proposed standards, and are expected to be ratified by the end of 2001.
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ISDN ISDN (Integrated Services Digital Network) is a set of CCITT/ITU standards for digital transmission over ordinary telephone copper wire as well as over other media. Home and business users who install an ISDN adapter (in place of a modem) can see highly-graphic Web pages arriving very
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quickly (up to 128 Kbps). ISDN requires adapters at both ends of the transmission so your access provider also needs an ISDN adapter. ISDN is generally available from your phone company in most urban areas in the United States and Europe. There are two levels of service: The Basic Rate Interface (BRI), intended for the home and small enterprise, and the Primary Rate Interface (PRI), for larger users. Both rates include a number of B-channels and a D-channels. Each Bchannel carries data, voice, and other services. Each Dchannel carries control and signaling information. The Basic Rate Interface consists of two 64 Kbps B-channels and one 16 Kbps D-channel. Thus, a Basic Rate user can have up to 128 Kbps service. The Primary Rate consists of 23 B-channels and one 64 Kpbs D-channel in the U.S. or 30 Bchannels and 1 D-channel in Europe. Integrated Services Digital Network in concept is the integration of both analog or voice data together with digital data over the same network. Although the ISDN you can install is integrating these on a medium designed for analog transmission, broadband ISDN (BISDN) will extend the integration of both services throughout the rest of the endto-end path using fiber optic and radio media. Broadband ISDN will encompass frame relay service for high-speed data that can be sent in large bursts, the Fiber DistributedData Interface (FDDI), and the Synchronous Opical Network (SONET). BISDN will support transmission from 2 Mbps up to much higher, but as yet unspecified, rates.
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One of the most commonly used routing protocols, the Intermediate System-to-Intermediate System protocol (IS-IS) is based on a routing method known as DECnet Phase V routing, in which routers known as intermediate systems exchange data about routing using a single metric to determine the network topology. IS-IS was developed by the International Organization for Standardization (ISO) as part of their Open Systems Interconnection (OSI) model. The first versions of IS-IS were used to manage routing within ISO Connectionless Network Protocol (CLNP) networks. IS-IS was ratified as a standard in 1990 (OSI IS-IS Intra-domain Routing Protocol, IETFRFC 1142 [2], ISO/IEC 10589 [3]). In the OSI context, an intermediate system refers to a router, as opposed to an end system (ES), which refers to a node. ES-IS protocols allow routers and nodes to identify each other; ISIS performs the same service between nodes for routing purposes. In common with other routing protocols such as OSPF (Open Shortest Path First), IS-IS is a link state protocol: it stores information about the state of links and uses that data to select paths. IS-IS is used to intermittently send out link state information across the network, so that each router can maintain a current picture of network topology. Optional metrics can be used to identify network delay, expense, and error involved with the use of a particular link.
ISO ISO (International Organization for Standardization), founded in 1947, is a worldwide federation of national standards bodies from some 100 countries, one from each country. Among the standards it fosters is Open Systems Interconnection (OSI), a universal reference model for communication protocols. Many countries have national standards organizations such as the American National Standards Institute (ANSI) that participate in and contribute to ISO standards making. ’’ISO’’ is not an abbreviation. It is a word, derived from the Greek isos, meaning ‘‘equal,’’ which is the root for the prefix ‘‘iso-’’ that occurs in a host of terms, such as ‘‘isometric’’ (of equal measure or dimensions) and ‘‘isonomy’’ (equality of laws, or of people before the law). The name ISO is used around the world to denote the organization, thus avoiding the assortment of abbreviations that would result from the translation of ‘‘International Organization for Standardization’’ into the different national languages of members. Whatever the country, the short form of the Organization’s name is always ISO.
ISO 14000 and 14001 ISO 14000 is a series of environmental management standards developed and published by the International Organization for Standardization (ISO) for organizations. The ISO 14000 standards provide a guideline or framework for organizations that need to systematize and improve their environmental management efforts. The ISO 14000 standards are not designed to aid the enforcement of environmental laws and do not regulate the environmental activities of organizations. Adherence to these standards is voluntary. The ISO 14001 standard is the most important standard within the ISO 14000 series. ISO 14001 specifies the requirements of an environmental management system (EMS) for small to large organizations. An EMS is a systemic approach to handling environmental issues within an organization. The ISO 14001 standard is based on the PlanCheck-Do-Review-Improve cycle. The Plan cycle deals with the beginning stages of an organization becoming ISO 14001-compliant. The Check cycle deals with checking and correcting errors. The Do cycle is the implementation and operation of the ISO 14001 standard within an organization. The Review cycle is a review of the entire process by the organization’s top management. And the Improve cycle is a cycle that never ends as an organization continually finds ways to improve their EMS. The entire process can take several months to several years depending on the size of the organization. If an organization is already ISO 9000-certified, the implementation of ISO 14001 does not take as long. When an organization is compliant, they can either register with a third-party
isotropic radiator registrar or self-declare their compliance. The ISO 14001 standard is the only ISO 14000 standard that allows an organization to be registered or ‘‘certified.’’ The Technical Committee (TC) behind ISO 14000 is TC 207.
also vary between countries. The question of how to express a date in numbers that precedes ‘‘1/1/1’’ also arises (how to express a date that is ‘‘B.C.’’). ISO 8601 provides a standard cross-national approach that says:
The Technical Committee (TC) behind ISO 9000 is TC 176.
ISO 9660 An ISO 9660 file system is a standard CD-ROM file system that allows you to read the same CD-ROM whether you’re on a PC, Mac, or other major computer platform. The standard, issued in 1988, was written by an industry group named High Sierra. Almost all computers with CD-ROM drives can read files from an ISO 9660 file system. There are several specification levels. In Level 1, file names must be in the 8.3 format (no more than eight characters in the name, no more than three characters in the suffix) and in capital letters. Directory names can be no longer than eight characters. There can be no more than eight nested directory levels. Level 2 and 3 specifications allow file names up to 32 characters long.
ISO date format The International Organization for Standardization (ISO) date format is a standard way to express a numeric calendar date that eliminates ambiguity. For example, North Americans usually write the month before the date. Europeans write the date before the month as in ‘‘30.3.1998’’ for March 30, 1998. The separators used between numbers
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Years expressed as ‘‘0’’ prior to year 1 and as ‘‘-1’’ for the year prior to year 0 (and so forth). Thus, March 30, 1998 would be: 1998-03-30.
ISO 9000
An organization can be ISO 9000-certified if it successfully follows the ISO 9000 standards for its industry. In order to be certified, the organization must submit to an examination by an outside assessor. The assessor interviews staff members to ensure that they understand their part in complying with the ISO 9000 standard, and the assessor examines the organization’s paperwork to ensure ISO 9000 compliance. The assessor then prepares a detailed report that describes the parts of the standard the organization missed. The organization then agrees to correct any problems within a specific time frame. When all problems are corrected, the organization can then be certified. Today, there are approximately 350,000 ISO 9000-certified organizations in over 150 countries.
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A general-to-specific approach, forming a date that is easier to process—thus, the year first, followed by month, then day
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ISO 9000 is a series of standards, developed and published by the International Organization for Standardization (ISO), that define, establish, and maintain an effective quality system for manufacturing and service industries. The ISO 9000 standard is the most widely known and has perhaps had the most impact of the 13,000 standards published by the ISO. It serves many different industries and organizations as a guide to quality products, service, and management.
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To express whether the date reflects the Julian calendar or the Gregorian calendar, the date can be followed with a ‘‘J’’ or a ‘‘G’’.
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In information technology, isochronous (from the Greek ‘‘equal’’ and ‘‘time’’; pronounced ‘‘eye-SAH-krun-us’’) pertains to processes that require timing coordination to be successful, such as voice and digital video transmission. A sound or picture going from a peripheral computer device or across a network into a computer or television set needs to arrive at close to the same rate of data flow as the source. In feeding digital image data from a peripheral device (such as a video camera) to a display mechanism within a computer, isochronous data transfer ensures that data flows continously and at a steady rate in close timing with the ability of the display mechanism to receive and display the image data. (FireWire, the IEEE 1394 High Performance Serial Bus, includes an isochronous interface.) Isochronous can be distinguished from asynchronous, which pertains to processes that proceed independently of each other until a dependent process has to ‘‘interrupt’’ the other process, and synchronous, which pertains to processes in which one process has to wait on the completion of an event in another process before continuing.
isotropic radiator An isotropic radiator is a transducer that produces useful electromagnetic field output in all directions with equal intensity, and at 100-percent efficiency, in three-dimensional space. If used for signal reception, the device is equally sensitive in all directions. In wireless communications and broadcasting, the isotropic radiator (also called an isotropic antenna) is a theoretical ideal. It is sometimes referred to as a point source. In wireless applications, an isotropic radiator can be approximated by constructing a single-turn square loop with a circumference of 1/2 wavelength, and then giving the loop a half-twist along an axis that lies in the plane of the loop.
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The isotropic radiator is useful primarily as a standard laboratory reference source. The behavior and performance of more complex antennas is measured against it. A dipole antenna, for example, has signal output and sensitivity approximately 2.15 decibels greater, in its favored directions, than the output and sensitivity of an isotropic antenna.
ISP An ISP (Internet service provider) is a company that provides individuals and other companies access to the Internet and other related services such as Web site building and virtual hosting. An ISP has the equipment and the telecommunication line access required to have a point-ofpresence on the Internet for the geographic area served. The larger ISPs have their own high-speed leased lines so that they are less dependent on the telecommunication providers and can provide better service to their customers. Among the largest national and regional ISPs are AT&T WorldNet, IBM Global Network, MCI, Netcom, UUNet, and PSINet. ISPs also include regional providers such as New England’s NEARNet and the San Francisco Bay area BARNet. They also include thousands of local providers. In addition, Internet users can also get access through online service providers (OSP) such as America Online and Compuserve. The larger ISPs interconnect with each other through MAE (ISP switching centers run by MCI WorldCom) or similar centers. The arrangements they make to exchange traffic are known as peering agreements. There are several very comprehensive lists of ISPs world-wide available on the Web. An ISP is also sometimes referred to as an IAP (Internet access provider). ISP is sometimes used as an abbreviation for independent service provider to distinguish a service provider that is an independent, separate company from a telephone company.
ISPF ISPF (Interactive System Productivity Facility) is the user interface and supporting programs that come with IBM’s OS/390 operating system and that allow a company to configure and manage its system, add new system or application programs and test them, and access system files (data sets). ISPF consists of the Dialog Manager (DM), the Program Development Facility (PDF), and the Software Configuration and Library Management (SCLM) facility. IBM also offers an ISPF that will run on an OS/2 system so that developers can work ‘‘offline’’ from the OS/390.
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See ‘‘ISP’’
ISRC See ‘‘International Standard Recording Code’’
ISV An ISV (independent software vendor) makes and sells software products that run on one or more computer hardware or operating system platforms. The companies that make the platforms like Microsoft, IBM, HewlettPackard, Apple, and others encourage and lend support to ISVs, often with special ‘‘business partner’’ programs. In general, the more applications that run on a platform, the more value it offers to customers. Of course, platform manufacturers such as Microsoft and IBM make applications, too, but don’t have the resources and, in many cases, the special knowledge required, to make them all. Think of all the programs that run on your Windows 2000 or Mac platform and you’ll realize how many ISVs there are. Some ISVs focus on a particular operating system like IBM’s small business AS/400 for which there are thousands of ISV applications. Other ISVs specialize in a particular application area, such as engineering, and develop software primarily for high-end UNIX-based workstation platforms. ISVs make and sell software that is added to platforms. Original equipment manufacturers (OEMs) use hardware platform components to build larger products. Value-added resellers (VARs) incorporate platform software into their own software product packages.
IT IT (information technology) is a term that encompasses all forms of technology used to create, store, exchange, and use information in its various forms (business data, voice conversations, still images, motion pictures, multimedia presentations, and other forms, including those not yet conceived). It’s a convenient term for including both telephony and computer technology in the same word. It is the technology that is driving what has often been called ‘‘the information revolution.’’
IT Infrastructure Library See ‘‘ITIL’’
Itanium Itanium is Intel’s first microprocessor that is based on the 64bit architecture known as IA-64. Developed under the code name of Merced, Itanium and its underlying architecture are expected to provide a foundation for the next-generation of software for the server and high-end workstation markets. Intel plans to follow Itanium with additional IA-64 microprocessors, which have the code names of McKinley, Madison, and Deerfield.
IT-ISAC In addition to supporting a 64-bit processor bus and a set of 28 registers, the 64-bit design allows access to a very large memory (VLM). In addition, the architecture exploits features in Explicitly Parallel Instruction Computing (EPIC), a joint Intel and Hewlett-Packard development effort. These provide advances in the parallel processing handling of computer instructions known as predication and speculation. An additional Itanium feature includes a Level 3 (L3) cache memory, to supplement the current L1 and L2 cache memories found in most of today’s microcomputers. Most applications in use today are based on a 32-bit microprocessor architecture, and are designed for up to 4 gigabytes of memory. However, with application access to ever-larger databases becoming more important, many of the leading software and hardware suppliers in the computer industry have already begun to develop systems and applications for the Itanium and its ability to handle 64bit address space. One feature of Itanium is its use of a ‘‘smart compiler’’ to optimize how instructions are sent to the processor. This approach allows Itanium and future IA-64 microprocessors to process more instructions per clock cycle (IPCs). (IPCs can be used along with clock speed in terms of megahertz (MHz) to indicate a microprocessor’s overall performance.) Itanium is viewed by Intel and industry observers as a new level of hardware platform for the UNIX and Windows server market and also for the high-end PC workstation market with its memory- and graphics-intensive scientific, technical, and high-end graphics applications. Observers say Itanium-based servers will be especially suited for today’s large enterprise computing environments with data warehouses and a requirement for intensive processing for complex queries and transactions. Virtually all of the major manufacturers and suppliers of today’s applications, high-end systems, operating systems, and computer hardware have endorsed Itanium and the IA64 line of processors. More than 30 Itanium-based systems are expected to come to the marketplace in 2001. Some reports indicate that Itanium’s IA-64 architecture will own its market through 2003, at which point some competitive ‘‘next-generation’’ microprocessors may begin to be seen. This window of opportunity allows Intel to capitalize on the popularity of IA-64, and to develop profitable original equipment manufacturer arrangements with other vendors, suppliers, and manufacturers in the computer industry. Itanium and IA-64 also support 32-bit applications.
iteration. If the sequence of instructions is executed repeatedly, it is called a loop, and we say that the computer iterates through the loop. 2) In software development, iterative is used to describe a heuristic planning and development process where an application is developed in small sections called iterations. Each iteration is reviewed and critiqued by the software team and potential end-users; insights gained from the critique of an iteration are used to determine the next step in development. Data models or sequence diagrams, which are often used to map out iterations, keep track of what has been tried, approved, or discarded, and eventually serve as a kind of blueprint for the final product. The challenge in iterative development is to make sure all the iterations are compatible. As each new iteration is approved, developers may employ a technique known as backwards engineering, which is a systematic review and check procedure to make sure each new iteration is compatible with previous ones. The advantage of using iterative development is that the end-user is involved in the development process. Instead of waiting until the application is a final product, when it may not be possible to make changes easily, problems are identified and solved at each stage of development. Iterative development is sometimes called circular or evolutionary development.
ITIL ITIL (Information Technology Infrastructure Library) is a set of best practices standards for information technology (IT) service management. The United Kingdom’s Central Computer and Telecommunications Agency (CCTA) created the ITIL in response to the growing dependence on information technology to meet business needs and goals. The ITIL provides businesses with a customizable framework of best practices to achieve quality service and overcome difficulties associated with the growth of IT systems. Hewlett-Packard and Microsoft are two businesses that use ITIL as part of their own best practices frameworks. The ITIL is organized into ‘‘sets’’ of texts which are defined by related functions: service support, service delivery, managerial, software support, computer operations, security management, and environmental. In addition to texts, which can be purchased online, ITIL services and products include training, qualifications, software tools, and user groups such as the IT Service Management Forum (itSMF).
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While owned by the CCTA since the mid-1980s, the ITIL is currently maintained and developed by The National Exam Institute for Informatics (EXIN), a non-proprietary and nonprofit organization based in the Netherlands.
Iterative (prounounced IT-ter-a-teev) is an adjective that means repetitious.
IT-ISAC
1) In computer programming, iterative is used to describe a situation in which a sequence of instructions can be executed multiple times. One pass through the sequence is called an
IT-ISAC (Information Technology Information Sharing and Analysis Center) is a facility founded in January 2001 by nineteen prominent IT industry companies (including
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Oracle, IBM, EDS, and Computer Sciences) to serve as a central repository for security-related information. The group’s purpose is to share each organization’s information about security attacks and vulnerabilities among all the members. Member companies are expected to report information concerning security problems that they have or solutions to such problems that they have found. IT-ISAC should increase security levels and decrease monetary losses for its membership, at a time when—according to FBI estimates—the average security attack can cost an organization $400,000. President Clinton, in May 1998, issued Presidential Decision Directive 63, appealing to U.S. industry leaders to form information sharing and analysis groups to protect the nation’s critical infrastructures against attacks, and establishing that purpose as a national security policy. Because much of the nation’s infrastructure is privately owned and operated, effective security measures depend upon collaboration between the public and private sectors. In response to the directive, the Financial Services industry formed FS-ISAC (Financial Services Information Sharing and Analysis Center) in 1999. The effectiveness of the new group was demonstrated in February 2000, when it saved its membership from falling victim to the widespread denial of service attacks that affected much of the industry. IT-ISAC is modeled on the financial services group, which distributes much of its information anonymously. Anonymity helps members to be more comfortable sharing information in a traditionally competitive industry where the security of organization-critical information has been very closely guarded.
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The ITU-T (for Telecommunication Standardization Sector of the International Telecommunications Union) is the primary international body for fostering cooperative standards for telecommunications equipment and systems. It was formerly known as the CCITT. It is located in Geneva, Switzerland.
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J/Direct J/Direct is an application programming interface (API) from Microsoft that allows applications written in the Java programming language to make programming requests for Windows operating system services. Introduced in June 1997, J/Direct allows Java applications to make calls on Windows dynamic link libraries (DLLs). The existing DLLs do not need to be changed manually, because J/Direct automatically converts many commonly-used data types, according to Microsoft. Unlike earlier Java-to-Windows tools, J/Direct eliminates the need for programmers to write wrapper code around existing Windows applications. J/Direct supports Microsoft’s Windows 32-bit APIs and has been included in the software developer’s kit (SDK) for Internet Explorer.
J2EE J2EE (Java 2 Platform, Enterprise Edition) is a Java platform designed for the mainframe-scale computing typical of large enterprises. Sun Microsystems (together with industry partners such as IBM) designed J2EE to simplify application development in a thin client tiered environment. J2EE simplifies application development and decreases the need for programming and programmer training by creating standardized, reusable modular components and by enabling the tier to handle many aspects of programming automatically. J2EE includes many components of the Java 2 Platform, Standard Edition (J2SE): L
The Java Development Kit (JDK) is included as the core language package.
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Write Once Run Anywhere technology is included to ensure portability.
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Support is provided for Common Object Request Broker Architecture (CORBA), a predecessor of Enterprise JavaBeans (EJB), so that Java objects can communicate with CORBA objects both locally and over a network through its interface broker.
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Java Database Connectivity 2.0 (JDBC), the Java equivalent to Open Database Connectivity (ODBC), is included as the standard interface for Java databases. A security model is included to protect data both locally and in Web-based applications.
J2EE also includes a number of components added to the J2SE model, such as the following:
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Full support is included for Enterprise JavaBeans. EJB is a server-based technology for the delivery of program components in an enterprise environment. It supports the Extensible Markup Language (XML) and has enhanced deployment and security features. The Java servlet API (application programming interface) enhances consistency for developers without requiring a graphical user interface (GUI). Java Server Pages (JSP) is the Java equivalent to Microsoft’s Active Server Pages (ASP) and is used for dynamic Web-enabled data access and manipulation.
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The J2EE Application Programming Model is the standard programming model used to facilitate the development of multi-tier, thin client applications.
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The J2EE Platform includes necessary policies and APIs such as the Java servlets and Java Message Service (JMS).
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The J2EE Compatibility Test Suite ensures that J2EE products are compatible with the platform standards. The J2EE Reference Implementation explains J2EE capabilities and provides its operational definition.
J2ME J2ME (Java 2 Platform, Micro Edition) is a technology that allows programmers to use the Java programming language and related tools to develop programs for mobile wireless information devices such as cellular phones and personal digital assistants (PDAs). J2ME consists of programming specifications and a special virtual machine, the K Virtual Machine, that allows a J2ME-encoded program to run in the mobile device. There are two programming specifications: Connected, Limited Device Configuration (CLDC) and the Mobile Information Device Profile (MIDP). CLDC lays out the application program interface (API) and virtual machine features needed to support mobile devices. MIDP adds to the CLDC the user interface, networking, and messaging details needed to interface with mobile devices. MIDP includes the idea of a midlet, a small Java application similar to an applet but one that conforms with CLDC and MIDP and is intended for mobile devices. Devices with systems that exploit J2ME are already available and are expected to become even more available in the next few years.
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jam sync
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Also see jabber (in networks).
In audio (sound) production, jam sync is a mode of device synchronization using SMPTE time code in which a slave device can furnish its own timing during the time that a master device is temporarily unstable. Jam sync is usually an optional mode that the user can select.
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Jabber is an initiative to produce an open source, XMLbased instant messaging platform. Similar to the Linux and Apache projects, Jabber developers volunteer their time to work with the code over the Internet. As a result of their efforts, anyone can download the Jabber client and server for free. Creator Jeremie Miller first started Jabber.org in 1998. Jabber operates differently than other proprietary instant messaging systems and works in a fashion similar to e-mail, using a distributed architecture. It adds a suffix to each address after the ‘‘@’’ sign (for instance user@msn) just like an e-mail addressing system. This enables a Jabber server to read addresses from different messaging systems and know where they can be found. Even though Jabber is offered for free, many businesses need a contact for packaged software, technical support, and other services. That’s the niche that Jabber.com hopes to fill. The company calls its relationship to Jabber.org similar to the one Red Hat has with Linux. Jabber.com has already released an instant messaging server that it vows will be compatible with many existing platforms, including America Online’s closely guarded ICQ and AOL Instant Messenger. Miller now works for Jabber.com. In September 2000, Jabber.com signed a deal with an opensource application service provider (ASP) and a wireless application developer to make a version for mobile devices.
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Also see Jabber, an open source program for instant messaging. In networks, a jabber is any device that is handling electrical signals improperly, usually affecting the rest of the network. In an Ethernet network, devices compete for use of the line, attempting to send a signal and then retrying in the event that someone else tried at the same time. A jabber can look like a device that is always sending, effectively bringing the network to a halt. A jabber is usually the result of a bad network interface card (NIC). Occasionally, it can be caused by outside electrical interference.
In a basic SMPTE synchronization scenario, time code is fed from one device (the master) to a second device (the slave). The slave device follows the time locations as transmitted by the master device. If the master device goes to one hour and thirteen minutes (expressed as 01:13:00:00; Hours:Minutes:Seconds:Frames), the slave device follows it there. The problem arises when the master device’s SMPTE time code has an error in it, caused by a small bad portion of tape, for example. In most situations, the slave device would not know what to do for a split second since it has missing instructions from its host. It would stutter or stop completely. By using jam syncing, the slave device can be instructed to ignore small dropouts of time code from the master, thus ensuring smooth synchronization. To do this, it generates its own time code whenever the master’s time code is missing.
Japanese emoticons In Japan, users have worked out emoticons (or keyboard ‘‘smiley faces’’) adapted to their culture. According to The New York Times (August 12, 1996), the Japanese are using emoticons even more than Westerners. Because their PC keyboards handle the two-byte characters of Kanji, users can choose between single- and double-byte versions of certain characters such as underscore characters, allowing a further degree of expression. Here are some examples of Japanese emoticons: ^_^ ^o^;> ^^; ^o^ *^o^* (^_^)/
Smile Excuse me Cold sweat Happy Exciting Banzai smiley
jam
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In an Ethernet network, a jam is a signal from one device to all other devices that a collision has occurred (it was trying to send a frame while another device was also trying to put a frame on the line). The jam signal results in additional collisions in any other frames in process of being put on the line and warns non-transmitting devices to wait. Effectively, a jam provides a way to force all devices to restart any bids to get control of the line.
A JAR (Java ARchive) file is a file that contains the class, image, and sound files for a Java applet gathered into a single file and compressed for faster downloading to your Web browser. An applet that comes as part of a Web page that you may happen to request may include several files, each of which would have to be downloaded along with the Web page. By putting the applet components in a single file and compressing that file, download time is saved.
Java Database Connectivity When a programmer gets a Java program development kit, a small program or utility called ‘‘jar’’ is included. The jar utility lets you create, list, and extract the individual files from a JAR file. Ordinarily, a browser user will not need to ‘‘open’’ or view a JAR file directly. It is opened when the Web page is received and the applet is in some manner initiated. The JAR format is based on the popular zip file format.
Java Java is a programming language expressly designed for use in the distributed environment of the Internet. It was designed to have the ‘‘look and feel’’ of the C++ language, but it is simpler to use than C++ and enforces an objectoriented programming model. Java can be used to create complete applications that may run on a single computer or be distributed among servers and clients in a network. It can also be used to build a small application module or applet for use as part of a Web page. Applets make it possible for a Web page user to interact with the page. The major characteristics of Java are: L
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The programs you create are portable in a network. (See portability.) Your source program is compiled into what Java calls bytecode, which can be run anywhere in a network on a server or client that has a Java virtual machine. The Java virtual machine interprets the bytecode into code that will run on the real computer hardware. This means that individual computer platform differences such as instruction lengths can be recognized and accommodated locally just as the program is being executed. Platform-specific versions of your program are no longer needed. The code is robust, here meaning that, unlike programs written in C++ and perhaps some other languages, the Java objects can contain no references to data external to themselves or other known objects. This ensures that an instruction can not contain the address of data storage in another application or in the operating system itself, either of which would cause the program and perhaps the operating system itself to terminate or ‘‘crash.’’ The Java virtual machine makes a number of checks on each object to ensure integrity. Java is object-oriented, which means that, among other characteristics, an object can take advantage of being part of a class of objects and inherit code that is common to the class. Objects are thought of as ‘‘nouns’’ that a user might relate to rather than the traditional procedural ‘‘verbs.’’ A method can be thought of as one of the object’s capabilities or behaviors. In addition to being executed at the client rather than the server, a Java applet has other characteristics designed to make it run fast. Relative to C++, Java is easier to learn. (However, it is not a language you’ll pick up in an evening!)
Java was introduced by Sun Microsystems in 1995 and instantly created a new sense of the interactive possibilities of the Web. Both of the major Web browsers include a Java virtual machine. Almost all major operating system developers (IBM, Microsoft, and others) have added Java compilers as part of their product offerings. The Java virtual machine includes an optional just-in-time compiler that dynamically compiles bytecode into executable code as an alternative to interpreting one bytecode instruction at a time. In many cases, the dynamic JIT compilation is faster than the virtual machine interpretation. JavaScript should not be confused with Java. JavaScript, which originated at Netscape, is interpreted at a higher level, is easier to learn than Java, but lacks some of the portability of Java and the speed of bytecode. Because Java applets will run on almost any operating system without requiring recompilation and because Java has no operating systemunique extensions or variations, Java is generally regarded as the most strategic language in which to develop applications for the Web. (However, JavaScript can be useful for very small applications that run on the Web client or server.)
Java 2 Platform, Enterprise Edition See ‘‘J2EE’’
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Java 2 Platform, Micro Edition See ‘‘J2ME’’
Java chip The Java chip is a microchip that, when included in or added to a computer, will accelerate the performance of Java programs (including the applets that are sometimes included with Web pages). The special chip can execute all or some of the instructions in the Java bytecode directly without requiring interpretation by a software virtual machine. The Java chip may be particularly important as part of embedded systems in handheld devices such as smart phones.
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Java Database Connectivity JDBC (Java Database Connectivity) is an application program interface (API) specification for connecting programs written in Java to the data in popular database. The application program interface lets you encode access request statements in structured query language (SQL) that are then passed to the program that manages the database. It returns the results through a similar interface. JDBC is very similar to the SQL Access Group’s Open Database Connectivity (ODBC) and, with a small ‘‘bridge’’ program, you can use the JDBC interface to access databases through
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Whatis?Com’s Encyclopedia of Technology Terms the ODBC interface. For example, you could write a program designed to access many popular database products on a number of operating system platforms. When accessing a database on a PC running Microsoft’s Windows 2000 and, for example, a Microsoft Access database, your program with JDBC statements would be able to access the Microsoft Access database. JDBC actually has two levels of interface. In addition to the main interface, there is also an API from a JDBC ‘‘manager’’ that in turn communicates with individual database product ‘‘drivers,’’ the JDBC-ODBC bridge if necessary, and a JDBC network driver when the Java program is running in a network environment (that is, accessing a remote database). When accessing a remote database, JDBC takes advantage of the Internet’s file addressing scheme and a file name looks much like a Web page address (or Uniform Resource Locator). For example, a Java SQL statement might identify the database as: jdbc:odbc://www.somecompany.com:400/databasefile
JDBC specifies a set of object-oriented classes for the programmer to use in building SQL requests. An additional set of classes describes the JDBC driver API. The most common SQL data types, mapped to Java data types, are supported. The API provides for implementation-specific support for Microsoft Transaction Server requests and the ability to commit or roll back to the beginning of a transaction.
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Using the Java programming language, Java Foundation Classes (JFC) are pre-written code in the form of class libraries (coded routines) that give the programmer a comprehensive set of graphical user interface (GUI) routines to use. The Java Foundation Classes are comparable to the Microsoft Foundation Class (MFC) library. JFC is an extension of the original Java Abstract Windowing Toolkit (AWT). Using JFC and Swing, an additional set of program components, a programmer can write programs that are independent of the windowing system within a particular operating system.
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Java Message Service (JMS) is an application program interface (API) from Sun Microsystems that supports the formal communication known as messaging between computers in a network. Sun’s JMS provides a common interface to standard messaging protocols and also to special messaging services in support of Java programs. Sun advocates the use of the Java Message Service for anyone developing Java applications, which can be run from any major operating system platform. The messages involved exchange crucial data between computers—rather than between users—and contain information such as event notification and service requests.
Messaging is often used to coordinate programs in dissimilar systems or written in different programming languages. Using the JMS interface, a programmer can invoke the messaging services of IBM’s MQSeries, Progress Software’s SonicMQ, and other popular messaging product vendors. In addition, JMS supports messages that contain serialized Java object and messages that contain Extensible Markup Language (XML) pages.
Java Naming and Directory Interface See ‘‘JNDI’’
Java Server Page Java Server Page (JSP) is a technology for controlling the content or appearance of Web pages through the use of servlets, small programs that are specified in the Web page and run on the Web server to modify the Web page before it is sent to the user who requested it. Sun Microsystems, the developer of Java, also refers to the JSP technology as the Servlet application program interface (API). JSP is comparable to Microsoft’s Active Server Page (ASP) technology. Whereas a Java Server Page calls a Java program that is executed by the Web server, an Active Server Page contains a script that is interpreted by a script interpreter (such as VBScript or JScript) before the page is sent to the user. An HTML page that contains a link to a Java servlet is sometimes given the file name suffix of .JSP.
JavaBeans JavaBeans is an object-oriented programming interface from Sun Microsystems that lets you build re-useable applications or program building blocks called components that can be deployed in a network on any major operating system platform. Like Java applet, JavaBeans components (or ‘‘Beans’’) can be used to give World Wide Web pages (or other applications) interactive capabilities such as computing interest rates or varying page content based on user or browser characteristics. From a user’s point-of-view, a component can be a button that you interact with or a small calculating program that gets initiated when you press the button. From a developer’s point-of-view, the button component and the calculator component are created separately and can then be used together or in different combinations with other components in different applications or situations. When the components or Beans are in use, the properties of a Bean (for example, the background color of a window) are visible to other Beans and Beans that haven’t ‘‘met’’ before can learn each other’s properties dynamically and interact accordingly.
JCL Beans are developed with a Beans Development Kit (BDK) from Sun and can be run on any major operating system platform (Windows, UNIX, Mac) inside a number of application environments (known as containers), including browsers, word processors, and other applications. To build a component with JavaBeans, you write language statements using Sun’s Java programming language and include JavaBeans statements that describe component properties such as user interface characteristics and events that trigger a bean to communicate with other beans in the same container or elsewhere in the network. Beans also have persistence, which is a mechanism for storing the state of a component in a safe place. This would allow, for example, a component (bean) to ‘‘remember’’ data that a particular user had already entered in an earlier user session. JavaBeans gives Java applications the compound document capability that the OpenDoc and ActiveX interfaces already provide.
JavaScript JavaScript is an interpreted programming or script language from Netscape. It is somewhat similar in capability to Microsoft’s Visual Basic, Sun’s Tcl, the UNIX-derived Perl, and IBM’s REX. In general, script languages are easier and faster to code in than the more structured and compiled languages such as C and C++. Script languages generally take longer to process than compiled languages, but are very useful for shorter programs. JavaScript is used in Web site development to do such things as: L
Automatically change a formatted date on a Web page
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Cause a linked-to page to appear in a popup window
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Cause text or a graphic image to change during a mouse rollover
JavaScript uses some of the same ideas found in Java, the compiled object-oriented programming derived from C++. JavaScript code can be imbedded in HTML pages and interpreted by the Web browser (or client). JavaScript can also be run at the server as in Microsoft’s Active Server Pages before the page is sent to the requestor. Both Microsoft and Netscape browsers support JavaScript, but sometimes in slightly different ways.
Jaz drive A Jaz drive is a small, portable hard disk drive used primarily for backing up and archiving personal computer files. The Jaz drive is sold by Iomega Corporation, the same company that developed the Zip drive. Both the Jaz drive and the disks come in two sizes, 1 GB and 2 GB. The two sizes look similar, but a 2 GB disk is not compatible with a 1 GB Jaz drive. The 2 GB Jaz drive can use both disk sizes.
Internal and external Jaz drives are available. The Jaz drive uses the Small Computer System Interface (SCSI) and requires a SCSI controller. A Jaz drive comes with software to back up the hard disk, to record and playback voice, music and video files, to temporarily use the Jaz drive on another computer, and to duplicate Jaz disks using one Jaz drive. Jaz drives are also used to exchange files with others, to store unusually large files, and to keep certain files separate from files on your hard disk. The Jaz drive is different from a Zip drive in that the computer recognizes it as another hard disk, which allows the user to install and use applications from the Jaz drive. The Jaz drive runs at speeds near or equal to traditional hard disks.
JBOD JBOD (for ‘‘just a bunch of disks,’’ or sometimes ‘‘just a bunch of drives’’) is a derogatory term—the official term is ‘‘spanning’’—used to refer to a computer’s hard disks that haven’t been configured according to the RAID (for ‘‘redundant array of independent disks’’) system to increase fault tolerance and improve data access performance. The RAID system stores the same data redundantly on multiple disks that nevertheless appear to the operating system as a single disk. Although JBOD also makes the disks appear to be a single one, it accomplishes that by combining the drives into one larger logical one. JBOD doesn’t deliver any advantages over using separate disks independently and doesn’t provide any of the fault tolerance or performance benefits of RAID.
JCL JCL (job control language) is a language for describing jobs (units of work) to the MVS, OS/390, and VSE operating systems, which run on IBM’s S/390 large server (mainframe) computers. These operating systems allocate their time and space resources among the total number of jobs that have been started in the computer. Jobs in turn break down into job steps. All the statements required to run a particular program constitute a job step. Jobs are background (sometimes called batch) units of work that run without requiring user interaction (for example, print jobs). In addition, the operating system manages interactive (foreground) user requests that initiate units of work. In general, foreground work is given priority over background work. One IBM manual compares a set of JCL statements to a menu order in a restaurant. The whole order is comparable to the job. Back in the kitchen, the chefs divide the order up and work on individual dishes (job steps). As the job steps complete, the meal is served (but it has to be served in the order prescribed just as some job steps depend on other job steps being performed first).
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JCL statements mainly specify the input data sets (files) that must be accessed, the output data set to be created or updated, what resources must be allocated for the job, and the programs that are to run, using these input and output data sets. A set of JCL statements for a job is itself stored as a data set and can be started interactively. MVS and OS/390 provide an interactive menu-like interface, ISPF, for initiating and managing jobs. In MVS and OS/390, the part of the operating system that handles JCL is called the Job Entry Subsystem (JES). There are two versions, JES2 and a later version with additional capabilities, JES3.
JDBC
F G H I J K L
See ‘‘Java Database Connectivity’’
JEDEC SDRAM JEDEC (Joint Electron Device Engineering Council) SDRAM is an industry standard synchronous DRAM. It has a dualbank architecture and several burst mode accesses that can be preset. JEDEC SDRAM chips operate at either 83 MHz or 100 MHz. JEDEC SDRAM is also known as PC66 SDRAM because it was originally rated for 66 MHz bus operation and to distinguish it from Intel’s PC100 architecture.
jello See ‘‘ice’’
JES
M N O
See ‘‘Job Entry Subsystem’’
JES2 See ‘‘Job Entry Subsystem’’
JES3
P Q R
See ‘‘Job Entry Subsystem’’
JFC See ‘‘Java Foundation Classes’’
JHTML
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JHTML (Java within Hypertext Markup Language) is a standard for including a Java program as part of a Web page (a page written using the Hypertext Markup Language, or HTML). A Web site developer can write a small program using the Java programming language and insert the program within a Web page. When a user requests the page, the Web site server, observing a request for a file with the .jhtml suffix, passes the code to a special Java program designed to handle JHML. This program, called the PageCompileServlet, calls the Java compiler, which quickly
compiles the code. (If the code has previously been compiled by an earlier page request, this step is unnecessary.) The code is then executed, typically modifying the contents of the Web page in some way before it is sent to the requestor. JHTML is comparable to Microsoft’s Active Server Page and to PHP in that all are approaches to modifying a Web page at the server before it is sent rather than at the client (the user’s Web browser) with JavaScript. To access a database from the Web page, the Java code might use the Java Database Connectivity (JDBC) interface. JHTML requires installing a Java compiler on the Web server. JHTML is part of JavaSoft’s Java WebServer Application Program Interface.
Jigsaw Jigsaw is a Web server from the World Wide Web Consortium (W3C) that is designed for the purpose of demonstrating new Web protocols and other features. Jigsaw is written in the Java programming language and made available as open source software. You can download it from the W3C site in either a UNIX or a Windows 2000 version. Although Jigsaw is intended to be a demonstration platform rather than a fully capable Web server, the W3C reports that it performs as well as the CERN server and provides much of the usual Web server support such as proxy server capability, virtual hosting, and support for common gateway interface. Jigsaw can also be used for delivering pages built or modified with Personal Home Page and JSP scripts.
Jikes Jikes is an open source Java compiler from IBM that adheres strictly to the Java specification and promises an ‘‘extremely fast’’ compilation. Although Sun Microsystems, the inventor of Java, offers its own proprietary compiler, IBM’s compiler uses the open source idea, meaning that anyone can acquire IBM’s Jikes source code for free and redistribute it with modifications (if these adhere to the Java specifications). Jikes is included as part of several different variations of Linux, including the popular version from Red Hat. IBM says that Jikes adheres to both the Java Language Specification and the Java Virtual Machine Specification, and does not support subsets, supersets, or other variations of the language. In addition to fast compilation, Jikes analyzes source code that is being compiled for dependencies on other source code files that may be needed. Jikes is available for most major operating system platforms. IBM points out that Jikes is not a Java development environment, but just a Java compiler.
JNDI
Jini
jitter
Jini (pronounced DJEE-nee like the Arabic word for ‘‘magician’’) is a new idea that Sun Microsystems calls ‘‘spontaneous networking.’’ Using the Jini architecture, users will be able to plug printers, storage devices, speakers, and any kind of device directly into a network and every other computer, device, and user on the network will know that the new device has been added and is available. Each pluggable device will define itself immediately to a network device registry. When someone wants to use or access the resource, their computer will be able to download the necessary programming from it to communicate with it. No longer will the special device support software known as a device driver need to be present in an operating system. The operating system will know about all accessible devices through the network registry.
Jitter is the deviation in or displacement of some aspect of the pulses in a high-frequency digital signal. As the name suggests, jitter can be thought of as shaky pulses. The deviation can be in terms of amplitude, phase timing, or the width of the signal pulse. Another definition is that it is ‘‘the period frequency displacement of the signal from its ideal location.’’ Among the causes of jitter are electromagnetic interference (EMI) and crosstalk with other signals. Jitter can cause a display monitor to flicker; affect the ability of the processor in a personal computer to perform as intended; introduce clicks or other undesired effects in audio signals, and loss of transmitted data between network devices. The amount of allowable jitter depends greatly on the application.
Jini can be viewed as the next step after the Java programming language toward making a network look like one large computer. Jini promises to enable manufacturers to make devices that can attach to a network independently of an operating system like Windows 95. Equipped with its own small, special-purpose and possibly microchipembedded operating system, a printer could be plugged into a network and immediately shared by users at a mix of computers: Windows, Macintosh, UNIX. Mobile devices could be transported and easily plugged into a network so that others could use the device.
How It Works Jini consists of four program layers: L
Directory Service
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JavaSpace
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Remote Method Invocation (RMI)
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Boot, Join, and Discover Protocol
Any device with an operating system that supports a Java can be plugged into the network. (For many devices, the operating system can be much smaller than Windows 2000, for example, since it is serving only the functions that device requires.) When a device is plugged into the Jini network, it is immediately registered by the Directory Service layer as a member of the network. Its necessary program objects are placed in a JavaSpace layer so that other network members can discover and download them when that network member wants to use the device. The actual communication with objects in JavaSpace is done using the Remote Method Invocation (RMI) interface and layer. The layer supporting the boot, join, and discover protocols enables devices, users, and applications to announce and register themselves and to discover others.
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See ‘‘Java Message Service’’
H JNDI JNDI (Java Naming and Directory Interface) enables Java platform-based applications to access multiple naming and directory services. Part of the Java Enterprise application programming interface (API) set, JNDI makes it possible for developers to create portable applications that are enabled for a number of different naming and directory services, including: file systems; directory services such as Lightweight Directory Access Protocol (LDAP), Novell Directory Services, and Network Information System (NIS); and distributed object systems such as the Common Object Request Broker Architecture (CORBA), Java Remote Method Invocation (RMI), and Enterprise JavaBeans (EJB). As an illustration of what JNDI does, Todd Sundsted (in a JavaWorld article, JNDI overview, Part 1: An introduction to naming services) uses the analogy of a library’s file system. Sundsted says that JNDI organizes and locates components within a distributed computing environment similarly to the way that card catalogs (and increasingly computer applications) organize and represent the locations of books within a library. A distributed application needs a means of locating components in the same way that the library patron needs a means of locating the book: Just rummaging around inside a library—or an application—is not an efficient way to find a particular object. JNDI makes it possible for application components to find each other. Because different naming and directory service providers can be seamlessly connected through the API, Java applications using it can be easily integrated into various environments and coexist with legacy applications. The current version, JNDI 1.2, was specified with input from Netscape, Novell, Tarantella, Sun, and BEA. JNDI is considered an industry standard.
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job
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In certain computer operating systems, a job is the unit of work that a computer operator (or a program called a job scheduler) gives to the operating system. For example, a job could be the running of an application program such as a weekly payroll program. A job is usually said to be run in batch (rather than interactive) mode. The operator or job scheduler gives the operating system a ‘‘batch’’ of jobs to do (payroll, cost analysis, employee file updating, and so forth) and these are performed in the background when timesensitive interactive work is not being done. In IBM mainframe operating systems (MVS, OS/390, and successors) a job is described with job control language (JCL). Jobs are broken down into job steps. An example of a job step might be to make sure that a particular data set or database needed in the job is made accessible.
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Typically, the development programmer who writes a program that is intended to be run as a batch job also writes the JCL that describes for the operating system how to run the job (for example, what data sets or databases it uses). The use of a job scheduler usually provides greater flexibility and the ability to monitor and report batch job operations.
K
A similar term is task, a concept usually applied to interactive work. A multitasking operating system serving one or more interactive users can at the same time perform batch jobs in the background.
L
job control language
J
See ‘‘JCL’’
M Job Entry Subsystem
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Job Entry Subsystem (JES) is a subsystem of the OS/390 and MVS mainframe operating systems that manages jobs (units of work) that the system does. Each job is described to the operating system by system administrators or other users in job control language (JCL). The operating system then sends the job to the JES program. The JES program receives the job, performs the job based on priority, and then purges the job from the system. There are two versions, JES2 and JES3. JES3 allows central control of the processing of jobs using a common work queue. Both OS/390 and MVS provide an interactive menu for initiating and managing jobs.
job scheduler A job scheduler is a program that enables an enterprise to schedule and, in some cases, monitor computer ‘‘batch’’ jobs (units of work, such as the running of a payroll program). A job scheduler can initiate and manage jobs automatically by processing prepared job control language statements or through equivalent interaction with a human operator.
Today’s job schedulers typically provide a graphical user interface and a single point of control for all the work in a distributed network of computers. Some features that may be found in a job scheduler include: L
Continously automatic monitoring of jobs and completion notification
L
Event-driven job scheduling
L
Performance monitoring
L
Report scheduling
job step In certain computer operating systems, a job step is part of a job, a unit of work that a computer operator (or a program called a job scheduler) gives to the operating system. For example, a job could be the running of an application program such as a weekly payroll program. A job is usually said to be run in batch (rather than interactive) mode. The operator or job scheduler gives the operating system a ‘‘batch’’ of jobs to do (payroll, cost analysis, employee file updating, and so forth) and these are performed in the background when time-sensitive interactive work is not being done. In IBM mainframe operating systems (MVS, OS/390, and successors) a job is described with job control language (JCL). Jobs are broken down into job steps. An example of a job step might be to make sure that a particular data set or database needed in the job is made accessible.
John von Neumann John von Neumann was the scientist who conceived a fundamental idea that serves all modern computers—that a computer’s program and the data that it processes do not have to be fed into the computer while it is working, but can be kept in the computer’s memory—a notion generally referred to as the stored-program computer. In his short life, von Neumann became one of the most acclaimed and lauded scientists of the 20th century. He left an indelible mark on the fields of mathematics, quantum theory, game theory, nuclear physics, and computer science. Born in Budapest, von Neumann was a child prodigy who went on to study chemistry in Berlin and Zurich, where he earned a Diploma in Chemical Engineering in 1926. His doctorate in mathematics (on set theory) from the University of Budapest followed in the same year. After lecturing at Berlin and Hamburg, von Neumann emigrated to the U.S. in 1930 where he worked at Princeton and was one of the founding members of the Institute for Advanced Studies. At Princeton, von Neumann lectured in the nascent field of quantum theory and through his work on rings of operators (later renamed Neumann algebras) he helped develop the mathematical foundations of that theory which were unveiled in the paper ’’Mathematische Grundlagen der
JPEG Quantenmechanik’’ (1932). His seminal publication on game theory, ’’Theory of Games and Economic Behaviour’’ was published in 1934 with co-author Oskar Morgenstern. Spurred by an interest in hydrodynamics and the difficulty of solving the non-linear partial differential equations involved, von Neumann turned to the emerging field of computing. His first introduction to computers was Howard Aiken’s Harvard Mark I. As a consultant to Eckert and Mauchly on the ENIAC, he devised a concept for computer architecture that remains with us to this day. Known subsequently as the ‘‘von Neumann architecture,’’ the stored-program computer (where both the instructions and the data they operate upon reside together in memory) with its central controller, I/O, and memory was outlined in a ‘‘Draft Report’’ and paved the way for the modern era of computing. von Neumann was a pioneer in the field of cellular automata (an n-dimensional array of cells where the contents of a cell depend of the contents of neighboring cells) and also popularized the binary digit as the unit of computer memory. von Neumann was constantly busy with both his extensive consulting career and his varied research interests. Among his employers was the U.S. military, for whom he worked on the development of the hydrogen bomb. He received the Enrico Fermi award in 1956, the latest in a long line of honors (including 7 honorary doctorates and 2 Presidential Awards). John von Neumann died on February 8, 1957 in Washington D.C.
Joint Photographic Experts Group See ‘‘JPEG’’
Jolt Jolt, an American soft drink that contains the equivalent of two cups of coffee, has a following with programmers, college students, Internet surfers, and anyone else who needs to keep awake in the late night or early morning hours. One of our contributors describes it as ‘‘the fuel on which the Internet is run.’’ A typical can or bottle of Jolt contains about twice the amount of caffeine as Pepsi Cola or Coca Cola. Yahoo lists a number of Web sites created by Jolt devotees.
joule The joule (pronounced DJOOL) is the standard unit of energy in electronics and general scientific applications. One joule is defined as the amount of energy exerted when a force of one newton is applied over a displacement of one meter. One joule is the equivalent of one watt of power radiated or dissipated for one second. In some applications, the British thermal unit (Btu) is used to express energy. One Btu is equivalent to approximately 1055 joules.
journaling file system A journaling file system is a fault-resilient file system in which data integrity is ensured because updates to directories and bitmaps are constantly written to a serial log on disk before the original disk log is updated. In the event of a system failure, a full journaling filesystem ensures that the data on the disk has been restored to its pre-crash configuration. It also recovers unsaved data and stores it in the location where it would have gone if the computer had not crashed, making it an important feature for missioncritical applications. Not all operating systems provide the same journaling technology. Windows NT offers a less robust version of the full system. If your Windows NT system crashes, you may not lose the entire disk volume, but you will likely lose all the data that hadn’t yet been written to the disk prior to the crash. By the same token, the default Linux system, ext2fs, does not journal at all. That means, a system crash—although infrequent in a Linux environment—can corrupt an entire disk volume. However, XFS, a journaling file system from Silicon Graphics, became a part of the open-source community in 1999 and, therefore, has had important implications for Linux developers, who previously lacked such insurance features. Capable of recovering from most unexpected interruptions in less than a second, XFS epitomizes the highperformance journaling filesystem of the future. The earliest journaling file systems, created in the mid-1980s, included Veritas, Tolerant, and IBM’s JFS. With increasing demands being placed on file systems to support terabytes of data, thousands upon thousands of files per directory and 64-bit capability, it is expected that interest will continue to grow in high-performance journaling file systems like XFS.
JPEG A JPEG (pronounced JAY-peg) is a graphic image file created by choosing from a range of compression qualities (actually, from one of a suite of compression algorithms). When you create a JPEG or convert an image from another format to a JPEG, you are asked to specify the quality of image you want. Since the highest quality results in the largest file, you can make a trade-off between image quality and file size. Formally, the JPEG file format is ISO standard 10918. The JPEG scheme includes 29 distinct coding processes although a JPEG implementor may not use them all. JPEG is an acronym for Joint Photographic Experts Group, the committee that established the baseline algorithms. Together with the Graphic Interchange Format (GIF) and Portable Network Graphics (PNG) file formats, the JPEG is one of the image file formats supported on the World Wide Web, usually with the file suffix of ‘‘.jpg’’. You can create a progressive JPEG that is similar to an interlaced GIF.
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JPG
A
See ‘‘JPEG’’
B
JScript
D
JScript is a script language from Microsoft that is expressly designed for use within Web pages. It adheres to the ECMAScript standard and is basically Microsoft’s equivalent to Netscape’s earlier and more widely used JavaScript.
E
JSP
C
See ‘‘Java Server Page’’
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Julian calendar The Julian calendar was the 365-day calendar that Julius Caesar made official in 46 B.C. It replaced a calendar based on lunar cycles. The Julian calendar provided for a leap year with an extra day every four years. Thus, the Julian calendar included an average of 365.25 days each year. By 1582 A.D., however, the Julian calendar had become out of step with the seasonal cycle by 10 days. The adjustment, ordered by Pope Gregory XIII in October, 1582, subtracted ten days from the calendar. The new Gregorian calendar removes a leap year every one hundred years except for years divisible by four hundred. The Julian calendar has no relation to the Julian date format sometimes used in computing.
Julian date 1) Not to be confused with the Julian calendar, a Julian date or day number is the number of elapsed days since the beginning of a cycle of 7,980 years invented by Joseph Scaliger in 1583. The purpose of the system is to make it easy to compute an integer (whole number) difference between one calendar date and another calendar date. The 7,980 year cycle was derived by combining several traditional time cycles (solar, lunar, and a particular Roman tax cycle) for which 7,980 was a common multiple. The starting point for the first Julian cycle began on January 1, 4713 B.C. (Gregorian calendar—expressed in the ISO date format as ‘‘-4713-01-01 G’’) and will end on January 22, 3268 (3268-0122 G). The following day will begin the first day of the second Julian date period (or 7,980 year cycle). A Julian date or day number for a certain time of day on January 9, 2001, looked like this: 2451919.3423000001348555 meaning 2,451,919 elapsed days since the beginning of the Julian cycle. The ‘‘.3423000001348555’’ represented the time of day (’’15:12:54 EST’’).
It is not certain whether the Julian date or day number system was named after Joseph Scaliger’s father, Julius Caesar Scaliger, or after the Julian calendar. Julian day numbers are widely used in astronomy. 2) Commonly in computer programming, Julian date has been corrupted to mean the number of elapsed days since the beginning of a particular year. For example, in this usage, the Julian date for the calendar date of 1998-02-28 would be day 59.
jumbogram Using the Internet Protocol version 6 (IPv6), a jumbogram is a transmission packet that contains a payload larger than 65,535 eight-bit bytes (also known as octets).
jump page 1) In Web site design, jump page is a term that is sometimes used to describe a Web site page that provides a ‘‘jumping off’’ point for users who need to locate information on the Web quickly and efficiently. Thus, a portal site or the home page for a site that specializes in gathering resources for a specific area of interest might be referred to as jump pages. whatis.com and other TechTarget.com sites can be considered jump pages for IT-specific information on the Web. 2) In Web site maintenance, a jump page can be a page that is posted temporarily to redirect the user when the page’s address (its URL) has been changed. 3) In Internet advertising and marketing, a jump page (also called a landing page) is a Web page that is posted temporarily to capture the user’s attention as a promotion or lead-in to a Web site. A jump page may be a splash page that contains a Flash animation. According to industry expert Robbin Zeff, a jump page on the Web is temporary, it is directly related to products or services that are currently being promoted, and it is designed to reinforce other ad promotions the user may have been exposed to.
jumper setting 1) In a computer, a jumper is a pair of prongs that are electrical contact points set into the computer motherboard or an adapter card. When you set a jumper, you place a plug on the prongs that completes a contact. Jumper settings tell the computer how it is configured and what operations can be performed. Computers come with jumpers preset. Instructions are sometimes provided so that the owner can reset the jumpers when new equipment is added. The latest trend, however, is plug and play equipment that does not require manual setting of jumpers. A group of jumpers is sometimes called a jumper block. Finding the right jumper settings for a particular computer configuration usually requires having some technical information that comes with each product for which a
just-in-time compiler jumper setting has to be set or changed. Where this information isn’t available, you may be able to find information at the Web site of the manufacturer.
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2) In electronic test equipment, a jumper cable is used to make a temporary contact between two points for the purpose of testing a circuit.
B
junk
C
See ‘‘spam’’
D just a bunch of disks See ‘‘JBOD’’
E
just-in-time compiler
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In the Java programming language and environment, a justin-time (JIT) compiler is a program that turns Java bytecode (a program that contains instructions that must be interpreted) into instructions that can be sent directly to the processor. After you’ve written a Java program, the source language statements are compiled by the Java compiler into bytecode rather than into code that contains instructions that match a particular hardware platform’s processor (for example, an Intel Pentium microprocessor or an IBM System/390 processor). The bytecode is platformindependent code that can be sent to any platform and run on that platform. In the past, most programs written in any language have had to be recompiled, and sometimes, rewritten for each computer platform. One of the biggest advantages of Java is that you only have to write and compile a program once. The Java on any platform will interpret the compiled bytecode into instructions understandable by the particular processor. However, the virtual machine handles one bytecode instruction at a time. Using the Java just-in-time compiler (really a second compiler) at the particular system platform compiles the bytecode into the particular system code (as though the program had been compiled initially on that platform). Once the code has been (re-)compiled by the JIT compiler, it will usually run more quickly in the computer. The just-in-time compiler comes with the virtual machine and is used optionally. It compiles the bytecode into platform-specific executable code that is immediately executed. Sun Microsystems suggests that it’s usually faster to select the JIT compiler option, especially if the method executable is repeatedly reused.
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K Desktop Environment K Desktop Environment (KDE) is an Open Source graphical desktop environment for UNIX workstations. Initially called the Kool Desktop Environment, KDE is an ongoing project with development taking place on the Internet and discussions held through the official KDE mailing list, numerous newsgroups, and Internet Relay Chat (IRC) channels. KDE has a complete graphical user interface (GUI) and includes a file manager, a window manager, a help system, a configuration system, tools and utilities, and several applications. The most popular suite of KDE applications is KOffice, which includes a word processor, a spreadsheet application, a presentation application, a vector drawing application, and image editing tools. KOffice was released with KDE version 2.0 in October 2000. On December 5, 2000, KDE 2.0.1 was released.
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*Note: In international English outside the U.S., the equivalent usage is ‘‘kbps’’ or ‘‘kbits s-1.’’ In the U.S., Kbps stands for kilobits per second (thousands of bits per second) and is a measure of bandwidth (the amount of data that can flow in a given time) on a data transmission medium. Higher bandwidths are more conveniently expressed in megabits per second (Mbps, or millions of bits per second) and in gigabits per second (Gbps, or billions of bits per second).
See ‘‘kilobyte’’
K-12, a term used in education and educational technology in the United States, Canada, and possibly other countries, is a short form for the publicly-supported school grades prior to college. These grades are kindergarten (K) and the 1st through the 12th grade (1-12). (If the term were used, ‘‘13th grade’’ would be the first year of college.)
KDE
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Kbps
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T
See ‘‘Pentium 3’’
k-business
P
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Matthias Ettrich launched the KDE project in October 1996 with the goal of making the UNIX platform more attractive and easy to use for computer users who are familiar with a graphical interface instead of typed commands. Today, KDE is used with Linux, Solaris, FreeBSD, OpenBSD, and LinuxPPC. Several hundred software programmers from all over the world contribute to the development of KDE.
Most communities in the United States and Canada (and wherever else the term is used) are just beginning to provide modern information technology at the K-12 levels.
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0.25 micron infrastructure that is used in the popular K6 microprocessor or, as for the 1 GHz model, with a 0.18 micron infrastructure.
See ‘‘knowledge management’’
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See ‘‘K Desktop Environment’’
Keep It Simple, Stupid See ‘‘KISS Principle’’
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In corporate culture, keiretsu refers to a uniquely Japanese form of corporate organization. A keiretsu is a grouping or family of affiliated companies that form a tight-knit alliance to work toward each other’s mutual success. The keiretsu system is also based on an intimate partnership between government and businesses. It can best be understood as the intricate web of relationships that links banks, manufacturers, suppliers, and distributors with the Japanese government.
K7 was the development name for the Athlon personal computer microprocessor from AMD, the microprocessor and flash memory maker. The Athlon became the first microprocessor for mainstream PCs to support a 200 MHz system bus and to achieve a 1 gigahertz clock speed. As Athlon, different versions of the K7 are built with either the
These ironclad corporate alliances have caused much debate and have been called ‘‘government-sponsored cartels.’’ While some think keiretsu are a menace to trade, others see them as a model for change. Features common to most keiretsu include ‘‘main bank,’’ stable shareholding, and seconded directors. Some keiretsu concepts have no
K6 (referred to by its manufacturer as AMD-K6) is a line of microprocessors from AMD that compete with Intel’s Pentium series of microprocessors. The successor to the K6 is the Athlon.
Kermit American parallel such as ‘‘general trading company.’’ The keiretsu system is one of the profound differences between Japanese and U.S. business structures. Keiretsu operate globally and are integrated both vertically and horizontally. They are organized around their own trading companies and banks. Each major keiretsu is capable of controlling nearly every step of the economic chain in a variety of industrial, resource and service sectors. There are horizontal and vertical keiretsu. Horizontal keiretsu are headed by major Japanese banks and include the ‘‘Big Six’’—Mitsui, Mitsubishi, Sumitomo, Fuyo, Sanwa, and Dai-Ichi Kangyo Bank Groups. Vertical keiretsu are industrial groups connecting manufacturers and part suppliers or manufacturers, wholesalers and retailers. These verticle keiretsu include car and electronics producers (Toyota, Nissan, Honda–Matsushita, Hitachi, Toshiba, Sony) and their ‘‘captive’’ subcontractors. Distribution keiretsu, a subgroup of vertical keiretsu, control much of Japanese retailing, determining what products will appear in stores and showrooms—and at what price.
kelvin The kelvin (abbreviation K), less commonly called the degree Kelvin (symbol, oK), is the International System (SI) unit of thermodynamic temperature. One kelvin is formally defined as 1/273.16 (3.6609 6 10-3) of the thermodynamic temperature of the triple point of pure water (H2O). The kelvin scale differs from the more familiar Celsius or centigrade (oC) temperature scale; there is no such thing as a below-zero Kelvin figure. A temperature of 0 K represents absolute zero, the absence of all heat. However, the size of the kelvin ‘‘degree’’ is the same as the size of the Celsius ‘‘degree.’’ A change of plus-or-minus 1 oC is the same as a change of plus-or-minus 1 K. At standard Earth-atmospheric sea-level pressure, water freezes at 0 oC or +273.15 K, and boils at +100 oC or +373.15 K. A temperature of 0 K thus corresponds to 7273.15 oC. To convert a kelvin temperature figure to Celsius, subtract 273.15. To convert a Celsius temperature figure to kelvin, add 273.15.
Kerberos Kerberos is a secure method for authenticating a request for a service in a computer network. Kerberos was developed in the Athena Project at the Massachusetts Institute of Technology (MIT). The name is taken from Greek mythology; Kerberos was a three-headed dog who guarded the gates of Hades. Kerberos lets a user request an encrypted ‘‘ticket’’ from an authentication process that can then be used to request a particular service from a server. The user’s password does not have to pass through the network. A version of Kerberos (client and server) can be downloaded from MIT or you can buy a commercial version.
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Briefly and approximately, here’s how Kerberos works:
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1. Suppose you want to access a server on another computer (which you may get to by sending a Telnet or similar login request). You know that this server requires a Kerberos ‘‘ticket’’ before it will honor your request.
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2. To get your ticket, you first request authentication from the Authentication Server (AS). The Authentication Server creates a ‘‘session key’’ (which is also an encryption key) basing it on your password (which it can get from your user name) and a random value that represents the requested service. The session key is effectively a ‘‘ticket-granting ticket.’’ 3. You next send your ticket-granting ticket to a ticketgranting server (TGS). The TGS may be physically the same server as the Authentication Server, but it’s now performing a different service. The TGS returns the ticket that can be sent to the server for the requested service. 4. The service either rejects the ticket or accepts it and performs the service. 5. Because the ticket you received from the TGS is timestamped, it allows you to make additional requests using the same ticket within a certain time period (typically, eight hours) without having to be reauthenticated. Making the ticket valid for a limited time period make it less likely that someone else will be able to use it later. The actual process is much more complicated than just described. The user procedure may vary somewhat according to implementation.
Kermit Kermit is a popular file transfer and management protocol and suite of communications software programs with advantages over existing Internet protocols such as File Transfer Protocol and Telnet. It is freeware, developed and maintained by members of the Kermit Project at Columbia University. (However, you’re invited to purchase shrinkwrapped versions and/or the manuals to help support the project.) The Kermit protocol is described as ‘‘fast, robust, extensible, tunable, and medium-independent.’’ In addition to the protocol support, the Kermit suite includes terminal emulation, character-set translation, and scripting. The suite can be installed on almost any operating system, including Windows, UNIX, DOS, VMS, OS/2, and a number of mainframe operating systems. Most versions support both direct or dialed serial connections (with a modem) and network connections (Telnet and often others such as Rlogin, LAT, or X.25).
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kernel The kernel is the essential center of a computer operating system, the core that provides basic services for all other parts of the operating system. A synonym is nucleus. A kernel can be contrasted with a shell, the outermost part of an operating system that interacts with user commands. Kernel and shell are terms used more frequently in UNIX and some other operating systems than in IBM mainframe systems. Typically, a kernel (or any comparable center of an operating system) includes an interrupt handler that handles all requests or completed I/O operations that compete for the kernel’s services, a scheduler that determines which programs share the kernel’s processing time in what order, and a supervisor that actually gives use of the computer to each process when it is scheduled. A kernel may also include a manager of the operating system’s address spaces in memory or storage, sharing these among all components and other users of the kernel’s services. A kernel’s services are requested by other parts of the operating system or by applications through a specified set of program interfaces sometimes known as system calls.
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Some kernels have been developed independently for use in any operating system that wants to use it. A well-known example is the Mach kernel, developed at Carnegie-Mellon University, and currently used in a version of the Linux operating system for Apple’s PowerMac computers.
key In cryptography, a key is a variable value that is applied using an algorithm to a string or block of unencrypted text to produce encrypted text, or to decrypt encrypted text. The length of the key is a factor in considering how difficult it will be to decrypt the text in a given message.
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as the Escape key, tab and cursor movement keys, shift and control keys, and sometimes other manufacturer-customized keys. The computer keyboard uses the same key arrangement as the mechanical and electronic typewriter keyboards that preceded the computer. The standard arrangement of alphabetic keys is known as the Qwerty (pronounced KWEHR-tee) keyboard, its name deriving from the arrangement of the five keys at the upper left of the three rows of alphabetic keys. This arrangement, invented for one of the earliest mechanical typewriters, dates back to the 1870s. Another well-known key arrangement is the Dvorak (pronounced duh-VOR-ak, not like the Czech composer) system, which was designed to be easier to learn and use. The Dvorak keyboard was designed with the most common consonants on one side of the middle or home row and the vowels on the other side so that typing tends to alternate key strokes back and forth between hands. Although the Dvorak keyboard has never been widely used, it has adherents. Because many keyboard users develop a cumulative trauma disorder, such as carpal tunnel syndrome, a number of ergonomic keyboards have been developed. Approaches include keyboards contoured to alleviate stress and footdriven pedals for certain keys or keyboard functions.
kHz The kilohertz, abbreviated kHz or KHz*, is a unit of alternating current (AC) or electromagnetic (EM) wave frequency equal to one thousand hertz (1,000 Hz). The unit is also used in measurements or statements of signal bandwidth. An AC signal having a frequency of 1 kHz is within the range of human hearing. If a signal at this frequency is input to a headset or loudspeaker, the resulting tone has a pitch that falls into the so-called ‘‘audio midrange.’’ An EM signal at a frequency of 1 kHz has a wavelength of 300 kilometers, or about 190 miles. The standard amplitude-modulation (AM) broadcast band extends from 535 kHz to 1,605 kHz. Some EM transmissions are made at millions of kHz. The kilohertz is a relatively small unit of frequency; more common units are the MHz, equal to 1,000,000 Hz or 1,000 kHz, and the GHz, equal to 1,000,000,000 Hz or 1,000,000 kHz. The kilohertz is often used to specify bandwidth for digital as well as analog signals. The bandwidth of a digital signal, in kilohertz, is related to the data speed in bits per second. In general, the greater the data speed, the larger the bandwidth. Data speed is not, however, the same thing as bandwidth. A modem operating at a speed of 28,800 bps has, in a certain sense, a nominal frequency of 28.8 kHz. But the bandwidth is generally much smaller, because it depends on variations in the individual data characters, not on the number of data bits per unit time.
kilogram *The engineer’s society, IEEE, and most other sources prefer ‘‘kHz’’ to ‘‘KHz.’’ This apparently makes it less likely that users will confuse ‘‘kilo’’ (decimal 1,000) with the computer ‘‘K’’ (1,024).
killer app A ‘‘killer app’’ is jargon in the computer industry for an application program that intentionally or unintentionally gets you to make the decision to buy the system the application runs on. A classic example of a killer app was the spreadsheet program, the first of which was called VisiCalc, followed later by Lotus 1-2-3. The spreadsheet application helped introduce the personal computer into the department level of large and small businesses. A killer app can refer to a generic type of application that hasn’t existed before, to a particular product that first introduces a new application type, or to any application with wide appeal. When a new kind of computer hardware product comes out, such as a hand-held computer, manufacturers offer or hope to entice others to develop what they believe will be the killer app that will motivate potential customers to buy the new computer. In recent ads, IBM says that the killer app for e-business (which is both a concept and an array of Internet products and services that IBM sells) is ‘‘an application deployed over the Web that makes it easier to do the things you already do.’’ Clearly, the Web browser and the Internet servers it communicates with became the killer app of the 1990s.
kilobit In data communications, a kilobit is a thousand (103) bits. It’s commonly used for measuring the amount of data that is transferred in a second between two telecommunication points. Kilobits per second is usually shortened to Kbps.* Some sources define a kilobit to mean 1,024 (that is, 210) bits. Although the bit is a unit of the binary number system, bits in data communications are discrete signal pulses and have historically been counted using the decimal number system. For example, 28.8 kilobits per second (Kbps) is 28,800 bits per second. Because of computer architecture and memory address boundaries, bytes are always some multiple or exponent of two. See kilobyte, etc. *Note: In international English outside the U.S., the equivalent usage is ‘‘kbps’’ or ‘‘kbits s-1.’’
kilobits per second See ‘‘Kbps’’
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As a measure of computer memory or storage, a kilobyte (KB or Kbyte*) is approximately a thousand bytes (actually, 2 to the 10th power, or decimal 1,024 bytes).
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kilogram per meter cubed The kilogram per meter cubed (symbolized kg/m3) is the standard unit of material density in the International System of Units (SI). Material density is expressed directly in base SI units, and cannot be further reduced.
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A substance with a density of 1 kg/m is extremely light for its size. Even styrofoam or bubble-wrap has a density greater than this. A cubic meter of water has a mass of about 1000 kg, more than a conventional English ton in the gravitational field of the earth. Alternative units are often used to express density. The most common of these is the gram per centimeter cubed (g/cm3). A substance with a density of 1 g/cm3 has a density of 1000 kg/m3. Conversely, a substance with a density of 1 kg/m3 has a density of 0.001 g/cm3. Also see kilogram, meter, specific gravity, and SI (International System of Units).
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kilogram The kilogram (abbreviation, kg) is the International System of Units (SI) unit of mass. It is defined as the mass of a particular international prototype made of platinum-iridium and kept at the International Bureau of Weights and Measures. It was originally defined as the mass of one liter (10-3 cubic meter) of pure water. At the Earth’s surface, a mass of 1 kg weighs approximately 2.20 pounds (lb). Conversely, an object that weighs 1 lb at the Earth’s surface has a mass of approximately 0.454 kg. There is a qualitative as well as a quantitative difference between kilograms and pounds. Kilograms denote mass, but pounds denote weight, the force a mass exerts against a barrier in the presence of an acceleration field acting perpendicular to the barrier. On the surface of Mars, where the gravitational acceleration is 37 percent of that on the surface of the Earth, a mass of 1 kg weighs 0.814 lb. In orbit, or in a vehicle coasting through space, the net acceleration is zero and a mass of 1 kg has no weight. In other words, it is weightless. Mass is often specified in smaller units than the kilogram, by changing the power-of-10 prefix multiplier. A mass of one gram (1 g) is 10-3 kg. A mass of one milligram (1 mg) is 10-6 kg, or 10-3 g. A mass of one microgram (1 mg) is 10-9 kg, or 10-6 g. A mass of one nanogram (1 ng) is 10-12 kg, or 10-9 g.
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kiosk
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In information technology, a kiosk (pronounced KEE-ahsk) is a small physical structure (often including a computer and a display screen) that displays information for people walking by. Kiosks are common near the entrances of shopping malls in North America where they provide shoppers with directions. Kiosks are also used at trade shows and professional conferences. The word is of Turkish and earlier Persian origin, where it meant an outdoor pavilion or a portico. The kiosks best known to travellers are those that display show and movie posters on the streets of Paris.
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More sophisticated kiosks let users interact and include touch screens, sound, and motion video. A number of companies specialize in creating multimedia kiosks. A simple kiosk can be created using HTML pages and graphics, setting the typesize large enough to attract people from a short distance, and removing the Web browser’s tool bar so that the display screen is effectively in ‘‘kiosk mode.’’ The presentation can be designed to simply loop through a series of pages or to allow user interaction and exploration. Having a separate printed sign that invites people to your home-made kiosk may help.
Kirchhoff’s Laws for current and voltage
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Also see Ohm’s Law. Kirchhoff’s Laws for current and voltage are two principles that apply to DC circuits and networks. The total current flowing into any DC circuit node, also called a branch point, is always the same as the total current flowing out of the node. An example is shown in the top illustration. There are four current-carrying conductors (a, b, c, and d) leading into the node (black dot), and two conductors (e and f) leading out. Direct currents in parallel add together arithmetically. Therefore, the total current flowing into the node is a + b + c + d, and the total current flowing out is e + f. These total currents, according to Kirchhoff’s First Law, must be equal. Kirchhoff’s Second Law deals with voltage. An example is shown in the bottom illustration. A source having voltage equal to a is connected in a circuit with five passive components having voltage differences b, c, d, e, and f across them. The voltages across the passive components add together arithmetically because they are connected in series. According to the Second Law, the total voltage across the set of passive components is always equal and opposite to the source voltage. Therefore, the sum of the voltage differences across all the circuit elements (including the source) is always zero.
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KISS Principle The KISS (Keep It Simple, Stupid) Principle is selfdescriptive and recognizes two things: 1. People (including product and service users) generally want things that are simple, meaning easy to learn and use. 2. A company that makes products or furnishes services may find simplicity an advantage for the company as well, since it tends to shorten time and reduce cost. (Where the company is trying to use the principle on behalf of users, however, design time may take longer and cost more, but the net effect will be beneficial since easy-to-learn-and-use products and services tend to be cheaper to produce and service in the long run.) The New Hacker’s Dictionary, edited by Eric Raymond, says the the KISS Principle is sometimes cited on a development project to fend off ‘‘creeping featurism.’’ The somewhat related idea of Ockham’s razor is about always looking for the simplest explanation.
KLOC KLOC (thousands of lines of code) is a traditional measure of how large a computer program is or how long or how many people it will take to write it. The code measured is usually source code. Since a higher-level source language (such as C++) compiles into more lines of machine code than a lower-level language (such as assembler language), a KLOC
knowledge base of C++ statements would generate a larger program (in number of bytes) than a KLOC of assembler language statements. KLOC has been used as a rough measure of programmer productivity, as in ‘‘How many lines of code can you write a day?’’ However, this measure does not consider the efficiency of the code. Many other factors obviously affect productivity. Defects per KLOC is a common measure used as a target or for evaluating code quality.
kludge In information technology, a kludge (pronounced KLOOdzh) is an awkward or clumsy (but at least temporarily effective) solution to a programming or hardware design or implementation problem. According to Eric Raymond, the term is indirectly derived from the German klug meaning clever. Raymond considers ‘‘kludge’’ an incorrect spelling of kluge, a term of the 1940s with the same general meaning and possibly inspired by the Kluge paper feeder, a ‘‘fiendishly complex assortment of cams, belts, and linkages...devilishly difficult to repair...but oh, so clever!’’ A kludge originates because another, more elegant or appropriate solution is not currently possible (perhaps because of time constraints). Hardware and software products are sometimes the result of adding a new and basically incompatible design to the original design rather than redesigning the product completely. What is a kludge can be a matter of opinion. Users often have a different opinion than the designers, who understand the problems that had to be overcome. To the extent that information technology products are combinations of elements originating from a variety of design philosophies and constraints, almost any product is bound to contain some element of kludginess.
knowbot A knowbot is a program that automatically searchs Internet sites and gathers information from them according to userspecified criteria. A knowbot is more frequently called an intelligent agent or simply an agent. A knowbot should not be confused with a search engine crawler or spider. A crawler or spider progam visits Web sites and gathers information according to some generalized criteria and this information is then indexed so that it can be used for searching by many individual users. A knowbot works with specific and easily changed criteria that conform to or anticipate the needs of the user or users. Its results are then organized for presentation but not necessarily for searching. An example would be a knowbot (sometimes also called a newsbot) that visited major news-oriented Web sites each morning and provided a digest of stories (or links to them) for a personalized news page.
knowledge In information technology, knowledge is, to an enterprise or an individual, the possession of information or the ability to quickly locate it. This is essentially what Samuel Johnson, compiler of the first comprehensive English dictionary, said when he wrote that: Knowledge is of two kinds: We know a subject ourselves, or we know where we can find information upon it. In the context of the business enterprise or the personal computer user, knowledge tends to connote possession of experienced ‘‘know-how’’ as well as possession of factual information or where to get it. Enterprises have recently begun to treat their accumulated knowledge as an asset and to develop knowledge management plans and applications. A new kind of application, called data mining, attempts to develop knowledge from a company’s accumulated business transactions and other data.
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In philosophy, the theory of knowledge is called epistemology and deals with such questions as how much knowledge comes from experience or from innate reasoning ability; whether knowledge needs to be believed or can simply be used; and how knowledge changes as new ideas about the same set of facts arise.
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In general, a knowledge base is a centralized repository for information: A public library, a database of related information about a particular subject, and whatis.com could all be considered to be examples of knowledge bases. In relation to information technology (IT), a knowledge base is a machine-readable resource for the dissemination of information, generally online or with the capacity to be put online. An integral component of knowledge management systems, a knowledge base is used to optimize information collection, organization, and retrieval for an organization, or for the general public. A well-organized knowledge base can save an enterprise money by decreasing the amount of employee time spent trying to find information about—among myriad possibilities—tax laws or company policies and procedures. As a customer relationship management (CRM) tool, a knowledge base can give customers easy access to information that would otherwise require contact with an organization’s staff; as a rule, this capacity should make the interaction simpler for both the customer and the organization. A number of software applications are available that allow users to create their own knowledge bases, either separately (these are usually called knowledge management software) or as part of another application, such as a CRM package.
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Whatis?Com’s Encyclopedia of Technology Terms In general, a knowledge base is not a static collection of information, but a dynamic resource that may itself have the capacity to learn, as part of an artificial intelligence (AI) expert system, for example. According to the World Wide Web Consortium (W3C), in the future the Internet may become a vast and complex global knowledge base. Tim Berners-Lee calls this the Semantic Web.
knowledge management Knowledge management is the name of a concept in which an enterprise consciously and comprehensively gathers, organizes, shares, and analyzes its knowledge in terms of resources, documents, and people skills. In early 1998, it was believed that few enterprises actually had a comprehensive knowledge management practice (by any name) in operation. Advances in technology and the way we access and share information have changed that; many enterprises now have some kind of knowledge management framework in place. Knowledge management involves data mining and some method of operation to push information to users. Some vendors are offering products to help an enterprise inventory and access knowledge resources. The Lotus Knowledge Discovery System, for example, advertises that it can locate and organize relevant content and expertise required to address specific business tasks and projects. It will analyze the relationships between content, people, topics, and activity, and produce a knowledge map report, based on a point system, that can be shared. In an Information Week article, Jeff Angus and Jeetu Patel describe a four-process view of knowledge management that we have put into a table: This major process...Includes these activities.... Gathering
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Data entry OCR and scanning Voice input Pulling information from various sources Searching for information to include Cataloging Indexing Filtering Linking Contextualizing Collaborating Compacting Projecting Mining Flow Sharing Alert Push
A knowledge management plan involves a survey of corporate goals and a close examination of the tools, both traditional and technical, that are required for addressing the needs of the company. The challenge is to select or build software that fits the context of the overall plan and encourage employees to share information.
knowledge worker A knowledge worker is anyone who works for a living at the tasks of developing or using knowledge. For example, a knowledge worker might be someone who works at any of the tasks of planning, acquiring, searching, analyzing, organizing, storing, programming, distributing, marketing, or otherwise contributing to the transformation and commerce of information and those (often the same people) who work at using the knowledge so produced. A term first used by Peter Drucker in his 1959 book, Landmarks of Tomorrow, the knowledge worker includes those in the information technology fields, such as programmers, systems analysts, technical writers, academic professionals, researchers, and so forth. The term is also frequently used to include people outside of information technology, such as lawyers, teachers, scientists of all kinds, and also students of all kinds.
Korn shell The Korn shell is the UNIX shell (command execution program, often called a command interpreter) that was developed by David Korn of Bell Labs as a comprehensive combined version of other major UNIX shells. Incorporating all the features of C shell (csh) and Tab C-shell (tcsh) with the script language features similar to that of the Bourne shell, the Korn shell is considered the most efficient shell. Korn, Bourne, and C are the three most commonly used UNIX shells. The Korn shell is considered a member of the Bourne shell family and uses as its shell prompt (character displayed to indicate readiness for user input) the $ symbol. Because it is the easiest shell to use, inexperienced users usually prefer the Korn shell and, not surprisingly, it is the one most often used in commercial environments. Sometimes known by its program name ksh, the Korn is the default shell on many UNIX systems.
KVM switch A KVM (keyboard, video, mouse) switch allows a single keyboard, video display monitor, and mouse to be switched to any of a number of computers when typically a single person interacts with all the computers but only one at a time. The switch provides more table space in addition to saving the cost of multiple keyboards and monitors. KVM switches are commonly used at Web and other server locations with multiple computers but usually a single administrator or Webmaster. The switches range in price
KVM switch from about $200 U.S. for a system in which up to eight computers can be daisy-chained to about $2,000 for a switch that controls up to 10 Sun workstations. Larger configurations can cost more.
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L1 and L2
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L1 and L2 are levels of cache memory in a computer. If the computer processor can find the data it needs for its next operation in cache memory, it will save time compared to having to get it from random access memory. L1 is ‘‘level-1’’ cache memory, usually built onto the microprocessor chip itself. For example, the Intel MMX microprocessor comes with 32 thousand bytes of L1.
Lambda switching (sometimes called photonic switching, or wavelength switching) is the technology used in optical networking to switch individual wavelengths of light onto separate paths for specific routing of information. In conjunction with technologies such as dense wavelength division multiplexing (DWDM)—which enables 80 or more separate light wavelengths to be transmitted on a single optical fiber—lambda switching enables a light path to behave like a virtual circuit.
L2 (that is, level-2) cache memory is on a separate chip (possibly on an expansion card) that can be accessed more quickly than the larger ‘‘main’’ memory. A popular L2 cache memory size is 1,024 kilobytes (one megabyte).
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Lambda, the 11th letter of the Greek alphabet, is the symbol for wavelength. In optical fiber networking, the word lambda is used to refer to an individual optical wavelength.
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Lambda calculus, considered to be the mathematical basis for programming language, is a calculus developed by Alonzo Church and Stephen Kleene in the 1930s to express all computable functions. In an effort to formalize the concept of computability (also known as constructibility and effective calculability), Church and Kleene developed a powerful language with a simple syntax and few grammar restrictions. The language deals with the application of a function to its arguments (a function is a set of rules) and expresses any entity as either a variable, the application of one function to another, or as a ‘‘lambda abstraction’’ (a function in which the Greek letter lambda is defined as the abstraction operator). Lambda calculus, and the closely related theories of combinators and type systems, are important foundations in the study of mathematics, logic, and computer programming language.
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Although the ability to redirect specific wavelengths intelligently is, in itself, a technological breakthrough, lambda switching works in much the same way as traditional routing and switching. Lambda routers—which are also called wavelength routers, or optical cross-connects (OXC)—are positioned at network junction points. The lambda router takes in a single wavelength of light from a specific fiber optic strand and recombines it into another strand that is set on a different path. Lambda routers are being manufactured by a number of companies, including Ciena, Lucent, and Nortel. Multiprotocol Lambda Switching is a variation of multiprotocol label switching (MPLS, confusingly, the abbreviation for both variants) in which specific wavelengths serve in place of labels as unique identifiers. The specified wavelengths, like the labels, make it possible for routers and switches to perform necessary functions automatically, without having to extract instructions regarding those functions from IP addresses or other packet information. Lambda switching gets its name from lambda, the 11th letter of the Greek alphabet, which has been adopted as the symbol for wavelength. In networking, the word is used to refer to an individual optical wavelength.
lamer Lamer is a term used on interactive Web sites to describe an irritating or immature participant. Among users of Internet Relay Chat (IRC), a lamer is someone who irritates other users by typing in all caps or iN pART cAPS, by insulting and flaming other users, by typing abbreviations for almost every word, or by performing other annoying acts in chat rooms. Some IRC channels use bots to monitor any ‘‘lame’’ behavior and kick lamers out. Lamer is also used by crackers and warez d00dz. Warez d00dz (that is, ‘‘wares dudes’’) are people who strip software of its copyright protection and then distribute the pirated software on the Internet for downloading. Warez d00dz sometimes compete to distribute the pirated software before other warez d00dz groups. People who create warez sites sometimes call them ‘‘warez sitez’’ and use ‘‘z’’ in other
laptop computer pluralizations. A lamer according to the warez d00dz culture is an individual who tries to distribute software that is several years old or software infected with a virus. Crackers use the term in a non-derogatory way to refer to cracker aspirants who are in the larval or beginning stage of computer cracking.
LAN See ‘‘local area network’’
LAN server A local area network (LAN) server is a program (and by implication usually the computer it runs in) that serves resources (files, storage, application programs, printers, and other devices) to a number of attached workstations. In an enterprise, a LAN server has an administrator (or someone who fills that role) and typically LAN services and the setting up and administration of the local area network are managed on behalf of users but not directly by the users. A LAN server can also be housed in a computer that has other servers such as a proxy server.
LANDesk Client Manager LDCM (LANDesk Client Manager) is a software product from Intel that lets a system administrator for a local area network (LAN) see the configurations and monitor the status of personal computers on the LAN. LDCM is an implementation of the Desktop Management Interface (DMI) standard established by the Desktop Management Task Force, an industry group. To take advantage of DMI, product components must provide a Management Information Format (MIF) file. Using LDCM, a LAN administrator can be notified automatically when a workstation is nearing problem status. LDCM’s ‘‘PC health monitoring’’ can notify the administrator of the status of low memory, any recoverable parity error, any boot virus that may be present, the motherboard and central processing unit temperatures, and other status information. LDCM also allows an inventory to be kept of all software and hardware characteristics of all workstations. The first releases of LDCM have been pre-installed on PCs by original equipment manufacturers. LDCM requires a BIOS that supports System Management BIOS Specification Version 2.0.
landing page See ‘‘jump page’’
Langmuir-Blodgett film A Langmuir-Blodgett (LB) film is a set of monolayers, or layers of organic material one molecule thick, deposited on a solid substrate. An LB film can consist of a single layer or many, up to a depth of several visible-light wavelengths. The term Langmuir-Blodgett comes from the names of a research scientist and his assistant, Irving Langmuir and Katherine Blodgett, who discovered unique properties of thin films in the early 1900s. Langmuir’s original work involved the transfer of monolayers from liquid to solid substrates. Several years later, Blodgett expanded on Langmuir’s research to include the deposition of multi-layer films on solid substrates. By transferring monolayers of organic material from a liquid to a solid substrate, the structure of the film can be controlled at the molecular level. Such films exhibit various electrochemical and photochemical properties. This has led some researchers to pursue LB films as a possible structure for integrated circuits (ICs). Ultimately, it might be possible to construct an LB-film memory chip in which each data bit is represented by a single molecule. Complex switching networks might be fabricated onto multilayer LB-films chips.
laptop computer This definition is very similar to our definition for notebook computer. Aso see portable computer. A laptop computer, usually called a notebook computer by manufacturers, is a battery-powered personal computer generally smaller than a briefcase that can easily be transported and conveniently used in temporary spaces such as on airplanes, in libraries, temporary offices, and at meetings. A laptop typically weighs less than 5 pounds and is 3 inches or less in thickness. Among the best-known makers of laptop computers are IBM, NEC, Dell, Toshiba, and Hewlett-Packard. Laptop computers generally cost more than desktop computers with the same capabilities because they are more difficult to design and manufacture. A laptop can effectively be turned into a desktop computer with a docking station, a hardware frame that supplies connections for peripheral input/output devices such as a printer or larger monitor. The less capable port replicator allows you to connect a laptop to a number of peripherals through a single plug. Laptops usually come with displays that use thin-screen technology. The thin film transistor or active matrix screen is brighter and views better at different angles than the STN or dual-scan screen. Laptops use several different approaches for integrating a mouse into the keyboard, including the touch pad, the trackball, and the pointing stick. A serial port also allows a regular mouse to be attached. The PC Card is insertable hardware for adding a modem or NIC to a laptop. CD-ROM and digital versatile disk drives may be built-in or attachable.
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laser
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The acronym laser stands for ‘‘light amplification by stimulated emission of radiation.’’ Lasers work as a result of resonant effects. The output of a laser is a coherent electromagnetic field. In a coherent beam of electromagnetic energy, all the waves have the same frequency and phase.
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Totally silvered mirror Laser
output Partially silvered mirror
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A simple laser works via resonant effects in a cavity with mirrors at either end.
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The helium-neon laser is another popular type, favored by electronics hobbyists because of its moderate cost. As its name implies, it has a cavity filled with helium and neon gases. The output of the device is bright crimson. Other gases can be used instead of helium and neon, producing beams of different wavelengths. Argon produces a laser with blue visible output. A mixture of nitrogen, carbon dioxide, and helium produces IR output.
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Coherent electromagnetic waves have identical frequency, and are aligned in phase.
In a basic laser, a chamber called a cavity is designed to internally reflect infrared (IR), visible-light, or ultraviolet (UV) waves so they reinforce each other. The cavity can contain gases, liquids, or solids. The choice of cavity material determines the wavelength of the output. At each end of the cavity, there is a mirror. One mirror is totally reflective, allowing none of the energy to pass through; the other mirror is partially reflective, allowing approximately 5 percent of the energy to pass through. Energy is introduced into the cavity from an external source; this is called pumping. As a result of pumping, an electromagnetic field appears inside the laser cavity at the natural (resonant) frequency of the atoms of the material that fills the cavity. The waves reflect back and forth between the mirrors. The length of the cavity is such that the reflected and re-reflected wavefronts reinforce each other in phase at the natural frequency of the cavity substance. Electromagnetic waves at this resonant frequency emerge from the end of the cavity having the partially-reflective mirror. The output may appear as a continuous beam, or as a series of brief, intense pulses. The ruby laser, a simple and common type, has a rod-shaped cavity made of a mixture of solid aluminum oxide and chromium. The output is in pulses that last approximately 500 microseconds each. Pumping is done by means of a helical flash tube wrapped around the rod. The output is in the red visible range.
Lasers are one of the most significant inventions developed during the 20th century. They have found a tremendous variety of uses in electronics, computer hardware, medicine, and experimental science.
laser diode Also see diode. A laser diode, also known as an injection laser or diode laser, is a semiconductor device that produces coherent radiation (in which the waves are all at the same frequency and phase) in the visible or infrared (IR) spectrum when current passes through it. Laser diodes are used in optical fiber systems, compact disc (CD) players, laser printers, remote-control devices, and intrusion detection systems. Laser diodes differ from conventional lasers, such as the helium-neon (He-Ne), ruby, and gas types, in several ways. L
Small size and weight: A typical laser diode measures less than one millimeter across and weighs a fraction of a gram, making it ideal for use in portable electronic equipment.
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Low current, voltage, and power requirements: Most laser diodes require only a few milliwatts of power at 3 to 12 volts DC and several milliamperes. Therefore, they can operate using small battery power supplies.
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Low intensity: A laser diode cannot be used for spectacular purposes such as burning holes in metal, bringing down satellites, or blinding aircraft pilots. Nevertheless, its coherent output results in high efficiency and ease of modulation for communications and control applications.
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Wide-angle beam: A laser diode produces a ‘‘cone’’ rather than a ‘‘pencil’’ of visible light or IR, although this ‘‘cone’’ can be collimated using convex lenses.
laserdisc
laser printer A laser printer is a popular type of personal computer printer that uses a non-impact (keys don’t strike the paper), photocopier technology. When a document is sent to the printer, a laser beam ‘‘draws’’ the document on a seleniumcoated drum using electrical charges. After the drum is charged, it is rolled in toner, a dry powder type of ink. The toner adheres to the charged image on the drum. The toner is transferred onto a piece of paper and fused to the paper with heat and pressure. After the document is printed, the electrical charge is removed from the drum and the excess toner is collected. Most laser printers print only in monochrome. A color laser printer is up to 10 times more expensive than a monochrome laser printer. IBM introduced the first laser printer in 1975 for use with its mainframe computers. In 1984, Hewlett-Packard revolutionized laser-printing technology with its first LaserJet, a compact, fast, and reliable printer that personal computer users could afford. Since then, laser printers have decreased further in price and increased in quality. Hewlett Packard continues to be the leading manufacturer with competitors including Lexmark, Okidata, and Xerox. The laser printer is different from an inkjet printer in a number of ways. The toner or ink in a laser printer is dry. In an inkjet, it is wet. Over time, an inkjet printer is about ten times more expensive to operate than a laser printer because ink needs replenishing more frequently. The printed paper from an inkjet printer will smear if wet, but a laser-printed document will not. Both types of printer operate quietly and allow fonts to be added by using font cartridges or installing soft fonts. If your printing needs are minimal, an inkjet printer is sufficient. But if your printing volume is high, consider buying a laser printer. When buying a laser printer, these are some important features to consider: Print capacity and speed: Personal laser printers are sufficient for printing an average of 200 pages per week. These are low-end and cost $200 and up. They can print up to eight ppm (pages per minute). A workgroup printer is needed if an average of 1000 pages per week is needed. These print up to 24 ppm and cost $1000 to $6000 and more. Production printers are needed for printing 50,000 or more pages per week. These are quite expensive and are used by commercial publishers. They can print up to 700 ppm and cost $100,000 and up. They can print 24 hours a day, seven days a week. Resolution: The standard resolution in most laser printers today is 600 dots-per-inch (dpi). This resolution is sufficient for normal everyday printing including small desktop publishing jobs. A high-end production printer might have a resolution of 2400 dpi. Some laser printers still use a resolution of 300 dpi. This resolution can cause jagged lines to appear on the outer edge of an image. Hewlett Packard created RET (Resolution Enhancement Technology) to
correct this. RET inserts smaller dots at the edges of lines and to smooth the rough edges. RET does not improve the resolution, but the document looks better. If you purchase a printer with 300 dpi, make sure it has RET. Printer languages: Printer Control Language (PCL) is the standard printer language for Hewlett Packard and most other laser printers (which are mostly HP-compatible). PCL is used for printing letters, database printouts, spreadsheets, and simple graphics. Postscript printers are used with desktop publishing software and drawing packages. Postscript printers are the norm for Apple Macintosh printers. A laser printer that comes with Postscript installed is more expensive. A laser printer that uses PCL can be upgraded to Postscript by installing a software driver provided by the manufacturer of the laser printer. The printer might require more memory when upgraded to use Postscript. This is because a laser printer needs the entire image in memory before printing, and a Postscript printer requires more memory to process than a PCL printer does. The application being used must support Postscript in order for the laser printer to print Postscript documents. Paper handling: Paper handling is important when shopping for a laser printer. Most laser printers use lettersize, cut-sheet paper. High-end production printers use continuous sheet paper. Laser printers can print on transparencies, adhesive labels, and lightweight cards. A laser printer with duplex printing can print on one side of the paper, turn the paper over, and print on the other side. Most laser printers, however, use simple printing with manual duplex printing. Manual duplex printing is achieved by changing the print options in the printer’s properties or printing one side and taking that same paper and reinserting it into the printer to print on the other side. FPOT and warm-up time: A final consideration in purchasing a printer is FPOT (first paper out time) and warm-up time. When a laser printer receives data from the computer to print, it takes 5 to 30 seconds to prepare the printer to print a new job. This is in addition to the time it takes to actually print the document. The warm-up time is as important. When the printer is turned on, it needs time to warm up the fuser to operating temperature. If the printer has a standby mode or is turned off between printing jobs, the warm-up time becomes even more important. Large workgroup and production printers can take 5 to 15 minutes to warm up. This waiting period can hinder overall productivity.
laserdisc Laserdisc is a technology and the physical medium used in storing and providing programmed access to a large database of text, pictures, and other objects, including motion video and full multimedia presentations. The laserdisc itself is 12 inches in diameter and holds much more information than a CD-ROM disc can currently hold. Laserdiscs require relatively expensive players and are more
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expensive to distribute than CD-ROM discs. However, for school and corporate education purposes and any presentation requiring a great deal of motion video and the ability to create scripted or programmed access to selected portions of the laserdisc, the technology can be useful.
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Last-mile technology is any telecommunications technology, such as wireless radio, that carries signals from the broad telecommunication along the relatively short distance (hence, the ‘‘last mile’’) to and from the home or business. Or to put it another way: the infrastructure at the neighborhood level. In many communities, last-mile technology represents a major remaining challenge to high-bandwidth applications such as on-demand television, fast Internet access, and Web pages full of multimedia effects. Today, in addition to ‘‘plain old telephone (dial-up) service,’’ last-mile technologies to deliver voice, data, and TV can include: L
ISDN, a somewhat faster technology than regular phone service
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Digital Subscriber Line (DSL) over existing telephone twisted pair lines
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Cable and the cable modem for data, using the same installed coaxial cable that already is used for television
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Wireless, including services such as DirecTV
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Less frequently because of the installation expense, optical fiber and its transmission technologies
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LATA
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See ‘‘local access and transport area’’
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latency
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1) In a network, latency, a synonym for delay, is an expression of how much time it takes for a packet of data to get from one designated point to another. In some usages (for example, AT&T), latency is measured by sending a packet that is returned to the sender and the round-trip time is considered the latency.
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The latency assumption seems to be that data should be transmitted instantly between one point and another (that is, with no delay at all). The contributors to network latency include:
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Propagation: This is simply the time it takes for a packet to travel between one place and another at the speed of light.
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Transmission: The medium itself (whether optical fiber, wireless, or some other) introduces some delay. The size of the packet introduces delay in a round trip since a larger packet will take longer to receive and return than a short one.
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Router and other processing: Each gateway node takes time to examine and possibly change the header in a packet (for example, changing the hop count in the timeto-live field).
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Other computer and storage delays: Within networks at each end of the journey, a packet may be subject to storage and hard disk access delays at intermediate devices such as switches and bridges. (In backbone statistics, however, this kind of latency is probably not considered.)
2) In a computer system, latency is often used to mean any delay or waiting that increases real or perceived response time beyond the response time desired. Specific contributors to computer latency include mismatches in data speed between the microprocessor and input/output devices and inadequate data buffers. Within a computer, latency can be removed or ‘‘hidden’’ by such techniques as prefetching (anticipating the need for data input requests) and multithreading, or using parallelism across multiple execution threads. 3) In 3D simulation, in describing a helmet that provides stereoscopic vision and head tracking, latency is the time between the computer detecting head motion to the time it displays the appropriate image.
layer See ‘‘layering’’
layer 2 Layer 2 refers to the Data Link layer of the commonlyreferenced multilayered communication model, Open Systems Interconnection (OSI). The Data Link layer is concerned with moving data across the physical links in the network. In a network, the switch is a device that redirects data messages at the layer 2 level, using the destination Media Access Control (MAC) address to determine where to direct the message. The Data-Link layer contains two sublayers that are described in the IEEE-802 LAN standards: L
Media Access Control (MAC) sublayer
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Logical Link Control (LLC) sublayer
The Data Link layer ensures that an initial connection has been set up, divides output data into data frames, and handles the acknowledgements from a receiver that the data arrived successfully. It also ensures that incoming data has been received successfully by analyzing bit patterns at special places in the frames.
layer 3 Layer 3 refers to the Network layer of the commonly referenced multilayered communication model, Open Systems Interconnection (OSI). The Network layer is
LDCM concerned with knowing the address of the neighboring nodes in the network, selecting routes and quality of service, and recognizing and forwarding to the Transport layer incoming messages for local host domains. A router is a layer 3 device, although some newer switches also perform layer 3 functions. The Internet Protocol (IP) address is a layer 3 address.
Layer Two Tunneling Protocol Layer Two Tunneling Protocol (L2TP) is an extension of the Point-to-Point Tunneling Protocol (PPTP) used by an Internet service provider (ISP) to enable the operation of a virtual private network (VPN) over the Internet. L2TP merges the best features of two other tunneling protocols: PPTP from Microsoft and L2F from Cisco Systems. The two main components that make up L2TP are the L2TP Access Concentrator (LAC), which is the device that physically terminates a call and the L2TP Network Server (LNS), which is the device that terminates and possibly authenticates the PPP stream. PPP defines a means of encapsulation to transmit multiprotocol packets over layer two (L2) point-to-point links. Generally, a user connects to a network access server (NAS) through ISDN, ADSL, dialup POTS or other service and runs PPP over that connection. In this configuration, the L2 and PPP session endpoints are both on the same NAS. L2TP uses packet-switched network connections to make it possible for the endpoints to be located on different machines. The user has an L2 connection to an access concentrator, which then tunnels individual PPP frames to the NAS, so that the packets can be processed separately from the location of the circuit termination. This means that the connection can terminate at a local circuit concentrator, eliminating possible long-distance charges, among other benefits. From the user’s point of view, there is no difference in the operation.
layering In computer programming, layering is the organization of programming into separate functional components that interact in some sequential and hierarchical way, with each layer usually having an interface only to the layer above it and the layer below it. Communication programs are often layered. The reference model for communication programs, Open System Interconnection (OSI) is a layered set of protocols in which two multilayered programs, one at either end of a communications exchange, use an identical set of layers. In the OSI model, each multilayer program contains seven layers, each reflecting a different function that has to be performed in order for program-to-program communication to take place between computers.
TCP/IP is an example of a two-layer (TCP and IP) set of programs that provide transport and network address functions for Internet communication. A set of TCP/IP and other layered programs is sometimes referred to as a protocol stack.
LB film
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See ‘‘Langmuir-Blodgett film’’
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LBA
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See ‘‘logical block addressing’’
LBS
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See ‘‘location-based services’’
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LCD (liquid crystal display) is the technology used for displays in notebook and other smaller computers. Like light-emitting diode (LED) and gas-plasma technologies, LCDs allow displays to be much thinner than cathode ray tube (CRT) technology. LCDs consume much less power than LED and gas-display displays because they work on the principle of blocking light rather than emitting it. An LCD is made with either a passive matrix or an active matrix display grid. The active matrix LCD is also known as a thin film transistor (TFT) display. The passive matrix LCD has a grid of conductors with pixels located at each intersection in the grid. A current is sent across two conductors on the grid to control the light for any pixel. An active matrix has a transistor located at each pixel intersection, requiring less current to control the luminance of a pixel. For this reason, the current in an active matrix display can be switched on and off more frequently, improving the screen refresh time (your mouse will appear to move more smoothly across the screen, for example). Some passive matrix LCD’s have dual scanning, meaning that they scan the grid twice with current in the same time that it took for one scan in the original technology. However, active matrix is still a superior technology.
LCP
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See ‘‘Link Control Protocol’’
LDAP
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legacy application
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LDIF (Lightweight Directory Interchange Format) is an ASCII file format used to exchange data and enable the synchronization of that data between Lightweight Directory Access Protocol (LDAP) servers called Directory System Agents (DSAs). LDAP is a software protocol for enabling anyone to locate organizations, individuals, and other resources such as files and devices in a network. An LDAP directory can be distributed among many servers. LDIF is used to synchronize each LDAP directory.
In information technology, legacy applications and data are those that have been inherited from languages, platforms, and techniques earlier than current technology. Most enterprises who use computers have legacy applications and databases that serve critical business needs. Typically, the challenge is to keep the legacy application running while converting it to newer, more efficient code that makes use of new technology and programmer skills. In the past, much programming has been written for specific manufacturers’ operating systems. Currently, many companies are migrating their legacy applications to new programming languages and operating systems that follow open or standard programming interfaces. Theoretically, this will make it easier in the future to update applications without having to rewrite them entirely and will allow a company to use its applications on any manufacturer’s operating system.
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The first step in synchronizing LDAP directories is extracting the full contents of or a portion of the original LDAP directory and formatting the contents into an LDIF file. The LDIF file is then mailed to a directory synchronization robot called DIRBOT. After several different steps, a final LDIF file is compared to the original LDIF file. The update instructions on what records to add, delete, or modify in the original directory are decided. These updates are then used to synchronize all LDAP directories.
leased line A leased line is a telephone line that has been leased for private use. In some contexts, it’s called a dedicated line. A leased line is usually contrasted with a switched line or dial-up line. Typically, large companies rent leased lines from the telephone message carriers (such as AT&T) to interconnect different geographic locations in their company. The alternative is to buy and maintain their own private lines or, increasingly perhaps, to use the public switched lines with secure message protocols. (This is called tunneling.)
LEC
length See ‘‘displacement’’
LEO See ‘‘satellite’’
leverage
LEC (local exchange carrier) is the term for a public telephone company in the U.S. that provides local service. Some of the largest LECs are the Bell operating companies (BOCs) which were grouped into holding companies known collectively as the regional Bell operating companies (RBOCs) when the Bell System was broken up by a 1983 consent decree. In addition to the Bell companies, there are a number of independent LECs, such as GTE.
In the physical sense, leverage is an assisted advantage. As a verb, to leverage means to gain an advantage through the use of a tool. For example, you can more easily lift a heavy object with a lever than you can lift it unaided. Leverage is commonly used in a metaphorical sense. For example, as a frequently used business or marketing term, leverage is any strategic or tactical advantage, and as a verb, means to exploit such an advantage, just as the use of a physical lever gives one an advantage in the physical sense.
LEC companies are also sometimes referred to as ‘‘telcos.’’ A ‘‘local exchange’’ is the local ‘‘central office’’ of an LEC. Lines from homes and businesses terminate at a local exchange. Local exchanges connect to other local exchanges within a local access and transport area (LATA) or to interexchange carriers (IXCs) such as long-distance carriers ATT, MCI, and Sprint.
Leverage is a very popular business term on the Web. B2B Web sites offer to leverage Web sites, code, interactivity, storage area networks, database technology, and many other things. In the world of finance, leverage is the use of borrowed money to make an investment and the return on an investment.
LED V,W See ‘‘light-emitting diode’’ X,Y,Z
In addition to moving to new languages, enterprises are redistributing the locations of applications and data. In general, legacy applications have to continue to run on the platforms they were developed for. Typically, new development environments account for the need to continue to support legacy applications and data. With many new tools, legacy databases can be accessed by newer programs.
LexiBot LexiBot is a specialized search tool developed by BrightPlanet, as a means of searching the deep Web (the hidden part of the Web that may contain 500 times the
Lightweight Directory Access Protocol content accessible to conventional search engines). LexiBot uses what BrightPlanet calls a comprehensive and intelligent search technology that enables users to conduct searches using simple text, natural language, or Boolean queries on hundreds of databases simultaneously, filter and analyze data, and publish the results as Web pages. LexiBot was designed to perform complex searches to identify and retrieve content from all areas of the Web, and to process the information. Deep Web content has been accessible only to those who knew the correct URL for the Web site. Then, even once a user had connected to a database, its data could only be accessed by single direct queries. LexiBot acts as an automated direct query engine to make dozens of queries simultaneously to multiple databases. Searches are supported on close to 600 databases. Once the links are found, LexiBot downloads the links, analyzes them, removes the irrelevant ones, then downloads the text portions of the documents to the user’s hard drive. The application can be used with desktop databases as well as the Internet, is customizable to user preferences, and can be set up for either simple or advanced usage.
transparent package, allowing visible or IR energy to pass through. Also, the LED or IRED has a large PN-junction area whose shape is tailored to the application. Benefits of LEDs and IREDs, compared with incandescent and fluorescent illuminating devices, include:
The opposite approach is known as dynamic scoping. Dynamic scoping creates variables that can be called from outside the block of code in which they are defined. A variable declared in this fashion is sometimes called a public variable.
light-emitting diode Also see laser diode. A light-emitting diode (LED) is a semiconductor device that emits visible light when an electric current passes through it. The light is not particularly bright, but in most LEDs it is monochromatic, occurring at a single wavelength. The output from an LED can range from red (at a wavelength of approximately 700 nanometers) to blue-violet (about 400 nanometers). Some LEDs emit infrared (IR) energy (830 nanometers or longer); such a device is known as an infraredemitting diode (IRED). An LED or IRED consists of two elements of processed material called P-type semiconductors and N-type semiconductors. These two elements are placed in direct contact, forming a region called the P-N junction. In this respect, the LED or IRED resembles most other diode types, but there are important differences. The LED or IRED has a
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Low power requirement: Most types can be operated with battery power supplies.
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High efficiency: Most of the power supplied to an LED or IRED is converted into radiation in the desired form, with minimal heat production.
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Long life: When properly installed, an LED or IRED can function for decades.
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Indicator lights: These can be two-state (i.e., on/off), bar-graph, or alphabetic-numeric readouts.
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LCD panel backlighting: Specialized white LEDs are used in flat-panel computer displays.
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Fiber optic data transmission: Ease of modulation allows wide communications bandwidth with minimal noise, resulting in high speed and accuracy.
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Remote control: Most home-entertainment ‘‘remotes’’ use IREDs to transmit data to the main unit.
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Optoisolator: Stages in an electronic system can be connected together without unwanted interaction.
lexical scoping Lexical scoping (sometimes known as static scoping) is a convention used with many programming languages that sets the scope (range of functionality) of a variable so that it may only be called (referenced) from within the block of code in which it is defined. The scope is determined when the code is compiled. A variable declared in this fashion is sometimes called a private variable.
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light-emitting polymer See ‘‘polymer LED’’
Lightweight Directory Access Protocol Lightweight Directory Access Protocol (LDAP) is a software protocol for enabling anyone to locate organizations, individuals, and other resources such as files and devices in a network, whether on the public Internet or on a corporate intranet. LDAP is a ‘‘lightweight’’ (smaller amount of code) version of Directory Access Protocol (DAP), which is part of X.500, a standard for directory services in a network. LDAP is lighter because in its initial version it did not include security features. LDAP originated at the University of Michigan and has been endorsed by at least 40 companies. Netscape includes it in its latest Communicator suite of products. Microsoft includes it as part of what it calls Active Directory in a number of products including Outlook Express. Novell’s NetWare Directory Services interoperates with LDAP. Cisco also supports it in its networking products. In a network, a directory tells you where in the network something is located. On TCP/IP networks (including the Internet), the domain name system (DNS) is the directory system used to relate the domain name to a specific network address (a unique location on the network). However, you
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may not know the domain name. LDAP allows you to search for an individual without knowing where they’re located (although additional information will help with the search). An LDAP directory is organized in a simple ‘‘tree’’ hierarchy consisting of the following levels: L
The root directory (the starting place or the source of the tree), which branches out to
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Countries, each of which branches out to
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Organizations, which branch out to
Linear ICs are employed in audio amplifiers, A/D (analogto-digital) converters, averaging amplifiers, differentiators, DC (direct-current) amplifiers, integrators, multivibrators, oscillators, audio filters, and sweep generators. Linear ICs are available in most large electronics stores. Some devices contain several amplifiers within a single housing.
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Organizational units (divisions, departments, and so forth), which branches out to (includes an entry for)
linear tape
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Individuals (which includes people, files, and shared resources such as printers)
See ‘‘Linear Tape-Open’’
An LDAP directory can be distributed among many servers. Each server can have a replicated version of the total directory that is synchronized periodically. An LDAP server is called a Directory System Agent (DSA). An LDAP server that receives a request from a user takes responsibility for the request, passing it to other DSAs as necessary, but ensuring a single coordinated response for the user.
Linear Tape-Open
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Li-Ion battery
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See ‘‘Lithium Ion battery’’
line information database
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A line information database (LIDB) is a database maintained by the local telephone company that contains subscriber information, such as a service profile, name and address, and credit card validation information.
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line print terminal
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See ‘‘LPT’’
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linear IC
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See ‘‘linear integrated circuit’’
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linear integrated circuit Also see integrated circuit (IC).
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to the non-inverting input produces a signal of identical phase at the output. A connection, through a variable resistance, between the output and the inverting input is used to control the amplification factor.
A linear integrated circuit (linear IC) is a solid-state analog device characterized by a theoretically infinite number of possible operating states. It operates over a continuous range of input levels. In contrast, a digital IC has a finite number of discrete input and output states. Within a certain input range, the amplification curve of a linear IC is a straight line; the input and output voltages are directly proportional. The best known, and most common, linear IC is the operational amplifier or op amp, which consists of resistors, diodes, and transistors in a conventional analog circuit. There are two inputs, called inverting and non-inverting. A signal applied to the inverting input results in a signal of opposite phase at the output. A signal applied
Linear Tape-Open (LTO) is an open standard for a backup tape system, providing formats for both fast data access and high storage capacity, developed jointly by HewlettPackard, IBM, and Seagate. IBM released the first LTO products in August 2000. One of the tape industry’s biggest problems has been a lack of standards, with each vendor providing its own technology. Standardization means that different manufacturers’ tapes and tape drives will interoperate (just as audio tape cassettes work in all tape players). Like existing tape systems, LTO uses a linear multi-channel bi-directional format. LTO adds to existing technologies timing-based servo (a device that automates a process of error correction for a mechanism), hardware data compression, enhanced track layouts, and efficient error correction code. LTO was developed in two different formats—one for fast data access and another for greater storage capacity. The Accelis format uses 8mm-wide tape on a two-reel cartridge that loads at the mid-point of the tape to provide fast data access, specifically for read-intensive applications, such as online searches and retrieval functions. The Ultrium format uses a single reel of half-inch wide tape to maximize storage capacity, specifically for write-intensive applications, such as archival and backup functions. Early products using the Accelis format offer a 25 gigabyte capacity for uncompressed data, while Ultrium based-products offer a 100 gigabyte capacity. Both formats provide transfer rates of 10720 Mbps. While these figures are not unheard of in other technologies, LTO specifications include plans for expected increases that will double current rates with each of the next three generations of products.
linearity Linearity is the behavior of a circuit, particularly an amplifier, in which the output signal strength varies in direct proportion to the input signal strength. In a linear device, the output-to-input signal amplitude ratio is always the same, no matter what the strength of the input signal (as long it is not too strong).
link checker In an amplifier that exhibits linearity, the output-versusinput signal amplitude graph appears as a straight line. Two examples are shown below. The gain, or amplification factor, determines the slope of the line. The steeper the slope, the greater the gain. The amplifier depicted by the higher line has more gain than the one depicted by the lower line. Both amplifiers are linear within the input-signal strength range shown, because both lines in the graph are straight.
Relative output 100
link 1) Using hypertext, a link is a selectable connection from one word, picture, or information object to another. In a multimedia environment such as the World Wide Web, such objects can include sound and motion video sequences. The most common form of link is the highlighted word or picture that can be selected by the user (with a mouse or in some other fashion), resulting in the immediate delivery and view of another file. The highlighted object is referred to as an anchor. The anchor reference and the object referred to constitute a hypertext link. Although most links do not offer the user a choice of types of link, it would be possible for the user to be provided a choice of link types, such as: a definition of the object, an example of it, a picture of it, a smaller or larger picture of it, and so forth. Links are what make the World Wide Web a web.
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Also see hypermedia and hyperlink. 2) In telecommunications, a link is a physical (and, in some usages, a logical) connection between two points.
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Relative input 10
Examples of linearity In analog applications such as amplitude-modululation (AM) wireless transmission and hi-fi audio, linearity is important. Nonlinearity in these applications results in signal distortion, because the fluctuation in gain affects the shape of an analog output waveform with respect to the analog input waveform. Even if an amplifier exhibits linearity under normal conditions, it will become nonlinear if the input signal is too strong. This situation is called overdrive. The amplification curve bends towards a horizontal slope as the input-signal amplitude increases beyond the critical point, producing distortion in the output. An example is a hi-fi amplifier whose gain is set to the point where the VU (volume-unit) meter needles kick into the red range. The red zone indicates that the amplifier is not operating in a linear fashion. This can degrade the fidelity of the sound.
line-of-business application See ‘‘LOB’’
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A link checker is a program that tests and reports on the validity of the hypertext links on the pages in a Web site. More advanced link checkers test links to other Web sites as well as links between pages on the same site. A link checker may be a separate program that specializes in this service, part of a larger program that provides a range of Web site publishing services, or provided periodically as a service from a remote application server.
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An example of a specialized link checker is NetMechanic Toolbox.
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Looks at each page in a Web site’s directory of pages and tries to connect to each link it finds on a page. If a link does not work, it identifies the reason (for example, ‘‘A connection could not be established’’ or ‘‘This site has been permanently moved’’) and prepares a report on ‘‘Pages with Bad Links’’ that you can view with your Web browser or print out. Summarizes the number of HTML and graphic files it found, their individual size, and the average download size.
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Lets you specify whether you want individual page reports or a summary.
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Lets you run the program automatically on a daily basis at a particular time.
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Link Control Protocol
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In the Point-to-Point Protocol (PPP), the Link Control Protocol (LCP) establishes, configures, and tests data-link Internet connections. Before establishing communications over a point-to-point link, each end of the PPP link must send out LCP packets. The LCP packet either accepts or rejects the identity of its linked peer, agrees upon packet size limits, and looks for common misconfiguration errors. Basically, the LCP packet checks the telephone line connection to see whether the connection is good enough to sustain data transmission at the intended rate. Once the LCP packet accepts the link, traffic can be transported on the network; if the LCP packet determines the link is not functioning properly, it terminates the link.
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LCP packets are divided into three classes:
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1. Link configuration packets used to establish and configure a link
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2. Link termination packets used to terminate a link
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3. Link maintenance packets used to manage and debug a link
link encryption Link encryption (sometimes called link level or link layer encryption) is the data security process of encrypting information at the data link level as it is transmitted between two points within a network. Data, which is plaintext in the host server, is encrypted when it leaves the host, decrypted at the next link (which may be a host or a relay point), and then reencrypted before it continues to the next link. Each link may use a different key or even a different algorithm for data encryption. The process is repeated until the data has reached the recipient. Link encryption takes place in the lowest protocol layers (layers 1 and 2 in the OSI model). Because the process protects the message in transit, link encryption is very useful in situations where the security of the transmission line is not assured. However, because the message is decrypted at each host in the transmission path, vulnerability can arise at a link when the message must be transmitted between hosts that are not known to be secure. Link encryption has been used successfully within organizations, including the military, where the security of each link can be assured. It isn’t feasible over the Internet, because intermediate links are neither accessible nor secure.
link type In Hyper-G and possibly other hypertext systems, a link type is the specification of the nature of the information object being linked to. A single link can have any number of defined link types. For example, for any word from which one might link to another information object, that object
could be an example of that word, or a graphic illustration of it, or a definition of it, or it might be associated with one person’s comments on that subject. For example, in a page on transmission technologies, the term ‘‘ISDN’’ might be highlighted as a link. When the link is created, the author might choose to let the reader link to an illustration of how ISDN works, or just a brief definition of it, or a whole book about it. Or a reviewer (rather than the author) might create the link for review purposes and the link would be to a particular reviewer comment. (Later, the author or other reviewers would find the link and the linked-to comments and perhaps create additional comments.) If link types are made generic and all links exist as separate objects apart from the files in which they are used, then different link types and links can be easily added, removed, or changed throughout a set of files. The examples described here are hypothetical and not necessarily exactly how Hyper-G or any other hypertext system works.
linkrot Linkrot is the tendency of hypertext links from one Web site to another site to become useless as other sites cease to exist or remove or reorganize their Web pages. A recent survey showed that almost one of every four Web pages in the survey sample contained a bad link. Observers note that the amount of linkrot that occurs can be correlated closely (and not surprisingly) with the passage of time. Links to the home page of large companies like IBM and Microsoft appear to be the least likely to ‘‘rot.’’ However, links to pages within companies often generate ‘‘Not found’’ messages as a result of site page restructures or ‘‘old’’ material being removed. Links to pages created by students often no longer work after the student graduates. A good practice followed by many sites is to leave the old pages in place with a message identifying the new page address or Uniform Resource Locator and specifying that an automatic refresh or redirection to the new page take place within a few seconds. A number of Web site development and testing products provide the ability to test all the links in a Web site and report on the failing links.
Linux logo See ‘‘Tux’’
Linux Linux (often pronounced LIH-nuhks with a short ‘‘i’’) is a UNIX-like operating system that was designed to provide personal computer users a free or very low-cost operating system comparable to traditional and usually more expensive UNIX systems. Linux has a reputation as a very
live cam efficient and fast-performing system. Linux’s kernel (the central part of the operating system) was developed by Linus Torvalds at the University of Helsinki in Finland. To complete the operating system, Torvalds and other team members made use of system components developed by members of the Free Software Foundation for the GNU project. Linux is a complete operating system, including a graphical user interface, an X Window System, TCP/IP, the Emacs editor, and other components usually found in a comprehensive UNIX system. Although copyrights are held by various creators of Linux’s components, Linux is distributed using the Free Software Foundation’s copyleft stipulations that mean any modified version that is redistributed must in turn be freely available. Unlike Windows and other proprietary systems, Linux is publicly open and extendible by contributors. Because it conforms to the Portable Operating System Interface standard user and programming interfaces, developers can write programs that can be ported to other operating systems. Linux comes in versions for all the major microprocessor platforms including the Intel, PowerPC, Sparc, and Alpha platforms. It’s also available on IBM’s S/390. Linux is distributed commercially by a number of companies. A magazine, Linux Journal, is published as well as a number of books and pocket references. Linux is sometimes suggested as a possible publicly developed alternative to the desktop predominance of Microsoft Windows. Although Linux is popular among users already familiar with UNIX, it remains far behind Windows in numbers of users.
liquid crystal display See ‘‘LCD’’
liquid See ‘‘ice’’
LISP LISP, an acronym for list processing, is a programming language that was designed for easy manipulation of data strings. Developed in 1959 by John McCarthy, it is a commonly used language for artificial intelligence (AI) programming. It is one of the oldest programming languages still in relatively wide use. In LISP, all computation is expressed as a function of at least one object. Objects can be other functions, data items (such as constants or variables), or data structures. LISP’s ability to compute with symbolic expressions rather than numbers makes it convenient for AI applications.
list server A list server (mailing list server) is a program that handles subscription requests for a mailing list and distributes new messages, newsletters, or other postings from the list’s members to the entire list of subscribers as they occur or are scheduled. (A list server should not be confused with a mail server, which handles incoming and outgoing e-mail for Internet users.) Two commonly used list servers are listserv and Majordomo. Lyris is a list server that is free for users maintaining very small mailing lists and scales up in price for those managing thousands of mailing list subscribers.
listserv Listserv, like Majordomo, is a small program that automatically redistributes e-mail to names on a mailing list. Users can subscribe to a mailing list by sending an e-mail note to a mailing list they learn about; listserv will automatically add the name and distribute future e-mail postings to every subscriber. (Requests to subscribe and unsubscribe are sent to a special address so that all subscribers do not see these requests.) These programs are also known as list servers.
liteware Liteware is a term for software that is distributed freely in a version having less capability than the full for-sale version. It’s usually designed to provide a potential customer with a sample of the ‘‘look-and-feel’’ of a product and a subset of its full capability. Liteware can be considered a type of shareware (where shareware also includes products distributed freely, usually on a trial basis, that do have full capability). An example of liteware is HTML Assistant Pro, an HTML editor. We tried their liteware version, which allows you to create HTML files (Web pages) and shows you, but doesn’t let you use, table and form creation facilities. Since we liked the liteware version and needed all the capabilities, we ordered the full product.
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Also see freeware and postcardware.
Lithium Ion battery
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A Lithium Ion (Li-Ion) battery is a rechargeable battery with twice the energy capacity of a Nickel-Cadmium battery and greater stability and safety.
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little-endian See ‘‘big-endian and little-endian’’
live cam
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load balancing
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LMDS (Local Multipoint Distribution System) is a system for broadband microwave wireless transmission direct from a local antenna to homes and businesses within a line-of-sight radius, a solution to the so-called last-mile technology problem of economically bringing high-bandwidth services to users. LMDS is an alternative to installing optical fiber all the way to the user or to adapting cable TV for broadband Internet service. Depending on the implementation, LMDS offers a bandwidth of up to 1.5 Gbps downstream to users and 200 Mbps upstream from the user. A more typical data rate is 38 Mbps downstream. Some services offer both downstream and upstream service (symmetrical service); others offer downstream only (asymmetrical service) with upstream being obtained using wire connections.
Load balancing is dividing the amount of work that a computer has to do between two or more computers so that more work gets done in the same amount of time and, in general, all users get served faster. Load balancing can be implemented with hardware, software, or a combination of both. Typically, load balancing is the main reason for computer server clustering.
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In addition to the investment by service providers for transmitters, users need to install transceivers costing about $125-225. However, the cost of installing LMDS is considered far lower than installing fiber optic cable or upgrading cable TV systems. The first markets for LMDS are seen as: L
High-speed data transmission for businesses
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Interactive television and streaming multimedia from Web sites
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Voice service (usually as a supplement to other services)
Because LMDS requires a more expensive and possibly larger transceiver than can conveniently be packaged in a handheld device, LMDS is not viewed as a replacement for or alternative to mobile wireless technologies such as cellular and GSM. On the other hand, LMDS offers much higher data rates because of its use of a higher range of frequencies with their wider bandwidths. In general, LMDS is for fixed locations and offers higher data rates; cellular digital such as GSM is for mobile users at lower data rates (although these will increase with technologies leading up to UMTS). LMDS uses the range of electromagnetic radiation spectrum in the vicinity of 28 GHz, with the allocated range differing slightly between the U.S., Canada, and other countries. In Europe, ETSI sponsors an equivalent technology. In Canada, it is called Local Multipoint Communication Service (LMCS). Like cellular telephone technologies, LMDS is point-to-multipoint. It is viewed as a future component in the convergence of data and telephony services. Ericsson’s LMDS system uses either Ethernet, ATM, or T-carrier system/E-carrier system network interfaces at the user end. ATM allows the user to select and pay for varying qualities of service.
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See ‘‘Local Number Portability’’
On the Internet, companies whose Web sites get a great deal of traffic usually use load balancing. For load balancing Web traffic, there are several approaches. For Web serving, one approach is to route each request in turn to a different server host address in a domain name system (DNS) table,round-robin fashion. Usually, if two servers are used to balance a work load, a third server is needed to determine which server to assign the work to. Since load balancing requires multiple servers, it is usually combined with failover and backup services. In some approaches, the servers are distributed over different geographic locations.
loader In a computer operating system, a loader is a component that locates a given program (which can be an application or, in some cases, part of the operating system itself) in offline storage (such as a hard disk), loads it into main storage (in a personal computer, it’s called random access memory), and gives that program control of the computer (allows it to execute its instructions). A program that is loaded may itself contain components that are not initially loaded into main storage, but can be loaded if and when their logic is needed. In a multitasking operating system, a program that is sometimes called a dispatcher juggles the computer processor’s time among different tasks and calls the loader when a program associated with a task is not already in main storage. (By program here, we mean a binary file that is the result of a programming language compilation, linkage editing, or some other program preparation process.)
loading coil A loading coil is an induction device placed on a local loop longer than 18,000 feet that carries analog signals. The device compensates for wire capacitance and boosts the frequencies carrying the voice information. Loading coils cause distortion at the higher frequencies used to carry digital information and so are not used on these local loops.
LOB An LOB (line-of-business) application is one of the set of critical computer applications that are vital to running an enterprise, such as accounting, supply chain management, and resource planning applications. LOB applications are
localization usually large progams that contain a number of integrated capabilities and tie into databases and database management systems. Increasingly, LOB applications are being connected with network applications with user interfaces on the Web and with personal applications such as e-mail and address books.
local access and transport area LATA (local access and transport area) is a term in the U.S. for a geographic area covered by one or more local telephone companies, which are legally referred to as local exchange carriers (LECs). A connection between two local exchanges within the LATA is referred to as intraLATA. A connection between a carrier in one LATA to a carrier in another LATA is referred to as interLATA. InterLATA is long-distance service. The current rules for permitting a company to provide intraLATA or interLATA service (or both) are based on the Telecommunications Act of 1996.
local area network A local area network (LAN) is a group of computers and associated devices that share a common communications line and typically share the resources of a single processor or server within a small geographic area (for example, within an office building). Usually, the server has applications and data storage that are shared in common by multiple computer users. A local area network may serve as few as two or three users (for example, in a home network) or as many as thousands of users (for example, in an FDDI network). The main local area network technologies are: L
Ethernet
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Token ring
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ARCNET
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FDDI (Fiber Distributed Data Interface)
Typically, a suite of application programs can be kept on the LAN server. Users who need an application frequently can download it once and then run it from their local hard disk. Users can order printing and other services as needed through applications run on the LAN server. A user can share files with others at the LAN server; read and write access is maintained by a LAN administrator. A LAN server may also be used as a Web server if safeguards are taken to secure internal applications and data from outside access.
local exchange carrier See ‘‘LEC’’
local loop In telephony, a local loop is the wired connection from a telephone company’s central office in a locality to its customers’ telephones at homes and businesses. This connection is usually on a pair of copper wires called twisted pair. The system was originally designed for voice transmission only using analog transmission technology on a single voice channel. Today, your computer’s modem makes the conversion between analog signals and digital signals. With Integrated Services Digital Network (ISDN) or Digital Subscriber Line (DSL), the local loop can carry digital signals directly and at a much higher bandwidth than they do for voice only.
Local Number Portability Local Number Portability (LNP) is the ability of a telephone customer in the U.S. to retain their local phone number if they switch to another local telephone service provider. The Telecommunications Act of 1996 required that the local exchange carriers (LECs) in the 100 largest metropolitan markets provide this capability by the end of 1998. The idea is that by removing the personal inconvenience of having to get a new phone number when changing service providers, competition among providers will be increased. LNP is one of the prices that local carriers must pay in order to be allowed to compete as well in the long-distance market. LNP is made possible by the Location Routing Number (LRN). In the future, phone number portability may be extended so that customers can retain their phone number when they move to another locality. LNPs and LRNs are supervised by the Number Portability Administration Center, operated by Lockheed Martin under the appointment of the Federal Communications Commission (FCC). When a customer moves their local service to a competitive local exchange carrier (CLEC), a new LRN is assigned to the telephone number being ported. Each local exchange and long distance carrier needs to know what that new LRN is so when someone in an another area dials the number being ported, the carrier knows what LRN to route to. This is accomplished through Local Service Management System (LSMS) databases distributed among the exchange carriers. The NPAC updates all of these databases with the newly assigned LRN. Thus, when the call is made from another area, that carrier refers to its LSMS database to obtain the current LRN for the number dialed.
localization Localization (sometimes shortened to ‘‘l10n’’) is the process of adapting a product or service to a particular language, culture, and desired local ‘‘look-and-feel.’’ Ideally, a product or service is developed so that localization is relatively easy to achieve—for example, by creating technical illustrations for manuals in which the text can easily be changed to another language and allowing some expansion room for
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Whatis?Com’s Encyclopedia of Technology Terms this purpose. This enabling process is termed internationalization. An internationalized product or service is therefore easier to localize. In localizing a product, in addition to idiomatic language translation, such details as time zones, money, national holidays, local color sensitivities, product or service names, gender roles, and geographic examples must all be considered. A successfully localized service or product is one that appears to have been developed within the local culture. Language translation, which is a large part of localization, can sometimes be facilitated with automatic language translation. However, much additional work is usually needed.
Location Routing Number
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In the U.S., a Location Routing Number (LRN) is a 10-digit number in a database called a Service Control Point (SCP) that identifies a switching port for a local telephone exchange. LRN is a technique for providing Local Number Portability (LNP). Using LRN, when a phone number is dialed, the local telephone exchange queries a routing database, usually the SCP, for the LRN associated with the subscriber. The LRN removes the need for the public telephone number to identify the local exchange carrier. If a subscriber changes to another telephone service provider, the current telephone number can be retained. Only the LRN needs to be changed. In addition to supporting service provider phone number portability, an LRN also supports the possibility of two other types of number portability: service portability (for example, ordinary service to ISDN) and geographic portability. LRN is an alternative to the current NPA-NXX format described in the North American Telephone Numbering System (NATNS). LNPs and LRNs are supervised by the Number Portability Administration Center, operated by Lockheed Martin under the appointment of the Federal Communications Commission (FCC).
location-based services
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Location-based services (LBS) are services that exploit knowledge about where an information device user is located. For example, the user of a wireless-connected smartphone could be shown ads specific to the region the user is traveling in. Location-based services exploit any of several technologies for knowing where a network user is geographically located. One is the Global Positioning System (GPS), based on a collection of 24 Navstar satellites developed originally for the U.S. Department of Defense. A land-based GPS receiver uses these satellites to determine its location, within 50 meters to 100 meters. A location-based service could require
that each of its users have a mobile device that contains a GPS receiver. A second approach is E911, an initiative of the Federal Communications Commission (FCC) that requires wireless carriers to pinpoint a caller’s telephone number to emergency dispatchers. E911 also ensures that carriers will be able to provide call locations from wireless phones. E911 is the most widely used location-based service in the U.S. Allied Business Intelligence estimates that the LBS industry will account for more than $40 billion in revenue by 2006. Most telecommunications carriers plan to pursue either network- or handset-based location fixing technologies in their networks. Sprint Corp. recently announced it planned to incorporate GPS chips into its handsets in 2001, which could provide a boost to the nascent industry. Several other companies are wrapping up initial LBS and location-relevant wireless advertising test markets. Another obstacle to immediate widespread adoption of LBS are concerns about privacy and unsolicited wireless advertising. The Cellular Telecommunications and Internet Association (CTIA) is asking the FCC to create specific rules about wireless location privacy. The CTIA proposal says a technical solution must include notice, consent, and security—and be technology-neutral.
log See ‘‘access log’’
logarithm See also exponential function and decibel. A logarithm is an exponent used in mathematical calculations to depict the perceived levels of variable quantities such as visible light energy, electromagnetic field strength, and sound intensity. Suppose three real numbers a, x, and y are related according to the following equation: x = ay Then y is defined as the base-a logarithm of x. This is written as follows: loga x = y As an example, consider the expression 100 = 102. This is equivalent to saying that the base-10 logarithm of 100 is 2; that is, log10 100 = 2. Note also that 1000 = 103; thus log10 1000 = 3. (With base-10 logarithms, the subscript 10 is often omitted, so we could write log 100 = 2 and log 1000 = 3). When the base-10 logarithm of a quantity increases by 1, the quantity itself increases by a factor of 10. A 10-to-1 change in the size of a quantity, resulting in a logarithmic increase or decrease of 1, is called an order of magnitude. Thus, 1000 is one order of magnitude larger than 100. Base-10 logarithms, also called common logarithms, are used in electronics and experimental science. In theoretical science and mathematics, another logarithmic base is encountered: the transcendental number e, which is
logic gate approximately equal to 2.71828. Base-e logarithms, written loge or ln, are also known as natural logarithms. If x = ey, then loge x = ln x = y
logic gate A logic gate is an elementary building block of a digital circuit. Most logic gates have two inputs and one output. At any given moment, every terminal is in one of the two binary conditions low (0) or high (1), represented by different voltage levels. The logic state of a terminal can, and generally does, change often, as the circuit processes data. In most logic gates, the low state is approximately zero volts (0 V), while the high state is approximately five volts positive (+5 V).
OR gate Input 1Input 2 0 0 1 1
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The XOR (exclusive-OR) gate acts in the same way as the logical ‘‘either/or.’’ The output is ‘‘true’’ if either, but not both, of the inputs are ‘‘true.’’ The output is ‘‘false’’ if both inputs are ‘‘false’’ or if both inputs are ‘‘true.’’ Another way of looking at this circuit is to observe that the output is 1 if the inputs are different, but 0 if the inputs are the same.
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There are seven basic logic gates: AND, OR, XOR, NOT, NAND, NOR, and XNOR. The AND gate is so named because, if 0 is called ‘‘false’’ and 1 is called ‘‘true,’’ the gate acts in the same way as the logical ‘‘and’’ operator. The following illustration and table show the circuit symbol and logic combinations for an AND gate. (In the symbol, the input terminals are at left and the output terminal is at right.) The output is ‘‘true’’ when both inputs are ‘‘true.’’ Otherwise, the output is ‘‘false.’’
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A logical inverter, sometimes called a NOT gate to differentiate it from other types of electronic inverter devices, has only one input. It reverses the logic state.
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AND gate Input 1Input 2 0 0 1 1
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N Output 0 0 0 1
The OR gate gets its name from the fact that it behaves after the fashion of the logical inclusive ‘‘or.’’ The output is ‘‘true’’ if either or both of the inputs are ‘‘true.’’ If both inputs are ‘‘false,’’ then the output is ‘‘false.’’
O Inverter or NOT gate Input Output 1 0
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The NAND gate operates as an AND gate followed by a NOT gate. It acts in the manner of the logical operation ‘‘and’’ followed by negation. The output is ‘‘false’’ if both inputs are ‘‘true.’’ Otherwise, the output is ‘‘true.’’
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logical block addressing
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The NOR gate is a combination OR gate followed by an inverter. Its output is ‘‘true’’ if both inputs are ‘‘false.’’ Otherwise, the output is ‘‘false.’’
Logical Link Control layer
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In the Open Systems Interconnection (OSI) model of communication, the Logical Link Control layer is one of two sublayers of the Data-Link layer and is concerned with managing traffic over the physical medium. The Logical Link Control layer identifies a line protocol, such as SDLC, NetBIOS, or NetWare, and may also assign sequence numbers to frames and track acknowledgements.
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NOR gate Input 1Input 2 0 0 1 1
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The XNOR (exclusive-NOR) gate is a combination XOR gate followed by an inverter. Its output is ‘‘true’’ if the inputs are the same, and ‘‘false’’ if the inputs are different.
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Logical block addressing is a technique that allows a computer to address a hard disk larger than 528 megabytes. A logical block address is a 28-bit value that maps to a specific cylinder-head-sector address on the disk. 28 bits allows sufficient variation to specify addresses on a hard disk up to 8.4 gigabytes in data storage capacity. Logical block addressing is one of the defining features of Enhanced IDE (EIDE), a hard disk interface to the computer bus or data paths.
XNOR gate Input 1Input 2 0 0 1 1
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Using combinations of logic gates, complex operations can be performed. In theory, there is no limit to the number of gates that can be arrayed together in a single device. But in practice, there is a limit to the number of gates that can be packed into a given physical space. Arrays of logic gates are found in digital integrated circuits (ICs). As IC technology advances, the required physical volume for each individual logic gate decreases and digital devices of the same or smaller size become capable of performing ever-morecomplicated operations at ever-increasing speeds.
The other Data-Link sublayer is the Media Access Control layer.
logical partition A logical partition (LPAR) is the division of a computer’s processors, memory, and storage into multiple sets of resources so that each set of resources can be operated independently with its own operating system instance and applications. The number of logical partitions that can be created depends on the system’s processor model and resources available. Typically, partitions are used for different purposes such as database operation or client/ server operation or to separate test and production environments. Each partition can communicate with the other partitions as if the other partition is in a separate machine. Logical partitioning was first studied by IBM in 1976 and later introduced by Amdahl and then IBM. Hitachi and Sun Microsystems also use forms of logical partitioning. Today, both IBM’s S/390 (now z/900 Series) and AS/400 products support logical partitioning.
logical unit In IBM’s Systems Network Architecture (SNA), a logical unit (LU) identifies an end user in an SNA network. By end user, IBM means either a human being that is interacting with the network or an application program that is indirectly representing such an end user. Two logical units that communicate depend on physical connections being established through associated physical units (PUs). The network point at each end of a communication that sets up the communication session between logical units is called the system services control point (SSCP). Typically, a logical unit is a unique connection to an application program. A physical unit is usually a hardware device, such as a
lossless and lossy compression terminal control unit. IBM’s Virtual Telecommunication Access Method (VTAM) is used by an application program to manage session and communication requests on behalf of logical units. In SNA, LU 6.2 is a type of logical unit used to represent end users in a distributed processing environment.
logical unit number The logical unit described here is different than the logical unit of IBM’s Systems Network Architecture (SNA). A logical unit number (LUN) is a unique identifier used on a SCSI bus that enables it to differentiate between up to eight separate devices (each of which is a logical unit). Each LUN is a unique number that identifies a specific logical unit, which may be an end user, a file, or an application program. A SCSI (Small System Computer Interface) is a parallel interface, that can have up to eight devices all attached through a single cable; the cable and the host (computer) adapter make up the SCSI bus. The bus allows the interchange of information between devices independently of the host. In the SCSI program, each device is assigned a unique number, which is either a number between 0 and 7 for an 8-bit (narrow) bus, or between 8 and 16 for a 16-bit (wide) bus. The devices that request input/output (I/O) operations are initiators and the devices that perform these operations are targets. Each target has the capacity to connect up to eight additional devices through its own controller; these devices are the logical units, each of which is assigned a unique number for identification to the SCSI controller for command processing.
logon In general computer usage, logon is the procedure used to get access to an operating system or application, usually in a remote computer. Almost always a logon requires that the user have (1) a user ID and (2) a password. Often, the user ID must conform to a limited length such as eight characters and the password must contain at least one digit and not match a natural language word. The user ID can be freely known and is visible when entered at a keyboard or other input device. The password must be kept secret (and is not displayed as it is entered). Some Web sites require users to register in order to use the site; registered users can then enter the site by logging on. Logon is also used as a modifier as in ‘‘logon procedure.’’ The verb form is two words: to log on. In UNIX-based operating systems, logon is called login. The procedure is called ‘‘the login procedure.’’ and the verb form is: to log in.
LOL See ‘‘chat abbreviations’’
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A long-distance carrier is a telephone company that provides connections between local exchanges in different geographic areas. Referred to in the U.S. as interexchange carriers (IXCs), long-distance carriers provides interlocal access and transport area (interLATA) service as described in the Telecommunications Act of 1996. Long-distance carriers include AT&T, MCI, Sprint, and others.
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In computer programming, a loop is a sequence of instructions that is continually repeated until a certain condition is reached. Typically, a certain process is done, such as getting an item of data and changing it, and then some condition is checked such as whether a counter has reached a prescribed number. If it hasn’t, the next instruction in the sequence is an instruction to return to the first instruction in the sequence and repeat the sequence. If the condition has been reached, the next instruction ‘‘falls through’’ to the next sequential instruction or branches outside the loop. A loop is a fundamental programming idea that is commonly used in writing programs. An infinite loop is one in which the programmer assumed that some condition would eventually terminate the loop and this condition failed to happen. The result is that the loop repeats continually until the operating system senses it and terminates the program with an error or until some other event occurs (such as having the program automatically terminate after a certain duration of time).
loopback In telephone systems, a loopback is a test signal sent to a network destination that is returned as received to the originator. The returned signal may help diagnose a problem. Sending a loopback test to each telephone system piece of equipment in succession, one at a time, is a technique for isolating a problem. (The loopback can be compared to the Internet’s ping utility, which lets you send a message out to a host computer on the Internet. The ping echo tells you whether or not the host computer is available and the time the signal took to return.) If you are an ISDN user with more than one B-channel, you can do a loopback test from your computer.
lossless and lossy compression Lossless and lossy compression are terms that describe whether or not, in the compression of a file, all original data can be recovered when the file is uncompressed. With lossless compression, every single bit of data that was originally in the file remains after the file is uncompressed. All of the information is completely restored. This is generally the technique of choice for text or spreadsheet files,
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where losing words or financial data could pose a problem. The Graphics Interchange File (GIF) is an image format used on the Web that provides lossless compression.
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On the other hand, lossy compression reduces a file by permanently eliminating certain information, especially redundant information. When the file is uncompressed, only a part of the original information is still there (although the user may not notice it). Lossy compression is generally used for video and sound, where a certain amount of information loss will not be detected by most users. The JPEG image file, commonly used for photographs and other complex still images on the Web, is an image that has lossy compression. Using JPEG compression, the creator can decide how much loss to introduce and make a trade-off between file size and image quality.
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See ‘‘Shimmer’’
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LPT (line print terminal) is the usual designation for a parallel port connection to a printer or other device on a personal computer. Most PCs come with one or two LPT connections designated as LPT1 and LPT2. Some systems support a third, LPT3. Whatever the number, LPT1 is the usual default. You can add a parallel port for a second printer or other device by buying and adding a parallel port adapter card to your computer. An LPT port can be used for an input device such as QuickCam, a video camera used with CU-SeeMe. Parallel computer connections traditionally have used the Centronics parallel interface for printer communication. A newer standard called EPP/ECP supports the older interface while providing faster communication for a range of devices, including scanners and video cameras.
LPT1 See ‘‘LPT’’
LPT2 See ‘‘LPT’’
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See ‘‘LPT’’
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LRN See ‘‘Location Routing Number’’
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LTO See ‘‘Linear Tape-Open’’
LU 6.2 See ‘‘APPC’’
LUHN formula The LUHN formula, also called modulus 10, is a simple algorithm used to validate the number on a credit card. It works on cards issued by all the major credit card companies, including American Express, Visa, Master Card, Discover, and Diner’s Club. Originally created by a group of mathematicians in the 1960s, the LUHN formula is in the public domain, and anyone can use it. The LUHN formula applies some simple arithemetic to a credit card number to calculate a number that must agree with the check digit, the last number that appears on the credit card. Here are the formula’s three steps: 1. Beginning with the second digit from the end (on the right), take every other digit and multiply it by two. 2. Proceeding right to left, take each of the digits skipped in step 1 and add them to the result digits from step 1. If the result of doubling a number in step 1 resulted in a twodigit number (such as 7 6 7 7 14), use each of these digits (1 and 4) in adding the digits in step 2. 3. Subtract the result obtained in step 2 from the next higher number that ends in 0. The result must agree with the check digit. The LUHN formula can be used by a business to make sure a credit card is valid before sending the number for debit authorization. In Canada, it is used to validate a person’s Social Insurance Number. It does not verify any other information on a credit card, including whether the card’s date is valid.
lumen The lumen (symbolized lm) is the International Unit of luminous flux. It is defined in terms of candela steradians (cd multiplied by sr). One lumen is the amount of light emitted in a solid angle of 1 sr, from a source that radiates to an equal extent in all directions, and whose intensity is 1 cd. One lumen is the equivalent of 1.46 milliwatt (1.46 6 10-3 W) of radiant electromagnetic (EM) power at a frequency of 540 terahertz (540 THz or 5.40 6 1014 Hz). Reduced to SI base units, one lumen is equal to 0.00146 kilogram meter squared per second cubed (1.46 6 10-3 kg multiplied by m2/s3). The lumen is a small unit. An electromagnetic field power level of 1.46 milliwatt is small; the radio-frequency (RF) output of a children’s toy two-way radio is several times that much. A frequency of 540 THz corresponds to a wavelength of about 556 nanometers (nm), which is in the middle of the
LZW compression visible-light spectrum. A steradian is the standard unit solid angle in three dimensions; a sphere encloses 4 pi (approximately 12.57) steradians. Also see candela, steradian, and International System of Units (SI).
luminous intensity Luminous intensity is an expression of the amount of light power emanating from a point source within a solid angle of one steradian. For reference, a frequency of 540 terahertz (540 THz or 5.40 6 1014 Hz) is specified. The quantities used to express luminous intensity are arcane to most nonscientists. A frequency of 540 THz corresponds to a wavelength of about 555 nanometers (nm), which is in the middle of the visible-light spectrum, and is generally accepted as the frequency and wavelength at which the average human eye is most sensitive. A steradian is the standard unit solid angle; a sphere encloses 4p (approximately 12.57) steradians. Decades ago, luminous intensity was measured in terms of a unit called the candle. This expression arose from the fact that one candle represented approximately the amount of visible radiation emitted by a candle flame. This was an inexact specification, because burning candles vary in brilliance. So, for a time, a specified amount of radiation from elemental platinum at its freezing temperature was used as the standard. Late in the 20th century, the candela was defined and adopted as the standard unit of luminous intensity. One candela (1 cd) is the magnitude of an electromagnetic field (EM-field), in a specified direction, that has a power level equivalent to a visible-light field of 1 /683 watt (1.46 6 10-3 W) per steradian at 540 THz. Also see candela, lumen, steradian, and the International System of Units (SI).
LUN See ‘‘logical unit number’’
lurking Lurking is the very common practice of reading an online or e-mail discussion without taking part in the discussion. Most visitors to discussions on Web sites, Usenet groups, e-mail discussion groups, Internet Relay Chat channels, or bulletin board systems understandably spend much more time reading or ‘‘listening’’ than writing or ‘‘speaking.’’ What this term seems to imply in its usage is that some people benefit a great deal from a discussion without ever offering to enrich it with their own information or ideas. It is well understood and accepted, however, that there will always be lurkers in any discussion. And people new to a discussion are sometimes advised to lurk until they become familiar with the discussion.
lux The lux (symbolized lx) is the unit of illuminance in the International System of Units (SI). It is defined in terms of lumens per meter squared (lm/m2). Reduced to SI base units, one lux is equal to 0.00146 kilogram per second cubed (1.46 6 10-3 kg/s3). One lux is the equivalent of 1.46 milliwatt (1.46 6 10-3 W) of radiant electromagnetic (EM) power at a frequency of 540 terahertz (540 THz or 5.40 6 1014 Hz), impinging at a right angle on a surface whose area is one square meter. A frequency of 540 THz corresponds to a wavelength of about 555 nanometers (nm), which is in the middle of the visiblelight spectrum. The lux is a small unit. An alternative unit is the watt per meter squared (W/m2). To obtain lux when the illuminance in watts per meter squared is known, multiply by 683. To obtain watts per meter squared when the illuminance in lux is known, divide by 683 or multiply by 0.00146. Illuminance varies inversely with the square of the distance from the source on a free-space line of sight. If the distance is doubled, the illuminance is cut to 1/4; if the distance increases by a factor of 10, the illuminance becomes 1/100 (0.01 times) as great.
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Also see candela, electromagnetic field, lumen, steradian, watt per meter squared, and International System of Units (SI).
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Lynx is a keyboard-oriented text-only Web browser that was developed at the University of Kansas primarily for students who used UNIX workstations. It has also been rewritten to run on VMS operating systems for users of VT100 terminals. If you use the UNIX shell interface and your access provider offers it, Lynx may be interesting for you since it has a succinct key- (not mouse-) driven user interface. Information about Lynx, including where to download it, is available at the lynx.browser.org Web site.
LZW compression LZW compression is the compression of a file into a smaller file using a table-based lookup algorithm invented by Abraham Lempel, Jacob Ziv, and Terry Welch. Two commonly-used file formats in which LZV compression is used are the GIF image format served from Web sites and the TIFF image format. LZW compression is also suitable for compressing text files. A particular LZW compression algorithm takes each input sequence of bits of a given length (for example, 12 bits) and creates an entry in a table (sometimes called a ‘‘dictionary’’ or ‘‘codebook’’) for that particular bit pattern, consisting of the pattern itself and a shorter code. As input is read, any pattern that has been read before results in the substitution
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Whatis?Com’s Encyclopedia of Technology Terms of the shorter code, effectively compressing the total amount of input to something smaller. Unlike earlier approaches, known as LZ77 and LZ78, the LZW algorithm does include the look-up table of codes as part of the compressed file. The decoding program that uncompresses the file is able to build the table itself by using the algorithm as it processes the encoded input.
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MAC address On a local area network (LAN) or other network, the MAC (Media Access Control) address is your computer’s unique hardware number. (On an Ethernet LAN, it’s the same as your Ethernet address.) When you’re connected to the Internet from your computer (or host as the Internet protocol thinks of it), a correspondence table relates your IP address to your computer’s physical (MAC) address on the LAN. The MAC address is used by the Media Access Control sublayer of the Data-Link Control (DLC) layer of telecommunication protocols. There is a different MAC sublayer for each physical device type. The other sublayer level in the DLC layer is the Logical Link Control sublayer.
Mac See ‘‘Macintosh’’
Mac OS Mac OS is the computer operating system for Apple Computer’s Macintosh line of personal computers and workstations. A popular feature of its latest version, Mac OS X, is a desktop interface with some 3-D appearance characteristics. OS X has a modular design intended to make it easier to add new features to the operating system in the future. It runs UNIX applications as well as older Mac applications. Mac OS comes with Apple Computer’s iMac and Power Macintosh line of computers.
machine code Machine code is the elemental language of computers, consisting of a stream of 0’s and 1’s. Ultimately, the output of any programming language analysis and processing is machine code. After you write a program, your source language statements are compiled or (in the case of assembler language) assembled into output that is machine code. This machine code is stored as an executable file until someone tells the computer’s operating system to run it. (In personal computer operating systems, these files often have the suffix of ‘‘.exe’’.) The computer’s microprocessor reads in and handles a certain number of 0’s and 1’s at a time. For example, it may be designed to read 32 bits of 0’s and 1’s at a time. Because it is designed to know how many bits (and which bits) tell it what operation to do, it can look at the right sequence of bits and perform the next operation. Then it reads the next instruction, and so on.
In analyzing problems or debugging programs, a tool to use is a dump of the program. A dump is a printout that shows the program in its machine code form, but since putting it in 0’s and 1’s would be hard to read, each four bits (of 0’s and 1’s) is represented by a single hexadecimal numeral. (Dumps also contain other information about the computer’s operation, such as the address of the instruction that was being executed at the time the dump was initiated.)
machine vision Machine vision is the ability of a computer to ‘‘see.’’ A machine-vision system employs one or more video cameras, analog-to-digital conversion (ADC), and digital signal processing (DSP). The resulting data goes to a computer or robot controller. Machine vision is similar in complexity to voice recognition. Two important specifications in any vision system are the sensitivity and the resolution. Sensitivity is the ability of a machine to see in dim light, or to detect weak impulses at invisible wavelengths. Resolution is the extent to which a machine can differentiate between objects. In general, the better the resolution, the more confined the field of vision. Sensitivity and resolution are interdependent. All other factors held constant, increasing the sensitivity reduces the resolution, and improving the resolution reduces the sensitivity.
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Human eyes are sensitive to electromagnetic wavelengths ranging from 390 to 770 nanometers (nm). Video cameras can be sensitive to a range of wavelengths much wider than this. Some machine-vision systems function at infrared (IR), ultraviolet (UV), or X-ray wavelengths.
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Binocular (stereo) machine vision requires a computer with an advanced processor. In addition, high-resolution cameras, a large amount of random access memory (RAM), and an artificial intelligence (AI) program are required for depth perception.
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Materials inspection
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The Macintosh (often called ‘‘the Mac’’), introduced in 1984 by Apple Computer, was the first widely-sold personal computer with a graphical user interface (GUI). The Mac was designed to provide users with a natural, intuitively understandable, and, in general, ‘‘user-friendly’’ computer interface. Many of the user interface ideas in the Macintosh derived from experiments at the Xerox Parc laboratory in the early 1970s, including the mouse, the use of icons or small visual images to represent objects or actions, the point-andclick and click-and-drag actions, and a number of window operation ideas. Microsoft was successful in adapting user interface concepts first made popular by the Mac in its first Windows operating system.
A macro virus is a computer virus that ‘‘infects’’ a Microsoft Word or similar application and causes a sequence of actions to be performed automatically when the application is started or something else triggers it. Macro viruses tend to be surprising but relatively harmless. A typical effect is the undesired insertion of some comic text at certain points when writing a line. A macro virus is often spread as an email virus. A well-known example in March 1999 was the Melissa virus.
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The Macintosh has its own operating system, Mac OS. Originally built on Motorola’s 68000 series microprocessors, Mac versions today are powered by the PowerPC microprocessor, which was developed jointly by Apple, Motorola, and IBM. The Mac is actually a line of personal computers, configured for individual users and businesses with different needs. A recent product, iMac, provides the Mac technology and interface in a low-cost package. While Mac users represent only about 5% of the total numbers of personal computer users, Macs are highly popular and almost a cultural necessity among graphic designers and online visual artists and the companies they work for. In general, Mac users tend to be enthusiasts.
macro 1) In Microsoft Word and other programs, a macro is a saved sequence of commands or keyboard strokes that can be stored and then recalled with a single command or keyboard stroke. 2) In computers, a macro (for ‘‘large’’; the opposite of ‘‘micro’’) is any programming or user interface that, when used, expands into something larger. The original use for ‘‘macro’’ or ‘‘macro definition’’ was in computer assembler language before higher-level, easier-to-code languages became more common. In assembler language, a macro definition defines how to expand a single language statement or computer instruction into a number of instructions. The macro statement contains the name of the macro definition and usually some variable parameter information. Macros were (and are) useful especially when a sequence of instructions is used a number of times (and possibly by different programmers working on a project). Some pre-compilers also use the macro concept. In general, however, in higher-level languages, any language statement is about as easy to write as an assembler macro statement. Assembler macros generate instructions inline with the rest of a program. More elaborate sequences of instructions that are used frequently by more than one program or programmer are encoded in subroutines that can be branched to from or assembled into a program.
MAE A MAE (pronounced MAY), originally an abbreviation for Metropolitan Area Exchange and now a service mark of MCI WorldCom, is a major center in the United States for switch traffic between Internet service providers (ISP). There are two major MAEs, MAE-East in the Washington, D.C. area and MAE-West in the San Jose, California area. MAE-East interconnects all of the major ISPs and also those from Europe. MAE-West interconnects ISPs in the Silicon Valley area. These two points along with several interconnection points previously identified by the National Science Foundation as network access points (NAPs) form what is sometimes considered the national commercial Internet backbone. In addition to MAE-East and MAE-West, there are five regional ‘‘Tier-2’’ MAEs: in Chicago, Dallas, Houston, Los Angeles, and New York. Additional MAEs appear to be in the planning stages. The MAEs and their services, originally developed by MFS Communications, are now owned and operated by MCI WorldCom. A MAE can be viewed as a giant local area network (LAN) switch. (In fact, the two major MAEs use an Fiber Distributed-Data Interface switch.) The only ISP device that can interconnect to a MAE switch is a router or a computer host acting as a router. The ISPs work out their own peering agreements and manage their own routing tables. Routers at the two major MAEs need very large routing tables. Cisco’s 7xxx series routers are examples of such routers. The regional MAEs consist of an Ethernet switch and an FDDI concentrator. Smaller routers (such as Cisco’s 4500-M) are required. The MAEs offer colocation space for ISP equipment on their premises.
magnetic field A magnetic field is generated when electric charge carriers such as electrons move through space or within an electrical conductor. The geometric shapes of the magnetic flux lines produced by moving charge carriers (electric current) are similar to the shapes of the flux lines in an electrostatic field. But there are differences in the ways electrostatic and magnetic fields interact with the environment.
main storage Electrostatic flux is impeded or blocked by metallic objects. Magnetic flux passes through most metals with little or no effect, with certain exceptions, notably iron and nickel. These two metals, and alloys and mixtures containing them, are known as ferromagnetic materials because they concentrate magnetic lines of flux. An electromagnet provides a good example. An air-core coil carrying direct current produces a magnetic field. If an iron core is substituted for the air core in a given coil, the intensity of the magnetic field is greatly increased in the immediate vicinity of the coil. If the coil has many turns and carries a large current, and if the core material has exceptional ferromagnetic properties, the flux density near the ends of the core (the poles of the magnet) can be such that the electromagnet can be used to pick up and move cars. When charge carriers are accelerated (as opposed to moving at constant velocity), a fluctuating magnetic field is produced. This generates a fluctuating electric field, which in turn produces another varying magnetic field. The result is a ‘‘leapfrog’’ effect, in which both fields can propagate over vast distances through space. Such a synergistic field is known as an electromagnetic field. This is the phenomenon that makes wireless communications and broadcasting possible.
magnetic permeability See ‘‘permeability’’
magneto-optical drive The magneto-optical (MO) drive is a popular way to back up files on a personal computer. As the term implies, an MO device employs both magnetic and optical technologies to obtain ultra-high data density. A typical MO cartridge is slightly larger than a conventional 3.5-inch magnetic diskette, and looks similar. But while the older type of magnetic diskette can store 1.44 megabytes (MB) of data, an MO diskette can store many times that amount, ranging from 100 MB up to several gigabytes (GB).
limitation of MO drives is that they are slower than hard disk drives, although they are usually faster than conventional 3.5-inch diskette drives.
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magnetoresistive head technology Magnetoresistive (MR) head technology is used in disk drives to allow higher storage densities than the older inductive-head technology. MR heads provide up to four times the storage densities possible with thin-film inductive heads. Almost all of the major storage vendors now sell drives based on this technology, which started to become popular in the late 1990s. MR heads have separate read/write elements—with each optimized for its particular function. With inductive-head drives, one head performs both tasks, leading to some performance problems as it tries to read and write data simultaneously. MR drives also have less noise, meaning fewer corrective reads or writes.
magnetoresistive RAM See ‘‘MRAM’’
mail bomb A mail bomb is the sending of a massive amount of e-mail to a specific person or system. A huge amount of mail may simply fill up the recipient’s disk space on the server or, in some cases, may be too much for a server to handle and may cause the server to stop functioning. In the past, mail bombs have been used to ‘‘punish’’ Internet users who have been egregious violators of netiquette (for example, people using e-mail for undesired advertising, or spam).
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Mail bombs not only inconvenience the intended target but they are also likely to inconvenience everybody using the server. Senders of mail bombs should be wary of exposing themselves to reciprocal mail bombs or to legal actions.
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An MO system achieves its high data density by using a laser and a magnetic read/write head in combination. Both the laser and the magnet are used to write data onto the diskette. The laser heats up the diskette surface so it can be easily magnetized, and also to allow the region of magnetization to be precisely located and confined. A less intense laser is used to read data from the diskette. Data can be erased and/or overwritten an unlimited number of times, as with a conventional 3.5-inch diskette.
A mailing list is a list of people who subscribe to a periodic mailing distribution on a particular topic. On the Internet, mailing lists include each person’s e-mail address rather than a postal address. Mailing lists have become a popular way for Internet users to keep up with topics they’re interested in. Many software producers and other vendors are now using them as a way to keep in touch with customers.
Examples of magneto-optical drives are the Fujitsu DynaMO, a 230 MB drive used in the PowerPC Apple Powerbook, a note book computer, and the Pinnacle Micro Vertex, a 2.6 GB drive.
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The chief assets of MO drives include convenience, modest cost, reliability, and (for some models) widespread availability approaching industry standardization. The chief
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Main storage is the main area in a computer in which data is stored for quick access by the computer’s processor. This term originated in the days of the mainframe computer to distinguish the more immediately accessible data storage from auxiliary storage. On today’s computers, especially personal computers and workstations, the term random
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Whatis?Com’s Encyclopedia of Technology Terms access memory (RAM) is usually used instead of main storage, and the hard disk, diskette, and CD-ROM collectively describe auxiliary storage.
commands and names it ‘‘makefile.’’ The commands are executed according to the rules in the makefile when the user types ‘‘make’’ while in the directory containing the file.
An earlier term for main storage was core in the days when the main data storage device contained ferrite cores.
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mainframe Mainframe is an industry term for a large computer, typically manufactured by a large company such as IBM for the commercial applications of Fortune 1000 businesses and other large-scale computing purposes. Historically, a mainframe is associated with centralized rather than distributed computing. Today, IBM refers to its larger processors as large servers and emphasizes that they can be used to serve distributed users and smaller servers in a computing network.
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Like listserv, Majordomo (from Latin: ‘‘master of the house’’) is a small program that automatically redistributes e-mail to names on a mailing list. Users can subscribe to a mailing list by sending an e-mail note to a mailing list they learn about; Majordomo will automatically add the name and distribute future e-mail postings to every subscriber. (Requests to subscribe and unsubscribe are sent to a special address so that other subscribers do not see these requests.) Majordomo is written in the Perl language. Although it originated in the UNIX culture, Majordomo can be run on any operating system platform with a Perl interpreter.
makefile A makefile is used with the UNIX make utility to determine which portions of a program to compile. A makefile is basically a script that guides the make utility to choose the appropriate program files that are to be compiled and linked together. The make utility keeps track of the last time files were updated so that it only updates the files containing changes. However, all of the files that are dependent on the updated files must be compiled as well, which can be very timeconsuming. With the help of makefile, the make utility automates this compilation to ensure that all files that have been updated—and only those—are compiled and that the most recent versions of files are the ones linked to the main program, without requiring the user to perform the tasks separately. A makefile contains three types of information for the make program: A target (the name of what the user is trying to construct); the rules (commands that tell how to construct the target from the sources) and a dependency (the reason that the target should be constructed, which is usually because it is out of date in respect to its components). To create a makefile, the user makes a file containing shell
See ‘‘metropolitan area network’’
man in the middle See ‘‘bucket brigade’’
managed service provider Also see management service provider. A managed service provider (MSP) provides delivery and management of network-based services, applications, and equipment to enterprises, residences, or other service providers. Managed service providers can be hosting companies or access providers that offer services that can include fully outsourced network management arrangements, including advanced features like IP telephony, messaging and call center, virtual private network (VPNs), managed firewalls, and monitoring/reporting of network servers. Most of these services can be performed from outside a company’s internal network with a special emphasis placed on integration and certification of Internet security for applications and content. MSPs serve as outsourcing agents for companies, especially other service providers like ISPs, that don’t have the resources to constantly upgrade or maintain faster and faster computer networks. In addition to such basic communication service as leased line wide area network (WAN) and frame relay service, an MSP can manage and integrate a range of activities associated with enterprise networks. The range of outsourcing services includes basic transport and access, managed premises, Web hosting, VPN, unified messaging, video networking, or other more sophisticated services. The market for managed services is forecast to grow about 20 percent annually, according to The Yankee Group, due largely to the need for enterprises to be more flexible and timely in getting to market and communicating with customers. Nortel Networks, Lucent, and Checkpoint are three of the best-known companies that provide MSP services. Managed service providers sometimes are referred to as management service providers, which also manage information technology services for companies. However, some industry experts say managed service providers provide a broader range of services than management service providers, which tend to limit themselves to monitoring services for servers, routers, firewalls, and other applications. Management service providers typically deliver infrastructure management services on a subscription basis, similar to the model used by application
maps service providers (ASPs). They most commonly offer network- and application-monitoring services to ebusinesses.
management information base A management information base (MIB) is a formal description of a set of network objects that can be managed using the Simple Network Management Protocol (SNMP). The format of the MIB is defined as part of the SNMP. MIB-I refers to the initial MIB definition; MIB-II refers to the current definition. SNMPv2 includes MIB-II and adds some new objects. There are MIBs (or more accurately, MIB extensions) for each set of related network entities that can be managed. For example, there are MIB definitions specified in the form of Requests for Comments (RFCs) for AppleTalk, domain name system (DNS), Fiber Distributed-Data Interface, and RS-232C network objects. Product developers can create and register new MIB extensions. Companies that have created MIB extensions for their sets of products include Cisco, Fore, IBM, Novell, QMS, and Onramp.
management information systems See ‘‘MIS’’
management service provider Also see managed service provider. On the Internet, a management service provider (MSP) is a company that manages information technology services for other companies. For example, a company could hire an MSP to configure and administer its business computers and related systems on a continuing basis, saving the company the need to hire, train, and pay its own administrators. Since almost all such systems today can be managed remotely using an interactive Web page as the user interface, the idea is that an administrator could just as easily be someone at an MSP as at your own company. The MSP can be viewed as similar to an application service provider (ASP), a company that provides one or more computer applications that can be used by other companies using an online interface such as a page at the ASP’s Web site. An ASP tends to become an MSP to the extent that it combines its application offering with additional service offerings. Another related service is the storage service provider (SSP), a company that provides computer storage and related services (such as automatic scheduled backup and archiving) for other companies. Some management service providers include Candle, InteQ, and McAfee.
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MAPI (Messaging Application Program Interface) is a Microsoft Windows program interface that enables you to send e-mail from within a Windows application and attach the document you are working on to the e-mail note. Applications that take advantage of MAPI include word processors, spreadsheets, and graphics applications. MAPIcompatible applications typically include a Send Mail or Send in the File pulldown menu of the application. Selecting one of these sends a request to a MAPI server. MAPI consists of a standard set of C language functions that are stored in a program library known as a dynamic link library (DLL). Developers who are using Microsoft’s Active Server Page (ASP) technology access the MAPI library by using Microsoft’s Collaboration Data Objects (CDO). The CDO library comes with Microsoft’s Internet Information Server (IIS). MAPI functions can be accessed by Visual Basic developers through a Basic-to-C translation layer. Eudora, one of the most popular e-mail programs, includes a MAPI server. To activate the Eudora MAPI server, select Options from the Tools pulldown menu and then doubleclick on MAPI and specify whether MAPI should always be active or whether it should be active only when Eudora is running. Microsoft Exchange and possibly some other programs may require that you turn the MAPI server off when they are in use.
maps A good map site is...
Get a map of your (or someone else’s) town, neighborhood, or street on ‘‘Find’’
MapQuest (http:// www.mapquest.com), enter your street address, and click Mapblast (http://www. mapblast.com) offers a similar service MapQuest (http:// www.mapquest.com) and select ‘‘TripQuest’’ Mapblast (http://www. mapblast.com) offers a similar service MapQuest (http://www. mapquest.com) Mapblast (http://www. mapblast.com) offers a similar service Java Online (http://www. indo.com/distance) (uses a University of Michigan database and calculator)
Create a personal map for your Web site
Find the great circle distance from one city to another
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Get detailed driving directions from one city to another in the U.S. Canada, and Mexico
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A good map site is...
Get 1990 U.S. census information; see and download a city map (which can be included in a Web site) Order U.S. Geological Survey maps and other products
U. S. Gazetteer (http:// tiger.census.gov/cgi-bin/ gazetteer)
U. S. Geological Service (http://www.usgs.gov) Or call 1-800-USA-MAPS.
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Marcom (sometimes spelled ‘‘marcomm’’) is an abbreviation for ‘‘marketing communications.’’ Marcom is targeted interaction with customers and prospects using one or more media, such as direct mail, newspapers and magazines, television, radio, billboards, telemarketing, and the Internet. A marketing communications campaign may use a single approach, but more frequently combines several.
markup
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Markup refers to the sequence of characters or other symbols that you insert at certain places in a text or word processing file to indicate how the file should look when it is printed or displayed or to describe the document’s logical structure. The markup indicators are often called ‘‘tags.’’ For example, this particular paragraph is preceded by a: (or paragraph tag) so that it will be separated by an empty line from the preceding line. There is now a standard markup definition for document structure (or really a description of how you can define markup) in the Standard Generalized Markup Language (SGML). Markup can be inserted by the document creator directly by typing the symbols in, by using an editor and selecting prepackaged markup symbols (to save keystrokes), or by using a more sophisticated editor that lets you create the document as you want it to appear (this is called a WYSIWYG editor).
marshalling Originally, to marshall was to tend horses or to arrange things in preparation for a feast. In heraldry, marshalling is the arrangement of several coats of arms to form a single composition. In the military, marshalling is the gathering and ordering of military forces in preparation for battle. In computer programming, marshalling is the process of gathering data from one or more applications or noncontiguous sources in computer storage, putting the data pieces into a message buffer, and organizing or converting the data into a format that is prescribed for a particular receiver or programming interface.
Marshalling is usually required when passing the output parameters of a program written in one language as input to a program written in another language.
masquerade In general, a masquerade is a disguise. In terms of communications security issues, a masquerade is a type of attack where the attacker pretends to be an authorized user of a system in order to gain access to it or to gain greater privileges than they are authorized for. A masquerade may be attempted through the use of stolen logon IDs and passwords, through finding security gaps in programs, or through bypassing the authentication mechanism. The attempt may come from within an organization, for example, from an employee; or from an outside user through some connection to the public network. Weak authentication provides one of the easiest points of entry for a masquerade, since it makes it much easier for an attacker to gain access. Once the attacker has been authorized for entry, they may have full access to the organization’s critical data, and (depending on the privilege level they pretend to have) may be able to modify and delete software and data, and make changes to network configuration and routing information.
mass Mass (symbolized m) is a dimensionless quantity representing the amount of matter in a particle or object. The standard unit of mass in the International System (SI) is the kilogram (kg). Mass is measured by determining the extent to which a particle or object resists a change in its direction or speed when a force is applied. Isaac Newton stated: A stationary mass remains stationary, and a mass in motion at a constant speed and in a constant direction maintains that state of motion, unless acted on by an outside force. For a given applied force, large masses are accelerated to a small extent, and small masses are accelerated to a large extent. The following formula applies: F = ma where F is the applied force in newtons, m is the mass of the object or particle in kilograms, and a is the resulting acceleration in meters per second squared. The mass of an object can be calculated if the force and the acceleration are known. Mass is not the same thing as weight. Weight has meaning only when an object having a specific mass is placed in an acceleration field, such as the gravitational field of the earth. At the earth’s surface, a kilogram mass weighs about 2.2 pounds, for example. But on Mars, the same kilogram mass would weigh only about 0.8 pounds, and on Jupiter it would weigh roughly 5.5 pounds.
matrix When expressing large or small masses, prefix multipliers are used. The table below shows the most common alternative mass units and their relationship to the kilogram. Unit To convert to Conversely, (and kilograms, multiply by: symbol) multiply by: metric ton (T) gram (g) milligram (mg) microgram (mg) nanogram (ng) picogram (pg)
1000 0.001 10-6 10-9 10-12 10-15
0.001 1000 106 109 1012 1015
massively parallel processing MPP (massively parallel processing) is the coordinated processing of a program by multiple processors that work on different parts of the program, using their own operating systems and memory. Typically, MPP processors communicate using some messaging interface. An ‘‘interconnect’’ arrangement of data paths allows messages to be sent between processors. Typically, the setup for MPP is more complicated, requiring thought about how to partition a common database among processors and how to assign work among the processors. An MPP system is also known as a ‘‘loosely coupled’’ or ‘‘shared nothing’’ system. MPP systems are considered better than SMP systems for applications that allow a number of databases to be searched in parallel. These include decision support system and data warehouse applications.
Master Boot Record The Master Boot Record (MBR) is the information in the first sector of any hard disk or diskette that identifies how and where an operating system is located so that it can be booted (loaded) into the computer’s main storage or random access memory. The Master Boot Record is also sometimes called the ‘‘partition sector’’ or the ‘‘master partition table’’ because it includes a table that locates each partition that the hard disk has been formatted into. In addition to this table, the MBR also includes a program that reads the boot sector record of the partition containing the operating system to be booted into RAM. In turn, that record contains a program that loads the rest of the operating system into RAM.
material density Material density, more often referred to simply as density, is a quantitative expression of the amount of mass contained per unit volume. The standard unit is the kilogram per meter cubed (kg/m3 or kg ˙ m-3). Density is sometimes expressed in grams per centimeter cubed (g/cm3 or gm ˙ cm-3). Pure liquid water at a temperature of 4 degrees Celsius has a density of 1 g/cm3, which is the equivalent of 1 kg per 1000 cm3, or 1 kilogram
per liter. To convert from kg/m3 to g/cm3, multiply by 0.001. Conversely, to convert from g/cm3 to kg/m3, multiply by 1000. Occasionally, density is expressed in unusual units such as pounds per cubic foot, pounds per cubic inch, or metric tons per cubic meter. Expressions such as this can be confusing unless reduced to standard form for comparison. Sometimes density is expressed in relative terms as the specific gravity (sp gr), which is the ratio of the density of a given substance to the density of pure liquid water at 4 degrees Celsius. Also see kilogram per meter cubed, specific gravity, and SI (International System of Units).
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MathML MathML is an application of XML (Extensible Markup Language) designed to facilitate the use of mathematical expressions in Web pages. Historically, mathematical content has been difficult to portray because standard typographic character sets do not provide for them and because they sometimes exceed a line width in size. On Web sites, GIF or JPG images must usually be created to portray all but the simplest mathematical expressions. This works, but it does not always look professional, and when the HTML files are saved and then viewed offline, the images do not show up unless they have been individually downloaded and their tags have been made consistent with the offline file. MathML allows the insertion of mathematical expressions within an HTML file as special data. The browser (which must support MathML) can then display the appropriate symbol or expression. For mathematicians, engineers, and scientists familiar with TeX or LaTeX, MathML can be thought of in this way: It is to HTML as TeX or LaTeX is to plain text. In fact, conversion programs are available that can be used to generate MathML documents from TeX or LaTeX documents. MathML is in the developmental stages. The ultimate goal is to render mathematical documents directly viewable in, and printable from, popular Web browsers such as Netscape or Microsoft Internet Explorer. Until that ideal is realized, the viewing of mathematical content in Web sites generally requires specialized plug-ins.
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See ‘‘MathML’’
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See ‘‘matrix’’
matrix 1) Apart from information technology, matrix (pronounced MAY-triks) has a number of special meanings. From the Latin word for womb (in turn from mater or mother), a
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Whatis?Com’s Encyclopedia of Technology Terms matrix is either the intercellular substance of a tissue, the material in which a fossil is embedded, or a mold from which a relief surface is made in printing or phonograph manufacturing. 2) In mathematics and computer science, a matrix is a set of numbers laid out in tabular form (in rows and columns). From this meaning, a less formal meaning is derived of a complex of lines intersecting at right angles. 3) In cyberculture, the Internet and other networks that flow into it are altogether sometimes called ‘‘the matrix. In William Gibson’s science-fiction novel, Neuromancer, the ‘‘matrix’’ is a vast sea of computing resources that can be visualized by the user, is accessible at many levels, and is lit up more intensely in the areas of greatest activity. The hero, Case, ‘‘jacks in’’ to the matrix through wiring that is (perhaps, since it’s not entirely clear) integrated with his brain and explores the matrix with a ‘‘deck’’ or computer console that provides a holographic view.
maximum transmission unit An MTU (maximum transmission unit) is the largest size packet or frame, specified in octets (eight-bit bytes), that can be sent in a packet- or frame-based network (such as a network using TCP/IP). Transmission Control Protocol uses the MTU to determine the size of each packet in any transmission. Too large an MTU size may mean retransmissions if the packet encounters a router that can’t handle that large a packet. Too small an MTU size means relatively more header overhead and more acknowledgements that have to be sent and handled. Most computer operating system provide a default MTU value that is suitable for most users. However, some users may need to change the MTU value. In general, Internet users should follow the advice of their Internet service provider (ISP) about whether to change the default value and what to change it to. For Windows 95 users, the default MTU is 1500 octets (eightbit bytes), partly because this is the Ethernet standard MTU. The Internet de facto standard MTU is 576, but ISPs often suggest using 1500. If you frequently access Web sites that encounter routers with an MTU size of 576, you may want to change to that size. (Apparently some users find that changing the setting to 576 improves performance and others do not find any improvement.) The minimum value that an MTU can be set to is 68. For Windows 98 users, the operating system is able to sense whether your connection should use 1500 or 576 and select the appropriate MTU for the connection. The default is ‘‘Automatic.’’ The user can also explicitly set the packet size to 1500, 1000, or 576. For protocols other than TCP, different MTU sizes may apply.
MB See ‘‘megabyte’’
MBone The MBone, now sometimes called the Multicast Internet, is an arranged use of a portion of the Internet for Internet Protocol (IP) multicasting (sending files—usually audio and video streams—to multiple users at the same time somewhat as radio and TV programs are broadcast over airwaves). Although most Internet traffic is unicast (one user requesting files from one source at another Internet address), the Internet’s IP protocol also supports multicasting, the transmission of data packets intended for multiple addresses. Since most IP servers on the Internet do not currently support the multicasting part of the protocol, the MBone was set up to form a network within the Internet that could transmit multicasts. The MBone was set up in 1994 as an outgrowth of earlier audio multicasts by the Internet Engineering Task Force (IETF) and has multicast a number of programs, including some well-publicized rock concerts. The MBone consists of known servers (mostly on UNIX workstations) that are equipped to handle the multicast protocol. Tunneling is used to forward multicast packets through routers on the network that don’t handle multicasting. An MBone router that is sending a packet to another MBone router through a non-MBone part of the network encapsulates the multicast packet as a unicast packet. The non-MBone routers simply see an ordinary packet. The destination MBone router unencapsulates the unicast packet and forwards it appropriately. The MBone consists of a backbone with a mesh topology which is used by servers that redistribute the multicast in their region in a star topology. The MBone network is intended to be global and includes nodes in Europe. The channel bandwidth for MBone multicasts is 500 kilobits per second and actual traffic is from 1007300 kilobits depending on content. MBone multicasts usually consist of streaming audio and video.
Mbps Mbps stands for millions of bits per second or megabits per second and is a measure of bandwidth (the total information flow over a given time) on a telecommunications medium. Depending on the medium and the transmission method, bandwidth is sometimes measured in the Kbps (thousands of bits or kilobits per second) range or the Gbps (billions of bits or gigabits per second) range. A megabit is a million binary pulses, or 1,000,000 (that is, 106) pulses (or ‘‘bits’’). For example, a U.S. phone company T-carrier system line is said to sustain a data rate of 1.544 megabits per second. Megabits per second is usually shortened to Mbps.
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Service/retail, as consumers are given the ability to place and pay for orders on-the-fly
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Information services, which include the delivery of financial news, sports figures and traffic updates to a single mobile device
Some sources define a megabit to mean 1,048,576 (that is, 220) bits. Although the bit is a unit of the binary number system, bits in data communications are discrete signal pulses and have historically been counted using the decimal number system. For example, 28.8 kilobits per second (Kbps) is 28,800 bits per second. Because of computer architecture and memory address boundaries, bytes are always some multiple or exponent of two. See kilobyte, etc.
IBM and other companies are experimenting with speech recognition software as a way to ensure security for mcommerce transactions.
MBR
MCSE
See ‘‘Master Boot Record’’
See ‘‘Microsoft Certified Systems Engineer’’
MCA
MD2
See ‘‘Micro Channel Architecture’’
MD2 is an earlier, 8-bit version of MD5, an algorithm used to verify data integrity through the creation of a 128-bit message digest from data input (which may be a message of any length) that is claimed to be as unique to that specific data as a fingerprint is to the specific individual. MD2, which was developed by Professor Ronald L. Rivest of MIT, is intended for use with digital signature applications, which require that large files must be compressed by a secure method before being encrypted with a secret key, under a public key cryptosystem. According to the RFC document, it is ‘‘computationally infeasible’’ that any two messages that have been input to the MD5 algorithm could have as the output the same message digest, or that a false message could be created through apprehension of the message digest. MD2, MD4 (a later version), and MD5, the latest version, have similar structures, but MD2 was optimized for 8-bit machines, in comparison with the two later formulas, which are optimized for 32-bit machines. The MD5 algorithm is an extension of MD4, which the critical review found to be fast, but possibly not absolutely secure. In comparison, MD5 is not quite as fast as the MD4 algorithm, but offers much more assurance of data security.
MCM See ‘‘multi-carrier modulation’’
m-commerce M-commerce (mobile commerce) is the buying and selling of goods and services through wireless handheld devices such as cellular telephones and personal digital assistants (PDAs). Known as next-generation e-commerce, mcommerce enables users to access the Internet without needing to find a place to plug in. The emerging technology behind m-commerce, which is based on the Wireless Application Protocol (WAP), has made far greater strides in Europe, where mobile devices equipped with Web-ready micro-browsers are much more common than in the U.S. In order to exploit the m-commerce market potential, handset manufacturers such as Nokia, Ericsson, Motorola, and Qualcomm are working with carriers such as AT&T Wireless and Sprint to develop WAP-enabled smart phones, the industry’s answer to the Swiss Army Knife, and ways to reach them. Using Bluetooth technology, smart phones offer fax, e-mail, and phone capabilities all in one, paving the way for m-commerce to be accepted by an increasingly mobile workforce. As content delivery over wireless devices becomes faster, more secure, and scalable, there is wide speculation that mcommerce will surpass wireline e-commerce as the method of choice for digital commerce transactions. The industries affected by m-commerce include: L
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Financial services, which includes mobile banking (when customers use their handheld devices to access their accounts and pay their bills) as well as brokerage services, in which stock quotes can be displayed and trading conducted from the same handheld device Telecommunications, in which service changes, bill payment and account reviews can all be conducted from the same handheld device
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MD4 MD4 is an earlier version of MD5, an algorithm used to verify data integrity through the creation of a 128-bit message digest from data input (which may be a message of any length) that is claimed to be as unique to that specific data as a fingerprint is to the specific individual. MD4, which was developed by Professor Ronald L. Rivest of MIT, is intended for use with digital signature applications, which require that large files must be compressed by a secure method before being encrypted with a secret key, under a public key cryptosystem. According to the MD5 specification, it is ‘‘computationally infeasible’’ that any two messages that have been input to the MD5 algorithm could have as the output the same message digest, or that a false message could be created through apprehension of the message digest. MD2, an earlier 8-bit version, MD4, and MD5, the latest version, have similar structures, but MD2 was optimized for 8-bit machines, in comparison with the two
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later formulas, which are optimized for 32-bit machines. The MD5 algorithm is an extension of MD4, which the critical review found to be fast, but possibly not absolutely secure. In comparison, MD5 is not quite as fast as the MD4 algorithm, but offers much more assurance of data security.
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In the Open Systems Interconnection (OSI) model of communication, the Media Access Control layer is one of two sublayers of the Data Link Control layer and is concerned with sharing the physical connection to the network among several computers. Each computer has its own unique MAC address. Ethernet is an example of a protocol that works at the Media Access Control layer level.
MD5 is a digital signature algorithm that is used to verify data integrity through the creation of a 128-bit message digest from data input (which may be a message of any length) that is claimed to be as unique to that specific data as a fingerprint is to the specific individual. MD5, which was developed by Professor Ronald L. Rivest of MIT, is intended for use with digital signature applications, which require that large files must be compressed by a secure method before being encrypted with a secret key, under a public key cryptosystem. MD5 is currently a standard, Internet Engineering Task Force (IETF) Request for Comments (RFC) 1321. According to the standard, it is ‘‘computationally infeasible’’ that any two messages that have been input to the MD5 algorithm could have as the output the same message digest, or that a false message could be created through apprehension of the message digest. MD5 is the third message digest algorithm created by Rivest. All three (the others are MD2 and MD4) have similar structures, but MD2 was optimized for 8-bit machines, in comparison with the two later formulas, which are optimized for 32-bit machines. The MD5 algorithm is an extension of MD4, which the critical review found to be fast, but possibly not absolutely secure. In comparison, MD5 is not quite as fast as the MD4 algorithm, but offers much more assurance of data security.
MDI See ‘‘Multiple Document Interface’’
MDRAM (Multibank Dynamic RAM)
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MDRAM (Multibank Dynamic RAM) is a type of video RAM, developed by MoSys, that divides memory into multiple 32-kilobyte parts or ‘‘banks’’ that can be accessed individually. Traditional video RAM is monolithic; the entire frame buffer is accessed at one time. Having individual memory banks allows accesses to be interleaved concurrently, increasing overall performance. It’s also cheaper since, unlike other forms of video RAM, cards can be manufactured with just the right amount of RAM for a given resolution capability instead of requiring it to be in multiples of megabytes.
meaning of life See ‘‘42’’
See ‘‘streaming video’’
Media Access Control layer
The other Data Link Control sublayer is the Logical Link Control layer.
media access management In the Open Systems Interconnection (OSI) communication reference model, media access management is performed by the Media Access Control (MAC) sublayer of the Data-Link layer. In an Ethernet network transmission, the function of media access management is to determine whether the transmission medium is free and available to send a frame. Media access management receives the frame from the Carrier Sense Multiple Access/Collision Detect (CSMA/CD) sublayer. It seeks to avoid collision with other traffic on the medium by monitoring the Carrier Sense Signal (CSNS) provided by the Physical Layer Signaling (PLS). Once media access management is satisfied that the transmission medium is available, transmission of the frame to the PLS can begin. If a collision does take place, media access management determines what action should be taken and when to try to send the frame again. Media access management is also responsible for checking a frame to determine its validity before passing it on to the data decapsulation function.
media attachment unit MAU is also sometimes used as the abbreviation for the token ring network multistation access unit (multistation access unit). In an Ethernet local area network (LAN), an MAU (media attachment unit) is a device that interconnects the attachment unit interface port on an attached host computer to the Ethernet network medium (such as unshielded twisted pair or coaxial cable). The MAU provides the services that correspond to the Physical layer of the Open Systems Interconnection (OSI) reference model. It provides an electrical connection and a transceiver that maps the digital bits between the computer and the network and detects collision and retries transmissions. An MAU can be built into the computer workstation or other device or it can be a separate device.
megahertz
media type On the Internet and as defined by Request for Comments (Request for Comments) 1521, a media type (also referred to as a content type) is a general category of data content, such as: application (executable program), audio content, an image, a text message, a video stream, and so forth. The media type tells the application that receives the message what kind of application is needed to process the content (for example, RealAudio or a similar program to play the audio content for a user). Each of these media types have subtypes. For example, the ‘‘text’’ media type has four subtypes: plain, richtext, enriched, and tab-separated values. E-mail users sometimes receive an attachment that describes its media type and subtype. For example, the media type and subtype of ‘‘application/octet-stream’’ describes an application organized in eight-bit units (octets). (Since this content is likely to be an executable program, it should be saved to disk so that it can be scanned by virus-scanning software before trying to run it.) Your browser may have assigned an application to handle each of several media types and subtypes or it may ask you to select an application. In the latter case, you need to know one or more applications that handle that particular media type and subtype.
medium A medium is a third-party or element through which a message is communicated. This seems to apply to information technology as well as to seances. In information technology, a medium can be: L
A physical transmission medium such as optical fiber
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A presentation medium (and thus the terms multimedia and advertising media)
In his influential book, Understanding Media, Marshall MacLuhan said ‘‘The medium is the message.’’ He seemed to mean that media technology itself transformed our lives more than the messages the media carried. He saw radio as a ‘‘hot medium’’ that imposed itself on you and television as a ‘‘cool’’ medium that pulled you into it. He envisioned that television would foster an electronic world community, a ‘‘global village.’’ Although he wrote before the Internet became widely known, the Internet almost seems like his best idea.
medium earth orbit satellite See ‘‘MEO satellite’’
megabit In data communications, a megabit is a million binary pulses, or 1,000,000 (that is, 106) pulses (or ‘‘bits’’). It’s commonly used for measuring the amount of data that is
transferred in a second between two telecommunication points. For example, a U.S. phone company T-carrier system line is said to sustain a data rate of 1.544 megabits per second. Megabits per second is usually shortened to Mbps. Some sources define a megabit to mean 1,048,576 (that is, 220) bits. Although the bit is a unit of the binary number system, bits in data communications are discrete signal pulses and have historically been counted using the decimal number system. For example, 28.8 kilobits per second (Kbps) is 28,800 bits per second. Because of computer architecture and memory address boundaries, bytes are always some multiple or exponent of two. See kilobyte, etc.
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megabyte 1) As a measure of computer processor storage and real and virtual memory, a megabyte (abbreviated MB) is 2 to the 20th power byte, or 1,048,576 bytes in decimal notation.
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2) According to the IBM Dictionary of Computing, when used to describe disk storage capacity and transmission rates, a megabyte is 1,000,000 bytes in decimal notation.
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According to the Microsoft Press Computer Dictionary, a megabyte means either 1,000,000 bytes or 1,048,576 bytes.
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According to Eric S. Raymond in The New Hacker’s Dictionary, a megabyte is always 1,048,576 bytes on the argument that bytes should naturally be computed in powers of two. Iomega Corporation uses the decimal megabyte in calling the Zip drive disk a ‘‘100MB disk’’ when it actually holds 100,431,872 bytes. If Iomega used the powers-of-two megabyte, the disk could be said to hold only 95.8 megabytes (if you divide 100,431,872 by 1,048,576).
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A megaflop is a measure of a computer’s speed and can be expressed as:
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10 to the 6th power floating-point operations per second
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megahertz A megahertz (MHz or sometimes Mhz) is a million cycles of electromagnetic currency alternation per second and is used as a unit of measure for the ‘‘clock speed’’ of computer microprocessor. In designing computer bus architectures, the microprocessor speed is considered together with the potential speed or amount of data that can come into the computer from I/O devices in order to optimize overall computer performance. The hertz as a unit of measure is named after Heinrich Hertz, German physicist.
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memory leak
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A meme is an idea that is passed on from one human generation to another. It’s the cultural equivalent of a gene, the basic element of biological inheritance. The term was coined in 1976 by Richard Dawkins in his book The Selfish Gene. Dawkins speculated that human beings have an adaptive mechanism that other species don’t have. In addition to genetic inheritance with its possibilities and limitations, humans, said Dawkins, can pass their ideas from one generation to the next, allowing them to surmount challenges more flexibly and more quickly than through the longer process of genetic adaptation and selection.
A memory leak is the gradual loss of available computer memory when a program (an application or part of the operating system) repeatedly fails to return memory that it has obtained for temporary use. As a result, the available memory for that application or that part of the operating system becomes exhausted and the program can no longer function. For a program that is frequently opened or called or that runs continuously, even a very small memory leak can eventually cause the program or the system to terminate. A memory leak is the result of a program bug.
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Examples of memes might include the idea of God; the importance of the individual as opposed to group importance; the belief that the environment can to some extent be controlled; or that technologies can create an electronically interconnected world community.
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Today, the word is sometimes applied ironically to ideas deemed to be of passing value. Dawkins himself described such short-lived ideas as memes that would have a short life in the meme pool.
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Memory is the electronic holding place for instructions and data that your computer’s microprocessor can reach quickly. When your computer is in normal operation, its memory usually contains the main parts of the operating system and some or all of the application programs and related data that are being used. Memory is often used as a shorter synonym for random access memory (RAM). This kind of memory is located on one or more microchips that are physically close to the microprocessor in your computer. Most desktop and notebook computers sold today include at least 32 megabytes of RAM, and are upgradeable to include more. The more RAM you have, the less frequently the computer has to access instructions and data from the more slowly accessed hard disk form of storage. Memory is sometimes distinguished from storage, or the physical medium that holds the much larger amounts of data that won’t fit into RAM and may not be immediately needed there. Storage devices include hard disks, floppy disks, CD-ROM, and tape backup systems. The terms auxiliary storage, auxiliary memory, and secondary memory have also been used for this kind of data repository. Additional kinds of integrated and quickly accessible memory are read-only memory (ROM), programmable ROM (PROM), and erasable programmable ROM (EPROM). These are used to keep special programs and data, such as the basic input/output system, that need to be in your computer all the time.
Some operating systems provide memory leak detection so that a problem can be detected before an application or the operating system crashes. Some program development tools also provide automatic ‘‘housekeeping’’ for the developer. It is always the best programming practice to return memory and any temporary file to the operating system after the program no longer needs it.
Memory Stick Developed by Sony, the Memory Stick is a digital data storage technology with up to 10 times the storage capacity of a 3.5 diskette. Sony is promoting the Memory Stick as a new way to share and transfer pictures, sound, and other data between different compact electronic devices such as digital cameras and camcorder. About the size of a flat AA battery, Memory Sticks are available in 4mb, 8mb, 16mb, 32mb and 64mb sizes. They are smaller in size than comparable data storage devices, including smart media and compact flash memory. In mid-2000, the Sony Vaio was the only laptop available with a Memory Stick slot, but any laptop with a PC card slot can use memory sticks with an adapter kit. Like its floppy disk predecessor, the Memory Stick has a lock mechanism to prevent the erasure of data. It has a 10-pin connector. In promoting the Memory Stick as a common portable storage medium, Sony has licensed the technology to companies such as Adobe, Pioneer, Casio, and Sharp. Sony recently unveiled the Memory Stick Walkman and the Memory Stick Digital Voice Recorder.
MEMS See ‘‘micro-electromechanical systems’’
mentor See ‘‘mentoring service’’
mentoring service A mentoring service is a company that provides professional counseling and advice for individuals and businesses for a fee. A mentoring service may provide career planning, skills training, project guidance, professional contacts, or
message queueing problem-solving for an individual’s career or within a company. A mentoring relationship may be for an unspecified or for a limited duration. A typical kind of mentoring in information technology is for the management of a software development project. A professional mentor may be hired for any or all of the phases of software development including the initial planning, architecture, design, development, implementation, debugging and testing, and deployment and installation Face-to-face mentoring is still the most common form of mentoring, but virtual mentoring is gaining popularity. Virtual mentoring uses videoconferencing, the Internet, and e-mail to mentor individuals or companies. This is beneficial for those who own a small business and are unable to leave their workplace and for those who live in rural or remote communities. Virtual mentoring is usually less expensive compared to face-to-face mentoring and provides an individual with more choices for mentors. Even if you choose virtual mentoring, it is recommended to meet with your mentor face-to-face at least once.
MEO satellite A medium earth orbit (MEO) satellite is one with an orbit within the range from a few hundred miles to a few thousand miles above the earth’s surface. Satellites of this type orbit higher than low earth orbit (LEO) satellites, but lower than geostationary satellites. The orbital periods of MEO satellites range from about two to 12 hours. Some MEO satellites orbit in near perfect circles, and therefore have constant altitude and travel at a constant speed. Other MEO satellites revolve in elongated orbits. The perigee (lowest altitude) of an elliptical-orbit satellite is much less than its apogee (greatest altitude). The orbital speed is much greater near perigee than near apogee. As seen from a point on the surface, a satellite in an elongated orbit crosses the sky in just a few minutes when it is near perigee, as compared to several hours when it is near apogee. Elliptical-orbit satellites are easiest to access near apogee, because the earth-based antenna orientation does not have to be changed often, and the satellite is above the horizon for a fairly long time. A fleet of several MEO satellites, with orbits properly coordinated, can provide global wireless communication coverage. Because MEO satellites are closer to the earth than geostationary satellites, earth-based transmitters with relatively low power and modest-sized antennas can access the system. Because MEO satellites orbit at higher altitudes than LEO satellites, the useful footprint (coverage area on the earth’s surface) is greater for each satellite. Thus a globalcoverage fleet of MEO satellites can have fewer members than a global-coverage fleet of LEO satellites.
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Merced was the code name during development for the microprocessor that Intel markets as the Itanium.
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1) Using e-mail, a message is an individual piece of mail. 2) In computer systems in general, a message is an information unit that the system sends back to the user or system operator with information about the status of an operation, an error, or other condition. 3) Using some telecommunication or data access methods, a message can be the unit of information or data that is transmitted from one program or user to another. 4) In messaging, which is the formal exchange of event notification, requests, or replies between programs through a messaging server, a message is data in a specified format that describes an event, a request, or a reply between programs. 5) In object-oriented programming, a message is the way that one program object requests an action from another object. A message specifies the name of the object to which the request is made, the action (or method) to be performed, and any parameter or value that needs to be specified for this request. Conceptually, a message for an object that incorporates all the methods of a automatic vaccuum cleaner might look like this:
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VacCleaner Chair Chair1LR which would request the object VacCleaner to use its chaircleaning procedure or method on ‘‘chair 1’’ in the living room.
message queueing In programming, message queueing is a method by which processes (or program instances) can exchange or pass data using an interface to a system-managed queue of messages. Messages can vary in length and be assigned different types or usages. A message queue can be created by one process and used by multiple processes that read and/or write messages to the queue. For example, a server process can read and write messages from and to a message queue created for client processes. The message type can be used to associate a message with a particular client process even though all messages are on the same queue. The message queue is managed by the operating system (or kernel). Application programs (or their processes) create message queues and send and receive messages using an application program interface (API). In UNIX systems, the C programming language msgget function is used with various parameters specifying the action requested, message queue ID, message type, and so forth.
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The maximum size of a message in a queue is limited by the operating system and is typically 8,192 bytes.
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Other forms of interprocess communication (IPC) include semaphores, sockets, and shared memory.
Meta is a prefix that in most information technology usages means ‘‘an underlying definition or description.’’ Thus, metadata is a definition or description of data and metalanguage is a definition or description of language. Meta (pronounced MEH-tah in the U.S. and MEE-tah in the U.K.) derives from Greek, meaning ‘‘among, with, after, change.’’ Whereas in some English words the prefix indicates ‘‘change’’ (for example, metamorphosis), in others, including those related to data and information, the prefix carries the meaning of ‘‘more comprehensive or fundamental.’’
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messages See ‘‘Internet problems’’
Messaging Application Program Interface See ‘‘MAPI’’
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messaging server A messaging server is a middleware program that handles messages that are sent for use by other programs using a messaging application program interface (API). A messaging server can usually queue and prioritize messages as needed and saves each of the client programs from having to perform these services. Messaging and messaging servers usually are organized in one or both of two models: Point-to-point messaging and subscribe/publish messaging.
messaging 1) More broadly, messaging (also called electronic messaging) is the creation, storage, exchange, and management of text, images, voice, telex, fax, e-mail, paging, and Electronic Data Interchange (EDI) over a communications network. You can find out more at the Web site of the Electronic Messaging Assocation (EMA). The EMA fosters the use of secure global electronic commerce. Its vendor members offer a wide range of telecommunications and computing services, including electronic mail, networks, directories, computer facsimile, electronic data interchange, paging, groupware, and voice mail. The EMA publishes Messaging magazine; back issues are available at the Web site. 2) In programming, messaging is the exchange of messages (specially-formatted data describing events, requests, and replies) with a messaging server, which acts as a message exchange program for client programs. There are two major messaging server models: the point-to-point model and the publish/subscribe model. Messaging allows programs to share common message-handling code, to isolate resources and interdependencies, and to easily handle an increase in message volume. Messaging also makes it easier for programs to communicate across different programming environments (languages, compilers, and operating systems) since the only thing that each environment needs to understand is the common messaging format and protocol. IBM’s MQSeries and Sun Microsystems Java Message Service (JMS) are examples of products that provide messaging interfaces and services.
The Standard Generalized Markup Language (SGML) defines rules for how a document can be described in terms of its logical structure (headings, paragraphs or idea units, and so forth). SGML is often referred to as a metalanguage because it provides a ‘‘language for how to describe a language.’’ A specific use of SGML is called a document type definition (DTD). A document type definition spells out exactly what the allowable language is. A DTD is thus a metalanguage for a certain type of document. (In fact, the Hypertext Markup Language (HTML) is an example of a document type definition. HTML defines the set of HTML tags that any Web page can contain.) The Extensible Markup Language (XML), which is comparable to SGML and modelled on it, describes how to describe a collection of data. It’s sometimes referred to as metadata. A specific XML definition, such as Microsoft’s Channel Definition Format (CDF), defines a set of tags for describing a Web channel. XML could be considered the metadata for the more restrictive metadata of CDF (and other future data definitions based on XML). In the case of SGML and XML, ‘‘meta’’ connotes ‘‘underlying definition’’ or set of rules. In other usages, ‘‘meta’’ seems to connote ‘‘description’’ rather than ‘‘definition.’’ For example, the HTML tag is used to enclose descriptive language about an HTML page. One could describe any computer programming or user interface as a metalanguage for conversing with a computer. And an English grammar and dictionary together could be said to define (and describe) the metalanguage for spoken and written English.
meta refresh On a Web site, a meta refresh is coding in the Hypertext Markup Language (HTML) that automatically directs the visitor to a new page after a specified number of seconds. It’s used when the address of a page is changed but you want visitors who know the old address to be able to find the new page. The page containing the meta refresh tag is known as a redirect page and the technique is known as redirection. Most Web surfers are familiar with the meta refresh. A user enters a Uniform Resource Locator on their browser’s address line and it takes them to a page that says something
MetaFrame like: ‘‘We have moved to a new location. Please change your bookmark. In five seconds, we will transfer you to the new page.’’ To make the transfer happen in five seconds, someone at that site has included an HTML line in the header section of the old page that looks like this:
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www.ournewsite.com’’>
In the above example, the ‘‘5’’ means change to the new page in five seconds. The ‘‘url=http://www.ournewsite.com’’ is an example of the URL for the new page. If you own a Web site that people have learned to use and you then change the name and location of a Web page, it’s strongly recommended that you make the original page a redirect page with a meta refresh tag to the new page. Otherwise, visitors familiar with the old page will get a 404 (Page Not Found) message.
meta tag A meta tag is a tag (that is, a coding statement) in the Hypertext Markup Language (HTML) that describes some aspect of the contents of a Web page. The information that you provide in a meta tag is used by search engines to index a page so that someone searching for the kind of information the page contains will be able to find it. The meta tag is placed near the top of the HTML in a Web page as part of the heading. There are several kinds of meta tags, but the most important for search engine indexing are the keywords meta tag and the description meta tag. The keywords meta tag lists the words or phrases that best describe the contents of the page. The description meta tag includes a brief one- or two-sentence description of the page. Both the keywords and the description are used by search engines in adding a page to their index. Some search engines also use the description to show the searcher a summary of the page’s contents. Although most search engines also use the contents of a page as a way to determine how to index it, the creator of a Web page should be sure to include meta tags with appropriate keywords and description. Well-written meta tags can help make the page rank higher in search results.
metacharacter A metacharacter (sometimes spelled meta character or metacharacter) is a special character in a program or data field that provides information about other characters. A metacharacter can express an idea about how to process the characters that follow the metacharacter, as the backslash character sometimes is used to indicate that the characters following it are to be treated in a special way. A common metacharacter usage is the wildcard character, which can represent either any one character or any string of characters. In UNIX shells, metacharacters include, but are not limited to these:
Each of these characters has a special meaning on the command line, and their use must be avoided for purposes other than their special meaning.
See ‘‘meta’’
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metafile A metafile is a file containing information that describes or specifies another file. Microsoft uses this term for its Windows Metafile (WMF) format. A WMF file contains a sequence of graphical-deviceinterface (GDI) function calls (‘‘commands’’ to the Windows operating system) that results in the presentation of a graphic image. Some of the function calls are equivalent to vector graphics statements and others identify stored bitmap or literal specifications of which bits to illuminate (raster graphics images). Using WMF files rather than already-built bitmaps saves space when many bitmaps are used repeatedly by different components of the operating system or of an application. The WMF file assumes a 16-bit operating system. Microsoft has an enhanced metafile (EMF) format for its 32-bit operating systems. Microsoft’s clipboard (CLP) file can contain a WMF file, an EMF file, or a bitmap (BMP) format file. Another example of a metafile is the Computer Graphics Metafile (CGM). The CGM file format is a standard (American National Standards Institute-approved) format that can be used on any operating system that supports it (unlike the WMF format which is designed only for Windows). The CGM file is commonly used in CAD and presentation graphics applications.
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In general, there are tools (such as HiJaak) that will convert the WMF, EMF, and CGM formats into other formats.
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MetaFrame is the name for a thin client/server software application from Citrix that is used to provide Microsoft’s Windows Terminal Server product (WTS) with additional server and client functionality by allowing any client, no matter what operating system (OS) they are using, to connect to a Windows Terminal Server and run a Windows application through their browser. MetaFrame is useful for its ability to ‘‘load balance’’ by automatically directing a client to the server with the lightest load in a server farm, and its ability to allow publishing and application management from a single server in that farm. MetaFrame also features a secure encryption option through a distributed Windows presentation protocol developed by Citrix, called ICA (Independent Computing Architecture).
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There is also a MetaFrame product for UNIX operating systems.
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metal-oxide semiconductor field-effect transistor
Any filename beginning with a metasyntactic variable denotes a scratch file. This means the file can be deleted at any time without affecting the program.
Also see gallium arsenide field-effect transistor.
Also see foo and bar.
MOSFET (metal-oxide semiconductor field-effect transistor, pronounced MAWS-feht) is a special type of field-effect transistor (FET) that works by electronically varying the width of a channel along which charge carriers (electrons or holes) flow. The wider the channel, the better the device conducts. The charge carriers enter the channel at the source, and exit via the drain. The width of the channel is controlled by the voltage on an electrode called the gate, which is located physically between the source and the drain and is insulated from the channel by an extremely thin layer of metal oxide. There are two ways in which a MOSFET can function. The first is known as depletion mode. When there is no voltage on the gate, the channel exhibits its maximum conductance. As the voltage on the gate increases (either positively or negatively, depending on whether the channel is made of P-type or N-type semiconductor material), the channel conductivity decreases. The second way in which a MOSFET can operate is called enhancement mode. When there is no voltage on the gate, there is in effect no channel, and the device does not conduct. A channel is produced by the application of a voltage to the gate. The greater the gate voltage, the better the device conducts. The MOSFET has certain advantages over the conventional junction FET, or JFET. Because the gate is insulated electrically from the channel, no current flows between the gate and the channel, no matter what the gate voltage (as long as it does not become so great that it causes physical breakdown of the metallic oxide layer). Thus, the MOSFET has practically infinite impedance. This makes MOSFETs useful for power amplifiers. The devices are also well suited to high-speed switching applications. Some integrated circuits (integrated circuit) contain tiny MOSFETs and are used in computers.
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Because the oxide layer is so thin, the MOSFET is susceptible to permanent damage by electrostatic charges. Even a small electrostatic buildup can destroy a MOSFET permanently. In weak-signal radio-frequency (RF) work, MOSFET devices do not generally perform as well as other types of FET.
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In programming, a metasyntactic (which derives from meta and syntax) variable is a variable (a changeable value) that is used to temporarily represent a function. Examples of metasyntactic variables include (but are by no means limited to) ack, bar, baz, blarg, wibble, foo, fum, and qux. Metasyntactic variables are sometimes used in developing a conceptual version of a program or examples of programming code written for illustrative purposes.
Metcalfe’s Law Metcalfe’s Law is expressed in two general ways: 1. The number of possible cross-connections in a network grow as the square of the number of computers in the network increases. 2. The community value of a network grows as the square of the number of its users increase. The original statement from Robert M. Metcalfe, inventor of Ethernet, was apparently (according to one source): ‘‘The power of the network increases exponentially by the number of computers connected to it. Therefore, every computer added to the network both uses it as a resource while adding resources in a spiral of increasing value and choice.’’ Metcalfe’s Law is often cited as an explanation for the rapid growth of the Internet (or perhaps more especially for the World Wide Web on the Internet). Together, with Moore’s Law about the rate at which computer power is accelerating, Metcalfe’s Law can be used to explain the rising wave of information technology that we are riding into the 21st century.
meter The meter (abbreviation, m; the British spelling is metre) is the International System of Units (SI) unit of displacement or length. One meter is the distance traveled by a ray of electromagnetic (EM) energy through a vacuum in 1/ 299,792,458 (3.33564095 6 10-9) of a second. The meter was originally defined as one ten-millionth (0.0000001 or 10-7) of the distance, as measured over the earth’s surface in a great circle passing through Paris, France, from the geographic north pole to the equator. One meter is a little more than three English feet, or about 39.37 inches. One foot is approximately 0.3048 meter. There are about 1609 meters in a statute mile. The official span was at one time formally defined as the separation between two scratches on a platinum bar in Paris. This was, of course, intended mainly for show, and not for use in the laboratory. Power-of-10 prefix multipliers facilitate the derivation of other, often more convenient, distance units from the meter. One centimeter (cm) is equal to 0.01 m, one millimeter (mm) is equal to 0.001 m, and one kilometer (km) is equal to 1000 m. These units are found in nonscientific as well as scientific literature. Smaller units are the realm of the scientist and engineer. One micrometer (symbolized mm or c¸), also called a micron, is equal to 0.000001 (10-6) m. One nanometer (nm) ¨ ) is is equal to 10-9 m. One Angstro¨m unit (symbolized A equal to 10-10 m, or 0.1 nm.
meter per second squared The meter and its kin are used to specify the wavelengths of EM fields. The so-called radio spectrum occupies an informally defined range of wavelengths from roughly a millimeter (microwaves) to several tens of kilometers (myriametric waves). A 3-m radio wave falls near the middle of the standard FM (frequency modulation) broadcast band; a 300-m radio wave is near the middle of the standard AM (amplitude-modulation) broadcast band. The range of visible light wavelengths is from approximately 390 nm (violet) to 770 nm (red). The speed of EM-field propagation in a vacuum, to nine significant figures, is 2.99792458 6 108 meters per second. In this sense, the meter can be derived from the second if the latter unit has been previously defined in absolute terms; one meter is the distance a ray of light travels through a vacuum in 3.33564095 6 10-9 second. In engineering applications, and also in an everyday sense, the term meter refers to any instrument used to measure the magnitude of a quantity. Examples include the volume-unit (VU) meter in home audio systems, the ammeter to measure electric current, and the kilowatt-hour meter to measure electrical energy consumed over a period of time. Also see International System of Units (SI) and prefix multipliers.
meter cubed See ‘‘cubic meter’’
meter cubed per kilogram The meter cubed per kilogram (symbolized m3/kg) is the standard unit of specific volume in the International System (SI). It is expressed directly in base SI units, and cannot be further reduced. Specific volume is the reciprocal of density. A substance with a specific volume of 1 m3/kg is extremely large for its mass. In fact, air at sea level has a specific volume less than this. Materials with high density have low specific volume; materials with low density have high specific volume. Alternative units are often used to express specific volume. The most common of these is the centimeter cubed per gram (cm3/g). A substance with a specific volume of 1 cm3/g has a specific volume of 0.001 m3/kg. Conversely, a substance with a specific volume of 1 m3/kg has a specific volume of 1000 cm3/g. Also see kilogram, meter, kilogram per cubic meter, and International System of Units (SI).
meter per second The meter per second (symbolized m/s or m/sec) is the International System of Units (SI) unit of linear speed. This quantity can be defined in either of two senses: average or instantaneous.
Average linear speed is obtained by measuring the distance in meters that an object travels in a certain number of seconds, and then dividing the distance by the time. If savg represents the average speed of an object (in meters per second) during a time interval t (in seconds), and the distance traveled in that time is equal to d (in meters), then:
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savg = d / t Instantaneous linear speed is more difficult to intuit, because it involves an expression of motion over an ‘‘infinitely short’’ interval of time. Let p represent a specific point in time. Suppose an object is in motion at about that time. The average speed can be measured over increasingly short time intervals centered at p, for example: [p74, p+4] [p73, p+3] [p72 , p+2] [p71, p+1] [p70.5, p+0.5] [p70.25, p+0.25] . . . [p7x, p+x] . . . where the added and subtracted numbers represent seconds. The instantaneous speed, sinst, is the limit of the measured average speed as x approaches zero. This is a theoretical value, because it cannot be obtained except by inference from measurements made over progressively shorter time spans. It is important to realize that speed is not the same thing as velocity. Speed is a scalar (dimensionless) quantity, while velocity is a vector quantity consisting of speed and direction. We might say a car is traveling at 20 m/s, and this tells us its speed. Or we might say the car is traveling at 20 m/s at a compass bearing of 25 degrees (north-by-northeast); this tells us its velocity. As with speed, we might specify either the average velocity over a period of time, or the instantaneous velocity at an exact moment in time.
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Also see meter per second squared and International System of Units (SI).
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The meter per second squared (symbolized m/s2 or m/sec2) is the International System of Units (SI) unit of acceleration vector magnitude. This quantity can be defined in either of two senses: average or instantaneous.
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For an object traveling in a straight line, the average acceleration magnitude is obtained by evaluating the object’s instantaneous linear speed (in meters per second) at two different points t1 and t2 in time, and then dividing the distance by the span of time t27t1 (in seconds). Suppose the
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Whatis?Com’s Encyclopedia of Technology Terms instantaneous speed at time t1 is equal to s1, and the instantaneous speed at time t2 is equal to s2. Then the average acceleration magnitude aavg (in meters per second squared) during the time interval [t1, t2] is given by:
meter-kilogram-second system of units See ‘‘metric system’’
aavg = (s27s1) / (t27t1)
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Instantaneous acceleration magnitude is more difficult to intuit, because it involves an expression of motion over an ‘‘infinitely short’’ interval of time. Let p represent a specific point in time. Suppose an object is in motion at about that time. The average acceleration magnitude can be determined over increasingly short time intervals centered at p, for example:
In object-oriented programming, a method is a programmed procedure that is defined as part of a class and included in any object of that class. A class (and thus an object) can have more than one method. A method in an object can only have access to the data known to that object, which ensures data integrity among the set of objects in an application. A method can be re-used in multiple objects.
[p74, p+4] [p73, p+3] [p72, p+2] [p71, p+1] [p70.5, p+0.5] [p70.25, p+0.25] . . . [p7x, p+x] . . . where the added and subtracted numbers represent seconds. The instantaneous acceleration magnitude, ainst, is the limit of the average acceleration magnitude as x approaches zero. This is a theoretical value, because it can be obtained only by inference from instantanous speed values determined at the starting and ending points of progressively shorter time spans. Acceleration, in its fullest sense, is a vector quantity, possessing direction as well as magnitude. For an object moving in a straight line and whose linear speed changes, the acceleration vector points in the same direction as the object’s direction of motion. But acceleration can be the result of a change in the direction of a moving object, even if the instantaneous speed remains constant. The classic example is given by an object in circular motion, such as a revolving weight attached to the rim of a wheel. If the rotational speed of the wheel is constant, the weight’s acceleration vector points directly inward toward the center of the wheel. Also see meter, second, meter per second, and International System of Units (SI).
meter squared See ‘‘square meter’’
metric 1) In software development, a metric (noun) is the measurement of a particular characteristic of a program’s performance or efficiency. Similarly in network routing, a metric is a measure used in calculating the next host to route a packet to. A metric is sometimes used directly and sometimes as an element in an algorithm. In programming, a benchmark includes metrics. 2) Metric (adjective) pertains to anything based on the meter as a unit of spatial measurement.
metric system The metric system, more formally called the meter-kilogramsecond (MKS or mks) system of units, is based on three fundamental units: the meter (m) that quantifies displacement, the kilogram (kg) that quantifies mass, and the second (s or sec) that quantifies time. The metric system is formally replaced by the very similar International System of Units (SI). The metric system was originally developed by scientists who were frustrated with the English (foot-pound-second) system. In theoretical and laboratory calculations, arithmetic involving English units is ‘‘messy.’’ There are, for example, 12 inches in a foot, three feet in a yard, and 5280 feet in a statute mile; there are 16 ounces in a pound. Less common English units such as the rod, furlong, peck, and bushel seem, in modern scientific terms, to have been dreamed up ages ago without concern for common sense, although they are used by some agricultural and industrial people to this day. In the metric or mks system, the meter and kilogram are divided into fractional units, and enlarged into multiple units, according to power-of-10 prefix multipliers. For example, there are 1000 millimeters (103 mm) in a meter, 1000 meters (103 m) in a kilometer, 1000 grams (103 g) in a kilogram, and 1000 milligrams (103 mg) in a gram. This makes these mks units easy to work with in scientific notation. Time, however, is denoted in the same way as in the English system. There are 60 seconds in a minute, 60 minutes in an hour, and 24 hours in a mean solar day in both systems.
Micro Channel Architecture The International System of Units (SI) provides formal definitions for the meter, the kilogram, and the second, and also specifies and defines four additional units: the Kelvin for temperature, the ampere for electric current, the candela for luminous intensity, and the mole for material quantity.
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Compare centimeter-gram-second (cgs) or small-unit metric system, English system of units, and International System of Units (SI). Also see prefix multiplier.
The megahertz, abbreviated MHz, is a unit of alternating current (AC) or electromagnetic (EM) wave frequency equal to one million hertz (1,000,000 Hz). The megahertz is commonly used to express microprocessor clock speed. The unit is occasionally used in measurements or statements of bandwidth for high-speed digital data, analog and digital video signals, and spread spectrum signals.
metropolitan area network A metropolitan area network (MAN) is a network that interconnects users with computer resources in a geographic area or region larger than that covered by even a large local area network (LAN) but smaller than the area covered by a wide area network (WAN). The term is applied to the interconnection of networks in a city into a single larger network (which may then also offer efficient connection to a wide area network). It is also used to mean the interconnection of several local area networks by bridging them with backbone lines. The latter usage is also sometimes referred to as a campus network. Examples of metropolitan area networks of various sizes can be found in the metropolitan areas of London, England; Lodz, Poland; and Geneva, Switzerland. Large universities also sometimes use the term to describe their networks.
mezzanine Mezzanine is a term used to describe the stacking of computer component cards into a single card that then plugs into the computer bus or data path. The bus itself is sometimes referred to as a mezzanine bus. The term derives from the Italian word, mezzano, which means middle. The more common use of this term is in architecture, where it is a low-ceilinged story between two main stories in a building. In theaters, a mezzanine is a balcony projecting partly over the ground floor below it. A mezzanine card is a smaller form of the more familiar Peripheral Component Interconnect (PCI) or Industry Standard Architecture (ISA) card. The original and still most popular mezzanine card is the Industry Pack (IP) card. An IP card provides a 16-bit data path. The IP card is 3.9 6 1.8 inches and has two 50-pin connectors that plug into an IP-toPCI adapter card. The IP-to-PCI adapter card usually holds up to three IP cards. Another popular mezzanine card is the PCI Mezzanine (PMC) card. This card provides 32 or 64-bit data paths and uses 64-pin connectors. Both types of mezzanine cards are widely used with VME bus, which is an expansion bus technology that supports up to 21 cards on a single backplane. VME bus is widely used in industrial, telecommunication, and military applications.
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An EM signal having a frequency of 1 MHz is near the center of the standard amplitude-modulation (AM) radio broadcast band, and has a wavelength of 300 meters, or about 980 feet. An EM signal of 100 MHz is near the middle of the standard frequency-modulation (FM) radio broadcast band, and has a wavelength of 3 meters, which is a little less than 10 feet. Some radio transmissions are made at frequencies up to many thousands of megahertz. Typical computer clock speeds are constantly increasing, but generally are on the order of a few hundred megahertz.
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Other units of frequency are the kHz, equal to 1,000 Hz or 0.001 MHz, and the gigahertz, equal to 1,000,000,000 Hz or 1,000 MHz.
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The bandwidth of a digital signal, in megahertz, is related to the data speed in bits per second. In general, the greater the data speed, the larger the bandwidth. Data speed is not, however, the same thing as bandwidth. A high-speed cable or fiberoptic modem operating at a speed of 5,000,000 bps has, in a certain sense, a nominal frequency of 5 MHz. But the bandwidth is generally much smaller, because it depends on variations in the individual data elements, not on the number of data bits per unit time.
MIB See ‘‘management information base’’
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Micro Channel Architecture Developed by IBM for its line of PS/2 desktop computers, Micro Channel Architecture is an interface between a computer (or multiple computers) and its expansion cards and their associated devices. MCA was a distinct break from previous bus architectures such as Industry Standard Architecture. The pin connections in MCA are smaller than other bus interfaces. For this and other reasons, MCA does not support other bus architectures. Although MCA offers a number of improvements over other bus architectures, its proprietary, nonstandard aspects did not encourage other manufacturers to adopt it. It has influenced other bus designs and it is still in use in PS/2s and in some minicomputer systems.
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microchip A microchip (sometimes just called a ‘‘chip’’) is a unit of packaged computer circuitry (usually called an integrated circuit) that is manufactured from a material such as silicon at a very small scale. Microchips are made for program logic (logic or microprocessor chips) and for computer memory (memory or RAM chips). Microchips are also made that include both logic and memory and for special purposes such as analog-to-digital conversion, bit slicing, and gateways.
microchip art Microchip art is a microscopic non-functioning drawing impressed on the surface of the design mask used in the production of microchips. The art, which grew out of the tradition of having chip designers ‘‘sign’’ their work, is created by etching into the upper metallic layers of the chip in an unused corner of the chip mask. Because microchip art is too small to see with the human eye, its existence was not widely known until 1998 when photographer Michal Davidson accidently stumbled on an example while photographing the geometric patterns of a microchip. Davidson, who makes his living photographing ordinary objects under a high-power Nikon optical mircroscope, was surprised to find the children’s book character ‘‘Waldo’’ hiding among the thousands of square microns of circuitry he was looking at. (The objective of the ‘‘Where’s Waldo?’’ books is for children to find the Waldo character who is hidden somewhere in each page’s illustration.) Davidson, who at first thought the image was a fluke, began to closely examine other microchips under his microscope and found what writer Michael Stroh has described as ‘‘the Lascaux Cave of the computer industry.’’ Since his initial discovery, Davidson has found and photographed a wide variety of examples of microchip art, including intricate sketches of hummingbirds, locomotives, and buffalo. After posting his microphotographs on a Web site, Davidson was pleased to have many of the chip designers contact him about their work and explain the symbolism behind the chosen design. For example, a bulldozer that appears on a chip designed in 1980 by Synertek was a mystery until it was learned that the chip was used in heavy equipment electonic monitoring systems. Microchip art is frowned upon in some corporations, noteably Intel, because the software used to create chips is programmed to spot design flaws, and, as microchips become more complex, the possibility exists that poorly executed microchip art could pose production problems. Chip designers, however, compare microchip art to the
‘‘Easter eggs’’ that programmers leave behind, and promise that somehow creative minds will continue to ‘‘make their mark’’ upon their work.
microcode Microcode is programming that is ordinarily not programaddressable but, unlike hardwired logic, is capable of being modified. Microcode may sometimes be installed or modified by a device’s user by altering programmable readonly memory (PROM) or erasable programmable read-only memory (EPROM). IBM uses this term in preference to firmware.
microcomputer A microcomputer is a complete computer on a smaller scale and is generally a synonym for the more common term, personal computer or PC, a computer designed for an individual. A microcomputer contains a microprocessor (a central processing unit on a microchip), memory in the form of read-only memory and random access memory, I/O ports and a bus or system of interconnecting wires, housed in a unit that is usually called a motherboard. In an ascending hierarchy of general computer sizes, we find: L
An embedded systems programming computer, which is embedded in something and doesn’t support direct human interaction but nevertheless meets all the other criteria of a microcomputer
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Workstation, as used to mean a more powerful personal computer for special applications
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Minicomputer, now restyled a ‘‘mid-range server’’
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Mainframe or mainframe computer, which is now usually referred to by its manufacturers as a ‘‘large server’’
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Supercomputer, formerly almost a synonym for ‘‘Cray supercomputer’’ but now meaning a very large server and sometimes including a system of computers using parallel processing
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A parallel processing system is a system of interconnected computers that work on the same application together, sharing tasks that can be performed concurrently
micro-electromechanical systems Micro-electromechanical systems (MEMS) is a technology that combines computers with tiny mechanical devices such as sensors, valves, gears, mirrors, and actuators embedded in semiconductor chips. Paul Saffo of the Institute for the Future in Palo Alto, California, believes MEMS or what he
micropayment calls analog computing will be ‘‘the foundational technology of the next decade.’’ MEMS is also sometimes called smart matter. MEMS are already used as accelerometers in automobile airbags. They’ve replaced a less reliable device at lower cost and show promise of being able to inflate a bag not only on the basis of sensed deceleration but also on the basis of the size of the person they are protecting. Basically, a MEMS device contains micro-circuitry on a tiny silicon chip into which some mechanical device such as a mirror or a sensor has been manufactured. Potentially, such chips can be built in large quantities at low cost, making them cost-effective for many uses. Among the presently available uses of MEMS or those under study are: L
Global position system sensors that can be included with courier parcels for constant tracking and that can also sense parcel treatment en route
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Sensors built into the fabric of an airplane wing so that it can sense and react to air flow by changing the wing surface resistance; effectively creating a myriad of tiny wing flaps
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Optical switching devices that can switch light signals over different paths at 20-nanosecond switching speeds
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Sensor-driven heating and cooling systems that dramatically improve energy savings
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Building supports with imbedded sensors that can alter the flexibility properties of a material based on atmospheric stress sensing
Saffo distinguishes between sensor-effector type microcomputing (which he calls ‘‘MEMS’’) and micro-devices containing gears, mirrors, valves, and other parts (which he calls ‘‘micro-machines’’). Much support for MEMS has come from Defense Advanced Research Projects Agency Research and Development Electronics Technology Office.
microfluidics Microfluidics is the science of designing, manufacturing, and formulating devices and processes that deal with volumes of fluid on the order of nanoliters (symbolized nl and representing units of 10-9 liter) or picoliters (symbolized pl and representing units of 10-12 liter). The devices themselves have dimensions ranging from millimeters (mm) down to micrometers (mm), where 1 mm = 0.001 mm. Microfluidics hardware requires construction and design that differs from macroscale hardware. It is not generally possible to scale conventional devices down and then expect them to work in microfluidics applications. When the dimensions of a device or system reach a certain size as the scale becomes smaller, the particles of fluid, or particles suspended in the fluid, become comparable in size with the apparatus itself. This dramatically alters system behavior.
Capillary action changes the way in which fluids pass through microscale-diameter tubes, as compared with macroscale channels. In addition, there are unknown factors involved, especially concerning microscale heat transfer and mass transfer, the nature of which only further research can reveal. The volumes involved in microfluidics can be understood by visualizing the size of a one-liter container, and then imagining cubical fractions of this container. A liter is slightly more than one U.S. fluid quart. A cube measuring 100 mm (a little less than four inches) on an edge has a volume of one liter. Imagine a tiny cube whose height, width, and depth are 1/1000 (0.001) of this size, or 0.1 mm. This is the size of a small grain of table sugar; it would take a strong magnifying glass to resolve it into a recognizable cube. That cube would occupy 1 nl. A volume of 1 pl is represented by a cube whose height, width, and depth are 1 /10 (0.1) that of a 1-nl cube. It would take a powerful microscope to resolve that.
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Microfluidic systems have diverse and widespread potential applications. Some examples of systems and processes that might employ this technology include inkjet printers, bloodcell-separation equipment, biochemical assays, chemical synthesis, genetic analysis, drug screening, electrochromatography, surface micromachining, laser ablation, and mechanical micromilling. Not surprisingly, the medical industry has shown keen interest in microfluidics technology.
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micron A micron (short for micrometer) is one-millionth of a meter. It can also be expressed as: L
10-6 meter
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One thousandth of a millimeter
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One 25-thousandth of an inch
The micron is a unit of measure for the core in a optical fiber cable, for which the most common diameter is 62.5 microns. It is also used to measure the line width on a microchip. Today’s microprocessors and memory chips are typically built using 0.35 and 0.25 line widths. Recently, a 0.18 micron line width has been achieved and is being used in the latest processors from Intel and AMD.
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A human hair is said to be about 50 microns wide.
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On the Web, micropayment is a business concept whose goal is to generate revenue by offering pay-per-view Web pages, Web links, or Web services for small amounts of money called ‘‘microcents.’’ Since it is not practical for individual users to charge small amounts of money (such as a penny or a fraction of a penny) to a major charge card, a different method of payment is needed for sites that wish to go
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Whatis?Com’s Encyclopedia of Technology Terms ‘‘micro.’’ Several methods of micropayment collection are being examined, many of which involve encoding per-feelinks inside HTML pages and some kind of Internet wallet account where individuals would establish a cash balance with a third-party application that would monitor, collect, and distribute micropayments. Once a common micropayment standard has been established, some visionaries predict that streaming media sites, sports access sites, and other specialized resources will pave the way for pay-per view Web use, just as they did for cable TV.
microprocessor A microprocessor is a computer processor on a microchip. It’s sometimes called a logic chip. It is the ‘‘engine’’ that goes into motion when you turn your computer on. A microprocessor is designed to perform arithmetic and logic operations that make use of small number-holding areas called registers. Typical microprocessor operations include adding, subtracting, comparing two numbers, and fetching numbers from one area to another. These operations are the result of a set of instructions that are part of the microprocessor design. When the computer is turned on, the microprocessor is designed to get the first instruction from the basic input/output system (BIOS) that comes with the computer as part of its memory. After that, either the BIOS, or the operating system that BIOS loads into computer memory, or an application progam is ‘‘driving’’ the microprocessor, giving it instructions to perform.
For comparison, a millisecond (ms or msec) is one thousandth of a second and is commonly used in measuring the time to read to or write from a hard disk or a CD-ROM player or to measure packet travel time on the Internet. A nanosecond (ns or nsec) is one billionth (10-9) of a second and is a common measurement of read or write access time to random access memory (RAM). A picosecond is one trillionth (10-12) of a second, or one millionth of a microsecond. A femtosecond is one millionth of a nanosecond or 10-15 of a second and is a measurement sometimes used in laser technology. An attosecond is one quintillionth (10-18) of a second and is a term used in photon research.
microsite As used in at least one leading Web design book, a microsite is a separately promoted part of a larger Web site. A microsite is designed to meet separate objectives and has a separate Web address (or Uniform Resource Locator) as its home page. Typically, a microsite resides on the same Web server and reflects the branding and overall visual design of the larger site with which it is associated. Occasionally, two Web sites will collaborate to produce a third, smaller site that both link to (and is probably located on one of the two main site’s servers). Minisite is also sometimes used with about the same meaning.
microrobot
Microsoft Certified Solution Developer
A microrobot is a miniaturized, sophisticated machine designed to perform a specific task or tasks repeatedly and with precision. Microrobots typically have dimensions ranging from a fraction of a millimeter up to several millimeters.
An MCSD (Microsoft Certified Solution Developer) is someone who has passed exams that test their ability to design and develop custom business applications with Microsoft development tools, technologies, and platform. To prepare for the exams, you can take courses at a certified training company location, in certified courses in a high school or college, or through self-study at Microsoft’s Web site or through certified training materials.
A microrobot, like its larger and smaller cousins, the robot and the nanorobot, can be either autonomous or insect-like. An autonomous microrobot contains its own on-board computer, which controls the machine and allows it to operate independently. The insect scheme is more common for microrobots. In an insect-microrobot arrangement, the machine is one of a fleet of several, or many, identical units that are all controlled by a single, central computer. (The term insect comes from the fact that such robots behave like ants in an anthill or bees in a hive.)
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A microsecond (us or Greek letter mu plus s) is one millionth (10-6) of a second.
The MCSD program is part of a set of Microsoft Certified Professional (MCP) training programs. In other MCP programs, you can gain certification as a Microsoft Certified Systems Engineer (MCSE), a Microsoft Certified Product Specialist (MCPS), or a Microsoft Certified Trainer (MCT).
Microsoft Certified Systems Engineer An MCSE (Microsoft Certified Systems Engineer) is someone who has passed exams about a specific Microsoft Windows operating system, related desktop systems, networking, and Microsoft’s BackOffice server products. To prepare for the exams, you can take courses at a certified training company location, in certified courses in a high school or college, or through self-study at Microsoft’s self-study Web site or through certified training materials.
MIDI The MCSE program is the most popular of a set of training programs that Microsoft calls the Microsoft Certified Professional (MCP). In other MCP programs, you can gain certification as a Microsoft Certified Solution Developer (MCSD), a Microsoft Certified Product Specialist (MCPS), or a Microsoft Certified Trainer (MCT).
Microsoft Foundation Class Library The Microsoft Foundation Class (MFC) Library is a collection of classes (generalized definitions used in objectoriented programming) that can be used in building application programs. The classes in the MFC Library are written in the C++ programming language. The MFC Library saves a programmer time by providing code that has already been written. It also provides an overall framework for developing the application program. There are MFC Library classes for all graphical user interface elements (windows, frames, menus, tool bars, status bars, and so forth), for building interfaces to databases, for handling events such as messages from other applications, for handling keyboard and mouse input, and for creating ActiveX control.
Microsoft Internet Explorer Microsoft Internet Explorer (MSIE) is the most widely used World Wide Web browser. It comes with the Microsoft Windows operating system and can also be downloaded from Microsoft’w Web site. The MSIE browser competes with an earlier browser, Netscape Navigator. Three other browsers are Mosaic (the browser on which Netscape’s browser was based), Lynx, and Opera.
Microsoft Management Console The Microsoft Management Console (MMC) is an application that provides a graphical-user interface (GUI) and a programming framework in which consoles (collections of administrative tools) can be created, saved, and opened. MMC was originally released as part of the Windows 98 Resource Kit, and is included on all later versions. It uses a Multiple Document Interface (MDI) in an environment similar to Microsoft’s Windows Explorer. MMC is considered to be a container for the actual operations, and is known as a ‘‘tools host.’’ It does not, itself, provide management, but rather a framework in which management tools can operate. Consoles are used to manage Windows-based hardware, software, and networking components, and include items such as controls, wizards, tasks, documentation, and snapins which may be from Microsoft or other software vendors, or user-defined. To create a console, the administrator runs the MMC executable file to open an empty console and chooses from among a list of all the tools installed on the system (such as, for example, certificate server manager, device
manager, and DNS manager). Because consoles exist as files, an administrator can create them and then send them as email attachments to developers responsible for specific tasks.
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Microsoft Transaction Server See ‘‘MTS’’
microwave The term microwave refers to electromagnetic energy having a frequency higher than 1 gigahertz (billions of cycles per second), corresponding to wavelengths shorter than 30 centimeters. Microwave signals propagate in straight lines and are affected very little by the troposphere. They are not refracted or reflected by ionized regions in the upper atmosphere. Microwave beams do not readily diffract around barriers such as hills, mountains, and large human-made structures. Some attenuation occurs when microwave energy passes through trees and frame houses. Radio-frequency (RF) energy at longer wavelengths is affected to a lesser degree by such obstacles. The microwave band is well suited for wireless transmission of signals having large bandwidths. This portion of the RF electromagnetic radiation spectrum encompasses many thousands of megahertz. Compare this with the so-called shortwave band that extends from 3 MHz to 30 MHz, and whose total available bandwidth is only 27 MHz. In communications, a large allowable bandwidth translates into high data speed. The short wavelengths allow the use of dish antennas having manageable diameters. These antennas produce high power gain in transmitting applications, and have excellent sensitivity and directional characteristics for reception of signals.
middleware In the computer industry, middleware is a general term for any programming that serves to ‘‘glue together’’ or mediate between two separate and usually already existing programs. A common application of middleware is to allow programs written for access to a particular database to access other databases.
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Messaging is a common service provided by middleware programs so that different applications can communicate. The systematic tying together of disparate applications is known as enterprise application integration (EAI).
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MIDI (Musical Instrument Digital Interface) is a protocol designed for recording and playing back music on digital synthesizers that is supported by many makes of personal computer sound cards. Originally intended to control one keyboard from another, it was quickly adopted for the personal computer. Rather than representing musical sound
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Whatis?Com’s Encyclopedia of Technology Terms directly, it transmits information about how music is produced. The command set includes note-ons, note-offs, key velocity, pitch bend and other methods of controlling a synthesizer. The sound waves produced are those already stored in a wavetable in the receiving instrument or sound card. Since a MIDI file only represents player information, it is far more concise than formats that the sound directly. An advantage is very small file size. A disadvantage is the lack of specific sound control. With a program that provides this interface, you can create music using a standard keyboard or other input device. You or others can then play your MIDI-conforming creation with the same or another program and a sound card as a music synthesizer. The MIDI program may come with a graphical user interface that looks like a sound studio control room. Many sound cards come as a package with MIDI software (for example, Media Vision’s Pro Audio Studio 16).
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The MIDI protocol uses eight-bit serial transmission with one start bit and one stop bit, has a 31.25 Kbs data rate, and is asynchronous. Connection is made through a five-pin DIN plug, of which three pins are used.
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In general, midrange refers to computers that are more powerful and capable than personal computers but less powerful and capable than mainframe computers. (Computer power is sometimes measured in terms of millions-of-instructions-per-second—MIPS. Capability includes, for example, how many devices can be connected to and interact with the computer at the same time.) The computer industry does not define exactly what characteristics constitute ‘‘midrange.’’
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Historically, midrange computers have been sold to small to medium-sized businesses as their main computer and to larger enterprises for branch or department-level operations. Makers of popular midrange computer lines include Hewlett-Packard, IBM, and Sun Microsystems. Today’s computers are almost universally known as servers to recognize that they often ‘‘serve’’ applications to end users at ‘‘client’’ computers, that they use a client/server computing model, and, by inference (since the client/server model developed in UNIX-based operating systems), that they support standard rather than proprietary programming interfaces.
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In information technology, migration is the process of moving from the use of one operating environment to another operating environment that is, in most cases, is thought to be a better one. For example, moving from Windows NT Server to Windows 2000 Server would usually be considered a migration because it involves making sure that new features are exploited, old settings do not require
changing, and taking steps to ensure that current applications continue to work in the new environment. Migration could also mean moving from Windows NT to a UNIX-based operating system (or the reverse). Migration can involve moving to new hardware, new software, or both. Migration can be small-scale, such as migrating a single system, or large-scale, involving many systems, new applications, or a redesigned network. One can migrate data from one kind of database to another kind of database. This usually requires converting the data into some common format that can be output from the old database and input into the new database. Since the new database may be organized differently, it may be necessary to write a program that can process the migrating files. Migration is also used to refer simply to the process of moving data from one storage device to another.
mil ’’mil’’ is one of the top-level domain names that can be used when choosing a domain name. It generally describes the entity owning the domain name as a military organization of the U.S government. Along with the second-level domain name (for example: ‘‘navy’’ in navy.mil), the top-level domain name is required in Web and e-mail addresses. For more information, see gTLD (generic top-level domain name).
millennium 1) A millennium is a period of one thousand years. It is similar to the terms biennium, a period of two years, and century, a period of one hundred years. The term derives from the Latin mille, meaning thousand, and annum, meaning year. 2) The millennium is the anniversary or celebration of a 1000year period. In the U.S., the official timekeeper at the Naval Observatory considers that the second millennium and the beginning of the third will be reached on January 1, 2001. This date is based on the Gregorian calendar, created in 1582 Anno Domini (A.D.) and which has since become a world standard for civil affairs. The Gregorian calendar uses the table of dates for Easter that was established by the sixthcentury scholar Dionysius Exiguus who marked the modern epoch as beginning on January 1, 1 A.D. For this reason, the second millennium is not actually reached until January 1, 2001. However, because the most dramatic change in the calendar occurred on January 1, 2000, much of the world celebrated the beginning of the third millennium a year early. 3) The millennium is also used to refer in general to a time when great achievements finally come to pass, great happiness prevails, or some other important objective is reached. In the Book of Revelations, the millennium is a period of a thousand years during which Christ is to rule on Earth.
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milliampere hour A milliampere hour (mAh) is 1000th of an ampere hour (Ah). Both measures are commonly used to describe the energy charge that a battery will hold and how long a device will run before the battery needs recharging. For example, an IBM Thinkpad 365 with an Nickel-Metal Hydride 9.6 voltage battery that is stated to provide a charge of 2800 mAh offers from 2.5 to 5 hours of battery use without recharge.
million instructions per second See ‘‘MIPS’’
millisecond (This definition follows U.S. usage in which a billion is a thousand million and a trillion is a 1 followed by 12 zeros.) A millisecond (ms or msec) is one thousandth of a second and is commonly used in measuring the time to read to or write from a hard disk or a CD-ROM player or to measure packet travel time on the Internet. For comparison, a microsecond (us or Greek letter mu plus s) is one millionth (10-6) of a second. A nanosecond (ns or nsec) is one billionth (10-9) of a second and is a common measurement of read or write access time to random access memory (RAM). A picosecond is one trillionth (10-12) of a second, or one millionth of a microsecond. -15
A femtosecond is one millionth of a nanosecond or 10 of a second and is a measurement sometimes used in laser technology. An attosecond is one quintillionth (10-18) of a second and is a term used in photon research.
MIM A MIM or MME file is a file in the Multi-Purpose Internet Mail Extensions (MIME) format that is created by some email programs, including that of America Online (AOL), to encapsulate e-mail that contains image or program attachments. The MIM or MME refers to the three-letter extension or suffix (‘‘.mim’’ or ‘‘.mme’’) at the end of the file name. AOL creates a MIM or MME file when a user sends a note with attachments to other users. Such notes sent to other than AOL users tend to be restored to their original form by the receiver’s e-mail software. However, AOL recipients receive a MIM or MME file that they need to ‘‘open’’ so that they can get the individual files inside. AOL users with Windows can use WinZip (using the ‘‘Classic Winzip’’ mode of operation). Mac users can use a similar utility. AOL users who enter the keyword ‘‘MIME’’ receive this explanation from AOL:
The Internet’s e-mail system handles basic text files nicely, but doesn’t reliably handle binary files – files like pictures or word processing documents. So, when you send an e-mail message with a file attachment to someone on the Internet, the AOL software automatically encodes or translates the attachment using a system called MIME. (MIME stands for Multipurpose Internet Mail Extensions.) MIME converts the binary attachment to a text format that can be handled by Internet e-mail. The message’s recipient needs a program that can decode the MIME e-mail and turn it back into a binary file that the computer can work with. When you receive MIME files... When someone sends you a MIME-encoded file in e-mail, it needs to be translated back into a format that your computer can understand. Depending on the mail system that the sender used, the AOL software may or may not be able to automatically decode the MIME file. If it was able to decode the file, the e-mail’s file attachment will be in its original, binary format: such as .GIF, or .ZIP. If AOL was unable to decode the MIME file, the attachment will be in MIME format, with a filename extension of .MME. Download this file–it is simple to use a utility to convert the .MME file back to a binary file. Windows users can use these applications to easily decode MIME files. * WinZip: http://www.winzip.com * MIME Decoders: MIME Help & Software After you’ve downloaded and installed these programs, please be sure to read the READ ME guide for information on how to use the decoder(s). Macintosh users can use these applications to easily decode MIME files. * Decoder: Harmony Software Home Page After you’ve downloaded and installed any of these programs, please be sure to read the READ ME guide for information on how to use the decoder. When you send an attachment to an Internet user... When you attach a file to an e-mail message that you send to an Internet user, it will automatically be MIME-encoded. In order to use the attachment, your message’s recipient must have a MIME-compliant e-mail program or use software that can decode MIME files–to translate it back into a format that his or her computer can understand. If the recipient has a MIME-compliant e-mail program, the MIME attachment will probably be automatically decoded for him. If not, the recipient can easily translate the file using a utility program.
MIME MIME (Multi-Purpose Internet Mail Extensions) is an extension of the original Internet e-mail protocol that lets people use the protocol to exchange different kinds of data files on the Internet: Audio, video, images, application
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programs, and other kinds, as well as the ASCII handled in the original protocol, the Simple Mail Transport Protocol (SMTP). In 1991, Nathan Borenstein of Bellcore proposed to the IETF that SMTP be extended so that Internet (but mainly Web) client and server could recognize and handle other kinds of data than ASCII text. As a result, new file types were added to ‘‘mail’’ as a supported Internet Protocol file type.
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Servers insert the MIME header at the beginning of any Web transmission. Clients use this header to select an appropriate ‘‘player’’ application for the type of data the header indicates. Some of these players are built into the Web client or browser (for example, all browser come with GIF and JPEG image players as well as the ability to handle HTML files); other players may need to be downloaded.
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New MIME data types are registered with the Internet Assigned Numbers Authority (IANA).
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MIME is specified in detail in Internet Request for Comments 1521 and 1522, which amend the original mail protocol specification, RFC 821 (the Simple Mail Transport Protocol) and the ASCII messaging header, RFC 822.
minicomputer
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A minicomputer, a term no longer much used, is a computer of a size intermediate between a microcomputer and a mainframe. Typically, minicomputers have been standalone computers (computer systems with attached terminals and other devices) sold to small and mid-size businesses for general business applications and to large enterprises for department-level operations. In recent years, the minicomputer has evolved into the ‘‘mid-range server’’ and is part of a network. IBM’s AS/400e is a good example.
minisite A minisite is a small Web site with a narrow subject focus or few pages that is sometimes a section in a larger Web site. Similar terms are microsite and sitelet. Typically, a minisite has a separate identity and users are encouraged to link to it directly.
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The number of MIPS (million instructions per second) is a general measure of computing performance and, by implication, the amount of work a larger computer can do. For large servers or mainframes, MIPS is a way to measure the cost of computing: The more MIPS delivered for the money, the better the value. Historically, the cost of computing measured in the number of MIPS has been reduced by half on an annual basis for a number of years.
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The number of MIPS attributed to a computer is usually determined by one or more benchmark runs.
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mirror site A mirror site is a Web site or set of file on a computer server that has been copied to another computer server in order to reduce network traffic, ensure better availability of the Web site or files, or make the site or downloaded files arrive more quickly for users close to the mirror site. Mirroring is the practice of creating and maintaining mirror sites. A mirror site is an exact replica of the original site and is usually updated frequently to ensure that it reflects the content of the original site. Mirror sites are used to make access faster when the original site may be geographically distant (for example, a much-used Web site in Germany may arrange to have a mirror site in the U.S.). In some cases, the original site (for example, on a small university server) may not have a high-speed connection to the Internet and may arrange for a mirror site at a larger site with higher-speed connection and perhaps closer proximity to a large audience. In addition to mirroring Web sites, you can also mirror files that can be downloaded from an File Transfer Protocol server. Netscape, Microsoft, Sun Microsystems, and other companies have mirror sites from which you can download their browser software. Mirroring could be considered a static form of content delivery.
MIS MIS (management information systems) is a general term for the computer systems in an enterprise that provide information about its business operations. It’s also used to refer to the people who manage these systems. Typically, in a large corporation, ‘‘MIS’’ or the ‘‘MIS department’’ refers to a central or centrally-coordinated system of computer expertise and management, often including mainframe systems but also including by extension the corporation’s entire network of computer resources. In the beginning, business computers were used for the practical business of computing the payroll and keeping track of accounts payable and receivable. As applications were developed that provided managers with information about sales, inventories, and other data that would help in managing the enterprise, the term ‘‘MIS’’ arose to describe these kinds of applications. Today, the term is used broadly in a number of contexts and includes (but is not limited to): decision support systems, resource and people management applications, project management, and database retrieval applications.
MITOCW See ‘‘OpenCourseWare’’
mks system of units See ‘‘metric system’’
Mobitex
MMC
MO drive
See ‘‘Microsoft Management Console’’
See ‘‘magneto-optical drive’’
MMDS
mobile satellite services
See ‘‘Multichannel Multipoint Distribution Service’’
Mobile satellite services (MSS) refers to networks of communications satellites intended for use with mobile and portable wireless telephones. There are three major types: AMSS (aeronautical MSS), LMSS (land MSS), and MMSS (maritime MSS).
MME See ‘‘MIM’’
MMX MMX is a Pentium microprocessor from Intel that is designed to run faster when playing multimedia applications. According to Intel, a PC with an MMX microprocessor runs a multimedia application up to 60% faster than one with a microprocessor having the same clock speed but without MMX. In addition, an MMX microprocessor runs other applications about 10% faster. The MMX technology consists of three improvements over the non-MMX Pentium microprocessor: 1. 57 new microprocessor instructions have been added that are designed to handle video, audio, and graphical data more efficiently. 2. A new process, Single Instruction Multiple Data (SIMD), makes it possible for one instruction to perform the same operation on multiple data items. 3. The memory cache on the microprocessor has increased to 32 thousand bytes, meaning fewer accesses to memory that is off the microprocessor.
mnemonic 1) In general, a mnemonic (from Greek mnemon or mindful; pronounced neh-MAHN-ik) is a word, abbreviation, rhyme, or similar verbal device you learn or create in order to remember something. The technique of developing these remembering devices is called ‘‘mnemonics.’’ Mnemonics is used to remember phone numbers, all your new department colleagues’ names, or the years of the reigns of the Kings and Queens of England. A number of approaches are used. Here’s a simple one for remembering a list of unrelated items in order: Start at the top of the list and make up an outlandish story connecting the first item to the next, continue by connecting the second item to the third, and so on. When your story is done and the list is removed, you’ll have a mental picture of a story that, as you recall its progression, will lead you from one remembered item to the next. 2) In computer assembler (or assembly) language, a mnemonic is an abbreviation for an operation. It’s entered in the operation code field of each assembler program instruction. For example, on an Intel microprocessor, inc (‘‘increase by one’’) is a mnemonic. On an IBM System/370 series computer, BAL is a mnemonic for ‘‘branch-and-link.’’
A telephone connection using MSS is similar to a cellular telephone link, except the repeaters are in orbit around the earth, rather than on the surface. MSS repeaters can be placed on geostationary, medium earth orbit (MEO), or low earth orbit (LEO) satellites. Provided there are enough satellites in the system, and provided they are properly spaced around the globe, an MSS can link any two wireless telephone sets at any time, no matter where in the world they are located. MSS systems are interconnected with landbased cellular networks. As an example of how MSS can work, consider telephones in commercial airliners. These sets usually link into the standard cellular system. This allows communication as long as the aircraft is on a line of sight with at least one landbased cellular repeater. Coverage is essentially continuous over most developed countries. But coverage is spotty over less well-developed regions, and is nonexistent at most points over the oceans. Using an MSS network, the aircraft can establish a connection from any location, no matter how remote.
Mobitex Mobitex is a wireless network architecture that specifies a framework for the fixed equipment necessary to support all the wireless terminals in a packet-switched, radio-based communication system. The three major components of a Mobitex network are the radio base station, the MX switch, and the network management center (NCC). Mobitex was developed in 1984 by Eritel, an Ericsson subsidiary, for the Swedish Telecommunication Administration. In a Mobitex network, a radio base station, with one or more switches (called MX switches), serves as the transmitter for each single cell (area of coverage) of up to 30 km. The base stations, among them, provide an area of coverage and determine the network capacity. Users of wireless devices, such as mobile phones and personal digital assistants (PDAs), communicate through the base station nearest to them and can move freely from one cell to another. The use of packet-switching technology for data transmission is less expensive than circuit-switching, which uses a dedicated path for each transmission. Mobitex packets (called MPAKs) are limited to 512 bytes of data. Each packet contains information about its origin and destination, size, type, and sequence within a transmission to ensure that
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it reaches its destination intact. Because packets can be sent on any route and in any order, they make more efficient use of channel capacity, supporting up to 50 times as many users per channel as a circuit-switched network. At the destination, packets are reorganized into the original transmission format. MX switches control communication routes to and from base stations and between wireless and fixed devices. Switches may be organized hierarchically into groupings of regional and area switches, all connected by fixed links. The MX switches also act as a gateway to other networks. A single network management center (NCC) takes care of maintenance and operations such as configuration and subscriber administration and billing. Currently at least twenty-eight Mobitex networks are in operation in twenty-two countries, mostly operating at either 80, 400, or 900 megahertz (MHz). In the US, Mobitex networks generally operate at 900 MHz, while European networks usually operate at 400 MHz. The Mobitex Operators Association (MOA) controls Mobitex specifications; Ericsson manufactures the infrastructure components.
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In recent years, the 2400 bits per second modem that could carry e-mail has become obsolete. 14.4 Kbps and 28.8 Kbps modems were temporary landing places on the way to the much higher bandwidth devices and carriers of tomorrow. From early 1998, most new personal computers came with 56 Kbps modems. By comparison, using a digital Integrated Services Digital Network adapter instead of a conventional modem, the same telephone wire can now carry up to 128 Kbps. With Digital Subscriber Line (DSL) systems, now being deployed in a number of communities, bandwidth on twisted-pair can be in the megabit range.
modem bonding See ‘‘modem doubling’’
modem doubling
Mobo is a short form for motherboard that is sometimes used in Usenet newsgroups and Web forum discussions.
Modem doubling is an inexpensive way for a user who wants a fast Internet connection, but can only connect with an analog telephone line, to use two 56kbps modems to double their bandwidth. Modem doubling makes it possible for the user to use one of two methods, modem bonding or modem teaming, to achieve a connection speed close to 112 Kbps.
model-view-controller In object-oriented programming development, model-viewcontroller (MVC) is the name of a methodology or design pattern for successfully and efficiently relating the user interface to underlying data models. The MVC pattern is widely used in program development with programming languages such as Java, Smalltalk, C, and C++. The MVC pattern has been heralded by many developers as a useful pattern for the reuse of object code and a pattern that allows them to significantly reduce the time it takes to develop applications with user interfaces. The model-view-controller pattern proposes three main components or objects to be used in software development: L
A Model, which represents the underlying, logical structure of data in a software application and the highlevel class associated with it. This object model does not contain any information about the user interface.
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A View, which is a collection of classes representing the elements in the user interface (all of the things the user can see and respond to on the screen, such as buttons, display boxes, and so forth).
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A Controller, which represents the classes connecting the model and the view, and is used to communicate between classes in the model and view.
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A modem (short for ‘‘modulator-demodulator’’) modulates outgoing digital signals from a computer or other digital device to analog signals on a conventional copper twisted pair telephone line and demodulates the incoming analog signal and converts it to a digital signal for the digital device.
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Modem bonding, which is supported by over ninety percent of Internet Service Providers (ISPs), takes advantage of Multilink Protocol Plus (MP+). Data packets are ‘‘inversemultiplexed’’ through the two modems, meaning that each of the modems receive half of the data packets as they are sent over the Internet and then they are recombined. An advantage of modem bonding is that if either of the modems gets disconnected for some reason, the other modem will take over so that the connection isn’t lost. Modem teaming, a good alternative for users whose IPS does not support MP+, allows the modems to work as separate connections by taking advantage of the ‘‘smart download’’ capability that most HTTP and FTP servers use in case a download is interrupted. Modem teaming requires a software application to instruct one modem to download the first part of the file while the other modem downloads the second part. A disadvantage of modem teaming is that it is not flexible if one modem should disconnect, nor is it useful when downloading streaming multimedia or conducting continuous sessions such as Telnet or PCAnywhere.
modem lights
modem error-correcting protocols
modem lights
The protocols that modems agree on and use for checking and correcting transmission errors have evolved toward accuracy, speed, and efficiency since 1978 when the Xmodem protocol became a de facto standard. To briefly summarize: In all protocols data is chunked into ‘‘blocks’’ of a certain byte size and sent to the destination modem which checks each block for errors and, depending on the results and the protocol, returns a positive (ACK) or negative (NAK) acknowledgement, the latter usually resulting in a retransmission. The type of checking (checksum or cyclic redundancy checking) and the frequency at which a response is sent vary by protocol. Today, new modems use a V.42 protocol, but the earlier protocols are still in use for older modems.
If you have an external modem, those flashing or steady lights tell you what’s happening in the ongoing ‘‘handshaking’’ or interaction between your computer (with its Universal Asynchronous Receiver/Transmitter or UART microchip) and the modem. Or, more formally, to use the RS-232C serial standard terms, between the Data Terminal Equipment (DTE) interface and the Data Communication Equipment (DCE) interface.
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Modem Light
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AA (auto answer)
The modem is ready to respond to an incoming call (assuming a communication program is also ready to handle the call). Since this light can be off even when the modem is ready, it’s not useful. This simply indicates that the computer and the modem have recognized each other and that a carrier connection is established. If a second call is received, carrier detect may be temporarily interrupted and, depending on a number of factors, the first connection may be dropped or lost. If on, indicates that the modem is ready to transfer data at its highest speed. Since this is often the case, this light is also not very useful. If on, indicates that your modem is ready to operate. If on, tells you that the phone line is ready for use. When flashing on, indicates that the modem is receiving data or signals from a remote computer. When flashing on, indicates that the modem is sending data or signals to another computer. When on, indicates that your computer’s communications program is active. If it’s not on, either the program or your computer may not be working.
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modem teaming
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modem V.xx standards See ‘‘V.xx’’
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modulation Modulation is the addition of information to an electronic or optical signal carrier. Modulation can be applied to direct current (mainly by turning it on and off), to alternating current, and to optical signals. One can think of blanket
waving as a form of modulation used in smoke signal transmission (the carrier being a steady stream of smoke). Morse code, invented for telegraphy and still used in amateur radio, uses a binary (two-state) digital code similar to the code used by modern computers. For most of radio and telecommunication today, the carrier is alternating current (AC) in a given range of frequencies. Common modulation methods include: L
Amplitude modulation (AM), in which the voltage applied to the signal is varied over time
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Frequency modulation (FM), in which the frequency of the carrier signal is transmitted is varied in small but meaningful amounts
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Phase modulation (PM), in which the natural flow of the alternating current waveform is delayed temporarily
These are sometimes known as continuous wave modulation methods to distinguish them from pulse code modulation (PCM), which is used to encode both digital and analog information in a binary way. Radio and television broadcast stations typically use AM or FM. Most two-way radios use FM, although some employ a mode known as single sideband (SSB). More complex forms of modulation are Phase Shift Keying (PSK) and Quadrature Amplitude Modulation (QAM). Optical signals are modulated by applying an electromagnetic current to vary the intensity of the laser beam.
Modem Modulation and Demodulation Any computer with an online or Internet connection includes a modem. This term is derived by combining the first three letters of the words modulator and demodulator. In a modem, the modulation process involves the conversion of the digital computer signals (high and low, or logic 1 and 0 states) to analog audio-frequency (AF) tones. Digital highs are converted to a tone having a certain constant pitch; digital lows are converted to a tone having a different constant pitch. These states alternate so rapidly that, if you listen to the output of a computer modem, it sounds like a hiss or roar. The demodulation process converts the audio tones back into digital signals that a computer can understand. directly.
Multiplexing More information can be conveyed in a given amount of time by dividing the bandwidth of a signal carrier so that more than one modulated signal is sent on the same carrier. Known as multiplexing, the carrier is referred to as a channel and each separate signal carried on it is called a subchannel. The device that puts the separate signals on the carrier and takes them off of received transmissions is a multiplexer. Common types of multiplexing include frequency-division multiplexing (FDM) and time-division multiplexing (TDM). FDM is usually used for analog
Molex communication and divides the main frequency of the carrier into separate subchannels, each with its own frequency band within the overall bandwidth. TDM is used for digital communication and divides the main signal into time-slots, with each time-slot carrying a separate signal.
module In computers, a module (pronounced MAHD-yewl, from Latin modulus, diminutive of modus, the measure or way of being) is, in general, a separate unit of software or hardware. The term was first used in architecture. 1) In computer programming, especially in older languages such as PL/1, the output of the language compiler was known as an object module to distinguish it from the set of source language statements, sometimes known as the source module. In mainframe systems such as IBM’s OS/360, the object module was then linked together with other object modules to form a load module. The load module was the executable code that you ran in the computer. Modular programming is the concept that similar functions should be contained within the same unit of programming code and that separate functions should be developed as separate units of code so that the code can easily be maintained and reused by different programs. Objectoriented programming is a newer idea that inherently encompasses modular programming. 2) In computer hardware and electronics, a module is a relatively compact unit in a larger device or arrangement that is designed to be separately installed, replaced, or serviced. For example, a single in-line memory module is a unit of random access memory (RAM) that you can add to a personal computer.
mole The mole (abbreviation, mol) is the International System of Units (SI) unit of material quantity. One mole is the number of atoms in precisely 12 thousandths of a kilogram (0.012 kg) of C-12, the most common naturally-occurring isotope of the element carbon. This number is equal to approximately 6.022169 6 1023, and is also called the Avogadro constant. The mole is the only fundamental SI unit that is dimensionless. That means it represents a simple numerical quantity or value; it is a term similar in nature to ‘‘dozen’’ (meaning 12) or ‘‘gross’’ (meaning 144). But 1 mol is, unlike a dozen or a gross, an incomprehensibly large number. Written out in decimal form, it appears as 602,216,900,000,000,000,000,000. Envision a huge cubical box measuring about 84 kilometers (84 km) on each edge. Then imagine that box being neatly packed full of sandgrain-size cubes measuring a millimeter (1 mm) on an edge. If this were done, the box would contain roughly 1 mol of cubes.
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The mole is of interest primarily to chemists and physicists. Sometimes, larger or smaller units are defined by attaching power-of-10 prefix multipliers. For example, a kilomole (1 kmol) is 103 mol, or approximately 6.022169 6 1026, and a millimole (1 mmol) is 10-3 mol, or approximately 6.022169 6 1020.
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Also see amount of substance, prefix multiplier, and International System of Units (SI).
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mole per meter cubed The mole per meter cubed (mol/m3) is the International Unit of amount-of-substance concentration. It is an expression of the number of atoms of a substance per unit volume, and is of interest primarily to chemists and physicists. One mole is equal to approximately 6.022169 6 1023, and is also called the Avogadro constant. Thus, 1 mol/m3 represents approximately 6.022169 6 1023 atoms of a substance in one cubic meter of space. Sometimes, larger or smaller units are defined by attaching power-of-10 prefix multipliers. For example, a kilomole per meter cubed (1 kmol/m3) is 103 mol/m3, or approximately 6.022169 6 1026 atoms per cubic meter; a nanomole per meter cubed (1 nmol/m3) is 10-9 mol/m3, or approximately 6.022169 6 1014 atoms per cubic meter.
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Also see meter cubed, mole, prefix multipliers, and SI (International System of Units).
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Molex is a company, in business since 1936, that manufactures electronic, electrical, and optical fiber connectors. Molex also makes a variety of switch and application tooling, especially crimp application tooling. Crimping is the metallurgical compression of a terminal around the conductor of a wire, which creates a common electrical path of low resistance and high current-carrying capabilities. A secondary crimp around the insulation of the wire provides support for insertion into a housing and allows the terminal to withstand shock and vibration. The crimp technology was developed as a substitute to soldering and the problems associated with it. Crimp terminations can be applied with a basic hand tool, a press and die set, a stripper crimper, or a fully automatic wire processing system.
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Molex’s Insulation Displacement Technology (IDT) is a wire termination technique in which an insulated wire is pressed into a terminal slot smaller than the conductor diameter, displacing the insulation and forming an electrical contact between the terminal and conductor.
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Other Molex products include thermal acoustic products (noise reduction and thermal component products for quieting and cooling personal computers) and cable assemblies.
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momentum
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Momentum is a vector quantity that is the product of the mass and the velocity of an object or particle. The standard unit of momentum magnitude is the kilogram-meter per second (kg ˙ m/s or kg ˙ m ˙ s-1). Alternatively, the gramcentimeter per second (g ˙ cm/s or g ˙ cm ˙ s-1) can be used to express momentum magnitude. The direction of a momentum vector can be expressed in various ways, depending on the number of dimensions involved, and is the same as the direction of the velocity vector.
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Momentum, like velocity, is relative. Consider a 1,000-kg car moving at 20 m/s with respect to the surface of a highway, traveling northward. If you are driving the car, the momentum of the car relative to your body is zero. If you stand by the side of the road, the momentum of the car relative to you is 20,000 kg ˙ m/s northward. If you are driving a 1,000-kg car at 15 m/s with respect to the road and are traveling northward, and a truck of mass 1,500 kg is moving 20 m/s with respect to the road and comes up behind you in the same direction, the truck’s momentum relative to you is the product of its relative velocity (5 m/s northward) and its mass (1,500 kg), or 7,500 kg ˙ m/s northward. Relative to the truck, the momentum of your car will be in the opposite direction, and will be smaller: 5 m/s 6 1,000 kg = 5,000 kg ˙ m/s southward. Thus, if a collision occurs, the danger is greater to the object that is less massive. If the above mentioned truck passes you going the opposite way on the road, its momentum relative to you is 35 m/s 6 1,500 kg southward, or 52,500 kg ˙ m/s southward. Relative to it, your momentum is 35 m/s 6 1,000 kg northward, or 35,000 kg m/s northward. In that situation, as with the rearend scenario, the peril is greater to the less massive vehicle in the event of a collision. Momentum, like velocity, can be expressed either as an average over a period of time or as an instantaneous value at a single moment in time. Also see mass, velocity, vector, SI (International System of Units), and Table of Physical Units.
moniker In general, a moniker is a name or a nickname and, in the simplest terms, that is what it is in computer terminology as well. A moniker is an object (or component) in Microsoft’s Component Object Model (COM) that refers to a specific instance of another object. Monikers originated in Microsoft’s Object Linking and Embedding (OLE) technology as a means of linking objects. A moniker may refer to any single object, or may be a composite made of a number of separate monikers, each of which refers to a particular instantiation of an object. The moniker is sometimes referred to as an ‘‘intelligent name,’’ because it retains information about how to create, initialize,
and bind to a single instance of an object. Once created, the moniker holds this information, as well as information about the object’s states in that specific instantiation. Since COM is not language-specific, a moniker can be used with any programming language. The programmer gives the instantiation of the object a name. By calling the moniker in code, a programmer can refer to the same object with the same states. If, for example, a moniker is created for a query, the programmer can reuse the query simply by calling the moniker in the code, because the moniker itself has the necessary information.
monitor In computers, a monitor is a computer display and related parts packaged in a physical unit that is separate from other parts of the computer. Notebook computers don’t have monitors because all the display and related parts are integrated into the same physical unit with the rest of the computer. In practice, the terms monitor and display are used interchangably.
MOO A MOO is an object-oriented MUD. According to Canton Becker, author of ‘‘The Unofficial Moo Guide Tutorial,’’ a MOO is: ‘‘...just a programming language in which you design objects. Everything is an object. Rooms are objects, exits are objects, possessions are objects, even your MOO alter-ego/avatar is an object. We’ll be looking at how you (1) make objects, and (2) write verbs that allow you to do Interesting Things with those objects. I’m not going to talk too much about the philosophy of object-oriented programming (that’s what the OO in MOO stands for) so let’s go ahead and make your first object:’’ Canton Becker’s first example (of creating an object) looks like this: ’@create $thing called kleenex’
You can then go on to relate the object to other objects and define the verbs that work with the object (which is given a number by the MOO server). Effectively, you can set up how other MOO participants will be able to use or view your object.
moof monster The moof monster is a vague and indefinable source of trouble for users of information technology. The term is used especially by people who frequent Internet Relay Chat (IRC) channels. If you’re suddenly disconnected from your channel, it can be attributed to the moof monster. You are said to have been ‘‘moofed.’’ The term seems reminiscent of the gremlin of the 1940 era.
Mosaic This term is apparently unrelated to the ‘‘moof!’’, the sound of the dogcow in an Apple Macintosh.
moonbounce Moonbounce, also called Earth-Moon-Earth (EME), is a form of wireless communication in which the moon is used as a passive satellite. To the uninitiated, this sounds a little like science fiction, but it has been done and continues to be done by experimentally-inclined amateur radio operators. There are several challenges and difficulties inherent in moonbounce operation. One of the most troublesome for two-way communication is the fact that the moon’s distance introduces lag time. The moon is approximately 250,000 miles away from the earth, and radio waves travel at 186,282 miles per second. A signal sent to the moon does not return until 2.7 seconds have elapsed. If two people are engaged in a conversation and one person asks a question, that person cannot expect a reply until at least 5.4 seconds later (the answer must travel to the moon and back, as must the question). Besides propagation delay, the path loss to and from the moon is considerable. The moon is a relatively poor reflector of electromagnetic rays at any wavelength, including radio waves. Its surface is irregular, and it scatters, rather than focusing, reflected energy. Because of this, sophisticated equipment is necessary to successfully bounce a signal off the moon and hear it return. Another problem with moonbounce communication is libration fading and Doppler shifting. The moon does not always present exactly the same face; it ‘‘wobbles’’ a few degrees back and forth. This ‘‘wobbling,’’ called libration, produces a constant change in every component of any signal reflected from the moon. The returned signal consists of the sum total of countless rays that have bounced off mountains, boulders, crater walls, and other lunar features. The relative phase of these components rapidly fluctuates because of libration, so any signal returning from the moon is ‘‘fluttery’’ and distorted. Amateur-radio following:
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A sensitive receiver with a narrowband filter
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A transmitter capable of operating on at least one amateur band above 144 MHz, and capable of producing 1500 watts of continuous radio-frequency output
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An antenna with high directivity and gain, capable of being rotated in both the azimuth and elevation planes
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A location in which the moon can be seen without obstruction for extended periods
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A location in which human-made radio noise is minimal
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Neighbors who will tolerate the presence of a large antenna and the proximity of a high-power radio transmitter
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A neighborhood without ordinances or covenants prohibiting large antennas and/or high-power radio transmitters
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Operating skill and patience
Moore’s Law Moore’s Law is that the pace of microchip technology change is such that the amount of data storage that a microchip can hold doubles every year or at least every 18 months. In 1965 when preparing a talk, Gordon Moore noticed that up to that time microchip capacity seemed to double each year. The pace of change having slowed down a bit over the past few years, the definition has changed (with Gordon Moore’s approval) to reflect that the doubling occurs only every 18 months.
Morphis Morphis is a Java-based open source wireless transcoding platform from Kargo, Inc., a wireless technology company based in New York City. The new application development platform will enable delivery of customized text and graphics to users of wireless devices, such as mobile phones, personal digital assistants (PDAs), and handheld computers. Morphis provides a framework for the transformation of binary, plain text, and text markup content to suitable formats for wireless applications. It can also process images, as well as Hypertext Markup Language (HTML), Wireless Markup Language (WML), and Wireless Abstract for XML (WAX), an integral part of Morphis. According to Kargo’s Morphis product description, the platform supplies a framework that allows users to retrieve, translate, and convert any electronic document. Morphis’ Extensible Markup Language (XML) processing framework uses Simple API for XML (SAX) event processing and XSL Transformations (XSLT). A single document may undergo multiple translations and filters; complex logic can be provided by writing XSLT extensions. Multiple complex transformations are performed through a pipeline SAX processing engine. Kargo claims that Morphis will allow content providers to develop both current and future wireless applications without requiring they use specific standards or technologies.
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Mosaic Mosaic was the first widely-distributed graphical browser or viewer for the World Wide Web. It is usually considered to have been the software that introduced the World Wide Web (and the Internet) to a wide general audience. Once Mosaic was available, the Web virtually exploded in numbers of users and content sites. (Of course, the software depended on the recent invention of the Hypertext Transport Protocol, or Hypertext Transfer Protocol, by Dr. Tim Berners-Lee.)
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Mosaic arrived in 1993. Marc Andreessen, then in his early 20’s, is credited with inventing or leading the development of Mosaic. He developed it at the National Center for Supercomputing Applications (NCSA) at the University of Illinois in Urbana, Illinois. Andreessen and others went on to become part of Netscape Communications, originally called Mosaic Communications, that developed what was initially the Web’s most popular browser, Netscape (its full name is Netscape Navigator).
The Motif Toolkit, also known as ‘‘Xm,’’ ensures consistency with the Motif guideline and is used with the X Toolkit Intrinsics (‘‘Xt’’). In turn, Xt is built on the lowest-level application program level, X library (‘‘Xlib’’). The Motif Style Guide, which describes how to conform to the guideline, is published as an industry standard (IEEE 1295, orderable from the Institute of Electrical and Electronics Engineers). Motif is the base graphical user interface toolkit for the Open Group’s Common Desktop Environment (CDE).
The original Mosaic, now in a later version, has since been licensed for commercial use and is provided to users by several Internet access providers.
2) Motif is also the name of a Microsoft Windows program object for a sound sequence or musical motif (short theme or iconic sound). Motifs are also known as Microsoft Interactive Music Controls.
Other browsers include Microsoft Internet Explorer (MSIE), a text-only browser, Lynx, and Opera.
motherboard
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A motherboard is the physical arrangement in a computer that contains the computer’s basic circuitry and components. On the typical motherboard, the circuitry is imprinted or affixed to the surface of a firm planar surface and usually manufactured in a single step. The most common motherboard design in desktop computers today is the AT, based on the IBM AT motherboard. A more recent motherboard specification, ATX, improves on the AT design. In both the AT and ATX designs, the computer components included in the motherboard are: L
The microprocessor
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Additional components can be added to a motherboard through its expansion slot. The electronic interface between the motherboard and the smaller boards or cards in the expansion slots is called the bus.
Motif 1) Motif is a graphical user interface (GUI) guideline and development toolkit for the X Window System. As such, it defines the rules and tools for the ‘‘look-and-feel’’ in the graphical user interface of most UNIX-based operating systems and their applications. Developed in 1989 by the Open Software Foundation (now part of The Open Group), Motif corresponds to IBM’s Common User Access (CUA) guideline for its OS/2 Presentation Manager and to the guidelines built into the Microsoft Windows application programming interfaces, including the Microsoft Foundation Class Library for user interface objects (often called widgets).
motion plan A motion plan is a multi-step process that can be used by a robot to precisely position itself to perform a specified task. Suppose you own an autonomous mobile personal robot. You tell it to switch on the light in a bedroom. The light switch is on the wall. The robot has a computer map of the house stored in its controller memory. This threedimensional map includes the exact coordinates of the bedroom light switch, and/or its position relative to other objects. The robot first proceeds to the general location of the switch. The program that facilitates this is known as a grossmotion plan. In order to find the switch, and to position its end-effector (robot hand) in the right place to toggle it, a fine-motion plan is required. Machine vision can facilitate this. An example is the so-called eye-in-hand system, which allows the robot to recognize the shape of the switch and guide the end effector into place, down to millimeter displacements or smaller. Another method of executing the fine-motion plan involves tactile sensing, in which the robot’s end effector gropes along the wall until it finds and throws the switch, just as you could find and throw it with your eyes closed once you were in its general vicinity. Also see artificial intelligence, eye-in-hand system, grasping plan, and robotics.
mount In computers, to mount is to make a group of files in a file system structure accessible to a user or user group. In some usages, it means to make a device physically accessible. Macintosh calls it mounting when a user inserts a disc into the machine. In a UNIX environment, the mount command attaches disks, or directories logically rather than physically. The Unix mount command makes a directory accessible by attaching a root directory of one file system to another directory, which makes all the file systems usable as if they were subdirectories of the file system they are attached to. Unix recognizes devices by their location, as compared to Windows, which recognizes them by their names (C: drive,
mousepad for example). Unix organizes directories in a tree-like structure, in which directories are attached by mounting them on the branches of the tree. The file system location where the device is attached is called a mount point. Mounts may be local or remote. A local mount connects disc drives on one machine so that they behave as one logical system. A remote mount uses Network File System (NFS) to connect to directories on other machines so that they can be used as if they were all part of the user’s file system.
mouse A mouse is a small device that a computer user pushes across a desk surface in order to point to a place on a display screen and to select one or more actions to take from that position. The mouse first became a widely-used computer tool when Apple Computer made it a standard part of the Apple Macintosh. Today, the mouse is an integral part of the graphical user interface (GUI) of any personal computer. The mouse apparently got its name by being about the same size and color as a toy mouse. A mouse consists of a metal or plastic housing or casing, a ball that sticks out of the bottom of the casing and is rolled on a flat surface, one or more buttons on the top of the casing, and a cable that connects the mouse to the computer. As the ball is moved over the surface in any direction, a sensor sends impulses to the computer that causes a mouseresponsive program to reposition a visible indicator (called a cursor) on the display screen. The positioning is relative to some variable starting place. Viewing the cursor’s present position, the user readjusts the position by moving the mouse. The most conventional kind of mouse has two buttons on top: the left one is used most frequently. In the Windows operating systems, it lets the user click once to send a ‘‘Select’’ indication that provides the user with feedback that a particular position has been selected for further action. The next click on a selected position or two quick clicks on it causes a particular action to take place on the selected object. For example, in Windows operating systems, it causes a program associated with that object to be started. The second button, on the right, usually provides some less-frequently needed capability. For example, when viewing a Web page, you can click on an image to get a popup menu that, among other things, lets you save the image on your hard disk. Some mouses have a third button for additional capabilities. Some mouse manufacturers also provide a version for lefthanded people. Windows and other operating systems let the user adjust the sensitivity of the mouse, including how fast it moves across the screen, and the amount of time that must elapse within a ‘‘double click.’’ In some systems, the user can also choose among several different cursor appearances. Some people use a mousepad to improve traction for the mouse ball.
Although the mouse has become a familiar part of the personal computer, its design continues to evolve and there continue to be other approaches to pointing or positioning on a display. Notebook computers include built-in mouse devices that let you control the cursor by rolling your finger over a built-in trackball. IBM’s ScrollPoint mouse adds a small ‘‘stick’’ between two mouse buttons that lets you scroll a Web page or other content up or down and right or left. Users of graphic design and CAD applications can use a stylus and a specially-sensitive pad to draw as well as move the cursor. Other display screen-positioning ideas include a video camera that tracks the user’s eye movement and places the cursor accordingly.
mouse miles Mouse miles is slang for user time at the computer (as in ‘‘I traveled a lot of mouse miles this week’’) and also an actual measure of how much activity a computer mouse has had over time. At least two companies offer shareware you can download that will tell you how many miles your mouse has traveled. One product provides a visual warning signal when it’s time to clean your mouse.
mouse potato A mouse potato is the computer equivalent of television’s couch potato: Someone who tends to spend a great deal of leisure time in front of the computer in much the same way the couch potato does in front of the television. Both activities tend to be accompanied by snacking. A recent survey by the American Snack Food Association found that 85% of Web surfers snack at the computer. It has been observed that this habitual nibbling and relative inactivity can lead to development of a characteristic potato-like body form. Television networks are concerned about the new phenomenon; they want to keep their potatoes planted on the couch. The Web’s main lure—and what television has been lacking—is the capacity for interaction, something that will be increasingly built into television broadcasting with the move to digital television and the cable modem. The convergence of television and the computer could perhaps be viewed as the convergence of the couch potato and the mouse potato.
mouseover
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See ‘‘rollover’’
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A mousepad is a small, portable surface that sometimes provides better traction for the ball on a computer mouse and, at the very least, provides a bounded area in which to move the mouse. On very smooth table or desktop surfaces,
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the mouse ball may tend to glide instead of roll unless pressure is continually applied. Some mousepads provide a slightly rougher surface so that it’s easier to get the mouse to move the screen cursor. A mousepad usually has a plastic surface and a thin rubber or plastic cushion.
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Mousepads are sometimes given away at computer and Internet trade shows or with Internet or computer magazine subscriptions.
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Mozilla was Netscape Communication’s nickname for Navigator, its Web browser, and, more recently, the name of an open source public collaboration aimed at making improvements to Navigator. This public collaboration was essentially launched and is still substantially supported by Netscape (now owned by AOL); however, the Mozilla project is independent. Netscape can use its code but so can anyone else. Mozilla originated as a name used by Navigator’s developers before the product had a commercial name and in varying degrees has continued to be nurtured by the company’s founders (of which the chief was Marc Andreessen, who designed Mosaic, the first Web browser with a graphical user interface). From time to time, Netscape has used Mozilla as the name of a kind of mascot or cartoon alterego created by illustrator Dave Titus.
MP3 MP3 (MPEG-1 Audio Layer-3) is a standard technology and format for compressing a sound sequence into a very small file (about one-twelfth the size of the original file) while preserving the original level of sound quality when it is played. MP3 files (identified with the file name suffix of ‘‘.mp3’’) are available for downloading from a number of Web sites. Many Windows users will find that they have a player built into their operating system. Otherwise, you can download a player from one of several popular MP3 sites. MP3 files are usually download-and-play files rather than streaming sound files that you link-and-listen-to with RealPlayer and similar products (However, streaming MP3 is possible.) Winamp (PC), MacAmp (Mac), and mpeg123 (UNIX) are popular MP3 players, but there are many others. To create an MP3 file, you use a program called a ripper to get a selection from a CD onto your hard disk and another program called an encoder to convert the selection to an MP3 file. Most people, however, simply download MP3s from someone else and play them. Digital audio is typically created by taking 16 bit samples a second of the analog signal. Since this signal is typically spread out over a spectrum of 44.1 thousand cycles per second (kHz), this means that one second of CD quality sound requires 1.4 million bits of data. Using their knowledge of how people actually perceive sound, the developers of MP3 devised a compression algorithm that
reduces data about sound that most listeners can not perceive. MP3 is currently the most powerful algorithm in a series of audio encoding standards developed under the sponsorship of the Motion Picture Experts Group (MPEG) and formalized by the International Organization for Standardization (ISO). Since it is relatively easy to create MP3 files from CD selections and make them available on Web sites for downloading, companies and sites that promote the MP3 format are sometimes accused of encouraging copyright violations. (It is illegal to copy music from a CD and redistribute it unless you have the copyright owner’s permission.) On the other hand, MP3 enthusiasts claim that what CD publishers are afraid of is any kind of non-CD distribution. While there are several proposals for how to discourage such piracy, there is currently no secure distribution and copyright management standard that publishers and other parties agree upon.
MPE/iX MPE/iX is the operating system for the line of e3000 midrange business servers from Hewlett-Packard (HP). Its latest version, MPE/iX Version 7.0, is marketed as ‘‘Webenabled’’ and ‘‘Internet-ready.’’ The operating system includes support for Java and HP describes it as providing ‘‘OLTP performance and functionality for business-critical applications.’’ Support also is provided for PCI I/O devices. HP emphasizes the value of an operating system that, since its first release in 1972 has built a reputation for being stable, reliable, and robust. MPE/iX provides support for applications that use standard POSIX C functions. In 2001, HP plans to run MPE-iX on e3000s that use the Intel Itanium (IA-64) microprocessor architecture. MPE originally stood for ‘‘Multi-Programming Executive.’’ The ‘‘iX’’ suggests its new support for POSIX (UNIX-compatible) applications and interfaces and a hierarchical file system. Unlike HP’s other major operating system, HP-UX (for the HP 9000 server line), MPE/iX supports all the legacy applications that have been written for HP 3000 computers over the years.
MPEG MPEG (pronounced EHM-pehg), the Moving Picture Experts Group, develops standards for digital video and digital audio compression. It operates under the auspices of the International Organization for Standardization (ISO). The MPEG standards are an evolving series, each designed for a different purpose. To use MPEG video files, you need a personal computer with sufficient processor speed, internal memory, and hard disk space to handle and play the typically large MPEG file (which has a file name suffix of .mpg). You also need an MPEG viewer or client software that plays MPEG files. (Note that .mp3 file suffixes indicate MP3 (MPEG-1 audio layer-3)
MRAM files, not MPEG-3 standard files.) You can download shareware or commercial MPEG players from a number of sites on the Web.
MPEG standards The MPEG standards are an evolving set of standards for video and audio compression developed by the Moving Picture Experts Group (MPEG). MPEG-1 was designed for coding progressive video at a transmission rate of about 1.5 million bits per second. It was designed specifically for Video-CD and CD-i media. MPEG1 audio layer-3 (MP3) has also evolved from early MPEG work. MPEG-2 was designed for coding interlaced images at transmission rates above 4 million bits per second. MPEG-2 is used for digital TV broadcast and digital versatile discs. An MPEG-2 player can handle MPEG-1 data as well. A proposed MPEG-3 standard, intended for High Definition TV (HDTV), was merged with the MPEG-2 standard when it became apparent that the MPEG-2 standard met the HDTV requirements. An MPEG-4 standard is in the final stages of development and release. It is a much more ambitious standard and addresses speech and video synthesis, fractal geometry, computer visualization, and an artificial intelligence (AI) approach to reconstructing images. An MPEG-7 is being discussed. MPEG-1 and -2 define techniques for compressing digital video by factors varying from 25:1 to 50:1. The compression is achieved using five different compression techniques: 1. The use of a frequency-based transform called Discrete Cosine Transform (DCT). 2. Quantization, a technique for losing selective information (sometimes known as lossy compression) that can be acceptably lost from visual information. 3. Huffman coding, a technique of lossless compression that uses code tables based on statistics about the encoded data. 4. Motion compensated predictive coding, in which the differences in what has changed between an image and its preceding image are calculated and only the differences are encoded. 5. Bi-directional prediction, in which some images are predicted from the pictures immediately preceding and following the image. The first three techniques are also used in JPEG file compression.
MPLS See ‘‘Multiprotocol Label Switching’’
MPP See ‘‘massively parallel processing’’
MQSeries MQSeries is an IBM software family whose components are used to tie together other software applications so that they can work together. This type of application is often known as business integration software or middleware.
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MQSeries Integrator, which centralizes and applies business operations rules
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MQSeries Workflow, which enables the capture, visualization, and automation of business processes
The point of business integration is to connect different computer systems, diverse geographical locations, and dissimilar IT infrastructures so that a seamless operation can be run. IBM’s MQSeries supplies communications between applications, or between users and a set of applications on dissimilar systems. It has grown in popularity as applications are made available over the Internet because of its support of over 35 platforms and its ability to integrate disparate automation systems. An additional helpful feature is that its messaging scheme requires the application that receives the the message to confirm receipt. If no confirmation materializes, the message is re-sent by the MQSeries. IBM asserts that MQSeries can connect any two commercial systems that are in current business use.
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See ‘‘modem lights’’
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MRAM MRAM (magnetoresistive random access memory) is a method of storing data bits using magnetic charges instead of the electrical charges used by DRAM (dynamic random access memory). Scientists define a metal as magnetoresistive if it shows a slight change in electrical resistance when placed in a magnetic field. By combining the high speed of static RAM and the high density of DRAM, proponents say MRAM could be used to significantly improve electronic products by storing greater amounts of data, enabling it to be accessed faster while consuming less battery power than existing electronic memory. Conventional random access memory (RAM) computer chips store information as long as electricity flows through them. Once power is turned off, the information is lost unless it has been copied to a hard drive or floppy disk. MRAM, however, retains data after a power supply is cut off. Replacing DRAM with MRAM could prevent data loss and enable computers that start instantly, without waiting for software to boot up.
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Whatis?Com’s Encyclopedia of Technology Terms The U.S. Defense Advanced Research Projects Agency (DARPA) has provided funding to help private industry conduct research into the potential of MRAM. Beginning in 1995, DARPA began funding three private consortia researching the viability of making MRAM a generalpurpose memory with high density, high speed, and low power usage. Leading the three consortia were IBM, Motorola, and Honeywell. Hewlett-Packard, Matsushita, NEC, Fujitsu, Toshiba, Hitachi, and Siemens also have invested in MRAM research. Motorola Labs says its ‘‘universal memory’’ allows the integration of several memory options within a single chip, resulting in a chip that uses less power. The chip is a threevolt MRAM with an address access time of about 15 nanoseconds. IBM and Infineon Technologies AG are working on a proposed 256-megabit chip they say could be on the market in 2004.
referred to their system as Disk Operating System. Like PCDOS, MS-DOS was (and still is) a non-graphical lineoriented command-driven operating system, with a relatively simple interface but not overly ‘‘friendly’’ user interface. Its prompt to enter a command looks like this: C:> The first Microsoft Windows operating system was really an application that ran on top of the MS-DOS operating system. Today, Windows operating systems continue to support DOS (or a DOS-like user interface) for special purposes by emulating the operating system. In the 1970s before the personal computer was invented, IBM had a different and unrelated DOS that ran on smaller business computers. It was replaced by IBM’s VSE operating system.
MSIE See ‘‘Microsoft Internet Explorer’’
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Development of MRAM basically followed two scientific schools: 1) spin electronics, the science behind giant magnetoresistive heads used in disk drives and 2) tunneling magnetic resistance, or TMR, which is expected to be the basis of future MRAM.
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An mrouter, or multicast router, is a router program that distinguishes between multicast and unicast packets and determines how they should be distributed along the Multicast Internet (sometimes known as the Multicast Backbone or MBone). Using an appropriate algorithm, an mrouter tells a switching device what to do with the multicast packet.
MTBF (mean time between failures) is a measure of how reliable a hardware product or component is. For most components, the measure is typically in thousands or even tens of thousands of hours between failures. For example, a hard disk drive may have a mean time between failures of 300,000 hours. A desired MTBF can be used as a quantifiable objective when designing a new product. The MTBF figure can be developed as the result of intensive testing, based on actual product experience, or predicted by analyzing known factors. The manufacturer may provide it as an index of a product’s or component’s reliability and, in some cases, to give customers an idea of how much service to plan for.
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Mrouters currently make up ‘‘islands’’ on the MBone separated by unicast routers. Thus, an mrouter can disguise multicast packets so that they can cross unicast routers. This is done by making each multicast packet look like a unicast packet; the destination address is the next mrouter. This process is called IP tunneling.
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There are two multicast routing protocols that mrouters use to distribute multicast packets. They are dense-mode routing and sparse-mode routing. The protocol used is determined by available bandwidth and the distribution of end users over the network. If the network has many end users and there is enough bandwidth, dense-mode routing is used. However, if bandwidth is limited and users are thinly distributed, sparse-mode routing is used.
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Also see Disk Operating System.
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MS-DOS (Microsoft Disk Operating System) was the Microsoft-marketed version of the first widely-installed operating system in personal computers. It was essentially the same operating system that Bill Gates’s young company developed for IBM as Personal Computer—Disk Operating System (PC-DOS). Most users of either DOS system simply
MSP See ‘‘management service provider’’
Most sources define this term to mean average time between failures.
M-theory M-theory (the ‘‘M’’ stands for the mother of all theories, magic, mystery, or matrix, depending on the source) is an adaptation of superstring theory developed by Ed Witten of Princeton and Paul Townsend of Cambridge. Townsend and Witten’s version could potentially be the unified field theory sought by Einstein for the last 40 years of his life: A simple equation that would reconcile incompatible aspects of his theory of relativity and quantum theory to explain the nature and behavior of all matter and energy. Applications of this knowledge could, through unlocking nature’s secrets, enable future technologies that currently are only spoken of within the realm of science fiction: an inexhaustible source of clean energy, and time travel, for example.
MUD Superstring theory (sometimes just called string theory) has as its basic premise the belief that the four fundamental forces of nature (gravity, electromagnetism, and strong and weak nuclear forces), as well as all matter are simply different manifestations of a single essence. This essence, the material making up all energy and matter, is thought to consist of tiny (a hundred billion billion times smaller than the nucleus of an atom) vibrating strings that exist in a multidimensional (10 or 26 dimensions) hyperspace. The extra dimensions (beyond the ones we recognize: three spatial dimensions and time) are thought to be compactified, or curled up, into tiny pockets inside observable space. The particular vibrations of the strings within this multidimensional hyperspace are thought to correspond to particles that form the basis of everything—all matter and energy—in existence.
Mtops Mtops (million theoretical operations per second) is a measure of computer performance used by the U.S. government and cooperating countries in determining whether a computer can be exported abroad to certain countries that might use it for nuclear arms development. (Mtops sometimes is written as ‘‘MTOPS’’ or ‘‘mtops.’’) In early November 1997, the U.S. Congress included in its 1997 Defense Appropriation bill an amendment to subject any manufacturer of computers in the 2,000 to 7,000 Mtops range to licensing restrictions on sales to many countries that have not agreed to stop nuclear arms development or testing. These include China, Russia, and India. Computers in the 2,000 Mtops range include workstation and may soon include powerful personal computers. The computer technology industry argued that computers in this range are already widely manufactured and sold abroad, even in a number of countries on the restricted list, and that the law would make it difficult for U.S. computer manufacturers to compete with other countries. While the law, which went into effect on February 2, 1998, doesn’t prohibit the shipment of computers in these classes to the designated ‘‘tier 3’’ countries, application for permission to ship can take up to 90 days. The calculation of Mtops involves counting the number of operations that could be performed by the computer during a second’s time, based on a specified formula. The actual number of operations that any computer performs over a given period of time depends on which operations are performed (some operations take longer than others or can be performed within the same clock cycle with other operations) and the real cycle speed.
MTS The Microsoft Transaction Server (MTS), called ‘‘Viper’’ while it was being developed, is a program that runs on an Internet or other network server with a Windows NT or later system and manages application and database transaction requests on behalf of a client computer user. The Transaction Server screens the user and client computer from having to formulate requests for unfamiliar databases and, if necessary, forwards the requests to database servers. It also manages security, connection to other servers, and transaction integrity. The Transaction Server is Microsoft’s bid to make distributed applications and data in a network relatively easy to create. It’s one of a category of programs sometimes known as middleware or multi-tier programming for the enterprise market that IBM has traditionally controlled with its CICS and similar transaction management products. Microsoft designed the Transaction Server to fit in with its overall object-oriented programming strategy. Using the Transaction Server, you can use a drag-and-drop interface to create a transaction model for a single user, then allow the Transaction Server to manage the model for multiple users, including the creation and management of user and task threads and processes.
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MTS runs on Windows NT Server 4.0 or higher.
K MUD A MUD or Multi-User Dungeon is an inventively structured social experience on the Internet, managed by a computer program and often involving a loosely organized context or theme, such as a rambling old castle with many rooms or a period in national history. Some MUDs are ongoing adventure games; others are educational in purpose; and others are simply social. MUDs existed prior to the World Wide Web, accessible through Telnet to a computer that hosted the MUD. Today, many MUDs can be accessed through a Web site and some are perhaps better known as 3D worlds or chatting. MUD participants adopt a character or avatar when they join or log in to a MUD. Typically, you can describe your avatar to the other participants. Each MUD has its own name, special character and ambience, and set of rules. MUDs are run by advanced participants or programmers called wizards. Although many MUDs continue to be entirely text-based, some new MUDs use virtual reality settings and you can see the characters. However, the focus is on the exchange of text between participants who are logged in at a particular time. There are a number of variations on the MUD, including MOO, MUCKs, and MUSHes, each associated with a server program of that name and varied mainly by the programming language used and the capabilities offered.
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multi-carrier modulation
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Multi-carrier modulation (MCM) is a method of transmitting data by splitting it into several components, and sending each of these components over separate carrier signals. The individual carriers have narrow bandwidth, but the composite signal can have broad bandwidth.
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The advantages of MCM include relative immunity to fading caused by transmission over more than one path at a time (multipath fading), less susceptibility than single-carrier systems to interference caused by impulse noise, and enhanced immunity to inter-symbol interference. Limitations include difficulty in synchronizing the carriers under marginal conditions, and a relatively strict requirement that amplification be linear. MCM was first used in analog military communications in the 1950s. Recently, MCM has attracted attention as a means of enhancing the bandwidth of digital communications over media with physical limitations. The scheme is used in some audio broadcast services. The technology lends itself to digital television, and is used as a method of obtaining high data speeds in asymmetric digital subscriber line (ADSL) systems. MCM is also used in wireless local area networks (WLANs). Also see orthogonal frequency-division multiplexing (OFDM), frequency-division multiplexing (FDM), and time-division multiplexing (TDM).
Multicast Backbone See ‘‘MBone’’
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Multichannel Multipoint Distribution Service Multichannel Multipoint Distribution Service (MMDS) is a broadcasting and communications service that operates in the ultra-high-frequency (UHF) portion of the radio spectrum between 2.1 and 2.7 GHz. MMDS is also known as wireless cable. It was conceived as a substitute for conventional cable television (TV). However, it also has applications in telephone/fax and data communications. In MMDS, a medium-power transmitter is located with an omindirectional broadcast antenna at or near the highest topographical point in the intended coverage area. The workable radius can reach up to 70 miles in flat terrain (significantly less in hilly or mountainous areas). Each subscriber is equipped with a small antenna, along with a converter that can be placed next to, or on top of, a conventional TV set. There is a monthly fee, similar to that for satellite TV service. The MMDS frequency band has room for several dozen analog or digital video channels, along with narrowband channels that can be used by subscribers to transmit signals to the network. The narrowband channels were originally intended for use in an educational setting (so-called wireless classrooms). The educational application has enjoyed some success, but conventional TV viewers prefer satellite TV services, which have more channels.
See ‘‘mrouter’’
Because of recent deregulation that allows cable TV companies to provide telephone and Internet services, along with the development of digital technologies that make efficient use of available bandwidth, MMDS has considerable future potential. An MMDS network can provide high-speed Internet access, telephone/fax, and TV together, without the constraints of cable connections.
multicast
Multics
Multicast is communication between a single sender and multiple receivers on a network. Typical uses include the updating of mobile personnel from a home office and the periodic issuance of online newsletters. Together with anycast and unicast, multicast is one of the packet types in the Internet Protocol Version 6 (IPv6).
Multics (Multiplexed Information and Computing Service) was a mainframe time-sharing operating system that was developed in the 1963-1969 period through the collaboration of the Massachusetts Institute of Technology (MIT), General Electric (GE), and Bell Labs. Multics was the first or one of the first operating systems that used page-segmented storage. The operating system was written in PL/I and ran on GE hardware. By 1970, Bell Labs had withdrawn from the project, and Honeywell, which had bought GE’s computer division, continued as the hardware provider. Support from the Advanced Research Projects Agency helped sustain the project.
Multicast Internet
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signals to be encapsulated as TCP/IP packets when passing through parts of the Internet that can not handle the multicast protocol directly.
See ‘‘MBone’’
multicast router
Multicast is supported through wireless data networks as part of the Cellular Digital Packet Data (CDPD) technology. Multicast is also used for programming on the MBone, a system that allows users at high-bandwidth points on the Internet to receive live video and sound programming. In addition to using a specific high-bandwidth subset of the Internet, Mbone multicast also uses a protocol that allows
In 1973, Honeywell announced a commercial system, the 6180, consisting of two processors that ran at 1 MIPS each, 768 kilobytes of memory, an 8 megabyte bulk store, a 1.6
multimedia gigabyte hard disk, 8 tape drives, and two communication controllers. The price was about $7 million. Later, a multiple disk system called the New Storage System (NSS) was added. In 1977, Honeywell offered the first commercial relational database, the Multics Relational Data Store (MRDS). Over time, Multics customers included General Motors, Ford, and Industrial Nucleonics (later AccuRay). By the late 1980s, efforts to migrate Multics to more strategic processor architectures such as Intel’s had failed and Honeywell transferred maintenance to one of its last customers, the University of Calgary, which has passed it on to a local company, CGI Group Inc. As of September, 1998, CGI Group continued to operate the one remaining Multics system.
multidimensional database A multidimensional database (MDB) is a type of database that is optimized for data warehouse and online analytical processing (OLAP) applications. Multidimensional databases are frequently created using input from existing relational databases. Whereas a relational database is typically accessed using a Structured Query Language (SQL) query, a multidimensional database allows a user to ask questions like ‘‘How many Aptivas have been sold in Nebraska so far this year?’’ and similar questions related to summarizing business operations and trends. A multidimensional database—or a multidimensional database management system (MDDBMS)—implies the processing ability to rapidly process the data in the database so that answers can be generated quickly. A number of vendors provide products that use multidimensional databases. Approaches to how data is stored and the user interface vary somewhat. Conceptually, a multidimensional database uses the idea of a cube to represent the dimensions of data available to a user. For example, ‘‘Sales’’ could be viewed in the dimensions of product, geography, time, or some additional dimension. Additionally, a database creator can define hierarchies and levels within a dimension (for example, state and city levels within a regional hierarchy).
multihomed Multihomed describes a computer host that has multiple IP addresses to connected networks. A multihomed host is physically connected to multiple data links that can be on the same or different networks. For example, a computer with a Windows NT 4.0 Server and multiple IP addresses can be referred to as ‘‘multihomed’’ and may serve as an IP router.
multilink bundle A multilink bundle is a collection of simultaneously opened bandwidth channels, including video and data links, that are coherently and logically controlled by preset commands. Links can be removed or added to the bundle when one Internet service provider (ISP) requests another ISP to do so. This process is known as link management. A simple example of a multilink bundle is the use of two 64 Kbps channels to effect a 128 Kbps ISDN channel. A multilink bundle enables an ISP to offer users bandwidthon-demand services, usually for a fee. By adding or removing channels, an ISP is able to change the available bandwidth. This may be done in response to changing line conditions or changing resource conditions. The complexity of the system can be easily understood when one thinks of it this way: One system tells another when it is capable of combining multiple physical links into a single, logical link. This coordination of multiple independent channels between a fixed pair of systems provides a virtual link that produces greater bandwidth than any of the individual physical links.
multimedia Multimedia is more than one concurrent presentation medium (for example, on CD-ROM or a Web site). Although still images are a different medium than text, multimedia is typically used to mean the combination of text, sound, and/ or motion video. Some people might say that the addition of animated images (for example, animated GIFs on the Web) produces multimedia, but it has typically meant one of the following:
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Text, sound, and still or animated graphic images
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Multiple display areas, images, or presentations presented concurrently
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In live situations, the use of a speaker or actors and ‘‘props’’ together with sound, images, and motion video
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Multimedia can arguably be distinguished from traditional motion pictures or movies both by the scale of the production (multimedia is usually smaller and less expensive) and by the possibility of audience interactivity or involvement (in which case, it is usually called interactive multimedia). Interactive elements can include: voice command, mouse manipulation, text entry, touch screen, video capture of the user, or live participation (in live presentations). Multimedia tends to imply sophistication (and relatively more expense) in both production and presentation than simple text-and-images. Multimedia presentations are possible in many contexts, including the Web, CD-ROMs,
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Whatis?Com’s Encyclopedia of Technology Terms and live theater. A rule-of-thumb for the minimum development cost of a packaged multimedia production with video for commercial presentation (as at trade shows) is: $1,000 a minute of presentation time. Since any Web site can be viewed as a multimedia presentation, however, any tool that helps develop a site in multimedia form can be classed as multimedia software and the cost can be less than for standard video productions.
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For multimedia Web sites, popular multimedia (sound or sound and motion video or animation) players include: MPEG, Quicktime, and RealAudio.
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In optical fiber technology, multimode fiber is optical fiber that is designed to carry multiple light rays or modes concurrently, each at a slightly different reflection angle within the optical fiber core. Multimode fiber transmission is used for relatively short distances because the modes tend to disperse over longer lengths (this is called modal dispersion) . For longer distances, single mode fiber (sometimes called monomode) fiber is used. Multimode fiber has a larger core than single mode.
Multiple Document Interface MDI (Multiple Document Interface) is a Microsoft Windows programming interface for creating an application that enables users to work with multiple documents at the same time. Each document is in a separate space with its own controls for scrolling. The user can see and work with different documents such as a spreadsheet, a text document, or a drawing space by simply moving the cursor from one space to another. An MDI application is something like the Windows desktop interface since both include multiple viewing spaces. However, the MDI viewing spaces are confined to the application’s window or client area. Within the client area, each document is displayed within a separate child window. MDI applications can be used for a variety of purposes—for example, working on one document while referring to another document, viewing different presentations of the same information, viewing multiple Web sites at the same time, and any task that requires multiple reference points and work areas at the same time.
Multiplexed Information and Computing Service
multiplexed using frequency-division multiplexing (FDM), in which the carrier bandwidth is divided into subchannels of different frequency widths, each carrying a signal at the same time in parallel. Digital signals are commonly multiplexed using time-division multiplexing (TDM), in which the multiple signals are carried over the same channel in alternating time slots. In some optical fiber networks, multiple signals are carried together as separate wavelengths of light in a multiplexed signal using dense wavelength division multiplexing(DWDM).
multiprocessing Multiprocessing is the coordinated processing of programs by more than one computer processor. Multiprocessing is a general term that can mean the dynamic assignment of a program to one of two or more computers working in tandem or can involve multiple computers working on the same program at the same time (in parallel). With the advent of parallel processing, multiprocessing is divided into symmetric multiprocessing (SMP) and massively parallel processing (MPP). In symmetric (or ‘‘tightly coupled’’) multiprocessing, the processors share memory and the I/O bus or data path. A single copy of the operating system is in charge of all the processors. SMP, also known as a ‘‘shared everything’’ system, does not usually exceed 16 processors. In massively parallel (or ‘‘loosely coupled’’) processing, up to 200 or more processors can work on the same application. Each processor has its own operating system and memory, but an ‘‘interconnect’’ arrangement of data paths allows messages to be sent between processors. Typically, the setup for MPP is more complicated, requiring thought about how to partition a common database among processors and how to assign work among the processors. An MPP system is also known as a ‘‘shared nothing’’ system. Multiprocessing should not be confused with multiprogramming, or the interleaved execution of two or more programs by a processor. Today, the term is rarely used since all but the most specialized computer operating systems support multiprogramming. Multiprocessing can also be confused with multitasking, the management of programs and the system services they request as tasks that can be interleaved, and with multithreading, the management of multiple execution paths through the computer or of multiple users sharing the same copy of a program.
See ‘‘Multics’’
multiprogramming
multiplexing
Multiprogramming is a rudimentary form of parallel processing in which several programs are run at the same time on a uniprocessor. Since there is only one processor, there can be no true simultaneous execution of different
Multiplexing is sending multiple signals or streams of information on a carrier at the same time in the form of a single, complex signal and then recovering the separate signals at the receiving end. Analog signals are commonly
multitasking programs. Instead, the operating system executes part of one program, then part of another, and so on. To the user it appears that all programs are executing at the same time. If the machine has the capability of causing an interrupt after a specified time interval, then the operating system will execute each program for a given length of time, regain control, and then execute another program for a given length of time, and so on. In the absence of this mechanism, the operating system has no choice but to begin to execute a program with the expectation, but not the certainty, that the program will eventually return control to the operating system. If the machine has the capability of protecting memory, then a bug in one program is less likely to interfere with the execution of other programs. In a system without memory protection, one program can change the contents of storage assigned to other programs or even the storage assigned to the operating system. The resulting system crashes are not only disruptive, they may be very difficult to debug since it may not be obvious which of several programs is at fault.
Multiprotocol Label Switching MPLS (Multiprotocol Label Switching) is a standardsapproved technology for speeding up network traffic flow and making it easier to manage. MPLS involves setting up a specific path for a given sequence of packets, identified by a label put in each packet, thus saving the time needed for a router to look up the address to the next node to forward the packet to. MPLS is called multiprotocol because it works with the Internet Protocol (IP), Asynchronous Transport Mode (ATM), and frame relay network protocols. With reference to the standard model for a network (the Open Systems Interconnection, or OSI model), MPLS allows most packets to be forwarded at the layer 2 (switching) level rather than at the layer 3 (routing) level. In addition to moving traffic faster overall, MPLS makes it easy to manage a network for quality of service (QoS). For these reasons, the technique is expected to be readily adopted as networks begin to carry more and different mixtures of traffic.
multisession CD A multisession CD is a recordable CD format (like a CD-R) that allows the recording of a compact disc to be conducted in more than one recording session. If there is free space left on the CD after the first session, additional data can be written to it at a later date. Each session has its own lead in, program area, and lead out. This takes up about 20 megabytes of space, and therefore, is less efficient than recording data all at once. Multisession CDs can be read in current CD-ROM drives, unless data is recorded track-by-track or sector by sector This process is known as packet writing and in this case only the newer CD-ROM drives, accompanied by appropriate software are able to read the disk.
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A multistation access unit is occasionally abbreviated as MAU. However, MAU is more frequently the abbreviation for the Ethernet media attachment unit.
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An MSAU (multistation access unit) is a hub or concentrator that connects a group of computers (‘‘nodes’’ in network terminology) to a token ring local area network. For example, eight computers might be connected to an MSAU in one office and that MSAU would be connected to an MSAU in another office that served eight other computers. In turn that MSAU could be connected to another MSAU in another office which would be connected back to the first MSAU. Such a physical configuration is called a star topology. However, the logical configuration is a ring topology because every message passes through every computer one at a time, each passing it on to the next in a continuing circle.
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An advantage of an MSAU is that if one computer fails in the ring, the MSAU can bypass it and the ring will remain intact.
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For larger networks, two additional ports on the MSAU (ring-in and ring-out) are available for connecting multiple MSAUs together. For example, using three MSAUs, the ringout port of MSAU 1 is connected to the ring-in port of MSAU 2. The ring-out port of MSAU 2 is connected to the ring-in port of MSAU 3 and the ring-out port of MSAU 3 is connected to the ring-in port of MSAU 1, completing the ring. In such an example, up to 24 computers can be connected to the token ring network. An MSAU is likely to be stored in a wiring closet and have cabling that extends out to a wall faceplate connection.
multitasking It is easy to confuse multitasking with multithreading, a somewhat different idea. In a computer operating system, multitasking is allowing a user to perform more than one computer task (such as the operation of an application program) at a time. The operating system is able to keep track of where you are in these tasks and go from one to the other without losing information. Microsoft Windows 2000, IBM’s OS/390, and Linux are examples of operating systems that can do multitasking (almost all of today’s operating systems can). When you open your Web browser and then open Word at the same time, you are causing the operating system to do multitasking. Being able to do multitasking doesn’t mean that an unlimited number of tasks can be juggled at the same time. Each task consumes system storage and other resources. As more tasks are started, the system may need to slow down or begin to run out of shared storage.
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Multithreading is the ability of a program or an operating system process to manage its use by more than one user at a time and to even manage multiple requests by the same user without having to have multiple copies of the program running in the computer. Each user request for a program or system service (and here a user can also be another program) is kept track of as a thread with a separate identity. As programs work on behalf of the initial request for that thread and are interrupted by other requests, the status of work on behalf of that thread is kept track of until the work is completed.
MULTOS MULTOS (which stands for ‘‘Multiple Operating System’’) is an operating system that allows multiple application programs to be installed and to reside separately and securely on a smart card. Each program is isolated by the operating system so that no application can interfere with another one. Whereas earlier smart card systems did not allow new applications to be installed or old ones deleted, MULTOS makes this possible. Updates or patches can also be installed as needed. Each application is platformindependent due to the implementation of a virtual machine. Developers write applications for MULTOS smart cards using the MULTOS Executable Language (MEL). Before MULTOS, application developers had to write a separate version of the application for each type of smart card and the consumer needed a separate smart card for each application. With MULTOS, several applications can reside on one smart card regardless of the microchip used. Security for MULTOS smart cards is enabled by the MULTOS Certification Authority (CA), which issues cryptographic keys for each MULTOS smart card and all MULTOS applications. These keys prevent unauthorized applications from being loaded into a card or deleted without the issuer’s permission. The MAOSCO Consortium, a group of leading international organizations, openly licenses the MULTOS specification. MasterCard, Mondex, Europay, and Discover favor MULTOS.
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MUMPs is a program that assists companies and academic researchers with ideas for micro-electromechanical systems (MEMS) to manufacture prototypes. Since the manufacture of silicon chips in which MEMS are imbedded is costly, MUMPs provides special manufacturing ‘‘runs’’ that qualified enterprises can be part of. MUMPs is administered by the MEMS Technology Application Center (MCNC).
Murphy’s Laws of Information Technology The original Murphy’s Law was ‘‘If there are two or more ways to do something, and one of those ways can result in a catastrophe, then someone will do it.’’ The law’s author was Edward A. Murphy, Jr., a U.S. Air Force engineer, who, in 1947, was involved in a rocket-sled experiment in which all 16 accelerator instruments were installed in the wrong way, resulting in Murphy’s observation. Murphy’s Law is commonly misstated to be ‘‘Anything that can go wrong, will,’’ a quote invented by science-fiction writer Larry Niven as Finagle’s Law of Dynamic Negatives. Extrapolating from the original, we arrive at Murphy’s Laws of Information Technology, a set of principles that may seem to be jokes but which events sometimes prove to be fundamental truths. Here are a few examples of Murphy’s Laws relative to hardware. L
Law of Inconvenient Malfunction: A device will fail at the least opportune possible moment.
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Law of Cable Compatibility: If you choose a cable and a connector at random, the probability that they are compatible is equal to zero.
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Law of Hardware Compatibility: The probability of a given peripheral being compatible with a PC is inversely proportional to the immediate need for that peripheral.
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Law of Bad Sectors: The probability that an untested diskette will have bad sectors is directly proportional to the importance of the data written onto the diskette.
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First Law of Selective Gravitation: When an object is dropped, it will fall in such a way as to cause the greatest possible damage to itself and/or other objects on which it lands.
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Second Law of Selective Gravitation: The tendency for an object to be dropped is directly proportional to its value.
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Law of Reality Change: Unalterable hardware specifications will change as necessary to maximize frustration for personnel affected by said specifications.
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Law of Noise: Noise bursts occur so as to cause the most, and/or most serious, errors in data communications, regardless of the actual amount of noise present.
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Law of Expectation: Consumer expectations always outpace advances in hardware technology.
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Law of the Titanic: If a device cannot malfunction, it will.
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Law of Debugging: The difficulty of debugging software is directly proportional to the number of people who will ultimately use it.
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Law of Neurosis: The chances of software being neurotic (developing bugs spontaneously without apparent reason) is directly proportional to the confusion such neurosis can cause.
2) On the Internet, a MUX is a multi-user text game played over the Internet, using the TinyMUX server. This kind of game started with the MUD (multi-user dungeon) movement.
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Law of Available Space: If there are n bytes in a crucial software program, the available space for its convenient storage or loading is equal to n-1 bytes.
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First Law of Bad Sectors: The probability of software being mutilated by bad sectors is directly proportional to the value and/or importance of the programs.
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Second Law of Bad Sectors: When a program is mutilated by bad sectors, the damage will occur at the point(s) that result in the most frequent and/or severe errors when the program is run.
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Law of Noise: When a downloaded program is corrupted by noise, the corruption will occur at the point(s) that result in the most frequent and/or severe errors when the program is run.
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Law of Software Compatibility: If two programs are chosen at random, the probability that they are compatible is equal to zero.
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Law of Option Preferences: When two people share a computer, their software option preferences will differ in every possible way.
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Law of Expectation: Consumer expectations always outpace advances in software technology.
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Law of the Titanic: Bug-free software isn’t.
mutex In computer programming, a mutex (mutual exclusion object) is a program object that is created so that multiple program threads can take turns sharing the same resource, such as access to a file. Typically, when a program is started, it creates a mutex for a given resource at the beginning by requesting it from the system and the system returns a unique name or ID for it. After that, any thread needing the resource must use the mutex to lock the resource from other threads while it is using the resource. If the mutex is already locked, a thread needing the resource is typically queued by the system and then given control when the mutex becomes unlocked (when once more, the mutex is locked during the new thread’s use of the resource).
mux 1) In communication transmission systems, mux (pronounce muks, sometimes spelled ‘‘MUX’’) is an abbreviation for multiplexing, a device that sends multiple signals on a carrier channel at the same time in the form of a single, complex signal to another device that recovers the separate signals at the receiving end. The receiver is sometimes called a demux (or ‘‘DEMUX’’).
MVS (Multiple Virtual Storage) is the operating system from IBM that is installed on most of its mainframe and large server computers. MVS has been said to be the operating system that keeps the world going. The payroll, accounts receivable, transaction processing, database management, and other programs critical to the world’s largest businesses are usually run on an MVS system. Although MVS tends to be associated with a monolithic, centrally-controlled information system, IBM has in recent years repositioned it as a ‘‘large server’’ in a network-oriented distributed environment, using a 3-tier application model. The latest versions of MVS, OS/390, and z/OS, no longer bear the ‘‘MVS’’ in its name. Since MVS represents a certain epoch and culture in the history of computing and since many older MVS systems still operate, the term ‘‘MVS’’ will probably continue to be used for some time. Since OS/390 also comes with UNIX user and programming interfaces built in, it can be used as both an MVS system and a UNIX system at the same time. MVS systems run older applications developed using COBOL and, for transaction programs, CICS. Older application programs written in PL/I and FORTRAN are still running. Older applications use the Virtual Storage Access Method access method for file management and Virtual Telecommunications Access Method for telecommunication with users. The most common program environment today uses the C and C++ languages. DB2 is IBM’s primary relational database management system (RDBMS). Java applications can be developed and run under OS/390’s UNIX environment.
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MVS is a generic name for specific products that included MVS/SP (MVS/System Product), MVS/XA (MVS/Extended Architecture), and MVS/ESA (MVS/Enterprise Systems Architecture). Historically, MVS evolved from OS/360, the operating system for the System/360, which was released in 1964. It later became the OS/370 and the System/370. OS/ 370 evolved into the OS/VS, OS/MFT, OS/MVT, OS/MVS, MVS/SP, MVS/XA, MVS/ESA, and finally OS/390. Throughout this evolution, application programs written for any operating system have always been able to run in any of the later operating systems. (This is called forward compatibility.)
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An MVS system is a set of basic products and a set of optional products. This allows a customer to choose the set of functions they need and exclude the rest. In practice, most customers probably use almost all of the functions. The main user interface in MVS systems is TSO (Time Sharing Option). The Interactive System Productivity Facility (ISPF) is a set of menus for compiling and managing programs and for configuring the system. The main work management system
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is either Job Entry Subsystem 2 or 3 (JES2 or JES3). Storage (DASD) management is performed by DFSMS (Distributed File Storage Management Subsystem). MVS is considerably more complex and requires much more education and experience to operate than smaller server and personal computer operating systems. The Virtual Storage in MVS refers to the use of virtual memory in the operating system. Virtual storage or memory allows a program to have access to the maximum amount of memory in a system even though this memory is actually being shared among more than one application program. The operating system translates the program’s virtual address into the real physical memory address where the data is actually located. The Multiple in MVS indicates that a separate virtual memory is maintained for each of multiple task partitions. Other IBM operating systems for their larger computers include or have included: the Transaction Processing Facility (TPF), used in some major airline reservation systems, and VM, an operating system designed to serve many interactive users at the same time.
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See also SAP. MySAP, a product of the German software company SAP, is an e-business software integration tool that delivers content to the user based on their role in the enterprise. More than 200 pre-defined role templates are available to provide the user with access to applications and resources that are taskappropriate to his or her communication needs in customer relationship management (CRM), supply chain management (SCM), e-procurement, business intelligence, product lifecycle management, human resources, or financial and marketplace channels. The user is able to work with the SAP applications they require by using a Web browser or Internet appliance to access the applications through their mySAP Web portal, called ‘‘Workplace.’’ Access to the mySAP.com Workplace portal can be based on a per-user or per-numberof-transactions fee model.
mySQL MySQL (pronounced ‘‘my ess cue el,’’ not ‘‘my sequel’’) is an open source relational database management system (RDBMS) that uses Structured Query Language (SQL), the most popular language for adding, accessing, and processing data in a database. Because it is open source, anyone can download mySQL and tailor it to their needs in accordance with the general public license. MySQL is noted mainly for its speed, reliability, and flexibility. Most agree, however, that it works best when managing content and not executing transactions. The mySQL relational database system was first released in January 1998. It is fully multi-threaded using kernel threads, provides application program interfaces (APIs) for C, C++,
Eiffel, Java, Perl, PHP, Python, and Tcl, allows for many column types, and offers full operator and function support in the SELECT and WHERE parts of queries. The development team working on future releases of mySQL plan to unveil mySQL 4.0 in mid-2001. Its features will include a new table definition file format, enhanced replication, and more functions for a full-text search. Later, mySQL developers hope to add fail-safe replication, a port of mySQL to BeOS, and an option to periodically flush key pages for tables with delayed keys. Over time, MySQL plans to be fully ANSI 92/ANSI 99-compliant. MySQL currently runs on the Linux, UNIX, and Windows platforms. Many Internet startups have been especially interested in mySQL as an alternative to the proprietary database systems from Oracle, IBM, and Informix. Yahoo’s news site uses mySQL.
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named pipe
Named for its creator, John Nagle, the Nagle algorithm is used to automatically concatenate a number of small buffer messages; this process (called nagling) increases the efficiency of a network application system by decreasing the number of packets that must be sent. Nagle’s algorithm, defined in 1984 as Ford Aerospace and Communications Corporation Congestion Control in IP/TCP Internetworks (IETF RFC 896) was originally designed to relieve congestion for a private TCP/IP network operated by Ford, but has since been broadly deployed.
In computer programming, a named pipe is a method for passing information from one computer process to other processes using a pipe or message holding place that is given a specific name. Unlike a regular pipe, a named pipe can be used by processes that do not have to share a common process origin and the message sent to the named pipe can be read by any authorized process that knows the name of the named pipe.
Nagle’s document specified a means of dealing with what he called the small packet problem, created when an application generates data one byte at a time, causing the network to be overloaded with packets (a situation often referred to as send-side silly window syndrome). A single character—one byte of data—originating from a keyboard could result in the transmission of a 41 byte packet consisting of one byte of useful information and 40 bytes of header data. This situation translates into 4000% overhead, which was considered to be acceptable for a lightly loaded network such as the Advanced Research Projects Agency Network (ARPANet—the precursor of the Internet operating at that time), but not so for a heavily loaded network such as Ford’s, where it could necessitate retransmissions, cause lost packets, and hamper propagation speed through excessive congestion in switching nodes and gateways. Throughput could be hampered to the extent that connections were aborted. Nagle’s algorithm—usually implemented through the insertion of two lines of code into a TCP program—instructs the sender to buffer (store) data if any unacknowledged data is outstanding. Any data sent subsequently is held until the outstanding data is acknowledged (ACKed) or until there is a full packet’s worth of data to send. Although Nagle’s algorithm addressed problems that were being experienced within Ford’s network, the same problems were beginning to be experienced by ARPANet. Nagling has been broadly implemented across networks, including the Internet, and is generally performed by default—although it is sometimes considered to be undesirable in highly interactive environments, such as some client/server situations. In such cases, nagling may be turned off through use of the TCP_NODELAY sockets option.
nagling See ‘‘Nagle’s algorithm’’
A named pipe is sometimes called a ‘‘FIFO’’ (first in, first out) because the first data written to the pipe is the first data that is read from it.
namespace In general, a namespace uniquely identifies a set of names so that there is no ambiguity when objects having different origins but the same names are mixed together. Using the Extensible Markup Language (XML), an XML namespace is a collection of element type and attribute names. These element types and attribute names are uniquely identified by the name of the unique XML namespace of which they are a part. In an XML document, any element type or attribute name can thus have a two-part name consisting of the name of its namespace and then its local (functional) name. For example, suppose the same XML document included the element type of OWNER for owners of motorcycles as well as for owners of automobiles. It might be necessary or desirable to know that an owner name was one of those who owned a motorcyle rather than an automobile. Having different motorcycle and automobile namespaces would make this possible. Effectively, it would make it possible to label motorcycle owners differently than automobile owners without having to create a different element type for each. In XML, a namespace is commonly given the name of a Uniform Resource Identifier (URI)—such as a Web site’s address—both because the namespace may be associated with the site or page of that URI (for example, a company name) and because a URI is conveniently likely to be a unique name. Note that the URI is not necessarily intended to be used other than as a name nor is there any namespace document or XML schema that must be accessed; the URI is simply used as a name (and part of the two-part name of any element type or attribute name so that the names are unique within the document).
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NAND See ‘‘logic gate’’
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nanite
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See ‘‘nanomachine’’
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nanoanalysis Nanoanalysis refers to techniques for determining the atomic structures of materials, especially crystals. The technology is similar to that used for microanalysis, except it is done on a nanometer scale. (One nanometer is 10-9 meter, or a millionth of a millimeter.) With any instrument used for nanoanalysis, there is a limit to the resolution (the diameter of the smallest object that can be resolved). This is true when the instrument works directly with electromagnetic radiation such as infrared (IR), ultraviolet (UV), visible light, or X rays, and also when the instrument employs high-speed subatomic particles such as electrons or ions. There is a minimum wavelength associated with any medium of observation. Objects whose diameters are less than this wavelength defy observation. In general, as the wavelength becomes shorter, the required particle or wave energy increases. This motivates scientists involved with nanoanalysis to seek ever-more-powerful machines with which to observe samples. The electron microscope is commonly used for nanoanalysis. There are two basic types that lend themselves to this application: the scanning electron microscope (SEM) and the high-voltage transmission analytical electron microscope. Also useful in nanoanalysis are X-ray and UV diffraction, IR microscopy, mass spectrometry, ion-beam machines, and precision optical microprobes.
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A nanochip is an integrated circuit (IC) that is so small, in physical terms, that individual particles of matter play major roles. Miniaturization of electronic and computer components has always been a primary goal of engineers. The smaller an electronic system can be made, the more processing power can fit into a given physical volume, the less energy is required to run it, and the faster it can work (because distances among components are reduced, minimizing charge-carrier transit time). The earliest computers, built in the middle of the 20th century, used vacuum tubes for switching. These machines were so massive and bulky, and demanded so much electricity to operate, that they required buildings and power plants of their own. Today, such a computer can be placed inside a microscopic capsule. The trend toward miniaturization shows no sign of slowing until some limit is forced on the manufacturing process. The scale of this limit ultimately depends only on the structure of matter. Traditional methods of IC manufacture involve etching unwanted semiconductor material from a preexisting chip, in the same way a sculptor carves a statue. Futurists suggest that, rather than etching switches and logic gates into
semiconductor material, chips ought to be mechanically assembled one molecule at a time. Some scientists believe ICs can be grown from biological seeds, just as a plant reproduces cells to create a defined structure such as a tree. Both of these processes involve nanotechnology, and would result in the greatest possible number of switches per unit volume of material substance. Nanochip is also the name of a nanotechnology researchand-development corporation based in San Jose, California as well as the trade name for a molecular biology workstation manufactured by Nanogen of San Diego, California.
nanocomputer A nanocomputer is a computer whose physical dimensions are microscopic. The field of nanocomputing is part of the emerging field of nanotechnology. Several types of nanocomputers have been suggested or proposed by researchers and futurists. Electronic nanocomputers would operate in a manner similar to the way present-day microcomputers work. The main difference is one of physical scale. More and more transistors are squeezed into silicon chips with each passing year; witness the evolution of integrated circuits (ICs) capable of ever-increasing storage capacity and processing power. The ultimate limit to the number of transistors per unit volume is imposed by the atomic structure of matter. Most engineers agree that technology has not yet come close to pushing this limit. In the electronic sense, the term nanocomputer is relative. By 1970s standards, today’s ordinary microprocessors might be called nanodevices. Chemical and biochemical nanocomputers would store and process information in terms of chemical structures and interactions. Biochemical nanocomputers already exist in nature; they are manifest in all living things. But these systems are largely uncontrollable by humans. We cannot, for example, program a tree to calculate the digits of pi, or program an antibody to fight a particular disease (although medical science has come close to this ideal in the formulation of vaccines, antibiotics, and antiviral medications). The development of a true chemical nanocomputer will likely proceed along lines similar to genetic engineering. Engineers must figure out how to get individual atoms and molecules to perform controllable calculations and data storage tasks. Mechanical nanocomputers would use tiny moving components called nanogears to encode information. Such a machine is reminiscent of Charles Babbage’s analytical engines of the 19th century. For this reason, mechanical nanocomputer technology has sparked controversy; some researchers consider it unworkable. All the problems inherent in Babbage’s apparatus, according to the naysayers, are magnified a millionfold in a mechanical nanocomputer. Nevertheless, some futurists are optimistic about the
nanomedicine technology, and have even proposed the evolution of nanorobots that could operate, or be controlled by, mechanical nanocomputers.
without necessarily modifying it. Either the SPM or the ATM can be used to etch, write, or print on a surface in singleatom dimensions.
A quantum nanocomputer would work by storing data in the form of atomic quantum states or spin. Technology of this kind is already under development in the form of singleelectron memory (SEM) and quantum dots. The energy state of an electron within an atom, represented by the electron energy level or shell, can theoretically represent one, two, four, eight, or even 16 bits of data. The main problem with this technology is instability. Instantaneous electron energy states are difficult to predict and even more difficult to control. An electron can easily fall to a lower energy state, emitting a photon; conversely, a photon striking an atom can cause one of its electrons to jump to a higher energy state.
Also see nanomachine and nanotechnology.
nanofabrication Nanofabrication is the design and manufacture of devices with dimensions measured in nanometers. One nanometer is 10-9 meter, or a millionth of a millimeter. Nanofabrication is of interest to computer engineers because it opens the door to super-high-density microprocessors and memory chips. It has been suggested that each data bit could be stored in a single atom. Carrying this further, a single atom might even be able to represent a byte or word of data. Nanofabrication has also caught the attention of the medical industry, the military, and the aerospace industry. There are several ways that nanofabrication might be done. One method involves scaling down integrated-circuit (IC) fabrication that has been standard since the 1970s, removing one atom at a time until the desired structure emerges. A more sophisticated hypothetical scheme involves the assembly of a chip atom-by-atom; this would resemble bricklaying. An extension of this is the notion that a chip might assemble itself atom-by-atom using programmable nanomachines. Finally, it has been suggested that a so-called biochip might be grown like a plant from a seed; the components would form by a process resembling cell division in living things. Also see nanolithography, nanotechnology, and selfassembly.
nanolithography Nanolithography is the art and science of etching, writing, or printing at the microscopic level, where the dimensions of characters are on the order of nanometers (units of 10-9 meter, or millionths of a millimeter). This includes various methods of modifying semiconductor chips at the atomic level for the purpose of fabricating integrated circuits (ICs). Instruments used in nanolithography include the scanning probe microscope (SPM) and the atomic force microscope (ATM). The SPM allows surface viewing in fine detail
nanomachine A nanomachine, also called a nanite, is a mechanical or electromechanical device whose dimensions are measured in nanometers (millionths of a millimeter, or units of 10-9 meter). Nanomachines are largely in the research-and-development phase, but some primitive devices have been tested. An example is a sensor having a switch approximately 1.5 nanometers across, capable of counting specific molecules in a chemical sample. The first useful applications of nanomachines will likely be in medical technology, where they could be used to identify pathogens and toxins from samples of body fluid. Another potential application is the detection of toxic chemicals, and the measurement of their concentrations, in the environment. The microscopic size of nanomachines translates into high operational speed. This is a result of the natural tendency of all machines and systems to work faster as their size decreases. Nanomachines could be programmed to replicate themselves, or to work synergistically to build larger machines or to construct nanochips. Specialized nanomachines called nanorobots might be designed not only to diagnose, but to treat, disease conditions, perhaps by seeking out invading bacteria and viruses and destroying them.
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Another asset of nanomachines is the fact that the individual units require only a tiny amount of energy to operate. Durability is another potential asset; nanites might last for centuries before breaking down. The main challenge lies in the methods of manufacture. It has been suggested that some nanomachines might be grown in a manner similar to the way plants evolve from seeds.
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Nanomedicine is the application of nanotechnology (the engineering of tiny machines) to the prevention and treatment of disease in the human body. This discipline is in its infancy. It has the potential to change medical science dramatically in the 21st century. The most elementary nanomedical devices will be used to diagnose illness. Chemical tests exist for this purpose; nanomachines could be employed to monitor the internal chemistry of the body. Mobile nanorobots, equipped with wireless transmitters, might circulate in the blood and lymph systems, and send out warnings when chemical
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Whatis?Com’s Encyclopedia of Technology Terms imbalances occur or worsen. Similar fixed nanomachines could be planted in the nervous system to monitor pulse, brain-wave activity, and other functions. A more advanced use of nanotechnology might involve implanted devices to dispense drugs or hormones as needed in people with chronic imbalance or deficiency states. Heart defibrillators and pacemakers have been around for some time; nanomedicine carries this to the next level down in terms of physical dimension, with the potential to affect the behavior of individual cells. Ultimately, artificial antibodies, artificial white and red blood cells, and antiviral nanorobots might be devised. The most advanced nanomedicine involves the use of nanorobots as miniature surgeons. Such machines might repair damaged cells, or get inside cells and replace or assist damaged intracellular structures. At the extreme, nanomachines might replicate themselves, or correct genetic deficiencies by altering or replacing DNA (deoxyribonucleic acid) molecules.
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A nanometer is a unit of spatial measurement that is 10-9 meter, or one millionth of a millimeter. It is commonly used in nanotechnology, the building of extremely small machines.
nanorobot A nanorobot is a specialized nanomachine designed to perform a specific task or tasks repeatedly and with precision. Nanorobots have dimensions on the order of nanometers (a nanometer is a millionth of a millimeter, or 10-9 meter). Nanorobots, like their larger counterparts, can be categorized into two groups, called autonomous robots and insect robots. An autonomous nanorobot contains its own on-board nanocomputer, which controls the machine and allows it to operate independently. An insect nanorobot is one of a fleet of several, or many, identical units that are all controlled by a single, central computer. (The term insect comes from the fact that such robots resemble ants in an anthill or bees in a hive.) Nanorobots are of special interest to researchers in the medical industry. It has been suggested that a fleet of selfreplicating insect nanorobots might act as a vaccine against disease. Such machines might even serve as antibodies or antiviral agents in patients with compromised immune systems, or in diseases that do not respond to more conventional measures. The nanorobots would work by seeking out and destroying specific bacteria, fungi, or viruses. Nanorobots have potential applications in the assembly of small-scale, sophisticated systems. They might function at the atomic level to build devices, machines, or circuits one particle at a time.
A major asset of nanorobots is the fact that the individual units require very little energy to operate. Durability is another potential asset; nanorobots might remain operational for years, decades, or centuries. High speed is also a significant consideration. Nanoscale systems can operate much faster than their larger counterparts because displacements are smaller; this allows mechanical and electrical events to occur in less time at a given absolute speed. Also see robot, nanotechnology, self-assembly, and selfreplication.
nanosecond (This definition follows U.S. usage in which a billion is a thousand million and a trillion is a 1 followed by 12 zeros.) A nanosecond (ns or nsec) is one billionth (10-9) of a second and is a common measurement of read or write access time to random access memory (RAM). For comparison, a millisecond (ms or msec) is one thousandth of a second and is commonly used in measuring the time to read to or write from a hard disk or a CD-ROM player or to measure packet travel time on the Internet. A microsecond (ms or Greek letter mu plus s) is one millionth (10-6) of a second. A picosecond is one trillionth (10-12) of a second, or one millionth of a microsecond. A femtosecond is one millionth of a nanosecond or 10-15 of a second and is a measurement sometimes used in laser technology. An attosecond is one quintillionth (10-18) of a second and is a term used in photon research.
nanotechnology Nanotechnology, or, as it is sometimes called, molecular manufacturing, is a branch of engineering that deals with the design and manufacture of extremely small electronic circuits and mechanical devices built at the molecular level of matter. The Institute of Nanotechnology in the U.K. expresses it as ‘‘science and technology where dimensions and tolerances in the range of 0.1 nanometer (nm) to 100 nm play a critical role.’’ Nanotechnology is often discussed together with micro-electromechanical systems (MEMS), a subject that usually includes nanotechnology but may also include technologies higher than the molecular level. There is a limit to the number of components that can be fabricated onto a semiconductor wafer or ‘‘chip.’’ Traditionally, circuits have been etched onto chips by removing material in small regions. However, it is also possible in theory to build chips up, one atom at a time, to obtain devices much smaller than those that can be manufactured by etching. With this approach, there would be no superfluous atoms; every particle would have a purpose. Electrical conductors, called nanowires, would be
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only one atom thick. A logic gate would require only a few atoms. A data bit could be represented by the presence or absence of a single electron.
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Nanotechnology holds promise in the quest for ever-morepowerful computers and communications devices. But the most fascinating (and potentially dangerous) applications are in medical science. So-called nanorobots might serve as programmable antibodies. As disease-causing bacteria and viruses mutate in their endless attempts to get around medical treatments, nanorobots could be reprogrammed to selectively seek out and destroy them. Other nanorobots might be programmed to single out and kill cancer cells.
Generally, narrowband describes telecommunication that carries voice information in a narrow band of frequencies. More specifically, the term has been used to describe a specific frequency range set aside by the U.S. FCC for mobile or radio services, including paging systems, from 50 cps to 64 Kbps. The term is usually contrasted with wideband or broadband.
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Two concepts associated with nanotechnology are positional assembly and self-replication. Positional assembly deals with the mechanics of moving molecular pieces into their proper relational places and keeping them there. Molecular robots are devices that do the positional assembly. Self-replication deals with the problem of multiplying the positional arrangements in some automatic way, both in building the manufacturing device and in building the manufactured product.
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nanotube In nanotechnology or the building of extremely small machines and computers, a nanotube (sometimes called a buckytube) is a long, cylindrical carbon structure consisting of hexagonal graphite molecules attached at the edges. The nanotube developed from the so-called Fullerene, a structure similar to the way geodesic domes, originally conceived by R. Buckminster Fuller, are built. Because of this, nanotubes are sometimes called buckytubes. Some nanotubes have a single cylinder; others have two or more concentric cylinders. Nanotubes have several characteristics: wall thickness, number of concentric cylinders, cylinder radius, and cylinder length. Some nanotubes have a property called chirality, an expression of longitudinal twisting. Researchers have suggested several applications for nanotubes. The structures might be assembled into microscopic mechanical systems called nanomachines. Because graphite can behave as a semiconductor, nanotubes might be used to build microscopic resistors, capacitors, inductors, diodes, or transistors. Concentric nanotubes might store electric charges because of capacitance among the layers, facilitating the construction of high-density memory chips.
NAP See ‘‘network access point’’
Also see bandwidth.
See ‘‘network-attached storage’’
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NAT See ‘‘Network Address Translation’’
National Computer Security Center The National Computer Security Center (NCSC) is a U.S. government organization within the National Security Agency (NSA) that evaluates computing equipment for high security applications to ensure that facilities processing classified or other sensitive material are using trusted computer systems and components. NCSC was founded in 1981 as the Department of Defense Computer Security Center and changed to its current name in 1985. The organization works with industry, education, and government agency partners to promote research and standardization efforts for secure information system development. The NCSC also functions in an educational capacity to disseminate information about issues surrounding secure computing, most significantly through its annual National Information Systems Security Conference. The NCSC’s computer evaluation program is carried out by another NSA organization, the Trusted Product Evaluation Program (TPEP), which tests commercial products against a comprehensive set of security-related criteria. NCSC issued the first Department of Defense (DoD) Trusted Computer System Evaluation Criteria (TCSEC) in August, 1983. The document, more commonly referred to as the ‘‘orange book,’’ was reissued in 1985 as a DoD standard that included the stated goals of providing manufacturers with securityrelated standards regarding features for inclusion in products, and providing DoD components with information about security metrics for the evaluation of trust levels to be accorded various products used for processing sensitive material.
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National Institute for Standards and Technology See ‘‘NIST’’
National Security Agency Also see cryptography. The National Security Agency (NSA) is the official U.S. cryptologic (the science of cryptographic design and decryption) organization. Under a directive (revised version of the National Security Council Intelligence Directive (NSCID) No. 9) from President Truman and the National Security Council, the NSA was formed in November of 1952. The organization exists to protect national communications systems integrity, and to seek information about foreign adversaries’ secret communications. The NSA works in close conjunction with the Central Security Service (CSS), which was established by Presidential Directive in 1972 to promote full partnership between the NSA and the cryptologic elements of the Armed Forces. The director of NSA/CSS, in accordance with a Department of Defense (DoD) directive, must be a high-ranking (at least 3 stars) commissioned officer of the military services. Although the organization’s number of employees (as well as its budget) falls into the category of ‘‘classified information,’’ the NSA lists among its workforce: analysts, engineers, physicists, linguists, computer scientists, researchers, customer relations specialists, security officers, data flow experts, managers, administrative and clerical assistants; it also claims to be the largest employer of mathematicians in the U.S., and possibly worldwide. NSA/ CSS mathematicians perform the Agency’s two critical functions: they design cryptographic systems to protect U.S. communications and search for weaknesses in the counterpart systems of U.S. adversaries. NSA denies rumours in circulation claiming that it has an unlimited ‘‘black budget’’—undisclosed even to other government agencies. Nevertheless, the Agency admits that, if it were judged as a corporation, it would rank in the top ten percent of Fortune 500 companies.
National Television Standards Committee The NTSC (National Television Standards Committee) was responsible for developing, in 1953, a set of standard protocols for television (TV) broadcast transmission and reception in the United States. Two other standards—Phase Alternation Line (PAL) and Sequential Couleur avec Memoire (SECAM)—are used in other parts of the world. The NTSC standards have not changed significantly since their inception, except for the addition of new parameters for color signals. NTSC signals are not directly compatible with computer systems.
An NTSC TV image has 525 horizontal lines per frame (complete screen image). These lines are scanned from left to right, and from top to bottom. Every other line is skipped. Thus it takes two screen scans to complete a frame: one scan for the odd-numbered horizontal lines, and another scan for the even-numbered lines. Each half-frame screen scan takes approximately 1/60 of a second; a complete frame is scanned every 1/30 second. This alternate-line scanning system is known as interlacing. Adapters exist that can convert NTSC signals to digital video that a computer can ‘‘understand.’’ Conversely, there are devices that can convert computer video to NTSC signals, allowing a TV receiver to be used as a computer display. But because a conventional TV receiver has lower resolution than a typical computer monitor, this does not work well for all computer applications, even if the TV screen is very large. In recent years, there has been increasing pressure to adopt a new set of TV standards. One of the proposed protocols is known as high-definition television (HDTV). Ideally, the HDTV standard that is ultimately adopted will be directly compatible with computer systems. However, there are engineering problems associated with this. Some industry experts fear such compatibility could dramatically increase the cost of a conventional TV set.
native In computer systems, native means ‘‘original’’ or ‘‘basic.’’ Here are several usages: 1) Microsoft stresses that its Internet Information Server (IIS) is ‘‘a native implementation of the current Internet standards for Web servers...’’ Microsoft is implying that its own implementation of a Web server is likely to perform better than Netscape’s (or someone else’s) Web server running on a Windows NT operating system. 2) In COBOL, a native character set is the default character set that comes with the computer specified in the OBJECTCOMPUTER paragraph. 3) In IBM’s Virtual Telecommunications Access Method (VTAM), native mode refers to its use directly in the VM operating system rather than in another operating system whose emulation is being supported by the VM operating system. 4) A computer program is compiled to run with a particular processor and its set of instructions. This is intrinsically running code in ‘‘native mode.’’ If the same program is run on a computer with a new processor, software can be provided so that the computer emulates the old processor. In this case, the same application runs in ‘‘emulation mode’’ and almost certainly more slowly than in native mode on the original processor. (The application can be rewritten and recompiled so that it runs on the new processor in native mode.)
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A natural number is a number that occurs commonly and obviously in nature. As such, it is a whole, non-negative number. The set of natural numbers, denoted N, can be defined in either of two ways:
NDIS (Network Driver Interface Specification) is a Windows specification for how communication protocol programs (such as TCP/IP) and network device drivers should communicate with each other. NDIS specifies interfaces for:
N = {0, 1, 2, 3, ...}
1. The program that sends and receives data by constructing or extracting it from the formatted units called frames (and sometimes packets or datagrams). This program, usually called a protocol stack, is layered and generally corresponds to layers 3 and 4 (the Network Addressing and Transport layers) of the Open Systems Interconnection (OSI) reference model. Examples are TCP/IP and Internetwork Packet Exchange.
N = (1, 2, 3, 4, ...} In mathematical equations, unknown or unspecified natural numbers are represented by lower-case, italicized letters from the middle of the alphabet. The most common is n, followed by m, p, and q. In subscripts, the lower-case i is sometimes used to represent a non-specific natural number when denoting the elements in a sequence or series. However, i is more often used to represent the positive square root of 71, the unit imaginary number. The set N, whether or not it includes zero, is a denumerable set. Denumerability refers to the fact that, even though there might be an infinite number of elements in a set, those elements can be denoted by a list that implies the identity of every element in the set. For example, it is intuitive from either the list {1, 2, 3, 4, ...} or the list {0, 1, 2, 3, ...} that 356,804,251 is a natural number, but 356,804,251.5, 2/3, and 723 are not. Both of the sets of natural numbers defined above are denumerable. They are also exactly the same size. It’s not difficult to prove this; their elements can be paired off oneto-one, with no elements being left out of either set. In infinite sets, the existence of a one-to-one correspondence is the litmus test for determining cardinality, or size. The set of integers and the set of rational numbers has the same cardinality as N. However, the sets of real numbers, imaginary numbers, and complex numbers have cardinality larger than that of N. See also: integer, rational number, real number, imaginary number, and complex number.
NCSA NCSA (National Center for Supercomputing Applications) at the University of Illinois in Urbana, Illinois is the home of the first Web browser that had a graphical user interface. Its inventor, Marc Andreessen, then 22 years old, later lead the creation of the Netscape browser and became a founder of that company. The original browser, Mosaic, exists in a more advanced version.
NCSC See ‘‘National Computer Security Center’’
2. The program, usually called a device driver, that interacts directly with the network interface card (NIC) or other adapter hardware, which sends or receives the data on the communications line in the form of electronic signals. The driver program and the NIC interact at the Media Access Control (MAC address) sublayer of layer2 level of OSI, which is called Data-Link Control. (Putting the signal on the line is the layer-1 or the Physical layer of OSI.) Examples of MAC drivers are those for Ethernet, Fiber Distributed-Data Interface, and token ring. 3. A program called the Protocol Manager that assists the protocol stack program and the MAC driver program by telling each of them the computer location of the other when the operating system is started or, in some cases, when a new device is added to the computer. This is called bind. A system file called PROTOCOL.INI identifies which protocol stacks use which MAC drivers and where each is located. A protocol stack can be bound to more than one MAC driver where a computer is connected to multiple networks. And a single MAC driver can be bound to more than one protocol stack in a computer. NDIS was developed by Microsoft and 3Com. Using NDIS, Windows software developers can develop protocol stacks that work with the MAC driver for any hardware manufacturer’s communications adapter. By the same token, any adapter maker can write a MAC driver software that can communicate with any protocol stack program. A similar interface, called Open Data-Link Interface (ODI), is provided by Novell for its NetWare local area network operating system.
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NDS See ‘‘Novell Directory Services’’
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Also see NEXT, a computer company that developed a personal computer with an advanced user interface and operating system.
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NEXT (near-end crosstalk) is an error condition that can occur when connectors are attached to twisted pair cabling. NEXT is usually caused by crossed or crushed wire pairs. The error condition does not require that the wires be crushed so much that the conductors inside become exposed. Two conductors only need to be close enough so that the radiating signal from one of the wires can interfere with the signal traveling on the other. Most medium- to high-end cable testers are capable of testing for NEXT errors.
A common alternative to nested is the prefix sub, as in subprocedure, substructure, subtransaction, and so forth.
A neologism (pronounced nee-AH-low-djism) is a newly invented word or term. Neologisms would seem to occur at a greater rate in cultures with rapidly changing technologies and with greater means for information dispersal.
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A neology is, according to our Webster’s, the use of a new word or the use of an existing word but given a new meaning. A second meaning given by Webster’s for neologism is that of ‘‘a meaningless word coined by a psychotic.’’
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Simply using CAT 5 cables, connectors, and patch panels does not guarantee that a network will support 100 Mb operation. You must actually certify your network for use at this speed with a CAT 5 cable tester after the network is fully installed.
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Dr. Seuss’s picture of the nerd looks much like the way computer nerds are portrayed today.
In general, something that is nested is fully contained within something else of the same kind. In programming, nested describes code that performs a particular function and that is contained within code that performs a broader function. One well-known example is the procedure known as the nested do-loop. In data structures, data organizations that are separately identifiable but also part of a larger data organization are said to be nested within the larger organization. A table within a table is a nested table. A list within a list is a nested list.
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And then, just to show them, I’ll sail to Ka-Troo And bring back an it-kutch, A preep, and a proo, A nerkle, a nerd, And a seersucker, too!
In order to certify a cable for Category 3 (CAT 3), the NEXT test must be performed in 1 MHz steps from 1 Mhz to 16 MHz. Since the level of signal loss or interference can change with frequency, a range of frequencies must be used to certify that the cable is acceptable over the entire range. In order to certify a cable for Category 5 (CAT 5) use, the test frequency range must be extended up to 100 MHz. A 1 MHz frequency step should still be used. In both cases, you must test both ends of the cable. This is because NEXT occurs on the ends of the cable right where the connector is attached.
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Although the term’s origin is obscure, one theory is that it derives from a children’s book by Dr. Seuss, If I Ran the Zoo, in which this passage is found:
A nerd is a technically bright but socially inept person. The classic image of the nerd has been the wild-haired genius kid with thick-lensed glasses surrounded by test tubes and computers. Microsoft’s Bill Gates is sometimes considered the walking embodiment of the older, successful nerd. As computer technology becomes less frightening and ‘‘nerdish’’ to larger numbers of people, society seems to be developing a more tolerant, even benevolent view of the nerd. Nerd is a more general term than geek (which always means computer geek) and weenie and is somewhat comparable to propellor head.
net 1) ‘‘net’’ is one of the top-level domain names that can be used when choosing a domain name. It generally describes the entity owning the domain name as an organization that administers or provides network connection services. Along with the second-level domain name (for example: ‘‘mhv’’ in mhv.net), the top-level domain name is required in Web and e-mail addresses. For more information, see gTLD (generic top-level domain name). 2) Net (capitalized or not) is sometimes used as a short form for ‘‘Internet.’’
.Net .Net is a business strategy from Microsoft that is aimed at a convergence of personal computing with the Web. The goal is to provide individual and business users with a seamlessly interoperable and Web-enabled interface for applications and computing devices and to make computing activities increasingly Web browser-oriented. Many of the details for .Net are not yet fully worked out; however, Microsoft has released information about the strategy in general and various aspects of the platform. The .Net platform will include servers; building-block services, such as Web-based data storage; and device software. The .Net platform is expected to provide
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The ability to make the entire range of computing devices work together and to have user information automatically updated and synchronized on all of them
A concept similar to the Net PC but with different ideas about what a low-cost network PC should be built with is called the network computer (NC).
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Increased interactive capability for Web sites, enabled by greater use of XML (Extensible Markup Language) rather than HTML
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A premium online subscription service, that will feature customized access and delivery of products and services to the user from a central starting point for the management of various applications, such as e-mail, for example, or software, such as Office .Net Centralized data storage, which will increase efficiency and ease of access to information, as well as synchronization of information among users and devices The ability to integrate various communications media, such as e-mail, faxes, and telephones For developers, the ability to create reusable modules, which should increase productivity and reduce the number of programming errors
According to Bill Gates, Microsoft expects that .Net will have as significant an effect on the computing world as the introduction of Windows. One concern being voiced is that although .Net’s services will be accessible through any browser, they are likely to function more fully on products designed to work with .Net code. The full release of .Net is expected to take a year or two to complete, with intermittent releases of products such as a personal security service and new versions of Windows and Office that implement the .Net strategy coming on the market separately. A technical preview of Visual Studio.Net has been available from the Microsoft developers Web site since July of 2000. The next version of Windows, Windows XP, to be released later in 2001, will include certain .Net capabilities.
Net PC The Net PC (also referred to as the Network PC) is an industry specification for a low-cost personal computer designed for businesses and their network applications. A Net PC, which is a design for what is also sometimes known as a thin client, is intended to be centrally managed and to lack a diskette drive, CD-ROM drive, or hardware expansion slots. It is designed to support specific business tasks and applications. Typical uses for Net PCs would be online transaction processing (OLTP) applications, retail point-ofsale, and banking services. Among companies that sell or plan to sell Net PCs are Acer, Compaq, Dell, Hewlett-Packard, NEC, and Unisys. Other companies including IBM plan to accomodate their existing personal computer line to compete. Net PCs are expected to cost $1,000 or less and offer Pentium microprocessors, 16 to 32 megabytes of random access memory, and at least 1.2 gigabyte hard drives. At its inception, the Net PC was forecast to achieve about 15% of total desktop PC sales.
NetBEUI (NetBIOS Extended User Interface) is an extended version of NetBIOS, the program that lets computers communicate within a local area network. NetBEUI (pronounced net-BOO-ee) formalizes the frame format (or arrangement of information in a data transmission) that was not specified as part of NetBIOS. NetBEUI was developed by IBM for its LAN Manager product and has been adopted by Microsoft for its Windows NT, LAN Manager, and Windows for Workgroups products. Hewlett-Packard and DEC use it in comparable products. NetBEUI is said to be the best performance choice for communication within a single LAN. Because, like NetBIOS, it does not support the routing of messages to other networks, its interface must be adapted to other protocols such as Internetwork Packet Exchange or TCP/IP. A recommended method is to install both NetBEUI and TCP/ IP in each computer and set the server up to use NetBEUI for communication within the LAN and TCP/IP for communication beyond the LAN.
NetBIOS NetBIOS (Network Basic Input/Output System) is a program that allows applications on different computers to communicate within a local area network (LAN). It was created by IBM for its early PC Network, was adopted by Microsoft, and has since become a de facto industry standard. NetBIOS is used in Ethernet, token ring, and Windows NT networks. It does not in itself support a routing mechanism so applications communicating on a wide area network (WAN) must use another ‘‘transport mechanism’’ (such as Transmission Control Protocol) rather than or in addition to NetBIOS. NetBIOS frees the application from having to understand the details of the network, including error recovery (in session mode). A NetBIOS request is provided in the form of a Network Control Block (NCB) which, among other things, specifies a message location and the name of a destination. NetBIOS provides the session and transport services described in the Open Systems Interconnection (OSI) model. However, it does not provide a standard frame or data format for transmission. A standard frame format is provided in the NetBIOS Extended User Interface (NetBEUI). NetBIOS provides two communication modes: session or datagram. Session mode lets two computers establish a connection for a ‘‘conversation,’’ allows larger messages to be handled, and provides error detection and recovery. Datagram mode is ‘‘connectionless’’ (each message is sent independently), messages must be smaller, and the
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Netcaster Netcaster, a component of Netscape’s Communicator (which includes the Netscape Navigator browser), allows a Web user to make preselected Web sites an integral part of the personal computer desktop and to have them automatically updated. Called channels and similar to the channels on Microsoft’s Windows desktops, the Netcaster channels include: CNET, CNN, Gartner, and Airius News. A user can request customized news and content updates on a preset schedule. Using push technology (sometimes referred to as Webcasting or netcasting), the updating occurs in the background while other computer work is going on and is then available when you want to read the updates. (You can read the updates offline.)
Netfinity IBM’s Netfinity is an Intel-based enterprise server line that is based on IBM’s X-architecture. IBM describes the Xarchitecture as a design blueprint that addresses the increasing need for communicating and managing huge amounts of information and the demands of business-toconsumer, business-to-business, and intra-business applications on technology platforms. Netfinity and another server line called xSeries are closely related. xSeries models are identified with an x preceding the model number (for example, the x430. Netfinity servers are identified without an x (for example, the Netfinity 7000). The Netfinity server line uses copper chip wiring and Silicon On Insulator (SOI) microprocessor technologies. Netfinity also uses 64-bit computing, which IBM began using before the introduction of Netfinity with both the AS/400 and RS/ 6000 servers. IBM also enhanced existing memory systems to support large amounts of memory in Netfinity servers. Currently, the Netfinity 7000 M10 supports up to 8 gigabytes (GBs) of memory. IBM plans to offer Netfinity servers with up to 64 GB of memory. Netfinity servers may be clustered for increased reliability and decreased downtime. Netfinity servers also interoperate with existing servers such as the AS/400 and the RS/6000. Netfinity includes the Communication Server for Windows NT that allows Netfinity applications to communicate with other applications, whether interoperable or not, using IBM’s own Systems Network Architecture (SNA) protocols as well as Transmission Control Protocol/Internet Protocol (TCP/IP). For diagnostic purposes, Netfinity includes an onboard systems management processor that allows for remote management and diagnostics of Netfinity servers even if turned off. If a server fails, it can connect to IBM’s Help Center using Mobile Service Terminal (MoST) connect for
diagnostic tests. RemoteConnect is another application that allows a Netfinity server to self-diagnose, issue an alert, call a service organization, and request a replacement part or a technician.
netiquette Netiquette is etiquette on the Internet. Since the Internet changes rapidly, its netiquette does too, but it’s still usually based on the Golden Rule. The need for a sense of netiquette arises mostly when sending or distributing e-mail, posting on Usenet groups, or chatting. To some extent, the practice of netiquette depends on understanding how e-mail, the Usenet, chatting, or other aspects of the Internet actually work or are practiced. So a little preliminary observation can help. Poor netiquette because you’re new is one thing, but such practices as spam and flaming are another matter.
netizen The word netizen seems to have two similar meanings. 1. A citizen who uses the Internet as a way of participating in political society (for example, exchanging views, providing information, and voting). 2. An Internet user who is trying to contribute to the Internet’s use and growth. As a powerful communications medium, the Internet seems to offer great possibilities for social change. It also creates a new culture and its own special issues, such as who shall have access to it. The implication is that the Internet’s users, who use and know most about it, have a responsibility to ensure that is used constructively while also fostering free speech and open access. Cybercitizen is a synonym.
netmask In administering Internet sites, a netmask is a string of 0’s and 1’s that mask or screen out the network part of an IP address (IP) so that only the host computer part of the address remains. The binary 1’s at the beginning of the mask turn the network ID part of the IP address into 0’s. The binary 0’s that follow allow the host ID to remain. A frequently-used netmask is 255.255.255.0. (255 is the decimal equivalent of a binary string of eight ones.) Used for a Class C subnet (one with up to 255 host computers), the ‘‘.0’’ in the ‘‘255.255.255.0’’ netmask allows the specific host computer address to be visible.
NetRexx NetRexx, a programming language designed to be easier to learn and use than Java, combines the syntax of the Rexx language and the semantics of Java. Rexx is a script language, developed at IBM, that is very popular among IBM’s corporate mainframe system users. NetRexx, developed by Rexx’s inventor, Mike Cowlishaw,
NetWare superimposes Rexx’s ease-of-use on the object-oriented programming concepts of Java—class, method, and properties. NetRexx source code statements are compiled into Java source statements, which are then compiled into Java bytecode that can be run in any computer operating system that includes a Java virtual machine (JVM). Thus, NetRexx programs can be run in Windows, OS/2, MVS, UNIX-based systems—in short, almost any operating system that supports the Java 1.1 toolkit and compiler and includes the JVM. According to IBM, a study shows that the Java source for a typical class has about 35% more lexical tokens and requires about 20% more keystrokes than it would in NetRexx. NetRexx has equivalent classes for all Java classes.
Netscape Netscape is one of the two most popular Web browsers and also the name of a company, Netscape Communications, now owned by America Online (AOL). Currently, almost all Internet users use either Netscape’s browser or Microsoft’s Internet Explorer (MSIE) browser, and many users use both. Although Netscape was initially the predominant product in terms of usability and number of users, Microsoft’s browser is now considered superior by many users and has taken a lead in usage. Netscape’s browser originally was called ‘‘Navigator,’’ and is still called that in the suite of software, Communicator, of which it is now a part. Navigator was developed in 1995 by a team led by Marc Andreessen, who created Mosaic, the first Web browser that had a graphical user interface, at the University of Illinois’ National Center for Supercomputing Applications (NCSA) in 1993. Navigator can be downloaded as part of the Communicator suite from Netscape’s Web site at no charge. CD-ROM versions can also be purchased in computer stores and are sometimes distributed freely as promotions.
Netscape Server Application Programming Interface NSAPI (Netscape Server Application Programming Interface) is an API (application program interface) that is provided with Netscape Web servers to help developers build faster and more complex Web-based applications by extending the server capabilities. NSAPI, the Common Gateway Interface (CGI), and the Java and JavaScript-based server API are the three components of Netscape’s Internet Application Framework. NSAPI was created as a more efficient and robust replacement for the Common Gateway Interface alone. It is often used to develop applications that involve custom authorization or logging, or to change aspects of server operation. NSAPI is used to handle Hypertext Transfer Protocol transactions in the Netscape Enterprise Server’s requestresponse process through its built-in Server Application
Functions (SAF). Netscape Server’s SAFs are written with the NSAPI and are functions called by the server to deal with requests received from a client. At start up, after initialization, the server waits for an HTTP request from the client for a resource such as an HTML file, a CGI program, or an image file, for example. The request consists of a Universal Resource Identifier (URI), an HTTP method, and, sometimes, information about the client or the request. The request-response process involves a sequence of six steps:
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1) AuthTrans, which verifies request information (name and password, for example);
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Each step may involve multiple operations, which are performed by Server Application Functions (SAF). Each SAF returns a result code to the server to indicate the success or failure of the operation. Based on the result code, the server either proceeds to the next step, carries out the next SAF in the current step, or aborts the process.
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In using an Internet Relay Chat (IRC) network, a netsplit is the loss of contact between two IRC servers. As a result, chat users at either end will see the users at the affected servers suddenly disappear. An IRC network usually has a linear topology. That is, each server is connected in line to the next server which is connected to the next one, and so forth. When two servers lose contact, the network essentially is split into two networks. Users remain in contact with other users on their side of the network split and lose contact with the other users, who appear to have quickly departed. If a network has a mesh topology, each server can route messages to a second server when it loses a server connection. Most netsplits are restored within a short period of time.
NetWare NetWare, made by Novell, is the most widely-installed network server operating system. Initially very successful in installing its products in large and small office local area networks (LANs), Novell has redesigned (or at least refeatured) NetWare to work successfully as part of larger and heterogeneous networks, including the Internet. NetWare’s primary competitor is the Microsoft Windows NT operating system.
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Whatis?Com’s Encyclopedia of Technology Terms The latest version of NetWare, NetWare 5, comes with support for both Novell’s own Internetwork Packet Exchange network protocol and for the Internet’s Internet Protocol as well as application-level support for a Web server. NetWare has integrated its own Novell Directory Services (NDS) with the industry standard Domain Name System (DNS) and the Dynamic Host Configuration Protocol (DHCP). NetWare supports Java applications and the Common Object Request Broker Architecture (CORBA) Object Request Broker (ORB). Its kernel supports multiprocessing. Additional features include what it calls ‘‘next generation’’ file system and printing services and advanced security (public-key cryptography and Secure Authentication Services—SAS).
network In information technology, a network is a series of points or nodes interconnected by communication paths. Networks can interconnect with other networks and contain subnetworks. The most common topology or general configurations of networks include the bus, star, and token ring topologies. Networks can also be characterized in terms of spatial distance as local area networks (LANs), metropolitan area networks (MANs), and wide area networks (WANs). A given network can also be characterized by the type of data transmission technology in use on it (for example, a TCP/IP or Systems Network Architecture network); by whether it carries voice, data, or both kinds of signals; by who can use the network (public or private); by the usual nature of its connections (dial-up or switched, dedicated or nonswitched, or virtual connections); and by the types of physical links (for example, optical fiber, coaxial cable, and unshielded twisted pair). Large telephone networks and networks using their infrastructure (such as the Internet) have sharing and exchange arrangements with other companies so that larger networks are created.
network access point In the United States, a network access point (NAP) is one of several major Internet interconnection points that serve to tie all the Internet access providers together so that, for example, an AT&T user in Portland, Oregon can reach the Web site of a Bell South customer in Miami, Florida. Originally, four NAPs—in New York, Washington, D.C., Chicago, and San Francisco—were created and supported by the National Science Foundation as part of the transition from the original U.S. government-financed Internet to a commercially operated Internet. Since that time, several new NAPs have arrived, including WorldCom’s ‘‘MAE West’’ site in San Jose, California and ICS Network Systems’ ‘‘Big East.’’
The NAPs provide major switching facilities that serve the public in general. Using companies apply to use the NAP facilities and make their own intercompany peering arrangements. Much Internet traffic is handled without involving NAPs, using peering arrangements and interconnections within geographic regions. The vBNS network, a separate network supported by the National Science Foundation for research purposes, also makes use of the NAPs.
network access server NAS is also the abbreviation for network-attached storage. A network access server (NAS) is a computer server that enables an independent service provider (ISP) to provide connected customers with Internet access. A network access server has interfaces to both the local telecommunication service provider such as the phone company and to the Internet backbone. The server authenticates users requesting login. It receives a dial-up call from each user host (such as your computer) that wants to access the Internet, performs the necessary steps to authenticate and authorize each user, usually by verifying a user name and password, and then allows requests to begin to flow between the user host and hosts (computers) elsewhere on the Internet. The term network access server may refer to a server devoted entirely to managing network access or to a server that also performs other functions as well. A network access server can be configured to provide a host of services such as VoIP, fax-over-IP, and voicemail-over-IP as well. One of the most well-known network access servers, the AS5800, is made by Cisco Systems. It is a workhorse product that is referred to as a carrier-class universal access server.
Network Address Translation NAT (Network Address Translation) is the translation of an Internet Protocol address (IP address) used within one network to a different IP address known within another network. One network is designated the inside network and the other is the outside. Typically, a company maps its local inside network addresses to one or more global outside IP addresses and unmaps the global IP addresses on incoming packets back into local IP addresses. This helps ensure security since each outgoing or incoming request must go through a translation process that also offers the opportunity to qualify or authenticate the request or match it to a previous request. NAT also conserves on the number of global IP addresses that a company needs and it lets the company use a single IP address in its communication with the world. NAT is included as part of a router and is often part of a corporate firewall. Network administrators create a NAT table that does the global-to-local and local-to-global IP address mapping. NAT can also be used in conjunction with
network interface card policy routing. NAT can be statically defined or it can be set up to dynamically translate from and to a pool of IP addresses. Cisco’s version of NAT lets an administrator create tables that map: L
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NAT is described in general terms in RFC 1631. which discusses NAT’s relationship to Classless Interdomain Routing (CIDR) as a way to reduce the IP address depletion problem. NAT reduces the need for a large amount of publicly known IP addresses by creating a separation between publicly known and privately known IP addresses. CIDR aggregates publicly known IP addresses into blocks so that fewer IP addresses are wasted. In the end, both extend the use of IPv4 IP addresses for a few more years before IPv6 is generally supported.
network computer A network computer (NC) is a concept from Oracle and Sun Microsystems for a low-cost personal computer for business networks that, like the Net PC, would be configured with only essential equipment, devoid of CD-ROM players, diskette drives, and expansion slots, and intended to be managed and maintained centrally (any new software would be downloaded). Unlike the Net PC, the network computer could be based on microprocessors other than Intel’s and might include a Java-based operating system rather than Windows. Along with the Net PC, the network computer is sometimes referred to as a thin client.
network encryption Network encryption (sometimes called network layer, or network level encryption) is a network security process that applies crypto services at the network transfer layer—above the data link level, but below the application level. The network transfer layers are layers 3 and 4 of the Open Systems Interconnection (OSI) reference model, the layers responsible for connectivity and routing between two end points. Using the existing network services and application software, network encryption is invisible to the end user and operates independently of any other encryption processes used. Data is encrypted only while in transit, existing as plaintext on the originating and receiving hosts. Network encryption is implemented through Internet Protocol Security (IPSec), a set of open Internet Engineering Task Force (IETF) standards that, used in conjunction, create a framework for private communication over IP networks.
IPSec works through the network architecture, which means that end users and applications don’t need to be altered in any way. Encrypted packets appear to be identical to unencrypted packets and are easily routed through any IP network. Network encryption products and services are offered by a number of companies, including Cisco, Motorola, and Oracle.
Network File System The Network File System (NFS) is a client/server application that lets a computer user view and optionally store and update files on a remote computer as though they were on the user’s own computer. The user’s system needs to have an NFS client and the other computer needs the NFS server. Both of them require that you also have TCP/IP installed since the NFS server and client use TCP/IP as the program that sends the files and updates back and forth. (However, the User Datagram Protocol, UDP, which comes with TCP/IP, is used instead of TCP with earlier versions of NFS.)
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NFS was developed by Sun Microsystems and has been designated a file server standard. Its protocol uses the Remote Procedure Call (RPC) method of communication between computers.
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NFS has been extended to the Internet with WebNFS, a product and proposed standard that is now part of Netscape’s Communicator browser. WebNFS offers what Sun believes is a faster way to access Web pages and other Internet files.
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A network interface card (NIC) is a computer circuit board or card that is installed in a computer so that it can be connected to a network. Personal computers and workstations on a local area network (LAN) typically contain a network interface card specifically designed for the LAN transmission technology, such as Ethernet or token ring. Network interface cards provide a dedicated, full-time connection to a network. Most home and portable computers connect to the Internet through as-needed dial-up connection. The modem provides the connection interface to the Internet service provider.
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A network interface unit (NIU) (sometimes called a network interface device) is a device that serves as a commmon interface for various other devices within a local area network (LAN), or as an interface to allow networked computers to connect to an outside network. The NIU enables communication between devices that use different protocols by supplying a common transmission protocol, which may be used instead of the devices’ own protocols, or may be used to convert the specific device protocol to the common one. To enable an interface between a LAN and another network, the NIU converts protocols and associated code and acts as a buffer between the connected hardware. A network interface card (NIC) is a type of NIU.
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In the Open Systems Interconnection (OSI) communications model, the Network layer knows the address of the neighboring nodes in the network, packages output with the correct network address information, selects routes and quality of service, and recognizes and forwards to the Transport layer incoming messages for local host domains. Among existing protocols that generally map to the OSI network layer are the Internet Protocol (IP) part of TCP/IP and NetWare IPX/SPX. Both IP Version 4 and IP Version 6 (IPv6) map to the OSI network layer.
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The Network Node Manager (NNM) is a program that helps a network administrator view and manage the conditions in a computer network. NNM is part of the OpenView suite of enterprise system management applications from HewlettPackard (HP). Using the Network Node Manager, an administrator can view the network in an easy-to-see graphical format. NNM ‘‘discovers’’ the devices that are in the network and shows their relative location and their status. When a device fails, NNM can analyze events associated with the failure and help to recommend action. BNM also provides some predictive information that helps identify potential failures before they occur.
network operating system A network operating system (NOS) is a computer operating system system that is designed primarily to support workstations, personal computers, and, in some instances, older terminals that are connected on a local area network (LAN). Artisoft’s LANtastic, Banyan VINES, Novell’s NetWare, and Microsoft’s LAN Manager are examples of network operating systems. In addition, some multi-purpose operating systems, such as Windows NT and Digital’s OpenVMS come with capabilities that enable them to be described as a network operating system.
A network operating system provides printer sharing, common file system and database sharing, application sharing, and the ability to manage a network name directory, security, and other housekeeping aspects of a network.
network operations center A network operations center (NOC) is a place from which a telecommunications network is supervised, monitored, and maintained. Enterprises with large networks as well as large network service providers such as GTE Internetworking typically have a network operations center, a room containing visualizations of the network or networks that are being monitored, workstations at which the detailed status of the network can be seen, and the necessary software to manage the networks. The network operations center is the focal point for network troubleshooting, software distribution and updating, router and domain name management, performance monitoring, and coordination with affiliated networks.
network PC A network PC (sometimes called an appliance) is a term used to denote a new kind of relatively low-cost PC designed for Internet access and specialized business use, but without the full capabilities ot today’s personal computer and software. The device is expected to cost in the $500 to $1,000 range. IBM, Apple, Oracle, Philips, and Sony are leading companies who sell or plan to sell network PCs.
Network Service Access Point The Network Service Access Point (NSAP) is one of two types of hierarchical addresses (the other type is the network entity title) used to implement Open Systems Interconnection (OSI) Network layer addressing. The NSAP is the logical point between the Network and Transport layers where network services are delivered to the Transport layer; the location of this point is identified to the OSI network service provider by the NSAP address. There are two NSAP address fields, Initial Domain Part (IDP) and the Domain-Specific Part (DSP). The IDP consists of the Authority Format Identifier (AFI) and the Initial Domain Identifier (IDI). AFI serves to provide information about the makeup of IDI and DSP, indicating, for example, whether the DSP uses decimal or binary notation, or whether or not the IDI may be of variable length. The DSP is made up of four fields: the Address Administration field, which enables further address administration tasks and task delegation; the Area field, which is used for routing and identifies the specific area within a domain; the Station field, also used for routing purposes, which specifies the station within the area; and the Selector field, also used for routing, which identifies a specific entity within the station called an n-selector, the last
network-attached storage byte of the NSAP address. If an OSI end system has multiple NSAP addresses—which is frequently the case—they will typically differ only in the n-selector.
network service provider A network service provider (NSP) is a company that provides backbone services to an Internet service provider (ISP), the company that most Web users use for access to the Internet. Typically, an ISP connects at a point called an Internet Exchange (IX) to a regional ISP that in turn connects to an NSP backbone. In the U.S., major NSPs include MCI, Sprint, UUNET, AGIS, and BBN.
Positioning System (GPS) and the governments of some nations. However, it is not practical or cost-effective to equip every computer with one of these receivers. Instead, computers designated as primary time servers are outfitted with the receivers and they use protocols such as NTP to synchronize the clock times of networked computers. Degrees of separation from the UTC source are defined as strata. A radio clock (which receives true time from a dedicated transmitter or satellite navigation system) is stratum-0; a computer that is directly linked to the radio clock is stratum-1; a computer that receives its time from a stratum-1 computer is stratum-2, and so on.
An ISP can purchase a wholesale dial access service from an NSP, which provides dialup connectivity for their customers. Customers then dial into their ISP’s network using a local access number, which in turn connects to the backbone of that Internet provider’s NSP. The NSP routes all traffic and basically provides the infrastructure needed for Internet connectivity. The NSP builds, maintains, and expands their infrastructure as Internet traffic demands. The ISP is responsible for its own network, sales and marketing, and customer service. An ISP can also purchase other services from an NSP that they in turn provide their customers such as e-mail service, Web-based e-mail service, personal Web hosting, discussion groups, and other enduser applications. All these services are provided under the ISP’s brand name rather than that of the NSP.
The term NTP applies to both the protocol and the client/ server programs that run on computers. The programs are compiled by the user as an NTP client, NTP server, or both. In basic terms, the NTP client initiates a time request exchange with the time server. As a result of this exchange, the client is able to calculate the link delay, its local offset, and adjust its local clock to match the clock at the server’s computer. As a rule, six exchanges over a period of about five to 10 minutes are required to initially set the clock. Once synchronized, the client updates the clock about once every 10 minutes, usually requiring only a single message exchange. Redundant servers and varied network paths are used to ensure reliability and accuracy. In addition to client/ server synchronization, NTP also supports broadcast synchronization of peer computer clocks. NTP is designed to be highly fault-tolerant and scalable.
network terminating unit
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See ‘‘network terminator 1’’
Network-attached storage (NAS) is hard disk storage that is set up with its own network address rather than being attached to the department computer that is serving applications to a network’s workstation users. By removing storage access and its management from the department server, both application programming and files can be served faster because they are not competing for the same processor resources. The network-attached storage device is attached to a local area network (typically, an Ethernet network) and assigned an IP address. File requests are mapped by the main server to the NAS file server.
Network Time Protocol Network Time Protocol (NTP) is a protocol that is used to synchronize computer clock times in a network of computers. Developed by David Mills at the University of Delaware, NTP is now an Internet standard. In common with similar protocols, NTP uses Coordinated Universal Time (UTC) to synchronize computer clock times to a millisecond, and sometimes to a fraction of a millisecond. Accurate time across a network is important for many reasons; even small fractions of a second can cause problems. For example, distributed procedures depend on coordinated times to ensure that proper sequences are followed. Security mechanisms depend on coordinated times across the network. File system updates carried out by a number of computers also depend on synchronized clock times. Air traffic control systems provide a graphic illustration of the need for coordinated times, since flight paths require very precise timing (imagine the situation if air traffic controller computer clock times were not synchronized). UTC time is obtained using several different methods, including radio and satellite systems. Specialized receivers are available for high-level services such as the Global
Network-attached storage consists of hard disk storage, including multi-disk RAID systems, and software for configuring and mapping file locations to the networkattached device. Network-attached storage can be a step toward and included as part of a more sophisticated storage system known as a storage area network (SAN). NAS software can usually handle a number of network protocols, including Microsoft’s Internetwork Packet Exchange and NetBEUI, Novell’s Netware Internetwork Packet Exchange, and Sun Microsystems’ Network File System. Configuration, including the setting of user access priorities, is usually possible using a Web browser.
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A networking chip is a microprocessor that provides the logic for sending and receiving data (including voice and video) on a telecommunications network so that additional devices are not needed for these functions. Among other manufacturers, IBM reportedly will sell networking chips that support asynchronous transfer mode (ATM) and token ring communication.
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In information technology, networking is the construction, design, and use of networks, including the physical (cabling, hubs, bridges, switches, routers, and so forth), the selection and use of telecommunication protocols and computer software for using and managing the network, and the establishment of operation policies and procedures related to the network.
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Netzip Netzip is a plug-in for Web browsers that allows you to view the files within a zip file from within your browser without the need to launch an external application. Netzip lets you create a zip file archive (including one that spans disks). It also lets you choose a compression algorithm when you create an archive. You can decompress and extract from ARJ, Lharc (LZH, LHA, LHZ), Z, GZ, and TAR (TAZ, TGZ, TAR.Z, TAR.GZ) files. Netzip also lets you temporarily ‘‘check out’’ files within a zip file and run installation utilities. If downloading from a site with a Netzip server and its I-ZIP technology, you can view the contents of a zip file before downloading the entire file. I-ZIP lets you download individual files within the zip file archive. Netzip is made by Software Builders International.
neural network In information technology, a neural network is a system of programs and data structures that approximates the operation of the human brain. A neural network usually involves a large number of processors operating in parallel, each with its own small sphere of knowledge and access to data in its local memory. Typically, a neural network is initially ‘‘trained’’ or fed large amounts of data and rules about data relationships (for example, ‘‘A grandfather is older than a person’s father’’). A program can then tell the network how to behave in response to an external stimulus (for example, to input from a computer user who is interacting with the network) or can initiate activity on its own (within the limits of its access to the external world). In making determinations, neural networks use several principles, including gradient-based training, fuzzy logic, genetic algorithms, and Bayesian methods. Neural networks are sometimes described in terms of knowledge layers, with,
in general, more complex networks having deeper layers. In feedforward systems, learned relationships about data can ‘‘feed forward’’ to higher layers of knowledge. Neural networks can also learn temporal concepts and have been widely used in signal processing and time series analysis. Current applications of neural networks include: oil exploration data analysis, weather prediction, the interpretation of nucleotide sequences in biology labs, and the exploration of models of thinking and consciousness. In his novel, Galatea 2.2, Richard Powers envisioned a neural network (named ‘‘Helen’’) that could be taught to pass a comprehensive exam in English literature.
neutron Also see electron. A neutron is a subatomic particle found in the nucleus of every atom except that of simple hydrogen. The particle derives its name from the fact that it has no electrical charge; it is neutral. Neutrons are extremely dense. If isolated, a single neutron would have a mass of only 1.675 ? 10-27 kilogram, but if a teaspoonful of tightly packed neutrons could be scooped up, the resulting chunk of matter would weigh millions of tons at the earth’s surface. The number of proton in an element’s nucleus is called the atomic number. This number gives each element its unique identity. In the atoms of any particular element, for example carbon, the number of protons in the nuclei is always the same, but the number of neutrons can vary. An atom of a given element having a specific number of neutrons in the nucleus is called an isotope. The isotope of an atom is denoted by writing the element’s name followed by the sum of the number of protons and neutrons. The nucleus of a carbon atom always has six protons and usually has six neutrons, but some carbon nuclei contain eight neutrons. Thus, carbon-12 is the most common isotope of carbon; carbon-14 is also found, but is less common. Neutrons need not be confined to the nuclei of atoms. They can exist all by themselves. When neutrons are found outside atomic nuclei, they acquire fascinating, bizarre, and potentially dangerous properties. When they travel at high speed, they produce deadly radiation. The so-called neutron bomb, known for its ability to kill people and animals while having a minimal effect on inanimate physical structures, works by producing a barrage of high-speed neutrons. The high density of these particles, combined with their speed, gives them extreme energy. As a result, they have the power to alter, or even break apart, the nuclei of atoms that they strike. When a large star explodes, blowing off its outer layers in a brilliant fireball called a supernova, the remaining matter is incredibly dense, and it collapses under its own gravitation. When this stellar remnant acquires a certain critical density, virtually all of the subatomic particles become neutrons. The resulting object is a neutron star that might have a diameter
newton smaller than that of the earth, yet have a mass hundreds of times that of the sun. Neutron stars can rotate at high speed, producing bursts of electromagnetic radiation that can be heard as periodic pulses in radio telescopes. These celestial objects are known as pulsars. If a neutron star is dense enough, it collapses into a black hole whose gravitation is so intense that nothing can escape, not even photons.
new media New media is a catch-all term for all forms of electronic communication that have appeared or will appear since the original mainly text-and-static picture forms of online communication. New media usually includes any and all of these: Special audiovisual effects of any kind Larger than 17 inch displays Streaming video and streaming audio 3-D and virtual reality environments and effects Highly interactive user interfaces (possibly including mere hypertext or not) Mobile presentation and computing capabilities Any kind of communication requiring high-bandwidth CD and DVD media Telephone and digital data integration Online communities Microdevices with embedded systems programming Live Internet broadcasting as on the MBone Person-to-person visual communication (as in CU-SeeMe) One-to-many visual communication as with Applications of any of these technology in particular fields such as medicine (telemedicine) and other fields Although the line is not easily drawn, users of the term often emphasize the visual and visual design aspects of the new digital technology experience. New media encompasses multimedia, a term originally used mainly for standalone (not online) presentations in various scales as well as hypermedia, which emphasizes interactivity and specifically means the ability to selectively link from one form of content to another.
newbie A newbie (pronounced NOO-bee) is any new user of a technology. The term is commonly applied to new users of personal computers and to new users of the Internet. According to Eric Raymond’s The New Hacker’s Dictionary, the term is a variant of the English public school term, new boy, someone in the first year or period of school. The term predates the Web and has been used for some time in Usenet newsgroups. Glenn Connery reports that he used the term in 1990 as a participant in Internet chatting groups.
newsgroup A newsgroup is a discussion about a particular subject consisting of notes written to a central Internet site and redistributed through Usenet, a worldwide network of news discussion groups. Usenet uses the Network News Transfer Protocol (NNTP). Newsgroups are organized into subject hierarchies, with the first few letters of the newsgroup name indicating the major subject category and sub-categories represented by a subtopic name. Many subjects have multiple levels of subtopics. Some major subject categories are: news, rec (recreation), soc (society), sci (science), comp (computers), and so forth (there are many more). Users can post to existing newsgroups, respond to previous posts, and create new newsgroups. Newcomers to newsgroups are requested to learn basic Usenet netiquette and to get familiar with a newsgroup before posting to it. A frequently-asked questions is provided. The rules can be found when you start to enter the Usenet through your browser or an online service. You can subscribe to the postings on a particular newsgroup. Some newsgroups are moderated by a designated person who decides which postings to allow or to remove. Most newsgroups are unmoderated.
newsXML See ‘‘XMLNews’’
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newton The newton is the International System of Units (SI) unit of force. In physics and engineering documentation, the term newton(s) is usually abbreviated N. One newton is the force required to cause a mass of one kilogram to accelerate at a rate of one meter per second squared in the absence of other force-producing effects. In general, force (F) in newtons, mass (m) in kilograms, and acceleration (a) in meters per second squared are related by a formula well known in physics: F = ma The formula also applies when F and a are vector quantities having magnitude and direction: F = ma where the direction of the force vector F is the same as the direction of the acceleration vector a. As an example of force as a function of mass and acceleration, suppose a mass of 4 kilograms is made to accelerate at 12 meters per second squared. Then the applied force in newtons is: F = ma = 4 6 12 = 48 N
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NeXT computer
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Also see near-end crosstalk (NEXT), a network wiring term.
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NeXT was a computer company formed by Steven Jobs, one of the founders of Apple Computer, and also the name of the advanced personal computer or workstation that the company developed and first offered in 1988. The NeXT computer was an industrial designer’s triumph, a futuristic black cube with a high-resolution display, a graphical user interface, and an operating system called NeXTStep. In 1993, the computer was taken off the market. Jobs offered the object-oriented programming components of NeXTStep as a proposed standard for a development environment called OpenStep. In 1996, Apple Computer bought what was now NeXT Software and hired Jobs to return to Apple and help run the company. In 1998, Apple continued to offer OpenStep as a development environment for object-oriented applications that will run on multiple platforms.
Next Hop Resolution Protocol In a computer network, NHRP (Next Hop Resolution Protocol) is a protocol or method that can be used so that a computer sending data to another computer can learn the most direct route (the fewest number of hops) to the receiving computer. If the receiving computer is in the same subnetwork, using NHRP will tell the sending computer and the sending computer can send subsequent data packets directly to the receiving computer using its subnetwork address rather than its global network address. If the receiving computer is not in the same subnetwork, using NHRP will tell the sending computer the computer in the subnetwork whose router provides the most direct path to the receiving computer and the sender can now forward subsequent data packets to that router. NHRP is basically a query-and-reply protocol and all parties through which reply information passes build a ‘‘network knowledge table’’ that can be used for all subsequent traffic. Using this knowledge, a computer can send subsequent packets directly to the destination computer (or to an egress router) using an acquired ‘‘machine’’ address rather than a network address. (This kind of operation at the Media Access Control sublayer of the Data-Link layer communication layer level—rather than at the Network layer—is sometimes referred to as ‘‘operating at close to wire speed’’ because fewer program instructions are required and these can sometimes be located in a switching device.) NHRP thus provides two benefits: It reduces the number of hops a packet may have to take within a subnetwork and it allows the packets to be forwarded using the faster ‘‘machine’’ address. NHRP does not currently shorten the number of hops a packet may take outside of the subnetwork. Currently, it only applies to Non-Broadcast Multi-Access (NBMA) subnetworks. That is, it applies to packet transmissions sent
only to a single receiver. It is possible that in the future, NHRP may be applied to Broadcast Multi-Access (BMA) subnetworks as well. NHRP was developed by the Internetworking Over NBMA working group of the Internet Engineering Task Force (IETF). It is a main component of Multiprotocol over ATM (MPOA) and is described in the IETF’s RFC (Request for Comments) 2332, ‘‘NBMA Next Hop Resolution Protocol.’’ 3Com refers to its implementation of NHRP as Fast IP.
NFS See ‘‘Network File System’’
Ni2 Ni2 (Net indexer 2) is the indexer for the AltaVista public search engine. According to AltaVista’s parent company, Digital Equipment Corporation (DEC), Ni2 can index ‘‘an astounding one gigabyte of text per hour.’’ Ni2 generates a link for each word that AltaVista’s Web crawler, Scooter, brings back. It eliminates duplicates and ranks entries so that queries will produce more effective results. In addition to AltaVista’s free public search service, DEC sells AltaVista for individual use (downloaded to your computer, it can search everything on your hard disk) and for enterprise intranets. Ni2, Scooter, and the AltaVista product offerings are described at DEC’s AltaVista Web site.
nibble In computers and digital technology, a nibble (pronounced NIHB-uhl; sometimes spelled nybble) is four binary digits or half of an eight-bit byte. A nibble can be conveniently represented by one hexadecimal digit. Like crumb, nibble carries on the ‘‘edible data’’ metaphor established with bit and byte. In communications, a nibble is sometimes referred to as a ‘‘quadbit.’’ or one of 16 possible four-bit combinations. A signal may be encoded in quadbits rather than one bit at a time. Nibble interleaving or multiplexing takes a quadbit or nibble from a lower-speed channel as input for a multiplexed signal on a higher-speed channel.
NIC See ‘‘network interface card’’
Ni-Cd battery See ‘‘Nickel-Cadmium battery’’
Nickel-Cadmium battery The Nickel Cadmium (Ni-Cd) battery is a type of battery commonly used in portable computers, camcorders, portable drills, and other small battery-powered devices, having an effective and even power discharge. Although Nickel-Metal
NLX Hydride and Lithium Ion batteries are becoming more popular in mobile computers, Ni-Cd batteries are still the most widely sold.
Nickel-Metal Hydride battery The Nickel-Metal Hydride (Ni-MH) battery is a rechargeable power source that is increasingly used in portable computers and other devices. The most advanced rechargeable battery offered, the Ni-MH battery provides up to 40 percent longer service life than Nickel-Cadmium batteries as well as greater charge capacity. Ni-MH batteries are also used in cellular phones and camcorders.
Ni-MH battery See ‘‘Nickel-Metal Hydride battery’’
NIS (Network Information System) NIS (Network Information System) is a network naming and administration system for smaller networks that was developed by Sun Microsystems. NIS+ is a later version that provides additional security and other facilities. Using NIS, each host client or server computer in the system has knowledge about the entire system. A user at any host can get access to files or applications on any host in the network with a single user identification and password. NIS is similar to the Internet’s domain name system (DNS) but somewhat simpler and designed for a smaller network. It’s intended for use on local area networks. NIS uses the client/server model and the Remote Procedure Call (RPC) interface for communication between hosts. NIS consists of a server, a library of client programs, and some administrative tools. NIS is often used with the Network File System (NFS). NIS is a UNIX-based program. Although Sun and others offer proprietary versions, most NIS code has been released into the public domain and there are freeware versions available. NIS was originally called Yellow Pages but because someone already had a trademark by that name, it was changed to Network Information System. It is still sometimes referred to by the initials: ‘‘YP’’. Sun offers NIS+ together with its NFS product as a solution for Windows PC networks as well as for its own workstation networks.
NIS See ‘‘NIS (Network Information System)’’
NIS+ See ‘‘NIS (Network Information System)’’
NIST NIST is the National Institute of Standards and Technology, a unit of the US Commerce Department. Formerly known as the National Bureau of Standards, NIST promotes and maintains measurement standards. It also has active programs for encouraging and assisting industry and science to develop and use these standards.
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nit 1) In information technology as elsewhere, a nit (pronounced NIHT) is a small, usually unimportant imperfection in something. People who have unusually high or unreasonable standards for the quality of a thing are sometimes referred to as nitpickers. 2) In lighting, the nit is a unit of visible-light intensity, commonly used to specify the brightness of a cathode ray tube or liquid crystal display computer display. One nit is equivalent to one candela per square meter. The candela, formerly called candlepower, is approximately the amount of light emitted by a common tallow candle; technically it is the quantity of radiation emitted by 1.667 6 10-6 square meter of a blackbody at the melting point of platinum. The candela is equal to one lumen per steradian (unit solid angle). (A blackbody is an object that radiates energy with 100% efficiency at all electromagnetic wavelengths. It also absorbs all electromagnetic energy that strikes it, hence the expression ‘‘black.’’ It is a theoretical ideal of interest in physics and engineering.) The nit is a comparatively small unit of brightness. A typical active-matrix LCD panel has an output between 200 and 300 nit, for example.
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3) In digital electronics, the term nit is occasionally used to represent an amount, or increment, of data equal to 1.44 bits.
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NLX is an industry-wide open specification for a spacesaving computer motherboard. The NLX specification defines the motherboard size, hole mounting locations, riser card location, and maximum component heights on motherboard. Companies such as IBM, Hewlett-Packard, Digital Equipment, Sony, NEC, Toshiba, Gateway, and Fujitsu support NLX. Traditionally a computer technician had to open the computer case using tools, but an NLX case is tool-free. Another feature of the NLX specification is the motherboard itself. It rests on rails and simply slides out without any screws to unscrew or card to remove. The motherboard plugs into a riser card that sits at a 90-degree angle from the motherboard. A riser card is an expansion card that physically extends a slot for more expansion cards. When the motherboard is removed, the riser card stays in place. This allows a technician to replace a motherboard or
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upgrade a component on the motherboard in about 30 minutes instead of the usual two hours, saving the customer money.
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NLX motherboards support current and future processor technologies, the Accelerated Graphics Port (AGP), and tall memory technology. NLX is beneficial because it improves access to components, supports new technology, and lowers the cost of ownership.
Noise is unwanted electrical or electromagnetic energy that degrades the quality of signals and data. Noise occurs in digital and analog systems, and can affect files and communications of all types, including text, programs, images, audio, and telemetry.
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See ‘‘Network Node Manager’’
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NNTP (Network News Transfer Protocol) is the predominant protocol used by computer clients and servers for managing the notes posted on Usenet newsgroups. NNTP replaced the original Usenet protocol, UNIX-to-UNIX Copy Protocol (UUCP) some time ago. NNTP servers manage the global network of collected Usenet newsgroups and include the server at your Internet access provider. An NNTP client is included as part of a Netscape, Internet Explorer, Opera, or other Web browser or you may use a separate client program called a newsreader.
no op A no op (or no-op), for no operation, is a computer instruction that takes up a small amount of space but specifies no operation. The computer processor simply moves to the next sequential instruction. The no op is included in most assembler languages. It may have a label and can serve as a placeholder for a useful instruction to be inserted later during code development. The New Hacker’s Dictionary reports as a derivative meaning ‘‘A person who contributes nothing to a project, or has nothing going on upstairs, or both. As in ’He’s a no-op.’’’
NOC See ‘‘network operations center’’
node
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In a network, a node is a connection point, either a redistribution point or an end point for data transmissions. In general, a node has programmed or engineered capability to recognize and process or forward transmissions to other nodes.
Also see audio noise.
In a hard-wired circuit such as a telephone-line-based Internet hookup, external noise is picked up from appliances in the vicinity, from electrical transformers, from the atmosphere, and even from outer space. Normally this noise is of little or no consequence. However, during severe thunderstorms, or in locations were many electrical appliances are in use, external noise can affect communications. In an Internet hookup it slows down the data transfer rate, because the system must adjust its speed to match conditions on the line. In a voice telephone conversation, noise rarely sounds like anything other than a faint hissing or rushing. Noise is a more significant problem in wireless systems than in hard-wired systems. In general, noise originating from outside the system is inversely proportional to the frequency, and directly proportional to the wavelength. At a low frequency such as 300 kHz, atmospheric and electrical noise are much more severe than at a high frequency like 300 megahertz. Noise generated inside wireless receivers, known as internal noise, is less dependent on frequency. Engineers are more concerned about internal noise at high frequencies than at low frequencies, because the less external noise there is, the more significant the internal noise becomes. Communications engineers are constantly striving to develop better ways to deal with noise. The traditional method has been to minimize the signal bandwidth to the greatest possible extent. The less spectrum space a signal occupies, the less noise is passed through the receiving circuitry. However, reducing the bandwidth limits the maximum speed of the data that can be delivered. Another, more recently developed scheme for minimizing the effects of noise is called digital signal processing (DSP). Using fiber optics, a technology far less susceptible to noise, is another approach.
non-disclosure agreement A non-disclosure agreement (NDA) is a signed formal agreement in which one party agrees to give a second party confidential information about its business or products and the second party agrees not to share this information with anyone else for a specified period of time. Non-disclosure agreements are common in technology companies where products are sometimes jointly developed. (In this case, the non-disclosure agreement is often mutual or two-way.) An NDA is also sometimes used when a company seeks venture capital from potential financial backers as a way to make
non-return-to-zero sure that proprietary secrets or ideas are not stolen or leaked to someone else by the prospective investors. Today, venture capitalists for Internet startups sometimes will not consider a company that requires a non-disclosure agreement because of the time and overhead involved (they may have dozens of business plans waiting to be read!) and because of the potential for legal liability.
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A number of different kinds of non-disclosure agreements can be found on the Web using popular search engines.
non-interlaced display
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A non-interlaced display is a cathode-ray tube (CRT) display in which the lines are scanned sequentially rather than in alternating lines. In a CRT display, there are several hundred horizontal lines in a frame (full screen). These lines are scanned from left to right, and from top to bottom. The refresh rate (number of frames scanned per second) varies, but it is normally between 60 and 100 hertz. Refresh rates slower than 60 Hz produce distracting screen flicker, which can cause headaches and eye fatigue.
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H I Nonlinearity can be tolerated in devices and systems that use digital modulation, and also in frequency modulation (FM) wireless transmitters. These signals are either full-on or full-off; the amplitude waveforms are not analog, so analog distortion cannot occur. In analog devices and systems, however, linearity is important. Nonlinear circuits generally cause distortion in applications such as amplitudemodulation (AM) wireless transmission and hi-fi audio.
For serious animated-graphics work and video editing, a non- interlaced CRT display is recommended. The refresh rate should be as high as the system will allow, ideally 70 Hz or more.
In general, nonrepudiation is the ability to ensure that a party to a contract or a communication cannot deny the authenticity of their signature on a document or the sending of a message that they originated. On the Internet, the digital signature is used not only to ensure that a message or document has been electronically signed by the person that purported to sign the document, but also, since a digital signature can only be created by one person, to ensure that a person cannot later deny that they furnished the signature.
Nonlinearity is the behavior of a circuit, particularly an amplifier, in which the output signal strength does not vary in direct proportion to the input signal strength. In a nonlinear device, the output-to-input amplitude ratio (also called the gain) depends on the strength of the input signal. In an amplifier that exhibits nonlinearity, the output-versusinput signal amplitude graph appears as a curved line over part or all of the input amplitude range. Two examples are shown below. The amplifier depicted by the upwardmoving curve has gain that increases as the input signal strength increases; the amplifier depicted by the flattening curve has gain that decreases as the input signal strength increases.
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Examples of nonlinearity
Most CRT computer monitors scan each line in turn from top to bottom at the lowest resolution levels (640 6 480 and 800 6 600 pixel). However, at the higher resolutions, such as 1024 6 768 or 1200 6 800, the frame is sometimes scanned in interlaced fashion: First the odd-numbered lines, and then the even-numbered lines. This allows for a lower refresh rate without producing flicker. With text and fixed graphics displays, this scheme can work well. However, with animated graphics—especially images that move or change form rapidly—interlacing can produce a fluttering effect at least as irritating as screen flicker.
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Since no security technology is absolutely fool-proof, some experts warn that the digital signature alone may not always guarantee nonrepudiation. It is suggested that multiple approaches be used, such as capturing unique biometric information and other data about the sender or signer that collectively would be difficult to repudiate.
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non-return-to-zero See also bipolar signaling, unipolar signaling, and returnto-zero.
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Whatis?Com’s Encyclopedia of Technology Terms Non-return-to-zero (NRZ) refers to a form of digital data transmission in which the binary low and high states, represented by numerals 0 and 1, are transmitted by specific and constant DC (direct-current) voltages. In positive-logic NRZ, the low state is represented by the more negative or less positive voltage, and the high state is represented by the less negative or more positive voltage. Examples are:
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Logic 0 = +0.5 volts Logic 1 = +5.0 volts
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In negative-logic NRZ, the low state is represented by the more positive or less negative voltage, and the high state is represented by the less positive or more negative voltage. Examples are: Logic 0 = +5.0 volts Logic 1 = +0.5 volts Logic 0 = 0.0 volts Logic 1 = 73.0 volts Some people wonder why the name of this mode is preceded by ‘‘non’’ when one of the logic states might be represented by zero voltage. The answer becomes apparent from the comparative definition of return-to-zero (RZ).
nonuniform rational B-spline NURBS (nonuniform rational B-splines) are mathematical representations of 2- or 3-dimensional objects, which can be standard shapes (such as a cone) or free-form shapes (such as a car). NURBS are used in computer graphics and the CAD/CAM industry and have come to be regarded as a standard way to create and represent complex objects. In addition to curves and surfaces, NURBS can also represent hypersurfaces. Most sophisticated graphic creation tools provide an interface for using NURBS, which are flexible enough to design a wide range of shapes—anything from points to straight lines to conic sections. NURBS are compact expressions that can be evaluated and displayed quickly. NURBS work especially well in 3-D modeling, allowing the designer to easily manipulate control vertices, called ISO curves, and control curvature and the smoothness of contours. NURBS are defined by both control points and weights. It takes very little data to define a NURB. A spline is a usually curvy pattern used to guide someone shaping something large, such as a boat hull. The B-spline is based (the B stands for ‘‘basis’’) on four local functions or control points that lie outside the curve itself. Nonuniform is the idea that some sections of a defined shape (between any two points) can be shortened or elongated relative to other sections in the overall shape. Rational describes the ability to give more weight to some points in the shape than to other
points in considering each positions relation to another object. (This is sometimes referred to as a 4th dimensional characteristic.)
non-virtual hosting Also see virtual hosting. Non-virtual hosting is offering to host a Web site for an Internet user or company within the same domain name as that of the service provider. Frequently, this kind of hosting provides the customer with a free Web site with the provision that the user’s Web pages will contain a certain amount of advertising. Depending on how the hosting company implements their service, the user’s Web site address may be a subdomain of the hosting site or, more typically, within a file directory name sequence that is part of the hosting site’s domain name. For example, Joe Smith’s Web site might be at www.thehost.com/JoeSmith/. In addition to carrying on your pages ad banners over which you have no control, a non-virtual hosted Web site may be required to conform to an overall design pattern and to carry the logo of the hosting company. Geocities and Tripod are two of the largest companies that provide non-virtual hosting.
nonvolatile memory Nonvolatile memory is a general term for all forms of solid state (no moving parts) memory that have a continuous source of power and do not need to have their memory contents periodically refreshed. This includes all forms of read-only memory (ROM) such as programmable read-only memory, erasable programmable read-only memory, electrically erasable programmable read-only memory, and flash memory. It also includes random access memory (RAM) that is powered with a battery.
nonvolatile RAM See ‘‘nonvolatile memory’’
nonvolatile storage See ‘‘nonvolatile memory’’
NOR See ‘‘logic gate’’
norm A norm (from norma, Latin for carpenter’s level) is a model of what should exist or be followed, or an average of what currently does exist in some context, such as an average salary among members of a large group.
NOT columns containing customer ID, product sold, and price of the product when sold, the price would be a function of the customer ID (entitled to a discount) and the specific product.
normal forms See ‘‘normalization’’
normalization In creating a database, normalization is the process of organizing it into tables in such a way that the results of using the database are always unambiguous and as intended. Normalization may have the effect of duplicating data within the database and often results in the creation of additional tables. (While normalization tends to increase the duplication of data, it does not introduce redundancy, which is unnecessary duplication.) Normalization is typically a refinement process after the initial exercise of identifying the data objects that should be in the database, identifying their relationships, and defining the tables required and the columns within each table. A simple example of normalizing data might consist of a table showing: Customer Item purchased Purchase price Thomas Maria Evelyn Pajaro
Shirt Tennis shoes Shirt Trousers
$40 $35 $40 $25
If this table is used for the purpose of keeping track of the price of items and you want to delete one of the customers, you will also delete a price. Normalizing the data would mean understanding this and solving the problem by dividing this table into two tables, one with information about each customer and a product they bought and the second about each product and its price. Making additions or deletions to either table would not affect the other. Normalization degrees of relational database tables have been defined and include: First normal form (1NF). This is the ‘‘basic’’ level of normalization and generally corresponds to the definition of any database, namely: L
It contains two-dimensional tables with rows and columns.
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Each column corresponds to a sub-object or an attribute of the object represented by the entire table.
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Each row represents a unique instance of that sub-object or attribute and must be different in some way from any other row (that is, no duplicate rows are possible).
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All entries in any column must be of the same kind. For example, in the column labeled ‘‘Customer,’’ only customer names or numbers are permitted.
Second normal form (2NF). At this level of normalization, each column in a table that is not a determiner of the contents of another column must itself be a function of the other columns in the table. For example, in a table with three
Third normal form (3NF). At the second normal form, modifications are still possible because a change to one row in a table may affect data that refers to this information from another table. For example, using the customer table just cited, removing a row describing a customer purchase (because of a return perhaps) will also remove the fact that the product has a certain price. In the third normal form, these tables would be divided into two tables so that product pricing would be tracked separately. Domain/key normal form (DKNF). A key uniquely identifies each row in a table. A domain is the set of permissible values for an attribute. By enforcing key and domain restrictions, the database is assured of being freed from modification anomalies. DKNF is the normalization level that most designers aim to achieve.
normative In general, normative—pertaining to a norm—has two related meanings: (1) a prescriptive meaning (for example, the rules specified in a standard or guideline), and (2) a descriptive meaning (for example, the median salary range in an particular occupation).
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In information technology standards, normative parts of a standard are those that specify what implementors should conform to and non-normative parts consist of examples, extended explanations, and other matter not dealing directly with the specifications.
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In economics, a normative statement is one that states how things ought to be and is contrasted with a positive statement, one that states factually how things are.
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Northbridge Northbridge is an Intel chipset that communicates with the computer processor and controls interaction with memory, the Peripheral Component Interconnect (PCI) bus, Level 2 cache, and all Accelerated Graphics Port (AGP) activities. Northbridge communicates with the processor using the frontside bus (FSB). Northbridge is one part of a two-part chipset called Northbridge/Southbridge. Southbridge handles the input/output (I/O) functions of the chipset. The Intel Hub Architecture (IHA) has replaced the Northbridge/Southbridge chipset. The IHA chipset also has two parts: the Graphics and AGP Memory Controller Hub (GMCH) and the I/O Controller Hub (ICH). The IHA architecture is used in Intel’s 800 series chipsets, which is the first chipset architecture to move away from the Northbridge/Southbridge design.
NOT See ‘‘logic gate’’
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A notebook computer is a battery-powered personal computer generally smaller than a briefcase that can easily be transported and conveniently used in temporary spaces such as on airplanes, in libraries, temporary offices, and at meetings. A notebook computer, sometimes called a laptop computer, typically weighs less than 5 pounds and is 3 inches or less in thickness. Among the best-known makers of notebook and laptop computers are IBM, NEC, Dell, Toshiba, and Hewlett-Packard.
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Notebook computers generally cost more than desktop computers with the same capabilities because they are more difficult to design and manufacture. A notebook can effectively be turned into desktop computers with a docking station, a hardware frame that supplies connections for peripheral input/output devices such as a printer or larger monitor. The less capable port replicator allows you to connect a notebook to a number of peripherals through a single plug. Notebooks usually come with displays that use thin-screen technology. The thin film transistor or active matrix screen is brighter and views better at different angles than the STN or dual-scan screen. Notebooks use several different approaches for integrating a mouse into the keyboard, including the touch pad, the trackball, and the pointing stick. A serial port also allows a regular mouse to be attached. The PC Card is insertable hardware for adding a modem or NIC to a notebook. CD-ROM and digital versatile disc drives may be built-in or attachable.
Notes Notes is a sophisticated groupware application from the Lotus Corporation, a subsidiary of IBM. Notes lets a corporation and its workers develop communications- and database-oriented applications so that users at different geographic locations can share files with each other, comment on them publicly or privately (to groups with special access), keep track of development schedules, work projects, guidelines and procedures, plans, white papers, and many other documents, including multimedia files. Notes keeps track of changes and makes updates to replications of all databases in use at any site. Changes are made at the field level to minimize network traffic. Notes runs on a special server called the Lotus Domino Server. The servers and workstations use the client/server model and replicated databases are updated using Remote Procedure Call (RPC) requests. Notes can be coordinated with Web servers and applications on a company’s intranet. e-mail, calendar, and some other basic applications come with Notes. However, Notes provides facilities so that users can create their own applications. (Notes applications are generally document-oriented, not transaction-oriented.) Notes was designed with large-scale work projects in mind. Notes comes with application development tools that make
it relatively easy to create applications. The Notes concepts and user interface are not intuitively understandable. Users as well as application developers usually require an assisted introduction. Microsoft Exchange is a somewhat comparable product. It does not have the application development facilities that Notes has. It does offer an easy-to-use interface.
Novell Directory Services NDS (Novell Directory Services) is a popular software product for managing access to computer resources and keeping track of the users of a network, such as a company’s intranet, from a single point of administration. Using NDS, a network administrator can set up and control a database of users and manage them using a directory with an easy-touse graphical user interface (GUI). Users of computers at remote locations can be added, updated, and managed centrally. Applications can be distributed electronically and maintained centrally. NDS can be installed to run under Windows NT, (and later) Sun Microsystem’s Solaris, and IBM’s OS/390 as well as under Novell’s own NetWare so that it can be used to control a multi-platform network. NDS is generally considered an industry benchmark against which other products, such as Microsoft’s Active Directory, must compete.
nroff nroff and troff are UNIX commands (and the utilities that support them) for formatting text files for printing. (There are other UNIX commands for initiating printing.) nroff is designed for formatting output for line printers and letterquality printers. troff is designed for formatting output for typesetters. troff contains some special functions that apply only to typesetters; otherwise, the commands are identical and either can be used. In general, a text file designed for use with nroff or troff contains embedded codes for line spacing, margin settings, centering, tab stops, keeping lines together, and so forth. The command itself includes options that apply to the entire file. nroff and troff provide about the same level of formatting control as IBM’s Script/VS language. In general, text formatting languages at this level have the drawback of making it difficult to reuse the text in a non-print medium (such as on the Web). This drawback led to the development of the non-output specific markup defined generally by Standard Generalized Markup Language in which a logical or functional description is applied to text elements that device-specific programs can interpret in an appropriate way. Examples of nroff/troff formatting codes include: .ce Center the next lines of text. .sp
NTFS Skip a line space. .ps 10 Use 10-point type. A popular UNIX replacement for nroff/troff is a formatter called TeX (pronounced ‘‘TEK’’). TeX is designed to give the user a great deal of control over font choice and text arrangement and especially to support text containing mathematical symbols. If you have inherited an nroff/troff file and need to convert it to HTML, you may be able to find a tool that will convert the file to HTML ‘‘preformatted’’ text (meaning that the file will look as though it has been brought in from somewhere). While this may be a temporary solution in some cases, it’s likely that eventually you will have to strip the old codes and start with new formatting (using HTML tags) manually.
NSA See ‘‘National Security Agency’’
NSAP See ‘‘Network Service Access Point’’
nslookup nslookup is the name of a program that lets an Internet server administrator or user enter a host name (for example, ‘‘whatis.com’’) and find out the corresponding IP address. It will also do reverse name lookup and find the host name for an IP address you specify. For example, if you entered ‘‘whatis.com’’, you would receive as a response our IP address, which happens to be: 216.34.126.200 Or if you entered ‘‘216.34.126.200’’, it would return ‘‘whatis.com’’. nslookup sends a domain name query packet to a designated (or defaulted) domain name system (DNS) server. Depending on the system you are using, the default may be the local DNS name server at your service provider, some intermediate name server, or the root server system for the entire domain name system hierarchy. Using the Linux and possibly other versions of nslookup, you can locate other information associated with the host name or IP address, such as associated mail services. nslookup is often included with a UNIX-based operating system. Free versions exist that can be added to Windows operating systems. A more limited alternative to nslookup for looking up an IP address is the ping command.
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NT1 Using the Integrated Services Digital Network (ISDN) Basic Rate Interchange (BRI) service, an NT1 (network terminating unit 1) is a device that accepts a two-wire signal from the phone company and converts it to a fourwire signal that sends and receives to and from devices within the home or business. In the U.K. and some other countries, the NT1 is located at the telephone company’s central office. In the U.S., the NT1 is a separate box at the home or business or it can be integrated into one device. If it is a separate box, up to eight devices, such as telephones and computers, can be attached to it. If the NT1 is built into one device, then only that one device can be served by the line coming in from the phone company. Additional devices would require one or more additional lines. The ISDN Basic Rate Interface is the most common service offered by ISDN providers. BRI supports two separate 64 Kbps B channels and one 16 Kbps D channel. The phone company replaces your conventional analog circuit with a BRI circuit. The NT1 provides the entry termination at your home or business that is required by the phone company. You must have one NT1 for each ISDN line. Some ISDN equipment such as an ISDN terminal adapter (the ISDN equivalent of a modem) may already have a built-in NT1. To find out if your ISDN device has a built-in NT1, check to see if it is designed to connect directly to the public ISDN network. If it is, it does not require a separate NT1. Most U.S. ISDN devices have the NT1 built-in. If you wish to have more than one ISDN device per line, consider purchasing a separate NT1 box with multiple jacks. Some NT1 boxes have built-in analog conversion so you can use both ISDN and analog equipment on the same line. Your ISDN provider may occasionally ‘‘talk’’ to your NT1 box to perform routine testing and maintenance.
NTFS NTFS (NT file system) is the system that the Windows NT operating system uses for storing and retrieving file on a hard disk. NTFS is the Windows NT equivalent of the Windows 95 file allocation table (file allocation table) and the OS/2 HPFS (high performance file system). However, NTFS offers a number of improvements over FAT and HPFS in terms of performance, extendibility, and security.
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Use of a b-tree directory scheme to keep track of file clusters
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Support for very large files (up to 2 to the 64th power or approximately 16 billion byte in size)
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An access control list (ACL) that lets a server administrator control who can access specific files
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Integrated file compression
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See ‘‘network service provider’’
NT See ‘‘Windows NT’’
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Support for names based on Unicode
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Data security on both removable and fixed disks
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NTSC See ‘‘National Television Standards Committee’’
The selection of the cluster size is a trade-off between efficient use of disk space and the number of disk accesses required to access a file. In general, using NTFS, the larger the hard disk the larger the default cluster size, since it’s assumed that a system user will prefer to increase performance (fewer disk accesses) at the expense of some amount of space inefficiency.
The standard RS-232C serial communications interface defines a signal protocol between a Data Terminal Equipment (DTE)—usually your PC—and a Data Communications Equipment (DCE)—or your modem. The signals are transmitted on a set of lines, each of which has a function in the ‘‘talk’’ that the DTE and DCE do back and forth. One line each way is for data; the other lines are for different ‘‘statements’’ that one end of the communication sends to the other. For example, the DTE sends the DCE (usually a modem) a ‘‘Request to Send’’ signal on the RTS line and the DCE replies with a ‘‘Clear to Send’’ signal on the CTS line. After a series of similar exchanges, the DTE sends data on the line devoted to transmitting data (which for the DCE is a line for receiving data from the DTE).
When a file is created using NTFS, a record about the file is created in a special file, the Master File Table (MFT). The record is used to locate a file’s possibly scattered clusters. NTFS tries to find contiguous storage space that will hold the entire file (all of its clusters). Each file contains, along with its data content, a description of its attributes (its metadata).
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When a hard disk is formatted (initialized), it is divided into partitions or major divisions of the total physical hard disk space. Within each partition, the operating system keeps track of all the files that are stored by that operating system. Each file is actually stored on the hard disk in one or more cluster or disk spaces of a predefined uniform size. Using NTFS, the sizes of clusters range from 512 byte to 64 kilobyte. Windows NT provides a recommended default cluster size for any given drive size. For example, for a 4 GB (gigabyte) drive, the default cluster size is 4 KB (kilobytes). Note that clusters are indivisible. Even the smallest file takes up one cluster and a 4.1 KB file takes up two clusters (or 8 KB) on a 4 KB cluster system.
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An n-tier application program is one that is distributed among three or more separate computers in a distributed network. The most common form of n-tier (meaning ’some number of tiers’) is the 3-tier application, in which user interface programming is in the user’s computer, business logic is in a more centralized computer, and needed data is in a computer that manages a database. N-tier application structure implies the client/server program model. Where there are more than three distribution levels or tiers involved, the additional tiers in the application are usually associated with the business logic tier. In addition to the advantages of distributing programming and data throughout a network, n-tier applications have the advantages that any one tier can run on an appropriate processor or operating system platform and can be updated independently of the other tiers. Communication between the program tiers uses special program interfaces such as those provided by the Common Object Request Broker Architecture (CORBA).
nuking See ‘‘blue bomb’’
null modem A null modem cable allows you to connect your PC to another nearby PC or serial device using its modem protocol. A popular use of null modem cables is for setting up ‘‘head-to-head’’ gaming between two players at different computers in the same room. (A null modem cable is limited to 30 feet in length.)
Since a modem or DCE is not really needed to interconnect your PC with another local serial device, the DTE interface can be used by both your PC and the attached serial device. However, the DTE interface is designed to work with a DCE device. What a null modem cable does is to make the other end of the PC or device’s DTE interface look like a DCE interface. A null modem cable is sometimes called a crossover cable.
NUMA NUMA (non-uniform memory access) is a method of configuring a cluster of microprocessors in a multiprocessing system so that they can share memory locally, improving performance and the ability of the system to be expanded. NUMA is used in a symmetric multiprocessing (SMP) system. An SMP system is a ‘‘tightly-coupled,’’ ‘‘share everything’’ system in which multiple processors working under a single operating system access each other’s memory over a common bus or ‘‘interconnect’’ path. Ordinarily, a limitation of SMP is that as microprocessors are added, the shared bus or data path get overloaded and becomes a performance bottleneck. NUMA adds an
nym intermediate level of memory shared among a few microprocessors so that all data accesses don’t have to travel on the main bus. NUMA can be thought of as a ‘‘cluster in a box.’’ The cluster typically consists of four microprocessors (for example, four Pentium microprocessors) interconnected on a local bus (for example, a Peripheral Component Interconnect bus) to a shared memory (called an ‘‘L3 cache’’) on a single motherboard (it could also probably be referred to as a card). This unit can be added to similar units to form a symmetric multiprocessing system in which a common SMP bus interconnects all of the clusters. Such a system typically contains from 16 to 256 microprocessors. To an application program running in an SMP system, all the individual processor memories look like a single memory. When a processor looks for data at a certain memory address, it first looks in the L1 cache on the microprocessor itself, then on a somewhat larger L1 and L2 cache chip nearby, and then on a third level of cache that the NUMA configuration provides before seeking the data in the ‘‘remote memory’’ located near the other microprocessors. Each of these clusters is viewed by NUMA as a ‘‘node’’ in the interconnection network. NUMA maintains a hierarchical view of the data on all the nodes. Data is moved on the bus between the clusters of a NUMA SMP system using scalable coherent interface (SCI) technology. SCI coordinates what is called ‘‘cache coherence’’ or consistency across the nodes of the multiple clusters. SMP and NUMA systems are typically used for applications such as data mining and decision support systems in which processing can be parceled out to a number of processors that collectively work on a common database. Sequent, Data General, and NCR are among companies that produce NUMA SMP systems.
NVRAM See ‘‘nonvolatile storage’’
NVS See ‘‘nonvolatile storage’’
In an Ethernet network, a device may have the capability to operate at different speeds. NWay currently supports at least the following technologies: 10BASE-T, 10BASE-T duplex, 100BASE-T, 100BASE-TX Full Duplex, and 100BASE-T4. Expanded support capability is likely in the near future. When a connection to a network device (known as the link partner) has been established, the NWay protocol determines what modes that device uses, sends information about its own capabilities, and automatically configures the highest common performance mode. NWay works by taking control of the cable connected to a network device and operating like a rotary switch (an electromechanical device with a rotating shaft at one terminal that makes or breaks connections with other terminals) to change to the best mode possible, and passing control of the cable to the appropriate technology. The benefits of NWay technology include: L
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NWay (Auto-Negotiation) is defined in Clause 28 of the D4 draft of the ANSI/IEEE Std 802.3 MAC Parameters, Physical Layer, Medium Attachment Units and Repeater for 100 Mb/s Operation.
nym A nym (pronounced NIHM and a shortened form of ‘‘pseudonym,’’) is a name invented by or provided for an Internet user in order to conceal the user’s real identity and, in some cases, to expressly create a new and separate Internet identity. Among reasons to have and use a ‘‘nym’’ are these: L
NWay NWay is a telecommunications protocol used with Ethernet networking devices (such as routers and switches) to automatically negotiate the highest possible common transmission speed between two devices. The NWay protocol (also known as auto-negotiation or auto-sensing) was developed by National Semiconductor in 1994 in response to the networking industry’s need for a mechanism to handle the connections between devices with varying connection speeds.
You want to comment or take part in controversial political or other discussions on the Internet without revealing your opinions to colleagues, friends, or employers
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You want to send spam (unrequested bulk e-mail) without anyone being able to trace it to you
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Whatis?Com’s Encyclopedia of Technology Terms There are a number of legitimate reasons for communicating anonymously. The example of a battered spouse seeking help in a Usenet news group or a Web site discussion is sometimes cited. To use a nym instead of your real e-mail name when sending and receiving e-mail, you can use the services of a remailer, a company with a Web server that accepts e-mail from you and forwards it to its destination with a different return address than yours. Incoming mail to your nym is forwarded to your real Internet address. Some remailing services forward mail through several remailers to make it even harder to trace the source of an e-mail note. There are free or public remailer services. Andre Bacard, author of The Anonymous Remailer FAQ and The Computer Privacy Handbook, divides remailers into: (1) pseudo-anonymous remailers, who know the names of the people for whom they are providing anonymity service, and (2) more serious anonymous remailers, who never know your name. Bacard believes that, for most people, the pseudo-anonymous is sufficient. It’s also much easier to use. Most remailer services disallow the use of spam. Remailing is often combined with encryption using Pretty Good Privacy (PGP). Some companies also allow you to surf the Web anonymously by first linking to an intermediary site that in turn connects you to any Web site you select without that site being able to learn anything about you from your request message.
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OASIS OASIS (Organization for Structured Information Standards) is a nonprofit, international consortium whose goal is to promote the adoption of product-independent standards for information formats such as Standard Generalized Markup Language (SGML), Extensible Markup Language (XML), and Hypertext Markup Language (HTML). Currently, OASIS (formerly known as SGML Open) is working to bring together competitors and industry standards groups with conflicting perspectives to discuss using XML as a common Web language that can be shared across applications and platforms. OASIS sponsors XML.org, a non-profit XML Web portal. The goal of OASIS is not to create structured information standards for XML, but to provide a forum for discussion, to promote the adoption of interoperability standards, and to recommend ways members can provide better interoperability for their users. OASIS has worked with the United Nations to sponsor ebXML, a global initiative for electronic business data exchange. EbXML, whose goal is to make it easier for companies of all sizes and locations to conduct business on the Internet, is currently focusing on the specific needs of business-to-business (B2B) and Internet security as it relates to XML.
OBI See ‘‘Open Buying on the Internet’’
object In object-oriented programming (OOP), objects are the things you think about first in designing a program and they are also the units of code that are eventually derived from the process. In between, each object is made into a generic class of object and even more generic classes are defined so that objects can share models and reuse the class definitions in their code. Each object is an instance of a particular class or subclass with the class’s own methods or procedures and data variables. An object is what actually runs in the computer.
object code Source code and object code refer to the ‘‘before’’ and ‘‘after’’ versions of a computer program that is compiled (see compiler) before it is ready to run in a computer. The source code consists of the programming statements that are created by a programmer with a text editor or a visual programming tool and then saved in a file. For example, a programmer using the C language types in a desired sequence of C language statements using a text editor and then saves them as a named file. This file is said to contain
the source code. It is now ready to be compiled with a C compiler and the resulting output, the compiled file, is often referred to as object code. The object code file contains a sequence of instructions that the processor can understand but that is difficult for a human to read or modify. For this reason and because even debugged programs often need some later enhancement, the source code is the most permanent form of the program. When you purchase or receive operating system or application software, it is usually in the form of compiled object code and the source code is not included. Proprietary software vendors usually don’t want you to try to improve their code since this may create additional service costs for them. Lately, there is a movement to develop software (Linux is an example) that is open to further improvement by anyone who wants to improve it, and here the source code is provided. In large program development environments, there are often management systems that help programmers separate and keep track of different states and levels of code files. For script (noncompiled or interpreted) program languages, such as JavaScript, the terms source code and object code do not apply since there is only one form of the code.
Object Linking and Embedding
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Object Management Group The OMG (Object Management Group) was formed in 1989 by a group of vendors for the purpose of creating a standard architecture for distributed objects (also known as ‘‘components’’) in networks. The architecture that resulted is the Common Object Request Broker Architecture (CORBA). A central element in CORBA is the Object Request Broker (ORB). An ORB makes it possible for a client object to make a server request without having to know where in a network the server object or component is located and exactly what its interfaces are. A number of middleware products have begun to use CORBA and it appears to be the strategic architecture for distributed objects. The OMG now boasts of over 500 member companies.
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Also see ORBS, a term easily confused with ORB.
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In Common Object Request Broker Architecture (CORBA), an Object Request Broker (ORB) is the programming that acts as a ‘‘broker’’ between a client request for a service from a distributed object or component and the completion of
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that request. Having ORB support in a network means that a client program can request a service without having to understand where the server is in a distributed network or exactly what the interface to the server program looks like. Components can find out about each other and exchange interface information as they are running.
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Licensing service, which allows the use of a component to be measured for purposes of compensation for use. Charging can be done by session, by node, by instance creation, and by site.
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Properties service, which lets a component contain a selfdescription that other components can use.
CORBA’s ORB may be thought of as strategic middleware that is more sophisticated conceptually and in its capabilities than earlier middleware, including Remote Procedure Calls (RPCs), message-oriented middleware, database stored procedures, and peer-to-peer services.
In addition, an ORB also can provide security and time services. Additional services for trading, collections, and change management are also planned. The requests and replies that originate in ORBs are expressed through the Internet Inter-ORB Protocol (IIOP) or other transport layer protocols.
An ORB uses the CORBA Interface Repository to find out how to locate and communicate with a requested component. When creating a component, a programmer uses either CORBA’s Interface Definition Language (IDL) to declare its public interfaces or the compiler of the programming language translates the language statements into appropriate IDL statements. These statements are stored in the Interface Repository as metadata or definitions of how a component’s interface works. In brokering a client request, an ORB may provide all of these services: L
Life cycle services, which define how to create, copy, move, and delete a component.
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Persistence service, which provide the ability to store data on object database, and plain files.
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Naming service, which allows a component to find another component by name and also supports existing naming systems or directories, including DCE, and Sun’s NIS (Network Information System).
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Event service, which lets components specify events that they want to be notified of.
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Concurrency control service, which allows an ORB to manage locks to data that transactions or threads may compete for.
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Relationship service, which creates dynamic associations between components that haven’t ‘‘met’’ before and for keeping track of these associations.
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Externalization service, which provides a way to get data to and from a component in a ‘‘stream.’’
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Query service, which allows a component to query a database. This service is based on the SQL3 specification and the Object Database Management Group’s (ODMG) Object Query Language (OQL).
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Transaction service, which ensures that when a transaction is completed, changes are committed, or that, if not, database changes are restored to their pretransaction state.
object-oriented database management system An object-oriented database management system (OODBMS), sometimes shortened to ODBMS for object database management system), is a database management system (DBMS) that supports the modelling and creation of data as objects. This includes some kind of support for classes of objects and the inheritance of class properties and methods by subclasses and their objects. There is currently no widely agreed-upon standard for what constitutes an OODBMS, and OODBMS products are considered to be still in their infancy. In the meantime, the object-relational database management system (ORDBMS), the idea that object-oriented database concepts can be superimposed on relational databases, is more commonly encountered in available products. An object-oriented database interface standard is being developed by an industry group, the Object Data Management Group (ODMG). The Object Management Group (OMG) has already standardized an object-oriented data brokering interface between systems in a network. In their influential paper, The Object-Oriented Database Manifesto, Malcolm Atkinson and others define an OODBMS as follows: An object-oriented database system must satisfy two criteria: it should be a DBMS, and it should be an object-oriented system, i.e., to the extent possible, it should be consistent with the current crop of objectoriented programming languages. The first criterion translates into five features: persistence, secondary storage management, concurrency, recovery and an ad hoc query facility. The second one translates into eight features: complex objects, object identity, encapsulation, types or classes, inheritance, overriding combined with late binding, extensibility and computational completeness. Their paper describes each of these features in detail.
Ockham’s razor
object-oriented programming A revolutionary concept that changed the rules in computer program development, object-oriented programming (OOP) is organized around ‘‘objects’’ rather than ‘‘actions,’’ data rather than logic. Historically, a program has been viewed as a logical procedure that takes input data, processes it, and produces output data. The programming challenge was seen as how to write the logic, not how to define the data. Objectoriented programming takes the view that what we really care about are the objects we want to manipulate rather than the logic required to manipulate them. Examples of objects range from human beings (described by name, address, and so forth) to buildings and floors (whose properties can be described and managed) down to the little widgets on your computer desktop (such as buttons and scroll bars). The first step in OOP is to identify all the objects you want to manipulate and how they relate to each other, an exercise often known as data modeling. Once you’ve identified an object, you generalize it as a class of objects (think of Plato’s concept of the ‘‘ideal’’ chair that stands for all chairs) and define the kind of data it contains and any logic sequences that can manipulate it. Each distinct logic sequence is known as a method. A real instance of a class is called (no surprise here) an ‘‘object’’ or, in some environments, an ‘‘instance of a class.’’ The object or class instance is what you run in the computer. Its methods provide computer instructions and the class object characteristics provide relevant data. You communicate with objects—and they communicate with each other—with well-defined interfaces called messages. The concepts and rules used in object-oriented programming provide these important benefits: L
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The concept of a data class makes it possible to define subclasses of data objects that share some or all of the main class characteristics. Called inheritance, this property of OOP forces a more thorough data analysis, reduces development time, and ensures more accurate coding. Since a class defines only the data it needs to be concerned with, when an instance of that class (an object) is run, the code will not be able to accidentally access other program data. This characteristic of data hiding provides greater system security and avoids unintended data corruption. The definition of a class is reuseable not only by the program for which it is initially created but also by other object-oriented programs (and, for this reason, can be more easily distributed for use in networks). The concept of data classes allows a programmer to create any new data type that is not already defined in the language itself.
One of the first object-oriented computer languages was called Smalltalk. C++ and Java are the most popular objectoriented languages today. The Java programming language is designed especially for use in distributed applications on corporate networks and the Internet.
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OC-1 – OC-2 – OC-3 – OC-12 – OC-48 – OC-192 – OC-256
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OC-768 OC-768 is currently the fastest synchronous optical network (SONET) standard rate for data transmission on optical fiber as part of the broadband ISDN (BISDN). OC stands for optical carrier and the number affixed is the multiple of the base rate bandwidth of 51.85 Mbps. OC-768 supports rates of 40 gigabits per second (Gbps) on a fiber optic carrier, a rate that translates to the equivalent of seven CD-ROM’s worth of data in one second. Developed to meet evergrowing demands for bandwidth, OC-768 uses dense wavelength division multiplexing (DWDM) to carry multiple channels of data on a single optic fiber. New DWDM systems are now in development to run at at 10 trillion bits per second (10 Tbps) per fiber. This translates into the theoretical capability of one fiber to support, simultaneously, an active Internet connection to every household in the U.S. Enkido was the first company to offer OC-768 service, which they currently provide for a number of clients including Deutsche Telecom.
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Ockham’s razor Ockham’s razor (also spelled Occam’s razor, pronounced AHK-uhmz RAY-zuhr) is the idea that, in trying to understand something, getting unnecessary information out of the way is the fastest way to the truth or to the best explanation. William of Ockham (1285-1349), English theologian and philosopher, spent his life developing a philosophy that reconciled religious belief with demonstratable, generally experienced truth, mainly by separating the two. Where earlier philosophers attempted to justify God’s existence with rational proof, Ockham declared religious belief to be incapable of such proof and a matter of faith. He rejected the notions preserved from Classical times of the independent existence of qualities such as truth, hardness, and durability and said these ideas had value only as descriptions of particular objects and were really
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characteristics of human cognition. Ockham was noted for his insistence on paying close attention to language as a tool for thinking and on observation as a tool for testing reality. His thinking and writing is considered to have laid the groundwork for modern scientific inquiry. Ockham’s insistence on the use of parsimony (we might call it minimalism) in thought resulted in some later writer’s invention of the term, Ockham’s razor. Among his statements (translated from his Latin) are: ‘‘Plurality is not to be assumed without necessity’’ and ‘‘What can be done with fewer [assumptions] is done in vain with more.’’ One consequence of this methodology is the idea that the simplest or most obvious explanation of several competing ones is the one that should be preferred until it is proven wrong.
OCR OCR (optical character recognition) is the recognition of printed or written text characters by a computer. This involves photoscanning of the text character-by-character, analysis of the scanned-in image, and then translation of the character image into character codes, such as ASCII, commonly used in data processing. In OCR processing, the scanned-in image or bitmap is analyzed for light and dark areas in order to identify each alphabetic letter or numeric digit. When a character is recognized, it is converted into an ASCII code. Special circuit boards and computer chips designed expressly for OCR are used to speed up the recognition process. OCR is being used by libraries to digitize and preserve their holdings. OCR is also used to process checks and credit card slips and sort the mail. Billions of magazines and letters are sorted every day by OCR machines, considerably speeding up mail delivery.
octal Octal (pronounced AHK-tuhl, from Latin octo or ‘‘eight’’) is a term that describes a base-8 number system. An octal number system consists of eight single-digit numbers: 0, 1, 2, 3, 4, 5, 6, and 7. The number after 7 is 10. The number after 17 is 20 and so forth. In computer programming, the octal equivalent of a binary number is sometimes used to represent it because it is shorter.
octet In computers, an octet (from the Latin octo or ‘‘eight’’) is a sequence of eight bits. An octet is thus an eight-bit byte. Since a byte is not eight bits in all computer systems, octet provides a nonambiguous term. This term should not be confused with octal, a term that describes a base-8 number system.
OCW See ‘‘OpenCourseWare’’
OCX An OCX is an Object Linking and Embedding (OLE) custom control, a special-purpose program that can be created for use by applications running on Microsoft’s Windows systems. OCXs provide such functions as handling scroll bar movement and window resizing. If you have a Windows system, you’ll find a number of files in your Windows directory with the OCX file name suffix. Object Linking and Embedding was designed to support compound documents (which contain multiple information types, such as text, graphic images, sound, motion video). The Windows desktop is an example of a compound document and Microsoft used OLE to build it. OLE and the Component Object Model (COM), a more general concept that succeeded OLE, support the development of ‘‘plug-andplay’’ programs that can be written in any language and used dynamically by any application in the system. These programs are known as components and the application in which they are run is known as a container. This component-based approach to application development reduces development time and improves the program capability and quality. Windows application development programs such as PowerBuilder and Microsoft Access take advantage of OCXs. Microsoft now calls an OCX an ActiveX control, the component object under Microsoft’s set of ActiveX technologies, of which the fundamental concept is the Component Object Model (COM) and, in a network, the Distributed Component Object Model (DCOM). An OCX or ActiveX control is actually implemented as a dynamic link library DLL module. (You can think of a DLL program as a ‘‘subprogram’’ that can be used by any number of application programs, each of which is a ‘‘container’’ for the DLL or OCX/ActiveX control ‘‘object.’’) Visual Basic and C++ are commonly used to write OCX or ActiveX controls.
OC-x See ‘‘Optical Carrier levels (OCx)’’
ODBC See ‘‘Open Database Connectivity’’
ODBMS See ‘‘object-oriented database management system’’
ODI See ‘‘Open Data-Link Interface’’
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ODL 1) Open and Distance Learning (ODL) is a general term for the use of telecommunication to provide or enhance learning. Around the world, the academic community is discovering and exploring the Internet, teleconferencing, and related means to achieve an extended classroom or learning experience. Students in Russia are earning degrees from a university in Australia. Primary and secondary-grade students are exchanging e-mail across continents as a supplement to their formal studies. Students and teachers at all levels are taking part in teleconferences and forming associations that would have been unlikely five years ago. A number of world conferences have been held on ODL and many experimental projects are underway. The United States Distance Learning Association has its own formal definition of ‘‘distance learning’’: The acquisition of knowledge and skills through mediated information and instruction, encompassing all technologies and other forms of learning at a distance. A few of the many educational organizations involved with ODL include: L
The University of Bergen in Bergen, Norway, which offers its own ODL Resource Page
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The Open University at the Institute for Educational Technology in the United Kingdom
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The Virtual Classroom at the New Jersey Institute of Technology
2) Object Description Language (ODL) is a simple language for defining data structures that is used in the United States by the National Aeronautics and Space Adinistration (NASA).
chapter, who has it ‘‘checked out,’’ and what draft level of the book it is stored in. The Open Document Management API provides vendors of DMS products and products such as word processors with a consistent interface. A word processor can integrate DMS support so that you can create, edit, and manage your documents through the word processor.
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DMS is also sometimes called EDMS (Electronic Document Management System).
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OEM See ‘‘original equipment manufacturer’’
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OFDM See ‘‘orthogonal frequency-division multiplexing’’
offline Offline is the condition of being currently disconnected from a computer or a telecommunications system. The term is frequently used to describe someone who has the ability to be connected to the Internet but who is not currently connected to it.
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Several somewhat similar languages have been created by other organizations for defining program or data objects. These include an Object Design Language and an Object Definition Language.
Deprecated spellings are off-line and off line. The opposite term is online.
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See ‘‘modem lights’’
ODMA (Open Document Management API or Application Program Interface) is an industry standard interface for managing documents that allows users to store, retrieve, and share them with security and version control. For example, you can collaborate with someone in another city on a book in which each of you write separate chapters and occasionally work on the same chapter. Using a document management system (DMS), you can create a chapter using your favorite word processor and store it in the document management system in the ‘‘first draft library.’’ Later, you can retrieve it from the ‘‘first draft library,’’ edit it, and put it back. Your collaborator can also retrieve your chapter to either look at it or make further changes to it, depending on how you wish to work together. The document management system manages who has read-only or update access to the
ohm The ohm is the standard unit of electrical resistance in the International System of Units (SI). Ohms are also used, when multiplied by imaginary numbers, to denote reactance in alternating-current (AC) and radio-frequency (RF) applications. Reduced to base SI units, one ohm is the equivalent of one kilogram meter squared per second cubed per ampere squared (1 kg times m2 ˙ s-3 ˙ A-2). The ohm is also the equivalent of a volt per ampere (V/A). In a direct-current (DC) circuit, a component has a resistance of one ohm when a potential difference of one volt produces a current of one ampere through the component. In AC and RF circuits, resistive ohms behave the same as they do in DC circuits, provided the root-mean-square (rms) AC voltage is
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specified. In AC and RF circuits, reactance exists only when there is a net capacitance or inductance. Capacitive reactances have negative imaginary ohmic values; inductive reactances have positive imaginary ohmic values. The reactance of a particular capacitor or inductor depends on the frequency. Resistances and reactances are sometimes expressed in units representing power-of-10 multiples of one ohm. A kilohm is equal to one thousand (103) ohms. A megohm is equal to one million (106) ohms. Fractional prefix multipliers are seldom used for resistance or reactances; rarely will you hear or read about a milliohm or a microhm. Extremely small resistances and reactances are usually referred to in terms of conductance. Also see conductance, Ohm’s Law, prefix multipliers, resistance, reactance, siemens, and International System of Units (SI).
Ohm’s Law Ohm’s Law is the mathematical relationship among electric current, resistance, and voltage. The principle is named after the German scientist Georg Simon Ohm.
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In direct-current (DC) circuits, Ohm’s Law is simple and linear. Suppose a resistance having a value of R ohms carries a current of I amperes. Then the voltage across the resistor is equal to the product IR. There are two corollaries. If a DC power source providing E volts is placed across a resistance of R ohms, then the current through the resistance is equal to E/R amperes. Also, in a DC circuit, if E volts appear across a component that carries I amperes, then the resistance of that component is equal to E/I ohms.
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Mathematically, Ohm’s Law for DC circuits can be stated as three equations:
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I = E/R R = E/I When making calculations, compatible units must be used. If the units are other than ohms (for resistance), amperes (for current), and volts (for voltage), then unit conversions should be made before calculations are done. For example, kilohms should be converted to ohms, and microamperes should be converted to amperes.
OK OK (pronounced oh-KAY and occasionally spelled okay) is a short way to say ‘‘I agree.’’ It’s sometimes encountered in computer messages that seek the user’s confirmation of a request.
OLAP See ‘‘online analytical processing’’
OLE OLE (Object Linking and Embedding) is Microsoft’s framework for a compound document technology. Briefly, a compound document is something like a display desktop that can contain visual and information objects of all kinds: text, calendars, animations, sound, motion video, 3-D, continually updated news, controls, and so forth. Each desktop object is an independent program entity that can interact with a user and also communicate with other objects on the desktop. Part of Microsoft’s ActiveX technologies, OLE takes advantage and is part of a larger, more general concept, the Component Object Model (COM) and its distributed version, DCOM. An OLE object is necessarily also a component (or COM object). Some main concepts in OLE and COM are: Microsoft terms are shown first; industry or alternative versions of those terms are shown in parentheses: Concept
What it is
OLE
A set of APIs to create and display a (compound) document A presentation of different items in an ‘‘animated desktop’’
Document (compound document) Item (object; also called a component) Container or container application Server or server application Embedding
ohnosecond An ohnosecond is that very short moment in time during which you realize that you have pressed the wrong key and deleted hours, days, or weeks of work.
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Visual editing Automation
An element in a document, such as an animated calendar, a video window, a sound player, a sound file... The program entity that holds a document or a control The program entity that holds an item within an OLE container Adding the source data for an item to a document; use the Paste command in a container application Adding a link to the source data for an item to a document; use the Paste Link command in a container application Activating an item that is embedded in a document and ‘‘editing’’ it Having one container or server application drive another application
one-armed router Concept
What it is
Compound files (structured storage)
A standard file format that simplifies the storing of (com pound) documents; consists of storages (similar to directories) and streams (similar to files) A single data transfer interface that accommodates drag-and-drop; clipboard; and dynamic data exchange (DDE) Provides the underlying support for OLE items (objects) and ActiveX controls to communicate with other OLE objects or ActiveX controls An item (object) that can be distributed and run on top of a COM A set of ready-made classes or templates that can be used to build container and server applications
Uniform Data Transfer (UDT)
Component Object Model (COM)
ActiveX control Microsoft Foundation Class (MCF) library
OLE contains about 660 new function calls or individual program interfaces in addition to those already in Win32. For this reason, Microsoft provides the Microsoft Foundation Class (MFC) Library, a set of ready-made classes that can be used to build container and server applications, and tools such as Visual C++. In the ‘‘Introduction to OLE’’ on its Developer Site, Microsoft says that ‘‘OLE’’ no longer stands for ‘‘Object Linking and Embedding,’’ but just for the letters ‘‘OLE.’’
OLE DB OLE DB is Microsoft’s strategic low-level application program interface (API) for access to different data sources. OLE DB includes not only the Structured Query Language (SQL) capabilities of the Microsoft-sponsored standard data interface Open Database Connectivity (ODBC) but also includes access to data other than SQL data. As a design from Microsoft’s Component Object Model (COM), OLE DB is a set of methods (in earlier days, these might have been called routines) for reading and writing data. The objects in OLE DB consist mainly of a data source object, a session object, a command object, and a rowset object. An application using OLE DB would use this request sequence: 1. Initialize OLE. 2. Connect to a data source. 3. Issue a command. 4. Process the results. 5. Release the data source object and uninitialize OLE. OLE once stood for ‘‘Object Link Embedding’’ and ‘‘DB’’ for database. However, Microsoft no longer ascribes these meanings to the letters ‘‘OLE’’ and ‘‘DB.’’
OLTP OLTP (online transaction processing) is a class of program that facilitates and manages transaction-oriented applications, typically for data entry and retrieval transactions in a number of industries, including banking, airlines, mailorder, supermarkets, and manufacturers. Probably the most widely installed OLTP product is IBM’s CICS (Customer Information Control System). Today’s online transaction processing increasingly requires support for transactions that span a network and may include more than one company. For this reason, new OLTP software uses client/server processing and brokering software that allows transactions to run on different computer platforms in a network.
OMG See ‘‘Object Management Group’’
on the fly In relation to computer technology, ‘‘on the fly’’ describes activities that develop or occur dynamically rather than as the result of something that is statically predefined. For example, the content of a page that is sent to you from a Web site can be developed (and varied) ‘‘on the fly’’ based on dynamic factors such as the time of day, what pages the user has looked at previously, and specific user input. The Web server calls an application program to produce the ‘‘on-thefly’’ page that is to be returned. There are several techniques for on-the-fly page development, including the server-side include, the use of cookies (information previously stored about you that is located in a special file on your hard disk), and Microsoft’s Active Server Page. In general usage, the expression often connotes a degree of haste and improvisation as in ‘‘I usually grab breakfast on the fly.’’ This usage is somewhat similar to the expression ‘‘catch as catch can.’’ The term also can simply connote being in a mobile or fluid situation. Our Webster’s New World Dictionary reminds us that the term probably originated with bird hunting and shooting birds on the fly (rather than on the ground). The dictionary says simply ‘‘in flight’’ and adds a colloquial meaning of ‘‘in a hurry.’’
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one-armed router A one-armed router is a router that routes traffic between virtual local area networks (VLANs). A one-armed router operates on the 80/20 rule, which states that 80% of traffic in a network remains within a virtual local area network and doesn’t need routing service. The other 20% of network traffic is between VLANs and goes through the one-armed router. Because the one-armed router takes care of the more intensive traffic between VLANs, it frees the primary data path in a network for inter-VLAN traffic.
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In order for a one-armed router to be beneficial, the VLAN must be configured to the 80/20 rule. One disadvantage of using the one-armed router structure is that it represents a single point of failure in a network. Another disadvantage is it can develop into a bottleneck if there are large amounts of traffic between VLANs.
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In cryptography, a one-time pad is a system in which a private key generated randomly is used only once to encrypt a message that is then decrypted by the receiver using a matching one-time pad and key. Messages encrypted with keys based on randomness have the advantage that there is theoretically no way to ‘‘break the code’’ by analyzing a succession of messages. Each encryption is unique and bears no relation to the next encryption so that some pattern can be detected. With a onetime pad, however, the decrypting party must have access to the same key used to encrypt the message and this raises the problem of how to get the key to the decrypting party safely or how to keep both keys secure. One-time pads have sometimes been used when the both parties started out at the same physical location and then separated, each with knowledge of the keys in the one-time pad. The key used in a one-time pad is called a secret key because if it is revealed, the messages encrypted with it can easily be deciphered. One-time pads figured prominently in secret message transmission and espionage before and during World War II and in the Cold War era. On the Internet, the difficulty of securely controlling secret keys led to the invention of public key cryptography.
How It Works
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Typically, a one-time pad is created by generating a string of characters or numbers that will be at least as long as the longest message that may be sent. This string of values is generated in some random fashion—for example, by someone pulling numbered balls out of a lottery machine or by using a computer program with a random number generator. The values are written down on a pad (or any device that someone can read or use). The pads are given to anyone who may be likely to send or receive a message. Typically, a pad may be issued as a collection of keys, one for each day in a month, for example, with one key expiring at the end of each day or as soon as it has been used once. When a message is to be sent, the sender uses the secret key to encrypt each character, one at a time. If a computer is used, each bit in the character (which is usually eight bits in length) is exclusively ‘‘OR’ed’’ with the corresponding bit in the secret key. (With a one-time pad, the encryption algorithm is simply the XOR operation. Where there is some concern about how truly random the key is, it is sometimes combined with another algorithm such as MD5.) One writer describes this kind of encryption as a ‘‘100% noise source’’ used to mask the message. Only the sender and receiver
have the means to remove the noise. Once the one-time pad is used, it can’t be reused. If it is reused, someone who intercepts multiple messages can begin to compare them for similar coding for words that may possibly occur in both messages.
one-way pager See ‘‘pager’’
online Online is the condition of being connected to a computer or a telecommunications system. The term is frequently used to describe someone who is currently connected to the Internet. Deprecated spellings are on-line and on line. The opposite term is offline.
online analytical processing Online analytical processing (OLAP) enables a user to easily and selectively extract and view data from different pointsof-view. For example, a user can request that data be analyzed to display a spreadsheet showing all of a company’s beach ball products sold in Florida in the month of July, compare revenue figures with those for the same products in September, and then see a comparison of other product sales in Florida in the same time period. To facilitate this kind of analysis, OLAP data is stored in a multidimensional database. Whereas a relational database can be thought of as two-dimensional, a multidimensional database considers each data attribute (such as product, geographic sales region, and time period) as a separate ‘‘dimension.’’ OLAP software can locate the intersection of dimensions (all products sold in the Eastern region above a certain price during a certain time period) and display them. Attributes such as time periods can be broken down into subattributes. OLAP can be used for data mining or the discovery of previously undiscerned relationships between data items. An OLAP database does not need to be as large as a data warehouse, since not all transactional data is needed for trend analysis. Using Open Database Connectivity (ODBC), data can be imported from existing relational databases to create a multidimensional database for OLAP. Two leading OLAP products are Arbor Software’s Essbase and Oracle’s Express Server. OLAP products are typically designed for multiple-user environments, with the cost of the software based on the number of users.
Online Public Access Catalog See ‘‘OPAC’’
Open Buying on the Internet
online service provider
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1) On the Internet, OSP (online service provider) has several different meanings.
See ‘‘object-oriented programming’’
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The term has had some currency in distinguishing Internet access providers that have their own online independent content, such as America Online (AOL), from Internet service providers (ISPs) that simply connect the user directly with the Internet. In general, the companies sometimes identified as OSPs (in this usage) offer an extensive online array of services of their own apart from the rest of the Internet and sometimes their own version of a Web browser. Connecting to the Internet through an OSP is an alternative to connecting through one of the national Internet service providers, such as AT&T or MCI, or a regional or local ISP. Some Internet service providers (ISPs) describe themselves as online service providers. In this usage, ISP and OSP are synonyms. America Online has used the term to refer to online content providers (usually Web sites) with which AOL has a business agreement.
2) Some companies use OSP in describing themselves as office service providers.
online transaction processing
An OPAC (Online Public Access Catalog) is an online bibliography of a library collection that is available to the public. OPACs developed as stand-alone online catalogs, often from VT100 terminals to a mainframe library catalog. With the arrival of the Internet, most libraries have made their OPAC accessible from a server to users all over the world. User searches of an OPAC make use of the Z39.50 protocol. This protocol can also be used to link disparate OPCS into a single ‘‘union’’ OPAC.
open 1) In information technology, a product or system is described as open when its workings are exposed to the public and capable of being modified or improved by anyone. The alternative is a proprietary product or system. Also see open source. 2) In programming, to open (verb) a file or unit of data is to make it accessible for reading and writing. An open file is one that is ready to be read or written to.
online voting
OBI (Open Buying on the Internet) is a proposed standard for business-to-business purchasing on the Internet, aimed particularly at high-volume, low-cost-per-item transactions. OBI uses a number of security technologies such as the digital certificate to allow orders to be placed and filled securely. On average, 80% of company purchasing is for non-production supplies such as office supplies, cleaning products, and computer equipment. Making and fulfilling an order can cost an organization up to $150 and the seller up to $50. In the fall of 1996, the Internet Purchasing Roundtable, a group of Fortune 500 buying organizations and their suppliers met to develop an open standard for business-to-business e-commerce. The result—OBI—is intended to eliminate redundant work in purchasing, minimize errors, and reduce labor and transaction costs.
In its general meaning, ontology (pronounced ahn-TAH-luhdjee) is the study or concern about what kinds of things exist—what entities there are in the universe. It derives from the Greek onto (being) and logia (written or spoken discourse). It is a branch of metaphysics, the study of first principles or the essence of things. In information technology, an ontology is the working model of entities and interactions in some particular domain of knowledge or practices, such as electronic commerce or ‘‘the activity of planning.’’ In artificial intelligence (AI), an ontology is, according to Tom Gruber, an AI specialist at Stanford University, ‘‘the specification of conceptualizations, used to help programs and humans share knowledge.’’ In this usage, an ontology is a set of concepts—such as things, events, and relations—that are specified in some way (such as specific natural language) in order to create an agreedupon vocabulary for exchanging information.
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There are four entities involved in an OBI transaction: The requisitioner, the buying organization, the selling organization, and the payment authority. The requisitioner is the person who places the order and must have a digital certificate for authentication. The buying organization maintains an OBI server that receives OBI order requests and approves them. The buying organization also negotiates and maintains contracts with selling organizations. It is the responsibility of the selling organization to provide a catalog tailored to each department in each company, maintain
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products and prices based on contracts with the buying organization, and authorize payments with the appropriate payment authority. An OBI transaction is done without the user needing special knowledge of the OBI process. With OBI, a purchaser needs only a computer, an Internet connection, a Web browser, and an OBI-compliant application. The following is a typical OBI scenario: L
A janitor needs to purchase some cleaning supplies. Using the stockroom computer, the janitor connects to the organization’s purchasing server using a Web browser.
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The janitor selects a merchant from a list on the purchasing department’s home page and is authenticated by the merchant’s server.
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The janitor views a catalog that is tailored to company needs and selects the needed cleaning supplies. The janitor clicks a button to submit the order, which is then formatted into the Electronic Data Interchange (EDI) format. The order is then encapsulated into an OBI object.
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The janitor’s order is sent to the buying organization in the company for approval using Hypertext Transfer Protocol (HTTP) and Secure Sockets Layer (SSL). The buying organization decodes the OBI object and approves the order.
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The order is sent to the merchant and fulfilled.
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The merchant submits the payment to a payment authority.
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The payment authority sends the billing data to the company’s billing department.
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OBI architecture, technical specifications, guidelines, and compliance and implementation information is available at the OBI Consortium Web site. Future OBI versions will include international support and the use of Extensible Markup Language (XML) data formats.
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See ‘‘OpenCourseWare’’
Open Database Connectivity Open Database Connectivity (ODBC) is an open standard application programming interface (API) for accessing a database. By using ODBC statements in a program, you can access files in a number of different databases, including Access, dBase, DB2, Excel, and Text. In addition to the ODBC software, a separate module or driver is needed for each database to be accessed. The main proponent and supplier of ODBC programming support is Microsoft. ODBC is based on and closely aligned with The Open Group standard Structured Query Language (SQL) CallLevel Interface. It allows programs to use SQL requests that
will access databases without having to know the proprietary interfaces to the databases. ODBC handles the SQL request and converts it into a request the individual database system understands. ODBC was created by the SQL Access Group and first released in September, 1992. Although Microsoft Windows was the first to provide an ODBC product, versions now exist for UNIX, OS/2, and Macintosh platforms as well. In the newer distributed object architecture called Common Object Request Broker Architecture (CORBA), the Persistent Object Service (POS) is a superset of both the Call-Level Interface and ODBC. When writing programs in the Java language and using the Java Database Connectivity (JDBC) application program interface, you can use a product that includes a JDBC-ODBC ‘‘bridge’’ program to reach ODBCaccessible databases.
Open Data-Link Interface ODI (Open Data-Link Interface) is a software interface that allows different Data-Link Layer protocols to share the same driver or adapter in a computer. ODI was introduced by Novell. For example, using ODI, both TCP/IP and IPX/SPX can share the same device adapter. The Data-Link Layer, part of the Open Systems Interconnect (OSI) model, provides a way to move data across a physical link.
Open Document Management API See ‘‘ODMA’’
Open eBook Forum The Open Electronic Book Forum (OEBF) is an organization whose purpose is to develop a specification for electronic content, based on existing HTML and XML standards, that allows electronic book content to be viewed on various devices (PC display, PDA, or eBook reader) and all platforms. The OEB Forum brings publishers, authors, agents, distributors, hardware and software vendors, and programmers together to work towards creating this common standard in ‘‘dual-stream publishing,’’ the simultaneous production of both electronic and print media. Goals for the OEB Forum include: L
Providing a forum for the discussion of issues and technologies related to electronic books
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Developing, publishing, and maintaining common specifications relating to electronic books and promoting the successful adoption of these specifications
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Promoting industry-wide participation of electronic publishing through training sessions, guidelines, and demonstrations of proven technology
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Identifying, evaluating and recommending standards created by other bodies related to electronic books
Open Shortest Path First protocol
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Encouraging interoperable implementations of electronic book related systems and providing a forum for resolution of interoperability issues
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Accommodating differences in language, culture, reading and learning styles, and individual abilities
Open Profiling Standard Open Profiling Standard (OPS) is a proposed standard for how Web users can control the personal information they share with Web sites. OPS has a dual purpose: (1) to allow Web sites to personalize their pages for the individual user and (2) to allow users to control how much personal information they want to share with a Web site. OPS was proposed to the Platform for Privacy Preferences Project (P3P) of the World Wide Web Consortium (W3C) in 1997 by Netscape Communications (now part of America Online), Firefly Network, and VeriSign.
How It Works 1. A Web user uses special software (or it could be combined with a Web browser) to create a Personal Profile that is stored in the user’s computer. (If desired, the Profile could also be placed in a corporate or global directory.) 2. When a Web user visits a Web site for the first time, the site could ask the user for information from the Personal Profile and the user could decide whether and how much information to give the site. 3. The site would store the information on its site. When the visitor returned, the Web site, after identifying the user, could use the previously-stored Profile to personalize the pages for that user (for example, provide occupation or hobby-related information on certain pages).
What the Personal Profile Contains One could think of the Personal Profile as an elaborate, userdefined cookie. Cookies are files that Web sites currently place on each user’s own hard disk so that they can recall some information about the user. This information is very limited and the user can control only whether cookies are permitted or not. A Personal Profile gives the user much finer control of personal information. In general, a Personal Profile would contain: L
A unique identifier for the Profile itself
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A unique identifier for each Web site that is visited (used to control how much of the Profile the site can access)
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Basic demographic data (country, zip code, age, and gender)
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Contact information (name, address, zip or postal code, telephone number, e-mail address, and so forth). This is based on the vCard specification.
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Additionally, one or more sections for e-commerce information, such as credit card numbers
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Detailed personal preferences (hobbies, favorite activities, favorite magazines, and so forth)
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The P3P Recommendations provide a formal way to implement the Personal Profile that uses the Resource Definition Framework (RDF) of the W3C.
Open Service Gateway Initiative OSGi (Open Service Gateway Initiative) is an industry plan for a standard way to connect devices such as home appliances and security systems to the Internet. With such a standard, home users could, for example, install a security system and be able to change from one monitoring service to another without having to install a new system of wires and devices. The ‘‘service gateway’’ would be an application server in a computer that was a gateway between the Internet and a home or small business’s network of device. The OSGi plans to specify the application program interface (API) for programmers to use to allow communication and control between service providers and the devices within the home or small business network. OSGi’s API will be built on the Java programming language. Java programs can generally be run on any computer operating system platform. OSGi is an open standard programming interface. Changes will evolve through the ‘‘Java Community Process.’’ OSGi is intended to connect new Jini ‘‘smart appliances,’’ Bluetooth wireless device groups, as well as TV set-top boxes, cable modems, alarm systems, energy management systems, and other devices to Internet sites that can be used to manage them remotely and interactively. The service gateway (SG) is intended to manage this interconnection with ‘‘zero administration.’’ Among some popular device-to-Internet applications are expected to be energy measurement and load management in the home; home security systems that a home owner can monitor and control away from home; continous monitoring of critical care and home-care patients; and predictive failure reporting for home appliances. The OSGi specification will be designed to complement existing residential standards, such as those of LonWorks (see control network), CAL, CEBus, HAVi, and others. The initial group of companies that formed the initiative were: Alcatel, Cable Wireless, Electricite de France, Enron Communications, Ericcson, IBM, Lucent Technologies, Motorola, NCI, Nortel Networks, Oracle, Philips Electronics, Sun Microsystems, Sybase, and Toshiba.
Open Shortest Path First protocol
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Open Source Development Labs (OSDL) is a nonprofit corporation founded by IBM, Intel, and Computer Associates to support Linux developers and users. The goal of OSDL is to provide a place where Linux and other Open Source developers can work together and create standardized and compatible Linux platform applications. Plans for a state-of-the-art laboratory based somewhere in Oregon were announced in August 2000 by Scott McNeil, who is widely credited with the concept for OSDL.
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open source 1) In general, open source refers to any program whose source code is made available for use or modification as users or other developers see fit. (Historically, the makers of proprietary software have generally not made source code available.) Open source software is usually developed as a public collaboration and made freely available. 2) Open Source is a certification mark owned by the Open Source Initiative (OSI). Developers of software that is intended to be freely shared and possibly improved and redistributed by others can use the Open Source trademark if their distribution terms conform to the OSI’s Open Source Definition. To summarize, the Definition model of distribution terms require that: L
The software being distributed must be redistributed to anyone else without any restriction
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The source code must be made available (so that the receiving party will be able to improve or modify it)
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The idea is very similar to the copyleft concept of the Free Software Foundation. Open Source is the result of a longtime movement toward software that is developed and improved by a group of volunteers cooperating together on a network. Many parts of the UNIX operating system were developed this way, including today’s most popular version, Linux. Linux uses applications from the GNU project, which was guided by Richard Stallman and the Free Software Foundation. The Open Source Definition, spearheaded by Eric Raymond (editor of The New Hacker’s Dictionary), is an effort to provide a branded model or guideline for this kind of software distribution and redistribution. The OSI considers the existing software distribution licenses used by GNU, BSD (a widely-distributed version of UNIX), X Window System, and Artistic to be conformant with the Open Source Definition. Prior to its acquisition by AOL, Netscape, in an effort to stay viable in its browser competition with Microsoft, made its browser source code (codenamed Mozilla) freely available, encouraging programmers to improve it. Possible enhancements will presumably be incorporated into future
versions. The open source movement has gained momentum as commercial enterprises have begun to consider Linux as an open alternative to Windows operating systems.
Open Systems Interconnection See ‘‘OSI’’
OpenCourseWare OpenCourseWare (OCW) is an educational initiative developed by the Massachusetts Institute of Technology (MIT) to make the core teaching materials for all MIT graduate and undergraduate classes available at no cost to Internet users around the world. OCW has been compared to the open source software movement because course materials on the OCW site will be ‘‘open and freely available worldwide for non-commerical purposes such as research and education, providing an extraordinary resource, free of charge, which others can adapt to their own needs.’’ MIT President, Charles M. Vest, anticipates that within ten years, lecture notes, course outlines, reading lists and assignments for over 2000 MIT classes will be freely available on the OCW Web site. President Vest has said that although OCW appears counterintuitive in a market-driven world, it is particularly appropriate for a research university such as MIT where ideas move quickly from the laboratory to the classroom before there is even time to publish it in textbooks. OCW is not seen as a substitute for revenue-generating distance education (which requires interaction between teacher and student) but rather as a Web-based resource for teachers and learners around the globe. MIT anticipates that OCW will initially cost between $7.5 million and $10 million per year and is actively seeking funding partners. MIT is known for its innovation in collaborative and distance learning projects, including long-distance education and collaborative research programs with the National University University of Singapore and Cambridge University in England. MIT faculty members will retain intellectual property ownership of most materials posted on the OCW site, following MIT’s current policy on textbook authorship. MIT faculty members have expressed the hope that OCW will encourage other universities to follow their initiative and join them in this ‘‘unprecedented step in challenging the privitization of knowledge.’’
OpenGL OpenGL (Open Graphics Library) is the computer industry’s standard application program interface (API) for defining 2-D and 3-D graphic images. Prior to OpenGL, any company developing a graphical application typically had to rewrite the graphics part of it for each operating system platform and had to be cognizant of the graphics hardware as well.
Opera With OpenGL, an application can create the same effects in any operating system using any OpenGL-adhering graphics adapter. OpenGL specifies a set of ‘‘commands’’ or immediately executed functions. Each command directs a drawing action or causes special effects. A list of these commands can be created for repetitive effects. OpenGL is independent of the windowing characteristics of each operating system, but provides special ‘‘glue’’ routines for each operating system that enable OpenGL to work in that system’s windowing environment. OpenGL comes with a large number of built-in capabilities requestable through the API. These include hidden surface removal, alpha blending (transparency), antialiasing, texture mapping, pixel operations, viewing and modeling transformations, and atmospheric effects (fog, smoke, and haze). Silicon Graphics, makers of advanced graphics workstations, initiated the development of OpenGL. Other companies on the industry-wide Architecture Review Board include DEC, Intel, IBM, Microsoft, and Sun Microsystems. There is no cost (other than learning) to developing an application using the OpenGL API. Microsoft offers free downloads of the OpenGL libraries for its Windows systems. Although OpenGL is not itself a development ‘‘toolkit,’’ such toolkits are available, including Silicon Graphics object-oriented programming 3D graphics toolkit, Open Inventor.
OpenType OpenType is a file format for scalable (outline) font files that extends the existing TrueType font file format used by Microsoft Windows and Apple Macintosh operating systems. OpenType was developed jointly by Microsoft and Adobe and allows an Adobe PostScript file to be part of a TrueType font file. Prior to OpenType, Adobe did not support TrueType fonts as well it did its own font format, Type 1, for printers that use PostScript. PostScript is an industry standard printer formatting language for higherquality and more sophisticated printers. OpenType is also known as TrueType Open v. 2.0. The main advantages of OpenType are: L
Improved cross-platform support
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The OpenType specification includes a convention for assigning suffixes (file name extensions) to OpenType file names. An OpenType file contains either a TrueType outline font file, which requires a suffix of TTF, or a PostScript outline font file, with a suffix of OTF. A collection of TrueType files packaged together has the suffix TTC.
OpenView See ‘‘HP OpenView’’
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OpenVMS OpenVMS is an operating system from the Digital Equipment Corporation (DEC) that runs in both its VAX and Alpha computers. OpenVMS evolved from VMS, which originated as the operating system for the VAX in 1979. VMS exploited the concept of virtual memory. DEC is now part of Compaq. OpenVMS is a multitasking and multiprocessing operating system based on VMS; it was renamed OpenVMS when it was redeveloped for the Alpha processor. (OpenVMS is also the name now used on the VAX computer.) The ‘‘Open’’ suggests the added support for the UNIX-like interfaces of the POSIX standard. Programs written to the POSIX standard, which includes a set of standard C language programming functions, can be ported to any POSIXsupporting computer platform. Formerly a 32-bit operating system, more recent versions of OpenVMS support 64-bit instructions. Among other features, OpenVMS can be used with special software that facilitates its use with Windows servers.
Opera Opera is a Web browser that provides some advantages over the two most popular browsers from Netscape and Microsoft. Much smaller in size, Opera is known for being fast and stable. Opera, which is available for BeOS, EPOC, Linux, Mac, OS/2 and Windows, offers the same capabilities of the more popular browsers including integrated searches and Instant Messaging, support for JavaScript, cascading style sheets, and mail. Because Opera is so compact, it is being promoted as the browser of choice for hand-held Internet devices. Opera for Windows is now free; there is still a purchase fee for other platforms. The free version of Opera contains ads, which are cached weekly to insure Opera’s fast speed is maintained. The other versions of Opera do not have ads, which is why Opera charges a modest one-time license fee. The feature you notice first after installing Opera is a menu or ‘‘hotlist’’ that serves as both a directory to the Web and a bookmark file. The hotlist can be easily removed and you can use the full viewing space to look at multiple Web sites at the same time, either tiling or cascading the windows. You can choose to have the sites you were last looking at restored the next time you open the Opera browser. Opera offers keyboard as well as mouse control of its features. Plug-ins such as RealAudio, RealVideo, and Shockwave can be added. Opera does not support Active-X or Visual Basic. Opera began in 1994 as a research project for the national phone company in Norway and is now considered to be the third most popular Web browser in use today.
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operand
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1) In computers, an operand is the part of a computer instruction that specifies data that is to be operating on or manipulated and, by extension, the data itself. Basically, a computer instruction describes an operation (add, subtract, and so forth) and the operand or operands on which the operation is to be performed.
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2) In mathematics, an operand is the object of a mathematical operation.
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operating system
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An operating system (sometimes abbreviated as ‘‘OS’’) is the program that, after being initially loaded into the computer by a boot program, manages all the other programs in a computer. The other programs are called applications or application programs. The application programs make use of the operating system by making requests for services through a defined application program interface (API). In addition, users can interact directly with the operating system through a user interface such as a command language or a graphical user interface (GUI).
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In a multitasking operating system where multiple programs can be running at the same time, the operating system determines which applications should run in what order and how much time should be allowed for each application before giving another application a turn. It manages the sharing of internal memory among multiple applications.
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It handles input and output to and from attached hardware devices, such as hard disks, printers, and dialup ports.
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It sends messages to each application or interactive user (or to a system operator) about the status of operation and any errors that may have occurred.
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It can offload the management of what are called batch jobs (for example, printing) so that the initiating application is freed from this work.
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On computers that can provide parallel processing, an operating system can manage how to divide the program so that it runs on more than one processor at a time.
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All major computer platforms (hardware and software) require and sometimes include an operating system. Linux, Windows 2000, VMS, OS/400, AIX, and z/OS are all examples of operating systems.
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An Operational Support System (OSS) is an application program that helps someone monitor, control, analyze, and manage problems with a telephone or computer network.
optical amplifier See ‘‘erbium amplifier’’
Optical Carrier levels (OCx) The Synchronous Optical Network (SONET) includes a set of signal rate multiples for transmitting digital signals on optical fiber. The base rate (OC-1) is 51.84 Mbps. Certain multiples of the base rate are provided as shown in the following table. Asynchronous transfer mode (ATM) makes use of some of the Optical Carrier levels. Optical Carrier Level
Data Rate
OC-1 OC-3 OC-12 OC-24 OC-48 OC-192 OC-256 OC-768
51.84 Mbps 155.52 Mbps 622.08 Mbps 1.244 Gbps 2.488 Gbps 10 Gbps 13.271 Gbps 40 Gbps
optical character recognition See ‘‘OCR’’
optical computer An optical computer is a hypothetical device that uses visible light or infrared (IR) beams, rather than electric current, to perform digital computations. An electric current flows at only about 10 percent of the speed of light. This limits the rate at which data can be exchanged over long distances, and is one of the factors that led to the evolution of optical fiber. By applying some of the advantages of visible and/or IR networks at the device and component scale, a computer might someday be developed that can perform operations 10 or more times faster than a conventional electronic computer. Visible-light and IR beams, unlike electric currents, pass through each other without interacting. Several (or many) laser beams can be shone so their paths intersect, but there is no interference among the beams, even when they are confined essentially to two dimensions. Electric currents must be guided around each other, and this makes threedimensional wiring necessary. Thus, an optical computer, besides being much faster than an electronic one, might also be smaller. Some engineers think optical computing will someday be common, but most agree that transitions will occur in specialized areas one at a time. Some optical integrated circuits have been designed and manufactured. Three-
optical mouse dimensional, full-motion video can be transmitted along a bundle of fibers by breaking the image into voxels (see voxel). Some optical devices can be controlled by electronic currents, even though the impulses carrying the data are visible light or IR.
because the installation of any new wiring is labor-intensive, few communities yet have optical fiber wires or cables from the phone company’s branch office to local customers (known as local loop).
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Single mode fiber is used for longer distances; multimode fiber fiber is used for shorter distances.
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optical media
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Optical media—such as the compact disc (CD)—are storage media that hold content in digital form and that are written and read by a laser; these media include all the various CD and DVD variations, as well as optical jukeboxes and autochangers. Optical media have a number of advantages over magnetic media such as the floppy disk. Optical disc capacity ranges up to 6 gigabytes; that’s 6 billion bytes compared to the 1.44 megabytes (MB)—1,440,000 bytes—of the floppy. One optical disc holds about the equivalent of 500 floppies worth of data. Durability is another feature of optical media; they last up to seven times as long as traditional storage media.
Electrical crossovers (top) require three dimensions, but optical crossovers (bottom require only two dimensions because light beams do not interact
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The Optical Storage Technology Association (OSTA) is an international trade organization dedicated to the promotion of standardized writable optical technologies and related products. Incorporated in 1992, OSTA is made up of members and associates from the leading optical media manufacturers and resellers of North America, Europe, and Asia. OSTA members include Adaptec, Hewlett-Packard, Philips, and Sony.
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optical mouse Optical technology has made its most significant inroads in digital communications, where fiber optic data transmission has become commonplace. The ultimate goal is the so-called photonic network, which uses visible and IR energy exclusively between each source and destination. Optical technology is employed in CD-ROM drives and their relatives, laser printers, and most photocopiers and scanners. However, none of these devices are fully optical; all rely to some extent on conventional electronic circuits and components.
optical fiber Optical fiber (or ‘‘fiber optic’’) refers to the medium and the technology associated with the transmission of information as light pulses along a glass or plastic wire or fiber. Optical fiber carries much more information than conventional copper wire and is in general not subject to electromagnetic interference and the need to retransmit signals. Most telephone company long-distance lines are now of optical fiber. Transmission on optical fiber wire requires repeaters at distance intervals. The glass fiber requires more protection within an outer cable than copper. For these reasons and
An optical mouse is an advanced computer pointing device that uses a light-emitting diode (LED), an optical sensor, and digital signal processing (DSP) in place of the traditional mouse ball and electromechanical transducer. Movement is detected by sensing changes in reflected light, rather than by interpreting the motion of a rolling sphere. The optical mouse takes microscopic snapshots of the working surface at a rate of more than 1,000 images per second. If the mouse is moved, the image changes. The tiniest irregularities in the surface can produce images good enough for the sensor and DSP to generate usable movement data. The best surfaces reflect but scatter light; an example is a blank sheet of white drawing paper. Some surfaces do not allow the sensor and DSP to function properly because the irregularities are too small to be detected. An example of a poor optical-mousing surface is unfrosted glass. In practice, an optical mouse does not need cleaning, because it has no moving parts. This all-electronic feature also eliminates mechanical fatigue and failure. If the device is used with the proper surface, sensing is more precise than is possible with any pointing device using the old electromechanical design. This is an asset in graphics applications, and it makes computer operation easier in general.
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Opt-in e-mail is a Web marketing term for promotional email that recipients have previously requested by signing up at a Web site or special ad banner. Typically, Web users are invited to sign up for promotional information about one or more categories of products or services. Those who sign up have thus ‘‘opted in.’’ Anyone sending them e-mail as a result hopes that the message will not be perceived as unwanted spam.
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Several companies gather sign-ups at their own site or through specially-designed banner ads and then sell marketers mailing lists of those who have signed up in various interest categories. The marketer sending opt-in email may remind the recipient that they have previously indicated they were interested in receiving such e-mail and that this is not spam. The recipient is given an opportunity to be removed from the mailing list if they so choose. The distribution model of sending unsolicited e-mail (spam) and allowing the recipient to request removal is sometimes referred to as ‘‘opt-out.‘‘ Opt-in e-mail has been endorsed as the best practice for marketers by the Internet Direct Marketing Bureau (IDMB).
optoelectronics Optoelectronics is a branch of electronics that overlaps with physics. The field concerns the theory, design, manufacture, and operation of hardware that converts electrical signals to visible or infrared radiation (infrared) energy, or vice-versa. Examples of optoelectronic components include photocells, solar cells, optoisolators (also called optical couplers or optocouplers), LEDs (light-emitting diodes), and laser diodes. Applications include electric eyes, photovoltaic power supplies, various monitoring and control circuits, and optical fiber communications systems.
optoisolator Also see diode. An optoisolator, also known as an optical coupler or optocoupler, is a semiconductor device that allows signals to be transferred between circuits or systems, while keeping those circuits or systems electrically isolated from each other. Optoisolators are used in a wide variety of communications, control, and monitoring systems. In its simplest form, an optoisolator consists of an lightemitting diode (LED), IRED (infrared-emitting diode), or laser diode for signal transmission, and a photosensor for signal reception. The ‘‘transmitter’’ takes the electrical signal and converts it into a a beam of modulated visible light or infrared (IR). This beam travels across a transparent gap and is picked up by the ‘‘receiver,’’ which converts the modulated light or IR back into an electrical signal. The electrical output waveform is identical to the electrical input waveform, although the input and output amplitudes
(signal strengths) often differ. The optoisolator is enclosed in a single package, and has the appearance of an integrated circuit (IC) or a transistor with extra leads. “Receiver” Photosensor
Visible In
Out or IR beam
“Transmitter” LED or IRED
A common application of an optoisolator is in a specialized modem that allows a computer to be connected to a telephone line without risk of damage from electrical transients (also called ‘‘surges’’ or ‘‘spikes’’). Two optoisolators are employed in the analog section of the device: one for upstream signals and the other for downstream signals. If a transient occurs on the telephone line (and these are common), the computer will be unaffected because the optical gap does not conduct electric current. For this reason, modems that use optoisolators provide superior protection against transients compared with modems incorporating electrical surge suppressors alone. (An electrical surge suppressor should be installed between the optoisolator and the telephone line for optimum protection.)
OR See ‘‘logic gate’’
Orange Book Orange Book is the informal name for Philips and Sony’s Recordable CD Standard. Published in 1990, the Orange Book is a follow-up to their Red Book CD-DA (Compact Disc—Digital Audio) specifications. The Orange Book is divided into two sections: Part I deals with magneto-optical (MO) drives, and Part II deals with the first recordable CD format CD-R (Compact Disc—Recordable). Part III, released separately, describes CD-RW (Compact Disc—Rewritable). In addition to specifying these CD forms, the Orange Book includes information about data organization, multisession and hybrid discs, pre-groove modulation (for motor control during writing), and recommendations for measuring reflectivity, environment, and light speed. Orange Book specifications enabled the first desktop disc writing. Formerly, CDs had been read-only music (CD-DA), to be played in CD players, and multimedia (CD-ROM), to
ordinal be played in computers. After the Orange Book, any user with a CD Recorder drive could create their own CDs from their desktop computers. Magneto-optical (CD-MO) technology allows tracks to be erased and rewritten on 12cm CDs that are rated to allow millions of rewrites. These drives use two heads (one to write and the other to erase), in a double-pass process. System information may be permanently written in a small, premastered area, but the rest of the area is available for recording, and re-recording many times. CD-R products can be written to only once, similarly to WORM (write once, read many) products. A CD-R drive records on CDs that have special recording layers and pregrooved tracks. The first tracks are a program calibration area, which is followed by the lead-in area (where the table of contents will be written), and the program area (where the user actually records), and a lead-out area. There are hybrid discs that include read-only and recordable areas. Rewritable CD (CD-RW) was developed by Philips and Sony in 1996, as an extension to the original Orange Book. This addition specifies the use of Phase Change technology and the Universal Disc Format (UDF) to produce a CD that can be rewritten in one pass. CD-RW makes it possible for the user to write and rewrite the disc.
ORB See ‘‘Object Request Broker’’
ORBS A similar but unrelated term is ORB (Object Request Broker). ORBS (Open Relay Behavior-modification System) is (or possibly was—as of June 7, 2001, it was not operating) a volunteer-run New Zealand-based organization that operates an anti-spam screening service. The ORBS database tracks e-mail (specifically SMTP) servers that allow third-party relay (TPR), a practice that makes it possible for any sender to connect to the server from anywhere and forward volumes of unsolicited bulk e-mail messages. As a further precaution, ORBS also tracks networks that have set up processes to prevent verification of third-party relay permission, since administrators sometimes find it easier to block ORBS testers than to address security problems. In the first years of the Internet’s operation, third-party relaying was a necessary and accepted means of routing messages. Although technological advances have made third-party relaying no longer required, many servers continue to maintain open relays, according to ORBS, in the ‘‘Internet’s spirit of cooperation.’’ According to ORBS, however, an open relay now falls into the category of ‘‘attractive nuisance.’’ The organization claims that since 1995, the culture of the Web has changed dramatically, with the result that open relays became vulnerable to spammers looking to make a quick profit through bulk junk mail.
ORBS maintains a blacklist of Internet service providers (ISPs) and other organizations found in violation of their criteria, a practice that is somewhat controversial because the targeted enterprises often believe they have been listed unfairly. In one recent instance, a New Zealand high court ruled that ORBS must remove Xtra mail servers (owned by Actrix, an New Zealand-based ISP) from their list of suspect servers. ORBS is in occasionally acrimonious competition with a similar system based in California, the Mail Abuse Prevention System (MAPS). The two organizations clashed when ORBS blacklisted Above.net (an ISP owned by Paul Vixie, who also runs MAPS) as an open relay.
order of magnitude An order of magnitude is an exponential change of plus-orminus 1 in the value of a quantity or unit. The term is generally used in conjunction with power-of-10 scientific notation. In base 10, the most common numeration scheme worldwide, an increase of one order of magnitude is the same as multiplying a quantity by 10. An increase of two orders of magnitude is the equivalent of multiplying by 100, or 102. In general, an increase of n orders of magnitude is the equivalent of multiplying a quantity by 10n. Thus, 2315 is one order of magnitude larger than 231.5, which in turn is is one order of magnitude larger than 23.15. As values get smaller, a decrease of one order of magnitude is the same as multiplying a quantity by 0.1. A decrease of two orders of magnitude is the equivalent of multiplying by 0.01, or 10-2. In general, a decrease of n orders of magnitude is the equivalent of multiplying a quantity by 10-n. Thus, 23.15 is one order of magnitude smaller than 231.5, which in turn is one order of magnitude smaller than 2315. In the International System of Units (SI), most quantities can be expressed in multiple or fractional terms according to the order of magnitude. For example, attaching the prefix ‘‘kilo-’’ to a unit increases the size of the unit by three orders of magnitude, or one thousand (103). Attaching the prefix ‘‘micro-’’ to a unit decreases the size of the unit by six orders of magnitude, the equivalent of multiplying it by one millionth (10-6). Scientists and engineers have designated prefix multipliers from septillionths (10-24) to septillions (1024), a span of 48 orders of magnitude.
ordinal Ordinal refers to the sequence in which something is in relation to others of its kind. Examples of ordinal numbers are first, third, 11th, and 123rd. Ordinal numbers can be contrasted to cardinal numbers.
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‘‘org’’ is one of the generic top-level domain names that can be used when choosing a domain name. It generally describes the entity owning the domain name as one that does not fit into other categories. In general, non-profit organizations and industry standard groups tend to use this generic name. Along with the second-level domain name (for example: ‘‘moma’’ in moma.org), the top-level domain name is required in Web and e-mail addresses.
In geometry, orthogonal means ‘‘involving right angles’’ (from Greek ortho, meaning right, and gon meaning angled). The term has been extended to general use, meaning the characteristic of being independent (relative to something else). It also can mean: non-redundant, non-overlapping, or irrelevant. In computer terminology, something—such as a programming language or a data object—is orthogonal if it can be used without consideration as to how its use will affect something else.
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Organization for Structured Information Standards See ‘‘OASIS’’
G original equipment manufacturer
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An OEM (original equipment manufacturer) is a company that uses product components from one or more other companies to build a product that it sells under its own company name and brand. (The term is sometimes mistakenly used to refer to the company that supplies the components.) IBM is an example of a supplier to the OEM market (and IBM is also an OEM itself since it uses other companies’ parts in some of its products). Many computer hardware manufacturers that have their own brand-name products derive considerable revenue by reselling the product or key parts of it to OEM companies that seem to be competing in the same market. Arguments for selling to an OEM are that you may be able to make money from a market sector that your competitor already owns (perhaps because they have an existing customer base) and that you can be a more efficient producer because you sell and manufacture more of your product. Frequently, an OEM company differentiates itself from the company it buys parts from by adding features or using different selling concepts. Many OEM companies are selling a ‘‘solution’’ tailored to a particular vertical market. Also see value-added reseller (VAR), a somewhat similar repackaging of software.
orphan file On a computer’s hard drive, an orphan file is a support file (such as a DLL file) that no longer serves a purpose because the ‘‘parent’’ application it is associated with has been moved or uninstalled. Orphan files can be deleted manually if the user is confident that the file is not being used by any other application.
In itself, a programming language is orthogonal if its features can be used without thinking about how that usage will affect other features. Pascal is sometimes considered to be an orthogonal language, while C++ is considered to be a non-orthogonal language. Features of a program that is compatible with its own earlier versions—this is called backward compatible—have an orthogonal relationship with the features of the earlier version, because they are mutually independent; you don’t have to worry about how the use of one version’s features will cause an unintended effect because of an interaction with those of the other version. Both the features and the programs can be said to be mutually orthogonal. The length of time data is kept in storage in a computer system is known as its persistence. Orthogonal persistence is the quality of a programming system that allows a programmer to treat data similarly without regard to the length of time the data is kept in storage. Data is stored for varying lengths of time; some is stored very briefly and some is stored relatively permanently. Frequently, a programmer must use different approaches and separate coding to access data depending on whether it is stored for a long time or a short time. Using a programming system with orthogonal data persistence allows the programmer to treat data the same way regardless of its persistence characteristic, saving programming time and making it easier to enforce referential integrity (a type of constraint applied to ensure correct data validity).
orthogonal frequency-division multiplexing Orthogonal frequency-division multiplexing (OFDM) is a method of digital modulation in which a signal is split into several narrowband channels at different frequencies. The technology was first conceived in the 1960s and 1970s during research into minimizing interference among channels near each other in frequency. In some respects, OFDM is similar to conventional frequency-division multiplexing (FDM). The difference lies in the way in which the signals are modulated and demodulated. Priority is given to minimizing the interference, or crosstalk, among the channels and symbols comprising the data stream. Less importance is placed on perfecting individual channels.
OS/400 OFDM is used in European digital audio broadcast services. The technology lends itself to digital television, and is being considered as a method of obtaining high-speed digital data transmission over conventional telephone lines. It is also used in wireless local area networks. Also see frequency-division multiplexing (FDM), timedivision multiplexing (TDM), and multi-carrier modulation (MCM).
OS See ‘‘operating system’’
OS 10 See ‘‘OS X’’
OS X OS X (pronounced OH-ESS-TEHN—the ‘‘X’’ is Roman number X) is version 10 of the Apple Macintosh operating system (OS). OS X is described by Apple as its first ‘‘complete revision’’ of the OS (the previous version is OS 9) and incorporates support for UNIX-based applications as well as for those written just for the Macintosh. The operating system is described as redesigned for modularity so that future changes will be easier to incorporate. A very visible difference in OS X from earlier Mac OS versions is a desktop with a 3-D appearance. OS X also includes the ability to play Quicktime movies in icon size and instant wake-from-sleep capability on portable computers. Features that Apple says will be added include support for DVD movies and the ability to create audio CDs. OS X can be installed on an existing Mac with OS 9 and the user can choose which one to use when the computer is started.
OS/2 OS/2 is an IBM operating system for the personal computer that was initially intended to provide an alternative to Microsoft Windows for both enterprise and personal PC users. The latest version of OS/2 Warp, Warp 4, has a Netscape-based Web browser that exploits OS/2’s existing speech recognition capability. IBM has also enhanced OS/2 to provide server functions for e-business.
OS/390 OS/390, recently renamed z/OS, is the IBM operating system most commonly installed on its S/390 line of mainframe server. It is an evolved version of MVS (Multiple Virtual Storage), IBM’s long-time, robust mainframe operating system. By whatever name, MVS has been said to be the operating system that keeps the world going. The payroll, accounts receivable, transaction processing,
database management, and other programs critical to the world’s largest businesses are usually run on an MVS system. Although MVS tends to be associated with a monolithic, centrally-controlled information system, IBM has in recent years repositioned it as a ‘‘large server’’ in a network-oriented distributed environment that would tend to use a 3-tier application model. Since MVS represents a certain epoch and culture in the history of computing and since many older MVS systems still operate, the term ‘‘MVS’’ will probably continue to be used for some time. Since OS/390 also comes with UNIX user and programming interfaces built in, it can be used as both an MVS system and a UNIX system at the same time. OS/390 (and earlier MVS) systems run older applications developed using COBOL and, for transaction programs, CICS. Older application programs written in PL/I and FORTRAN are still running. Older applications use the Virtual Storage Access Method for file management and Virtual Telecommunications Access Method for telecommunication with users. The most common program environment today uses the C and C++ languages. DB2 is IBM’s primary. Java applications can be developed and run under OS/390’s UNIX environment.
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For additional information about major components of OS/ 390, see MVS. Other IBM operating systems for their larger computers include or have included: the Transaction Processing Facility (TPF), used in some major airline reservation systems, and virtual machine, an operating system designed to serve many interactive users at the same time.
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OS/400 is IBM’s operating system for its AS/400 and AS/ 400e line of business computers. Because OS/400 is closely attuned to the AS/400 hardware design and generally comes as part of the basic package, there is no alternative operating system to compete with it. OS/400 is built to operate with the AS/400 logical partition (LPAR) architecture, in which multiple instances of the operating system can run concurrently in different partitions. Among other uses, LPAR is useful when migrating to a new release. The old production system can keep operating in one partition while a new system is being tested. As the AS/400 has evolved to meet the latest trends in business and information technology, OS/400 and its related software has added support for: L
Applications written in the Java programming language
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The Portable Application Solutions Environment (PASE), which supports a subset of the AIX environment so that UNIX applications can be ported and run on the AS/400
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Lotus Domino, which provides groupware and e-mail from Lotus Notes applications or a standard Web browser
oscillator An oscillator is an electronic device used for the purpose of generating a signal. Oscillators are found in computers, wireless receivers and transmitters, and audio-frequency equipment, particularly music synthesizers. There are many types of oscillator devices, but they all operate according to the same basic principle: an oscillator always employs a sensitive amplifier whose output is fed back to the input in phase. Thus, the signal regenerates and sustains itself. This is known as positive feedback. It is the same process that sometimes causes unwanted ‘‘howling’’ in public-address systems. The frequency at which an oscillator works is usually determined by a quartz crystal. When a direct current is applied to such a crystal, it vibrates at a frequency that depends on its thickness, and on the manner in which it is cut from the original mineral rock. Some oscillators employ combinations of inductors, resistors, and/or capacitors to determine the frequency. However, the best stability (constancy of frequency) is obtained in oscillators that use quartz crystals.
oscilloscopes employ computers to process and display waveforms. These computers can use any type of display, including CRT, LCD, and gas plasma. In any oscilloscope, the horizontal sweep is measured in seconds per division (s/div), milliseconds per division (ms/ div), microseconds per division (s/div), or nanoseconds per division (ns/div). The vertical deflection is measured in volts per division (V/div), millivolts per division (mV/div), or microvolts per division (mV/div). Virtually all oscilloscopes have adjustable horizontal sweep and vertical deflection settings. The illustration shows two common waveforms as they might appear when displayed on an oscilloscope screen. The signal on the top is a sine wave; the signal on the bottom is a ramp wave. It is apparent from this display that both signals have the same, or nearly the same, frequency. They also have approximately the same peak-to-peak amplitude. Suppose the horizontal sweep rate in this instance is 1 ms/ div. Then these waves both complete a full cycle every 2 ms, so their frequencies are both approximately 0.5 MHz or 500 kilohertz (kHz). If the vertical deflection is set for, say, 0.5 mV/div, then these waves both have peak-to-peak amplitudes of approximately 2 mV.
In a computer, a specialized oscillator, called the clock, serves as a sort of pacemaker for the microprocessor. The clock frequency (or clock speed) is usually specified in megahertz (MHz), and is an important factor in determining the rate at which a computer can perform instructions.
oscilloscope An oscilloscope is a laboratory instrument commonly used to display and analyze the waveforms of electronic signals. In effect, the device draws a graph of the instantaneous signal voltage as a function of time. A typical oscilloscope can display alternating current (AC) or pulsating direct current (DC) waveforms having a frequency as low as approximately 1 hertz (Hz) or as high as several megahertz (MHz). High-end oscilloscopes can display signals having frequencies up to several hundred gigahertz (GHz). The display is broken up into so-called horizontal divisions (hor div) and vertical divisions (vert div). Time is displayed from left to right on the horizontal scale. Instantaneous voltage appears on the vertical scale, with positive values going upward and negative values going downward. The oldest form of oscilloscope, still used in some labs today, is known as the cathode-ray oscilloscope. It produces an image by causing a focused electron beam to travel, or sweep, in patterns across the face of a cathode ray tube (CRT). More modern oscilloscopes electronically replicate the action of the CRT using a liquid crystal display (LCD) similar to those found on notebook computers. The most sophisticated
These days, typical high-end oscilloscopes are digital devices. They connect to personal computers and use their displays. Although these machines no longer employ scanning electron beams to generate images of waveforms in the manner of the old cathode-ray ‘‘scope,’’ the basic principle is the same. Software controls the sweep rate, vertical deflection, and a host of other features which can include:
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Storage of waveforms for future reference and comparison
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OSDL See ‘‘Open Source Development Labs’’
OSGi OSGi (Open Service Gateway Initiative) is an industry plan for a standard way to connect devices such as home appliances and security systems to the Internet. With such a standard, home users could, for example, install a security system and be able to change from one monitoring service to another without having to install a new system of wires and devices. The ‘‘service gateway’’ would be an application server in a computer that was a gateway between the Internet and a home or small business’s network of device. The OSGi plans to specify the application program interface (API) for programmers to use to allow communication and control between service providers and the devices within the home or small business network. OSGi’s API will be built on the Java programming language. Java programs can generally be run on any computer operating system platform. OSGi is an open standard programming interface. Changes will evolve through the ‘‘Java Community Process.’’ OSGi is intended to connect new Jini ‘‘smart appliances,’’ Bluetooth wireless device groups, as well as TV set-top boxes, cable modems, alarm systems, energy management systems, and other devices to Internet sites that can be used to manage them remotely and interactively. The service gateway (SG) is intended to manage this interconnection with ‘‘zero administration.’’ Among some popular device-to-Internet applications are expected to be energy measurement and load management in the home; home security systems that a home owner can monitor and control away from home; continous monitoring of critical care and home-care patients; and predictive failure reporting for home appliances. The OSGi specification will be designed to complement existing residential standards, such as those of LonWorks (see control network), CAL, CEBus, HAVi, and others.
The initial group of companies that formed the initiative were: Alcatel, Cable Wireless, Electricite de France, Enron Communications, Ericcson, IBM, Lucent Technologies, Motorola, NCI, Nortel Networks, Oracle, Philips Electronics, Sun Microsystems, Sybase, and Toshiba.
OSI OSI (Open Systems Interconnection) is a standard description or ‘‘reference model’’ for how messages should be transmitted between any two points in a telecommunication network. Its purpose is to guide product implementors so that their products will consistently work with other products. The reference model defines seven layers of functions that take place at each end of a communication. Although OSI is not always strictly adhered to in terms of keeping related functions together in a welldefined layer, many if not most products involved in telecommunication make an attempt to describe themselves in relation to the OSI model. It is also valuable as a single reference view of communication that furnishes everyone a common ground for education and discussion. Developed by representatives of major computer and telecommunication companies beginning in 1983, OSI was originally intended to be a detailed specification of interfaces. Instead, the committee decided to establish a common reference model for which others could develop detailed interfaces, that in turn could become standards. OSI was officially adopted as an international standard by the International Organization of Standards (ISO). Currently, it is Recommendation X.200 of the ITU-TS.
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The main idea in OSI is that the process of communication between two end points in a telecommunication network can be divided into layers, with each layer adding its own set of special, related functions. Each communicating user or program is at a computer equipped with these seven layers of function. So, in a given message between users, there will be a flow of data through each layer at one end down through the layers in that computer and, at the other end, when the message arrives, another flow of data up through the layers in the receiving computer and ultimately to the end user or program. The actual programming and hardware that furnishes these seven layers of function is usually a combination of the computer operating system, applications (such as your Web browser), TCP/IP or alternative transport and network protocols, and the software and hardware that enable you to put a signal on one of the lines attached to your computer.
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OSI divides telecommunication into seven layers. The layers are in two groups. The upper four layers are used whenever a message passes from or to a user. The lower three layers (up to the network layer) are used when any message passes through the host computer. Messages intended for this computer pass to the upper layers. Messages destined for some other host are not passed up to the upper layers but are forwarded to another host. The seven layers are:
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Layer 7: The Application layer...This is the layer at which communication partners are identified, quality of service is identified, user authentication and privacy are considered, and any constraints on data syntax are identified. (This layer is not the application itself, although some applications may perform application layer functions.) Layer 6: The Presentation layer...This is a layer, usually part of an operating system, that converts incoming and outgoing data from one presentation format to another (for example, from a text stream into a popup window with the newly arrived text). Sometimes called the syntax layer. Layer 5: The Session layer...This layer sets up, coordinates, and terminates conversations, exchanges, and dialogs between the applications at each end. It deals with session and connection coordination. Layer 4: The Transport layer...This layer manages the endto-end control (for example, determining whether all packets have arrived) and error-checking. It ensures complete data transfer. Layer 3: The Network layer...This layer handles the routing of the data (sending it in the right direction to the right destination on outgoing transmissions and receiving incoming transmissions at the packet level). The network layer does routing and forwarding. Layer 2: The Data-Link layer...This layer provides synchronization for the physical level and does bit-stuffing for strings of 1’s in excess of 5. It furnishes transmission protocol knowledge and management. Layer 1: The Physical layer...This layer conveys the bit stream through the network at the electrical and mechanical level. It provides the hardware means of sending and receiving data on a carrier.
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Rather than simply counting the number of hops, OSPF bases its path descriptions on ‘‘link states’’ that take into account additional network information. OSPF also lets the user assign cost metrics to a given host router so that some paths are given preference. OSPF supports a variable network subnet mask so that a network can be subdivided. RIP is supported within OSPF for router-to-end station communication. Since many networks using RIP are already in use, router manufacturers tend to include RIP support within a router designed primarily for OSPF.
out of the box 1) ‘‘Out of the box’’ is an expression that describes nonconformal, creative thinking. The term is used as an adverb to describe the thinking or as an adjective to describe the ideas. Although the origin of the term is unknown to us, it would seem to suggest that we tend to carry around an existing and conventional structure of thought that it is hard to escape. One also thinks of the expression ‘‘boxed-in,’’ or having reduced choices. In the fast-paced world of information technology, employers often say they are looking for someone who ‘‘thinks out of the box.’’ Older, related but really different terms include blue sky, far out, and off the wall. A variation is ‘‘outside the box.’’ As you might expect, the term ‘‘in the box’’ is sometimes used to describe conformal thinking. For example, in a recent magazine article about MP3 and music pirating, a manager of some music groups is quoted as saying of major label recording companies who have been slow to adapt to the Internet, ‘‘They’re always thinking inside of the box.’’ 2) ‘‘Out of the box’’ is also used as a synonym for ‘‘off the shelf,’’ meaning a ready-made software, hardware, or combination package that meets a need that would otherwise require an ad hoc development effort.
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out-of-band signaling
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Out-of-band signaling is telecommunication signaling (exchange of information in order to control a telephone call) that is done on a channel that is dedicated for the purpose and separate from the channels used for the telephone call. Out-of-band signaling is used in Signaling System 7 (SS7), the latest standard for the signaling that controls the world’s phone calls.
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OSPF (Open Shortest Path First) is a router protocol used within larger autonomous system networks in preference to the Routing Information Protocol (RIP), an older routing protocol that is installed in many of today’s corporate networks. Like RIP, OSPF is designated by the Internet Engineering Task Force (IETF) as one of several Interior Gateway Protocols (IGPs). Using OSPF, a host that obtains a change to a routing table or detects a change in the network immediately multicasts the information to all other hosts in the network so that all will have the same routing table information. Unlike the RIP in which the entire routing table is sent, the host using OSPF sends only the part that has changed. With RIP, the routing table is sent to a neighbor host every 30 seconds. OSPF multicasts the updated information only when a change has taken place.
output feedback Output feedback (OFB) is a mode of operation for a block cipher. It has some similarities to the ciphertext feedback mode in that it permits encryption of differing block sizes, but has the key difference that the output of the encryption block function is the feedback (instead of the ciphertext). The XOR (exclusive OR) value of each plaintext block is created independently of both the plaintext and ciphertext. It is this mode that is used when there can be no tolerance for error propagation, as there are no chaining dependencies.
overclocking Like the ciphertext feedback mode, it uses an initialization vector (IV). Changing the IV in the same plaintext block results in different ciphertext. In terms of error correction, output feedback can tolerate ciphertext bit errors, but is incapable of self-synchronization after losing ciphertext bits, as it disturbs the synchronization of the aligning keystream. A problem with output feedback is that the plaintext can be easily altered but using a digital signature scheme can overcome this problem.
outsourcing Outsourcing is an arrangement in which one company provides services for another company that could also be or usually have been provided in-house. Outsourcing is a trend that is becoming more common in information technology and other industries for services that have usually been regarded as intrinsic to managing a business. In some cases, the entire information management of a company is outsourced, including planning and business analysis as well as the installation, management, and servicing of the network and workstations. Outsourcing can range from the large contract in which a company like IBM manages IT services for a company like Xerox to the practice of hiring contractors and temporary office workers on an individual basis.
overclocking Overclocking is resetting your computer so that the microprocessor runs faster than the manufacturer-specified speed (for example, setting an Intel 166 MHz (megahertz) microprocessor to run at 200 Mhz). Somewhat surprisingly, this is possible. However, it’s more likely to work with an Intel microprocessor than with those of other manufacturers because, according to Tom’s Hardware Guide, Intel labels the speed of its microprocessors more conservatively. Factors that favor your ability to successfully ‘‘upgrade by resetting’’ include (in addition to having an Intel microprocessor): having a well-designed motherboard with a fast enough bus and having a fan or other cooling device that will keep your system cool enough. The procedure for ‘‘overclocking’’ depends on your combination of factors. The first and most commonly applicable step is to reset your computer’s bus speed. The microprocessor is often able to adjust successfully to a slightly higher bus speed. Resetting the bus speed may require resetting jumpers inside your computer. In newer systems with SoftMenu BIOS, the bus speed can be set through your system setup interface.
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P2P See ‘‘peer-to-peer’’
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A packet is the unit of data that is routed between an origin and a destination on the Internet or any other packetswitched network. When any file (e-mail message, HTML file, Graphics Interchange Format file, Uniform Resource Locator request, and so forth) is sent from one place to another on the Internet, the Transmission Control Protocol (TCP) layer of TCP/IP divides the file into ‘‘chunks’’ of an efficient size for routing. Each of these packets is separately numbered and includes the Internet address of the destination. The individual packets for a given file may travel different routes through the Internet. When they have all arrived, they are reassembled into the original file (by the TCP layer at the receiving end).
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A packet-switching scheme is an efficient way to handle transmissions on a connectionless network such as the Internet. An alternative scheme, circuit-switched, is used for networks allocated for voice connections. In circuitswitching, lines in the network are shared among many users as with packet-switching, but each connection requires the dedication of a particular path for the duration of the connection. ’’Packet’’ and ‘‘datagram’’ are similar in meaning. A protocol similar to TCP, the User Datagram Protocol(UDP) uses the term datagram.
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On the Internet, a packet monkey is someone (see cracker, hacker, and script kiddy) who intentionally inundates a Web site or network with data packets, resulting in a denial-of-service situation for users of the attacked site or network. Packet monkeys typically use tools created and made available on the Internet by hackers.
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According to one writer’s distinction, a packet monkey, unlike a script kiddy, leaves no clues as to who is making the exploit, making the identity of a packet monkey more
difficult to trace. In addition, a denial-of-service attack can be launched on a wider scale than attacks performed by script kiddies, making them more difficult to investigate. Hackers look down on packet monkeys and often describe them as ‘‘bottom feeders.’’ Because a packet monkey uses tools created by others, the packet monkey has little understanding of the harm that may be caused. Typically, packet monkey exploits are random and without any purpose other than the thrill of making an effect.
packet writing Packet writing is a technology that enables the writing of data to a recordable compact disc (CD-R) incrementally, in multiple small blocks of data, rather than in a single block (as in disc-at-once recording), or in blocks defined by tracks (as in track-at-once recording), the other two common methods. Packet writing software makes it possible for the user to save data to a CD-R or CD-RW in the same way as it would be saved to the hard drive or a floppy. Not all CD recorders support packet writing.
PacketHound PacketHound is a product that aims to help an enterprise regulate traffic that might otherwise slow services down for all users of a local area network. PacketHound is intended to address the concern of businesses and universities about the downloading of large files, especially music files in the MP3 format, using Napster, Gnutella, and similar approaches. PacketHound can also monitor and reduce the number of streaming media files that are downloaded by network users. Both businesses and universities are concerned not only about such traffic slowing down the network for other uses, but also about the possibility of being sued by music companies for loss of intellectual property. According to Palisade Systems, the developers of PacketHound, Napster and Gnutella traffic has had a significant effect in performance on a number of university networks. Although Gnutella uses a known port number that a network firewall server could screen for, the port number can be changed by a sophisticated user. The makers of PacketHound claim to be the only product that can monitor and block certain traffic based on the characteristics of the request and response flow, although they do not describe their approach in detail. PacketHound customers can also use the product to monitor without blocking and to also permit or block given traffic at different times of the day. PacketHound is not installed in a firewall server but as a separate PC with an Ethernet card and running NetBSD. The machine’s presence is said to be transparent to the network. When PacketHound discerns a pattern of traffic that meets
pagefile the blocking criteria, it returns a reset packet to the requesting machine. The user sees a ‘‘Connection reset by host’’ message. The company also sells PacketPup, a downloadable program that lets a company monitor bandwidth usage on a network. A similar product called PacketShaper (from Packeteer) analyzes and classifies applications in use on the network in terms of their bandwidth and other behavior.
Packet-Level Procedure 1) PAP (Packet-Level Procedure) is a full-duplex protocol for transferring packets between parties in an X.25 network. PAP supports data sequencing, flow control, accountability, and error detection and recovery. 2) PAP (Password Authentication Procedure) is a procedure used by Point-to-Point Protocol servers to validate a connection request. PAP works as follows: 1. After the link is established, the requestor sends a password and an id to the server. 2. The server either validates the request and sends back an acknowledgement, terminates the connection, or offers the requestor another chance. Passwords are sent without security and the originator can make repeated attempts to gain access. For these reasons, a server that supports Challenge-Handshake Authentication Protocol will offer to use that protocol before using PAP. PAP protocol details can be found in RFC 1334.
packet-switched Packet-switched describes the type of network in which relatively small units of data called packets are routed through a network based on the destination address contained within each packet. Breaking communication down into packets allows the same data path to be shared among many users in the network. This type of communication between sender and receiver is known as connectionless (rather than dedicated). Most traffic over the Internet uses packet switching and the Internet is basically a connectionless network. Contrasted with packet-switched is circuit-switched, a type of network such as the regular voice telephone network in which the communication circuit (path) for the call is set up and dedicated to the participants in that call. For the duration of the connection, all resources on that circuit are unavailable for other users. Voice calls using the Internet’s packet-switched system are possible. Each end of the conversation is broken down into packets that are reassembled at the other end. Another type of digital network that uses packet-switching is the X.25 network, a widely-installed commercial wide area network protocol. Internet protocol packets can be carried on an X.25 network. The X.25 network can also support a virtual circuit in which a logical connection is established for
two parties on a dedicated basis for some duration. A permanent virtual circuit (PVC) reserves the path on an ongoing basis and is an alternative for corporations to a system of leased lines. A permanent virtual circuit is a dedicated logical connection but the actual physical resources can be shared among multiple logical connections or users.
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page 1) On the World Wide Web, a page is a file notated with the Hypertext Markup Language (HTML). Usually, it contains text and specifications about where image or other multimedia files are to be placed when the page is displayed. You can think of a Web site as a book (albeit a hypertext kind of book rather than a sequentially arranged kind of book) that arrives a page at a time as you request each one. Each page is an individual HTML file with its own Web address (URL). The first page you usually request at a site is known as the home page. (Most home pages have a default name that doesn’t have to be specified; you only need to enter the domain name for the site itself.) With frames, multiple pages (HTML files) can be downloaded to a browser and presented on designated sections of the display screen at the same time. 2) In a computer’s random access memory (RAM), a page is a group of memory cells that are accessed as part of a single operation. That is, all the bits in the group of cells are changed at the same time. In some kinds of RAM, a page is all the memory cells in the same row of cells. In other kinds of RAM, a page may represent some other group of cells than all those in a row. 3) In computer systems that use virtual memory (also known as virtual storage), a page is a unit of data storage that is brought into real storage (on a personal computer, RAM) from auxiliary storage (on a personal computer, usually the hard disk) when a requested item of data is not already in real storage (RAM). 4) In printing, a page is what is printed on a single piece of paper.
pagefile In storage, a pagefile is a reserved portion of a hard disk that is used as an extension of random access memory (RAM) for data in RAM that hasn’t been used recently. A pagefile can be read from the hard disk as one contiguous chunk of data and thus faster than re-reading data from many different original locations. Windows administrators or users can reset the system-provided default size value of the pagefile to meet their particular needs.
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In other operating systems, the pagefile is called a swap file or a swap partition.
pagejacking Pagejacking is stealing the contents of a Web site by copying some of its pages, putting them on a site that appears to be the legitimate site, and then inviting people to the illegal site by deceptive means—for example, by having the contents indexed by major search engines whose results in turn link users to the illegal site. By moving enough of a Web site’s content as well as the page descriptor information (known as information) within each page, pagejackers can then submit the illegal site to major search engines for indexing. Users of the search engine sites may then receive results from both the illegitimate as well as the legitimate site and can easily be misled to link to the wrong one. Users linking to the illegitimate site may find themselves redirected to a pornographic or other unwanted site. As an additional annoyance, users subjected to pagejacking may also encounter mousetrapping, in which clicking the Back button with the mouse does not lead out of the illegal site but only to the viewing of additional unwanted pages. To escape, the user may need to close the browser or even restart the operating system. Web users who enter Web page addresses (known as URLs) directly on their Web browser address line, by selecting it from a bookmark, or by clicking on a properly coded link on another site will not be subject to pagejacking. The problem most typically occurs when clicking site descriptions that result from searches at major search engine sites. Although the practice was not new at the time, the New York Times on September 23, 1999, carried a page one story about an Australian company that had pagejacked a number of corporate sites, adding pornographic links or ads, and then mousetrapping visitors. Australian officials were reported to be considering civil or criminal charges and a U.S. Federal judge in Virginia, where the original Internet site registration company is located, ordered the sites to lose their Web registrations.
pager A pager is a small telecommunications device that receives (and, in some cases, transmits) alert signals and/or short messages. This type of device is convenient for people expecting telephone calls, but who are not near a telephone set to make or return calls immediately. A typical one-way pager fits easily in a shirt pocket; some are as small as a wristwatch. A miniature, short-range wireless receiver captures a message, usually accompanied by a beep. (This is why the device is also known as a beeper). The simplest one-way pagers display the return-call telephone number of the person who sent the message. Alternatively, a code can be displayed that indicates which
of several designated parties is requesting a return phone call. Sophisticated one-way pagers can display short text messages. Until recently, pagers were designed as receive-only devices. There are at least two reasons for this. First, if two-way communication is needed, cell phones are available for that purpose. Second, it is difficult to engineer an efficient wireless transmitter that can fit inside a tiny package and provide enough signal range to reach repeaters from all points within the coverage zone. Despite the engineering challenge, a two-way pager, also called a two-way messaging device or two-way interactive system, has been developed. This system employs large numbers of repeaters, allowing low-power wireless transmitters with subminiature antennas to reach at least one repeater from any location within the coverage area. A typical unit is about the size of a pocket calculator and has a built-in, miniature keyboard and a liquid crystal display (LCD) screen that can display several lines of text and/or simple graphics.
PAL See ‘‘Phase Alternation Line’’
palette A palette is both the board on which an artist puts selected colors and also the set of colors themselves. On the Web, choosing the colors you use not only involves understanding which colors work well together but also understanding the capabilities of display screens and browser for displaying the colors you choose. (In the case of pre-created images such as photographs someone else has taken, you don’t choose the colors that were captured initially, but you can control which colors are saved for transmission.) In computer display technology, a color is set for each individual pixel or addressable illumination element on the screen. Each pixel has a red, a green, and a blue (RGB) component. By specifying the amount of intensity for each of these components, a distinct color is given to that pixel. (A good way to remember this if you create Web page is to think of the way the background, text, or link colors are specified in the BODY tag as a string of eight binary digits, where each two-digit sequence (of eight bits, or a byte) represents one of the RGB components.) For Web presentation, you (or the artist you’re working with) will usually want to choose from a palette that is limited to the 256 colors that most computer users can display. Users with very high-quality display monitors and adapters that provides a 24-bit variation for each pixel can view up to 16,777,216 different colors. However, most of us have computers that can only handle an 8-bit variation, limiting us to a (still pretty graphic) 256 color-palette.
panel If you do use a larger range of colors or pallette than someone’s display or browser can handle, the browser will dithering the colors (that is, the browser will find colors within its palette that it can substitute for any color that is outside its palette). As a designer, you will also want to consider that Mac and Windows browsers do not have identical palettes. In the usual 256 color palette, 216 are common to both types of browsers, but 40 are different and require dithering by one of the browsers.
Palm Palm is the trade name for a popular personal digital assistant (PDA), a form of handheld device that that is also known as a palmtop computer. Originally the Palm, which is used mainly for personal organization, wireless e-mail, notetaking, and electronic games, was called the PalmPilot. It was introduced in 1996 by Palm Computing, Inc. The Palm is about the same size as a stenographer’s tablet. Data appears on an liquid crystal display screen. One of the outstanding features of the Palm is its user-friendly method of data entry. The device uses an optical character recognition (OCR) scheme capable of converting handwriting into digital bits that the microprocessor can understand. A writing device, called a stylus, can be pointed at icons on the display to select items for entry. The stylus can also be used to enter alphanumeric data (words and numbers) by manually scrawling them across the face of the display. This is known as Graffiti. The Palm can not only be used to originate, store, and process data on its own, but it can download data from a desktop or notebook computer or from the Internet, process it, and then upload the new data back. Although the Palm is more convenient for some tasks than others, the pace and nature of technological evolution is likely to change and expand its role in the near future. The Palm is ideally suited for downloading data, storing it, and displaying it as electronic books and magazines. In the long term, the potential of the PDA is limited only by consumer demand, and by the skill and ingenuity of hardware and software engineers.
Palm OS Palm OS is the computer operating system that provides a software platform for the PalmPilot series of handheld personal digital assistants made by Palm, Inc. Microsoft’s Windows CE and Symbian’s EPOC (originated by Psion) are also operating systems for handheld devices, but are designed to serve a broader range of devices. Palm OS uses multitasking, but only one task is for applications. The user uses one application at a time, one application program must finish before the next can be selected. This constraint allows the operating system to devote full attention to the application that is open. The
space needed by the system for any application that is running is kept in dynamic, reusable random access memory (RAM). The application and its related database are kept in what is called permanent storage, but here the permanent storage is RAM (rather than a hard disk) that cannot be reused as the dynamic RAM can. Palm OS divides an application into runnable code and different types of data elements, such as user interface elements and icons. The data elements can be easily changed without necessarily having to rewrite code. Palm chose not to include a keyboard in the PalmPilot in order to produce a truly palm-size device. Learning from Apple’s Newton, an earlier attempt at a pen-and-notepad interface, the company also chose not to provide full handwriting recognition code. Instead, PalmPilot users learn to use a more quickly recognized but restrictive set of pen strokes. These decisions helped keep Palm OS small in size. Palm OS comes with these applications built-in: Dates, Address Book, To Do List, Memo Pad, Calculator, and Password Protection. New applications can be written and added using several facilities that accelerate development. Palm supports Metrowerks’ CodeWarrior as the official software development kit (SDK), using a Macintosh or Windows environment. UNIX platform users can use a kit called GCC, which is available through the Free Software Foundation. Programmers can use C, C++, assembler, or scripting. The PalmPilot user interface is emulated within a window in the desktop environment, encouraging rapid application development. Simpler applications can be developed using Palm Computing’s forms interface. Palm OS comes with communication interfaces to infrared transmission devices, TCP/IP (for Web connection through wireless or wireline devices), and, optionally, barcode recognition scanners.
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See ‘‘personal area network’’
panel In computer program development, a panel is a representation of what information will be sent to a user’s display screen in given circumstances. Typically, when designing a program, the user interface is specified by portraying what information (text and pictures) will be presented to the user at different stages of using the program. For example, each menu, help page, or other form of content constitutes a panel of information that is to be implemented by developers and tested by early users. Since most applications are developed against the context of an operating system graphical user interface (GUI), these elements can sometimes be assumed in describing specific panels. Generally, in a windowed user interface, a panel is designed for each window of information.
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paradigm
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A paradigm (pronounced PEHR-uh-daim, from Greek paradeiknyai—to show side by side) is a pattern or an example of something. The word also connotes the ideas of a mental picture and pattern of thought. Thomas Kuhn uses the word to mean the model that scientists hold about a particular area of knowledge. Kuhn’s famous book, The Structure of Scientific Revolutions, is his view of the stages through which a science goes in getting from one paradigm to the next.
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paradox
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A paradox is a statement or concept that contains conflicting ideas. In logic, a paradox is a statement that contradicts itself; for example, the statement ‘‘I never tell the truth’’ is a paradox because if the statement is true (T), it must be false (F) and if it is false (F), it must be true (T). In everyday language, a paradox is a concept that seems absurd or contradictory, yet is true. In a Windows environment, for instance, it is a paradox that when a user wants to shut down their computer, it is necessary to click ‘‘Start.’’
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In the context of the Internet and computing, parallel means more than one event happening at a time. It is usually contrasted with serial, meaning only one event happening at a time. In data transmission, the techniques of time division and space division are used, where time separates the transmission of individual bits of information sent serially and space (in multiple lines or paths) can be used to have multiple bits sent in parallel. In the context of computer hardware and data transmission, serial connection, operation, and media usually indicate a simpler, slower operation (think of your serial mouse attachment). Parallel connection and operation (think of multiple characters being sent to your printer) indicates faster operation. This indication doesn’t always hold since a serial medium (for example, fiber optic cable) can be much faster than a slower medium that carries multiple signals in parallel. A conventional phone connection is generally thought of as a serial line since its usual transmission protocol is serial. Conventional computers and their programs operate in a serial manner, with the computer reading a program and performing its instructions one after the other. However, some of today’s computers have multiple processors that divide up the instructions and perform them in parallel.
parallel presence detect Parallel presence detect (PPD) is a method of using resistors to communicate a memory module’s speed and density to the basic input/output system (BIOS) when a computer boots (starts or restarts). The BIOS uses this information to
configure the memory modules to run correctly or to detect if an incorrect memory module has been installed. Today, serial presence detect (SPD) is used on newer memory modules because SPD can store up to 400 times more information than PPD. PPD is used with 30- and 72-pin SIMMs. PPD indicates the memory module’s specifications using parallel pins on the module that transmit a binary code. If a pin has a resistor connected to it, it’s a logical zero. If a pin does not have a resistor connected to it, it’s a logical one. The combination of resistors connected and not connected make up the binary code that the BIOS uses.
parallel processing In computers, parallel processing is the processing of program instructions by dividing them among multiple processors with the objective of running a program in less time. In the earliest computers, only one program ran at a time. A computation-intensive program that took one hour to run and a tape copying program that took one hour to run would take a total of two hours to run. An early form of parallel processing allowed the interleaved execution of both programs together. The computer would start an I/O operation, and while it was waiting for the operation to complete, it would execute the processor-intensive program. The total execution time for the two jobs would be a little over one hour. The next improvement was multiprogramming. In a multiprogramming system, multiple programs submitted by users were each allowed to use the processor for a short time. Then the operating system would allow the next program to use the processor for a short time, and so on. To users it appeared that all of the programs were executing at the same time. Problems of resource contention first arose in these systems. Explicit requests for resources led to the problem of deadlock. Competition for resources on machines with no tie-breaking instructions lead to critical section routine. Vector processing was another attempt to increase performance by doing more than one thing at a time. In this case, capabilities were added to machines to allow a single instruction to add (or subtract, or multiply, or ...) two arrays of numbers. This was valuable in certain engineering applications where data naturally occurred in the form of vectors or matrices. In applications with less well-formed data, vector processing was not so valuable. The next step in parallel processing was the introduction of multiprocessing. In these systems, two or more processors shared the work to be done. The earliest versions had a master/slave configuration. One processor (the master) was programmed to be responsible for all of the work in the system; the other (the slave) performed only those tasks it was assigned by the master. This arrangement was
parallel processing software necessary because it was not then understood how to program the machines so they could cooperate in managing the resources of the system. Solving these problems led to symmetric multiprocessing systems (SMP). In an SMP system, each processor is equally capable and responsible for managing the flow of work through the system. Initially, the goal was to make SMP systems appear to programmers to be exactly the same as single processor, multiprogramming systems. (This standard of behavior is known as sequential consistency). However, engineers found that system performance could be increased by someplace in the range of 10-20% by executing some instructions out of order and requiring programmers to deal with the increased complexity. (The problem can become visible only when two or more programs simultaneously read and write the same operands; thus the burden of dealing with the increased complexity falls on only a very few programmers and then only in very specialized circumstances.) The question of how SMP machines should behave on shared data is not yet resolved. As the number of processors in SMP systems increases, the time it takes for data to propagate from one part of the system to all other parts grows also. When the number of processors is somewhere in the range of several dozen, the performance benefit of adding more processors to the system is too small to justify the additional expense. To get around the problem of long propagation times, message passing systems were created. In these systems, programs that share data send messages to each other to announce that particular operands have been assigned a new value. Instead of a broadcast of an operand’s new value to all parts of a system, the new value is communicated only to those programs which need to know the new value. Instead of a shared memory, there is a network to support the transfer of messages between programs. This simplification allows hundreds, even thousands, of processors to work together efficiently in one system. (In the vernacular of systems architecture, these systems ‘‘scale well.’’) Hence such systems have been given the name of massively parallel processing (MPP) systems. The most successful MPP applications have been for problems that can be broken down into many separate, independent operations on vast quantities of data. In data mining, there is a need to perform multiple searches of a static database. In artificial intelligence, there is the need to analyze multiple alternatives, as in a chess game. Often MPP systems are structured as clusters of processors. Within each cluster the processors interact as in a SMP system. It is only between the clusters that messages are passed. Because operands may be addressed either via messages or via memory addresses, some MPP systems are called NUMA machines, for Non-Uniform Memory Addressing.
SMP machines are relatively simple to program; MPP machines are not. SMP machines do well on all types of problems, providing the amount of data involved is not too large. For certain problems, such as data mining of vast data bases, only MPP systems will serve.
parallel processing software Parallel processing software manages the execution of a program on parallel processing hardware with the objectives of obtaining unlimited scalability (being able to handle an increasing number of interactions at the same time) and reducing execution time. Applications that benefit from parallel processing divide roughly into business data processing and technical/scientific processing. Business data processing applications are characterized by record processing, and the size of the data typically creates significant I/O performance issues as well as the need for fast computation. Parallel processing software assists business applications in two significant ways: 1) Frameworks—Dataflow frameworks provide the highest performance and simplest method for expressing recordprocessing applications so that they are able to achieve high scalability and total throughput. Dataflow frameworks underlie the internals of most relational database management systems (RDBMSs) as well as being available for direct use in construction of data warehouse, business intelligence, and analytic CRM (customer relationship management) applications. Frameworks hide most or all the details of inter-process and inter-processor communications from application developers, making it simpler to create these applications than it would be using low-level message passing. 2) RDBMS—As the most common repositories for commercial record-oriented data, RDBMS systems have evolved so that the Structured Query Language (SQL) that is used to access them is executed in parallel. The nature of the SQL language lends itself to faster processing using parallel techniques. Technical and scientific applications tend to be ‘‘computebound’’ (they require much processor computation) and have tended to be associated with the supercomputer. There are two primary techniques used in the creation of most of these applications—message passing and parallelizing compilers. Message Passing—Application programs can be built using mechanisms for communication between one processor operating concurrently with others. This is the lowest-level mechanism available and can lead to the highest possible performance at the greatest implementation cost and complexity. (Note that message passing for parallel computation should not be confused with the term messaging which is also used to describe transactional communication systems for distributed client-to-server and inter-server business applications.)
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Parallelizing Compilers—For technical and mathematical applications dominated by matrix algebra, there are compilers that can create parallel execution from seemingly sequential program source code. These compilers can decompose a program and insert the necessary message passing structures and other parallel constructs automatically.
parallel sysplex
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See ‘‘sysplex and Parallel Sysplex’’
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parameter
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In information technology, a parameter (pronounced puhRAA-meh-tuhr, from Greek for, roughly, through measure) is an item of information—such as a name, a number, or a selected option—that is passed to a program by a user or another program. Parameters affect the operation of the program receiving them.
H para-site
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A para-site is a Web site that frames other Web sites or pages within its own site. For example, a Web site whose purpose was simply to act as a starting place for all sites about gardening-related home pages might have a framed site with a menu in the left frame. You would select a specific gardening Web site by clicking on the menu and that site would then appear in the right frame. This controversial practice has both proponents and detractors. For Web users who just want to survey all possible gardening site offerings fairly quickly, the para-site approach may be appreciated. The owner of the para-site may get more page views because users leave the site less frequently. On the other hand, it can be argued that the sites that are framed by the para-site are being ‘‘republished’’ within someone else’s ‘‘publication.’’ Furthermore, the framed sites are designed for full screen viewing, not for framing within a smaller space by someone else. If the framed site is itself framed (for example, as whatis.com is!), the result may be not only less than pleasing to the eye but confusing to the user.
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Some observers believe that, assuming there is a valid reason to build a para-site, the para-site builder should first request and receive permission from any site it plans to frame.
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In computers, parity (from the Latin paritas: equal or equivalent) refers to a technique of checking whether data has been lost or written over when it’s moved from one place in storage to another or when transmitted between computers. Here’s how it works: An additional bit, the parity bit, is added to a group of bits that are moved together. This bit is used only for the purpose of identifying whether the bits being moved arrived successfully. Before the bits are sent,
they are counted and if the total number of data bits is even, the parity bit is set to one so that the total number of bits transmitted will form an odd number. If the total number of data bits is already an odd number, the parity bit remains or is set to 0. At the receiving end, each group of incoming bits is checked to see if the group totals to an odd number. If the total is even, a transmission error has occurred and either the transmission is retried or the system halts and an error message is sent to the user. The description above describes how parity checking works within a computer. Specifically, the Peripheral Component Interconnect bus and the I/O bus controller use the odd parity method of error checking. Parity bit checking is not an infallible error-checking method since it’s possible that two bits could be in error in a transmission, offsetting each other. For transmissions within a personal computer, this possibility is considered extremely remote. In some large computer systems where data integrity is seen as extremely important, three bits are allocated for parity checking. Parity checking is also used in communication between modems. Here, parity checking can be selected to be even (a successful transmission will form an even number) or odd. Users may also select no parity, meaning that the modems will not transmit or check a parity bit. When no parity is selected (or defaulted), it’s assumed that there are other forms of checking that will detect any errors in transmission. No parity also usually means that the parity bit can be used for data, speeding up transmission. In modem-to-modem communication, the type of parity is coordinated by the sending and receiving modems before the transmission takes place.
parse To parse is to analyze something in an orderly way. In linguistics, to parse is to divide words and phrases into different parts in order to understand relationships and meaning. For example, English students are sometimes asked to parse a sentence by dividing it into subject and predicate, and then into dependent phrases, modifiers, and so forth. In general, to parse someone’s writing or speech simply means to interpret it. In computers, to parse is to divide a computer language statement into parts that can be made useful for the computer. A parser in a program compiler is a program that takes each program statement that a developer has written and divides it into parts (for example, the main command, options, target objects, their attributes, and so forth) that can then be used for developing further actions or for creating the instructions that form an executable program.
Pascal
parser
pascal (unit of pressure or stress)
In computer technology, a parser is a program, usually part of a compiler, that receives input in the form of sequential source program instructions, interactive online commands, markup tags, or some other defined interface and breaks them up into parts (for example, the nouns (objects), verbs (methods), and their attributes or options) that can then be managed by other programming (for example, other components in a compiler). A parser may also check to see that all input has been provided that is necessary.
The pascal (pronounced pass-KAL and abbreviated Pa) is the International System of Units (SI) unit of pressure or stress. It is named after the scientist Blaise Pascal. One pascal is equivalent to one newton (1 N) of force applied over an area of one meter squared (1 m2). That is, 1 Pa = 1 N˚ m-2. Reduced to base units in SI, one pascal is one kilogram per meter per second squared; that is, 1 Pa = 1 kg˚ m-1˚ s-2.
partition In personal computers, a partition is a logical division of a hard disk created so that you can have different operating systems on the same hard disk or to create the appearance of having separate hard drives for file management, multiple users, or other purposes. A partition is created when you format the hard disk. Typically, a one-partition hard disk is labelled the ‘‘C:’’ drive (’’A:’’ and ‘‘B:’’ are typically reserved for diskette drives). A two-partition hard drive would typically contain ‘‘C:’’ and ‘‘D:’’ drives. (CD-ROM drives typically are assigned the last letter in whatever sequence of letters have been used as a result of hard disk formatting, or typically with a two-partition, the ‘‘E:’’ drive.) When you boot an operating system into your computer, a critical part of the process is to give control to the first sector on your hard disk. It includes a partition table that defines how many partitions the hard disk is formatted into, the size of each, and the address where each partition begins. This sector also contains a program that reads in the boot sector for the operating system and gives it control so that the rest of the operating system can be loaded into random access memory. Boot viruses can put the wrong information in the partition sector so that your operating system can’t be located. For this reason, you should have a back-up version of your partition sector on a diskette known as a bootable floppy.
partner relationship management Partner relationship management (PRM) is a business strategy for improving communication between companies and their channel partners. Web-based PRM software applications enable companies to customize and streamline administrative tasks by making shipping schedules and other real-time information available to all the partners over the Internet. Several CRM providers have incorporated PRM features, such as Web-enabled spreadsheets shared through an extranet, in their software applications. PRM is often compared to customer relationship management (CRM) and there is some argument over whether the complex relationships of channel partnerships makes it necessary for PRM to be a separate entity, or merely a component of CRM.
If a pressure p in pascals exists on an object or region whose surface area is A meters squared, then the force F, in newtons, required to produce p is given by the following formula:
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F = pA Suppose a small rocket engine produces 100,000 (105) Pa of pressure, and the nozzle has a cross-sectional area of 1 /10,000 of a square meter (10-4 m2). Then the force F, in newtons, produced by the engine is: F = pA = 105 x 10-4 = 10 Imagine that this engine is used in a propellant pack for a space walker whose mass is 50 kg. How fast will the person accelerate relative to nearby objects in the weightless environment of earth orbit? The answer is found by the familiar formula stating that force is equal to mass times acceleration (F = ma). This can be manipulated to obtain:
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a=F/m
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where a is the acceleration in meters per second squared, F is the force in newtons, and m is the mass in kilograms. Plugging in the known numbers:
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a = 10 / 50 = 0.5 The acceleration is 0.5 m/s2. If the rocket engine is fired continuously by a space walker who is stationary relative to another object nearby, then after one second the space walker will be moving at a speed of 0.5 m/s with respect to that object; after two seconds, moving at 1 m/s; after three seconds, moving at 1.5 m/s; and so on. Obviously, the space walker will want to fire the engine only in short bursts! Also see kilogram, meter, meter per second, meter per second squared, newton, second, and SI.
Pascal
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Also see pascal (unit of pressure or stress). Pascal is a strongly-typed third-generation language (3GL) with a one-pass compiler. Designed for instructional purposes about 1967-68 by Nicholas Wirth, Pascal requires a programmer to define all routines and variables fully, including the nature of their use, before using them. Pascal is the language on which many programmers first learn how to write structured, compiled programs. While commercial versions of Pascal have been made available, it has had limited success in the business world. While Pascal has been extended by compiler makers to address special-purpose needs, many programmers feel that it is too restrictive in its
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passive FTP
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Passive FTP (sometimes referred to as PASV FTP because it involves the FTP PASV command) is a more secure form of data transfer in which the flow of data is set up and initiated by the File Transfer Program (FTP) client rather than by the FTP server program. Separate FTP client programs, such as WS_FTP Pro, usually allow the user to select passive FTP. Most Web browsers (which act as FTP clients) use passive FTP by default because corporations prefer it as a safety measure. As a general rule, any corporate firewall server, which exists in order to protect an internal network from the outside world, recognizes input from the outside only in response to user requests that were sent out requesting the input. The use of passive FTP ensures all data flow initiation comes from inside the network rather than from the outside.
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Using normal or passive FTP, a client begins a session by sending a request to communicate through TCP port 21, the port that is conventionally assigned for this use at the FTP server. This communication is known as the Control Channel connection. Using ‘‘normal’’ FTP communication, the client requestor also includes in the same PORT command packet on the Control Channel a second port number that is to be used when data is to be exchanged; the port-to-port exchange for data is known as the Data Channel. The FTP server then initiates the exchange from its own port 20 to whatever port was designated by the client. However, because the serverinitiated communication is no longer controlled by the client and can’t be correlated by a firewall to the initial request, the potential exists for uninvited data to arrive from anywhere posing as a normal FTP transfer. Using passive FTP, a PASV command is sent instead of a PORT command. Instead of specifying a port that the server can send to, the PASV command asks the server to specify a port it wishes to use for the Data Channel connection. The server replies on the Control Channel with the port number which the client then uses to initiate an exchange on the Data Channel. The server will thus always be responding to client-initiated requests on the Data Channel and the firewall can correlate these.
to-the-building (FTTB), or fiber-to-the-home (FTTH). A PON consists of an Optical Line Termination (OLT) at the communication company’s office and a number of Optical Network Units (ONUs) near end users. Typically, up to 32 ONUs can be connected to an OLT. The passive simply describes the fact that optical transmission has no power requirements or active electronic parts once the signal is going through the network. In a stand-alone system, a PON could deliver up to 622 Mbps downstream to the user and up to 155 Mbps upstream. Multiple users of a PON could be allocated portions of this bandwidth. A PON could also serve as a trunk between a larger system, such as a CATV system, and a neighborhood, building, or home Ethernet network on coaxial cable. In Japan, Nippon Telephone and Telegraph (NTT) is planning to install passive optical networks throughout its system, using equipment made by Lucent Technologies.
passphrase A passphrase is a string of characters longer than the usual password (which is typically from four to 16 characters long) that is used in creating a digital signature (an encoded signature that proves to someone that it was really you who sent a message) or in an encryption or a decryption of a message. For example, Phil Zimmermann’s popular encryption program, Pretty Good Privacy, requires a passphrase when you sign or decrypt a message. Passphrases are often up to 100 characters in length.
password A password is an unspaced sequence of characters used to determine that a computer user requesting access to a computer system is really that particular user. Typically, users of a multiuser or securely protected single-user system claim a unique name (often called a user ID) that can be generally known. In order to verify that someone entering that user ID really is that person, a second identification, the password, known only to that person and to the system itself, is entered by the user. A password is typically somewhere between four and 16 characters, depending on how the computer system is set up. When a password is entered, the computer system is careful not to display the characters on the display screen, in case others might see it. Good criteria when choosing a password or setting up password guidelines include the following: L
Don’t pick a password that someone can easily guess if they know who you are (for example, not your Social Security number, birthday, or maiden name)
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Don’t pick a word that can be found in the dictionary (since there are programs that can rapidly try every word in the dictionary!)
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Don’t pick a word that is currently newsworthy
passive optical network A passive optical network (PON) is a system that brings optical fiber cabling and signals all or most of the way to the end user. Depending on where the PON terminates, the system can be described as fiber-to-the-curb (FTTC), fiber-
path to profitability
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Don’t pick a password that is similar to your previous password
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Do pick a mixture of letters and at least one number
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Do pick a word that you can easily remember
Many networks require that you change your password on some periodic basis.
password cracker A password cracker is an application program that is used to identify an unknown or forgotten password to a computer or network resources. It can also be used to help a human cracker obtain unauthorized access to resources. Password crackers use two primary methods to identify correct passwords: brute-force and dictionary searches. When a password cracker uses brute-force, it runs through combinations of characters within a predetermined length until it finds the combination accepted by the computer system. When conducting a dictionary search, a password cracker searches each word in the dictionary for the correct password. Password dictionaries exist for a variety of topics and combinations of topics, including politics, movies, and music groups. Some password cracker programs search for hybrids of dictionary entries and numbers. For example, a password cracker may search for ants01; ants02; ants03, etc. This can be helpful where users have been advised to include a number in their password. A password cracker may also be able to identify encrypted passwords. After retrieving the password from the computer’s memory, the program may be able to decrypt it. Or, by using the same algorithm as the system program, the password cracker creates an encrypted version of the password that matches the original.
PASV FTP
A patch is usually developed and distributed as a replacement for or an insertion in compiled code (that is, in a binary file or object module). In larger operating systems, a special program is provided to manage and keep track of the installation of patches.
patch panel A patch panel is a mounted hardware unit containing an assembly of port locations in a communications or other electronic or electrical system. In a network, a patch panel serves as a sort of static switchboard, using cables to interconnect computers within the area of a local area network (LAN) and to the outside for connection to the Internet or other wide area network (WAN). A patch panel uses a sort of jumper cable called a patch cord to create each interconnection.
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1) In a computer operating system, a path is the route through a file system to a particular file. A pathname (or path name) is the specification of that path. Each operating system has its own format for specifying a pathname.
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In all operating systems, an absolute pathname (or fully qualified path name) specifies the complete path name. A relative pathname specifies a path relative to the directory to which the operating system is currently set. The World Wide Web’s HTTP program uses a pathname as part of a Uniform Resource Locator (URL). 2) In a network, a path is a route between any two points or nodes. 3) In a number of products or applications, a path is a route to or between points within a given organized structure. 4) In IBM’s Virtual Telecommunication Access Method (VTAM), a path identifies a particular dial-out port.
See ‘‘passive FTP’’
path control
patch
See ‘‘pathing’’
A patch (sometimes called a ‘‘fix’’) is a quick-repair job for a piece of programming. During a software product’s beta test distribution or try-out period and later after the product is formally released, problems (called bug) will almost invariably be found. A patch is the immediate solution that is provided to users; it can sometimes be downloaded from the software maker’s Web site. The patch is not necessarily the best solution for the problem and the product developers often find a better solution to provide when they package the product for its next release.
path to profitability Path to profitability (sometimes abbreviated as P2P, which also stands for peer-to-peer) is a term that refers to a business plan that is designed to take an enterprise from startup to turning a profit. In Internet business, the prevalent emphasis on profitability, especially in the e-business world, is in contrast to the attitude prevalent in recent years, when dotcom ventures were often encouraged to open for business, ‘‘burn’’ enough venture capital to dominate a particular business niche, and worry about profits later. Industry and stock market analysts suggest that the popularity of this term indicates a return to traditional business practices and a new, more mature stage in the evolution of the Internet.
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Whatis?Com’s Encyclopedia of Technology Terms Before the market slump of the spring of 2000, almost any entrepreneur with a concept and a PowerPoint presentation could obtain venture capital funding for a dotcom enterprise. Optimism about the future of e-business led to a suspension of traditional business principles and practices: an entrepreneur was not necessarily required to demonstrate a clear business plan—with profits in the foreseeable future—because the ultimate payoff was expected to be so large. With the slowdown in technology stocks, there has been a return to standard business practices. Investors are much more cautious than they were in the early days of the dotcom boom. In order to obtain funding, entrepreneurs are expected to have a well-organized business plan with a clearly articulated—and hopefully short—path to profitability. Some analysts believe that the market change was, in a literal sense, a correction, since the practices employed did not lead to sustainable growth. Many are still optimistic about the future of e-business, although that must be in a world in which the old business rules—such an enterprise’s need for a path to profitability—still apply.
pathing Pathing (sometimes called path control) is a networking approach used to address the specific needs of storage networks (as compared to ordinary message networks) by changing the way that communication paths are managed and organized. Connection failures in message networks generally just mean that the connection must be retried; in storage networks, however, a failure is more likely to cause a system crash. Also, the margin for error is smaller for storage networks; whereas error recovery within minutes is acceptable for message networks, for storage networks it should be within seconds—and preferably milliseconds. Because of these differences, storage network routing must include redundant, well-defined paths, enable fast path changes, and have the altered topology information updated rapidly. According to Marc Farley, author of Building Storage Networks, node routing provides more precise connection control than is afforded by switches and routers. Message networks typically have a very large number of nodes, any of which may have to connect to any other and connection requests are randomly designated. As a consequence, it is considered that routing activities would overload end nodes. In comparison, storage networks have fewer nodes and fewer connections, and connection requests are specifically assigned. In these conditions, it is possible for nodes to manage paths. Pathing is among the most promising technologies behind the expansion of the storage market—which is projected by Adam Couture, senior analyst at Gartner Dataquest, to grow to $7 billion by 2003, up from $10 million in 1999. Storage area networks (SANs) and network attached storage (NAS) have arisen as solutions to the rapidly increasing need for
storage of enterprise data. Originally, enterprise computers and storage devices had one-to-one relationships, not a practical arrangement for multi-user environments with complex data-sharing needs. SANs allow multiple computers to share a single storage device, with technologies like path control, mirroring, and virtualization used to meet the enterprise’s needs for reliability and constant data availability.
pattern In software development, a pattern (or design pattern) is a written document that describes a general solution to a design problem that recurs repeatedly in many projects. Software designers adapt the pattern solution to their specific project. Patterns use a formal approach to describing a design problem, its proposed solution, and any other factors that might affect the problem or the solution. A successful pattern should have established itself as leading to a good solution in three previous projects or situations. In object-oriented programming, a pattern can contain the description of certain objects and object classes to be used, along with their attributes and dependencies, and the general approach to how to solve the problem. Often, programmers can use more than one pattern to address a specific problem. A collection of patterns is called a pattern framework. The design patterns methodology has become increasingly popular among software developers since the early 1990s, due largely to some ground-breaking presentations and books on the subject released to the object-oriented world at trade shows and conventions, notably OOPSLA ’94 (the Object-Oriented Programming Systems, Languages, and Applications conference). A book, ‘‘Design Patterns: Elements of Reusable Object-Oriented Software,’’ by E. Gamma, R. Helm, R. Johnson, and J. Vlissides (known in the industry as the Gang of Four, or GOF) is generally credited with sparking the growing interest in design patterns for use in object-oriented programming. The book contains twentythree patterns, each with a solution for a recurring problem faced in object-oriented design. Design patterns include the following types of information: L
Name that describes the pattern
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Problem to be solved by the pattern
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Context, or settings, in which the problem occurs
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Forces that could influence the problem or its solution
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Solution proposed to the problem
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Rationale behind the solution (examples and stories of past successes or failures often go here)
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Known uses and related patterns
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Author and date information
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References and keywords used or searching
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Sample code related to the solution, if it helps
The usage of patterns also goes beyond the area of software design and development, and can be applied to any field where solutions have been derived from the experience of several projects or events.
payload On the Internet, a payload is either: 1) The essential data that is being carried within a packet or other transmission unit. The payload does not include the ‘‘overhead’’ data required to get the packet to its destination. Note that what constitutes the payload may depend on the point-of-view. To a communications layer that needs some of the overhead data to do its job, the payload is sometimes considered to include the part of the overhead data that this layer handles. However, in more general usage, the payload is the bits that get delivered to the end user at the destination. 2) The eventual effect of a software virus that has been delivered to a user’s computer.
pay-per-view See ‘‘micropayment’’
PBX See ‘‘private branch exchange’’
PC 1) In its more general usage, a PC (personal computer) is a computer designed for use by one person at a time. Prior to the PC, computers were designed for (and only affordable by) companies who attached terminals for multiple users to a single large computer whose resources were shared among all users. Beginning in the late 1980s, technology advances made it feasible to build a small computer that an individual could own and use. 2) The term ‘‘PC’’ is also commonly used to describe an ‘‘IBM-compatible’’ personal computer in contradistinction to an Apple Macintosh computer. The distinction is both technical and cultural. The ‘‘IBM-compatible’’ PC is one with an Intel microprocessor architecture and an operating system such as DOS or Windows that is written to use the Intel microprocessor. The Apple Macintosh uses a Motorola microprocessor architecture and a proprietary operating system. The ‘‘IBM-compatible’’ PC is associated with business (as well as home) use. The ‘‘Mac,’’ known for its more intuitive user interface, is associated with graphic design and desktop publishing.
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A PC Card (previously known as a PCMCIA card) is a credit card-size memory or I/O device that fits into a personal computer, usually a notebook or laptop computer. Probably the most common example of a PC Card is the 28.8 Kbps modem for notebook computers. There are 16-binary digit and 32-bit (CardBus) varieties of PC Cards. Another type of PC card is the ZV port Card. The PC Card is based on standards published by the Personal Computer Memory Card International Association (PCMCIA), an industry group organized in 1989 to promote standards for both memory and I/O integrated circuit card. The PCMCIA 2.1 Standard was published in 1993. As a result, PC users can be assured of standard attachments for any peripheral device that follows the standard. A PC Card has a 68-pin connector that connects into a slot in the PC. There are three sizes (or ‘‘types’’) of PC Cards: Type Thickness (mm) Typical use I II III
3.3 5.0 10.5
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The Type I and II cards work in a Type III slot and a Type I card will work in a Type II slot. (On the other hand, the thicker cards can’t be fitted into the slots for the thinner cards.) The PCMCIA standard is most commonly applied to portable PCs but it can also be used on desktop computers. The PC Card is not to be confused with another credit-size electronic card, the smart card.
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PC100 Synchronous Dynamic Random Access Memory PC100 SDRAM is synchronous DRAM (dynamic random access memory) that states that it meets the PC100 specification from Intel. Intel created the specification to enable RAM manufacturers to make chips that would work with Intel’s i440BX processor chipset. The i440BX was designed to achieve a 100 MHz system bus speed. Ideally, PC100 SDRAM would work at the 100 MHz speed, using a 4-1-1-1 access cycle. It’s reported that PC100 SDRAM will improve performance by 10-15% in an Intel Socket 7 system (but not in a Pentium II because its L2 cache speed runs at only half of processor speed).
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PC-DOS (Personal Computer—Disk Operating System) was the first widely-installed operating system used in personal computers. It was developed for IBM by Bill Gates and his fledgling Microsoft Corporation for installation in IBM’s first lines of PCs. Gates marketed an almost identical version of the operating system called MS-DOS (Microsoft—Disk Operating System). Most users of either DOS system have usually referred to their system as just DOS. Like MS-DOS, PC-DOS was (and still is) a non-graphical line-oriented command-driven operating system, with a relatively simple interface but not overly ‘‘friendly’’ user interface. Its prompt to enter a command looks like this: C:>
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The first Microsoft Windows operating system was really an application that ran on top of the MS-DOS operating system. Today, Windows operating systems continue to support DOS (or a DOS-like user interface) for special purposes by emulating the operating system. In the 1970s before the personal computer was invented, IBM had a different and unrelated DOS (Disk Operating System) that ran on smaller business computers. It was replaced by IBM’s VSE operating system.
PCI See ‘‘Peripheral Component Interconnect’’
PCI-X PCI-X (Peripheral Component Interconnect Extended) is a new computer bus technology (the ‘‘data pipes’’ between parts of a computer) that increases the speed that data can move within a computer from 66 MHz to 133 MHz. The technology was developed jointly by IBM, HP and Compaq. PCI-X doubles the speed and amount of data exchanged between the computer processor and peripherals. With the current PCI design, one 64-bit bus runs at 66 MHz and additional buses move 32 bits at 66 MHz or 64 bits at 33 MHz. The maximum amount of data exchanged between the processor and peripherals using the current PCI design is 532 MB per second. With PCI-X, one 64-bit bus runs at 133 MHz with the rest running at 66 MHz, allowing for a data exchange of 1.06 GB per second. PCI-X is backwardscompatible, meaning that you can, for example, install a PCIX card in a standard PCI slot but expect a decrease in speed to 33 MHz. You can also use both PCI and PCI-X cards on the same bus but the bus speed will run at the speed of the slowest card. PCI-X is more fault tolerant than PCI. For example, PCI-X is able to reinitialize a faulty card or take it offline before computer failure occurs. IBM, HP and Compaq designed PCI-X for servers to increase performance for high bandwidth devices such as Gigabit Ethernet cards, Fibre Channel, Ultra3 Small Computer
System Interface, and processors that are interconnected as a cluster. Compaq, IBM, and HP submitted PCI-X to the PCI Special Interest Group (Special Interest Group of the Association for Computing Machinery) in 1998. PCI SIG approved PCI-X, and it is now an open standard that can be adapted and used by all computer developers. PCI SIG controls technical support, training and compliance testing for PCI-X. IBM, Intel, Microelectronics and Mylex plan to develop chipsets to support PCI-X. 3Com and Adaptec intend to develop PCI-X peripherals. To accelerate PCI-X adoption by the industry, Compaq offers PCI-X development tools at their Web site. The site includes a PCI-X Core Source Code, a Reference Design Guide, interface documents, PCI-X Architectural and Partitioning guidelines, and PCI-to PCI-X Enhancement guidelines. Compaq is also working with Synopsis, a leading supplier of electronic design automation solutions, to offer a structured PCI-X test environment.
PCL See ‘‘Printer Control Language’’
PCM See ‘‘pulse code modulation’’
PCMCIA card See ‘‘PC Card’’
PCMCIA See ‘‘Personal Computer Memory Card International Association’’
PCS See ‘‘personal communications services’’
PDA See ‘‘personal digital assistant’’
PDF See ‘‘Portable Document Format’’
peak Also see peak-to-peak. Peak (pk) is the maximum value, either positive (pk+) or negative (pk-), that a waveform attains. Peak values can be expressed for voltage (the usual case), current, or power. Alternating current (AC) waveforms reach peaks in two directions of polarity or current flow. These peaks are usually of equal amplitude and opposite polarity. However,
peer-to-peer if a direct current (DC) component exists, pk+ differs from pk-. An example of an AC wave with a DC component, showing the pk+ and pk- values, is shown below. For an AC sine wave without a DC component, the peak amplitude is equal to approximately 1.414 times the rootmean-square amplitude. Amplitude
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Time pk–
peak-to-peak Peak-to-peak (pk-pk) is the difference between the maximum positive and the maximum negative amplitudes of a waveform, as shown below. If there is no direct current (DC) component in an alternating current (AC) wave, then the pk-pk amplitude is twice the peak amplitude. Amplitude
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For an AC sine wave with no DC component, the peak-topeak amplitude is equal to approximately 2.828 times the root-mean-square amplitude. Peak-to-peak values can be expressed for voltage (the usual case), current, or power.
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peering
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Peering is the arrangement of traffic exchange between Internet service providers (ISPs). Larger ISPs with their own backbone networks agree to allow traffic from other large ISPs in exchange for traffic on their backbones. They also exchange traffic with smaller ISPs so that they can reach regional end points. Essentially, this is how a number of individual network owners put the Internet together. To do this, network owners and access providers, the ISPs, work out agreements that describe the terms and conditions to which both are subject. Bilateral peering is an agreement between two parties. Multilateral peering is an agreement between more than two parties. Peering requires the exchange and updating of router information between the peered ISPs, typically using the Border Gateway Protocol (BGP). Peering parties interconnect at network focal points such as the network access points (NAP) in the United States and at regional switching points. Initially, peering arrangements did not include an exchange of money. More recently, however, some larger ISPs have charged smaller ISPs for peering. Each major ISP generally develops a peering policy that states the terms and conditions under which it will peer with other networks for various types of traffic. Private peering is peering between parties that are bypassing part of the public backbone network through which most Internet traffic passes. In a regional area, some ISPs exchange local peering arrangements instead of or in addition to peering with a backbone ISP. In some cases, peering charges include transit charges, or the actual line access charge to the larger network. Properly speaking, peering is simply the agreement to interconnect and exchange routing information.
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1) Peer-to-peer is a communications model in which each party has the same capabilities and either party can initiate a communication session. Other models with which it might be contrasted include the client/server model and the master/ slave model. In some cases, peer-to-peer communications is implemented by giving each communication node both server and client capabilities. In recent usage, peer-to-peer has come to describe applications in which users can use the Internet to exchange files with each other directly or through a mediating server. IBM’s Advanced Peer-to-Peer Networking (APPN) is an example of a product that supports the peer-to-peer communication model.
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2) On the Internet, peer-to-peer (referred to as P2P) is a type of transient Internet network that allows a group of computer users with the same networking program to connect with each other and directly access files from one another’s hard drives. Napster and Gnutella are examples of this kind of peer-to-peer software. Corporations are looking at the advantages of using P2P as a way for employees to share files without the expense involved in maintaining a centralized server and as a way for businesses to exchange information with each other directly.
motion video and 3-D images. Among the Pentium II’s features are a 512 KB (kilobyte) level-two (L1 and L2) memory cache and a 32 KB L1 and L2 cache, twice that of the Pentium Pro processor. The L2 cache can include error correcting code (ECC).
How Does Internet P2P Work?
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The user must first download and execute a peer-to-peer networking program. (Gnutellanet is currently one of the most popular of these decentralized P2P programs because it allows users to exchange all types of files.) After launching the program, the user enters the IP address of another computer belonging to the network. (Typically, the Web page where the user got the download will list several IP addresses as places to begin). Once the computer finds another network member on-line, it will connect to that user’s connection (who has gotten their IP address from another user’s connection and so on).
The Pentium III is a microprocessor designed by Intel as a successor to its Pentium II. The Pentium III is faster, especially for applications written to take advantage of its ‘‘Katmai New Instructions’’ (the code name for the Pentium III during development was ‘‘Katmai’’). The 70 new computer instructions make it possible to run 3-D, imaging, streaming video, speech recognition, and audio applications more quickly. In addition, the Pentium III offers clock speeds up to 800 MHz.
Users can choose how many member connections to seek at one time and determine which files they wish to share or password protect. Generally, the P2P network consists of no more than four users at any given time because each additional member slows down the transmission of data over the Internet.
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Pentium Also see the Pentium MMX and the Pentium 3.
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The Pentium is a widely-used personal computer microprocessor from the Intel Corporation. First offered in 1993, the Pentium quickly replaced Intel’s 486 microprocessor as the microchip-of-choice in manufacturing a personal computer. The original Pentium model includes two processors on one chip that contains 3.1 million transistors. The Pentium Pro, released in 1995, was designed for PC servers and workstation that needed to serve multiple users or needed the speed required for graphics-intensive applications. In addition to the microprocessor, the Pentium Pro includes another microchip containing cache memory that, being closer to the processor than the computer’s main memory (RAM), speeds up computer operation. The Pentium Pro contains 5.5 million transistors. The Pentium II is a Pentium Pro with Intel’s MMX technology included. It comes in microprocessor clock speeds of 233 MHz (millions of cycles per second), 266 MHz, and 300 MHz. It’s suitable for applications that include
The latest Pentium II’s are Celeron, a low-end Pentium without the L2 cache, and Xeon, a high-end Pentium that replaces the Pentium Pro for enterprise server and workstation computers.
The Katmai New Instructions are similar to the instructions optimized for multimedia applications called MMX and now included in most Pentiums. However, unlike the MMX instruction set, the Katmai instructions support floating point units as well as integer calculations, a type of calculation often required when still or video images are modified for display. The Katmai instructions also support Single Instruction Multiple Data instructions. These allow a single instruction to cause data to be modified in multiple memory locations simultaneously, a kind of parallel processing. For 3-D applications, changing values in parallel for a given 3-D scene means that users can see smoother and more realistic effects. Application developers can create effects that the slower instructions could not support, such as scenes with subtle and complex lighting. Animated effects and streaming video should also be less choppy for the viewer. The new instructions also specifically include some that will make speech recognition faster and more accurate and allow the creation of more complex audio effects.
Pentium 4 Pentium 4 (P4) is the latest Intel processor (codenamed Willamette), released in November 2000. The P4 processor has a viable clock speed of 1.5 gigahertz (GHz)—as compared to the 1 GHz of the Pentium 3—and reached 2 GHz in August 2001. P4 has the first totally new chip architecture since the 1995 Pentium Pro. The major difference involves structural changes that affect the way processing takes place within the chip, something Intel calls NetBurst microarchitecture. Aspects of the changes include: A 20-stage pipeline, which boosts performance by increasing processor frequency; a rapidexecution engine, which doubles the core frequency and
perigee reduces latency by enabling each instruction to be executed in a half (rather than a whole) clock cycle; a 400 MHz system bus, which enables transfer rates of 3.2 gigabytes per second (GBps); an execution trace cache, which optimizes cache memory efficiency and reduces latency by storing decoded sequences of micro-operations; and improved floating point and multimedia unit and advanced dynamic execution, all of which enable faster processing for especially demanding applications, such as digital video, voice recognition, and online gaming. P4’s main competition for processor market share is the AMD Athlon processor. A number of industry test results (such as those from PC Magazine Labs and Tom’s Hardware Guide) have found the P4 was faster than the Athlon for streaming video, 3-D graphics programs, and some similarly demanding applications, but that the Athlon processor—which is significantly less expensive—was faster for the types of tasks that make up the bulk of the typical home or business PC’s workload.
Peoplesoft PeopleSoft is a leading provider of e-business application software and claims to be the only software company to provide e-business solutions purely over the Internet for Fortune 1000 corporations. The company was founded in 1987 by Dave Duffield and Ken Morris, whose goal was to build client/server applications that empower the user, are easily adaptable in a changing marketplace, and are supported by superior customer service.
performance
Based in Pleasanton, California, PeopleSoft in 2001 employed more than 7,000 people worldwide and had over 4,000 customers. PeopleSoft (PSFT) is publicly traded on the NASDAQ.
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Performance seems to have two meanings: 1) The speed at which a computer operates, either theoretically (for example, using a formula for calculating Mtops—millions of theoretical instructions per second) or by counting operations or instructions performed (for example, (MIPS)—millions of instructions per second) during a benchmark test. The benchmark test usually involves some combination of work that attempts to imitate the kinds of work the computer does during actual use. Sometimes performance is expressed for each of several different benchmarks. 2) The total effectiveness of a computer system, including throughput, individual response time, and availability.
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When a satellite follows a non-circular orbit around the earth, the satellite’s path is an ellipse with the center of the earth at one focus. Such a satellite has variable altitude and variable orbital speed. The point of lowest altitude is called perigee. The term also applies to the minimum distance in kilometers or miles between the satellite and the center of the earth. (Perigee can be measured between the satellite and the earth’s surface, although this is a less precise specification because the earth is not a perfect sphere. The difference is approximately 4,000 miles or 6,400 kilometers.) Orbit of satellite
PeopleSoft originally offered human resources and finance applications. Over the years, it has developed tools and applications for general business processes such as materials management and e-business in addition to applications for specific industries, such as the automotive, communications, and higher-education fields. In 1999, the company shifted its focus to the Internet. In 2000, the company launched PeopleSoft8, the first pure Internet software, as well as PeopleSoft e-center, its in-house application service provider (ASP). Its Web-based applications are intended to integrate systems easily so that a company can connect customers, employees, and suppliers more cost-effectively. An organization can streamline operations due to the fact that the information is readily accessible by a wide-variety of people anytime, anywhere, which includes mobile equipment such as personal digital assistants (PDAs) and mobile phones.
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At perigee, a satellite travels faster than at any other point in its orbit. When viewed from the earth’s surface, a satellite at or near perigee traverses the sky at a rapid pace. In communications, perigee is the least desirable time to access a satellite. Although its proximity means that the signal path is short, the fact that the satellite is rapidly moving means that it is accessible for only a brief time. In addition, if a directional antenna is used at a ground-based station, it is
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Compare apogee.
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A peripheral (pronounced peh-RIHF-uh-ruhl, a noun truncation of peripheral device) is any computer device that is not part of the essential computer (the processor, memory, and data paths) but is situated relatively close by. A near synonym is input/output (I/O) device. Some peripherals are mounted in the same case with the main part of the computer as are the hard disk drive, CD-ROM drive, and NIC. Other peripherals are outside the computer case, such as the printer and image scanner, attached by a wired or wireless connection.
Peripheral Component Interconnect Extended See ‘‘PCI-X’’
Peripheral Component Interconnect PCI is an interconnection system between a microprocessor and attached devices in which expansion slot are spaced closely for high speed operation. Using PCI, a computer can support both new PCI cards while continuing to support Industry Standard Architecture (ISA) expansion cards, currently the most common kind of expansion card. Designed by Intel, the original PCI was similar to the VESA Local Bus. However, PCI 2.0 is no longer a local bus and is designed to be independent of microprocessor design. PCI is designed to be synchronized with the clock speed of the microprocessor, in the range of 20 to 33 MHz. PCI is now installed on most new desktop computers, not only those based on Intel’s Pentium processor but also those based on the PowerPC. PCI transmits 32 bits at a time in a 124-pin connection (the extra pins are for power supply and grounding) and 64 bits in a 188-pin connection in an expanded implementation. PCI uses all active paths to transmit both address and data signals, sending the address on one clock cycle and data on the next. Burst data can be sent starting with an address on the first cycle and a sequence of data transmissions on a certain number of successive cycles. Also see Extended Industry Standard Architecture (EISA) and Micro Channel Architecture (MCA).
Perl Perl (Practical Extraction and Reporting Language) is a script programming language that is similar in syntax to the C language and that includes a number of popular UNIX facilities such as sed, awk, and tr. Perl is an interpreted language that can optionally be compiled just before execution into either C code or cross-platform bytecode. When compiled, a Perl program is almost (but not quite) as fast as a fully precompiled C language program. Perl is regarded as a good choice for developing common gateway interface (CGI) programs because it has good text manipulation facilities (although it also handles binary files). It was invented by Larry Wall. In general, Perl is easier to learn and faster to code in than the more structured C and C++ languages. Perl programs can, however, be quite sophisticated. Perl tends to have devoted adherents. Plug-ins can be installed for some servers (Apache, for example) so that Perl is loaded permanently in memory, thus reducing compile time and resulting in faster execution of CGI Perl scripts.
permanent virtual circuit A permanent virtual circuit (PVC) is a software-defined logical connection in a network such as a frame relay network. A feature of frame relay that makes it a highly flexible network technology is that users (companies or clients of network providers) can define logical connections and required bandwidth between end points and let the frame relay network technology worry about how the physical network is used to achieve the defined connections and manage the traffic. In frame relay, the end points and a stated bandwidth called a Committed Information Rate (CIR) constitute a PVC, which is defined to the frame relay network devices. The bandwidth may not exceed the possible physical bandwidth. Typically, multiple PVCs share the same physical paths at the same time. To manage the variation in bandwidth requirements expressed in the CIRs, the frame relay devices use a technique called statistical multiplexing.
permeability Permeability, also called magnetic permeability, is a constant of proportionality that exists between magnetic induction and magnetic field intensity. This constant is equal to approximately 1.257 6 10-6 henry per meter (H/m) in free space (a vacuum). In other materials it can be much different, often substantially greater than the free-space value, which is symbolized mo. Materials that cause the lines of flux to move farther apart, resulting in a decrease in magnetic flux density compared with a vacuum, are called diamagnetic. Materials that concentrate magnetic flux by a factor of more than 1 but less
personal area network than or equal to 10 are called paramagnetic; materials that concentrate the flux by a factor of more than 10 are called ferromagnetic. The permeability factors of some substances change with rising or falling temperature, or with the intensity of the applied magnetic field. In engineering applications, permeability is often expressed in relative, rather than in absolute, terms. If mo represents the permeability of free space (that is, 1.257 6 10-6 H/m) and m represents the permeability of the substance in question (also specified in henrys per meter), then the relative permeability, mr, is given by: mr = m / mo = m (7.958 6 105) Diamagnetic materials have mr less than 1, but no known substance has relative permeability much less than 1. Certain ferromagnetics, especially powdered or laminated iron, steel, or nickel alloys, have mr that can range up to about 1,000,000. When a paramagnetic or ferromagnetic core is inserted into a coil, the inductance is multiplied by mr compared with the inductance of the same coil with an air core. This effect is useful in the design of transformers and chokes for alternating current (AC), audio frequencies (AF), and radio frequencies (RF). Also see henry per meter, inductor, and magnetic field.
permission marketing Permission marketing is an approach to selling goods and services in which a prospect explicitly agrees in advance to receive marketing information. Opt-in e-mail, where Internet users sign up in advance for information about certain product categories, is a good example of permission marketing. Advocates of permission marketing argue that it is effective because the prospect is more receptive to a message that has been requested in advance and more costefficient because the prospect is already identified and targetted. In a world of information overload, automated telemarketing, and spam, most people welcome the idea of permission marketing.
permittivity Permittivity, also called electric permittivity, is a constant of proportionality that exists between electric displacement and electric field intensity. This constant is equal to approximately 8.85 6 10-12 farad per meter (F/m) in free space (a vacuum). In other materials it can be much different, often substantially greater than the free-space value, which is symbolized eo. In engineering applications, permittivity is often expressed in relative, rather than in absolute, terms. If eo represents the permittivity of free space (that is, 8.85 6 10-12 F/m) and e represents the permittivity of the substance in question (also specified in farads per meter), then the relative permittivity, also called the dielectric constant er, is given by:
er = e / eo = e (1.13 6 1011)
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Various substances have dielectric constants er greater than 1. These substances are generally called dielectric materials, or simply dielectrics. Commonly used dielectrics include glass, paper, mica, various ceramics, polyethylene, and certain metal oxides. Dielectrics are used in capacitors and transmission lines in alternating current (AC), audio frequency (AF), and radio frequency (RF) applications.
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Personal Access Communications System Personal Access Communications System (PACS) is a type of wireless telephone network compatible with telephone sets, answering machines, fax machines, and computers. A PACS can be used like a local area network (LAN) with voice capability and can be part of a larger network or can be connected into the telephone system. A typical PACS resembles a cellular telephone network in miniature. It contains numerous radio port control units (RCPUs), each of which is the equivalent of a cellular repeater, but with a shorter communications range, linking subscriber sets within a radius of a few hundred feet. RPCUs are located on utility poles, atop buildings, and in other unobtrusive places that offer good coverage for several hundred feet in all directions. RPCU transmitter power is limited to 800 milliwatts. The operating frequency is in the UHF (ultra-high-frequency) radio range at 1.9 GHz. The subscriber sets in a PACS can be fixed, mobile, or portable. Voice subscriber sets use 32 Kbps or 64 Kbps digital speech coding. Computer modems can be supported at speeds of up to 28.8 kbps or 57.6 kbps, respectively. Transmitter output power is limited to 200 milliwatts, but is often much less, on the order of a few tens of milliwatts. This low power level minimizes the likelihood of electromagnetic interference (EMI) to other electronic devices that might be located near the subscriber set.
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personal area network A personal area network (PAN) is a technology that could enable wearable computer devices to communicate with other nearby computers and exchange digital information using the electrical conductivity of the human body as a data network. For example, two people each wearing business card-size transmitters and receivers conceivably could exchange information by shaking hands. The transference of data through intra-body contact, such as handshakes, is known as linkup. The human body’s natural salinity makes it a good conductor of electricity. An electric field passes tiny currents, known as Pico amps, through the body when the two people shake hands. The handshake completes an electric circuit and each person’s data, such as e-mail addresses and phone numbers, are transferred to the other
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person’s laptop computer or a similar device. A person’s clothing also could act as a mechanism for transferring this data.
Personal Computer Memory Card International Association card
The concept of a PAN first was developed by Thomas Zimmerman and other researchers at M.I.T.’s Media Lab and later supported by IBM’s Almaden research lab. In a research paper, Zimmerman explains why the concept might be useful:
See ‘‘PCMCIA card’’
As electronic devices become smaller, lower in power requirements, and less expensive, we have begun to adorn our bodies with personal information and communication appliances. Such devices include cellular phones, personal digital assistants (PDAs), pocket video games, and pagers. Currently there is no method for these devices to share data. Networking these devices can reduce functional I/O redundancies and allow new conveniences and services.
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PCS (personal communications services) is a wireless phone service somewhat similar to cellular telephone telephone service but emphasizing personal service and extended mobility. It’s sometimes referred to as digital cellular (although cellular systems can also be digital). Like cellular, PCS is for mobile users and requires a number of antennas to blanket an area of coverage. As a user moves around, the user’s phone signal is picked up by the nearest antenna and then forwarded to a base station that connects to the wired network. The phone itself is slightly smaller than a cellular phone. PCS is being introduced first in highly urban areas for large numbers of users.
PDA (personal digital assistant) is a term for any small mobile hand-held device that provides computing and information storage and retrieval capabilities for personal or business use, often for keeping schedule calendars and address book information handy. The term handheld is a synonym. Many people use the name of one of the popular PDA products as a generic term. These include HewlettPackard’s Palmtop and 3Com’s PalmPilot.
The ‘‘personal’’ in PCS distinguishes this service from cellular by emphasizing that, unlike cellular, which was designed for car phone use with transmitters emphazing coverage of highways and roads, PCS is designed for greater user mobility. It generally requires more cell transmitters for coverage, but has the advantage of fewer blind spots. Technically, cellular systems in the U.S. operate in the 824849 megahertz (MHz) frequency bands; PCS operates in the1850-1990 MHz bands. Several technologies are used for PCS in the U.S., including Cellular Digital Packet Data (CDPD) and Global System for Mobile (GSM) communication. GSM is more commonly used in Europe and elsewhere.
personal computer See ‘‘PC’’
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The PCMCIA (Personal Computer Memory Card International Association) is an industry group organized in 1989 to promote standards for a credit card-size memory or I/O device that would fit into a personal computer, usually a notebook or laptop computer. The PCMCIA 2.1 Standard was published in 1993. As a result, PC users can be assured of standard attachments for any peripheral device that follows the standard. The initial standard and its subsequent releases describe a standard product, the PC Card.
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Most PDAs have a small keyboard. Some PDAs have an electronically sensitive pad on which handwritng can be received. Apple’s Newton, which has been withdrawn from the market, was the first widely-sold PDA that accepted handwriting. Typical uses include schedule and address book storage and retrieval and note-entering. However, many applications have been written for PDAs. Increasingly, PDAs are combined with telephones and paging systems. Some PDAs offer a variation of the Microsoft Windows operating system called Windows CE. Other products have their own or another operating system.
personal firewall A personal firewall (sometimes called a desktop firewall) is a software application used to protect a single Internetconnected computer from intruders. Personal firewall protection is especially useful for users with ‘‘always-on’’ connections such as DSL or cable modem. Such connections use a static IP address that makes them especially vulnerable to potential hackers. Often compared to anti-virus applications, personal firewalls work in the background at the device (link layer) level to protect the integrity of the system from malicious computer code by controlling Internet connections to and from a user’s computer, filtering inbound and outbound traffic, and alerting the user to attempted intrusions.
personalization Several companies have announced plans to develop personal firewall solutions that will go right in the chips used in DSL and cable modems. It is generally believed that personal firewall protection will become standard issue for new home computers in the not-too-distant future.
links; to create directories; and to set permissions. PWS is frequently used as part of the trend towardpeer-to-peer exchange and publishing.
Personal Handyphone System
personality profile
Developed by the Nippon Telegraph and Telephone Corporation, the Personal Handyphone is a lightweight portable wireless telephone that functions as a cordless phone in the home and as a mobile phone elsewhere. The Personal Handyphone also handles voice, fax, and video signals. The phone is now being marketed in other Asian countries.
A personality profile is a knowledge management tool used to provide an evaluation of an employee’s personal attributes, values and life skills in an effort to maximize his or her job performance and contribution to the company. Questions in a personality profile test, which can be taken traditionally or online, are designed to seek out information about an employee’s temperament, decision-making methods, communication style and general attitude towards work and recreation. The information is used to match the right employee to the right project or task, especially when group work or telecommuting is involved. There are two generally accepted categories of personality profile tests, trait and type.
Personal Home Page In Web programming, Personal Home Page (PHP) is a script language and interpreter, similar to JavaScript and Microsoft’s VBScript, that is freely available and used primarily on Linux Web servers. PHP (the initials come from the earliest version of the program, which was called ‘‘Personal Home Page Tools’’) is a cross-platform alternative to Microsoft’s Active Server Page (ASP) technology (which runs only on Microsoft’s Windows NT servers). As with ASP, the PHP script is embedded within a Web page along with its HTML. Before the page is sent to a user that has requested it, the Web server calls PHP to interpret and perform the operations called for in the PHP script. An HTML page that includes a PHP script is typically given a file name suffix of ‘‘.php’’ ‘‘.php3,’’ or ‘‘.phtml’’. Like ASP, PHP can be thought of as ‘‘dynamic HTML pages,’’ since content will vary based on the results of interpreting the script. PHP is free and offered under an Open Source license.
personal portal See ‘‘bookmark portal’’
Personal Web Server PWS (Personal Web Server) is Microsoft’s version of a Web server program for individual PC users who want to share Web pages and other files from their hard drive. PWS is a scaled-down version of Microsoft’s more robust Web server, Internet Information ServerIIS. PWS can be used with a fulltime Internet connection to serve Web pages for a Web site with limited traffic. It can also be used for testing a Web site offline or from a ‘‘staging’’ site before putting it on a main Web site that is exposed to larger traffic. PWS can be used together with Microsoft’s FrontPage, a Web site design product, to upload Web pages from a remote location or to the local hard drive; to check for dead
The equivalent program for the Macintosh is called Personal Web Sharing.
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Trait personality profile tests, such as Orpheus, 16 PF, and OPQ, operate on the assumption that personality is made up of a number of characteristics. The goal of the test is to document the employee’s characteristics and match the characteristics to appropriate roles within the company. Type personality profile tests, such as Myers-Briggs, Insights Discovery, and the Keirsey Temperament Sorter, propose that people fall into well-defined categories. The goal of the test is to identify the category the employee belongs to, share the information, and build team skills by having team members become aware of the talents associated with each category. Advocates of personality profiling claim that it’s a valuable reality check when viewed in the context of an employee’s job performance. Critics claim that the advent of sophisticated knowledge management technology could put too much emphasis on the process involved in gathering and mining employee data, especially in large companies, and recommend that face-to-face communication and evaluation be valued above all else.
personalization On a Web site, personalization is the process of tailoring pages to individual users’ characteristics or preferences. Commonly used to enhance customer service or e-commerce sales, personalization is sometimes referred to as one-to-one marketing, because the enterprise’s Web page is tailored to specifically target each individual consumer. Personalization is a means of meeting the customer’s needs more effectively and efficiently, making interactions faster and easier and, consequently, increasing customer satisfaction and the likelihood of repeat visits. There are a number of personalization software products available, including those from Broadvision, ResponseLogic, and Autonomy.
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Personalization in some ways harkens back to an earlier day, by making consumer relationships more closely tailored to the individual. If you’ve ever bought a book from Amazon, for example, the next time you visit they will—like a friendly and helpful sales clerk—greet you by name and tell you about products in stock that they think you might like (such as more books by the same author, or books purchased by other people who also bought the book that you purchased). Many portal sites, such as Yahoo allow site visitors to customize the page with selected news categories, local weather reports, and other features. In addition to use of the cookie, the technologies behind personalization include: L
Ccollaborative filtering, in which a filter is applied to information from different sites to select relevant data that may apply to the specific e-commerce experience of a customer or specific group of customers
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Data analysis tools used to predict likely future interactions
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Because personalization depends on the gathering and use of personal user information, privacy issues are a major concern. The Personalization Consortium is an international advocacy group organized to promote and guide the development of responsible one-to-one marketing practices. Founding members include Pricewaterhouse Coopers, American Airlines, and DoubleClick. The consortium has established ethical information and privacy management objectives; these include, for example, the suggestion that enterprises should inform users about the information being gathered, and the purposes for which it is sought. According to a March 2000 Consortium survey of over 4,500 Web users, 73% of respondents find it helpful to have Web sites retain their personal information, while only 15% refuse to supply personal information online. 63% of respondents disliked having to reenter information that they had already supplied.
PERT chart A PERT chart is a project management tool used to schedule, organize, and coordinate tasks within a project. PERT stands for Program Evaluation Review Technique, a methodology developed by the U.S. Navy in the 1950s to manage the Polaris submarine missile program. A similar methodology, the Critical Path Method (CPM), which was developed for project management in the private sector at about the same time, has become synonymous with PERT, so that the technique is known by any variation on the names: PERT, CPM, or PERT/CPM.
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A PERT chart presents a graphic illustration of a project as a network diagram consisting of numbered nodes (either circles or rectangles) representing events, or milestones in the project linked by labelled vectors (directional lines) representing tasks in the project. The direction of the arrows on the lines indicates the sequence of tasks. In the diagram, for example, the tasks between nodes 1, 2, 4, 8, and 10 must be completed in sequence. These are called dependent or serial tasks. The tasks between nodes 1 and 2, and nodes 1 and 3 are not dependent on the completion of one to start the other and can be undertaken simultaneously. These tasks are called parallel or concurrent tasks. Tasks that must be completed in sequence but that don’t require resources or completion time are considered to have event dependency. These are represented by dotted lines with arrows and are called dummy activities. For example, the dashed arrow linking nodes 6 and 9 indicates that the system files must be converted before the user test can take place, but that the resources and time required to prepare for the user test (writing the user manual and user training) are on another path. Numbers on the opposite sides of the vectors indicate the time allotted for the task. The PERT chart is sometimes preferred over the Gantt chart, another popular project management charting method, because it clearly illustrates task dependencies. On the other hand, the PERT chart can be much more difficult to interpret, especially on complex projects. Frequently, project managers use both techniques.
PERT/CPM See ‘‘PERT chart’’
phantom dialing
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A petabit is one quadrillion (10 ) bits and is used in discussing possible volumes of data traffic per second in a large telecommunications network. A petabit is one thousand terabits. Petabits per second can be shortened to Pbps. Although the bit is a unit of the binary number system, bits in data communications have historically been counted using the decimal number system. For example, 28.8 kilobits per second (Kbps) is 28,800 bits per second. Because of computer architecture and memory address boundaries, bytes are always some multiple or exponent of two. See kilobyte, etc.
petabyte A petabyte is a measure of memory or storage capacity and is 2 to the 50th power bytes or, in decimal, approximately a thousand terabytes. In recently announcing how many Fibre Channel storage arrays they had sold, Sun Microsystems stated that it had shipped an aggregate of two petabytes of storage or the equivalent of 40 million four-drawer filing cabinets full of text. IBM says that it has shipped four petabytes of SSA Storage. Also see petaflop.
petaflop Computer architects have begun to envision how a petaflop computer might work. A petaflop is a theoretical measure of a computer’s speed and can be expressed as: L
A thousand trillion floating point operations per second
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Today’s fastest parallel computing operations are capable of teraflop speeds. The National Science Foundation, together with NASA and DARPA, has funded eight research projects for envisioning a petaflop computer. A petaflop computer would actually require a massive number of computers working in parallel on the same problem. Applications might include real-time nuclear magnetic resonance imaging during surgery, computer-based drug design, astrophysical simulation, the modeling of environmental pollution, and the study of long-term climate changes.
PGP See ‘‘Pretty Good Privacy’’
Ph is an Internet facility that lets you search for someone’s email address if their e-mail provider has a Ph server program. A Ph client program comes with Eudora, a popular e-mail program (click on ‘‘Tools,’’ then ‘‘Directory Services’’), as well as with other programs. To use Ph, you need to know at what university or other organization the person you’re looking for is located. A large number of universities and research institutions maintain a directory of e-mail users. In addition, several large Web portal sites, including AltaVista and Bigfoot, have Ph directories. To search using Ph, you need to enter the server name for the location you’re searching at and then the person’s name. A number of places provide lists of servers. Several Web sites provide a forms interface that includes the server list in order to make searching easy and quick. Some places use the Lightweight Directory Access Protocol (LDAP) and convert a Ph query to LDAP and return the result in Ph. Ph is somewhat similar to another Internet facility, Finger, which lets you search for a name if you know the e-mail address. Ph is sometimes referred to as Qi or Ph/Qi. A Ph response may include other information besides an e-mail address.
phantom dialing 1) On a computer using a dial-up connection, phantom (meaning ghost) dialing is a term used to describe what occurs when a computer’s auto-connect feature has been enabled and the computer attempts to dial out and establish an Internet connection on its own. 2) In mobile wireless communication, phantom dialing is a term used to describe what occurs when a user unintentionally presses a pre-programmed auto-dial number on their cellular telephone keypad and unintentionally initiates a phone call. In the U.S., phantom dialing is a problem for 911 emergency centers, especially since many cell phones are configured to dial 911 (the emergency center) automatically when either a ‘‘9’’ or a ‘‘1’’ is pressed. When emergency services receives a phone call, the operator must, by law, remain on the phone long enough to determine whether or not the call is an emergency. If the operator listens and determines that the call is probably a result of phantom dialing, they may terminate the call, but must dial back the caller and verbally confirm that there is no emergency. Operators across the U.S. report thousands of such calls daily, and say that phantom dialers are almost always unaware they have made a call. Users report that they may have dropped the phone, sat on it, put it in their pocket, or otherwise jostled it, hitting auto-dial and inadvertently initiating the call. Phantom dialing can be prevented by using the cell phone’s keyguard, a feature that locks the keypad, or by disabling the auto-dial feature.
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In electronic signaling, phase is a definition of the position of a point in time (instant) on a waveform cycle. A complete cycle is defined as 360 degrees of phase as shown in Illustration A below. Phase can also be an expression of relative displacement between or among waves having the same frequency.
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Phase difference, also called phase angle, in degrees is conventionally defined as a number greater than 7180, and less than or equal to +180. Leading phase refers to a wave that occurs ‘‘ahead’’ of another wave of the same frequency. Lagging phase refers to a wave that occurs ‘‘behind’’ another wave of the same frequency. When two signals differ in phase by 790 or +90 degrees, they are said to be in phase quadrature. When two waves differ in phase by 180 degrees (7180 is technically the same as +180), the waves are said to be in phase opposition. Illustration B shows two waves that are in phase quadrature. The wave depicted by the dashed line leads the wave represented by the solid line by 90 degrees.
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tdeg = 1/(360f) The time trad (in seconds) corresponding to one radian of phase is approximately: trad = 1/(6.28f)
Phase Alternation Line Phase Alternation Line (PAL) is the analog television display standard that is used in Europe and certain other parts of the world. PAL is one of the three major TV standards together with the American National Television Systems Committee (NTSC) color television system and the French Sequential Couleur avec Memoire (SECAM). NTSC is also used in Japan. SECAM is used in countries of the former Soviet Union. Like SECAM, PAL scans the cathode ray tube (CRT) horizontally 625 times to form the video image. NTSC scans 525 lines. Color definitions between the systems vary slightly.
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The time interval for one degree of phase is inversely proportional to the frequency. If the frequency of a signal (in hertz) is given by f, then the time tdeg (in seconds) corresponding to one degree of phase is:
Phase is sometimes expressed in radians rather than in degrees. One radian of phase corresponds to approximately 57.3 degrees. Engineers and technicians generally use degrees; physicists more often use radians.
Phase change is a type of compact disc (CD) recording technology that enables the discs to be written, erased, and rewritten through the use of a layer of a special material (called the phase change layer) that can be changed repeatedly from an amorphous (formless) to a crystalline state, or phase-through exposure to variably-powered laser beams. The technology, which was developed in the late 1960s by Stanford Ovshinsky, is used in rewritable CD (CD-RW) and DVD (DVD-RW) formats. A different type of phase change technology used in CD-Magneto Optical (CD-MO) uses magnetic fields as well as lasers. Like CD-Recordable (CD-R), CD-RW has a polycarbonate substrate, which is preformed with a spiral groove to guide the laser. The alloy phase-change recording layer, which is commonly a mix of silver, indium, antimony and tellurium, is sandwiched between two dielectric layers that draw excess heat from the recording layer. After heating to one particular temperature, the alloy will become crystalline when it is cooled; after heating to a higher temperature, it will become amorphous when it is cooled. By controlling the temperature of the laser, crystalline areas and noncrystalline areas are formed. The crystalline areas will reflect the laser, while the other areas will absorb it. The differences will register as binary data that can be unencoded for playback. To erase or write over recorded data, the higher temperature laser is used, which results in the amorphous form, which can then be reformed by the lower temperature laser. According to Sony, the process can be repeated up to 1000 times.
phase-shift keying
phase modulation Also see modulation and phase-shift keying. Phase modulation (PM) is a method of impressing data onto an alternating-current (AC) waveform by varying the instantaneous phase of the wave. This scheme can be used with analog or digital data. In analog PM, the phase of the AC signal wave, also called the carrier, varies in a continuous manner. Thus, there are infinitely many possible carrier phase states. When the instantaneous data input waveform has positive polarity, the carrier phase shifts in one direction; when the instantaneous data input waveform has negative polarity, the carrier phase shifts in the opposite direction. At every instant in time, the extent of carrier-phase shift (the phase angle) is directly proportional to the extent to which the signal amplitude is positive or negative. In digital PM, the carrier phase shifts abruptly, rather than continuously back and forth. The number of possible carrier phase states is usually a power of 2. If there are only two possible phase states, the mode is called biphase modulation. In more complex modes, there can be four, eight, or more different phase states. Each phase angle (that is, each shift from one phase state to another) represents a specific digital input data state. Phase modulation is similar in practice to frequency modulation (FM). When the instantaneous phase of a carrier is varied, the instantaneous frequency changes as well. The converse also holds: When the instantaneous frequency is varied, the instantaneous phase changes. But PM and FM are not exactly equivalent, especially in analog applications. When an FM receiver is used to demodulate a PM signal, or when an FM signal is intercepted by a receiver designed for PM, the audio is distorted. This is because the relationship between phase and frequency variations is not linear; that is, phase and frequency do not vary in direct proportion.
phased antenna system A phased antenna system consists of two or more active antenna elements, arranged so the electromagnetic fields effectively add in some directions and cancel in other directions. This produces enhanced transmission and reception in the directions where the fields add, and reduces the strength of radiated and received signals in the directions where the fields cancel. Many AM (amplitude-modulation) broadcast stations use sets of two, three, or four phased vertical antennas. This results in a directional pattern that optimizes coverage, so the station can reach the greatest possible number of listeners in its designated area. The nulls in the pattern reduce or eliminate interference with other AM broadcast stations.
In two-way radio communications, several vertical dipole antenna can be placed end-to-end and fed in phases. This is known as a collinear antenna and is a specialized type of phased array. At low elevation angles, the radiation and response are enhanced relative to a single vertical dipole. This gain occurs at the expense of radiation and response at higher elevation angles, increasing the range of communication for surface-to-surface communications.
phaser A phaser is an electronic sound processor, that creates a sweeping effect by modulating a narrow notch signal filter. This hardware or software device is also known as a phase shifter. Its sound is similar to that of flanging. Phase shifting is a popular effect for keyboards and guitars. It is included in the special effects processors of a number of popular keyboards and digital effect devices. A phase shifter works by applying a narrow signal filter to a sound source. This causes various amounts of canceling of frequency components in the original signal. The filter’s output is mixed back in with the original source audio signal. To get the sweeping effect, a low frequency oscillator (LFO) is used to slowly move the frequency center of the notch filter. Blending the altered signal back into the signal path (called ‘‘regeneration’’ or ‘‘feedback’’) intensifies the effect.
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phase-shift keying See also frequency-shift keying (FSK). Phase-shift keying (PSK) is a method of transmitting and receiving digital signals in which the phase of a transmitted signal is varied to convey information. There are several schemes that can be used to accomplish PSK. The simplest method uses only two signal phases: 0 degrees and 180 degrees. The digital signal is broken up timewise into individual bits (binary digits). The state of each bit is determined according to the state of the preceding bit. If the phase of the wave does not change, then the signal state stays the same (low or high). If the phase of the wave changes by 180 degrees—that is, if the phase reverses—then the signal state changes (from low to high, or from high to low). Because there are two possible wave phases, this form of PSK is sometimes called biphase modulation. More complex forms of PSK employ four or eight wave phases. This allows binary data to be transmitted at a faster rate per phase change than is possible with biphase modulation. In four-phase modulation, the possible phase angles are 0, +90, 790, and 180 degrees; each phase shift can represent two signal elements. In eight-phase modulation, the possible phase angles are 0, +45, 745, +90, 790, +135, 7135, and 180 degrees; each phase shift can represent four signal elements.
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Photo CD Photo CD is a process from Kodak that puts film images (including scanned prints and slides) on a compact disc as digitally stored images that you can view or work with at your computer. The images can also be printed out on photographic paper with a special Kodak machine. Also see digital camera.
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reduced. For example, visible light travels more slowly through glass than through outer space. Radio waves travel more slowly through the polyethylene in a transmission line than they do through the atmosphere. The ratio of the speed of the photons in a particular medium to their speed in a vacuum is called the velocity factor. This factor is always between 0 and 1 (or 0 and 100%), and it depends to some extent on the wavelength. The shorter the wavelength of an electromagnetic disturbance, the more energy each photon contains. In fact, this relationship is so precise that a mathematical formula applies. If e represents the energy (the unit of measurement is the joule) contained in each photon and s represents the electromagnetic wavelength (in meters), then
See also photosensor.
e = hc/s
Photoconductivity is the tendency of a substance to conduct electricity to an extent that depends on the intensity of lightradiant energy (usually infrared transmission or visible light) striking the surface of a sample. Most semiconductor materials have this property.
where h is Planck’s constant (approximately equal to 6.626 times 10-34 joule-second) and c is the speed of electromagnetic-field propagation in the medium in question (approximately 2.998 times 108 meters per second in a vacuum). A simpler formula applies to frequency. If f represents the frequency of an electromagnetic field (in hertz), then
When there is no illumination, a photoconductive sample has a conductance that depends on its dimensions, on the specific material(s) from which it is made, and on the temperature. In most cases, the greater the radiant energy of a specific wavelength that strikes the surface, the higher the conductance of the sample becomes, up to a certain maximum. When the maximum conductance is reached for a particular sample, further increases in irradiation produce no change in the conductance. Photoconductive materials are used in the manufacture of photoelectric devices. Typical photoconductive substances consist of germanium, gallium, selenium, or silicon with impurities, also known as dopants, added. Other common materials include metal oxides and sulfides.
photon In some ways, visible light behaves like a wave phenomenon, but in other respects it acts like a stream of high-speed, submicroscopic particles. Isaac Newton was one of the first scientists to theorize that light consists of particles. Modern physicists have demonstrated that the energy in any electromagnetic field is made up of discrete packets. The term photon (meaning ‘‘visible-light particle’’) has been coined for these energy packets. Particle-like behavior is not restricted to the visible-light portion of the electromagnetic radiation spectrum, however. Radio waves, infrared rays, visible light, ultraviolet rays, X rays, and gamma rays all consist of photons, each of which contains a particular amount of energy that depends on the wavelength. Photons travel through empty space at a speed of approximately 186,282 miles (299,792 kilometers) per second. This is true no matter what the electromagnetic wavelength. In media other than a vacuum, the speed is
e = hf The energy contained in a single photon does not depend on the intensity of the radiation. At any specific wavelength—say, the wavelength of light emitted by a helium-neon laser—every photon contains exactly the same amount of energy, whether the source appears as dim as a candle or as bright as the sun. The brilliance or intensity is a function of the number of photons striking a given surface area per unit time.
photonic network A photonic (or optical) network is a communications network in which information is transmitted entirely in the form of optical or infrared transmission(IR) signals. In a true photonic network, every switch and every repeater works with IR or visible-light energy. A recent development in this field is the erbium amplifier. Conversion to and from electrical impulses is not done except at the source and destination (origin and end point). Optical or IR data transmission has several advantages over electrical transmission. Perhaps most important is the greatly increased bandwidth provided by photon signals. Because the frequency of visible or IR energy is so high (on the order of millions of megahertz), thousands or millions of signals can be impressed onto a single beam by means of frequency division multiplexing (FDM). In addition, a single strand of fiber can carry IR and/or visible light at several different wavelengths, each beam having its own set of modulating signals. This is known as wave-division multiplexing (WDM).
phreaking A subtle, but potentially far-reaching, advantage of photonic systems over electronic media results from the fact that visible and IR energy actually moves several times faster than electricity. Electric current propagates at about 10 percent of the speed of light (18,000 to 19,000 miles or 30,000 kilometers per second), but the energy in fiber optic systems travels at the speed of light in the glass or plastic medium, which is a sizable fraction of the speed of light in free space (186,000 miles or 300,000 kilometers per second). This results in shorter data-transmission delay times between the end points of a network. This advantage is especially significant in systems where the individual computers or terminals continuously share data. It affects performance at all physical scales, whether components are separated by miles or by microns. It is even significant within microchips, a phenomenon of interest to research-and-development engineers in optical computer technology.
photonic switching See ‘‘lambda switching’’
photosensor A photosensor is an electronic component that detects the presence of visible light, infrared transmission (IR), and/or ultraviolet (UV) energy. Most photosensors consist of semiconductors having a property called photoconductivity, in which the electrical conductance varies depending on the intensity of radiation striking the material. The most common types of photosensor are the photodiode, the bipolar phototransistor, and the photoFET (photosensitive field-effect transistor). These devices are essentially the same as the ordinary diode, bipolar transistor, and field-effect transistor, except that the packages have transparent windows that allow radiant energy to reach the junctions between the semiconductor materials inside. Bipolar and field-effect phototransistors provide amplification in addition to their sensing capabilities. Photosensors are used in a great variety of electronic devices, circuits, and systems, including: L
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PHP In Web programming, PHP is a script language and interpreter that is freely available and used primarily on Linux Web servers. PHP (the initials come from the earliest version of the program, which was called ‘‘Personal Home Page Tools’’) is an alternative to Microsoft’s Active Server Page (ASP) technology. As with ASP, the PHP script is embedded within a Web page along with its HTML. Before the page is sent to a user that has requested it, the Web server calls PHP to interpret and perform the operations called for in the PHP script. An HTML page that includes a PHP script is typically given a file name suffix of ‘‘.php’’ ‘‘.php3,’’ or ‘‘.phtml’’. Like ASP, PHP can be thought of as ‘‘dynamic HTML pages,’’ since content will vary based on the results of interpreting the script.
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PHP is free and offered under an open source license. The latest version is PHP4.
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A phreak is someone who breaks into the telephone network illegally, typically to make free long-distance phone calls or to tap phone lines. The term is now sometimes used to include anyone who breaks or tries to break the security of any network. Recently, the phone companies have introduced new security safeguards, making phreaking more difficult. According to Eric Raymond’s The New Hacker’s Dictionary, phreaking was originally a more innocent occupation and hackers would sometimes take up the challenge. The typical phreak was or is usually equipped with a specially-made ‘‘box’’ designed to ‘‘fool’’ the network in some way. Different boxes, somehow named for different colors but not necessarily painted any color, are used for different phreak approaches. A ‘‘black box’’ allows you to make free calls from a home phone; a ‘‘red box’’ to make free calls on a pay phone; and the infamous ‘‘blue box’’ provides complete control over the telephone system. If you look hard enough on the Web, you’ll probably find directions on how to make all of these boxes. (But note that using the boxes as directed is probably illegal and any directions you find may be outof-date.)
phreaking See ‘‘phreak’’
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PHTML and phtml suffix
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A PHTML (or it’s sometimes called a PHP) page is a Web page that includes a script written in Personal Home Page, a language comparable to JavaScript or Microsoft’s VBScript. Like Microsoft’s Active Server Page (ASP) page, a PHTML page contains programming that is executed at the Web server rather than at the Web client (which is usually your Web browser). You may sometimes see a Web site whose address or URL ends with a file with a suffix of ‘‘.phtml’’ or ‘‘.php3’’. Either of these suffixes indicate an HTML page that includes a PHP script.
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In the Open Systems Interconnection (OSI) communications model, the Physical layer supports the electrical or mechanical interface to the physical medium. For example, this layer determines how to put a stream of bits from the upper (Data Link) layer on to the pins for a parallel printer interface, an optical fiber transmitter, or a radio carrier.
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The Physical layer is usually a combination of software and hardware programming and may include electromechanical devices. It does not include the physical media as such.
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In IBM’s Systems Network Architecture (SNA), a physical unit (PU) identifies a network node that supports communication sessions between logical units (LU). Logical units represent end users. Two logical units that communicate depend on physical connections being established through associated physical units. The network point at each end of a communication that sets up the communication session between logical units is called the system services control point (SSCP). The SSCP sets up its own session with the PU on behalf of the LU-LU session. Typically, a logical unit is a unique connection to an application program. A physical unit is the hardware and routing aspect of a network node. IBM’s Virtual Telecommunication Access Method (VTAM) is used by an application program to manage session and communication requests on behalf of logical units.
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In SNA, there are different node types (formerly called physical unit types) that characterize routing, pooling, and other network capabilities.
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Pi is a numerical constant that represents the ratio of a circle’s circumference to its diameter on a flat plane surface. The value is the same regardless of the size of the circle. The decimal expansion of pi is a nonterminating, nonrepeating
sequence of digits. For most calculations, the value can be taken as 3.14159. This means, for example, that a circle with a diameter of 10 centimeters, as measured on a flat surface, has a circumference of approximately 31.4159 centimeters. The number pi is also the ratio of the diameter of a sphere to the length of any great circle (geodesic) on the sphere. So, for example, if the earth is considered to be a perfect sphere with a diameter of 8,000 miles, then the distance around the earth, as measured along the equator or along any great circle, is approximately 8,000 6 3.14159, or 25,133 miles. Pi is an irrational number. It cannot be precisely defined as the ratio of any two whole numbers. Thus, its decimal expansion has no pattern and never ends. The first few hundred, thousand, million, or billion digits of pi can be calculated using a computer to add up huge initial sequences of the terms of an infinite sum known as a Fourier series. Mathematically, it can be shown that the following equation holds: π = 4 – 4/3 + 4/5 – 4/7 = 4/9 – 4/11…
The symbol to the left of the equal sign is the lowercase Greek letter used in mathematics, physics, and engineering to represent pi.
Pick Pick is a data base management system (DMS) based on a business model of data and its organization and traditionally associated with minicomputer systems for small- to medium-size businesses. Because it lets developers view data much as a business naturally uses data, Pick is reputed to be easy to learn and use as well as cost-efficient. Originally, Pick was viewed as a data- and business-oriented operating system supported by an assembler language virtual machine for particular minicomputer hardware. Based on concepts from TRW’s Generalized Information Retrieval Language and System, Dick Pick and Don Nelson formed the idea of a computer system that would be based on the user’s data-oriented and business process perspective rather than the usual perspective of how the system itself worked. Pick’s package of hardware and software was sold through value-added resellers (VARs) who could adapt it for many kinds of business and hardware platforms. Today, Pick is still sold by the original company and its family of VARs as a database management system on UNIX and Windows systems. Although Pick apparently missed the initial industry shift to client/server programming for distributed network applications, Pick Systems now offers a new version of Pick that is compliant with Structured Query Language and Open Database Connectivity for use in networks with distributed programming. Pick Systems states that over 4,000 Pick-based applications have been built and that over 2,000 are currently available. Familiarity with Pick is still a requirement in some companies, especially those with legacy applications built for Pick.
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picosecond
pie graph
(This definition follows U.S. usage in which a billion is a thousand million and a trillion is a 1 followed by 12 zeros.)
Compare with histogram.
A picosecond is one trillionth (10-12) of a second, or one millionth of a microsecond. For comparison, a millisecond (ms or msec) is one thousandth of a second and is commonly used in measuring the time to read to or write from a hard disk or a CD-ROM player or to measure packet travel time on the Internet. A microsecond (ms or Greek letter mu plus s) is one millionth (10-6) of a second. A nanosecond (ns or nsec) is one billionth (10-9) of a second and is a common measurement of read or write access time to random access memory (RAM). -15
A femtosecond is one millionth of a nanosecond or 10 of a second and is a measurement sometimes used in laser technology. If the term attosecond is used, it would mean one quintillionth (10-18) of a second.
The illustration is a pie graph depicting the results of a final exam given to a hypothetical class of students. Each grade is denoted by a ‘‘slice.’’ The total of the percentages is equal to 100 (this is important; if it were not, the accuracy of the graph would be suspect). The total of the arc measures is equal to 360 degrees.
pie chart See ‘‘pie graph’’
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PICTIVE (Plastic Interface for Collaborative Technology Initiatives through Video Exploration) is a paper mock-up technique that allows users to participate in the development process. A PICTIVE is a representation of a graphical user interface (GUI) or a Web page on paper. A PICTIVE prototype gives a user a sense of what a system or a piece of software will look like and how it will behave once it is finished. PICTIVE enables a non-technical person to contribute ideas to the development process.
Developers at Bell Communications Research (Bellcore) are credited with creating PICTIVE in the early 1990s.
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A pie graph (or pie chart) is a specialized graph used in statistics. The independent variable is plotted around a circle in either a clockwise direction or a counterclockwise direction. The dependent variable (usually a percentage) is rendered as an arc whose measure is proportional to the magnitude of the quantity. Each arc is depicted by constructing radial lines from its ends to the center of the circle, creating a wedge-shaped ‘‘slice.’’ The independent variable can attain a finite number of discrete values (for example, five). The dependent variable can attain any value from zero to 100 percent.
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A PICTIVE is usually made from simple office supplies like pens, paper, Post-It stickers, and paper clips. The developer uses those supplies to represent elements of the project, including drop-down boxes, menu bars, and special icons. During a design session, users manipulate the mock-up so it becomes easier for them to use. The development team takes notes and incorporates user ideas into its work. The development team also uses a video camera to record the physical changes a user might make to the PICTIVE. The team then reviews the ideas generated and incorporates them into the project. The ultimate goal of a PICTIVE is to simplify the design process enough that non-technical users are empowered to participate in it.
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From this graph, one might gather that the professor for this course was not especially lenient nor severe. It is evident that grading was not done on a ‘‘pure curve’’ (in which case all the arcs would have equal measures of 72 degrees, corresponding to 20%). If this graph were compared with those of classes from other years that received the same test from the same professor, some conclusions might be drawn about intelligence changes among students over the years. If this graph were compared with those of other classes in the same semester who had received the same final exam but who had taken the course from different professors, one might draw conclusions about the relative competence and/ or grading whims of the professors.
pin A pin is a pronged contact as part of a signal interface in a computer or other communications device. Pins are part of a male connector and plug into a female connector. The number of pins in a connector is sometimes used in describing the
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connector (for example, ‘‘a 25-way D-type connector’’). The description of the signal associated with each pin is called the pinout.
ping Tip: To find out the dot address (such as 205.245.172.72) for a given domain name, Windows users can go to their MS DOS prompt screen and enter: ping xxx.yyy where xxx is the second-level domain name like ‘‘whatis’’ and yyy is the toplevel domain name like ‘‘com’’). Ping is a basic Internet program that lets you verify that a particular IP address exists and can accept requests. The verb ping means the act of using the ping utility or command. Ping is used diagnostically to ensure that a host computer you are trying to reach is actually operating. If, for example, a user can’t ping a host, then the user will be unable to use the File Transfer Protocol (FTP) to send files to that host. Ping can also be used with a host that is operating to see how long it takes to get a response back. Using ping, you can learn the number form of the IP address from the symbolic domain name (see ‘‘Tip’’). Loosely, ping means ‘‘to get the attention of’’ or ‘‘to check for the presence of’’ another party online. Ping operates by sending a packet to a designated address and waiting for a response. The computer acronym was contrived to match the submariners’ term for the sound of a returned sonar pulse. Ping can also refer to the process of sending a message to all the members of a mailing list requesting an ACK (acknowledgement code). This is done before sending e-mail in order to confirm that all of the addresses are reachable.
ping storm A ping storm is a condition in which the ping program is used to send a flood of packets to a server to test its ability to handle a high amount of traffic or, maliciously, to make the server inoperable. Although the ping support in Windows operating systems does not allow someone to mount a ping storm, the ping command in at least some UNIX-based systems offers two options: ‘‘ping -f’’ which specifies to output ping packets back as fast as they are returned, and ‘‘ping -s]packetsize[’’, which causes the size of the outgoing packet to be padded by some specified size in order to increase the load on the receiving server. A ping storm is one form of packet storm.
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Ping strangeness is a term used in a popular book on network design and troubleshooting to describe the incidence of an unusual pattern or frequency of ping messages in a network. Pings are used to determine the presence of particular Internet Protocol (IP) addresses on host computers in a network and the time it takes for the
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ping packet to return. Besides being a useful diagnostic tool, pings are sometimes used by router program ICMP requests so that network router tables can be kept up-to-date. The issuance of a ping request by a user or a program results in an Internet Control Message Protocol (ICMP) echo request. If the remote IP address is active, it responds with an ICMP echo reply. Ping can be used as an alternative to the traceroute utility to trace the hop or path that the ping echo takes through the network. Ping is faster, however, and generates less network traffic than traceroute. Using a network monitor like LANalyzer for Windows, a network administrator can follow a packet exchange. If, for example, the monitor shows a consistent pattern of unexplained pings occurring in the same time period as the packet exchange, this ‘‘strange’’ pattern may be worth investigating. A consistent and continual pattern would suggest that the pings were not the result of someone trying to attack the network (since the attack would be made too obvious) but rather that the hosts originating the pings had been programmed to issue them for some reason. For example, Synoptic hubs sometimes seek a network management station, an optional facility that Synoptic offers. The hubs will look for the management station using ICMP pings at frequent time intervals. If the management station doesn’t exist, the pings will continue, causing unnecessary traffic. In this example, one solution would be to disable the Internet Protocol on each hub.
pink noise See ‘‘white noise’’
pinout A pinout is a description of the purpose of each pin in a multi-pin hardware connection interface. The description can be entirely in text, often in table form, or it can be an illustration. Not all pins are used in every connection.
PIO Programmed Input/Output (PIO) is a way of moving data between devices in a computer in which all data must pass through the processor. The Advanced Technology Attachment/Integrated Drive Electronics standard specifies three PIO data transfer rates (mode 0 at 3.3 MBps, mode 1 at 5.2 MBps, and mode 2 at 8.3 MBps). The newer Advanced Technology Attachment standard specifies two higher data transfer rates (mode 3 at 11.1 MBps and mode 4 at 16.6 MBps). A newer alternative to PIO is Direct Memory Access (DMA). It’s expected that PIO will be dropped from future industry standards, replaced entirely by DMA and Ultra DMA.
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pipelining
Also see named pipe (or FIFO).
In computers, a pipeline is the continuous and somewhat overlapped movement of instructions to the processor or in the arithmetic steps taken by the processor to perform an instruction. Pipelining is the use of a pipeline. Without a pipeline, a computer processor gets the first instruction from memory, performs the operation it calls for, and then goes to get the next instruction from memory, and so forth. While fetching (getting) the instruction, the arithmetic part of the processor is idle. It must wait until it gets the next instruction. With pipelining, the computer architecture allows the next instructions to be fetched while the processor is performing arithmetic operations, holding them in a buffer close to the processor until each instruction operation can be performed. The staging of instruction fetching is continuous. The result is an increase in the number of instructions that can be performed during a given time period.
In computer programming, especially in UNIX operating systems, a pipe is a technique for passing information from one program process to another. Unlike other forms of interprocess communication (IC), a pipe is one-way communication only. Basically, a pipe passes a parameter such as the output of one process to another process which accepts it as input. The system temporarily holds the piped information until it is read by the receiving process. Using a UNIX shell (the UNIX interactive command interface), a pipe is specified in a command line as a simple vertical bar ]|[ between two command sequences. The output or result of the first command sequence is used as the input to the second command sequence. The pipe system call is used in a similar way within a program. For two-way communication between processes, two pipes can be set up, one for each direction. A limitation of pipes for interprocess communication is that the processes using pipes must have a common parent process (that is, share a common open or initiation process and exist as the result of a fork system call from a parent process). A pipe is fixed in size and is usually at least 4,096 bytes.
pipeline burst cache A pipeline burst cache is a cache or storage area for a computer processor that is designed to be read from or written to in a pipelining succession of four data transfers (or bursts) in which later bursts can start to flow or transfer before the first burst has arrived at the processor. A pipeline burst cache is often used for the static RAM (SRAM) that serves as the L1 and L2 cache in a computer. It was introduced in 1996 with Intel’s Pentium series of processors. A pipeline burst cache is an alternative to an asynchronous cache or a synchronous burst cache. In most personal computer processors, data is transferred along a path (or bus) that is 64 bits (8 bytes) wide. Since each line of storage in a cache is 32 bytes long, it takes four successive transfers to transfer the storage line. With a pipeline burst cache, the first transfer takes 3 of the processor’s clock speed cycles, and the remaining three transfers take only one cycle each (since no time is required to locate the storage location for the remaining transfers). The adjectives pipelining and burst describe the idea that once the storage location has been addressed and accessed and the first read or write transfer is started, the subsequent transfers come rapidly in discrete bursts down (or up) the pipe or data path. The timing for the total transfer of a storage line using pipeline burst cache is usually shown as ‘‘3-1-1-1’’ or a total of 6 clock cycles.
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Pipelining is sometimes compared to a manufacturing assembly line in which different parts of a product are being assembled at the same time although ultimately there may be some parts that have to be assembled before others are. Even if there is some sequential dependency, the overall process can take advantage of those operations that can proceed concurrently.
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Computer processor pipelining is sometimes divided into an instruction pipeline and an arithmetic pipeline. The instruction pipeline represents the stages in which an instruction is moved through the processor, including its being fetched, perhaps buffered, and then executed. The arithmetic pipeline represents the parts of an arithmetic operation that can be broken down and overlapped as they are performed.
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Pipelines and pipelining also apply to computer memory controllers and moving data through various memory staging places.
piracy Software piracy is the illegal copying, distribution, or use of software. According to one source, about 40% of all software in current use is stolen. In 1998, revenue losses from software piracy were estimated at $11 billion worldwide. North America, Asia, and Western Europe account for 80% of revenue losses with the U.S. ranking highest in dollar losses. It is such a profitable ‘‘business’’ that it has caught the attention of organized crime groups in a number of countries. Software piracy causes significant lost revenue for publishers, which in turn results in higher prices for the consumer. Some software publishers go out of business because of software piracy. Others are discouraged from entering markets where software piracy rates are high. When you purchase a commercial software package, a licensing agreement is included to protect that software program from copyright infringement. Typically, the license
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states that you can install the original copy of software you bought on one computer and that you can make a backup copy in case the original is lost or damaged. You agree to the licensing agreement when you open the software package (this is called a shrink wrap license), when you open the envelope that contains the software disks, or when you install the software.
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Software piracy applies mainly to full-function commercial software. The time-limited or function-restricted versions of commercial software called shareware are less likely to be pirated since they are freely available. Freeware, a type of software that is copyrighted but freely distributed at no charge, also offers little incentive for piracy.
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Types of Software Piracy
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Types of software piracy include:
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Softlifting: This is the most common form of piracy. Sharing a program with a friend or associate is softlifting. To stop this kind of piracy in businesses and institutions, software publishers are offering multi-user licenses, which are cheaper than buying single copies for every machine.
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Hard-disk loading: This piracy is committed by computer dealers who install unauthorized copies of software on computers that they sale. When purchasing a computer, request that all the software installed be provided in a hard disk copy too. If the dealer refuses, do not buy that computer and report the incident to an antipiracy organization.
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Counterfeiting: With today’s technology, making counterfeit copies of software is easy. Microsoft is the most targeted for software piracy because their software is so widespread. Microsoft investigators have found pirated software on the computers of police who were conducting piracy investigations. Some counterfeit copies are copied onto floppy disks and CDs and sold on street corners. Other counterfeit copies are sold through retail stores usually unknowingly. In these cases, significant efforts were made to duplicate packaging and logos. Notice the heat sensitive strip at the top of a Microsoft manual. Rub it or hold it near a hot bulb. The strip will disappear and reveal the word ‘‘genuine.’’ This is a genuine Microsoft product. Online piracy: Because software is now available on the Internet, online piracy is becoming popular. In the past, the only place to download software was from a bulletin board system and these were limited to local areas because of long distance charges while online. Microsoft and other companies regularly patrol the Web for pirated software. Buying software from online auctions can be dangerous. Lately, online piracy has been extended to music, too, especially in the MP3 format. It is not difficult to copy music from a CD, convert it to the MP3 format, and make it available for anyone to download. Unfortunately, this deprives composers of
royalties that they are entitled to. Although much pirated music is available in the MP3 format, MP3.com has recently made arrangements to charge users for downloads and pay royalties to composers who make music available.
How to Fight Piracy The first step in fighting software piracy is to make sure you don’t contribute to it knowingly or unknowingly. Buy software from an authorized dealer and don’t use any program offered illegally. If you discover that you might have a counterfeit copy of software, contact the dealer you bought it from, but keep in mind that they may have been fooled, too. Enterprises with networks need policies that state what is acceptable in using software, stipulating consequences for illegal use. An enterprise may want to consider purchasing an anti-piracy program. For example, KeyServer keeps track of how many copies are in use at one time on a network. Another solution certain publishers have used is the dongle, a device that attaches to a serial or parallel port of a desktop or into the PC-Card host of a laptop. A dongle uses codes or passwords that are embedded inside to control access to a software application. A program used with a dongle will only work when the dongle is attached to the computer. Most dongles can be daisy-chained with a printer or other dongles. There are potentially stiff penalties for software piracy. In the U.S., software pirates can be punished with statutory damages of up to $100,000. If you are convicted of a felony charge of software piracy, you can get up to a five-year prison sentence plus fines of up to $250,000 for each work that is infringed.
pixel A pixel (a word invented from ‘‘picture element‘‘) is the basic unit of programmable color on a computer display or in a computer image. Think of it as a logical—rather than a physical—unit. The physical size of a pixel depends on how you’ve set the resolution for the display screen. If you’ve set the display to its maximum resolution, the physical size of a pixel will equal the physical size of the dot pitch (let’s just call it the dot size) of the display. If, however, you’ve set the resolution to something less than the maximum resolution, a pixel will be larger than the physical size of the screen’s dot (that is, a pixel will use more than one dot). The specific color that a pixel describes is some blend of three components of the color spectrum—RGB. Up to three bytes of data are allocated for specifying a pixel’s color, one byte for each color. A true color or 24-bit color system uses all three bytes. However, most color display systems use only eight-bits (which provides up to 256 different colors). A bitmap is a file that indicates a color for each pixel along the horizontal axis or row (called the x coordinate) and a color for each pixel along the vertical axis (called the y
PKI coordinate). A Graphics Interchange Format file, for example, contains a bitmap of an image (along with other data). Screen image sharpness is sometimes expressed as dots per inch (dpi). (In this usage, the term dot means pixel, not dot as in dot pitch.) Dots per inch is determined by both the physical screen size and the resolution setting. A given image will have lower resolution—fewer dots per inch—on a larger screen as the same data is spread out over a larger physical area. On the same size screen, the image will have lower resolution if the resolution setting is made lower—resetting from 800 by 600 pixels per horizontal and vertical line to 640 by 480 means fewer dots per inch on the screen and an image that is less sharp. (On the other hand, individual image elements such as text will be larger in size.) Pixel has generally replaced an earlier contraction of picture element, pel.
pixels per inch In computers, pixels per inch (ppi) is a measure of the sharpness (that is, the density of illuminated points) on a display screen. The dot pitch determines the absolute limit of the possible pixels per inch. However, the displayed resolution of pixels (picture elements) that is set up for the display is usually not as fine as the dot pitch. The pixels per inch for a given picture resolution will differ based on the overall screen size since the same number of pixels are being spread out over a different space. The term ‘‘dots per inch’’ (dpi), extended from the print medium, is sometimes used instead of pixels per inch.
pixie dust Pixie dust is the informal name that IBM is using for its antiferromagnetically-coupled (AFC) media technology, which can increase the data capacity of hard drives to up to four times the density possible with current drives. AFC overcomes limits of current hard drives caused by a phenomenon called the superparamagnet effect (basically, alterations in magnetic orientation). The ‘‘pixie dust’’ used is a 3-atom thick magnetic coating composed of the element ruthenium sandwiched between two magnetic layers. The technology is expected to yield 400 GB (gigabyte) hard drives for desktop computers, and 200 GB hard drives for laptops by 2003. In information technology, the term ‘‘pixie dust’’ is often used to refer to a technology that seemingly does the impossible. IBM’s use of AFC for hard drives overcomes what was considered an insuperable problem for storage: the physical limit for data stored on hard drives. Hard drive capacities have more or less doubled in each of the last five years, and it was assumed in the storage industry that the upper limit would soon be reached. The superparamagnetic effect has long been predicted to appear when densities reached 20 to 40 gigabits per square inch—close to the data
density of current products. AFC increases possible data density, so that capacity is increased without using either more disks or more heads to read the data. Current hard drives can store 20 gigabits of data per square inch. IBM began shipping Travelstar hard drives in May 2001 that are capable of storing 25.7 gigabits per square inch. Drives shipped later in the year are expected to be capable of 33% greater density. Because smaller drives will be able to store more data and use less power, the new technology may also lead to smaller and quieter devices.
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IBM discovered a means of adding AFC to their standard production methods so that the increased capacity costs little or nothing. The company, which plans to implement the process across their entire line of products, chose not to publicize the technology in advance. Many companies have focused research on the use of AFC in hard drives; a number of vendors, such as Seagate Technology and Fujitsu, are expected to follow IBM’s lead.
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pk
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See ‘‘peak’’
I PKCS See ‘‘Public-Key Cryptography Standards’’
PKI A PKI (public key infrastructure) enables users of a basically unsecure public network such as the Internet to securely and privately exchange data and money through the use of a public and a private cryptographic key pair that is obtained and shared through a trusted authority. The public key infrastructure provides for a digital certificate that can identify an individual or an organization and directory services that can store and, when necessary, revoke the certificates. Although the components of a PKI are generally understood, a number of different vendor approaches and services are emerging. Meanwhile, an Internet standard for PKI is being worked on. The public key infrastructure assumes the use of public key cryptography, which is the most common method on the Internet for authenticating a message sender or encrypting a message. Traditional cryptography has usually involved the creation and sharing of a secret key for the encryption and decryption of messages. This secret or private key system has the significant flaw that if the key is discovered or intercepted by someone else, messages can easily be decrypted. For this reason, public key cryptography and the public key infrastructure is the preferred approach on the Internet. (The private key system is sometimes known as symmetric cryptography and the public key system as asymmetric cryptography.)
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A registration authority (RA) that acts as the verifier for the certificate authority before a digital certificate is issued to a requestor
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One or more directories where the certificates (with their public keys) are held
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A certificate management system
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A certificate authority (CA) that issues and verifies digital certificate. A certificate includes the public key or information about the public key
How Public and Private Key Cryptography Works In public key cryptography, a public and private key are created simultaneously using the same algorithm (a popular one is known as RSA) by a certificate authority (CA). The private key is given only to the requesting party and the public key is made publicly available (as part of a digital certificate) in a directory that all parties can access. The private key is never shared with anyone or sent across the Internet. You use the private key to decrypt text that has been encrypted with your public key by someone else (who can find out what your public key is from a public directory). Thus, if I send you a message, I can find out your public key (but not your private key) from a central administrator and encrypt a message to you using your public key. When you receive it, you decrypt it with your private key. In addition to encrypting messages (which ensures privacy), you can authenticate yourself to me (so I know that it is really you who sent the message) by using your private key to encrypt a digital certificate. When I receive it, I can use your public key to decrypt it. Here’s a table that restates it: To do this
Use whose
Kind of key
Send an encrypted message Send an encrypted signature Decrypt an encrypted message Decrypt an encrypted signature (and authenticate the sender)
Use the receiver’s
Public key
Use the sender’s
Private key
Use the receiver’s
Private key
Use the sender’s
Public key
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RSA, which has developed the main algorithms used by PKI vendors
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Verisign, which acts as a certificate authority and sells software that allows a company to create its own certificate authorities
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GTE CyberTrust, which provides a PKI implementation methodology and consultation service that it plans to vend to other companies for a fixed price
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Check Point, which offers a product, VPN-1 Certificate Manager, that is based on the Netscape Directory Server
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Xcert, whose Web Sentry product that checks the revocation status of certificates on a server, using the Online Certificate Status Protocol (OCSP)
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Netscape, whose Directory Server product is said to support 50 million objects and process 5,000 queries a second; Secure E-Commerce, which allows a company or extranet manager to manage digital certificates; and Meta-Directory, which can connect all corporate directories into a single directory for security management
Pretty Good Privacy For e-mail, the Pretty Good Privacy (PGP) product lets you encrypt a message to anyone who has a public key. You encrypt it with their public key and they then decrypt it with their private key. PGP users share a directory of public keys that is called a key ring. (If you are sending a message to someone that doesn’t have access to the key ring, you can’t send them an encrypted message.) As another option, PGP lets you ‘‘sign’’ your note with a digital signature using your private key. The recipient can then get your public key (if they get access to the key ring) and decrypt your signature to see whether it was really you who sent the message.
PKUNZIP See ‘‘PKZIP’’
PKZIP
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A number of products are offered that enable a company or group of companies to implement a PKI. The acceleration of e-commerce and business-to-business commerce over the
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Internet has increased the demand for PKI solutions. Related ideas are the virtual private network (VPN) and the IP Security (IPsec) standard. Among PKI leaders are:
PKZIP (also capitalized as PKZip) is a program, originally written for the DOS operating system, that gathers multiple files into a single compressed file that can be stored in less space and transmitted more quickly from one place to another. A companion program, PKUNZIP, uncompresses and restores the original files when they are needed. The compressed file archive is known as a zip file and conventionally has a .zip file name suffix. The ‘‘PK’’ derives from the name of the program’s inventor, Phillip W. Katz. Katz is said to have developed the idea in 1986, working at
plasma display his mother’s kitchen table. PKZIP became one of the most popular shareware programs ever written and, with various platform distributions, an industry standard. The original owner, PKWARE, still distributes the program in versions for a number of different operating system platforms. Several other companies also distribute crossplatform versions that enable files to be economically exchanged between locations. For example, a company with an IBM OS/390 (MVS) platform can use PKZIP MVS to build an archive that can be transmitted to a Sun Solaris (UNIX) platform or to an IBM OS/400 platform where it can be unzipped using PKUNZIP UNIX. Windows users often use WinZip, another popular program that also uses the .zip file format.
PL/I PL/I is a third-generation (3GL) programming language developed in the early 1960s as an alternative to assembler language (for low-level computer processing functions), COBOL (for large-scale business applications), and FORTRAN (for scientific and algorithmic applications). PL/I stands for ‘‘Programming Language 1.’’ PL/I was an antecedent of the C programming language, which essentially replaced it as an all-purpose serious programming language. PL/S, which IBM evolved from PL/I, was, until the advent of C, IBM’s language of choice for its operating systems and application subsystems.
PL/S PL/S is a language that IBM designed for use in developing system programs, especially in mainframe operating systems and application subsystems. It evolved from or was at least influenced by PL/I.
PL/SQL In Oracle database management, PL/SQL is a procedural language extension to Structured Query Language (SQL). The purpose of PL/SQL is to combine database language and procedural programming language. The basic unit in PL/SQL is called a block, which is made up of three parts: a declarative part, an executable part, and an exceptionbuilding part. Because PL/SQL allows you to mix SQL statements with procedural constructs, it is possible to use PL/SQL blocks and subprograms to group SQL statements before sending them to Oracle for execution. Without PL/SQL, Oracle must process SQL statements one at a time and, in a network environment, this can affect traffic flow and slow down response time. PL/SQL blocks can be compiled once and stored in executable form to increase response time.
A PL/SQL program that is stored in a database in compiled form and can be called by name is referred to as a stored procedure. A PL/SQL stored procedure that is implicitly started when an INSERT, UPDATE or DELETE statement is issued against an associated table is called a trigger.
plaintext In cryptography, plaintext is ordinary readable text before being encrypted into ciphertext or after being decrypted.
planogram A planogram is a diagram that shows how and where specific retail products should be placed on retail shelves or displays in order to increase customer purchases. Planogramming is a skill developed in the fields of merchandising and retail space planning. A person with this skill can be referred to as a planogrammer. Planograms themselves are also referred to as POGs. Planograms can be developed for different types of retail merchandising displays (such as shelf displays, pegboards, or slatboard displays). They ar developed using other information about products, such as the amount of inventory left for the product, volume of sales per square foot of retail space, and other specific information about products (such as stockkeeping unit numbers, product codes, and so forth). By analyzing past and current sales patterns, a planogrammer can make successful recommendations about the number of ‘‘facings’’ a certain product should have on a retail display, how high or low it should be on the display, as well as which products should surround it. (Facings represent how many of the products should be facing straight out toward the customer.) The resulting planogram can be printed out as a visual to be followed by the part-time help that is often hired to restock retail shelves and displays. This gives executive management of a retail store or chain more control over how products are displayed, and allows them to track and improve on the success of their planograms. Planogram software is available for somewhere between $750 to well over $10,000. On the high end of the market, a planogramming component also is included with other larger space planning and retail space management software applications. Some consulting firms specialize in retail space management and planogramming.
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plasma display A plasma display is a computer video display in which each pixel on the screen is illuminated by a tiny bit of plasma or charged gas, somewhat like a tiny neon light. Plasma displays are thinner than cathode ray tube (CRT) displays and brighter than liquid crystal displays (LCD). Plasma displays are sometimes marketed as ‘‘thin-panel’’ displays and can be used to display either analog video signals or display digital computer input.
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Whatis?Com’s Encyclopedia of Technology Terms In addition to the advantage of slimness, a plasma display is flat rather than slightly curved as a CRT display is and therefore free of distortion on the edges of the screen. Unlike many LCD displays, a plasma display offers a very wide viewing angle. Plasma displays come in conventional PC displays sizes and also in sizes up to 60 inches for home theater and high definition television. IBM built a monochrome plasma display in the 1980s that displayed orange letters against a black screen. Today’s displays contain a grid of cells in which gas reacts with phosphors in varying degrees in red, green, or blue subpixels, making it possible to produce over 16 million different colors.
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Plastic Interface for Collaborative Technology Initiatives
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See ‘‘PICTIVE’’
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plastic pin grid array See ‘‘PPGA’’
I Platform for Privacy Preferences Project
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Sponsored by the World Wide Web Consortium, P3P (Platform for Privacy Preferences Project) is a framework for products and practices that will let World Wide Web users control the amount of personal information they share with Web sites. It’s described as a ‘‘privacy assistant.’’ Using a P3P application, a user can enter personal information once and not have to repeatedly reenter it at different Web sites. The P3P application can inform a user (or a programmed agent that is operating for the user) of a Web site’s practices with regard to gathering and reusing its visitors’ personal information. Users will be able to define the information that a specific site can be provided or not provided. P3P is related to and encompasses the Open Profiling Standard (OPS) proposed by Netscape Communications (now part of America Online). It also considers Microsoft’s formal proposal for privacy and profiling on the Web. P3P defines a vocabulary and a standard data format for expressing personal information within the W3C’s Resource Definition Framework (RDF), which uses the syntax of the Extensible Markup Language (XML). P3P will not necessarily replace the use of cookies (individual files that a Web site puts on the user’s own hard disk to keep track of interaction). However, it may change the way that cookies are used. The P3P Recommendations are available at the W3C Web site. Companies that have worked on or contributed to P3P include AT&T Labs, Center for Democracy and Technology, Digital Equipment Corporation, DISA, DoubleClick, Engage Technologies, Ernst & Young LLP, Firefly Network Inc, IBM, Intermind Corporation, MatchLogic, Microsoft, MIT,
Narrowline, NEC, Netscape Communications, Open Market Inc., Open Sesame, Oracle Corporation, Sony, The DMA, TRUSTe, and VeriSign.
platform 1) In computers, a platform is an underlying computer system on which application programs can run. On personal computers, Windows 2000 and the Macintosh are examples of two different platforms. On enterprise servers or mainframes, IBM’s S/390 is an example of a platform. A platform consists of an operating system, the computer system’s coordinating program, which in turn is built on the instruction set for a processor or microprocessor, the hardware that performs logic operations and manages data movement in the computer. The operating system must be designed to work with the particular processor’s set of instructions. As an example, Microsoft’s Windows 2000 is built to work with a series of microprocessors from the Intel Corporation that share the same or similar sets of instructions. There are usually other implied parts in any computer platform such as a motherboard and a data bus, but these parts have increasingly become modularized and standardized. Historically, most application programs have had to be written to run on a particular platform. Each platform provided a different application program interface for different system services. Thus, a PC program would have to be written to run on the Windows platform and then again to run on the Macintosh platform. Although these platform differences continue to exist and there will probably always be proprietary differences between them, new open or standards-conforming interfaces now allow many programs to run on different platforms or to interoperate with different platforms through mediating or ‘‘broker’’ programs. 2) A platform is any base of technologies on which other technologies or processes are built.
PLEDM PLEDM (phase-state low electron drive memory) is a new memory microchip technology that offers vastly greater capacity than memory devices commonly used in the past. The images and sound for a full-length movie can be stored on a single PLEDM device. Because the memory is nonvolatile memory (data remains when power is removed), PLEDM chips, which contain no moving parts, offer the possibility of replacing the electromechanical disk drives familiar to today’s PC users. The first PLEDM chips, developed in 1999 at Hitachi’s laboratory at the University of Cambridge, had a read/write time of less than 10 nanoseconds (10-8 seconds). In the future, these chips promise to combine the high speed currently available with DRAM (dynamic random-access memory) and the nonvolatility of so-called flash memory. It is envisioned that the chips could also be used in mobile and
point-of-presence portable telephone sets and in digital video recorders. Ultimately, this technology might lead to single-electron memory (SEM), in which each data bit is determined by the presence or absence of one electron in an atom or at a particular energy level within an atom. Products using PLEDM are expected to become available at the consumer level by 2005.
plenum In building construction, a plenum (pronounced PLEHnuhm, from Latin meaning full) is a separate space provided for air circulation for heating, ventilation, and airconditioning (sometimes referred to as HVAC) and typically provided in the space between the structural ceiling and a drop-down ceiling. A plenum may also be under a raised floor. In buildings with computer installations, the plenum space is often used to house connecting communication cables. Because ordinary cable introduces a toxic hazard in the event of fire, special plenum cabling is required in plenum areas. In the U.S., typical plenum cable sizes are American Wire Gauge (AWG) sizes 22 and 24. Plenum cabling is often made of Teflon and is more expensive than ordinary cabling. In the event of fire, its outer material is more resistant to flames and, when burning, produces less smoke than ordinary cabling. Both twisted pair and coaxial cable are made in plenum cable versions.
plesiochronous Plesiochronous (pronounced plee-see-AH-krun-us, from Greek plesos, meaning close, and chronos, meaning time) is an adjective that describes operations that are almost, but not quite, in synchronization—in other words, almost synchronous. The term is used in the Plesiochronous Digital Hierarchy (PDH), the widely-used system in which the transmissions from one continent (such as North America) are internetworked with transmissions in other continents (such as Europe) by making small adjustments in the differing data rates between the systems. A plesiochronous system could arise as the result of two systems having slightly different clock readings over time. In such a system, one of the systems or a third system would need to notice the mismatch and make some compensating adjustment, such as repeating or deleting a data packet or frame. Plesiochronous can be compared with asynchronous, isochronous, and synchronous.
Plug and Play Plug and Play (PnP) is a capability developed by Microsoft for its Windows 95 and later operating systems that gives users the ability to plug a device into a computer and have the computer recognize that the device is there. The user
doesn’t have to tell the computer. In many earlier computer systems, the user was required to explicitly tell the operating system when a new device had been added. Microsoft made Plug and Play a selling point for its Windows operating systems. A similar capability had long been built into Macintosh computers. With Microsoft’s participation, Plug and Play has been replaced by an open, industry standard, Universal Plug and Play (UPnP), which uses Internet protocols for seamless device plug-in. Also see plug-in.
plug-and-play See ‘‘Plug and Play’’
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plug-in Plug-in applications are programs that can easily be installed and used as part of your Web browser. Initially, the Netscape browser allowed you to download, install, and define supplementary programs that played sound or motion video or performed other functions. These were called helper applications. However, these applications run as a separate application and require that a second window be opened. A plug-in application is recognized automatically by the browser and its function is integrated into the main HTML file that is being presented.
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Among popular plug-ins to download are Adobe’s Acrobat, a document presentation and navigation program that lets you view documents just as they look in the print medium, RealNetworks’ streaming video player, and Macromedia’s Shockwave for DIrector, an interactive animation and sound player. There are now hundreds of possible plug-ins. Most users wait until they need a particular plug-in before they download it.
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See ‘‘phase modulation’’
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See ‘‘Portable Network Graphics’’
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See ‘‘Plug and Play’’
point-of-presence A point-of-presence (POP) is an access point to the Internet. A POP necessarily has a unique Internet Protocol (IP) address. Your Internet service provider (ISP) or online service provider (such as AOL) has a point-of-presence on
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the Internet and probably more than one. The number of POPs that an ISP or OSP has is sometimes used as a measure of its size or growth rate.
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A POP may actually reside in rented space owned by the telecommunications carrier (such as Sprint) to which the ISP is connected. A POP usually includes routers, digital/analog call aggregators, servers, and frequently frame relays or ATM switches.
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Point-to-Point Protocol
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See ‘‘PPP’’
E Point-to-Point Tunneling Protocol
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PPTP (Point-to-Point Tunneling Protocol) is a protocol (set of communication rules) that allows corporations to extend their own corporate network through private ‘‘tunnels’’ over the public Internet. Effectively, a corporation uses a widearea network as a single large local area network. A company no longer needs to lease its own lines for wide-area communication but can securely use the public networks. This kind of interconnection is known as a virtual private network (VPN). PPTP, a proposed standard sponsored by Microsoft and other companies, and Layer 2 Forwarding, proposed by Cisco Systems, are among the most likely proposals as the basis for a new Internet Engineering Task Force (IETF) standard. With PPTP, which is an extension of the Internet’s Point-to-Point Protocol (PPP), any user of a PC with PPP client support is able to use an independent service provider (ISP) to connect securely to a server elsewhere in the user’s company.
poison reverse In a computer network that uses the Routing Information Protocol (RIP) or other distance vector routing protocols, a poison reverse is a way in which a gateway node tells its neighbor gateways that one of the gateways is no longer connected. To do this, the notifying gateway sets the number of hops to the unconnected gateway to a number that indicates ‘‘infinite’’ (meaning ‘‘You can’t get there’’). Since RIP allows up to 15 hops to another gateway, setting the hop count to 16 would mean ‘‘infinite.’’
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Polarity is a term used in electricity, magnetism, and electronic signaling. Suppose there is a constant voltage, also called an electric potential or electromotive force (EMF), between two objects or points. In such a situation, one of the objects or points (poles) has more electrons than the other. The pole with relatively more electrons is said to have negative polarity; the other is assigned positive polarity. If the two poles are connected by a conductive path such as a
Also see bipolar signaling and unipolar signaling.
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wire, electrons flow from the negative pole toward the positive pole. This flow of charge carriers constitutes an electric current. In physics, the theoretical direction of current flow is considered to be from positive to negative by convention, opposite to the flow of electrons. The movement of electric charge carriers inevitably produces a magnetic field. Conversely, any magnetic field is the result of the motion of charge carriers. In a permanent magnet, a magnetic field is produced by the composite motions of electrons in geometrically aligned atoms. A magnetic field is characterized by poles called north and south. Magnetic polarity refers to the orientation of these poles in space. In digital communications, data is composed of shortduration pulses called bits. There are two possible states for each bit: Logic 0 (also called low) and logic 1 (also called high). In a closed circuit, these logic elements are represented by direct current voltages. A high-speed data signal varies rapidly between the low and high states. Common values are approximately +0.5 volts for low and +5 volts for high. In some cases different values are used, for example, 73 volts for low and +3 volts for high, or 75 volts for low and 70.5 volts for high. If both voltages have the same polarity, the signal is called unipolar; if the voltages have opposite polarity, the signal is called bipolar.
policy 1) In policy-based networking, a policy is a formal set of statements that define how the network’s resources are to be allocated among its clients. Clients can be individual users, departments, host computers, or applications. Resources can be allocated based on time of day, client authorization priorities, availability of resources, and other factors. Allocation can be static or dynamic (based on variations in traffic). Policies and policy statements are created by network managers and stored in a policy repository. During network operation, the policies are retrieved and used by network management software to make decisions. Ideally, policy statements can be written in natural language like this: Give voice-over-IP users the highest priority. Give teleconference users the next highest priority.
In practice, most policy-based networking software today requires specific knowledge of the network configuration and the use of a more artificial language. 2) In Hierarchical Storage Management (HSM), a policy is a set of statements used to guide HSM software about when to backup or archive files or to migrate them from one form of backup storage to another.
polymer LED
policy-based management Policy-based management is an administrative approach that is used to simplify the management of a given endeavor by establishing policies to deal with situations that are likely to occur. Policies are operating rules that can be referred to as a means of maintaining order, security, consistency, or other ways of successfully furthering a goal or mission. For example, a town council might have in place a policy against hiring the relatives of council members for civic positions. Then, each time that situation arose, council members could refer to the policy rather than deciding on the merits of individual cases. In the computing world, policy-based management is used as an administrative tool throughout an enterprise or a network, or on workstations that have multiple users. Policy-based management includes policybased network management, the use of delineated policies to control access to and priorities for the use of resources. Policy-based management may be used in systems management, or the creation and operation of an efficient computing environment. Policy-based management of a multi-user workstation typically includes setting individual policies for such things as access to files or applications, various levels of access (such as ‘‘read-only’’ permission, or permission to update or delete files), the appearance and makeup of individual users’ desktops, and so on. Specific user settings are activated when that person logs in to the computer. There are a number of software packages available to automate some elements of policy-based management. In general, the way these work is as follows: business policies are input to the products, and the software communicates to network hardware how to support those policies.
policy-based networking Policy-based networking is the management of a network so that various kinds of traffic—data, voice, and video—get the priority of availability and bandwidth needed to serve the network’s users effectively. With the convergence of data, telephone, and video traffic in the same network, companies will be challenged to manage traffic so that one kind of service doesn’t preempt another kind. Using policy statements, network administrators can specify which kinds of service to give priority at what times of day on what parts of their Internet Protocol (IP)-based network. This kind of management is often known as Quality of Service (QoS) and is controlled using policy-based network software. Theoretically, a policy statement could be as natural as: ‘‘Provide the fastest forwarding for all voice traffic to Chicago between 9 am and 3pm.’’ In actuality, most policy-based networking software today requires a much more detailed and network-aware statement. Currently, the Internet Engineering Task Force (IETF) is working on a standard policy framework and related protocols. A typical policy-based network includes:
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A network management console at which policies are entered, edited, or called from a policy repository
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A server, referred to as the policy decision point (PDP), that retrieves policies from the policy repository and acts on the policies on behalf of Policy Enforcement Points (PEPs)
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The Policy Enforcement Points (PEPs), which are the routers, switches, and other network devices that enforce the policies, using access control list, queue management algorithms, and so forth The Policy Repository, a directory server of policies that is based on the Lightweight Directory Access Protocol (LDAP)
Among companies that currently provide proprietary policy-based networking software are Cabletron, Cisco, Nortel, and 3Com. These companies are aware of (and perhaps contributing to) the developing standard. Their products are likely to continue to evolve in support of the standard.
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polling In electronic communication, ‘‘polling’’ is the continuous checking of other programs or devices by one progam or device to see what state they are in, usually to see whether they are still connected or want to communicate.
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Specifically, in multipoint or multidrop communication (a controlling device with multiple devices attached that share the same line), the controlling device sends a message to each device, one at a time, asking each whether it has anything to communicate (in other words, whether it wants to use the line).
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polyLED
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O polymer LED Polymer LED (light-emitting diode)—sometimes called light-emitting polymer or polyLED—is a technology based on the use of polymer as the semiconductor material in LEDs. Dow Chemical Corporation announced the discovery of light-emitting polymers at the spring meeting of the Materials Research Society in 1998. Polymers are chemical substances that consist of large molecules that are, themselves, made from many smaller and simpler molecules: Proteins and DNA are examples of naturally occuring polymers; many others, such as nylon, are artificially created. Because of their flexibility and strength, polymers are used for products such as car bumpers and bullet-proof vests. Because polymers are lightweight and flexible, they can simply and inexpensively be used for the creation of thin film displays for future technologies such as the electronic newspaper. Polymer LEDs have a number of inherent
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qualities that are ideally suited to such applications: they enable full-spectrum color displays, high brightness at low drive voltages, glare-free viewing, and long operating lifetimes. It is currently possible to create a small text display by structuring the polymer material and electrodes. The technology also offers a great deal of promise as a basis for cheaper, simpler lighting sources. According to Edmund Woo, of Dow’s Materials Researched and Development Laboratory, polymer LEDs could be used to create a brightness similiar to that of fluorescent lighting on a 5-volt power supply.
download any mail. POP3 is built into the Netmanage suite of Internet products and one of the most popular e-mail products, Eudora. It’s also built into the Netscape and Microsoft Internet Explorer browsers.
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POP and IMAP deal with the receiving of e-mail and are not to be confused with the Simple Mail Transfer Protocol (SMTP), a protocol for transferring e-mail across the Internet. You send e-mail with SMTP and a mail handler receives it on your recipient’s behalf. Then the mail is read using POP or IMAP.
A polynomial is a mathematical expression consisting of a sum of terms, each term including a variable or variables raised to a power and multiplied by a coefficient. The simplest polynomials have one variable. A one-variable (univariate) polynomial of degree n has the following form: anxn + an-1xn-1 + ... + a2x2 + a1x1 + a0x0 where the a’s represent the coefficients and x represents the variable. Because x1 = x and x0 = 1 for all complex numbers x, the above expression can be simplified to: n
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anx + an-1x -1 + ... + a2x + a1x + a0 When an nth-degree univariate polynomial is equal to zero, the result is a univariate polynomial equation of degree n: anxn + an-1xn-1 + ... + a2x2 + a1x + a0 = 0 There may be several different values of x, called roots, that satisfy a univariate polynomial equation. In general, the higher the order of the equation (that is, the larger the value of n), the more roots there are. A univariate polynomial equation of degree 1 (n = 1) constitutes a linear equation. When n = 2, it is a quadratic equation; when n = 3, it is a cubic equation; when n = 4, it is a quartic equation; when n = 5, it is a quintic equation. The larger the value of n, the more difficult it is to find all the roots of a univariate polynomial equation.
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Some polynomials have two, three, or more variables. A two-variable polynomial is called bivariate; a three-variable polynomial is called trivariate.
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PON See ‘‘passive optical network’’
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POP3 (Post Office Protocol 3) is the most recent version of a standard protocol for receiving e-mail. POP3 is a client/ server protocol in which e-mail is received and held for you by your Internet server. Periodically, you (or your client email receiver) check your mail-box on the server and
An alternative protocol is Internet Message Access Protocol (IMAP). With IMAP, you view your e-mail at the server as though it was on your client computer. An e-mail message deleted locally is still on the server. E-mail can be kept on and searched at the server. POP can be thought of as a ‘‘store-and-forward’’ service. IMAP can be thought of as a remote file server.
The conventional port number for POP3 is 110.
pop-up A pop-up is a graphical user interface (GUI) display area, usually a small window, that suddenly appears (’’pops up’’) in the foreground of the visual interface. Pop-ups can be initiated by a single or double mouse click or rollover (sometimes called a mouseover), and also possibly by voice command or can simply be timed to occur. A pop-up window must be smaller than the background window or interface; otherwise, it’s a replacement interface. On the World Wide Web, JavaScript (and less commonly Java applets) are used to create interactive effects including pop-up and full overlay windows. A menu or taskbar pulldown can be considered a form of pop-up. So can the little message box you get when you move your mouse over taskbars in many PC applications.
port 1) On computer and telecommunication devices, a port (noun) is generally a specific place for being physically connected to some other device, usually with a socket and plug of some kind. Typically, a personal computer is provided with one or more serial ports and usually one parallel port. The serial port supports sequential, one bit-ata-time transmission to peripheral devices such as scanners and the parallel port supports multiple-bit-at-a-time transmission to devices such as printers. 2) In programming, a port (noun) is a ‘‘logical connection place’’ and specifically, using the Internet’s protocol, TCP/ IP, the way a client program specifies a particular server program on a computer in a network. Higher-level applications that use TCP/IP such as the Web protocol, Hypertext Transfer Protocol, have ports with preassigned numbers. These are known as ‘‘well-known ports’’ that have been assigned by the Internet Assigned Numbers Authority
port replicator (IANA). Other application processes are given port numbers dynamically for each connection. When a service (server program) initially is started, it is said to bind to its designated port number. As any client program wants to use that server, it also must request to bind to the designated port number. Port numbers are from 0 to 65536. Ports 0 to 1024 are reserved for use by certain privileged services. For the HTTP service, port 80 is defined as a default and it does not have to be specified in the Uniform Resource Locator (URL). 3) In programming, to port (verb) is to move an application program from an operating system environment in which it was developed to another operating system environment so it can be run there. Porting implies some work, but not nearly as much as redeveloping the program in the new environment. Open standard programming interfaces (such as those specified in X/Open’s 1170 C language specification and Sun Microsystem’s Java programming language) minimize or eliminate the work required to port a program. Also see portability.
port 80 On a Web server or Hypertext Transfer Protocol daemon, port 80 is the port that the server ‘‘listens to’’ or expects to receive from a Web client, assuming that the default was taken when the server was configured or set up. A port can be specified in the range from 0-65536. However, the server administrator configures the server so that only one port number can be recognized. By default, the port number for a Web server is 80. Experimental services are sometimes run at port 8080.
port mirroring Port mirroring, also known as a roving analysis port, is a method of monitoring network traffic that forwards a copy of each incoming and outgoing packet from one port of a network switch to another port where the packet can be studied. A network administrator uses port mirroring as a diagnostic tool or debugging feature, especially when fending off an attack. It enables the administrator to keep close track of switch performance and alter it if necessary. Port mirroring can be managed locally or remotely.
Port mirroring can consume significant CPU resources while active. Better choices for long-term monitoring may include a passive tap like an optical probe or an Ethernet repeater.
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port number A port number is a way to identify a specific process to which an Internet or other network message is to be forwarded when it arrives at a server. For the Transmission Control Protocol and the User Datagram Protocol, a port number is a 16-bit integer that is put in the header appended to a message unit. This port number is passed logically between client and server Transport layers. For example, a request from a client (perhaps on behalf of you at your PC) to a server on the Internet may request a file be served from that host’s File Transfer Protocol (FTP) server or process. In order to pass your request to the FTP process in the remote server, the Transmission Control Protocol (TCP) software layer in your computer identifies the port number of 21 (which by convention is associated with an FTP request) in the 16-bit port number integer that is appended to your request. At the server, the TCP layer will read the port number of 21 and forward your request to the FTP program at the server. Some services or processes have conventionally assigned permanent port numbers. These are known as well-known port numbers. In other cases, a port number is assigned temporarily (for the duration of the request and its completion) from a range of assigned port numbers. This is called an ephemeral port number.
port replicator A port replicator is an attachment for a notebook computer that allows a number of devices such as a printer, large monitor, and keyboard to be simultaneously connected. Each of the devices is attached to the port replicator and when the notebook user wants access to one or more of the devices, the user simply attaches the port replicator rather than having to connect one device at a time. The port replicator duplicates each of the notebook’s port, including parallel and serial ports. In addition, it may provide some extra ports for devices such as joysticks or Musical Instrument Digital Interface (MIDI) devices.
An administrator configures port mirroring by assigning a port from which to copy all packets and another port where those packets will be sent. A packet bound for or heading away from the first port will be forwarded onto the second port as well. The administrator places a protocol analyzer on the port receiving the mirrored data to monitor each segment separately. The analyzer captures and evaluates the data without affecting the client on the original port.
A port replicator is similar to a docking station, but does not include expansion slots.
The monitor port may be a port on the same SwitchModule with an attached RMON probe, a port on a different SwitchModule in the same hub, or the SwitchModule processor.
Connecting a notebook to a port replicator is a simple process:
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Attach all devices needed to the port replicator.
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Close the notebook’s display panel and align the connectors of the port replicator to the notebook ports.
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The notebook computer can be on or off during the connection and disconnection process. To disconnect the computer from the port replicator, press the notebook release buttons and lift the notebook away from the replicator.
port scan
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A port scan is a series of messages sent by someone attempting to break into a computer to learn which computer network services, each associated with a ‘‘wellknown’’ port number, the computer provides. Port scanning, a favorite approach of computer crackers, gives the assailant an idea where to probe for weaknesses. Essentially, a port scan consists of sending a message to each port, one at a time. The kind of response received indicates whether the port is used and can therefore be probed for weakness. Types of port scans include: L
Vanilla—An attempt to connect to all ports (there are 65,536)
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Strobe—An attempt to connect to only selected ports (typically, under 20)
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Stealth scan—Several techniques for scanning that attempt to prevent the request for connection being logged
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FTP Bounce Scan—Attempts that are directed through an FTP server to disguise the cracker’s location
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Fragmented Packets—Scans by sending packet fragments that can get through simple packet filters in a firewall
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UDP—Scans for open User Datagram Protocol ports
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Sweep—Scans the same port on a number of computers
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Portability is a characteristic attributed to a computer program if it can be used in an operating systems other than the one in which it was created without requiring major rework. Porting is the task of doing any work necessary to make the computer program run in the new environment. In general, programs that adhere to standard program interfaces such as the X/Open UNIX 95 standard C language interface are portable. Ideally, such a program needs only to be compiled for the operating system to which it is being ported. However, programmers using standard interfaces also sometimes use operating system extensions or special capabilities that may not be present in the new operating system. Uses of such extensions have to be removed or replaced with comparable functions in the new operating system. In addition to language differences, porting may also require data conversion and adaptation to new system procedures for running an application. Portability has usually meant some work when moving an application program to another operating system. Recently, the Java programming language and runtime environment
has made it possible to have programs that run on any operating system that supports the Java standard (from Sun Microsystems) without any porting work. Java applets in the form of precompiled bytecode can be sent from a server program in one operating system to a client program (your Web browser) in another operating system without change.
portable computer A portable computer is a personal computer that is designed to be easily transported and relocated, but is larger and less convenient to transport than a notebook computer. The earliest PCs designed for easy transport were called portables. As the size and weight of most portables decreased, they became known as laptop computers and later as notebook computers. Today, larger transportable computers continue to be called portable computers. Most of these are special-purpose computers—for example, those for use in industrial environments where they need to be moved about frequently.
Portable Document Format PDF (Portable Document Format) is a file format that has captured all the elements of a printed document as an electronic image that you can view, navigate, print, or forward to someone else. PDF files are created using Adobe Acrobat, Acrobat Capture, or similar products. To view and use the files, you need the free Acrobat Reader, which you can easily download. Once you’ve downloaded the Reader, it will start automatically whenever you want to look at a PDF file. PDF files are especially useful for documents such as magazine articles, product brochures, or flyers in which you want to preserve the original graphic appearance online. A PDF file contains one or more page images, each of which you can zoom in on or out from. You can page forward and backward. The Acrobat product that lets you create PDF files sells in the $200-300 range. A non-Adobe alternative is a product called Niknak from 5D, a company in the UK. (The Reader itself is free and can be used as a plug-in with your Web browser or can be started by itself.) Some situations in which PDF files are desirable include: L
Graphic design development in which team members are working at a distance and need to explore design ideas online
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Help desk people who need to see the printed book that users are looking at
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The online distribution of any printed document in which you want to preserve its printed appearance
Acrobat’s PDF files are more than images of documents. Files can embed type fonts so that they’re available at any viewing location. They can also include interactive elements such as buttons for forms entry and for triggering sound and
portal software Quicktime or AVI movies. PDF files are optimized for the Web by rendering text before graphic images and hypertext links.
Portable Network Graphics PNG (pronounced PEENG) is a file format for image compression that, in time, is expected to replace the Graphics Interchange Format format that is widely used on today’s Internet. Owned by Unisys, the GIF format and its usage in image-handling software involves licensing or other legal considerations. (Web users can make, view, and send GIF files freely but they can’t develop software that builds them without an arrangement with Unisys.) The PNG format, on the other hand, was developed by an Internet committee expressly to be patent-free. It provides a number of improvements over the GIF format. Like a GIF, a PNG file is compressed in lossless fashion (meaning all image information is restored when the file is decompressed during viewing). A PNG file is not intended to replace the JPEG format, which is ‘‘lossy’’ but lets the creator make a trade-off between file size and image quality when the image is compressed. Typically, an image in a PNG file can be 10 to 30% more compressed than in a GIF format. The PNG format includes these features: L
You can not only make one color transparent, but you can control the degree of transparency (this is also called ‘‘opacity’’).
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Interlacing of the image is supported and is faster in developing than in the GIF format.
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Gamma correction allows you to ‘‘tune’’ the image in terms of color brightness required by specific display manufacturers.
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Images can be saved using true color as well as in the palette and gray-scale formats provided by the GIF.
Unlike the GIF89a, the PNG format doesn’t support animation since it can’t contain multiple images. The PNG is described as ‘‘extensible,’’ however. Software houses will be able to develop variations of PNG that can contain multiple, scriptable images.
Portable Operating System Interface See ‘‘POSIX’’
portal 1) Portal is a term, generally synonymous with gateway, for a World Wide Web site that is or proposes to be a major starting site for users when they get connected to the Web or that users tend to visit as an anchor site. There are general portals and specialized or niche portals. Some major general portals include Yahoo, Excite, Netscape, Lycos, CNET, Microsoft Network, and America Online’s AOL.com.
Examples of niche portals include Garden.com (for gardeners), Fool.com (for investors), and SearchNetworking.com (for network administrators). A number of large access providers offer portals to the Web for their own users. Most portals have adopted the Yahoo style of content categories with a text-intensive, faster loading page that visitors will find easy to use and to return to. Companies with portal sites have attracted much stock market investor interest because portals are viewed as able to command large audiences and numbers of advertising viewers. Typical services offered by portal sites include a directory of Web sites, a facility to search for other sites, news, weather information, e-mail, stock quotes, phone and map information, and sometimes a community forum. Excite is among the first portals to offer users the ability to create a site that is personalized for individual interests. The term portal space is used to mean the total number of major sites competing to be one of the portals. 2) In fantasy games, science-fiction, and some ‘‘New Age’’ philosophies, a portal is a gateway to another world of the past, present, or future, or to an expanded awareness. 3) In 3-D graphics development, portal rendering is a technique that increases the effect of realism and speeds up presentation.
Portal Markup Language Portal Markup Language (PML), an application of the Extensible Markup Language (XML), describes the characteristics of a product that is used to create a portal Web site (sometimes referred to as an enterprise information portal). PML allows manufacturers to exchange information in a standard way about ‘‘information objects, users, groups, access control subscriptions and notifications managed by the system.’’ PML is spelled out in a formal document tag definition (DTD). PML does not describe syndication (how content from one site could be published on other sites).
portal software Portal software is a type of development tool used to create a portal (starting point) on a company’s intranet so that employees can find a centralized starting place for access to consolidated enterprise-related functions, such as e-mail, customer relationship management (CRM) tools, company information, workgroup systems, and other applications. The package may be customized to varying degrees of enterprise or individual specificity. Portal software is similar to intranet software, but the end product typically features more complexity, automation, organization, and interactivity. Although the end product is sometimes referred to as an intranet portal, it is usually called an enterprise information portal.
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Whatis?Com’s Encyclopedia of Technology Terms Portal software packages generally fall into one of the four following categories: Digital dashboard, pure-play, application, or infrastructure portals. A digital dashboard portal package creates a centralized starting point for various applications and provide a summary of information pertaining to those applications, just as a car’s dashboard provides centralized access to summarized information about various aspects of the car’s critical details. A pure-play portal package concentrates on creating a portable interface for a specific portal offering across multiple platforms. Pureplay provides more functionality than the digital dashboard in a centralized access point for various business productivity applications, such as e-mail, collaboration tools, and resource planning tools. An application portal package includes a wide variety of tools—such as workgroup software—that have been adapted to go with a portal interface. Most common applications now fall into this category, since most are built to work in a portal environment. An infrastructure portal package is similar to a pure-play package, but is specifically geared towards technologies developed within the company rather than towards portability; for example, Oracle’s portal product is geared toward Oracle technologies. Corechange, Epicentric, Hummingbird, and Plumtree are among the leading portal software vendors. Most major software companies have released some type of portal software. Although the current generation of portal software is oriented towards the enterprise itself, it is likely that applications will increasingly encompass enterprisecustomer interactions as well.
positional assembly Positional assembly is one of several techniques that have been developed or suggested to construct machines on an atomic scale, where dimensions are measured in nanometers. One nanometer is 10-9 meter, or a millionth of a millimeter. Two things are necessary for positional assembly to be practical. The first requirement is that a technician be able to see, and analyze, what is taking place as the process is carried out. This is known as nanoanalysis, and requires the use of precision observation equipment with high magnification and excellent resolving power. Electron microscopes are commonly used; alternative devices include ultraviolet (UV) and infrared (IR) microscopes, ion-beam devices, and X-ray microscopes. Computer processing of the images makes it possible to get a three-dimensional view. The second requirement for positional assembly is the ability to manipulate tiny objects at will. This sometimes involves attaching individual atoms to, or detaching them from, other atoms one by one. In some cases this might be done by direct manipulation with microscopic tools. It might also be accomplished with the aid of special adhesives, or by
introducing electrical charges to cause atoms to stick together or come apart. Also see exponential assembly, nanoanalysis, and self-assembly.
POSIX POSIX (Portable Operating System Interface) is a set of standard operating system interfaces based on the UNIX operating system. The need for standardization arose because enterprises using computers wanted to be able to develop programs that could be moved among different manufacturer’s computer systems without having to be recoded. UNIX was selected as the basis for a standard system interface partly because it was ‘‘manufacturerneutral.’’ However, several major versions of UNIX existed so there was a need to develop a common denominator system. Informally, each standard in the POSIX set is defined by a decimal following the POSIX. Thus, POSIX.1 is the standard for an application program interface in the C language. POSIX.2 is the standard shell and utility interface (that is to say, the user’s command interface with the operating system). These are the main two interfaces, but additional interfaces, such as POSIX.4 for thread management, have been developed or are being developed. The POSIX interfaces were developed under the auspices of the Institute of Electrical and Electronics Engineers (IEEE). POSIX.1 and POSIX.2 interfaces are included in a somewhat larger interface known as the X/Open Programming Guide 4.2 (also known as the ‘‘Single UNIX Specification’’ and ‘‘UNIX 95’’). The Open Group, an industry standards group, owns the UNIX trademark and can thus ‘‘brand’’ operating systems that conform to the interface as ‘‘UNIX’’ systems. IBM’s OS/390 is an example of an operating system that includes a branded UNIX interface.
POST When power is turned on, POST (Power-On Self-Test) is the diagnostic testing sequence that a computer’s basic input/ output system (or ‘‘starting program’’) runs to determine if the computer keyboard, random access memory, disk drives, and other hardware are working correctly. If the necessary hardware is detected and found to be operating properly, the computer begins to boot. If the hardware is not detected or is found not to be operating properly, the BIOS issues an error message which may be text on the display screen and/or a series of coded beeps, depending on the nature of the problem. Since POST runs before the computer’s video card is activated, it may not be possible to progress to the display screen. The pattern of beeps may be a variable numbers of short beeps or a mixture of long and short beeps, depending on what type of BIOS is installed.
PowerBuilder The patterns of beeps contain messages about the nature of the problem detected. For example, if the keyboard is not detected, a particular pattern of beeps will inform you of that fact. An error found in the POST is usually fatal (that is, it causes current program to stop running) and will halt the boot process, since the hardware checked is absolutely essential for the computer’s functions.
postcardware Postcardware is freeware (no-charge software that is freely shared) that requires only that the user send the software provider a postcard as a form of payment. The idea is to humanize the transaction, remind the user that someone else shared something freely, and remind the provider that someone is actually using the creation.
Postscript Postscript is a programming language that describes the appearance of a printed page. It was developed by Adobe in 1985 and has become an industry standard for printing and imaging. All major printer manufacturers make printers that contain or can be loaded with Postscript software, which also runs on all major operating system platforms. A Postscript file can be identified by its ‘‘.ps’’ suffix. Postscript describes the text and graphic elements on a page to a black-and-white or color printer or other output device, such as a slide recorder, imagesetter, or screen display. Postscript handles industry-standard, scalable typeface in the Type 1 and TrueType formats. Users can convert Postscript files to the Adobe Portable Document Format (PDF) using the Adobe Acrobat product. PDF files present the document’s printed appearance on a display screen. (You’ll find many PDF documents for downloading and viewing from Web sites; you’ll need to download the Acrobat viewer as a plug-in.) Adobe sells a more sophisticated product called Supra for print-on-demand and production printing. Note that the average home laser printer is not a Postscript printer; which is somewhat more expensive and more frequently purchased for business use. (However, they aren’t that expensive and Postscript printers for home or small business use are worth considering.)
POTS POTS (plain old telephone service) is a term sometimes used in discussion of new telephone technologies in which the question of whether and how existing voice transmission for ordinary phone communication can be accommodated. For example, Asymmetric Digital Subscriber Line and Integrated Services Digital Network connections provide some part of their channels for ‘‘plain old telephone service’’ while providing most of their bandwidth for digital data transmission.
power Also see current, voltage, resistance, and Ohm’s Law. Electrical power is the rate at which electrical energy is converted to another form, such as motion, heat, or an electromagnetic field. The common symbol for power is the uppercase letter P. The standard unit is the watt, symbolized by W. In utility circuits, the kilowatt (kW) is often specified instead; 1 kW = 1000 W. One watt is the power resulting from an energy dissipation, conversion, or storage process equivalent to one joule per second. When expressed in watts, power is sometimes called wattage. The wattage in a direct current (DC) circuit is equal to the product of the voltage in volts and the current in amperes. This rule also holds for low-frequency alternating current (AC) circuits in which energy is neither stored nor released. At high AC frequencies, in which energy is stored and released (as well as dissipated or converted), the expression for power is more complex. In a DC circuit, a source of E volts, delivering I amperes, produces P watts according to the formula:
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When a potential difference of E volts appears across a component having a resistance of R ohms, then the power in watts dissipated or converted by that component is given by:
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P = E2/R In a DC circuit, power is a scalar (one-dimensional) quantity. In the general AC case, the determination of power requires two dimensions, because AC power is a vector quantity. Assuming there is no reactance (opposition to AC but not to DC) in an AC circuit, the power can be calculated according to the above formulas for DC, using root-mean-square values for the alternating current and voltage. If reactance exists, some power is alternately stored and released by the system. This is called apparent power power or reactive power. The resistance dissipates power as heat or converts it to some other tangible form; this is called true power. The vector combination of reactance and resistance is known as impedance.
PowerBuilder PowerBuilder is a popular rapid application development (RAD) tool for building object-oriented programming client/server applications the parts of which can be distributed within a network. PowerBuilder is a product of Sybase, a company that has historically been a leader in selling products for building distributed and replicated database in networks. A major feature of PowerBuilder (and its competitors) is the ability to create databases using an
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object-oriented interface. Applications created with PowerBuilder can access other popular types of databases on other major platforms using Open Database Connectivity (ODC). PowerBuilder includes:
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A set of utilities (called Painters) for managing database objects. These include:
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A table utility with a graphical interface that lets you create and modify data table structures and associate business rules on individual table columns
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A utility that lets you test Structured Query Language statements against the server you are building
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PowerScript, an object-oriented scripting language that lets you link everything together
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PowerBuilder emphasizes its ability to provide an end-user interface through a Web browser. Data from PowerBuilder tables can be converted into HTML tables. With a Window plug-in, a PowerBuilder application can be run from within a Web browser. And PowerBuilder’s Web.PB allows client applications to be put on servers accessible to network PC (‘‘thin client’’) workstations equipped with browsers but with the clients on the local server.
power-of-10 notation See ‘‘scientific notation’’
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PowerPC PowerPC is a microprocessor architecture, developed jointly by Apple, IBM, and Motorola, that uses reduced instructionset computing (RISC). The PowerPC chip is used principally in IBM’s RS/6000 workstation with its UNIX-based operating system, AIX, and in Apple Computer’s Macintosh personal computers with their Mac OS operating system. The three developing companies, calling themselves the PowerPC Alliance, have made the PowerPC architecture an open standard, inviting other companies to build the architecture further.
The PowerPC architecture provides an alternative for any computer maker to the extremely popular processor architectures from Intel, including the Pentium PC architecture. (Microsoft builds its Windows operating system offerings to run on Intel processors, and this widelysold combination is sometimes known as ‘‘Wintel’’.) Developed at IBM, reduced instruction-set computing (RISC) is based on studies showing that the simpler computer instructions are the ones most frequently performed. Traditionally, processors have been designed to accommodate the more complex instructions as well. RISC performs the more complex instructions using combinations of simple instructions. The timing for the processor can then be based on simpler and faster operations, enabling the microprocessor to perform more instructions for a given clock speed. Typically, the PowerPC can perform one instruction for each clock cycle. The PowerPC architecture handles 32-bit instructions.
PPD See ‘‘parallel presence detect’’
PPGA PPGA (plastic pin grid array) is a microchip design from Intel that has the silicon core of the microchip facing down toward the computer motherboard. The core is covered by a heat slug, which helps to dissipate the heat to the heatsink. The chip includes 370 pins that plug into a Socket 370 connector on the motherboard. The PPGA chip is less expensive to manufacture that the slot-based chip, which is why PPGA chips are used in sub-$1000 desktop computers. Intel has also developed another chip design called FC-PGA (flip chip-pin grid array). FC-PGA packages have the processor core flipped up on the back of the chip, facing away from the motherboard. In order to use either type of these microprocessors with its associated 370 socket, a computer’s motherboard must support certain guidelines, known as VRM specifications. (For PPGA processors, the motherboard must support VRM 8.2 specifications. For FCPGA processors, the motherboard must support VRM 8.4 specifications.) Both chip designs are used for the Celeron processor and use the Zero Insertion Force (ZIF) feature that helps the chip to come out of its socket easily.
ppi See ‘‘pixels per inch’’
PPP PPP (Point-to-Point Protocol) is a protocol for communication between two computers using a serial interface, typically a personal computer connected by phone line to a server. For example, your Internet server provider may provide you with a PPP connection so that the
Preboot Execution Environment provider’s server can respond to your requests, pass them on to the Internet, and forward your requested Internet responses back to you. PPP uses the Internet protocol (IP) (and is designed to handle others). It is sometimes considered a member of the TCP/IP suite of protocols. Relative to the Open Systems Interconnection (OSI) reference model, PPP provides layer 2 (data-link layer) service. Essentially, it packages your computer’s TCP/IP packets and forwards them to the server where they can actually be put on the Internet. PPP is a full-duplex protocol that can be used on various physical media, including twisted pair or fiber optic lines or satellite transmission. It uses a variation of High Speed Data Link Control (HDLC) for packet encapsulation. PPP is usually preferred over the earlier de facto standard Serial Line Internet Protocol (SLIP) because it can handle synchronous as well as asynchronous communication. PPP can share a line with other users and it has error detection that SLIP lacks. Where a choice is possible, PPP is preferred.
PPPoE PPPoE (Point-to-Point Protocol over Ethernet) is a specification for connecting multiple computer users on an Ethernet local area network to a remote site through common customer premises equipment, which is the telephone company’s term for a modem and similar devices. PPPoE can be used to have an office or building-full of users share a common Digital Subscriber Line (DSL), cable modem, or wireless connection to the Internet. PPPoE combines the Point-to-Point Protocol (PPP), commonly used in dialup connections, with the Ethernet protocol, which supports multiple users in a local area network. The PPP protocol information is encapsulated within an Ethernet frame. PPPoE has the advantage that neither the telephone company nor the Internet service provider (ISP) needs to provide any special support. Unlike dialup connections, DSL and cable modem connections are ‘‘always on.’’ Since a number of different users are sharing the same physical connection to the remote service provider, a way is needed to keep track of which user traffic should go to and which user should be billed. PPPoE provides for each user-remote site session to learn each other’s network addresses (during an initial exchange called ‘‘discovery’’). Once a session is established between an individual user and the remote site (for example, an Internet service provider), the session can be monitored for billing purposes. Many apartment houses, hotels, and corporations are now providing shared Internet access over DSL lines using Ethernet and PPPoE.
PPTP See ‘‘Point-to-Point Tunneling Protocol’’
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PRAM (parameter RAM or parameter random access memory) is a special battery-powered form of random access memory in certain Macintosh computers where vital system information such as the date and time are stored. PRAM also contains computer configuration information, such as what’s connected to each port on the computer. Because PRAM is powered by an internal battery, the information isn’t lost when you turn the computer off as it is with regular RAM. PRAM occasionally gets corrupted as the result of an application failure and needs to be zapped or restored. The procedure of ‘‘zapping the PRAM’’ is easily located by searching for it on major search engines. However, be aware that it may not apply to more recent Apple computers or operating systems. We suggest contacting Apple to find out the right procedure for your particular system.
preamble A preamble is a signal used in network communications to synchronize the transmission timing between two or more systems. Proper timing ensures that all systems are interpreting the start of the information transfer correctly. This is something like how a rock band drummer ‘‘lays down a beat’’ prior to beginning a song. By setting up the proper tempo, the band members are more likely to be synchronized and begin the song at the same moment. A preamble defines a specific series of transmission pulses that is understood by communicating systems to mean ‘‘someone is about to transmit data’’. This ensures that systems receiving the information correctly interpret when the data transmission starts. The actual pulses used as a preamble vary depending on the network communication technology in use.
Preboot Execution Environment The Preboot Execution Environment (PXE) is an industry standard client/server interface that allows networked computers that are not yet loaded with an operating system to be configured and booted remotely by an administrator. The PXE code is typically delivered with a new computer on a read-only memory chip or boot disk that allows the computer (a client) to communicate with the network server so that the client machine can be remotely configured and its operating system can be remotely booted. PXE provides three things: 1) The Dynamic Host Configuration Protocol (DHCP), which allows the client to receive an IP address to gain access to the network servers.
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2) A set of application program interfaces (API) that are used by the client’s Basic Input/Output Operating System (BIOS) or a downloaded Network Bootstrap Program (NBP) that automates the booting of the operating system and other configuration steps.
Not to be confused with an automatic dialer, a predictive dialer is programmed to predict when a human caller is available to pick up a call, A somewhat related system is the lead generator, which dials a list of telephone numbers and, when a live voice answers, delivers a recorded message.
3) A standard method of initializing the PXE code in the PXE ROM chip or boot disk.
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The PXE process consists of the client notifying the server that it uses PXE. If the server uses PXE, it sends the client a list of boot servers that contain the operating systems available. The client finds the boot server it needs and receives the name of the file to download. The client then downloads the file using Trivial File Transfer Protocol (TFTP) and executes it, which loads the operating system. If a client is equipped with PXE and the server is not, the server ignores the PXE code preventing disruption in the DHCP and Bootstrap Protocol (BP) operations. The advantages of using PXE include:
Preemptive multitasking is multitasking in which a computer operating system uses some criteria to decide how long to allocate to any one task before giving another task a turn to use the operating system. The act of taking control of the operating system from one task and giving it to another task is called preempting. A common criterion for preempting is simply elapsed time (this kind of system is sometimes called time sharing or time slicing). In some operating systems, some applications can be given higher priority than other applications, giving the higher priority programs control as soon as they are initiated and perhaps longer time slices.
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The client machine can be rebooted in the event of hardware or software failure. This allows the administrator to diagnose and perhaps fix the problem.
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Since PXE is vendor-independent, new types of computers can easily be added to the network.
Presence technology is a type of application that makes it possible to locate and identify a computing device (including, for example, handheld computers as well as desktop models) wherever it might be, as soon as the user connects to the network. One application of presence technology, instant messaging (IM), is already very popular. Presence technology is expected to be an integral part of third generation (3G) wireless networks, and is likely to be employed across a wide variety of communication devices, including cellphones, PDAs (personal digital assistants), television sets, and pagers. A number of wireless application service providers (WASPs) are developing platforms for mobile presence applications, called m-presence.
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Predication (also called branch predication) is a process implemented in Explicitly Parallel Instruction Computing (EPIC)-based processors and their compilers to increase performance by eschewing branch prediction (a common technique used in modern processors), where a wrong guess by the processor brings a performance penalty. In predication, all the possible code branches are executed in parallel before the branch condition is proved. Applications like data mining and data warehousing that depend on large back-end database stand to gain the most from predicative methods.
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Intel’s IA-64 architecture is based on EPIC and the first processor in the IA-64 line, the Itanium, uses predication.
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A predictive dialer is a telephone control system that automatically calls a list of telephone numbers in sequence and screens out no-answers, busy signals, answering machines, and disconnected numbers while predicting at what point a human caller will be able to handle the next call. Predictive dialers are commonly used for telemarketing, surveys, appointment confirmation, payment collection, and service follow-ups. Sellers of predictive dialer systems claim that they greatly increase caller productivity. The phone calls you receive from ‘‘no one there’’ are often predictive dialer calls in which a manual caller isn’t ready yet.
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Future applications of presence technology could take any number of forms. For example, a driver with a wireless phone enabled with GPS (global positioning system) could be tracked, sent messages warning about traffic delays and suggesting alternate routes. Among other possibilities, users of the technology could in the future automatically set up an impromptu teleconference by connecting all the parties as soon as they were detected to be available. Privacy issues will be addressed by allowing a high degree of user-defined control, allowing people to select conditions in which they would be detectable, for example. The Internet Engineering Task Force’s (IETF) Instant Messaging and Presence Protocol (IMPP) Working Group was formed to establish core standards that could be used to make presence technologies interoperable, a challenge that is currently slowing their development. Many of the current IM systems, for example, don’t make it possible for users to exchange messages with the customers of other systems, a situation which has been compared to a long distance telephone service provider making it impossible for users to communicate with another long distance provider’s
pressure customers. Lucent and Novell have joined forces to form the Presence and Availability Management Forum (PAM), a venue for collaboration within the industry.
presence This definition is also listed under site, Web site and Website. A Web presence (or Web site) is a collection of Web files on a particular subject that includes a beginning file called a home page. For example, most companies, organizations, or individuals that have Web sites have a single address that they give you. This is their home page address. From the home page, you can get to all the other pages on their site. For example, the Web site for IBM has the home page address of http://www.ibm.com. (In this case, the actual file name of the home page file doesn’t have to be included because IBM has named this file index.html and told the server that this address really means http://www.ibm.com/ index.html.) Since it sounds like geography is involved, a Web site is rather easily confused with a Web server. A server in this context is a computer that holds the files for one or more sites. On one hand, a very large Web site may reside on a number of servers that may in different geographic locations. IBM is a good example; its Web site consists of thousands of files spread out over many servers in worldwide locations. But a more typical example is probably the site you are looking at, whatis.com. We reside on a commercial space provider’s server with a number of other sites that have nothing to do with Internet glossaries. ’’Web presence’’ seems to express the idea that a site is not tied to a specific geographic location, but is ‘‘somewhere in cyberspace.’’ However, ‘‘Web site’’ seems to be used much more frequently. Some publications have begun using the term ‘‘Website.’’ We prefer Web site.
Presentation layer In the Open Systems Interconnection (OSI) communications model, the Presentation layer ensures that the communications passing through are in the appropriate form for the recipient. For example, a Presentation layer program may format a file transfer request in binary code to ensure a successful file transfer. Programs in the presentation layer address three aspects of presentation: L
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Encapsulation of data into message ‘‘envelopes’’ for transmission through the network
An example of a program that generally adheres to the Presentation layer of OSI is the program that manages the Web’s Hypertext Transfer Protocol (HTTP). This program, sometimes called the HTTP daemon, usually comes
included as part of an operating system. It forwards user requests passed to the Web browser on to a Web server elsewhere in the network. It receives a message back from the Web server that includes a Multi-Purpose Internet Mail Extensions (MIME) header. The MIME header indicates the kind of file (text, video, audio, and so forth) that has been received so that an appropriate player utility can be used to present the file to the user.
presentation software Presentation software (sometimes called ‘‘presentation graphics’’) is a category of application program used to create sequences of words and pictures that tell a story or help support a speech or public presentation of information. Presentation software can be divided into business presentation software and more general multimedia authoring tools, with some products having charactertistics of both. Business presentation software emphasizes easeand quickness-of-learning and use. Multimedia authoring software enables you to create a more sophisticated presentation that includes audio and video sequences. Business presentation software usually enables you to include images and sometimes audio and video developed with other tools. Some very popular presentation software, such as Microsoft’s Powerpoint and Lotus’s Freelance Graphics, are sold stand-alone or can come as part of office-oriented suites or packages of software. Other popular products include Adobe Persuasion, Astound, Asymetrix Compel, Corel Presentations, and Harvard Graphics. Among the most popular multimedia authoring tools are Macromedia Director and Asymetrix’s Multimedia Toolbook. These authoring tools also include presentation capability as well. Most if not all of these products come in both PC and Mac versions. Recently, a new presentation tool has arrived: your Web browser and the tools for creating Web pages, such as Microsoft’s FrontPage and Adobe’s PageMill. The ubiquity of these tools and the browser as a playback device make this a popular approach, especially when a presentation can combine HTML pages on the hard disk with links to outside sites (if you have a live Internet connection).
pressure Pressure is an expression of force exerted on a surface per unit area. The standard unit of pressure is the pascal (Pa), equivalent to one newton per meter squared (N/m2 or N ˙ m-2). Alternatively, pressure can be measured in dynes per centimeter squared (dyn/cm2 or dyn ˙ cm-2). To convert from pascals to dynes per centimeter squared, multiply by 10. Conversely, multiply by 0.1. Consider an enclosed chamber filled with a gas and surrounded by a vacuum. The pressure exerted on the walls of the chamber by the gas depends on three factors: (1) the
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Whatis?Com’s Encyclopedia of Technology Terms amount of gas in the chamber, (2) the temperature of the gas, and (3) the volume of the chamber. As the amount of gas increases, assuming the volume of the chamber and the temperature remain constant, the pressure increases. As the temperature increases, assuming the amount of gas and the size of the chamber remain constant, the pressure increases. As the volume of the chamber increases, assuming the amount of gas in the chamber and the temperature remain constant, the pressure decreases. These are idealized examples; in practical scenarios these three factors often interact.
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The pressure of the Earth’s atmosphere at sea level is approximately 105 Pa. A unit of 100 Pa is known as a millibar (mb); atmospheric pressure is generally about 1,000 mb at sea level.
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Also see newton, pascal, SI (International System of Units), and Table of Physical Units.
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pressure sensing Pressure sensing allows a robot to tell when it collides with something, or when something pushes against it. Pressure sensors can be used to measure force, and in some cases, to determine the contour of an applied force. A capacitive pressure sensor employs two metal plates separated by a layer of nonconductive foam. This forms a capacitive transducer. The transducer is connected in series or parallel with an inductor. The resulting inductancecapacitance (LC) combination determines the frequency of an oscillator. If an object strikes or presses against the transducer, the plate spacing decreases, causing the capacitance to increase. This lowers the oscillator frequency. When the pressure is removed, the foam springs back, the plates return to their original spacing, and the oscillator returns to its original frequency. A capacitive pressure sensor can be fooled by metallic objects. If a good electrical conductor comes very close to the plates, the capacitance might change even if physical contact is not made. If this occurs, the sensor will interpret this proximity as pressure. An elastomer pressure sensor solves the proximity problem inherent in the capacitive device. The elastomer is a foam pad with resistance that varies depending on how much it is compressed. An array of electrodes is connected to the top of the pad; an identical array is connected to the bottom. Each electrode in the top matrix receives a negative voltage, and its mate in the bottom matrix receives a positive voltage. When pressure appears at some point on the pad, the material compresses at and near that point, reducing the resistance between certain electrode pairs. This causes a current increase in a particular region in the pad. The location of the pressure can be determined according to which electrode pairs experience the increase in current. The extent of the pressure can be determined by how much the current increases. If the electrode matrix is fine enough, the
contour of the pressure-producing object can be determined by a microprocessor that evaluates the electrode current profile. The output of a pressure sensor is analog, but it can be converted to digital data using an analog-to-digital converter (ADC). This signal can be used by a robot controller. Pressure on a transducer in the front of a robot might cause the machine to back up; pressure on the right side might make the machine turn left. The presence of pressure might be used to actuate an alarm, or to switch a device on or off. Calibrated pressure sensors can be used to measure applied force, mass, weight, or acceleration. Also see back-pressure sensor, proximity sensing, and robotics.
pressure, standard See ‘‘standard temperature and pressure’’
Pretty Good Privacy Pretty Good Privacy (PGP) is a popular program used to encrypt and decrypt e-mail over the Internet. It can also be used to send an encrypted digital signature that lets the receiver verify the sender’s identity and know that the message was not changed en route. Available both as freeware and in a low-cost commercial version, PGP is the most widely used privacy-ensuring program by individuals and is also used by many corporations. Developed by Philip R. Zimmermann in 1991, PGP has become a de facto standard for e-mail security. PGP can also be used to encrypt files being stored so that they are unreadable by other users or intruders.
How It Works PGP uses a variation of the public key system. In this system, each user has a publicly known encryption key and a private key known only to that user. You encrypt a message you send to someone else using their public key. When they receive it, they decrypt it using their private key. Since encrypting an entire message can be time-consuming, PGP uses a faster encryption algorithm to encrypt the message and then uses the public key to encrypt the shorter key that was used to encrypt the entire message. Both the encrypted message and the short key are sent to the receiver who first uses the receiver’s private key to decrypt the short key and then uses that key to decrypt the message. PGP comes in two public key versions—Rivest-ShamirAdleman (RSA) and Diffie-Hellman. The RSA version, for which PGP must pay a license fee to RSA, uses the IDEA algorithm to generate a short key for the entire message and RSA to encrypt the short key. The Diffie-Hellman version uses the CAST algorithm for the short key to encrypt the message and the Diffie-Hellman algorithm to encrypt the short key.
Primary Rate Interface For sending digital signatures, PGP uses an efficient algorithm that generates a hash (or mathematical summary) from the user’s name and other signature information. This hash code is then encrypted with the sender’s private key. The receiver uses the sender’s public key to decrypt the hash code. If it matches the hash code sent as the digital signature for the message, then the receiver is sure that the message has arrived securely from the stated sender. PGP’s RSA version uses the MD5 algorithm to generate the hash code. PGP’s Diffie-Hellman version uses the SHA-1 algorithm to generate the hash code. To use PGP, you download or purchase it and install it on your computer system. Typically, it contains a user interface that works with your customary e-mail program. You also need to register the public key that your PGP program gives you with a PGP public-key server so that people you exchange messages with will be able to find your public key. Network Associates maintains an LDAP/HTTP public key server that has 300,000 registered public keys. This server has mirror sites around the world. Where Can You Use PGP? Originally, the U.S. government restricted the exportation of PGP technology. Today, however, PGP encrypted e-mail can be exchanged with users outside the U.S. if you have the correct versions of PGP at both ends. Unlike most other encryption products, the international version is just as secure as the domestic version. The freely available PGP cannot legally be used for commercial purposes—for that, one must obtain the commercial version from Network Associates (formerly PGP, Inc.). There are several versions of PGP in use. Addons can be purchased that allow backwards compatibility for newer RSA versions with older versions. However, the Diffie-Hellman and RSA versions of PGP do not work with each other since they use different algorithms.
PRI See ‘‘Primary Rate Interface’’
primary domain controller and backup domain controller Primary domain controller (PDC) and backup domain controller (BDC) are roles that can be assigned to a server in a network of computers that use the Windows NT operating system. Windows NT uses the idea of a domain to manage access to a set of network resources (applications, printers, and so forth) for a group of users. The user need only to log in to the domain to gain access to the resources, which may be located on a number of different servers in the network. One server, known as the primary domain controller, manages the master user database for the domain. One or more other servers are designated as backup domain controllers. The primary domain controller periodically sends copies of the database to the backup domain
controllers. A backup domain controller can step in as primary domain controller if the PDC server fails and can also help balance the workload if the network is busy enough. In Windows NT, a domain combines some of the advantages of a workgroup (a group of users who exchange access to each others’ resources on different computers) and a directory (a group of users who are managed centrally by an administrator). The domain concept not only allows a user to have access to resources that may be on different servers, but it also allows one domain to be given access to another domain in a trust relationship. In this arrangement, the user need only log in to the first domain to also have access to the second domain’s resources as well. In a Windows NT network, not all servers need to be a PDC or BDC. A server can be designated as a member server whose resources become part of a domain without having a role in the logon process. Setting up and maintaining PDCs and BDCs and domain information is a major activity for the administrator of a Windows NT network. In Windows 2000, the domain controller concept is retained but the PDC and BDC server roles are generally replaced by the Active Directory.
Primary Rate Interface In the Integrated Services Digital Network (ISDN), there are two levels of service: The Basic Rate Interface (BRI), intended for the home and small enterprise, and the Primary Rate Interface (PRI), for larger users. Both rates include a number of B-channels and a D-channel. Each B-channel carries data, voice, and other services. The D-channel carries control and signaling information. The Basic Rate Interface consists of two 64 Kbps B-channels and one 16 Kbps D-channel. Thus, a Basic Rate Interface user can have up to 128 Kbps service. The Primary Rate Interface consists of 23 B-channels and one 64 Kpbs D-channel using a T-1 line or 30 B-channels and 1 D-channel using an E1 line. Thus, a Primary Rate Interface user on a T-1 line can have up to 1.544 Mbps service or up to 2.048 Mbps service on an E1 line. PRI uses the Q.931 protocol over the D-channel. The Primary Rate Interface channels are carried on a Tcarrier system line (in the U.S., Canada, and Japan) or an Ecarrier line (in other countries) and are typically used by medium to large enterprises. The 23 (or 30) B-channels can be used flexibly and reassigned when necessary to meet special needs such as videoconferences. The Primary Rate user is hooked up directly to the telephone company central office. For more information, see ISDN.
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A prime number is a whole number greater than 1, whose only two whole-number factors are 1 and itself. The first few prime numbers are 2, 3, 5, 7, 11, 13, 17, 19, 23, and 29. As we proceed in the set of natural numbers N = {1, 2, 3, ...}, the primes become less and less frequent in general. However, there is no largest prime number. For every prime number p, there exists a prime number p’ such that p’ is greater than p. This was demonstrated in ancient times by the Greek mathematician Euclid.
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Suppose n is a whole number, and we want to test it to see if it is prime. First, we take the square root (or the 1/2 power) of n; then we round this number up to the next highest whole number. Call the result m. We must find all of the following quotients:
Personal computer printers can be distinguished as impact or non-impact printers. Early impact printers worked something like an automatic typewriter, with a key striking an inked impression on paper for each printed character. The dotmatrix printer was a popular low-cost personal computer printer. It’s an impact printer that strikes the paper a line at a time. The best-known non-impact printers are the inkjet printer, of which several makes of low-cost color printers are an example, and the laser printer. The inkjet sprays ink from an ink cartridge at very close range to the paper as it rolls by. The laser printer uses a laser beam reflected from a mirror to attract ink (called toner) to selected paper areas as a sheet rolls over a drum. The four printer qualities of most interest to most users are: L
Color: Color is important for users who need to print pages for presentations or maps and other pages where color is part of the information. Color printers can also be set to print only in black-and-white. Color printers are more expensive to operate since they use two ink cartridges (one color and one black ink) that need to be replaced after a certain number of pages. Users who don’t have a specific need for color and who print a lot of pages will find a black-and-white printer cheaper to operate.
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Resolution: Printer resolution (the sharpness of text and images on paper) is usually measured in dots per inch (dpi). Most inexpensive printers provide sufficient resolution for most purposes at 600 dpi.
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Speed: If you do much printing, the speed of the printer becomes important. Inexpensive printers print only about 3 to 6 sheets per minute. Color printing is slower. More expensive printers are much faster.
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Memory: Most printers come with a small amount of memory (for example, one megabyte) that can be expanded by the user. Having more than the minimum amount of memory is helpful and faster when printing out pages with large images or tables with lines around them (which the printer treats as a large image).
qm = n / m q(m-1) = n / (m-1)
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q(m-2) = n / (m-2) q(m-3) = n / (m-3)
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q2 = n / 2 The number n is prime if and only if none of the q’s, as derived above, are whole numbers.
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A computer can be used to test extremely large numbers to see if they are prime. But, because there is no limit to how large a natural number can be, there is always a point where testing in this manner becomes too great a task even for the most powerful supercomputers. Various algorithms have been formulated in an attempt to generate ever-larger prime numbers. These schemes all have limitations.
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1) In computer programming, a primitive (pronounced PRIH-muh-teev) is a basic interface or segment of code that can be used to build more sophisticated program elements or interfaces. 2) In computer graphics, a primitive is an image element, such as an arc, a square, or a cone, from which more complicated images can be constructed.
printer In computers, a printer is a device that accepts text and graphic output from a computer and transfers the information to paper, usually to standard size sheets of paper. Printers are sometimes sold with computers, but more frequently are purchased separately. Printers vary in size, speed, sophistication, and cost. In general, more expensive printers are used for higher-resolution color printing.
Printer I/O Interfaces The most common I/O interface for printers has been the parallel Centronics interface with a 36-pin plug. In the future, however, new printers and computers are likely to use a serial interface, especially Universal Serial Bus or FireWire with a smaller and less cumbersome plug.
Printer Languages Printer languages are commands from the computer to the printer to tell the printer how to format the document being printed. These commands manage font size, graphics, compression of data sent to the printer, color, etc. The two most popular printer languages are Postscript and Printer Control Language.
privacy Postscript is a printer language that uses English phrases and programmatic constructions to describe the appearance of a printed page to the printer. This printer language was developed by Adobe in 1985. It introduced new features such as outline fonts and vector graphics. Printers now come from the factory with or can be loaded with Postscript support. Postscript is not restricted to printers. It can be used with any device that creates an image using dots such as screen displays, slide recorders, and image setters. PCL (Printer Command Language) is an escape code language used to send commands to the printer for printing documents. Escape code language is so-called because the escape key begins the command sequence followed by a series of code numbers. Hewlett Packard originally devised PCL for dot matrix and inkjet printers. Since its introduction, it has become an industry standard. Other manufacturers who sell HP clones have copied it. Some of these clones are very good, but there are small differences in the way they print a page compared to real HP printers. In 1984, the original HP Laserjet printer was introduced using PCL. PCL helped change the appearance of low-cost printer documents from poor to exceptional quality.
Fonts Fonts are characters of a specific style and size within an overall typeface design. Printers use resident fonts and soft fonts to print documents. Resident fonts are built into the hardware of a printer. They are also called internal fonts or built-in fonts. All printers come with one or more resident fonts. Additional fonts can be added by inserting a font cartridge into the printer or installing soft fonts to the hard drive. Resident fonts cannot be erased unlike soft fonts. Soft fonts are installed onto the hard drive and then sent to the computer’s memory when a document is printed that uses the particular soft font. Soft fonts can be purchased in stores or downloaded from the Internet. There are two types of fonts used by the printer and screen display, bitmap fonts and outline fonts. Bitmap fonts are digital representations of fonts that are not scalable. This means they have a set size or a limited set of sizes. For example, if a document using a bitmap font sized to 24 point is sent to the printer and there is not a bitmap font of that size, the computer will try to guess the right size. This results in the text looking stretched-out or squashed. Jagged edges are also a problem with bitmap fonts. Outline fonts are mathematical descriptions of the font that are sent to the printer. The printer then rasterizes (see raster graphics) or converts them to the dots that are printed on the paper. Because they are mathematical, they are scalable. This means the size of the font can be changed without losing the sharpness or resolution of the printed text. TrueType and Type 1 fonts are outline fonts. Outline fonts are used with Postscript and PCL printer languages.
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Printer Control Language (PCL) is a language (a set of command codes) that enable applications to control Hewlett-Packard DeskJet, LaserJet, and other HP printers. Many personal computer users find themselves in need of PCL drivers after purchasing a new HP or HP-compatible printer and attaching it to their existing PC and operating system. These drivers (which are small programs that work between the operating system and the printer) are available for downloading from Hewlett-Packard’s Web site.
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On the Internet, privacy, a major concern of users, can be divided into these concerns:
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What personal information can be shared with whom
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Whether messages can be exchanged without anyone else seeing them
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Whether and how one can send messages anonymously
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Personal Information Privacy Most Web users want to understand that personal information they share will not be shared with anyone else without their permission. An annual survey conducted by the Graphics, Visualization and Usability Center of the Georgia Institute of Technology showed that 70% of the Web users surveyed cited concerns about privacy as the main reason for not registering information with Web sites. 86% indicated that they wanted to be able to control their personal information. A study by TRUSTe revealed that 78% of users surveyed would be more likely to provide information to sites that offered privacy assurance. The World Wide Web Consortium’s Platform for Personal Privacy Project (P3P) offers specific recommendations for practices that will let users define and share personal information with Web sites that they agree to share it with. The P3P incorporates a number of industry proposals, including the Open Profiling Standard (OPS). Using software that adheres to the P3P recommendations, users will be able to create a personal profile, all or parts of which can be made accessible to a Web site as the user directs. A tool that will help a user decide whether to trust a given Web site with personal information is a Statement of Privacy Policy that a Web site can post.
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Message Privacy In an open network such as the Internet, message privacy, particularly for e-commerce transactions, requires encryption. The most common approach on the Web is through a public key infrastructure (PKI). For e-mail, many people use Pretty Good Privacy (PGP), which lets an individual encrypt a message or simply send a digital signature that can be used to verify that the message was not tampered with en route.
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Although this form of privacy is not usually needed or wanted, there are occasions when a user may want anonymity (for example, to report a crime). The need is sometimes met through the use of a site—called a remailer—that reposts a message from its own address, thus disguising the originator of the message. (Unfortunately, many spam distributors also take advantage of remailers.)
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A private automatic branch exchange (PABX) is an automatic telephone switching system within a private enterprise. Originally, such systems—called private branch exchanges (PBX)—required the use of a live operator. Since almost all private branch exchanges today are automatic, the abbreviation ‘‘PBX’’ usually implies a ‘‘PABX.’’ Some manufacturers of PABX (PBX) systems distinguish their products from others by creating new kinds of private branch exchanges. Rolm offers a Computerized Branch Exchange (CABX) and Usha Informatics offers an Electronic Private Automatic Branch Exchange (EPABX). For additional information, see private branch exchange.
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private branch exchange A PBX (private branch exchange) is a telephone system within an enterprise that switches calls between enterprise users on local lines while allowing all users to share a certain number of external phone lines. The main purpose of a PBX is to save the cost of requiring a line for each user to the telephone company’s central office. The PBX is owned and operated by the enterprise rather than the telephone company (which may be a supplier or service provider, however). Private branch exchanges used analog technology originally. Today, PBXs use digital technology (digital signals are converted to analog for outside calls on the local loop using plain old telephone service). A PBX includes:
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Telephone trunk (multiple phone) lines that terminate at the PBX
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A computer with memory that manages the switching of the calls within the PBX and in and out of it
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The network of lines within the PBX
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Usually a console or switchboard for a human operator
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In some situations, alternatives to a PBX include centrex service (in which a pool of lines are rented at the phone company’s central office), key telephone systems, and, for very small enterprises, primary rate Integrated Services Digital Network.
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Among the larger manufacturers of PBXs are Lucent Technologies, Northern Telecom (NORTEL), Rolm/Siemens, NEC, GTE, Intecom, Fujitsu, Hitachi, and Mitel.
private key In cryptography, a private or secret key is an encryption/ decryption key known only to the party or parties that exchange secret messages. In traditional secret key cryptography, a key would be shared by the communicators so that each could encrypt and decrypt messages. The risk in this system is that if either party loses the key or it is stolen, the system is broken. A more recent alternative is to use a combination of public and private keys. In this system, a public key is used together with a private key. See public key infrastructure (PKI) for more information.
private port numbers See ‘‘dynamic port numbers’’
PRM See ‘‘partner relationship management’’
probability Probability is a branch of mathematics that deals with calculating the likelihood of a given event’s occurrence, which is expressed as a number between 1 and 0. An event with a probability of 1 can be considered a certainty: For example, the probability of a coin toss resulting in either ‘‘heads’’ or ‘‘tails’’ is 1, because there are no other options, assuming the coin lands flat. An event with a probability of .5 can be considered to have equal odds of occurring or not occurring: For example, the probability of a coin toss resulting in ‘‘heads’’ is .5, because the toss is equally as likely to result in ‘‘tails.’’ An event with a probability of 0 can be considered an impossibility: For example, the probability that the coin will land (flat) without either side facing up is 0, because either ‘‘heads’’ or ‘‘tails’’ must be facing up. A little paradoxical, probability theory applies precise calculations to quantify uncertain measures of random events. In its simplest form, probability can be expressed mathematically as: The number of occurrences of a targeted event divided by the number of occurrences plus the number of failures of occurrences (this adds up to the total of possible outcomes): p(a) = p(a)/p(a) + p(b) Calculating probabilities in a situation like a coin toss is straightforward, because the outcomes are mutually exclusive: Either one event or the other must occur. Each coin toss is an independent event; the outcome of one trial has no effect on subsequent ones. No matter how many consecutive times one side lands facing up, the probability that it will do so at the next toss is always .5 (50-50). The mistaken idea that a number of consecutive results (six ‘‘heads’’ for example) makes it more likely that the next toss will result in a ‘‘tails’’ is known as the gambler’s fallacy, one that has led to the downfall of many a bettor.
program temporary fix Probability theory had its start in the 17th century, when two French mathematicians, Blaise Pascal and Pierre de Fermat carried on a correspondence discussing mathematical problems dealing with games of chance. Contemporary applications of probability theory run the gamut of human inquiry, and include aspects of computer programming, astrophysics, music, weather prediction, and medicine.
probe 1) In telecommunications, a probe is an action taken or an object used for the purpose of learning something about the state of the network. For example, an empty message can be sent simply to see whether the destination actually exists. ping is a common utility for sending such a probe. A probe is also something inserted by a hardware device or software program at a key juncture in a network for the purpose of monitoring or collecting data about network activity. 2) In semiconductor testing, a probe card is a microchip placed in a circuit in order to test its signals.
problem program Now seldom used, the term problem program is used to distinguish a computer program that directly supports a user application from an operating system, a utility, or any other underlying support programming. This term has generally been replaced by the term application program.
problems See ‘‘Internet problems’’
process A process is an instance of a program running in a computer. In UNIX and some other operating systems, a process is started when a program is initiated (either by a user entering a shell command or by another program). A process is a running program with which a particular set of data is associated so that the process can be kept track of. An application that is being shared by multiple users will generally have one process at some stage of execution for each user. A process can initiate a subprocess, which is a called a child process (and the initiating process is sometimes referred to as its parent). A child process is a replica of the parent process and shares some of its resources, but cannot exist if the parent is terminated. Processes can exchange information or synchronize their operation through several methods of interprocess communication (IC).
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A processor is the logic circuitry that responds to and processes the basic instructions that drive a computer.
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The term processor has generally replaced the term central processing unit (CPU). The processor in a personal computer or that is embedded in small devices is often called a microprocessor.
processor serial number A PSN (processor serial number) is a software-readable unique serial number that Intel has stamped into its Pentium 3 microprocessor. Intel offers this as a feature that can be optionally used to provide certain network management and e-commerce benefits. Basically, it lets a program identify individual PCs. Pentiums are not the the first processors to be shipped with software-readable serial numbers. Sun RISC chips have, for many years, shipped with serial numbers, and it is a commonplace for software keys for applications software to be generated with reference to the processor ID, to provide a mechanism for copyright protection. However, Intel’s PSN has caused concern to privacy advocates, on grounds that it may undercut individual user efforts to maintain their anonymity, especially when using the Internet. There is a concern that Web pages could covertly acquire PSNs which, if matched with user identities disclosed through a registration process, would facilitate detailed monitoring of the use of the Internet by the individual. For example, chat rooms could use PSNs to prevent unwelcome users from posting.
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Reacting to the objections, Intel announced that they would ship Pentium IIIs with the PSN function turned off. However, since the function appears to be softwareconfigurable without need for a reboot, this assurance seems to be of little comfort.
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program temporary fix In IBM, a program temporary fix (PTF) is a temporary solution to a bug in an IBM software product that is made available for customers to install. A PTF is developed after a customer or someone at IBM encounters the problem and an authorized program analyis report (APAR) is written. The PTF effectively ‘‘closes’’ the APAR and then is made available for delivery to customers. PTFs can be applied individually but are usually made available as part of a fix package that includes a number of PTFs. A fix package can be distributed as one or more downloadable compressed files or on a CD-ROM.
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Whatis?Com’s Encyclopedia of Technology Terms Meanwhile, the PTFs are provided to the development group that is working on the next product release. The developers may or may not use the PTFs (since these are temporary fixes), but will determine and design the correct problem solution to incorporate in the next product release.
See ‘‘PIO’’
In computing, a program is a specific set of ordered operations for a computer to perform. In the modern computer that John von Neumann outlined in 1945, the program contains a one-at-a-time sequence of instructions that the computer follows. Typically, the program is put into a storage area accessible to the computer. The computer gets one instruction and performs it and then gets the next instruction. The storage area or memory can also contain the data that the instruction operates on. (Note that a program is also a special kind of ‘‘data’’ that tells how to operate on ‘‘application or user data.’’)
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Programs can be characterized as interactive or batch in terms of what drives them and how continuously they run. An interactive program receives data from an interactive user (or possibly from another program that simulates an interactive user). A batch program runs and does its work, and then stops. Batch programs can be started by interactive users who request their interactive program to run the batch program. A command interpreter or a Web browser is an example of an interactive program. A program that computes and prints out a company payroll is an example of a batch program. Print jobs are also batch programs. When you create a program, you write it using some kind of computer language. Your language statements are the source program. You then ‘‘compile’’ the source program (with a special program called a language compiler) and the result is called an object program (not to be confused with objectoriented programming). There are several synonyms for object program, including object module and compiled program. The object program contains the string of 0s and 1s called machine language that the logic processor works with.
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The machine language of the computer is constructed by the language compiler with an understanding of the computer’s logic architecture, including the set of possible computer instructions and the length (number of bits) in an instruction.
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by users use erasable programmable read-only memory (EPROM) or electrically erasable programmable read-only memory (EEPROM).
Programmable read-only memory (PROM) is read-only memory (ROM) that can be modified once by a user. PROM is a way of allowing a user to tailor a microcode program using a special machine called a PROM programmer. This machine supplies an electrical current to specific cells in the ROM that effectively blows a fuse in them. The process is known as burning the PROM. Since this process leaves no margin for error, most ROM chips designed to be modified
In the computer industry, these abbreviations are widely used to represent major steps or ‘‘generations’’ in the evolution of programming languages. 1GL or first-generation language was (and still is) machine language or the level of instructions and data that the processor is actually given to work on (which in conventional computers is a string of 0s and 1s). 2GL or second-generation language is assembler (sometimes called ‘‘assembly’’) language. A typical 2GL instruction looks like this: ADD 12,8
An assembler converts the assembler language statements into machine language. 3GL or third-generation language is a ‘‘high-level’’ programming language, such as PL/I, C, or Java. Java language statements look like this: public boolean handleEvent (Event evt) { switch (evt.id) { case Event.ACTION_EVENT: { if (’’Try me’’ .equald(evt.arg)) {
A compiler converts the statements of a specific high-level programming language into machine language. (In the case of Java, the output is called bytecode, which is converted into appropriate machine language by a Java virtual machine that runs as part of an operating system platform.) A 3GL language requires a considerable amount of programming knowledge. 4GL or fourth-generation language is designed to be closer to natural language than a 3GL language. Languages for accessing databases are often described as 4GLs. A 4GL language statement might look like this: EXTRACT ALL CUSTOMERS WHERE ‘‘PREVIOUS PURCHASES’’ TOTAL MORE THAN $1000
5GL or fifth-generation language is programming that uses a visual or graphical development interface to create source language that is usually compiled with a 3GL or 4GL language compiler. Microsoft, Borland, IBM, and other companies make 5GL visual programming products for developing applications in Java, for example. Visual programming allows you to easily envision object-oriented programming class hierarchies and drag icons to assemble program components.
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Programming Language/System PL/S is a language that IBM designed for use in developing system programs, especially in mainframe operating system and application subsystems. It evolved from or was at least influenced by PL/I.
progressive JPEG A progressive JPEG is the JPEG equivalent of the interlaced GIF (Graphics Interchange Format). It’s an image created using the JPEG suite of compression algorithms that will ‘‘fade in’’ in successive waves of lines until the entire image has completely arrived. Like the interlaced GIF, a progressive JPEG is a more appealing way to deliver an image at modem connection speeds. Users with faster connections are not likely to notice the difference.
ProLiant ProLiant, an ‘‘industry standard’’ computer server from Compaq, typifies the popularly-priced server that is designed to satisfy general computing needs at various levels in an enterprise. Servers like the ProLiant are sometimes purchased in large orders when an enterprise is upgrading its servers, creating a new building installation, or simply becoming larger. Based on several versions of Intel’s Pentium III microprocessor, the ProLiant can be ordered with enterprise versions of Windows 2000, Linux, or several other UNIX-based operating systems. The ProLiant can also be ordered in configurations containing up to eight Intel microprocessors. Like other servers designed for industry, the ProLiant comes in models that can be rack-mounted. The ProLiant with the least amount of storage comes with 73 gigabytes. A server can also be equipped with connections for Fibre Channel and similar fast-connect technologies that enable clustering and storage area networks.
next LAN device until the device with the correct network address is reached. That device then receives and reads the data.
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A propeller head (also spelled propellor head, and sometimes shortened to prop head or prophead) is jargon for someone who is exceptionally, perhaps weirdly bright or knowledgeable, especially in some technical field. In computers, according to The New Hacker’s Dictionary, it’s a synonym for computer geek. The term refers to the child’s beanie cap that comes with a spinning propeller sticking out of the top. The New Hacker’s Dictionary says that the propeller cap somehow became a self-parody symbol of the out-of-thisworldness of science fiction fans. It attributes the idea to science fiction writer Ray Faraday Nelson. We like this tongue-in-cheek definition from The Web Developer’s Journal: ‘‘The term ’prophead’ is a holdover from the days when the nerd kids on the block wore caps with little propellers on top. This fashion gave way to the pencil pocket protector. Here at the WDJ, ’propheads’ refers to programmers, developers and other technicallyoriented types. A weenie doesn’t even use a regular keyboard, just a little one with two keys: 1 and 0. Weenies talk among themselves in continuous data streams, which sound to mortal ears like a modem logging on.’’
See ‘‘programmable read-only memory’’
Also see open and Open Source.
Promiscuous mode is the opposite of non-promiscuous mode. When a data packet is transmitted in nonpromiscuous mode, all the LAN devices ‘‘hear’’ the data to determine if the network address included in the data packet is theirs. If it isn’t, the data packet is passed onto the
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In a local area network (LAN), promiscuous mode is a mode of operation in which every data packet transmitted is received and read by every network adapter. Promiscuous mode must be supported by each network adapter as well as by the input/output driver in the host operating system. Promiscuous mode is often used to monitor network activity.
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Promiscuous mode is often used by hackers because sensitive data such as user names, passwords, and corporate information are transmitted in the data packets and is open to all LAN devices allowing the data to be easily intercepted. Promiscuous mode also allows a hacker access to the mainframe. It is recommended to encrypt sensitive data before sending over a LAN especially when in promiscuous mode.
PROM promiscuous mode
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In information technology, proprietary describes a technology or product that is owned exclusively by a single company that carefully guards knowledge about the technology or the product’s inner workings. Some proprietary products can only function properly if at all when used with other products owned by the same company. An example of a proprietary product is Adobe Acrobat, whose Portable Document Format (PDF) files can only be read with the Acrobat Reader. Microsoft is often held up as the best example of a company that takes the proprietary approach. It should be observed that the proprietary approach is a traditional approach. Throughout history, the knowledge of how an enterprise makes its
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extender included with Windows 3.0 (which was itself a DOS application). Microsoft later gave the standard to an industry organization, the DPMI Committee.
A prime motivation behind development of products using proprietary technology is straightforward—buyers are compelled to use other products marketed by the same company. Microsoft is often held up as the exemplary company in terms of proprietary sophistication. Nevertheless, the strongest reason in favor of using proprietary standards leads to the strongest reason against: Customers may be disinclined to buy one product that limits their choice of others. The IT world is increasingly moving toward open standardization.
Today’s personal computers, using microprocessors that succeeded the 8088, typically contain eight or more megabytes of RAM. Today’s operating systems (including the latest DOS versions) come with extended memory management that frees the programmer from the original addressing constraints.
Criteria for open standard products include: Absence of specificity to a particular vendor, wide distribution of standards, and easy and free or low-cost accessibility. One of the best known examples of an open standard product is the Linux operating system. (There are free versions of Linux and for-cost versions. The latter come with accessability to service and updating.)
protected mode Protected mode is a mode of program operation in a computer with an Intel-based microprocessor in which the program is restricted to addressing a specific contiguous area of 640 kilobytes. Intel’s original PC microprocessor, the 8088, provided a one megabyte (1 Mbyte) random access memory (RAM). The memory was divided into several areas for basic input/output system data, signals from your display, and other system information. The remainder or 640 kilobytes of contiguous space was left for the operating system and application programs. The 8088 ensured that any instruction issued by a program running in protected mode would not be able to address space outside of this contiguous 640 kilobytes. Typically, much operating system code and almost all application programs run in protected mode to ensure that essential data is not unintentionally overwritten. Real mode is program operation in which an instruction can address any space within the 1 megabyte of RAM. Typically, a program running in real mode is one that needs to get to and use or update system data and can be trusted to know how to do this. Such a program is usually part of the operating system or a special application subsystem. As new microprocessors (such as the 80386) with larger RAM followed the 8088, DOS continued to preserve the 640 kilobyte addressing limitation so that newly-written application programs could continue to run on both the old as well as new microprocessors. Several companies developed DOS ‘‘extenders’’ that allowed DOS applications to be freed from the 640K constraint by inserting memory management code into the application. Microsoft developed the DOS Protected Mode Interface to go with a DOS
Besides the DPMI standard interface, two other standard extended memory management interfaces exist. Extended Memory Specification (XMS) is a program added to more recent versions of DOS and Windows when the system is loaded. It’s called HIMEM.SYS. Another extended memory manager is the Virtual Control Program Interface (VCPI). A copy of the DPMI Specification is viewable at the Tenberry Web site.
protocol In information technology, a protocol (pronounced PROHtuh-cahl, from the Greek protocollon, which was a leaf of paper glued to a manuscript volume, describing its contents) is the special set of rules that end points in a telecommunication connection use when they communicate. Protocols exist at several levels in a telecommunication connection. There are hardware telephone protocols. There are protocols between each of several functional layers and the corresponding layers at the other end of a communication. Both end points must recognize and observe a protocol. Protocols are often described in an industry or international standard. On the Internet, there are the TCP/IP protocols, consisting of: L
Transmission Control Protocol (TCP), which uses a set of rules to exchange messages with other Internet points at the information packet level.
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Internet Protocol (IP), which uses a set of rules to send and receive messages at the Internet address level.
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Additional protocols that are usually packaged with a TCP/IP suite, including the Hypertext Transfer Protocol (HTTP) and File Transfer Protocol (FTP), each with defined sets of rules to use with corresponding programs elsewhere on the Internet.
proton A proton is a subatomic particle found in the nucleus of every atom. The particle has a positive electrical charge, equal and opposite to that of the electron. If isolated, a single proton would have a mass of only 1.673 6 10-27 kilogram, just slightly less than the mass of a neutron. The number of protons in an element’s nucleus is called the atomic number. This number gives each element its unique identity. In the atoms of any particular element, the number of protons in the nuclei is always the same. An atom of
proximity sensing simple hydrogen has a nucleus consisting of a single proton all by itself. The nuclei of all other elements nearly always contain neutrons in addition to protons. Protons need not be confined to the nuclei of atoms. When protons are found outside atomic nuclei, they acquire fascinating, bizarre, and potentially dangerous properties, similar to those of neutrons in similar circumstances. But protons have an additional property. Because they carry an electric charge, they can be accelerated by electric and/or magnetic fields. High-speed protons, and atomic nuclei containing them, are emitted in large numbers during solar flares. The particles are accelerated by the earth’s magnetic field, causing ionospheric disturbances known as geomagnetic storms.
prototyping model The prototyping model is a systems development method (SDM) in which a prototype (an early approximation of a final system or product) is built, tested, and then reworked as necessary until an acceptable prototype is finally achieved from which the complete system or product can now be developed. This model works best in scenarios where not all of the project requirements are known in detail ahead of time. It is an iterative, trial-and-error process that takes place between the developers and the users. There are several steps in the Prototyping Model: 1. The new system requirements are defined in as much detail as possible. This usually involves interviewing a number of users representing all the departments or aspects of the existing system. 2. A preliminary design is created for the new system. 3. A first prototype of the new system is constructed from the preliminary design. This is usually a scaled-down system, and represents an approximation of the characteristics of the final product. 4. The users thoroughly evaluate the first prototype, noting its strengths and weaknesses, what needs to be added, and what should to be removed. The developer collects and analyzes the remarks from the users. 5. The first prototype is modified, based on the comments supplied by the users, and a second prototype of the new system is constructed. 6. The second prototype is evauated in the same manner as was the first prototype. 7. The preceding steps are iterated as many times as necessary, until the users are satisfied that the prototype represents the final product desired.
8. The final system is constructed, based on the final prototype. 9. The final system is thoroughly evaluated and tested. Routine maintenance is carried out on a continuing basis to prevent large-scale failures and to minimize downtime.
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C provisioning In general, provisioning means ‘‘providing.’’ In telecommunications terminology, provisioning means providing a product or service, such as wiring or bandwidth. The term has a number of varied meanings when used in telecommunications: 1) Providing telecommunications service to a user, including everything necessary to set up the service, such as equipment, wiring, and transmission. 2) Used as a synonym for configuring, as in ‘‘Telecommunications lines must be correctly provisioned to work with the customer’s equipment and enabled for various options the customer has chosen.’’ 3) In a traditional telecommunications environment, there are three separate types of provisioning: Circuit provisioning, service provisioning, and switch provisioning. 4) In a wireless environment, provisioning refers to service activation and involves programming various network databases with the customer’s information. 5) In a slightly different sense, network provisioning systems are intermediary systems that are used to provide customer services, log transactions, carry out requests, and update files. 6) Provisioning is the fourth step of the telecommunications sequence called OAM&P: Operations, Administration, Maintenance, and Provisioning.
proximity sensing Proximity sensing is the ability of a robot to tell when it is near an object, or when something is near it. This sense keeps a robot from running into things. It can also be used to measure the distance from a robot to some object. The simplest proximity sensors do not measure distance. A bumper can be passive, simply making the robot bounce away from things it hits. More often, a bumper has a switch that closes when it makes contact, sending a signal to the controller causing the robot to back away. When whiskers hit something, they vibrate. This can be detected, and a signal sent to the robot controller.
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A photoelectric proximity sensor uses a light-beam generator, a photodetector, a special amplifier, and a microprocessor. The light beam reflects from an object and is picked up by the photodetector. The light beam is modulated at a specific frequency, and the detector has a frequency-sensitive amplifier that responds only to light modulated at that frequency. This prevents false imaging that might otherwise be caused by lamps or sunlight. If the robot is approaching a light-reflecting object, its microprocessor senses that the reflected beam is getting stronger. The robot can then steer clear of the object. This method of proximity sensing won’t work for black objects, or for things like windows or mirrors approached at a sharp angle. An acoustic proximity sensor works on the same principle as sonar. A pulsed signal, having a frequency somewhat above the range of human hearing, is generated by an oscillator. This signal is fed to a transducer that emits ultrasound pulses at various frequencies in a coded sequence. These pulses reflect from nearby objects and are returned to another transducer, which converts the ultrasound back into high-frequency pulses. The return pulses are amplified and sent to the robot controller. The delay between the transmitted and received pulses is timed, and this will give an indication of the distance to the obstruction. The pulse coding prevents errors that might otherwise occur because of confusion between adjacent pulses. A capacitive proximity sensor uses a radio-frequency (RF) oscillator, a frequency detector, and a metal plate connected into the oscillator circuit. The oscillator is designed so that a change in the capacitance of the plate, with respect to the environment, causes the frequency to change. This change is sensed by the frequency detector, which sends a signal to the apparatus that controls the robot. In this way, a robot can avoid bumping into things. Objects that conduct electricity to some extent, such as house wiring, animals, cars, or refrigerators, are sensed more easily by capacitive transducers than are things that do not conduct, like wood-frame beds and dry masonry walls. Also see robotics, pressure sensing, and texture sensing.
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In an enterprise that uses the Internet, a proxy server is a server that acts as an intermediary between a workstation user and the Internet so that the enterprise can ensure security, administrative control, and caching service. A proxy server is associated with or part of a gateway server that separates the enterprise network from the outside network and a firewall server that protects the enterprise network from outside intrusion.
A proxy server receives a request for an Internet service (such as a Web page request) from a user. If it passes filtering requirements, the proxy server, assuming it is also a cache server, looks in its local cache of previously downloaded Web pages. If it finds the page, it returns it to the user without needing to forward the request to the Internet. If the page is not in the cache, the proxy server, acting as a client on behalf of the user, uses one of its own IP addresses to request the page from the server out on the Internet. When the page is returned, the proxy server relates it to the original request and forwards it on to the user. To the user, the proxy server is invisible; all Internet requests and returned responses appear to be directly with the addressed Internet server. (The proxy is not quite invisible; its IP address has to be specified as a configuration option to the browser or other protocol program.) An advantage of a proxy server is that its cache can serve all users. If one or more Internet sites are frequently requested, these are likely to be in the proxy’s cache, which will improve user response time. In fact, there are special servers called cache servers. A proxy can also do logging. The functions of proxy, firewall, and caching can be in separate server programs or combined in a single package. Different server programs can be in different computers. For example, a proxy server may in the same machine with a firewall server or it may be on a separate server and forward requests through the firewall.
pseudocode Pseudocode (pronounced SOO-doh-kohd) is a detailed yet readable description of what a computer program or algorithm must do, expressed in a formally-styled natural language rather than in a programming language. Pseudocode is sometimes used as a detailed step in the process of developing a program. It allows designers or lead programmers to express the design in great detail and provides programmers a detailed template for the next step of writing code in a specific programming language. Because pseudocode is detailed yet readable, it can be inspected by the team of designers and programmers as a way to ensure that actual programming is likely to match design specifications. Catching errors at the pseudocode stage is less costly than catching them later in the development process. Once the pseudocode is accepted, it is rewritten using the vocabulary and syntax of a programming language. Pseudocode is sometimes used in conjunction with computer-aided software engineeringbased methodologies. It is possible to write programs that will convert a given pseudocode language into a given programming language.
PSK See ‘‘phase-shift keying’’
pulse code modulation
PSTN PSTN (public switched telephone network) is the world’s collection of interconnected voice-oriented public telephone networks, both commercial and government-owned. It’s also referred to as the ‘‘plain old telephone service’’ (POTS). It’s the aggregation of circuit-switching telephone networks that has evolved from the days of Alexander Graham Bell (’’Doctor Watson, come here!’’). Today, it is almost entirely digital in technology except for the final link from the central (local) telephone office to the user.
as well as the digital signature of a certification authority for authentication. The certification authority attests that the sender’s name is the one associated with the public key in the document. A user ID packet, containing the sender’s unique identifier, is sent after the certificate packet. There are different types of public key certificates for different functions, such as authorization for a specific action or delegation of authority. Public key certificates are part of a public key infrastructure that deals with digitally signed documents. The other components are public key encryption, trusted third parties (such as the certification authority), and mechanisms for certificate publication and issuing.
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In relation to the Internet, the PSTN actually furnishes much of the Internet’s long-distance infrastructure. Because Internet service providers ISPs pay the long-distance providers for access to their infrastructure and share the circuits among many users through packet-switching, Internet users avoid having to pay usage tolls to anyone other than their ISPs.
public key infrastructure public switched telephone network
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Public-Key Cryptography Standards
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The Public-Key Cryptography Standards (PKCS) are a set of intervendor standard protocols for making possible secure information exchange on the Internet using a public key infrastructure (PKI). The standards include RSA encryption, password-based encryption, extended certificate syntax, and cryptographic message syntax for S/MIME, RSA’s proposed standard for secure e-mail. The standards were developed by RSA Laboratories in cooperation with a consortium that included Apple, Microsoft, DEC, Lotus, Sun, and MIT.
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public domain software Programs that are uncopyrighted because their authors intended to share them with everyone else are in the public domain. The UNIX community has developed a number of such programs over the years. Programs in the public domain can be used without restriction as components of other programs. When reusing such code, it is good to understand its history so that you can be sure it really is in the public domain. Also see shareware, which is programming that is ‘‘free’’ but more or less on a trial basis, and freeware.
public key A public key is a value provided by some designated authority as a key that, combined with a private key derived from the public key, can be used to effectively encrypt messages and digital signatures. The use of combined public and private keys is known as asymmetric cryptography. A system for using public keys is called a public key infrastructure (PKI).
public key certificate A public key certificate is a digitally signed document that serves to validate the sender’s authorization and name. The document consists of a specially formatted block of data that contains the name of the certificate holder (which may be either a user or a system name) and the holder’s public key,
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PCM (pulse code modulation) is a digital scheme for transmitting analog data. The signals in PCM are binary; that is, there are only two possible states, represented by logic 1 (high) and logic 0 (low). This is true no matter how complex the analog waveform happens to be. Using PCM, it is possible to digitize all forms of analog data, including fullmotion video, voices, music, telemetry, and virtual reality (VR). To obtain PCM from an analog waveform at the source (transmitter end) of a communications circuit, the analog signal amplitude is sampled (measured) at regular time intervals. The sampling rate, or number of samples per second, is several times the maximum frequency of the analog waveform in cycles per second or hertz. The instantaneous amplitude of the analog signal at each sampling is rounded off to the nearest of several specific, predetermined levels. This process is called quantization. The number of levels is always a power of 2—for example, 8, 16, 32, or 64. These numbers can be represented by three,
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new type of error correction (known as CIRC7), an adaptation of the ISO 9660 file format, and a scanning velocity of 0.9 meters per second.
At the destination (receiver end) of the communications circuit, a pulse code demodulator converts the binary numbers back into pulses having the same quantum levels as those in the modulator. These pulses are further processed to restore the original analog waveform.
The Purple Book-defined products are expected to be released in recordable and rewritable formats, rather than read-only. Because of the specific requirements for reading DDCD, they cannot be read by other drives currently on the market. Sony has plans to manufacture DDCD-R/RW drives that can read any type of CD, record CD-R, rewrite CD-RW, and perform all three tasks on DDCD. Some upcoming CD-R and CD-RW drives, such as Adaptec’s new version of Easy CD Creator, are being designed to be DDCD-compatible.
Also see modulation.
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A pulsing zombie is a computer whose security has been compromised without its owner’s knowledge by a cracker so that it intermittently carries out a denial-of-service attack on target computers in a network. Unlike a regular zombie, the pulsing zombie doesn’t completely paralyze its targets, but merely weakens them, in what some call degradation-ofservice attacks. Whereas the usual zombie attack consists of a steady (and therefore more easily traced) stream of attack traffic intended to overwhelm one or more target computers, the pulsing zombie attack consists of irregular bursts of traffic intended to hamper service. It is more difficult to locate the source of an attack from a pulsing zombie, or even to know that an attack has taken place. Pulsing zombie attacks have been known to go on for months before they are detected; in one case, a victim received six times its normal traffic volume for several months, according to Asta Networks CEO Joe Devich.
pure-play Pure-play, a term used in stock trading (especially on the Internet), refers to ownership in companies that focus on and specialize in a particular product or service area to the exclusion of other market opportunities in order to obtain a large market share and brand identity in one area. Examples of such companies are said to include SAP for corporate back-office applications; Siebel Systems in customer relationship management (CRM) applications; i2 Technologies in supply chain management (SCM); and Ariba in eprocurement. A conglomerate is usually considered to be the opposite of a pure-play company.
Purple Book The Purple Book is the informal name for Philips and Sony’s specification document for the double-density compact disc (DDCD) format. By narrowing the track pitch (to 1.1 micron from 1.6 micron), and shortening the minimum pit length (to 0.623 micron from 0.833 micron), the Purple Book enables a CD to hold 1.3 gigabytes, roughly twice the capacity of a standard CD. Other Purple Book specifications include a
push Push (or ‘‘server-push’’) is the delivery of information on the Web that is initiated by the information server rather than by the information user or client, as it usually is. An early Web service that specialized in ‘‘pushing’’ information rather than having it ‘‘pulled’’ as the result of requests for Web pages was Pointcast, a site that provided up-to-date news and other information tailored to a previously defined user profile. Marimba was a somewhat similar site (and product) that pushed information to the user on a predefined schedule. In fact, the information pushed from a server to a user actually comes as the result of a programmed request from the client in your computer. That is, any information pusher on the Web requires that you download a client program. This program captures your profile and then periodically initiates requests for information on your behalf from the server. A truer form of push is broadcast information. In this case, the information is pushed to everyone that has access to a particular channel or frequency. Broadcast usually (but not always) involves a continuous flow of information. Another form of ‘‘pushed’’ information is e-mail. Although the e-mail client in your computer has to occasionally go to your local e-mail server to ‘‘pick up’’ the e-mail, the e-mail arrived because someone sent it (pushed) it to you without a one-for-one request having been made.
push technology Also see Webcasting, another usage. In both usages, netcasting is a synonym. Push technology (Webcasting) is the prearranged updating of news, weather, or other selected information on a computer user’s desktop interface through periodic and generally unobtrusive transmission over the World Wide Web (including the use of the Web protocol on intranet). Webcasting is a feature of the Microsoft Internet Explorer browser and Netscape’s Netcaster, part of its Communicator suite. Webcasting is also available through separate applications such as Pointcast and Backweb that run on current browsers.
Python Webcasting uses so-called push technology in which the Web server ostensibly ‘‘pushes’’ information to the user rather than waiting until the user specifically requests it. (In actuality, most of the push is triggered by user or administrator preselection and arrives only as the result of client requests.) In addition to changing the Web for the home user, new Webcasting products offer corporations an organized way to manage information for their intranet users.
PVC See ‘‘permanent virtual circuit’’
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PWS See ‘‘Personal Web Server’’
Python Python is an interpreted, object-oriented programming language similar to PERL, that has gained popularity because of its clear syntax and readability. Python is said to be relatively easy to learn and portable, meaning its statements can be interpreted in a number of operating systems, including UNIX-based systems, Mac OS, MSDOS, OS/2, and various versions of Microsoft Windows 98. Python was created by Guido van Rossum, a former resident of the Netherlands, whose favorite comedy group at the time was Monty Python’s Flying Circus. The source code is freely available and open for modification and reuse. Python has a significant number of users. A notable feature of Python is its indenting of source statements to make the code easier to read. Python offers dynamic data type, ready-made class, and interfaces to many system calls and libraries. It can be extended, using the C or C++ language. Python can be used as the script in Microsoft’s Active Server Page (ASP) technology. The scoreboard system for the Melbourne (Australia) Cricket Ground is written in Python. Zope, a popular Web application server, is also written in the Python language.
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QAM QAM (quadrature amplitude modulation) is a method of combining two amplitude-modulated (AM) signals into a single channel, thereby doubling the effective bandwidth. QAM is used with pulse amplitude modulation (PAM) in digital systems, especially in wireless applications. In a QAM signal, there are two carriers, each having the same frequency but differing in phase by 90 degrees (one quarter of a cycle, from which the term quadrature arises). One signal is called the I signal, and the other is called the Q signal. Mathematically, one of the signals can be represented by a sine wave, and the other by a cosine wave. The two modulated carriers are combined at the source for transmission. At the destination, the carriers are separated, the data is extracted from each, and then the data is combined into the original modulating information. Also see bandwidth, modulation, and wireless.
QDOS QDOS was the forerunner of DOS (Disk Operating System), the first widely-used personal computer operating system. In 1980, when IBM was making plans to enter the personal computer market, it asked Bill Gates, the young owner of a small company called Microsoft, if they could locate an operating system for the new PC that IBM was developing. Microsoft, which had previously furnished IBM with a BASIC language product for the IBM PC, looked around and found an operating system called 86-DOS at a small company called Seattle Computer Products. 86-DOS—often referred to as QDOS, or Quick and Dirty Operating System—was written in six weeks by Tim Paterson, based on ideas in CP/M (Control Program for Microcomputers), an operating system popular with early personal computer users. 86-DOS was designed for use with Seattle Computer’s Intel 8086-based computers. It contained about 4,000 lines of assembler language code. Microsoft bought 86-DOS from Seattle Computer Products for $50,000, revised it, renaming it MS-DOS, and then delivered it to IBM for its new PC. IBM rewrote MS-DOS after finding 300 bugs in it and renamed it PC-DOS, which is why both IBM and Microsoft hold a copyright for it. Bill Gates saw the potential for MSDOS and persuaded IBM to let Microsoft sell it separately from IBM’s PC projects. The initial IBM PC actually offered the user a choice of one of three operating systems: PC-DOS, CP/M 86, and UCSD p-System, a Pascal-based system. PCDOS, which was cheaper, proved the most popular and began to come bundled with the IBM PC in its second product release. The IBM PC brought personal computing to the business world for the first time and was successful
beyond IBM’s imaginings. In 18 months, IBM introduced the PC-XT, which included a hard drive loaded with a newer version of DOS. Microsoft promised a multitasking DOS, but that never happened. Instead, Microsoft developed Windows with multitasking features.
QFE See ‘‘Quick Fix Engineering’’
QoS On the Internet and in other networks, QoS (Quality of Service) is the idea that transmission rates, error rates, and other characteristics can be measured, improved, and, to some extent, guaranteed in advance. QoS is of particular concern for the continuous transmission of high-bandwidth video and multimedia information. Transmitting this kind of content dependably is difficult in public networks using ordinary ‘‘best effort’’ protocols. Using the Internet’s Resource Reservation Protocol (RSVP), packets passing through a gateway host can be expedited based on policy and reservation criteria arranged in advance. Using ATM, which also lets a company or user preselect a level of quality in terms of service, QoS can be measured and guaranteed in terms of the average delay at a gateway, the variation in delay in a group of cells (cells are 53-byte transmission units), cell losses, and the transmission error rate. The Common Open Policy Service (COPS) is a relatively new protocol that allows routers and layer 3 switches to get QoS policy information from the network policy server.
quad A quad (pronounced KWAHD) is a unit in a set of something that comes in four units. The term is sometimes used to describe each of the four numbers that constitute an Internet Protocol (IP) address. Thus, an Internet address in its numeric form (which is also sometimes called a dot address) consists of four quads separated by ‘‘dots’’ (periods). For example: 192.68.00.21 192, 68, 00, and 21 are each quads of the entire address. A quad also means ‘‘a quarter’’ in some usages.
Quad FastEthernet Quad FastEthernet (QFE) is a network interface card (NIC) manufactured by Sun Microsystems that is designed to enhance the bandwidth of a Peripheral Component Interconnect (PCI)-based server using Sun Microsystem’s Solaris 8 or later operating environment. Speeds of up to 100
quantum computer megabits per second (Mbps) are provided by converting PCI data streams into Fast Ethernet traffic. QFE cards are hotswappable, minimizing downtime, and comply with the IEEE 802/3U Ethernet standard. A single card can work with up to four network interfaces at a time and provide support for multihoming.
than one bit at a time. According to Harry Newton, nibble interleaving or multiplexing takes a quadbit or nibble from a lower-speed channel as input for a multiplexed signal on a higher-speed channel. In the IEEE 1284 Parallel Port Interface standard, data can be sent in nibbles (a sequence of two four-bit units) across the line.
quad tree
quadrature amplitude modulation
A quad tree is a method of placing and locating files (called records or keys) in a database. The algorithm finds data by repeatedly dividing the number of ultimately accessible records in four parts until only one remains.
See ‘‘QAM’’
In a tree, records are stored in locations called leaves. This name derives from the fact that records always exist at end points; there is nothing beyond them. Branch points are called nodes. The order of a tree is the number of branches (called children) per node. In a quad tree, there are always four children per node, so the order is 4. The number of leaves in a quad tree is always a power of 4. The number of access operations required to reach the desired record is called the depth of the tree. The image below shows a quad tree of depth 3.
In a practical tree, there can be thousands, millions, or billions of records. Not all leaves necessarily contain a record, but more than half do. A leaf that does not contain a record is called a null. In the example shown here, the eighth, twelfth, and sixteenth leaves are nulls, indicated by open circles. The quad tree is uniquely suited as an algorithm to locate pixels in a two-dimensional image. The reason for this is that, in two dimensions (the usual way in which graphics are depicted), square pixels can be divided into four square parts repeatedly. The depth of such a tree depends on the image resolution, the computer memory, and the complexity of the image. Also see binary tree, B-tree, splay tree, and tree structure.
quadbit A quadbit, sometimes called a nibble, is one of 16 possible four-bit combinations used in some communication signals. A signal may be encoded in quadbit (nibble) units rather
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Quantum is the Latin word for amount and, in modern understanding, means the smallest possible discrete unit of any physical property, such as energy or matter. Quantum came into the latter usage in 1900, when the physicist Max Planck used it in a presentation to the German Physical Society. Planck had sought to discover the reason that radiation from a glowing body changes in color from red, to orange, and, finally, to blue as its temperature rises. He found that by making the assumption that radiation existed in discrete units in the same way that matter does, rather than just as a constant electromagnetic wave, as had been formerly assumed, and was therefore quantifiable, he could find the answer to his question. Planck wrote a mathematical equation involving a figure to represent individual units of energy. He called the units quanta. Planck assumed there was a theory yet to emerge from the discovery of quanta, but in fact, their very existence defined a completely new and fundamental law of nature. Quantum theory and Einstein’s theory of relativity, together, explain the nature and behavior of all matter and energy on earth and form the basis for modern physics. Quantum is sometimes used loosely, in an adjectival form, to mean on such an infinitessimal level as to be infinite, as, for example, you might say ‘‘Waiting for pages to load is quantumly boring.’’
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quantum computer A quantum computer is a machine, as-yet hypothetical, that performs calculations based on the behavior of particles at the sub-atomic level. Such a computer will be, if it is ever developed, capable of executing far more millions of instructions per second (MIPS) than any previous computer. Such an exponential advance in processing capability would be due to the fact that the data units in a quantum computer, unlike those in a binary computer, can exist in more than one state at a time. In a sense, the machine ‘‘thinks’’ several ‘‘thoughts’’ simultaneously, each ‘‘thought’’ being independent of the others even though they all arise from the same set of particles.
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Engineers have coined the term qubit (pronounced KYEWbit) to denote the fundamental data unit in a quantum computer. A qubit is essentially a bit (binary digit) that can take on several, or many, values simultaneously. The theory behind this is as bizarre as the theory of quantum mechanics, in which individual particles appear to exist in multiple locations. One way to think of how a qubit can exist in multiple states is to imagine it as having two or more aspects or dimensions, each of which can be high (logic 1) or low (logic 0). Thus if a qubit has two aspects, it can have four simultaneous, independent states (00, 01, 10, and 11); if it has three aspects, there are eight possible states, binary 000 through 111, and so on.
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Quantum computers might prove especially useful in the following applications:
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Factoring large numbers
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Solving optimization problems in many variables
The main difficulty that the research-and-development engineers have encountered is the fact that it is extremely difficult to get particles to behave in the proper way for a significant length of time. The slightest disturbance will cause the machine to cease working in quantum fashion and revert to ‘‘single-thought’’ mode like a conventional computer. Stray electromagnetic fields, physical movement, or a tiny electrical discharge can disrupt the process.
quantum computing Quantum computing is the area of study focused on developing computer technology based on the principles of quantum theory, which explains the nature and behavior of energy and matter on the quantum (atomic and subatomic) level. Development of a quantum computer, if practical, would mark a leap forward in computing capability far greater than that from the abacus to a modern day supercomputer, with performance gains in the billion-fold realm and beyond. The quantum computer, following the laws of quantum physics, would gain enormous processing power through the ability to be in multiple states, and to perform tasks using all possible permutations simultaneously. Current centers of research in quantum computing include MIT, IBM, Oxford University, and the Los Alamos National Laboratory. The essential elements of quantum computing originated with Paul Benioff, working at Argonne National Labs, in 1981. He theorized a classical computer operating with some quantum mechanical principles. But it is generally accepted that David Deutsch of Oxford University provided the critical impetus for quantum computing research. In 1984, he was at a computation theory conference and began to wonder about the possibility of designing a computer that
was based exclusively on quantum rules, then published his breakthrough paper a few months later. With this, the race began to exploit his ideas. However, before we delve into what he started, it is beneficial to have a look at the background of the quantum world.
Quantum Theory Quantum theory’s development began in 1900 with a presentation by Max Planck to the German Physical Society, in which he introduced the idea that energy exists in individual units (which he called ‘‘quanta’’), as does matter. Further developments by a number of scientists over the following thirty years led to the modern understanding of quantum theory. The Essential Elements of Quantum Theory: L
Energy, like matter, consists of discrete units, rather than solely as a continuous wave.
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Elementary particles of both energy and matter, depending on the conditions, may behave like either particles or waves.
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The movement of elementary particles is inherently random, and, thus, unpredictable.
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The simultaneous measurement of two complementary values, such as the position and momentum of an elementary particle, is inescapably flawed; the more precisely one value is measured, the more flawed will be the measurement of the other value.
Further Developments of Quantum Theory Niels Bohr proposed the Copenhagen interpretation of quantum theory, which asserts that a particle is whatever it is measured to be (for example, a wave or a particle) but that it cannot be assumed to have specific properties, or even to exist, until it is measured. In short, Bohr was saying that objective reality does not exist. This translates to a principle called superposition that claims that while we do not know what the state of any object is, it is actually in all possible states simultaneously, as long as we don’t look to check. To illustrate this theory, we can use the famous and somewhat cruel analogy of Schrodinger’s Cat. First, we have a living cat and place it in a thick lead box. At this stage, there is no question that the cat is alive. We then throw in a vial of cyanide and seal the box. We do not know if the cat is alive or if it has broken the cyanide capsule and died. Since we do not know, the cat is both dead and alive, according to quantum law—in a superposition of states. It is only when we break open the box and see what condition the cat is in that the superposition is lost, and the cat must be either alive or dead. The second interpretation of quantum theory is the multiverse or many-worlds theory. It holds that as soon as a potential exists for any object to be in any state, the universe of that object transmutes into a series of parallel universes equal to the number of possible states in which the object can
quantum computing exist, with each universe containing a unique single possible state of that object. Furthermore, there is a mechanism for interaction between these universes that somehow permits all states to be accessible in some way and for all possible states to be affected in some manner. Stephen Hawking and the late Richard Feynman are among the scientists who have expressed a preference for the many-worlds theory. Whichever argument one chooses, the principle that, in some way, one particle can exist in numerous states opens up profound implications for computing.
computations that a quantum computer could undertake is 2^n, where n is the number of qubits used. A quantum computer comprised of 500 qubits would have a potential to do 2^500 calculations in a single step. This is an awesome number—2^500 is infinitely more atoms than there are in the known universe (this is true parallel processing—classical computers today, even so called parallel processors, still only truly do one thing at a time: There are just two or more of them doing it). But how will these particles interact with each other? They would do so via quantum entanglement.
A Comparison of Classical and Quantum Computing
Entanglement
Classical computing relies, at its ultimate level, on principles expressed by Boolean algebra, operating with a (usually) 7mode logic gate principle, though it is possible to exist with only three modes (which are AND, NOT, and COPY). Data must be processed in an exclusive binary state at any point in time—that is, either 0 (off/false) or 1 (on/true). These values are binary digits, or bits. The millions of transistors and capacitors at the heart of computers can only be in one state at any point. While the time that the each transistor or capacitor need be either in 0 or 1 before switching states is now measurable in billionths of a second, there is still a limit as to how quickly these devices can be made to switch state. As we progress to smaller and faster circuits, we begin to reach the physical limits of materials and the threshold for classical laws of physics to apply. Beyond this, the quantum world takes over, which opens a potential as great as the challenges that are presented.
Particles (such as photons, electrons, or qubits) that have interacted at some point retain a type of connection and can be entangled with each other in pairs, in a process known as correlation. Knowing the spin state of one entangled particle—up or down—allows one to know that the spin of its mate is in the opposite direction. Even more amazing is the knowledge that, due to the phenomenon of superpostition, the measured particle has no single spin direction before being measured, but is simultaneously in both a spin-up and spin-down state. The spin state of the particle being measured is decided at the time of measurement and communicated to the correlated particle, which simultaneously assumes the opposite spin direction to that of the measured particle. This is a real phenomenon (Einstein called it ‘‘spooky action at a distance’’), the mechanism of which cannot, as yet, be explained by any theory—it simply must be taken as given. Quantum entanglement allows qubits that are separated by incredible distances to interact with each other instantaneously (not limited to the speed of light). No matter how great the distance between the correlated particles, they will remain entangled as long as they are isolated.
The Quantum computer, by contrast, can work with a twomode logic gate: XOR and a mode we’ll call QO1 (the ability to change 0 into a superposition of 0 and 1, a logic gate which cannot exist in classical computing). In a quantum computer, a number of elemental particles such as electrons or photons can be used (in practice, success has also been achieved with ions), with either their charge or polarization acting as a representation of 0 and/or 1. Each of these particles is known as a quantum bit, or qubit, the nature and behavior of these particles form the basis of quantum computing. The two most relevant aspects of quantum physics are the principles of superposition and entanglement.
Superposition Think of a qubit as an electron in a magnetic field. The electron’s spin may be either in alignment with the field, which is known as a spin-up state, or opposite to the field, which is known as a spin-down state. Changing the electron’s spin from one state to another is achieved by using a pulse of energy, such as from a laser—let’s say that we use 1 unit of laser energy. But what if we only use half a unit of laser energy and completely isolate the particle from all external influences? According to quantum law, the particle then enters a superposition of states, in which it behaves as if it were in both states simultaneously. Each qubit utilized could take a superposition of both 0 and 1. Thus, the number of
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Taken together, quantum superposition and entanglement create an enormously enhanced computing power. Where a 2-bit register in an ordinary computer can store only one of four binary configurations (00, 01, 10, or 11) at any given time, a 2-qubit register in a quantum computer can store all four numbers simultaneously, because each qubit represents two values. If more qubits are added, the increased capacity is expanded exponentially.
Quantum Programming Perhaps even more intriguing than the sheer power of quantum computing is the ability that it offers to write programs in a completely new way. For example, a quantum computer could incorporate a programming sequence that would be along the lines of ‘‘take all the superpositions of all the prior computations’’—something which is meaningless with a classical computer—which would permit extremely fast ways of solving certain mathematical problems, such as factorization of large numbers, one example of which we discuss below.
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There have been two notable successes thus far with quantum programming. The first occurred in 1994 by Peter Shor, (now at AT&T Labs) who developed a quantum algorithm that could efficiently factorize large numbers. It centers on a system that uses number theory to estimate the periodicity of a large number sequence. The other major breakthrough happened with Lov Grover of Bell Labs in 1996, with a very fast algorithm that is proven to be the fastest possible for searching through unstructured databases. The algorithm is so efficient that it requires only, on average, roughly N square root (where N is the total number of elements) searches to find the desired result, as opposed to a search in classical computing, which on average needs N/2 searches.
The Problems—And Some Solutions The above sounds promising, but there are tremendous obstacles still to be overcome. Some of the problems with quantum computing are as follows: L
Interference—During the computation phase of a quantum calculation, the slightest disturbance in a quantum system (say a stray photon or wave of EM radiation) causes the quantum computation to collapse, a process known as de-coherence. A quantum computer must be totally isolated from all external interference during the computation phase. Some success has been achieved with the use of qubits in intense magnetic fields, with the use of ions.
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Error correction—Because truly isolating a quantum system has proven so difficult, error correction systems for quantum computations have been developed. Qubits are not digital bits of data, thus they cannot use conventional (and very effective) error correction, such as the triple redundant method. Given the nature of quantum computing, error correction is ultra critical—even a single error in a calculation can cause the validity of the entire computation to collapse. There has been considerable progress in this area, with an error correction algorithm developed that utilizes 9 qubits (1 computational and 8 correctional). More recently, there was a breakthrough by IBM that makes do with a total of 5 qubits (1 computational and 4 correctional).
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Output observance—Closely related to the above two, retrieving output data after a quantum calculation is complete risks corrupting the data. In an example of a quantum computer with 500 qubits, we have a 1 in 2500 chance of observing the right output if we quantify the output. Thus, what is needed is a method to ensure that, as soon as all calculations are made and the act of observation takes place, the observed value will correspond to the correct answer. How can this be done? It has been achieved by Grover with his database search algorithm, that relies on the special ‘‘wave’’ shape of the probability curve inherent in quantum computers, that
ensures, once all calculations are done, the act of measurement will see the quantum state decohere into the correct answer. Even though there are many problems to overcome, the breakthroughs in the last 15 years, and especially in the last 3, have made some form of practical quantum computing not unfeasible, but there is much debate as to whether this is less than a decade away or a hundred years into the future. However, the potential that this technology offers is attracting tremendous interest from both the government and the private sector. Military applications include the ability to break encryptions keys via brute force searches, while civilian applications range from DNA modeling to complex material science analysis. It is this potential that is rapidly breaking down the barriers to this technology, but whether all barriers can be broken, and when, is very much an open question.
quantum cryptography Quantum cryptography uses our current knowledge of physics to develop a cryptosystem that is not able to be defeated—that is, one that is completely secure against being compromised without knowledge of the sender or the receiver of the messages. The word quantum itself refers to the most fundamental behavior of the smallest particles of matter and energy: quantum theory explains everything that exists and nothing can be in violation of it. Quantum cryptography is different from traditional cryptographic systems in that it relies more on physics, rather than mathematics, as a key aspect of its security model. Essentially, quantum cryptography is based on the usage of individual particles/waves of light (photon) and their intrinsic quantum properties to develop an unbreakable cryptosystem—essentially because it is impossible to measure the quantum state of any system without disturbing that system. It is theoretically possible that other particles could be used, but photons offer all the necessary qualities needed, their behavior is comparatively wellunderstood, and they are the information carriers in optical fiber cables, the most promising medium for extremely high-bandwidth communications.
How It Works in Theory In theory, quantum cryptography works in the following manner (this view is the ‘‘classical’’ model developed by Bennett and Brassard in 1984—some other models do exist): Assume that two people wish to exchange a message securely, traditionally named Alice and Bob. Alice initiates the message by sending Bob a key, which will be the mode for encrypting the message data. This is a random sequence of bits, sent using a certain type of scheme, which can see two different initial values represent one particular binary value (0 or 1).
quantum cryptography Let us assume that this key is a stream of photons travelling in one direction, with each of these photon particles representing a single bit of data (either a 0 or 1). However, in addition to their linear travel, all of these photons are oscillating (vibrating) in a certain manner. These oscillations can occur in any 360-degree range across any conceivable axis, but for the purpose of simplicity (at least as far as it is possible to simplify things in quantum cryptography), let us assume that their oscillations can be grouped into 4 particular states: We’ll define these as UP/DOWN, LEFT/ RIGHT, UPLEFT/RIGHTDOWN and UPRIGHT/LEFTDOWN. The angle of this vibration is known as the polarization of the photon. Now, let us introduce a polarizer into the equation. A polarizer is simply a filter that permits certain photons to pass through it with the same oscillation as before and lets others pass through in a changed state of oscillation (it can also block some photons completely, but let’s ignore that property for this exercise). Alice has a polarizer that can transmit the photons in any one of the four states mentioned—in effect, she can choose either rectilinear (UP/DOWN and LEFT/RIGHT) or diagonal (UPLEFT/ RIGHTDOWN and UPRIGHT/LEFTDOWN) polarization filters. Alice swaps her polarization scheme between rectilinear and diagonal filters for the transmission of each single photon bit in a random manner. In doing so, the transmission can have one of two polarizations represent a single bit, either 1 or 0, in either scheme she uses. When receiving the photon key, Bob must choose to measure each photon bit using either his rectilinear or diagonal polarizer: Sometimes he will choose the correct polarizer and at other times he will choose the wrong one. Like Alice, he selects each polarizer in a random manner. So what happens with the photons when the wrong polarizer is chosen? The Heisenberg Uncertainty Principle states that we do not know exactly what will happen to each individual photon, for in the act of measuring its behavior, we alter its properties (in addition to the fact that if there are two properties of a system that we wish to measure, measuring one precludes us from quantifying the other). However, we can make a guess as to what happens with them as a group. Suppose Bob uses a rectilinear polarizer to measure UPLEFT/RIGHTDOWN and UPRIGHT/LEFTDOWN (diagonal) photons. If he does this, then the photons will pass through in a changed state—that is, half will be transformed to UP/DOWN and the other half to LEFT/RIGHT. But we cannot know which individual photons will be transformed into which state (it is also a reality that some photons may be blocked from passing altogether in a real world application, but this is not relevant to the theory). Bob measures some photons correctly and others incorrectly. At this point, Alice and Bob establish a channel of communication that can be insecure—that is, other people can listen in. Alice then proceeds to advise Bob as to which polarizer she used to send each photon bit—but not how she
polarized each photon. So she could say that photon number 8597 (theoretically) was sent using the rectilinear scheme, but she will not say whether she sent an UP/DOWN or LEFT/RIGHT. Bob then confirms if he used the correct polarizer to receive each particular photon. Alice and Bob then discard all the photon measurements that he used the wrong polarizer to check. What they have, is, on average, a sequence of 0s and 1s that is half the length of the original transmission...but it will form the basis for a one-time pad, the only cryptosystem that, if properly implemented, is proven to be completely random and secure. Now, suppose we have an eavesdropper, Eve, who attempts to listen in, has the same polarizers that Bob does and must also randomly choose whether to use the rectilinear or diagonal one for each photon. However, she also faces the same problem that Bob does, in that half the time she will choose the wrong polarizer. But Bob has the advantage of speaking to Alice to confirm which polarizer type was used for each photon. This is useless to Eve, as half the time she used the wrong detector and will misinterpret some of the photons that will form that final key, rendering it useless.
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Furthermore, there is another level of security inherent in quantum cryptography—that of intrusion detection. Alice and Bob would know if Eve was eavesdropping on them. The fact that Eve is on the ‘‘photon highway’’ can become obvious because of the following.
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Let’s say that Alice transmits photon number 349 as an UPRIGHT/LEFTDOWN to Bob, but for that one, Eve uses the rectilinear polarizer, which can only measure UP/ DOWN or LEFT/RIGHT photons accurately. What Eve will do is transform that photon into either UP/DOWN or LEFT/ RIGHT, as that is the only way the photon can pass. If Bob uses his rectilinear polarizer, then it will not matter what he measures as the polarizer check Alice and Bob go through above will discard that photon from the final key. But if he uses the diagonal polarizer, a problem arises when he measures its polarization; he may measure it correctly as UPRIGHT/LEFTDOWN, but he stands an equal chance, according to the Heisenberg Uncertainty Principle, of measuring it incorrectly as UPLEFT/RIGHTDOWN. Eve’s use of the wrong polarizer will warp that photon and will cause Bob to make errors even when he is using the correct polarizer.
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To discover Eve’s nefarious doings, they must perform the above procedures, with which they will arrive at an identical key sequence of 0s and 1s—unless someone has been eavesdropping, whereupon there will be some discrepancies. They must then undertake further measures to check the validity of their key. It would be foolish to compare all the binary digits of the final key over the unsecured channel discussed above, and also unnecessary. Let us assume that the final key comprises 4,000 binary digits. What needs to be done is that a subset of these digits be selected randomly by Alice and Bob, say 200 digits, in terms of both position (that is, digit sequence number 2, 34,
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Whatis?Com’s Encyclopedia of Technology Terms 65, 911 etc) and digit state (0 or 1). Alice and Bob compare these—if they match, then there is virtually no chance that Eve was listening. However, if she was listening in, then her chances of being undiscovered are one in countless trillions, that is, no chance in the real world. Alice and Bob would know someone was listening in and then would not use the key—they would need to start the key exchange again over a secure channel inaccessible to Eve, even though the comparisons between Alice and Bob discussed above can still be done over an insecure channel. However, even if Alice and Bob have concluded that the their key is secure, since they have communicated 200 digits over an un-secure channel, these 200 digits should be discarded from the final key, turning it from a 4,000 into a 3,800 bit key). Thus, quantum cryptography is a way to combine the relative ease and convenience of key exchange in public key cryptography with the ultimate security of a onetime pad.
How It Works in Practice
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In practice, quantum cryptography has been demonstrated in the laboratory by IBM and others, but over relatively short distances. Recently, over longer distances, fiber optic cables with incredibly pure optic properties have successfully transmitted photon bits up to 60 kilometers. Beyond that, BERs (bit error rates) caused by a combination of the Heisenberg Uncertainty Principle and microscopic impurities in the fiber make the system unworkable. Some research has seen successful transmission through the air, but this has been over short distances in ideal weather conditions. It remains to be seen how much further technology can push forward the distances at which quantum cryptography is practical.
quantum dot A quantum dot is a particle of matter so small that the addition or removal of an electron changes its properties in some useful way. All atoms are, of course, quantum dots, but multi-molecular combinations can have this characteristic. In biochemistry, quantum dots are called redox groups. In nanotechnology, they are called quantum bits or qubits. Quantum dots typically have dimensions measured in nanometers, where one nanometer is 10-9 meter or a millionth of a millimeter. The fields of biology, chemistry, computer science, and electronics are all of interest to researchers in nanotechnology. An example of the overlapping of these disciplines is a hypothetical biochip, which might contain a sophisticated computer and be grown in a manner similar to the way a tree evolves from a seed. In this scenario, the terms redox group and qubit are equally applicable; it is hard to classify such a chip as either animate or inanimate. The quantum dots in a biochip would each account for at least one data bit, and possibly several.
In the extreme, the position of a single electron in a quantum dot might attain several states, so that a quantum dot could represent a byte of data. Alternatively, a quantum dot might be used in more than one computational instruction at a time. Other applications of quantum dots include nanomachines, neural networks, and high-density memory or storage media.
quantum mirage The term quantum mirage refers to a phenomenon that may make it possible to transfer data without conventional electrical wiring. Instead of forcing charge carriers through solid conductors, a process impractical on a microscopic scale, electron wave phenomena are made to produce effective currents. Leading the research are physicists Donald Eigler, Hari Manoharan, and Christopher Lutz of the IBM facility in San Jose, California. All moving particles have a wavelike nature. This is rarely significant on an everyday scale. But in atomic dimensions, where distances are measured in nanometers (nm), moving particles behave like waves. This phenomenon is what makes the electron microscope workable. It is of interest to researchers in nanotechnology, who are looking for ways to deliver electric currents through circuits too small for conventional wiring. A quantum mirage is a spot where electron waves are focused so they reinforce each other. The result is an energy hot zone, similar to the acoustical hot zones observed in concrete enclosures, or the electromagnetic wave focus of a dish antenna. In the case of electron waves, the enclosure is called a quantum corral. An elliptical corral produces mirages at the foci of the ellipse. A typical quantum corral measures approximately 20 nm long by 10 nm wide. By comparison, the range of visible wavelengths is approximately 390 nm (violet light) to 750 nm (red light). One nanometer is 10-9 meter, or a millionth of a millimeter.
quantum theory Quantum theory is the theoretical basis of modern physics that explains the nature and behavior of matter and energy on the atomic and subatomic level. Quantum theory dates to 1900, with physicist Max Planck’s presentation to the German Physical Society. Planck had sought to discover the reason that radiation from a glowing body changes in color from red, to orange, and, finally, to blue as its temperature rises. He found that by making the assumption that energy existed in individual units in the same way that matter does, rather than just as a constant electromagnetic wave—as had been formerly assumed—and was therefore quantifiable, he could find the answer to his question. The existence of these units became the first assumption of quantum theory. Planck wrote a mathematical equation involving a figure to represent these individual units of energy, which he called quanta. The equation explained the phenomenon very well;
qubit Planck found that at certain discrete temperature levels (exact multiples of a basic minimum value), energy from a glowing body will occupy different areas of the color spectrum. Planck assumed there was a theory yet to emerge from the discovery of quanta, but, in fact, their very existence implied a completely new and fundamental understanding of the laws of nature. Planck won the Nobel Prize in Physics for his theory in 1918, but developments by various scientists over a thirty-year period all contributed to the modern understanding of quantum theory.
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In 1900, Planck made the assumption that energy was made of individual units, or quanta.
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In 1905, Albert Einstein theorized that not just the energy, but the radiation itself was quantized in the same manner.
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In 1924, Louis de Broglie proposed that there is no fundamental difference in the makeup and behavior of energy and matter; on the atomic and subatomic level either may behave as if made of either particles or waves. This theory became known as the principle of wave-particle duality: Elementary particles of both energy and matter behave, depending on the conditions, like either particles or waves.
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In 1927, Werner Heisenberg proposed that precise, simultaneous measurement of two complementary values—such as the position and momentum of a subatomic particle—is impossible. Contrary to the principles of classical physics, their simultaneous measurement is inescapably flawed; the more precisely one value is measured, the more flawed will be the measurement of the other value. This theory became known as the uncertainty principle, which prompted Albert Einstein’s famous comment, ‘‘God does not play dice.’’
The Copenhagen Interpretation and the Many-Worlds Theory The two major interpretations of quantum theory’s implications for the nature of reality are the Copenhagen interpretation and the many-worlds theory. Niels Bohr proposed the Copenhagen interpretation of quantum theory, which asserts that a particle is whatever it is measured to be (for example, a wave or a particle), but that it cannot be assumed to have specific properties, or even to exist, until it is measured. In short, Bohr was saying that objective reality does not exist. This translates to a principle called superposition that claims that while we do not know what the state of any object is, it is actually in all possible states simultaneously, as long as we don’t look to check. To illustrate this theory, we can use the famous and somewhat cruel analogy of Schrodinger’s Cat. First, we have a living cat and place it in a thick lead box. At this stage,
there is no question that the cat is alive. We then throw in a vial of cyanide and seal the box. We do not know if the cat is alive or if it has broken the cyanide capsule and died. Since we do not know, the cat is both dead and alive, according to quantum law—in a superposition of states. It is only when we break open the box and see what condition the cat is that the superposition is lost, and the cat must be either alive or dead. The second interpretation of quantum theory is the manyworlds (or multiverse theory. It holds that as soon as a potential exists for any object to be in any state, the universe of that object transmutes into a series of parallel universes equal to the number of possible states in which that the object can exist, with each universe containing a unique single possible state of that object. Furthermore, there is a mechanism for interaction between these universes that somehow permits all states to be accessible in some way and for all possible states to be affected in some manner. Stephen Hawking and the late Richard Feynman are among the scientists who have expressed a preference for the manyworlds theory.
Quantum Theory’s Influence Although scientists throughout the past century have balked at the implications of quantum theory—Planck and Einstein among them—the theory’s principles have repeatedly been supported by experimentation, even when the scientists were trying to disprove them. Quantum theory and Einstein’s theory of relativity form the basis for modern physics. The principles of quantum physics are being applied in an increasing number of areas, including quantum optics, quantum chemistry, quantum computing, and quantum cryptography.
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A qubit is a quantum bit, the counterpart in quantum computing to the binary digit or bit of classical computing. Just as a bit is the basic unit of information in a classical computer, a qubit is the basic unit of information in a quantum computer.
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In a quantum computer, a number of elemental particles such as electrons or photons can be used (in practice, success has also been achieved with ions), with either their charge or polarization acting as a representation of 0 and/or 1. Each of these particles is known as a qubit; the nature and behavior of these particles (as expressed in quantum theory) form the basis of quantum computing. The two most relevant aspects of quantum physics are the principles of superposition and entanglement.
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Superposition Think of a qubit as an electron in a magnetic field. The electron’s spin may be either in alignment with the field, which is known as a spin-up state, or opposite to the field, which is known as a spin-down state. Changing the electron’s
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spin from one state to another is achieved by using a pulse of energy, such as from a laser—let’s say that we use 1 unit of laser energy. But what if we only use half a unit of laser energy and completely isolate the particle from all external influences? According to quantum law, the particle then enters a superposition of states, in which it behaves as if it were in both states simultaneously. Each qubit utilized could take a superposition of both 0 and 1. Thus, the number of computations that a quantum computer could undertake is 2n, where n is the number of qubits used. A quantum computer comprised of 500 qubits would have a potential to do 2500 calculations in a single step. This is an awesome number—2500 is infinitely more atoms than there are in the known universe (this is true parallel processing—classical computers today, even so called parallel processors, still only truly do one thing at a time: there are just two or more of them doing it). But how will these particles interact with each other? They would do so via quantum entanglement.
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Particles that have interacted at some point retain a type of connection and can be entangled with each other in pairs, in a process known as correlation. Knowing the spin state of one entangled particle—up or down—allows one to know that the spin of its mate is in the opposite direction. Even more amazing is the knowledge that, due to the phenomenon of superposition, the measured particle has no single spin direction before being measured, but is simultaneously in both a spin-up and spin-down state. The spin state of the particle being measured is decided at the time of measurement and communicated to the correlated particle, which simultaneously assumes the opposite spin direction to that of the measured particle. This is a real phenomenon (Einstein called it ‘‘spooky action at a distance’’), the mechanism of which cannot, as yet, be explained by any theory—it simply must be taken as given. Quantum entanglement allows qubits that are separated by incredible distances to interact with each other instantaneously (not limited to the speed of light). No matter how great the distance between the correlated particles, they will remain entangled as long as they are isolated. Taken together, quantum superposition and entanglement create an enormously enhanced computing power. Where a 2-bit register in an ordinary computer can store only one of four binary configurations (00, 01, 10, or 11) at any given time, a 2-qubit register in a quantum computer can store all four numbers simultaneously, because each qubit represents two values. If more qubits are added, the increased capacity is expanded exponentially.
query In general, a query (noun) is a question, often required to be expressed in a formal way. The word derives from the Latin quaere (the imperative form of quaerere, meaning to ask or
seek). In computers, what a user of a search engine or database enters is sometimes called the query. To query (verb) means to submit a query (noun). A database query can be either a select query or an action query. A select query is simply a data retrieval query. An action query can ask for additional operations on the data, such as insertion, updating, or deletion. Languages used to interact with databases are called query languages, of which the Structured Query Language (SQL) is the well-known standard.
Query by Example Query by Example (QBE) is a method of query creation that allows the user to search for documents based on an example in the form of a selected text string or in the form of a document name or a list of documents. Because the QBE system formulates the actual query, QBE is easier to learn than formal query languages, such as the standard Structured Query Language (SQL), while still enabling powerful searches. To conduct a search for similar documents based on matching text, the user enters or copies selected text into the form search field. This is then passed to the QBE parser for processing. A query is created using the relevant words (common words such as ‘‘and,’’ ‘‘is’’ and ‘‘the’’ are ignored by default) and a search is carried out for documents containing them. Because the meaning of the selected text is less precise than a formal query, results may be more variable than those in a formal query entry. To conduct a search for similar documents based on full document text, the user submits documents or lists of documents to the QBE results template. The QBE parser performs an analysis of these and formulates a query to submit to the search engine, which in turn conducts a search for similar material. In terms of database management system, QBE can be thought of as a ‘‘fill-in-the blanks’’ method of query creation. The Microsoft Access Query Design Grid is an example. To conduct a search for field data matching particular conditions, the user enters criteria into the form, creating search conditions for as many fields as desired. A query is automatically generated to search the database for matching data.
queue In general, a queue is a line of people or things waiting to be handled, usually in sequential order starting at the beginning or top of the line or sequence. In computer technology, a queue is a sequence of work objects that are waiting to be processed. The possible factors, arrangements, and processes related to queues is known as queueing theory.
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queueing theory In computer science, queueing theory is the study of queues as a technique for managing processes and objects in a computer. A queue can be studied in terms of: The source of each queued item, how frequently items arrive on the queue, how long they can or should wait, whether some items should jump ahead in the queue, how multiple queues might be formed and managed, and the rules by which items are enqueued and dequeued. The queues that a computer manages are sometimes viewed as being in stacks. In most systems, an item is always added to the top of a stack. A process that handles queued items from the bottom of the stack first is known as a first-in firstout (FIFO) process. A process that handles the item at the top of the stack first is known as a last-in first-out (LIFO) process.
Quick Fix Engineering Quick Fix Engineering (QFE) is a Microsoft term for the delivery of individual service updates to its operating systems and application programs such as Word. Formerly called a hotfix, ‘‘QFE’’ can be used to describe both the method of delivering and applying a patch or fix, and also to refer to any individual fix. Because of the complexity and sheer number of lines of code in most application programs and operating systems, the delivery of temporary fixes to users has long been provided by major software manufacturers. Typically, not all fixes are necessarily applied by an enterprise since they can occasionally introduce new problems. All of the fixes in any given system are usually incorporated (so they don’t have to be reapplied) whenever a new version of a program or operating system comes out.
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information about a resource that has been built by the system remains where it is during the quiescence. The reverse of quiesce is usually unquiesce, but reset and other terms are also used.
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The QWERTY (pronounced KWEHR-tee) keyboard is the standard typewriter and computer keyboard in countries that use a Latin-based alphabet. QWERTY refers to the first six letters on the upper row of the keyboard. The key arrangement was devised by Christopher Latham Sholes whose ‘‘Type-Writer,’’ as it was then called, was first massproduced in 1874. Since that time, it has become what may be the most ubiquitous machine-user interface of all time. The QWERTY arrangement was intended to reduce the jamming of typebars as they moved to strike ink on paper. Separating certain letters from each other on the keyboard reduced the amount of jamming. In 1932, August Dvorak developed what was intended to be a faster keyboard, putting the vowels and the five most common consonants in the middle row, with the idea that an alternating rhythm would be established between left and right hands. Although the Dvorak keyboard has many adherents, it has never overcome the culture of learning to type on a QWERTY.
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Periodically, all current QFEs (or hotfixes) are delivered together as a service pack, which can be applied more efficiently than applying fixes one at a time.
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Quicktime is a multimedia development, storage, and playback technology from Apple. Quicktime files combine sound, text, animation, and video in a single file. Using a Quicktime player that either comes with a Web browser or can be downloaded from Apple or the browser company, you can view and control brief multimedia sequences. Quicktime files can be recognized by their file name extensions: qt, mov, and moov.
quiesce To quiesce is to put a computer, a program, a thread, or some other computer resource into a temporarily inactive or inhibited state. A resource that is in a quiesced state can be reactivated more quickly than one that has been completely removed from the system. Typically, any descriptive
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R/3 R/3 is the comprehensive set of integrated business applications from SAP, the German company that states it is the market and technology leader in business application software. R/3 replaced an earlier system, R/2, which is still in use. R/3 uses the client-server model and provides the ability to store, retrieve, analyze, and process in many ways corporate data for financial analysis, production operation, human resource management, and most other business processes. A recent release, R/3 3.1, makes it possible to get to the R/3 database and applications through Internet access and Web browsers. A sales representative can initiate the workflow for a sales order by filling out an electronic form on a laptop that will be ‘‘translated’’ into input for the R/3 system. Other interfaces such as Lotus Notes can also be used. The Web implementation adheres to the Workflow Client API standard of the Workflow Management Coalition (WfMC). A more recent version of R/3 adds features designed to speed product delivery by helping to manage the supply chain.
RA See ‘‘registration authority’’
RACF See ‘‘Resource Access Control Facility’’
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Radar is an acronym for ‘‘radio detection and ranging.’’ A radar system usually operates in the ultra-high-frequency (UHF) or microwave part of the radio-frequency (RF) spectrum, and is used to detect the position and/or movement of objects. Radar can track storm systems, because precipitation reflects electromagnetic fields at certain frequencies. Radar can also render precise maps. Radar systems are widely used in air-traffic control, aircraft navigation, and marine navigation. High-power radar, using large dish antennas, has been used to measure distances to the moon, other planets, asteroids, and artificial satellites. From unmanned space probes, radar has been used to map Venus, whose surface is obscured at visible wavelengths by a thick layer of clouds. Radar has been employed by NASA (the U.S. National Aeronautics and Space Administration) to make highly detailed topographical maps of the earth’s surface as well.
Most radar systems determine position in two dimensions: Azimuth (compass bearing) and radius (distance). The display is in polar coordinates. A rotating antenna transmits RF pulses at defined intervals. The delay between a transmitted pulse and the echo, or return pulse, determines the radial position of the plotted point(s) for each azimuth direction on the display. The greater the echo delay from a particular object in space, the farther from the display center its point appears. The maximum range of a UHF or microwave radar system depends on the height of the antenna above average terrain, the topography of the surface in the region, the atmospheric conditions in the region, and in some cases the level of radio background noise. Radar is known to the general public for its use by law enforcement in determining the speeds of motor vehicles. This type of radar does not display the exact position of an object, but determines its radial speed vector from the Doppler effect. A radar detector, which consists of a simple UHF/microwave broadband receiver, can be used in a car or truck to warn drivers of the presence of police radar. Radar detectors are illegal in some states. The Weather Service uses so-called Doppler radar to determine not only the positions and extent of storm systems, but wind patterns and velocities aloft. Doppler radar employs a combination of position-sensing and speedsensing radar, making it possible to ascertain the locations and intensity of severe thunderstorms, hurricanes, and tornadoes. Radar has been used on the high-frequency (HF) radio bands, between approximately 5 MHz and 20 MHz, in an attempt to obtain early warning in the event of a nuclear assault via ballistic missiles. The ionosphere refracts HF waves, allowing much greater system range than is possible with radar at UHF or microwave frequencies. During the 1970s and early 1980s, the signals from these systems became infamous because of the interference they caused. Radio amateurs coined the term woodpecker to describe the sound of HF over-the-horizon radar pulses in communications receivers.
radian The radian is the International System of Units (SI) unit of plane angular measure. There are 2 pi, or approximately 6.28318, radians in a complete circle. Thus, one radian is about 57.296 angular degrees. The term radian arises from the fact that the length of a circular arc, corresponding to an angle of one radian, is equal to the radius of the arc. This is shown in the illustration. Point P represents the center of the circle. The
radian per second angle q, representing one radian, is such that the length of the subtended circular arc is equal to the radius, r, of the circle.
r q P
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The radian is used by mathematicians, physicists, and engineers. It arises in natural phenomena and in equations which, unlike the angular degree, were invented for human convenience. Also see International System of Units (SI).
radian per second squared The radian per second squared (symbolized rad/s2 or rad/ sec2) is the International System of Units (SI) unit of angular (rotational) acceleration, which is the rate of change of angular speed or velocity. This quantity can be defined in either of two senses: Average or instantaneous. The average angular acceleration magnitude can be obtained by evaluating an object’s instantaneous angular speed (in radians per second) at two different points t1 and t2 in time, and then dividing the distance by the span of time t27t1 (in seconds). Suppose the instantaneous angular speed at time t1 is equal to u1, and the instantaneous angular speed at time t2 is equal to u2. Then the average angular acceleration magnitude bavg (in meters per second squared) during the time interval [t1, t2] is given by: bavg = (u27u1) / (t27t1) Instantaneous angular acceleration magnitude is more difficult to intuit, because it involves an expression of rotational motion over an ‘‘infinitely short’’ interval of time. Let p represent a specific point in time. Suppose an object is in rotational motion at about that time. The average angular acceleration magnitude can be determined over increasingly short time intervals centered at p, for example:
[p-4, p+4] [p-3, p+3] [p-2, p+2] [p-1, p+1] [p-0.5, p+0.5] [p-0.25, p+0.25] . . . [p-x, p+x] . . . where the added and subtracted numbers represent seconds. The instantaneous angular acceleration magnitude, binst, is the limit of the average angular acceleration magnitude as x approaches zero. This is a theoretical value, because it can be obtained only by inference from instantanous speed values determined at the starting and ending points of progressively shorter time spans. In the complete sense, angular acceleration is a vector quantity having direction as well as magnitude, and representing the rate of change of angular velocity. Suppose, for example, that a wheel’s rate of rotation is increasing at 0.5 rad/s2 in the counterclockwise sense; this might be the case for a car or truck moving from right to left (relative to the viewer) with increasing speed. This would produce an angular acceleration vector with a magnitude of 0.5 rad/s2, pointing toward the viewer in line with the wheel’s axle. But if rate of rotation were decreasing at 0.5 rad/s2 in the counterclockwise sense (the same car or truck slowing down while moving from right to left), the angular acceleration vector would have a magnitude of 0.5 rad/s2 in the opposite direction, that is, away from the viewer in line with the wheel’s axle.
radian per second The radian per second (symbolized rad/s or rad/sec) is the International System of Units (SI) unit of angular (rotational) speed. This quantity can be defined in either of two senses: average or instantaneous. Average angular speed is obtained by measuring the angle in radians through which an object rotates in a certain number of seconds, and then dividing the total angle by the time. If uavg represents the average angular speed of an object (in radians per second) during a time interval t (in seconds), and the angle through which the object rotates in that time is equal to q (in radians), then:
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Whatis?Com’s Encyclopedia of Technology Terms Instantaneous angular speed is more difficult to intuit, because it involves an expression of motion over an ‘‘infinitely short’’ interval of time. Let p represent a specific point in time. Suppose an object is in rotational motion at about that time. The average angular speed can be measured over increasingly short time intervals centered at p, for example: [p74, p+4] [p73, p+3] [p72 , p+2] [p71, p+1] [p70.5, p+0.5] [p70.25, p+0.25] . . . [p7x, p+x] . . . where the added and subtracted numbers represent seconds. The instantaneous angular speed, uinst, is the limit of the measured average speed as x approaches zero. This is a theoretical value, because it cannot be obtained except by inference from measurements made over progressively shorter time spans. It is important to realize that angular speed is not the same thing as angular velocity. Angular speed is a scalar (dimensionless) quantity, while angular velocity is a vector quantity consisting of angular speed and direction in the form of a rotational sense (clockwise or counterclockwise). We might say the earth rotates at at 7.272 6 10-5 rad/s, and this tells us its angular speed. Or we might say the earth rotates at at 0.00007272 (7.272 6 10-5) rad/s counterclockwise relative to the sun as viewed from above the north geographic pole; this tells us the earth’s angular velocity. As with angular speed, we might specify either the average angular velocity over a period of time, or the instantaneous angular velocity at an exact moment in time. In the case of the earth, the instantaneous angular velocity is essentially constant, and it can be said that uinst = uavg. In the case of a moving car or truck, the angular speeds of the wheels, axles, and drive shaft often change, and the statement uinst = uavg is, in general, not true. Also see radian, second, radian per second squared, and International System of Units (SI).
radiation shield In mobile phones, a radiation shield is an accessory that serves to decrease the amount of radio frequency (RF) energy being absorbed by a mobile phone user. Mobile phones are wireless communications devices that use frequencies in the microwave band. Microwave energy can be absorbed by water and converted to heat energy. There is growing concern, especially in Europe, about the potential for harm arising out of the effects of the absorbtion of mobile phone microwave radiation in the brain of the user. In particular, both the use of unidirectional aerials and their very close proximity with the cranium have been identified as special risks. It is suggested that a significant proportion (perhaps 40%) of the output of unidirectional aerials is absorbed by the head of the user; and that energy is concentrated around the aerial, which tends to be bare millimeters from the brain. Mobile phones have been linked with a number of phenomena including short-term and long-term memory loss, increased blood pressure, and brain tumors. Radiation shields, designed to clip around the body and aerial of mobile phones, absorb and/or reflect microwave energy. Manufacturers claim that the action of placing the user in what amounts to a radiation shadow, can diminish the quantity of radiation absorbed to perhaps 2% of the level of an unshielded phone. Radio frequency energy limits for mobile phones are specified in international standards EN55022b & IEC1000-43. However, some current research suggests these standards may be inadequate and incomplete.
radio detection and ranging See ‘‘radar’’
radio frequency See also audio frequency (AF). Radio frequency (abbreviated RF, rf, or r.f.) is a term that refers to alternating current (AC) having characteristics such that, if the current is input to an antenna, an electromagnetic (EM) field is generated suitable for wireless broadcasting and/or communications. These frequencies cover a significant portion of the electromagnetic radiation spectrum, extending from nine kilohertz (9 kHz), the lowest allocated wireless communications frequency (it’s within the range of human hearing), to thousands of gigahertz (GHz).
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Frequencies
Free-space Wavelengths
Very Low Frequency Low Frequency Medium Frequency High Frequency Very High Frequency Ultra High Frequency Super High Frequency Extremely High Frequency
VLF LF MF HF VHF UHF SHF EHF
9 kHz—30 kHz 30 kHz—300 kHz 300 kHz—3 MHz 3 MHz—30 MHz 30 MHz—300 MHz 300 MHz—3 GHz 3 GHz—30 GHz 30 GHz—300 GHz
33 km710 km 10 km71 km 1 km7100 m 100 m710 m 10 m71 m 1 m7100 mm 100 mm710 mm 10 mm71 mm
When an RF current is supplied to an antenna, it gives rise to an electromagnetic field that propagates through space. This field is sometimes called an RF field; in less technical jargon it is a ‘‘radio wave.’’ Any RF field has a wavelength that is inversely proportional to the frequency. In the atmosphere or in outer space, if f is the frequency in megahertz and s is the wavelength in meters, then s = 300/f The frequency of an RF signal is inversely proportional to the wavelength of the EM field to which it corresponds. At 9 kHz, the free-space wavelength is approximately 33 kilometers (km) or 21 miles (mi). At the highest radio frequencies, the EM wavelengths measure approximately one millimeter (1 mm). As the frequency is increased beyond that of the RF spectrum, EM energy takes the form of infrared (IR), visible, ultraviolet (UV), X rays, and gamma rays. Many types of wireless devices make use of RF fields. Cordless and cellular telephone, radio and television broadcast stations, satellite communications systems, and two-way radio services all operate in the RF spectrum. Some wireless devices operate at IR or visible-light frequencies, whose electromagnetic wavelengths are shorter than those of RF fields. Examples include most television-set remotecontrol boxes, some cordless computer keyboards and mice, and a few wireless hi-fi stereo headsets. The RF spectrum is divided into several ranges, or bands. With the exception of the lowest-frequency segment, each band represents an increase of frequency corresponding to an order of magnitude (power of 10). The table depicts the eight bands in the RF spectrum, showing frequency and bandwidth ranges. The SHF and EHF bands are often referred to as the microwave spectrum.
radio paper
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See ‘‘e-paper’’
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See ‘‘Remote Authentication Dial-In User Service’’
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RAID (redundant array of independent disks; originally redundant array of inexpensive disks) is a way of storing the same data in different places (thus, redundantly) on multiple hard disks. By placing data on multiple disks, I/O operations can overlap in a balanced way, improving performance. Since multiple disks increases the mean time between failure (MTBF), storing data redundantly also increases fault-tolerance. A RAID appears to the operating system to be a single logical hard disk. RAID employs the technique of striping, which involves partitioning each drive’s storage space into units ranging from a sector (512 bytes) up to several megabytes. The stripes of all the disks are interleaved and addressed in order. In a single-user system where large records, such as medical or other scientific images, are stored, the stripes are typically set up to be small (perhaps 512 bytes) so that a single record spans all disks and can be accessed quickly by reading all disks at the same time. In a multi-user system, better performance requires establishing a stripe wide enough to hold the typical or maximum size record. This allows overlapped disk I/O across drives. There are at least nine types of RAID plus a non-redundant array (RAID-0): L
RAID-0. This technique has striping but no redundancy of data. It offers the best performance but no faulttolerance.
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RAID-2. This type uses striping across disks with some disks storing error checking and correcting (ECC) information. It has no advantage over RAID-3.
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RAID-3. This type uses striping and dedicates one drive to storing parity information. The embedded error checking (ECC) information is used to detect errors. Data recovery is accomplished by calculating the exclusive OR (XOR) of the information recorded on the other drives. Since an I/O operation addresses all drives at the same time, RAID-3 cannot overlap I/O. For this reason, RAID3 is best for single-user systems with long record applications.
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RAID-4. This type uses large stripes, which means you can read records from any single drive. This allows you to take advantage of overlapped I/O for read operations. Since all write operations have to update the parity drive, no I/O overlapping is possible. RAID-4 offers no advantage over RAID-5.
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RAID-5. This type includes a rotating parity array, thus addressing the write limitation in RAID-4. Thus, all read and write operations can be overlapped. RAID-5 stores parity information but not redundant data (but parity information can be used to reconstruct data). RAID-5 requires at least three and usually five disks for the array. It’s best for multi-user systems in which performance is not critical or which do few write operations.
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RAID-6. This type is similar to RAID-5 but includes a second parity scheme that is distributed across different drives and thus offers extremely high fault- and drivefailure tolerance. There are few or no commercial examples currently.
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RAID-7. This type includes a real-time embedded operating system as a controller, caching via a highspeed bus, and other characteristics of a stand-alone computer. One vendor offers this system.
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RAID-10. This type offers an array of stripes in which each stripe is a RAID-1 array of drives. This offers higher performance than RAID-1 but at much higher cost.
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RAID-53. This type offers an array of stripes in which each stripe is a RAID-3 array of disks. This offers higher performance than RAID-3 but at much higher cost.
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RAID-1. This type is also known as disk mirroring and consists of at least two drives that duplicate the storage of data. There is no striping. Read performance is improved since either disk can be read at the same time. Write performance is the same as for single disk storage. RAID-1 provides the best performance and the best faulttolerance in a multi-user system.
rain fade Rain fade is an interruption of wireless communication signals as a result of rain or snow droplets whose separation approximates the signal wavelengths. The phenomenon can affect satellite Internet connections as well as satellite television and other systems. Most satellite communication takes place in the microwave portion of the electromagnetic radiation spectrum. Signals at these wavelengths, typically on the order of a few inches, are affected by heavy concentrations of water droplets or ice crystals in the atmosphere. When the mean distance between water droplets or crystals is comparable to the wavelength of the electromagnetic signals, severe attenuation can occur. The observed effect is a degradation or loss of communications during heavy downpours, snow squalls, and blizzards. Rain fade usually does not last long. Once a heavy shower or squall has passed, normal communications returns. However, during tropical storms or severe winter storms at northern latitudes, fadeouts can persist for hours at a time. The phenomenon occurs with all types of satellite systems, including geostationary (GEO), low-earth-orbit (LEO), and medium-earth-orbit (MEO). It can also affect the Global Positioning System (GPS). Compare solar fade.
RAM RAM (random access memory) is the place in a computer where the operating system, application programs, and data in current use are kept so that they can be quickly reached by the computer’s processor. RAM is much faster to read from and write to than the other kinds of storage in a computer, the hard disk, floppy disk, and CD-ROM. However, the data in RAM stays there only as long as your computer is running. When you turn the computer off, RAM loses its data. When you turn your computer on again, your operating system and other files are once again loaded into RAM, usually from your hard disk. RAM can be compared to a person’s short-term memory and the hard disk to the long-term memory. The short-term memory focuses on work at hand, but can only keep so many facts in view at one time. If short-term memory fills up, your brain sometimes is able to refresh it from facts stored in long-term memory. A computer also works this way. If RAM fills up, the processor needs to continually go to the hard disk to overlay old data in RAM with new, slowing down the computer’s operation. Unlike the hard disk which can become completely full of data so that it won’t accept any more, RAM never runs out of memory. It keeps operating, but much more slowly than you may want it to.
Rambus How Big is RAM? RAM is small, both in physical size (it’s stored in microchips) and in the amount of data it can hold. It’s much smaller than your hard disk. A typical computer may come with 32 million bytes of RAM and a hard disk that can hold 4 billion bytes. RAM comes in the form of ‘‘discrete’’ (meaning separate) microchips and also in the form of modules that plug into holes in the computer’s motherboard. These holes connect through a bus or set of electrical paths to the processor. The hard drive, on the other hand, stores data on a magnetized surface that looks like a phonograph record. Today’s personal computers come with 16 or more megabytes of RAM, usually increasing in multiples of 8 megabytes. Users of graphic applications usually need 32 or 64 megabytes of memory. Most personal computers are designed to allow you to add additional RAM modules up to a certain limit (for example, up to 64 or 128 megabytes). Having more RAM in your computer reduces the number of times that the computer processor has to read data in from your hard disk, an operation that takes much longer than reading data from RAM. (RAM access time is in nanoseconds; hard disk access time is in milliseconds.)
Why Random Access? RAM is called ‘‘random access’’ because any storage location can be accessed directly. Originally, the term distinguished regular core memory from offline memory, usually on magnetic tape in which an item of data could only be accessed by starting from the beginning of the tape and finding an address sequentially. Perhaps it should have been called ‘‘nonsequential memory’’ because RAM access is hardly random. RAM is organized and controlled in a way that enables data to be stored and retrieved directly to specific locations. A term IBM has preferred is direct access storage or memory. Note that other forms of storage such as the hard disk and CD-ROM are also accessed directly (or ‘‘randomly’’) but the term random access is not applied to these forms of storage. In addition to disk, floppy disk, and CD-ROM storage, another important form of storage is read-only memory (ROM), a more expensive kind of memory that retains data even when the computer is turned off. Every computer comes with a small amount of ROM that holds just enough programming so that the operating system can be loaded into RAM each time the computer is turned on.
RAMAC (IBM had an earlier system called RAMAC. See RAMAC (original).) RAMAC (pronounced RAY-mac; IBM does not spell out the initials) is a multiple-disk storage subsystem from IBM that emphasizes fault-tolerance and is intended for large enterprises. IBM claims that its RAMAC product family ‘‘provides the highest data availability of any DASD
subsytem.’’ The RAMAC product family uses redundant array of independent disks technology (RAID)—in particular, the RAID-5 storage technique. RAMAC also includes redundant (backup) power and cooling subsystems. RAMAC also uses multilevel caching to improve performance and it uses predictive failure analysis to identify potential disk failures.
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The RAMAC subsystem can be addressed by any operating system (such as IBM’s OS/390) that currently works with IBM’s main storage controller products. RAMAC is scalable from nine gigabytes up to 90 gigabytes of storage.
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(Also see RAMAC, a current line of storage products from IBM.)
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RAMAC (which stood for random access method of accounting and control) was the world’s first computer disk storage system, developed by IBM engineers in San Jose and introduced in 1957. Prior to this, computer storage was largely reliant on magnetic tape. The disk-based storage introduced moveable read/write heads, which enabled a semi-random access capability; this ability was a momentous achievement, both for IBM and for the computing world in general, because fast random access to large volumes of data now made it practical to have interactive computer systems. RAMAC’s development also led to IBM opening its first storage manufacturing plant. As part of the IBM 350, RAMAC was a system that used fifty metallic disks, each two feet in diameter, arranged on a rotating spindle. Data was recorded on both sides of the disks, each holding a hundred concentric tracks, for a storage capacity of 5 megabytes. An access arm under servo control moved two read/write heads to access to any of the tracks. Originally, RAMAC’s purchase price worked out to about $10,000 per megabyte of storage capacity; by 1997 the price per megabyte had dropped to about ten cents.
Rambus Rambus Dynamic Random Access Memory (RDRAM) is a memory subsystem that promises to transfer up to 1.6 billion bytes per second. The subsystem consists of the random access memory, the RAM controller, and the bus (path) connecting RAM to the microprocessor and devices in the computer that use it. High-speed RAM is expected to accelerate the growth of visually intensive interfaces such as 3-D, interactive games, and streaming multimedia. Rambus is intended to replace the current main memory technology of dynamic random access memory (DRAM). Much faster data transfer rates from attached devices such as videocams using FireWire and the Accelerated Graphics Port (AGP) make it important to reduce the bottleneck in getting data into the computer, staging it in RAM, and moving it throught the microprocessor and to the display or other output devices.
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Whatis?Com’s Encyclopedia of Technology Terms Direct Rambus (DRDRAM) provides a two-byte (16 bit) bus rather than DRAM’s 8-bit bus. At a RAM speed of 800 megahertz (800 million cycles per second), the peak data transfer rate is 1.6 billion bytes per second. Direct Rambus uses pipelining to move data from RAM to cache memory levels that are closer to the microprocessor or display. Up to eight operations may be underway at the same time. Rambus is designed to fit into existing motherboard standards. The components that are inserted into motherboard connections are called Rambus in-line memory modules (RIMMs). They can replace conventional dual inline memory modules.
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The latest version of RDRAM is Direct RDRAM (DRDRAM). A proposed alternative to DRDRAM is SyncLink DRAM (SDRAM).
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Rambus DRAM See ‘‘DRDRAM’’
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RAMDAC RAMDAC (random access memory digital-to-analog converter) is a microchip that converts digital image data into the analog data needed by a computer display. A RAMDAC microchip is built into the video adapter in a computer. It combines a small static RAM (SRAM) containing a color table with three digital-to-analog converters that change digital image data into analog signals that are sent to the display’s color generators, one for each primary color—red, green, and blue. In a cathode ray tube (CRT) display, an analog signal is sent to each of three electron guns. With displays using other technologies, the signals are sent to a corresponding mechanism.
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Gathering requirements using workshops or focus groups
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Prototyping and early, reiterative user testing of designs
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The re-use of software components
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A rigidly paced schedule that defers design improvements to the next product version
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Less formality in reviews and other team communication
Some companies offer products that provide some or all of the tools for RAD software development. (The concept can be applied to hardware development as well.) These products include requirements gathering tools, prototyping tools, computer-aided software engineering tools, language development environments such as those for the Java platform, groupware for communication among development members, and testing tools. RAD usually embraces object-oriented programming methodology, which inherently fosters software re-use. The most popular objectoriented programming languages, C++ and Java, are offered in visual programming packages often described as providing rapid application development.
RARP See ‘‘Reverse Address Resolution Protocol’’
RAS In computer memory technology, RAS (row address strobe) is a signal sent to a dynamic random access memory (DRAM) that tells it that an associated address is a row address. A data bit in DRAM is stored in a cell located by the intersection of a column address and a row address. A column address strobe (CAS) signal is used to validate the column address.
How It Works
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The SRAM part of the RAMDAC contains a color palette table. A logical color number in the digital data input to SRAM is used to generate three separate values obtained from the table—one for each of red, green, and blue—that are output to one of three digital-to-analog converters. The analog signal output from the converter is input directly to the display electron guns or other image projecting mechanisms. For displays with true color, the digital color data is fed directly to the DACs, bypassing the SRAM table, which is not needed.
random access memory See ‘‘RAM’’
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RAD (rapid application development) is a concept that products can be developed faster and of higher quality through:
raster Also see frame, raster graphics, and raster image processor. The term raster refers to the region of a cathode ray tube (CRT) or liquid crystal display (LCD) monitor that is capable of rendering images. In a CRT, the raster is a sequence of horizontal lines that are scanned rapidly with an electron beam from left to right and top to bottom, in much the same way as a TV picture tube is scanned. However, there are certain differences. In general, the resolution is better in a computer CRT than in a TV picture tube. Also, a TV raster scan is interlaced, while the raster scan in a computer CRT is almost always noninterlaced. In a CRT, the raster is slightly smaller than the full screen size of the monitor. The height and width of the raster can be adjusted, as can the horizontal and vertical position. Other parameters such as pincushioning, horizontal linearity, and vertical linearity can be adjusted in some CRT monitors.
rational number In an LCD, the raster (usually called a grid) is scanned differently than in a CRT; image elements are displayed individually. The raster normally matches the screen monitor in size. But if low resolution is used (for example, 640 6 480 pixels on an LCD intended for 800 6 600), the displayed image may fill only part of the screen. If high resolution is used (such as 1024 6 768 pixels on an LCD intended for 800 6 600), the displayed image may exceed the area of the screen, and scrolling will be necessary to view all portions of the raster.
raster graphics Raster graphics are digital images created or captured (for example, by scanner in a photo) as a set of samples of a given space. A raster is a grid of x and y coordinates on a display space. (And for three-dimensional images, a z coordinate.) A raster image file identifies which of these coordinates to illuminate in monochrome or color values. The raster file is sometimes referred to as a bitmap because it contains information that is directly mapped to the display grid.
For the set of natural numbers N and the set of integers Z, neither of which are ‘‘dense,’’ denumeration lists are straightforward. For Q, it is less obvious how such a list might be constructed. An example appears below. The matrix includes all possible numbers of the form p/q, where p is an integer and q is a nonzero natural number. Every possible rational number is represented in the array. Following the line with the arrow, think of 0 as the ‘‘first stop,’’ 1/1 as the ‘‘second stop,’’ -1/1 as the ‘‘third stop,’’ 1/2 as the ‘‘fourth stop,’’ and so on. This defines a sequential (although redundant) list of the rational numbers. There is a one-to-one correspondence between the elements of the array and the set of natural numbers N.
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-3/3 …
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Also see Routing Information Protocol (RIP).
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-2/4
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-3/4 …
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A raster file is usually larger than a vector graphics image file. A raster file is usually difficult to modify without loss of information, although there are software tools that can convert a raster file into a vector file for refinement and changes. Examples of raster image file types are: BMP, TIFF, GIF, and JPEG files.
If r and t are rational numbers such that r t, then there exists a rational number s such that r s t. This is true no matter how small the difference between r and t, as long as the two are not equal. In this sense, the set Q is ‘‘dense.’’ Nevertheless, Q is a denumerable set. Denumerability refers to the fact that, even though a set might contain an infinite number of elements, and even though those elements might be ‘‘densely packed,’’ the elements can be defined by a list that assigns them each a unique number in a sequence corresponding to the set of natural numbers N = {1, 2, 3, ...}..
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RIPs are also used to enlarge images for printing. They use special algorithms (such as error diffusion and schochastic) to provide large blow-ups without loss of clarity.
rational number A rational number is a number determined by the ratio of some integer p to some nonzero natural number q. The set of rational numbers is denoted Q, and represents the set of all possible integer-to-natural-number ratios p/q. In mathematical expressions, unknown or unspecified rational numbers are represented by lowercase, italicized letters from the late middle or end of the alphabet, especially r, s, and t, and occasionally u through z. Rational numbers are primarily of interest to theoreticians. Theoretical mathematics has potentially far-reaching applications in communications and computer science, especially in data encryption and security.
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A raster image processor (RIP) is a hardware or combination hardware/software product that converts images described in the form of vector graphics statements into raster graphics images or bitmaps. For example, laser printers use RIPs to convert images that arrive in vector form (for example, text in a specified font) into rasterized and therefore printable form.
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To demonstrate a true one-to-one correspondence between Q and N, a modification must be added to the algorithm shown in the illustration. Some of the elements in the matrix are repetitions of previous numerical values. For example, 2/4 = 3/6 = 4/8 = 5/10, and so on. These redundancies can be eliminated by imposing the constraint, ‘‘If a number represents a value previously encountered, skip over it.’’ In this manner, it can be rigorously proven that the set Q has exactly the same number of elements as the set N. Some people find this hard to believe, but the logic is sound. In contrast to the natural numbers, integers, and rational numbers, the sets of irrational numbers, real numbers, imaginary numbers, and complex numbers are nondenumerable. They have cardinality greater than that of the set N. This leads to the conclusion that some ‘‘infinities’’ are larger than others!
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Rational Rose is an object-oriented Unified Modeling Language (UML) software design tool intended for visual modeling and component construction of enterprise-level software applications. In much the same way a theatrical director blocks out a play, a software designer uses Rational Rose to visually create (model) the framework for an application by blocking out classes with actors (stick figures), use case elements (ovals), objects (rectangles) and messages/relationships (arrows) in a sequence diagram using drag-and-drop symbols. Rational Rose documents the diagram as it is being constructed and then generates code in the designer’s choice of C++, Visual Basic, Java, Oracle8, Corba or Data Definition Language.
See ‘‘Resource Description Framework’’
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Two popular features of Rational Rose are its ability to provide iterative development and round-trip engineering. Rational Rose allows designers to take advantage of iterative development (sometimes called evolutionary development) because the new application can be created in stages with the output of one iteration becoming the input to the next. (This is in contrast to waterfall development where the whole project is completed from start to finish before a user gets to try it out.) Then, as the developer begins to understand how the components interact and makes modifications in the design, Rational Rose can perform what is called ‘‘round-trip engineering’’ by going back and updating the rest of the model to ensure the code remains consistent.
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raw spool files See ‘‘EMF’’
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See ‘‘relational database management system’’
RDO See ‘‘Remote Data Objects’’
RDRAM See ‘‘DRDRAM’’
reach In Internet marketing, reach is how many different people visit a Web site to see an ad and also what percentage of these people fall into the audience to which an ad is targeted. A common measure of reach for a Web site is its ‘‘unique visitors per month.’’ For example, assume an advertising banner campaign was targeted to car owners. On a Web site visited by 100,000 unique visitors a month where 90% of the audience owned a car, an ad running on such the site would reach 90,000 different car owners in a one-month run. The term is used in other advertising media as well as on the Web.
reactance Reactance, denoted X, is a form of opposition that electronic components exhibit to the passage of AC (alternating current) because of capacitance or inductance. In some respects, reactance is like an AC counterpart of DC (direct current) resistance. But the two phenomena are different in important ways, and they can vary independently of each other. Resistance and reactance combine to form impedance, which is defined in terms of two-dimensional quantities known as complex number. When alternating current passes through a component that contains reactance, energy is alternately stored in, and released from, a magnetic field or an electric field. In the case of a magnetic field, the reactance is inductive. In the case of an electric field, the reactance is capacitive. Inductive reactance is assigned positive imaginary number values. Capacitive reactance is assigned negative imaginary-number values. As the inductance of a component increases, its inductive reactance becomes larger in imaginary terms, assuming the frequency is held constant. As the frequency increases for a given value of inductance, the inductive reactance increases in imaginary terms. If L is the inductance in henries (H) and f is the frequency in hertz (Hz), then the inductive reactance +jXL, in imaginary-number ohms, is given by: +jXL = +j(6.2832fL)
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real number where 6.2832 is approximately equal to 2 times pi, a constant representing the number of radians in a full AC cycle, and j represents the unit imaginary number (the positive square root of 71). The formula also holds for inductance in microhenries (mH) and frequency in megahertz (MHz). As a real-world example of inductive reactance, consider a coil with an inductance of 10.000 mH at a frequency of 2.0000 MHz. Using the above formula, +jXL is found to be +j125.66 ohms. If the frequency is doubled to 4.000 MHz, then +jXL is doubled, to +j251.33 ohms. If the frequency is halved to 1.000 MHz, then +jXL is cut in half, to +j62.832 ohms. As the capacitance of a component increases, its capacitive reactance becomes smaller negatively (closer to zero) in imaginary terms, assuming the frequency is held constant. As the frequency increases for a given value of capacitance, the capacitive reactance becomes smaller negatively (closer to zero) in imaginary terms. If C is the capacitance in farads (F) and f is the frequency in Hz, then the capacitive reactance 7jXC, in imaginary-number ohms, is given by:
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If you ever do the hardware setup procedure with your computer, you effectively will be writing to ROM.
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Real life is a term used to describe what happens outside the Internet and implies that somehow the Internet furnishes us with a virtual life. Depending on the context, usage of the term may imply one or more of the following: L
This formula also holds for capacitance in microfarads (mF) and frequency in megahertz (MHz). L
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reactive power In a direct current (DC) circuit, or in an alternating current (AC) circuit whose impedance is a pure resistance, the voltage and current are in phase, and the following formula holds: P = ErmsIrms where P is the power in watts, Erms is the root-mean-square (rms) voltage in volts, and Irms is the rms current in amperes. But in an AC circuit whose impedance consists of reactance as well as resistance, the voltage and current are not in phase. This complicates the determination of power. In the absence of reactance, the product ErmsIrms represents true power because it is manifested in tangible form (radiation, dissipation, and/or mechanical motion). But when there is reactance in an AC circuit, the product ErmsIrms is greater than the true power. The excess is called reactive power, and represents energy alternately stored and released by inductors and/or capacitors. The vector sum of the true and reactive power is known as apparent power.
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ROM is ‘‘built-in’’ computer memory containing data that normally can only be read, not written to. ROM contains the programming that allows your computer to be ‘‘booted up’’ or regenerated each time you turn it on. Unlike a computer’s random access memory (RAM), the data in ROM is not lost when the computer power is turned off. The ROM is sustained by a small long-life battery in your computer.
7jXC = 7j (6.2832fC)-1
As a real-world example of capacitive reactance, consider a capacitor with a value of 0.0010000 mF at a frequency of 2.0000 MHz. Using the above formula, 7jXC is found to be 7j79.577 ohms. If the frequency is doubled to 4.0000 MHz, then 7jXC is cut in half, to 7j39.789 ohms. If the frequency is cut in half to 1.0000 MHz, then 7jXC is doubled, to 7j159.15 ohms.
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The Internet is an artificial reflection or weak imitation of life that, in spite of its ability to let us quickly be in contact with knowledge and people at great distance, lacks the ability to engage the full range of our emotions and powers. In this sense, real life is seen as superior to virtual life.
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Increasing numbers of us spend time working on or using the Internet and somehow our work life on or frequent preoccupation with the Internet is seen as an intrusion into our lives. In this sense, real life is seen as a welcome respite from the keyboard and display.
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For those who become happily and fully engaged with the Internet – for example, gamers and those who get involved with 3-D environments or chatting—real life can be seen as a letdown, meaning back to the mundane.
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For anyone who communicates with others on the Internet, there is an air of the virtual or ‘‘made-up’’ in one’s e-mail or chat voice or persona (often exemplified by the e-mail address or the chat name that one adopts). In chat environments, there is even the possibility of using a mask or avatar to represent you. In the aspect of personal identity, real life means returning to your ‘‘real’’ name and place in the world.
Paradoxically, perhaps especially for those who work on it every day, Internet activity can come to seem more like one’s real life and feeding the cat can come to seem surreal or virtual.
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real number A real number is any element of the set R, which is the union of the set of rational number and the set of irrational number. In mathematical expressions, unknown or unspecified real numbers are usually represented by lowercase italic letters u through z. The set R gives rise to other sets such as the set of imaginary numbers and the set of complex numbers. The idea of a real number (and what
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Whatis?Com’s Encyclopedia of Technology Terms makes it ‘‘real’’) is primarily of interest to theoreticians. Abstract mathematics has potentially far-reaching applications in communications and computer science, especially in data encryption and security. If x and z are real numbers such that x < z, then there always exists a real number y such that x < y < z. The set of reals is ‘‘dense’’ in the same sense as the set of irrationals. Both sets are nondenumerable. There are more real numbers than is possible to list, even by implication. The set R is sometimes called the continuum because it is intuitive to think of the elements of R as corresponding oneto-one with the points on a geometric line. This notion, first proposed by Georg Cantor who also noted the difference between the cardinalities (sizes) of the sets of rational and irrational numbers, is called the Continuum Hypothesis. This hypothesis can be either affirmed or denied without causing contradictions in theoretical mathematics.
real time Also see real-time clock and real-time operating system.
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Real time is a level of computer responsiveness that a user senses as sufficiently immediate or that enables the computer to keep up with some external process (for example, to present visualizations of the weather as it constantly changes). Real-time is an adjective pertaining to computers or processes that operate in real time. Real time describes a human rather than a machine sense of time. In the days when mainframe batch computers were predominant, an expression for a mainframe that interacted immediately with users working from connected terminals was online in real time.
RealAudio RealAudio is a continous or streaming sound technology from Progressive Networks’ RealAudio. A RealAudio player or client program may come included with a Web browser or can be downloaded from the RealAudio or other Web sites. To deliver RealAudio sound from your own Web site, you (or your space provider) need to have a RealAudio server.
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A RealAudio file can be recognized by its file name extensions of: ra, ram.
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A real-time clock (RTC) is a battery-powered clock that is included as part of a microchip in a computer motherboard. This microchip is usually separate from the microprocessor and other chips and is often referred to simply as ‘‘the CMOS.’’ A small memory on this microchip stores system description or setup values—including current time values stored by the real-time clock. The time values are for the year, month, date, hours, minutes, and seconds. When the computer is turned on, the basic input-output operating
system (BIOS) that is stored in the computer’s read-only memory (ROM) microchip reads the current time from the memory in the chip with the real-time clock.
real-time operating system A real-time operating system (RTOS) is an operating system that guarantees a certain capability within a specified time constraint. For example, an operating system might be designed to ensure that a certain object was available for a robot on an assembly line. In what is usually called a ‘‘hard’’ real-time operating system, if the calculation could not be performed for making the object available at the designated time, the operating system would terminate with a failure. In a ‘‘soft’’ real-time operating system, the assembly line would continue to function but the production output might be lower as objects failed to appear at their designated time, causing the robot to be temporarily unproductive. Some real-time operating systems are created for a special application and others are more general purpose. Some existing general purpose operating systems claim to be a real-time operating systems. To some extent, almost any general purpose operating system such as Microsoft’s Windows 2000 or IBM’s OS/390 can be evaluated for its realtime operating system qualities. That is, even if an operating system doesn’t qualify, it may have characteristics that enable it to be considered as a solution to a particular realtime application problem. In general, real-time operating systems are said to require: L
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Real-time operating systems are often required in small embedded operating systems that are packaged as part of microdevices. Some kernels can be considered to meet the requirements of a real-time operating system. However, since other components, such as device drivers, are also usually needed for a particular solution, a real-time operating system is usually larger than just the kernel.
recency, frequency, monetary analysis See ‘‘RFM analysis’’
reciprocal meter The reciprocal meter is the standard unit of wave number for electromagnetic fields (EM fields). In terms of International Units (SI), the reciprocal meter is equal to m-1. The wave number is the number of complete wave cycles that exist in a linear span of 1 meter (m). At the speed of light in free space, 2.99792 6 108 m/s, the frequency f in hertz (Hz) is related to the wave number y in reciprocal meters (m-1) according to the following formula: y = f / (2.99792 6 108)
Red Book To cite a few examples, an EM wave at 300 megahertz (MHz) in free space has a wave number of almost exactly 1 m-1. If the frequency is doubled to 600 MHz, the wave number also doubles, to 2 m-1. If the frequency becomes 1/10 as great, that is, it is decreased to 30 MHz, then the wave number is reduced to 0.1 m-1. As a point of reference, it is easy to remember that the wave number of a 300-MHz signal in free space is very close to 1 m-1.
detects that some condition has been satisified. Here is a simple example (using a made-up computer source language): CODELINE1 N=0; CODELINE2 IF N=<10 THEN DO WRITE LETTER; CODELINE3 ELSE GOTO CODELINE6; CODELINE4 N=N+1;
In general, if the speed of propagation in meters per second for a specific medium is given by c, then:
CODELINE5 GOTO CODELINE2; CODELINE6 ...some other instruction
y=f/c
Here, the instructions labeled CODELINE2 through CODELINE5 are recursive until the condition of N having the value of 10. ‘‘IF N=<10’’ means ‘‘If N has a value less than 10.’’ ‘‘N=N+1’’ means ‘‘Add 1 to the current value of N.’’
where y is in reciprocal meters, and f is in hertz. In radio-frequency (RF) transmission lines such as coaxial cables and waveguides, the speed of propagation is less than 2.99792 6 108 m/s. This increases the wave number, although it has no effect on the frequency. Also see frequency, electromagnetic field, meter, wavelength, and International System of Units (SI).
record 1) In computer data processing, a record is a collection of data items arranged for processing by a program. Multiple records are contained in a file or data set. The organization of data in the record is usually prescribed by the programming language that defines the record’s organization and/or by the application that processes it. Typically, records can be of fixed-length or be of variable length with the length information contained within the record. 2) In Virtual Telecommunications Access Method, IBM’s proprietary telecommunications access method for mainframes and part of its Systems Network Architecture (SNA), a record is the unit of data that is transmitted from sender to receiver.
recursion In computer programming, a recursion (noun, pronounced ree-KUHR-zhion) is programming that is recursive (adjective), and recursive has two related meanings: 1) A recursive procedure or routine is one that has the ability to call itself. This usually means that it has the capability to save the condition it was in or the particular process it is serving when it calls itself (otherwise, any variable values that have been developed in executing the code are overlaid by the next iteration or go-through). Typically, this is done by saving values in registers or data area stacks before calling itself or at the beginning of the sequence where it has just been reentered. 2) A recursive expression is a function, algorithm, or sequence of instructions (typically, an IF, THEN, ELSE sequence) that loops back to the beginning of itself until it
In mathematics, recursion has similar but more complicated meanings than it does when used in programming.
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Red Book The Red Book is the 1980 document that provides the specifications for the standard compact disc (CD) developed by Sony and Philips. According to legend, the document was in a binder with red covers, originating the tradition for subsequent adaptations of CD specifications to be referred to as variously colored books. The Red Book described the CD’s physical specifications, such as the tracks, sector and block layout, coding, and sampling. Sony and Philips referred to the discs as CD-DA (digital audio), defined as a content medium for audio data digitized at 44,100 samples per second (44.1KHz) and in a range of 65,536 possible values (16 bits).
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The CD Format According to Red Book specifications, a standard CD is 120 mm (4.75 inches) in diameter and 1.2 mm (0.05 inches) thick and is composed of a polycarbonate plastic substrate (underlayer—this is the main body of the disc), one or more thin reflective metal (usually aluminum) layers, and a lacquer coating. CDs are divided into a lead-in area, which contains the table of contents (TOC), a program area, which contains the audio data, and a lead-out area, which contains no data. An audio CD can hold up to 74 minutes of recorded sound, and up to 99 separate tracks. Data on a CD-DA is organized into sectors (the smallest possible separately addressable block) of information. The audio information is stored in frames of 1/75 second length. 44,100 16-bit samples per second are stored, and there are two channels (left and right). This gives a sector size of 2,352 bytes per frame, which is the total size of a physical block on a CD. CD data is not arranged in distinct physical units; data is organized into frames (consisting of 24 bytes of user data, plus synchronization, error correction, and control and display bits) which are intricately interleaved so that damage to the disc will not destroy any single frame, but only small parts of many frames.
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The Red Book specifications form the basis for all later CD technologies.
Red Hat Red Hat is a leading software company in the business of assembling open source components for the Linux operating system and related programs into a distribution package that can easily be ordered. Red Hat provides over 400 different software packages, including the C language compiler from Cygnus, a Web server from Apache, and the X Window System from X Consortium. The advantages to buying the distribution from Red Hat rather than assembling it at no cost yourself from various sources is that you get it as a single assembled package. Red Hat also offers service that isn’t provided as quickly by the individual component developers, including members of the Free Software Foundation. Like all free software, Red Hat’s packages allow the buyer to modify and even resell modified versions of code as long as they do not restrict anyone else from further modification. Red Hat was one of the first companies to realize that ‘‘free’’ software could be sold as a product. After examining the successful marketing campaign of Evian water, Red Hat concluded that to achieve success, the company had to create more Linux users and brand Red Hat as the Linux name that customers preferred. Today, the ‘‘Red Hat Plan’’ is discussed as a model in business schools.
On a Web site, redirection is a technique for moving visitors to a Web page when its address has been changed and visitors are familiar with the old address. Web users often encounter redirection when they visit the Web site of a company whose name has been changed or which has been acquired by another company. In either case, the Web site probably will include a new domain name and it will have a new Uniform Resource Locator. To make sure that visitors familiar with the old company get to the new site, the company will turn the original Web site home page into a redirect page, containing a message that says something like: ‘‘We have moved to a new location. Please change your bookmark. In five seconds, we will transfer you to the new page.’’
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In addition to replacing the content of the old page with the redirect text, the company adds a meta refresh line in the header section of the page that looks something like this:
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reduced instruction set computer See ‘‘RISC’’
redundant In information technology, the term redundant has several usages: 1) Redundant describes computer or network system components, such as fans, hard disk drives, servers, operating systems, switches, and telecommunication links that are installed to back up primary resources in case they fail. A well-known example of a redundant system is the redundant array of independent disks (RAID). 2) Redundant information is unneeded or duplicated information. 3) Redundant bits are extra bits that are generated and transferred along with a data transfer to ensure that no bits were lost during the data transfer. Redundancy is the quality of a system, an item of information, or a bit that is redundant.
redundant array of independent disks See ‘‘RAID’’
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redirect page with a meta refresh tag to the new page. Otherwise, visitors familiar with the old page will get a 404 (Page Not Found) message.
In the above example, the ‘‘5’’ means change to the new page in five seconds. The ‘‘url=http://www.ournewsite.com’’ is an example of the URL for the new page. If you own a Web site that people have learned to use and you then change the name and location of a Web page, it’s strongly recommended that you make the original page a
redundant array of inexpensive disks See ‘‘RAID’’
reentrant Reentrant is an adjective that describes a computer program or routine that is written so that the same copy in memory can be shared by multiple users. Reentrant code is commonly required in operating systems and in applications intended to be shared in multi-use systems. A programmer writes a reentrant program by making sure that no instructions modify the contents of variable values in other instructions within the program. Each time the program is entered for a user, a data area is obtained in which to keep all the variable values for that user. The data area is in another part of memory from the program itself. When the program is interrupted to give another user a turn to use the program, information about the data area associated with that user is saved. When the interrupted user of the program is once again given control of the program, information in the saved data area is recovered and the program can be reentered without concern that the previous user has changed some instruction within the program.
register
refresh rate The refresh rate is the amount of times a display’s image is repainted or refreshed per second. The refresh rate is expressed in hertz so a refresh rate of 75 means the image is refreshed 75 times in a second. The refresh rate for each display depends on the video card used. You can change the refresh rate in the display properties. However, if you change the refresh rate to a setting that the display or video card cannot support, the display goes blank or the image becomes distorted. It is recommended to consult the display and video card manuals before changing the settings to determine the supported refresh rates. An older refresh rate standard, developed by the Video Electronics Standards Association (VESA Local Bus), was only 60 Hz. This refresh rate caused the display’s image to flicker causing eye fatigue and headaches in users. A new standard set the refresh rate to 75 Hz. It is believed that 70 Hz or higher eliminates the flicker. When purchasing a monitor, look for a refresh rate of 75 to 85 Hz. Dynamic random access memory (DRAM) memory modules also have a refresh rate. A DRAM module is made up of electrical cells. These cells must be recharged or refreshed thousands of times per second or they lose their data. The refresh cycles depend on the number of rows that must be refreshed. For example, a DRAM module that has 4 rows of cells has a refresh cycle of 4K. Some DRAM modules are able to refresh themselves independently of the processor or external refresh circuits. Since this reduces power consumption, this kind of DRAM is commonly used in notebook computers.
refurbished Refurbish, in everyday language, is ‘‘to renew or to restore to a new condition and/or appearance.’’ In the computer world, refurbished equipment is not necessarily defective in any way; it may just be ‘‘old’’ (a relative term in the world of computers). When hardware is refurbished, the components are examined and non-working parts are replaced. Companies that lease computers may use refurbishing as a means of dealing with units that are returning from longterm leases; after the computers are refurbished, they may be sold at a reduced price or donated to charity—either of which is good for business. Corporations may buy refurbished computers and hardware for a number of reasons. Refurbished hardware costs less than new hardware, which makes it possible for small businesses to make the move to computerized operations for the first time or for businesses with limited means to buy higher quality computers for less money. Another reason for a corporation to buy refurbished equipment is to maintain corporate standards by ensuring that all employees have the same equipment, even when the model being used is not being sold any longer.
There are additional reasons for choosing refurbished hardware if a company is already computerized. For example, there may be a corporate plan to use desktop computers for three years before upgrading to newer ones. In the latter part of the three-year period, equipment purchases may be needed for new staff or to replace a component that no longer works. Since the corporation intends to replace all desktop computers in the near future, the purchasing department may not want to pay full price for any new equipment at the end of this three-year period. Refurbished computers provide a cost-effective alternative. A number of organizations, such as StRUT (Students Recycling Used Technology), the National Cristina Foundation, and the Resource Area for Teachers (RAFT) collect and refurbish donated computer equipment for redistribution to schools and charities around the world.
regional Bell operating company Regional Bell operating company (RBOC) is a term describing one of the U.S. regional telephone companies (or their successors) that were created as a result of the breakup of American Telephone and Telegraph Company (AT&T, known also as the Bell System or ‘‘Ma Bell’’) by a U.S. Federal Court consent decree on December 31, 1983. The seven original regional Bell operating companies were Ameritech, Bell Atlantic, BellSouth, NYNEX, Pacific Bell, Southwestern Bell, and US WEST. Each of these companies owned at least two Bell operating companies (BOCs). The BOCs were given the right to provide local phone service while AT&T was allowed to retain its long-distance service. The RBOCs and their constituent BOCs are part of the class of local exchange carriers (LECs). In addition to the RBOCs, there are more than 100 other franchised local telephone companies classed as local exchange carriers. Competitive local exchange carriers (CLECs) are additional companies allowed to compete with the LECs. These include AT&T in some localities and power companies. An interexchange carrier (IC) is a long-distance carrier that carries traffic between LECs. Under the Telecommunications Act of 1996, RBOCs and LECs are allowed to compete for long-distance telephone traffic under certain circumstances. RBOCs are generally in competition for digital data and Internet traffic with wireless service providers and cable TV companies. RBOCs are gradually making available new telephone carrier technologies such as ISDN and DSL.
register In a computer, a register is one of a small set of data holding places that are part of a computer microprocessor and that provide a place for passing data from one instruction to the next sequential instruction or to another program that the operating system has just given control to. A register must be large enough to hold an instruction—for example, in a
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Whatis?Com’s Encyclopedia of Technology Terms 32-bit instruction computer, a register must be 32 bits in length. In some computer designs, there are smaller registers—for example, half-registers—for shorter instructions. Depending on the processor design and language rules, registers may be numbered or have arbitrary names.
registered port numbers
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The registered port numbers are the port numbers that companies and other users register with the Internet Corporation for Assigned Names and Numbers (ICANN) for use by the applications that communicate using the Internet’s Transmission Control Protocol (TCP) or the User Datagram Protocol (UDP). In most cases, these applications run as ordinary programs that can be started by nonprivileged users. The registered port numbers are in the range from 1024 through 49151. They follow in sequence the well-known port numbers, which are, in most cases, applications that can only be started by privileged users, such as the Hypertext Transfer Protocol (HTTP) and Post Office Protocol Version 3 (POP3) applications. When one application communicates with another application at another host computer on the Internet, it specifies that application in each data transmission by using its port number. Examples of applications with registered port numbers include Sun’s NEO Object Request Broker (port numbers 1047 and 1048) and Shockwave (port number 1626). Besides the well-known port numbers and the registered port numbers, the remaining ports in the port number spectrum are referred to as dynamic ports or private ports and are numbered from 49152 through 65535. Before the arrival of ICANN, the port numbers were administered by the Internet Internet Assigned Numbers Authority (IANA).
registration authority A registration authority (RA) is an authority in a network that verifies user requests for a digital certificate and tells the certificate authority (CA) to issue it. RAs are part of a public key infrastructure (PKI), a networked system that enables companies and users to exchange information and money safely and securely. The digital certificate contains a public key that is used to encrypt and decrypt messages and digital signatures.
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In the Microsoft Windows 95, Windows 98, Windows NT, and Windows 2000 operating systems, the registry is a single place for keeping such information as what hardware is attached, what system options have been selected, how computer memory is set up, and what application programs are to be present when the operating system is started. The registry is somewhat similar to and a replacement for the simpler INI (initialization) and configuration files used in
earlier Windows (DOS-based) systems. INI files are still supported, however, for compatibility with the 16-bit applications written for earlier systems. In general, the user updates the registry indirectly using Control Panel tools, such as TweakUI. When you install or uninstall application programs, they also update the registry. In a network environment, registry information can be kept on a server so that system policies for individuals and workgroups can be managed centrally.
regression testing Regression testing is the process of testing changes to computer programs to make sure that the older programming still works with the new changes. Regression testing is a normal part of the program development process and, in larger companies, is done by code testing specialists. Test department coders develop code test scenarios and exercises that will test new units of code after they have been written. These test cases form what becomes the test bucket. Before a new version of a software product is released, the old test cases are run against the new version to make sure that all the old capabilities still work. The reason they might not work is because changing or adding new code to a program can easily introduce errors into code that is not intended to be changed.
relational database management system A relational database management system (RDBMS) is a program that lets you create, update, and administer a relational database. An RDBMS takes Structured Query Language (SQL) statements entered by a user or contained in an application program and creates, updates, or provides access to the database. Some of the best-known RDBMS’s include Oracle’s database product line, Computer Associates’ CA-OpenIngres, and IBM’s DB2. The majority of new corporate, small business, and personal databases are being created for use with an RDBMS. Meanwhile, the idea of object-orientation has begun to contend with the RDBMS as the database management system of the future, sometimes in hybrid implementations.
relational database A relational database is a collection of data items organized as a set of formally-described tables from which data can be accessed or reassembled in many different ways without having to reorganize the database tables. The relational database was invented by E. F. Codd at IBM in 1970. The standard user and application program interface to a relational database is the Structured Query Language (SQL). SQL statements are used both for interactive queries for information from a relational database and for gathering data for reports.
remailer In addition to being relatively easy to create and access, a relational database has the important advantage of being easy to extend. After the original database creation, a new data category can be added without requiring that all existing applications be modified. A relational database is a set of tables containing data fitted into predefined categories. Each table (which is sometimes called a relation) contains one or more data categories in columns. Each row contains a unique instance of data for the categories defined by the columns. For example, a typical business order entry database would include a table that described a customer with columns for name, address, phone number, and so forth. Another table would describe an order: product, customer, date, sales price, and so forth. A user of the database could obtain a view of the database that fitted the user’s needs. For example, a branch office manager might like a view or report on all customers that had bought products after a certain date. A financial services manager in the same company could, from the same tables, obtain a report on accounts that needed to be paid. When creating a relational database, you can define the domain of possible values in a data column and further constraints that may apply to that data value. For example, a domain of possible customers could allow up to ten possible customer names but be constrained in one table to allowing only three of these customer names to be specifiable. The definition of a relational database results in a table of metadata or formal descriptions of the tables, columns, domains, and constraints.
relational online analytical processing Relational online analytical processing (ROLAP) is a form of online analytical processing (OLAP) that performs dynamic multidimensional analysis of data stored in a relational database rather than in a multidimensional database (which is usually considered the OLAP standard). Data processing may take place within the database system, a mid-tier server, or the client. In a two-tiered architecture, the user submits a Structure Query Language (SQL) query to the database and receives back the requested data. In a three-tiered architecture, the user submits a request for multidimensional analysis and the ROLAP engine converts the request to SQL for submission to the database. Then the operation is performed in reverse: the engine converts the resulting data from SQL to a multidimensional format before it is returned to the client for viewing. As is typical of relational databases, some queries are created and stored in advance. If the desired information is available, then that query will be used, which saves time. Otherwise, the query is created on the fly from the user request. Microsoft Access’s PivotTable is an example of a three-tiered architecture.
Since ROLAP uses a relational database, it requires more processing time and/or disk space to perform some of the tasks that multidimensional databases are designed for. However, ROLAP supports larger user groups and greater amounts of data and is often used when these capacities are crucial, such as in a large and complex department of an enterprise.
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Reliability is an attribute of any computer-related component (software, or hardware, or a network, for example) that consistently performs according to its specifications. It has long been considered one of three related attributes that must be considered when making, buying, or using a computer product or component. Reliability, availability, and serviceability—RAS, for short—are considered to be important aspects to design into any system. In theory, a reliable product is totally free of technical errors; in practice, however, vendors frequently express a product’s reliability quotient as a percentage. Evolutionary products (those that have evolved through numerous versions over a significant period of time) are usually considered to become increasingly reliable, since it is assumed that bugs have been eliminated in earlier releases. For example, IBM’s z/OS (an operating system for their S/390 server series), has a reputation for reliability because it evolved from a long line of earlier MVS and OS/390 operating system versions.
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The Institute of Electrical and Electronics Engineers (IEEE) sponsors an organization devoted to reliability in engineering, the IEEE Reliability Society (IEEE RS). The Reliability Society promotes industry-wide acceptance of a systematic approach to design that will help to ensure reliable products. To that end, they promote reliability not just in engineering, but in maintenance and analysis as well. The Society encourages collaborative effort and information sharing among its membership, which encompasses organizations and individuals involved in all areas of engineering, including aerospace, transportation systems, medical electronics, computers, and communications.
remailer A remailer is an Internet site to which you can send e-mail for forwarding to an intended destination while concealing your own e-mail address. E-mail sent through a remailer is sometimes known as anonymous e-mail. There may be valid reasons for wanting to conceal your e-mail address (and personal identity) from an e-mail recipient. There are, of course, unworthy reasons, too. However, advocates of anonymous e-mail and remailer services remind us that having the right to conceal your identity in a note can, on occasion, be socially useful and the practice should be possible.
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Whatis?Com’s Encyclopedia of Technology Terms A small number of Web sites provide remailer services. Until it was recently closed, the best-known remailer was the Finland-based anon.penet.fi. It reportedly handled about 10,000 messages a day and had almost 700,000 registered users.
Remote Access Server Application Program Interface RAS API (Remote Access Server Application Program Interface) is an application program interface (API) in Microsoft Windows 95/98/NT systems that supports remote access for users with dial-up connections. The Windows program component with which a program must interface is called rasapi32.dll.
remote access server
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See ‘‘remote access’’
remote access Remote access is the ability to get access to a computer or a network from a remote distance. In corporations, people at branch offices, telecommuters, and people who are travelling may need access to the corporation’s network. Home users get access to the Internet through remote access to an Internet service provider (ISP). Dial-up connection through desktop, notebook, or handheld computer modem over regular telephone lines is a common method of remote access. Remote access is also possible using a dedicated line between a computer or a remote local area network and the ‘‘central’’ or main corporate local area network. A dedicated line is more expensive and less flexible but offers faster data rates. Integrated Services Digital Network (ISDN) is a common method of remote access from branch offices since it combines dial-up with faster data rates. Wireless, cable modem, and Digital Subscriber Line (DSL) technologies offer other possibilities for remote access. A remote access server is the computer and associated software that is set up to handle users seeking access to network remotely. Sometimes called a communication server, a remote access server usually includes or is associated with a firewall server to ensure security and a router that can forward the remote access request to another part of the corporate network. A remote access server may include or work with a modem pool manager so that a small group of modems can be shared among a large number of intermittently present remote access users. A remote access server may also be used as part of a virtual private network (VPN).
Remote Authentication Dial-In User Service RADIUS (Remote Authentication Dial-In User Service) is a client/server protocol and software that enables remote access servers to communicate with a central server to authenticate dial-in users and authorize their access to the requested system or service. RADIUS allows a company to maintain user profiles in a central database that all remote servers can share. It provides better security, allowing a company to set up a policy that can be applied at a single administered network point. Having a central service also means that it’s easier to track usage for billing and for keeping network statistics. Created by Livingston (now owned by Lucent), RADIUS is a de facto industry standard used by Ascend and other network product companies and is a proposed IETF standard.
Remote Data Objects RDO (Remote Data Objects) is an application program interface (API) from Microsoft that lets programmers writing Windows applications get access to and from both Microsoft and other database providers. In turn, RDO statements in a program use Microsoft’s lower-layer Data Access Objects (DAO) for actual access to the database. Database providers write to the DAO interface. RDO has evolved into ActiveX Data Objects (ADO) which is now the program interface Microsoft recommends for new programs. ADO also provides access to nonrelational databases and is somewhat easier to use.
Remote Function Call RFC is also an abbreviation for Request for Comments. A Remote Function Call (RFC) is an application program interface to R/3 applications from SAP, the German company that sells a coordinated set of applications and databases to Fortune 1000 companies. SAP customers who wish to write other applications that communicate with R/3 applications and databases can use the RFC interface to do so.
Remote Installation Service Remote Installation Service (RIS) is a feature included in Microsoft’s Windows 2000 server that allows network administrators to install the Windows 2000 Professional operating system and its upgrades to any number of client computers at one time from a centralized location. If the client computer is connected to the server through a local area network (LAN), the computer’s hardware will find the RIS server and request a copy of the operating system. The network administrator can use the RISrep imaging option to copy several versions, or images, of a company’s desktop configuration to the server so that if a request is made, the server can issue an ‘‘image’’ for a particular computer or
Remote Procedure Call user. Microsoft promotes the use of RIS as a way to configure new computers right out of the box and to restore the operating system on a computer that has failed.
Remote Method Invocation RMI (Remote Method Invocation) is a way that a programmer, using the Java programming language and development environment, can write object-oriented programming in which objects on different computers can interact in a distributed network. RMI is the Java version of what is generally known as a remote procedure call (RPC), but with the ability to pass one or more objects along with the request. The object can include information that will change the service that is performed in the remote computer. Sun Microsystems, the inventors of Java, calls this ‘‘moving behavior.’’ For example, when a user at a remote computer fills out an expense account, the Java program interacting with the user could communicate, using RMI, with a Java program in another computer that always had the latest policy about expense reporting. In reply, that program would send back an object and associated method information that would enable the remote computer program to screen the user’s expense account data in a way that was consistent with the latest policy. The user and the company both would save time by catching mistakes early. Whenever the company policy changed, it would require a change to a program in only one computer. Sun calls its object parameter-passing mechanism object serialization. An RMI request is a request to invoke the method of a remote object. The request has the same syntax as a request to invoke an object method in the same (local) computer. In general, RMI is designed to preserve the object model and its advantages across a network. RMI is implemented as three layers: L
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A stub program in the client side of the client/server relationship, and a corresponding skeleton at the server end. The stub appears to the calling program to be the program being called for a service. (Sun uses the term proxy as a synonym for stub.) A Remote Reference Layer that can behave differently depending on the parameters passed by the calling program. For example, this layer can determine whether the request is to call a single remote service or multiple remote programs as in a multicast. A Transport Connection Layer, which sets up and manages the request. A single request travels down through the layers on one computer and up through the layers at the other end.
RMI is supplied as part of Sun Microsystem’s Java Development Kit (JDK).
Remote Network Access Application RNAAPP (Remote Network Access Application) is one of those occasionally visible yet mysterious little programs that come with the Microsoft Windows 95/98 operating system. It is part of Windows’ Dial Up Networking capability. Users can see it in Windows Explorer as rnaapp.exe. (The exe stands for executable, meaning it’s a file that is a program.) RNAAPP can be the source of a problem when trying to get Internet access. It sometimes remains in memory from a previous connection, making the system believe that a connection is already established.
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One approach if you get the message that ‘‘Dial-up Networking could not establish a compatible set of network protocols’’ or a message similar to it is to simply press ctrlalt-delete, find RNAAPP in the task list, select it, and then click on the End Task button. There may also be other approaches, too.
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Remote Procedure Call Remote Procedure Call (RPC) is a protocol that one program can use to request a service from a program located in another computer in a network without having to understand network details. (A procedure call is also sometimes known as a function call or a subroutine call.) RPC uses the client/server model. The requesting program is a client and the service-providing program is the server. Like a regular or local procedure call, an RPC is a synchronous operation requiring the requesting program to be suspended until the results of the remote procedure are returned. However, the use of lightweight processes or threads that share the same address space allows multiple RPCs to be performed concurrently. When program statements that use RPC are compiled into an executable program, a stub is included in the compiled code that acts as the representative of the remote procedure code. When the program is run and the procedure call is issued, the stub receives the request and forwards it to a client runtime program in the local computer. The client runtime program has the knowledge of how to address the remote computer and server application and sends the message across the network that requests the remote procedure. Similarly, the server includes a runtime program and stub that interface with the remote procedure itself. Results are returned the same way. There are several RPC models and implementations. A popular model and implementation is the Distributed Computing Environment (DCE). The IEEE defines RPC in its ISO Remote Procedure Call Specification, ISO/IEC CD 11578 N6561, ISO/IEC, November 1991.
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RPC spans the Transport layer and the Application layer in the Open Systems Interconnection (OSI) model of network communication. RPC makes it easier to develop an application that includes multiple programs distributed in a network.
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Alternative methods for client/server communication include message queueing and IBM’s Advanced Programto-Program Communication (APPC).
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Remote-control software is programming in a central or server computer that is used to control other computers (or their users) at a distance, either under the control of an administrator or at the request of the user. Although remotecontrol software existed before the World Wide Web (for remote diagnosis of computer problems and other purposes), the Web has essentially built a platform on which anyone can build a new remote-control application that can reach millions of computers and their users. Remote-control software can be viewed as one class of application furnished by application service providers. Remote-control software can be divided into applications for use within a private network (such as an intranet) or for use on the public network. In a private network, remote-control software can be used to configure and administer all computers from a central point. On the public network, users can request such services as name lookup or arrange to have their files backed up automatically once a day. Remotecontrol software can also be used in a classroom system where one PC becomes the ‘‘master’’ of student computers, automatically reconfiguring them or turning them off at night.
render To render (a verb, pronounced REHN-dir, from the medieval French rendre meaning ‘‘to give back or yield‘‘) has a number of usages along the lines of forming something out of something else originally given. A jury renders a verdict given evidence and the rules of law. Animal fat can be rendered into lard. Out of loyalty to the king, a service is rendered. An artist can render an idea in the mind into a drawing on paper. A translator renders one language into another. In computer graphics technology, computer software can be used to render special 3-D effects given the right programming statements. A computer display system renders an image that is sent to it in the form of a bitmap or streaming image. A rendering (noun) is a term sometimes used to describe a drawing, sketch, plan, or other artistic or engineered effort to depict or portray something on paper or in another medium.
repeater 1) In telecommunication networks, a repeater is a device that receives a signal on an electromagnetic or optical transmission medium, amplifies the signal, and then retransmits it along the next leg of the medium. Repeaters overcome the attenuation caused by free-space electromagnetic-field divergence or cable loss. A series of repeaters make possible the extension of a signal over a distance. Repeaters are used to interconnect segments in a local area network (LAN). They’re also used to amplify and extend wide area network transmission on wire and wireless media. Because digital signals depend on the presence or absence of voltage, they tend to dissipate more quickly than analog signals and need more frequent repeating. Whereas analog signal amplifiers are spaced at 18,000 meter intervals, digital signal repeaters are typically placed at 2,000 to 6,000 meter intervals. In a cable system, a repeater can be simple, consisting of an amplifier circuit and a couple of signal transformers. The impedance of the cable must be matched to the input and output of the amplifier to optimize the efficiency of the amplifier. Impedance matching also minimizes reflection of signals along the cable. Such reflection can produce undesirable echo effects. In a wireless communications system, a repeater consists of a radio receiver, an amplifier, a transmitter, an isolator, and two antennas. The transmitter produces a signal on a frequency that differs from the received signal. This socalled frequency offset is necessary to prevent the strong transmitted signal from disabling the receiver. The isolator provides additional protection in this respect. A repeater, when strategically located on top of a high building or a mountain, can greatly enhance the performance of a wireless network by allowing communications over distances much greater than would be possible without it. In a fiber optic network, a repeater consists of a photocell, an amplifier, and a light-emitting diode (LED) or infraredemitting diode (IRED) for each light or IR signal that requires amplification. Fiber optic repeaters operate at power levels much lower than wireless repeaters, and are also much simpler and cheaper. However, their design requires careful attention to ensure that internal circuit noise is minimized. 2) A bus repeater links one computer bus to a bus in another computer chassis, essentially chaining one computer to another. 3) Repeaters are commonly used by amateur and commercial radio operators to extend signals in the radio frequency range from one receiver to another. These consist of drop repeaters, similar to the cells in cellular radio, and hub repeaters, which receive and retransmit signals from and to a number of directions.
Resilient Packet Ring
replication Replication (pronounced rehp-lih-KA-shun) is the process of making a replica (a copy) of something. A replication (noun) is a copy. The term is used in fields as varied as microbiology (cell replication), knitwear (replication of knitting patterns), and information distribution (CD-ROM replication).
or changes are permitted. Change can occur, however, through subsequent RFCs that supersede or elaborate on all or parts of previous RFCs.
Reseau Numerique a Integration de Services
Using the groupware product, Lotus Notes, replication is the periodic electronic refreshing (copying) of a database from one computer server to another so that all users in the Notes network constantly share the same level of information.
In information technology, a reseller, also sometimes known as a value-added reseller (VAR), is a company that typically buys products such as computers in bulk from a manufacturer and then ‘‘adds value’’ to the original equipment by including specific software applications or other components. The reseller then markets and sells the customized product under its own name. For example, a VAR might take an operating system such as IBM’s OS/390 with UNIX services and, adding its own proprietary UNIX application designed for architects, resell the package to architectural firms. Depending on sales and installation requirements, the reseller could choose whether or not to identify OS/390 as part of the package.
RPG (Report Program Generator) is a programming language that originated as a report-building program used in DEC and IBM minicomputer operating systems and evolved into a fully procedural programming language. Its latest version, RPG IV, is supported by IBM’s leading minicomputer system, the AS/400. Historically, RPG has probably been the second most used programming language, after COBOL, for commercial applications on mid-range computers.
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RPG also stands for role-playing game.
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On the Internet, a Web site that has been replicated in its entirety and put on another site is called a mirror site.
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reseller hosting Reseller hosting is the provision of Web hosting services to companies that in turn act as Web hosts for other companies, typically providing Web site design and management services as well as acting as host for the site and serving its pages to users. For example, a hypothetical Sports Site Server Inc. could provide a package for professional or amateur sports teams that included an easy-to-use approach to creating a Web site as well as the space for the Web site’s pages and an easy-to-use interface for keeping the site up-todate. Sports Site Server would actually use the computer and storage facilities of Giant Host, Inc., a hosting company that offered similar reselling capabilities to many companies like Sports Site Server. A large hosting company that allowed reselling (like Giant Host) might actually host hundreds of Web sites.
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In information technology, a repository (pronounced reePAHZ-ih-tor-i) is a central place in which an aggregation of data is kept and maintained in an organized way, usually in computer storage. The term is from the Latin repositorium, a vessel or chamber in which things can be placed, and it can mean a place where things are collected. Depending on how the term is used, a repository may be directly accessible to users or may be a place from which specific databases, files, or documents are obtained for further relocation or distribution in a network. A repository may be just the aggregation of data itself into some accessible place of storage or it may also imply some ability to selectively extract data. Related terms are data warehouse and data mining.
In general, a reseller is a company that buys a product or service from another company and repackages it, usually adding something to it, to other companies under its own company or brand name.
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Request for Comments
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RFC is also an abbreviation for Remote Function Call.
Resilient Packet Ring (RPR) is a network topology being developed as a new standard for fiber optic rings. The Institute of Electrical and Electronic Engineers (IETF) began the RPR standards (IETF 802.17) development project in December 2000 with the intention of creating a new Media Access Control layer for RPR. The IETF working group is part of the IETF’s Local Area Network (LAN) and Metropolitan Area Network (MAN) Committee. Fiber optic rings are widely deployed in both MANs and wide area
A Request for Comments (RFC) is a formal document from the Internet Engineering Task Force (IETF) that is the result of committee drafting and subsequent review by interested parties. Some RFCs are informational in nature. Of those that are intended to become Internet standards, the final version of the RFC becomes the standard and no further comments
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networks (WANs); however, these topologies are dependent on protocols that aren’t optimized or scalable to meet the demands of packet-switched networks. The working group intends to actively promote RPR as a technology for the networking market as a whole, and as a technology to enable connectivity among various computing and telecommunications devices. The working group will specify an RPR access protocol and physical layer interfaces to enable high-speed data transmission in a fiber optic ring topology. Among the issues the RPR working group is addressing are bandwidth allocation and throughput, speed of deployment, and equipment and operational costs.
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Also see current, voltage, and Ohm’s Law. Resistance is the opposition that a substance offers to the flow of electric current. It is represented by the uppercase letter R. The standard unit of resistance is the ohm, sometimes written out as a word, and sometimes symbolized by the uppercase Greek letter omega. When an electric current of one ampere passes through a component across which a potential difference (voltage) of one volt exists, then the resistance of that component is one ohm. In general, when the applied voltage is held constant, the current in a direct-current (DC) electrical circuit is inversely proportional to the resistance. If the resistance is doubled, the current is cut in half; if the resistance is halved, the current is doubled. This rule also holds true for most lowfrequency alternating-current (AC) systems, such as household utility circuits. In some AC circuits, especially at high frequencies, the situation is more complex, because some components in these systems can store and release energy, as well as dissipating or converting it. The electrical resistance per unit length, area, or volume of a substance is known as resistivity. Resistivity figures are often specified for copper and aluminum wire, in ohms per kilometer.
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Opposition to AC, but not to DC, is a property known as reactance. In an AC circuit, the resistance and reactance combine vectorially to yield impedance.
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A resistor is an electrical component that limits or regulates the flow of electrical current in an electronic circuit. Resistors can also be used to provide a specific voltage for an active device such as a transistor. All other factors being equal, in a direct-current (DC) circuit, the current through a resistor is inversely proportional to its resistance, and directly proportional to the voltage across it. This is the well-known Ohm’s Law. In alternating-current (AC) circuits, this rule also applies as long as the resistor does not contain inductance or capacitance.
Resistors can be fabricated in a variety of ways. The most common type in electronic devices and systems is the carboncomposition resistor. Fine granulated carbon (graphite) is mixed with clay and hardened. The resistance depends on the proportion of carbon to clay; the higher this ratio, the lower the resistance. Another type of resistor is made from winding Nichrome or similar wire on an insulating form. This component, called a wirewound resistor, is able to handle higher currents than a carbon-composition resistor of the same physical size. However, because the wire is wound into a coil, the component acts as an inductors as well as exhibiting resistance. This does not affect performance in DC circuits, but can have an adverse effect in AC circuits because inductance renders the device sensitive to changes in frequency.
resolution Resolution is the number of pixels (individual points of color) contained on a display monitor, expressed in terms of the number of pixels on the horizontal axis and the number on the vertical axis. The sharpness of the image on a display depends on the resolution and the size of the monitor. The same pixel resolution will be sharper on a smaller monitor and gradually lose sharpness on larger monitors because the same number of pixels are being spread out over a larger number of inches. A given computer display system will have a maximum resolution that depends on its physical ability to focus light (in which case the physical dot size—the dot pitch—matches the pixel size) and usually several lesser resolutions. For example, a display system that supports a maximum resolution of 1280 by 1023 pixels may also support 1024 by 768, 800 by 600, and 640 by 480 resolutions. Note that on a given size monitor, the maximum resolution may offer a sharper image but be spread across a space too small to read well. Display resolution is not measured in dots per inch as it usually is with printers. However, the resolution and the physical monitor size together do let you determine the pixels per inch. Typically, PC monitors have somewhere between 50 and 100 pixels per inch. For example, a 15-inch VGA monitor has a resolution of 640 pixels along a 12-inch horizontal line or about 53 pixels per inch. A smaller VGA display would have more pixels per inch.
Resource Access Control Facility RACF (Resource Access Control Facility) is the IBM security management product for its mainframe (large server) operating system, OS/390 (MVS) as well as for its VM operating system. RACF allows an enterprise to manage the biggest security threat in most enterprises: destruction of computing resources by its own employees. Since its first
response time release in 1976, RACF has screened millions of mainframe users who are familiar with their logon identification as their ‘‘RACF userid.’’ RACF gives access to a computer system only to users who have the authorization to use a requested resource (such as a file, a printer queue, space to run a program, and so forth). To do this, RACF identifies and authenticates a user, determines the resources to which the user is authorized, and logs and reports attempts to get access to protected resources by unauthorized users.
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By providing a consistent framework, RDF will encourage the providing of metadata about Internet resources.
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Because RDF will include a standard syntax for describing and querying data, software that exploits metadata will be easier and faster to produce.
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Resource Description Framework
A certain amount of metadata is already provided for Web site resources using the Hypertext Markup Language (HTML). For example, when we wrote this page, we added HTML statements containing key words that describe the content of this definition and that are used by search engines for indexing. We also added a one-sentence description that can be shown by search engines. (These statements, called META tag statements, are invisible to you unless you click on this page and then right-click on ‘‘View source’’). Less formally, the ‘‘Created on’’ or ‘‘Updated on’’ date at the bottom of this definition is also metadata—that is, data that tells you something about the data or content on this page. These are simply a few examples of many possible existing and future resource descriptions needed about a Web resource. Originally conceived as an extension of the content rating PICS Recommendation, the RDF will in time subsume it, with the idea that it can express any data that a PICS-1.1 label can express. However, both RDF and the equivalent PICS expression are expected to be in use for a while.
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Here are some of the likely benefits:
In OS/390, RACF is part of the Security Server.
The Resource Description Framework (RDF) is a general framework for how to describe any Internet resource such as a Web site and its content. An RDF description (such descriptions are often referred to as metadata, or ‘‘data about data’’) can include the authors of the resource, date of creation or updating, the organization of the pages on a site (the sitemap), information that describes content in terms of audience or content rating, key words for search engine data collection, subject categories, and so forth. The Resource Description Framework will make it possible for everyone to share Web site and other descriptions more easily and for software developers to build products that can use the metadata to provide better search engines and directories, to act as intelligent agents, and to give Web users more control of what they’re viewing. The RDF is an application of another technology, the Extensible Markup Language (XML), and is being developed under the auspices of the World Wide Consortium (W3C).
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The standard syntax and query capability will allow applications to exchange information more easily.
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Searchers will get more precise results from searching, based on metadata rather than on indexes derived from full text gathering.
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Intelligent software agents will have more precise data to work with.
How RDF Works An Internet resource is defined as any resource with a Uniform Resource Identifier (URI). This includes the Uniform Resource Locators (URL) that identify entire Web sites as well as specific Web pages. As with today’s HTML META tags, the RDF description statements, encased as part of an Extensible Markup Language (XML) section, could be included within a Web page (that is, a Hypertext Markup Language—HTML—file) or could be in separate files. RDF is now a formal W3C Recommendation, meaning that it is ready for general use. Currently, a second W3C recommendation, still at the Proposal stage, proposes a system in which the descriptions related to a particular purpose (for example, all descriptions related to security and privacy) would constitute a class of such like descriptions (using class here much as it is used in object-oriented programming data modeling and programming). Such classes could fit into a schema or hierarchy of classes, with subclasses of a class able to inherit the descriptions of the entire class. The schema of classes proposal would save having to repeat descriptions since a single reference to the class of which a particular RDF description was a part would suffice. The scheme or description of the collection of classes could itself be written in RDF language.
Resource Reservation Protocol
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See ‘‘RSVP’’
S response time According to the IBM Dictionary of Computing (which cites International Organization for Standardization Information Technology Vocabulary as the source), response time is:
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The elapsed time between the end of an inquiry or demand on a computer system and the beginning of a response; for example, the length of the time between an indication of the end of an inquiry and the display of the first character of the response at a user terminal.
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Since user input and system response can now be in terms other than text, the examples in the ISO definition could be expanded. There is also the concept of perceived response time, which is the time a user senses as the beginning of input and the end of the response.
return on investment For a given use of money in an enterprise, the ROI (return on investment) is how much ‘‘return,’’ usually profit or cost saving, results. An ROI calculation is sometimes used along with other approaches to develop a business case for a given proposal. The overall ROI for an enterprise is sometimes used as a way to grade how well a company is managed.
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Latency is any characteristic of a computer system that increases response time beyond the time desired for an operation.
If an enterprise has immediate objectives of getting market revenue share, building infrastructure, positioning itself for sale, or other objectives, a return on investment might be measured in terms of meeting one or more of these objectives rather than in immediate profit or cost saving.
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return merchandise authorization
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An RMA (return merchandise authorization) is a numbered authorization provided by a mail-order merchant to permit the return of a product. Most mail-order businesses have a policy concerning returns. Some companies allow only defective products to be returned; others allow any software to be returned if it is unopened. To return a defective product, a typical process is:
See also bipolar signaling, unipolar signaling, and nonreturn-to-zero.
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First, the customer must call the technical support office and speak with a technician. The technician helps to determine whether the product is indeed defective. This usually involves the customer answering several questions and following the technician’s directions for testing the product in question. If the technician feels that the product is defective, the technician issues an RMA. This process is especially important concerning software. Most merchants do not allow the return of software unless the packaging is unopened. This policy prevents software piracy. Some merchants allow the customer to send an e-mail message requesting an RMA instead of having to call. Secondly, the customer must write the RMA number on the outside of the box that the product is being shipped in. It is important to mail all the original boxes, manuals, and any other items along with the product. If a return is sent without an RMA number, the merchant can return the product to the customer or charge a restocking fee. Finally, the customer mails the product. Most merchants recommend using a shipping company that can track packages. RMA numbers are important to both the merchant and customer. An RMA number tells the merchant that a return is being made and offers protection against fraudulent returns. The customer can use the RMA number to inquire on the progress of a return. For example, if the customer hasn’t received any information about the return, the customer can call the merchant and use the RMA number as a reference.
Return-to-zero (RZ) refers to a form of digital data transmission in which the binary low and high states, represented by numerals 0 and 1, are transmitted by voltage pulses having certain characteristics. The signal state is determined by the voltage during the first half of each data binary digit. The signal returns to a resting state (called zero) during the second half of each bit. The resting state is usually zero volts, although it does not have to be. In positive-logic RZ, the low state is represented by the more negative or less positive voltage, and the high state is represented by the less negative or more positive voltage. Examples are: Logic 0 = 0 volts for 1 bit Logic 1 = +5 volts for 1/2 bit, then 0 volts for 1/2 bit Logic 0 = -4 volts for 1/2 bit, then 0 volts for 1/2 bit Logic 1 = 0 volts for 1 bit In negative-logic RZ, the low state is represented by the more positive or less negative voltage, and the high state is represented by the less positive or more negative voltage. Examples are: Logic 0 = +5 volts for 1/2 bit, then 0 volts for 1/2 bit Logic 1 = 0 volts for 1 bit Logic 0 = 0 volts for 1 bit Logic 1 = -4 volts for 1/2 bit, then 0 volts for 1/2 bit
reverb Reverb (short for reverberation) is the acoustic environment that surrounds a sound. Natural reverb exists everywhere. Whether the space being described is a bathroom or a gymnasium, the essential characteristics remain the same. Reverb is composed of a series of tightly-spaced echoes. The number of echoes and the way that they decay play a major role in shaping the sound that you hear. Many other factors influence the sound of a reverberant space. These include the dimensions of the actual space (length, width, and height),
reverse Telnet the construction of the space (such as whether the walls are hard or soft and whether the floor is carpetted), and diffusion (what the sound bounces off of). In addition to natural reverb, software synthesis of reverberation is also possible. Many audio cards, synthesizers, dedicated effects processors, and digital audio applications can create reverb, simulating both natural and supernatural environments. For example, one could create the reverb for a room fifty feet long, five feet wide, with a four-foot ceiling, lined with carpet. The synthesis of reverb by a digital signal processing algorithm usually attempts to mimic the way a real acoustic space works. The algorithm designers simulate the early reflections, the compounding of echoes, and the decay of high versus low frequencies when designing their product. Of course, the more processing power and speed available, the more complex and potentially realistic a reverb signal can be created.
Reverse Address Resolution Protocol RARP (Reverse Address Resolution Protocol) is a protocol by which a physical machine in a local area network can request to learn its IP address from a gateway server’s Address Resolution Protocol (ARP) table or cache. A network administrator creates a table in a local area network’s gateway router that maps the physical machine (or Media Access Control—MAC address) addresses to corresponding Internet Protocol addresses. When a new machine is set up, its RARP client program requests from the RARP server on the router to be sent its IP address. Assuming that an entry has been set up in the router table, the RARP server will return the IP address to the machine which can store it for future use. RARP is available for Ethernet, Fiber Distributed-Data Interface, and token ring LANs.
reverse engineering Reverse engineering is taking apart an object to see how it works in order to duplicate or enhance the object. It’s a practice taken from older industries that is now frequently used on computer hardware and software. In the automobile industry, for example, a manufacturer may purchase a competitor’s vehicle, disassemble it, and examine the welds, seals, and other components of the vehicle for the purpose of enhancing their vehicles with similar components. Software reverse engineering involves reversing a program’s machine code (the string of 0s and 1s that are sent to the logic processor) back into the source code that it was written in, using program language statements. Software reverse engineering is done to retrieve the source code of a program because the source code was lost, to study how the program performs certain operations, to improve the performance of a program, to fix a bug (correct an error in the program when the source code is not available), to identify malicious
content in a program such as a virus, or to adapt a program written for use with one microprocessor for use with a differently-designed microprocessor. Reverse engineering for the sole purpose of copying or duplicating programs constitutes a copyright violation and is illegal. In some cases, the licensed use of software specifically prohibits reverse engineering. Someone doing reverse engineering on software may use several tools to disassemble a program. One tool is a hexadecimal dumper, which prints or displays the binary numbers of a program in hexadecimal format (which is easier to read than a binary format). By knowing the bit patterns that represent the processor instructions as well as the instruction lengths, the reverse engineer can identify certain portions of a program to see how they work. Another common tool is the disassembler. The disassembler reads the binary code and then displays each executable instruction in text form. A disassembler cannot tell the difference between an executable instruction and the data used by the program so a debugger is used, which allows the disassembler to avoid disassembling the data portions of a program. These tools might be used by a cracker to modify code and gain entry to a computer system or cause other harm. Hardware reverse engineering involves taking apart a device to see how it works. For example, if a processor manufacturer wants to see how a competitor’s processor works, they can purchase a competitor’s processor, disassemble it, and then make a processor similar to it. However, this process is illegal in many countries. In general, hardware reverse engineering requires a great deal of expertise and is quite expensive.
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Another type of reverse engineering involves producing 3-D images of manufactured parts when a blueprint is not available in order to remanufacture the part. To reverse engineer a part, the part is measured by a coordinate measuring machine (CMM). As it is measured, a 3-D wire frame image is generated and displayed on a monitor. After the measuring is complete, the wire frame image is dimensioned. Any part can be reverse engineered using these methods.
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The term forward engineering is sometimes used in contrast to reverse engineering.
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reverse Telnet Reverse Telnet (sometimes called direct Telnet) is the initiation of a Telnet session from a computer system to one of its remote users. Usually, a Telnet session is initiated by a user who wishes to access and share resources on a remote computer system. The user must have permission and is prompted to provide a user name and password. Reverse Telnet is when the host computer initiates a Telnet session instead of accepting one. Reverse Telnet is typically used by a system administrator to configure or to troubleshoot a remote computer.
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REXX
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REXX is an interpreted script language developed by IBM originally for use by personal users of large operating systems. It was designed for ease of learning and use and to make programming accessible to non-programmers. It offers powerful character-manipulation, automatic data typing, manipulation of words, numbers, and names, and debugging capabilities.
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REXX can be compared with these interpreted script languages: Visual Basic Netscape’s JavaScript Sun Microsystem Tool Command Language Larry Wall’s Perl In general, script languages are easier and faster to code in than the more structured, compiled languages such as C and C++ and are ideal for programs of very limited capability or that can reuse and tie together existing compiled programs.
RF See ‘‘radio frequency’’
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RFC See ‘‘Request for Comments’’
RFM See ‘‘RFM analysis’’
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RFM (recency, frequency, monetary) analysis is a marketing technique used to determine quantitatively which customers are the best ones by examining how recently a customer has purchased (recency), how often they purchase (frequency), and how much the customer spends (monetary). RFM analysis is based on the marketing axiom that ‘‘80% of your business comes from 20% of your customers.’’
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For more than 30 years, direct mailing marketers for nonprofit organizations have used an informal RFM analysis to target their mailings to customers most likely to make donations. The reasoning behind RFM was simple: People who donated once were more likely to donate again. With the advent of e-mail marketing campaigns and customer relationship management software, RFM ratings have become an important tool. Using RFM analysis, customers are assigned a ranking number of 1,2,3,4, or 5 (with 5 being highest) for each RFM parameter. The three scores together are referred to as an RFM ‘‘cell.’’ The database is sorted to determine which customers were ‘‘the best customers’’ in the past, with a cell ranking of ‘‘555’’ being ideal.
Although RFM analysis is a useful tool, it does have its limitations. A company must be careful not to oversolicit customers with the highest rankings. Experts also caution marketers to remember that customers with low cell rankings should not be neglected, but instead should be cultivated to become better customers.
RGB Also see palette. RGB (red, green, and blue) refers to a system for representing the colors to be used on a computer display. Red, green, and blue can be combined in various proportions to obtain any color in the visible spectrum. Levels of R, G, and B can each range from 0 to 100 percent of full intensity. Each level is represented by the range of decimal numbers from 0 to 255 (256 levels for each color), equivalent to the range of binary numbers from 00000000 to 11111111, or hexadecimal 00 to FF. The total number of available colors is 256 6 256 6 256, or 16,777,216 possible colors. In the Hypertext Markup Language (HTML), the color for a page background or text font is specified by an RGB value, expressed with six digits in hexadecimal format. The first and second digits represent the red level; the third and fourth digits represent the green level; the fifth and sixth digits represent the blue level. In order to actually display the colors for all possible values, the computer display system must have 24 bits to describe the color in each pixel. In display systems or modes that have fewer bits for displaying colors, an approximation of the specified color will be displayed. In creating Web pages, the number of RGB values that are recommended for use is considerably reduced—first, by the fact that many displays can handle only 256 colors and, secondly, because PC and Mac Web browsers handle 40 of these 256 colors slightly differently. In order to ensure that your colors will be consistent on both browsers, a palette of the 216 colors common to both PC and Web browsers is recommended. Any color outside of these will be dithered (approximated).
rich media Rich media is an Internet advertising term for a Web page ad that uses advanced technology such as streaming video, downloaded applets (programs) that interact instantly with the user, and ads that change when the user’s mouse passes over it. For example:
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An ad for a Hollywood movie that inclues a streaming video sample of a scene from the movie
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A mouse cursor that is changed to an image on a particular Web site if the user requests it (for example, a cursor that changes to a tiny red question mark on a site like whatis.com)
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A standard-size banner ad that includes an inquiry form about ISDN installation, capturing the user’s filled-in personal information, and telling the user they will be contacted by a company representative—all simply by interacting with an ad on an online publisher’s Web page
Advertiser servers of rich media ads that use Java applets or components may also serve regular GIF images to people whose browsers don’t support Java well.
Rich Text Format RTF (Rich Text Format) is a file format that lets you exchange text file between different word processors in different operating systems. For example, you can create a file using Microsoft Word 97 in Windows 95, save it as an RTF file (it will have a ‘‘.rtf’’ file name suffix), and send it to someone who uses WordPerfect 6.0 on Windows 3.1 and they will be able to open the file and read it. (In some cases, the RTF capability may be built into the word processor. In others, a separate reader or writer may be required.) The RTF Specification uses the ANSI, PC-8, Macintosh, and IBM PC character sets. It defines control words and symbols that serve as ‘‘common denominator’’ formatting commands. When saving a file in the Rich Text Format, the file is processed by an RTF writer which converts the word processor’s markup to the RTF language. When being read, the control words and symbols are processed by an RTF reader that converts the RTF language into formatting for the word processor that will display the document. The Specification, a copy of which is located in the archives at the World Wide Web Consortium, is used to create an RTF reader or writer.
Rijndael Rijndael (pronounced rain-dahl) is the algorithm that has been selected by the U.S. National Institute of Standards and Technology (NIST) as the new Advanced Encryption Standard (AES). It was selected from a list of five finalists, that were themselves selected from an original list of more than 15 submissions. Rijndael will begin to supplant the Data Encryption Standard (DES)—and later Triple DES—over the next few years in many cryptography applications. The algorithm was designed by two Belgian cryptologists, Vincent Rijmen and Joan Daemen, whose surnames are reflected in the cipher’s name. Rijndael has its origins in Square, an earlier collaboration between the two cryptologists.
The Rijndael algorithm is a new generation symmetric block cipher that supports key sizes of 128, 192 and 256 bits, with data handled in 128-bit blocks—however, in excess of AES design criteria, the block sizes can mirror those of the keys. Rijndael uses a variable number of rounds, depending on key/block sizes, as follows:
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9 rounds if the key/block size is 128 bits 11 rounds if the key/block size is 192 bits
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13 rounds if the key/block size is 256 bits Rijndael is a substitution linear transformation cipher, not requiring a Feistel network. It use triple discreet invertible uniform transformations (layers). Specifically, these are: Linear Mix Transform; Non-linear Transform and Key Addition Transform. Even before the first round, a simple key addition layer is performed, which adds to security. Thereafter, there are Nr-1 rounds and then the final round. The transformations form a State when started but before completion of the entire process. The State can be thought of as an array, structured with 4 rows and the column number being the block length divided by bit length (for example, divided by 32). The cipher key similarly is an array with 4 rows, but the key length divided by 32 to give the number of columns. The blocks can be interpreted as unidimensional arrays of 4-byte vectors. The exact transformations occur as follows: The byte subtransformation is nonlinear and operates on each of the State bytes independently—the invertible S-box (substitution table) is made up of 2 transformations. The shiftrow transformation sees the State shifted over variable offsets. The shift offset values are dependent on the block length of the State. The mixcolumn transformation sees the State columns take on polynomial characteristics over a Galois Field values (28), multiplied x4 + 1 (modulo) with a fixed polynomial. Finally, the roundkey transform is XORed to the State. The key schedule helps the cipher key determine the round keys through key expansion and round selection. Overall, the structure of Rijndael displays a high degree of modular design, which should make modification to counter any attack developed in the future much simpler than with past algorithm designs.
RIMM In a computer, a RIMM is a memory module developed by Kingston Technology Corp. that takes up less space inside the computer than the older DIMM module and has different pin characteristics. A RIMM has a 184-pin connector and an SO-RIMM module has a 160-pin connector. An SO-RIMM is smaller and is used in systems that require smaller form factors. While RIMM is commonly believed to stand for ‘‘Rambus inline memory module,’’ Kingston Technology has trademarked ‘‘RIMM’’ and uses only that term.
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A RIMM module consists of RDRAM chips that are attached using a thin layer of solder, a metal alloy that, when melted, fuses metals to each other. Solder balls on each chip create a metal pathway used to conduct electricity.
ring Also see Web ring. A ring is a network topology or circuit arrangement in which each device is attached along the same signal path to two other devices, forming a path in the shape of a ring. Each device in the ring has a unique address. Information flow is unidirectional and a controlling device intercepts and manages the flow to and from the ring. The token ring is the most prevalent form of ring network.
The RISC concept has led to a more thoughtful design of the microprocessor. Among design considerations are how well an instruction can be mapped to the clock speed of the microprocessor (ideally, an instruction can be performed in one clock cycle); how ‘‘simple’’ an architecture is required; and how much work can be done by the microchip itself without resorting to software help. Besides performance improvement, some advantages of RISC and related design improvements are: L
A new microprocessor can be developed and tested more quickly if one of its aims is to be less complicated.
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Operating system and application programmers who use the microprocessor’s instructions will find it easier to develop code with a smaller instruction set.
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The simplicity of RISC allows more freedom to choose how to use the space on a microprocessor.
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Higher-level language compilers produce more efficient code than formerly because they have always tended to use the smaller set of instructions to be found in a RISC computer.
RIP See ‘‘raster image processor’’
ripper In digital audio technology, a ripper is a program that moves a sound sequence from a compact disk onto a computer hard drive as a Wave file, often as a step toward converting it to an MP3-encoded file. The program that converts the WAV file to an MP3 file is known as an encoder. The program that plays the MP3 file is called a player. Some audio products provide all three programs together as a package.
RIS See ‘‘Remote Installation Service’’
RISC A RISC (reduced instruction set computer) is a microprocessor that is designed to perform a smaller number of types of computer instruction so that it can operate at a higher speed (perform more million instructions per second, or millions of instructions per second). Since each instruction type that a computer must perform requires additional transistors and circuitry, a larger list or set of computer instructions tends to make the microprocessor more complicated and slower in operation. John Cocke of IBM Research in Yorktown, New York, originated the RISC concept in 1974 by proving that about 20% of the instructions in a computer did 80% of the work. The first computer to benefit from this discovery was IBM’s PC/XT in 1980. Later, IBM’s RISC System/6000, made use of the idea. The term itself (RISC) is credited to David Patterson, a teacher at the University of California in Berkeley. The concept was used in Sun Microsystems’ SPARC microprocessors and led to the founding of what is now MIPS Technologies, part of Silicon Graphics. DEC’s Alpha microchip also uses RISC technology.
Rivest-Shamir-Adleman RSA is an Internet encryption and authentication system that uses an algorithm developed in 1977 by Ron Rivest, Adi Shamir, and Leonard Adleman. The RSA algorithm is the most commonly used encryption and authentication algorithm and is included as part of the Web browser from Netscape and Microsoft. It’s also part of Lotus Notes, Intuit’s Quicken, and many other products. The encryption system is owned by RSA Security. The company licenses the algorithm technologies and also sells development kits. The technologies are part of existing or proposed Web, Internet, and computing standards.
How the RSA System Works The mathematical details of the algorithm used in obtaining the public and private keys are available at the RSA Web site. Briefly, the algorithm involves multiplying two large prime numbers (a prime number is a number divisible only by that number and 1) and through additional operations deriving a set of two numbers that constitutes the public key and another set that is the private key. Once the keys have been developed, the original prime numbers are no longer important and can be discarded. Both the public and the private keys are needed for encryption/decryption but only the owner of a private key ever needs to know it. Using the RSA system, the private key never needs to be sent across the Internet. The private key is used to decrypt text that has been encrypted with the public key. Thus, if I send you a message, I can find out your public key (but not your private key) from a central administrator and encrypt a message to you using your public key. When you receive it, you decrypt it with your private key. In addition to encrypting messages
roaming service (which ensures privacy), you can authenticate yourself to me (so I know that it is really you who sent the message) by using your private key to encrypt a digital certificate. When I receive it, I can use your public key to decrypt it. A table might help us remember this. To do this
Use whose
Kind of key
Send an encrypted message Send an encrypted signature Decrypt an encrypted message Decrypt an encrypted signature (and authenticate the sender)
Use the receiver’s
Public key
Use the sender’s
Private key
Use the receiver’s
Private key
Use the sender’s
Public key
RJ-11 – RJ-14 – RJ-45 – RJ-XX See ‘‘telephone jacks’’
rlogin Rlogin (remote login) is a UNIX command that allows an authorized user to login to other UNIX machines (hosts) on a network and to interact as if the user were physically at the host computer. Once logged in to the host, the user can do anything that the host has given permission for, such as read, edit, or delete files. Rlogin is similar to the better known Telnet command. Rlogin is considered useful for simple logins that don’t require a lot of control over the client/host interaction, but is thought to be less useful than Telnet where a lot of customization is desired, for multiple sessions, for connections between very distant terminals or to terminals that are not running UNIX, for that matter, since rlogin can only connect to UNIX hosts. A benefit of rlogin is the ability to use a file called .rhosts that resides on the host machine and maintains a list of terminals allowed to login without a password. A secure version of rlogin (slogin) was combined with two other UNIX utility, ssh and scp, in the Secure Shell suite, an interface and protocol created to replace the earlier utilities.
RMI See ‘‘Remote Method Invocation’’
RMON RMON (Remote Network Monitoring) provides standard information that a network administrator can use to monitor, analyze, and troubleshoot a group of distributed local area networks (LANs) and interconnecting T-1/E-1 and T-2/E-3 lines from a central site. RMON specifically defines the information that any network monitoring system will be able to provide. It’s specified as part of the Management Information Base (MIB) in Request for Comments 1757 as an extension of the Simple Network Management Protocol (SNMP). The latest level is RMON Version 2 (sometimes referred to as ‘‘RMON 2’’ or ‘‘RMON2’’). RMON can be supported by hardware monitoring devices (known as ‘‘probes’’) or through software or some combination. For example, Cisco’s line of LAN switches includes software in each switch that can trap information as traffic flows through and record it in its MIB. A software agent can gather the information for presentation to the network administrator with a graphical user interface. A number of vendors provide products with various kinds of RMON support. RMON collects nine kinds of information, including packets sent, bytes sent, packets dropped, statistics by host, by conversations between two sets of addresses, and certain kinds of events that have occurred. A network administrator can find out how much bandwidth or traffic each user is imposing on the network and what Web sites are being accessed. Alarms can be set in order to be aware of impending problems.
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See ‘‘root-mean-square’’
N RNIS RNIS (Reseau Numerique a Integration de Services) is the European name for Integrated Services Digital Network (ISDN).
roaming service Roaming service is the ability to get access to the Internet when away from home at the price of a local call or at a charge considerably less than the regular long-distance charges. For example, if you normally get access to the Internet from an access provider in Brooklyn, New York and are travelling to Hong Kong, you can call a designated access provider in Hong Kong. Instead of paying long distance charges to your local provider in Brooklyn, you pay the local phone connection charge in Hong Kong and possibly a modest additional charge for the service.
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Roaming service is made possible through Internet service providers (ISPs) who have cooperative agreements to grant each others’ customers local access to the Internet. Special software allows cooperating ISPs to keep track of and calculate prearranged payments for usage differences. Here’s how it works for the user:
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2. Assuming the ISP does, the user can determine a cooperating ISP in a city to which the user is travelling.
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3. In the travel location, the user can call the local ISP’s designated phone number through the computer modem, entering information during login that will identify the user’s home ISP. 4. The ‘‘foreign’’ ISP will contact the ISP and determine that the user is a valid user. 5. The ‘‘foreign’’ ISP will grant the user access to the Internet. The user will be able to access e-mail from the home mail server. 6. The user will be charged at local phone rates. In addition, depending on the particular service arrangement, the home ISP may levy an additional hourly usage charge of several dollars an hour or a monthly charge in case the service is used during that month. A similar roaming service is provided by some cooperating cellular telephone telephone or personal digital assistant (PDA) service providers. If you are travelling and simply need to be able to exchange e-mail, you can consider getting a freemail membership (usually free) from HotMail, Rocketmail, or other freemail providers. Hotmail also offers POP3 server accounts for access to up to four e-mail accounts you may already have, assuming you remember the POP3 server name and your user IDs and passwords. If you subscribe to a somewhat global service such as AT&T’s WorldNet or the IBM Global Network, you may already be able to access your account in certain cities through your provider’s local point-of-presence (POP) on the Internet without having to pay for a long-distance call.
roaming See ‘‘roaming service’’
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robot A robot is a machine designed to execute one or more tasks repeatedly, with speed and precision. There are as many different types of robots as there are tasks for them to perform. A robot can be controlled by a human operator, sometimes from a great distance. But most robots are controlled by computer, and fall into either of two categories: Autonomous robots and insect robots. An autonomous
robot acts as a stand-alone system, complete with its own computer (called the controller). Insect robots work in fleets ranging in number from a few to thousands, with all fleet members under the supervision of a single controller. The term insect arises from the similarity of the system to a colony of insects, where the individuals are simple but the fleet as a whole can be sophisticated. Robots are sometimes grouped according to the time frame in which they were first widely used. First-generation robots date from the 1970s and consist of stationary, nonprogrammable, electromechanical devices without sensors. Second-generation robots were developed in the 1980s and can contain sensors and programmable controllers. Third-generation robots were developed between approximately 1990 and the present. These machines can be stationary or mobile, autonomous or insect type, with sophisticated programming, speech recognition and/or synthesis, and other advanced features. Fourthgeneration robots are in the research-and-development phase, and include features such as artificial intelligence, self-replication, self assembly, and nanoscale size (physical dimensions on the order of nanometers, or units of 10-9 meter). Some advanced robots are called androids because of their superficial resemblance to human beings. Androids are mobile, usually moving around on wheels or a track drive (robots legs are unstable and difficult to engineer). The android is not necessarily the end point of robot evolution. Some of the most esoteric and powerful robots do not look or behave anything like humans. The ultimate in robotic intelligence and sophistication might take on forms yet to be imagined. Also see artificial intelligence, nanotechnology, and robotics.
robotics Robotics is a branch of engineering that involves the conception, design, manufacture, and operation of robots. This field overlaps with electronics, computer science, artificial intelligence, mechatronics, nanotechnology, and bioengineering. Science-fiction author Isaac Asimov is often given credit for being the first person to use the term robotics in a short story composed in the 1940s. In the story, Asimov suggested three principles to guide the behavior of robots and smart machines. Asimov’s Three Laws of Robotics, as they are called, have survived to the present: 1. Robots must never harm human beings. 2. Robots must follow instructions from humans without violating rule 1. 3. Robots must protect themselves without violating the other rules.
rollover
robust
ROI
Robust (pronounced RO-buhst) is an adjective commonly applied in marketing literature to information technology products in several ways. It derives from the Latin robustus, meaning ‘‘strength.’’
See ‘‘return on investment’’
1) A robust product can be one that doesn’t break easily. Thus, an operating system in which any individual application can fail without disturbing the operating system or other applications can be said to be robust. 2) Robust is also sometimes used to mean a product or system of products designed with a full complement of capabilities. Thus, in the context of the business world, early UNIX systems were not considered as robust as IBM’s mainframe operating systems, such as MVS, which were designed for continuous operation with a very low failure rate and features such as automatic backup of file systems.
Roentgen Roentgen is IBM’s development name for a liquid crystal display (LCD) that has 2560 6 2048 pixels and is described as providing an image ‘‘as precise as the original paper document.’’ Roentgen can provide an image resolution of up to 200 pixels per inch. The Roentgen display is a form of active matrix LCD (AMLCD). Aluminum and copper are used, instead of the traditional molybdenum and tungsten. Aluminum and copper are better conductors, and make possible a higher resolution at moderate cost. Display sizes vary from approximately 16 inches to 22 inches diagonal measure. The images are more than four times as sharp as those in a highend high-definition television (HDTV) display. AMLCD displays such as Roentgen are used mainly in highend graphics applications. However, this type of display also has advantages in text applications. Because it renders text more cleanly than displays having lower resolution, Roentgen allows some people to read online text faster and with less fatigue than is possible with older technologies. In addition, the expanded effective screen area makes it possible to render a large amount of information on a single screen. A typical Roentgen display contains more than 8600 feet of thin-film wiring. The backlight consumes 44 watts of power, about the same as a conventional cathode-ray tube (CRT). The entire unit weighs about 20 pounds and is housed in a box measuring less than 10 inches deep. The development name apparently is derived from Wilhelm Ro¨ntgen, the physicist who discovered X-rays.
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ROLAP See ‘‘relational online analytical processing’’
role-playing game RPG also stands for Report Program Generator. A role-playing game (RPG) is a game in which each participant assumes the role of a character (such as an ogre or a futuristic spaceship captain) that can interact within the game’s imaginary world. More popular games include Dungeons and Dragons, one of the early online games, Battletech, and Star Wars. The online environments known as MUD and MOO include software for playing and developing role-playing games. A different but somewhat related idea is the ‘‘world’’ in which a participant defines an appearance or avatar and interacts with others in the chat world using that avatar.
rollback A rollback is the undoing of partly completed database changes when a database transaction is determined to have failed.
rollout A rollout is a staged series of activities that often accumulate meaning as they occur. Computer product makers and marketers use the term to describe a series of related product announcements that are staged over time. Public relations campaigns use the term to describe the revelation of a major company theme, event, or other message over a period of time calculated to lead to ideal results. When a company installs new equipment, the installation staging is sometimes called a rollout.
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rollover In creating page for a Web site, a rollover (some people call it a ‘‘mouseover’’) is a technique using JavaScript that lets you change a page element (usuallly a graphic image) when the user rolls the mouse over something on the page (like a line of text or a graphic image). The term rollover recognizes that there is a little ball in your mouse that you roll on a surface.
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Inviting user interaction by causing text elements in a menu to ‘‘pop up’’ or otherwise become lively as you roll the mouse over each line. This is done by creating a small graphic image for each text line and a second graphic image for each line that is substituted when a given text line is rolled over with a mouse. The effect invites users to click on one of the rolled over lines. Causing a previously invisible graphic image or other element to suddenly appear when the user rolls the mouse over a particular page element. Raymond Pirouz’s home page site for his book, click here, uses this technique. Try it. Have one element on a page change color as you roll the mouse over other elements. It’s a way to reinforce the user’s recognition that certain page elements are ‘‘alive’’ and clickable.
rootkit A rootkit is a collection of tools (programs) that a hacker uses to mask intrusion and obtain administrator-level access to a computer or computer network. The intruder installs a rootkit on a computer after first obtaining user-level access, either by exploiting a known vulnerability or cracking a password. The rootkit then collects userids and passwords to other machines on the network, thus giving the hacker root or privileged access. A rootkit may consist of utilities that also: Monitor traffic and keystrokes; create a ‘‘backdoor’’ into the system for the hacker’s use; alter log files; attack other machines on the network; and alter existing system tools to circumvent detection.
See ‘‘read-only memory’’
The presence of a rootkit on a network was first documented in the early 90s. At that time Sun and Linux operating systems were the primary targets for a hacker looking to install a rootkit. Today, rootkits are available for a number of operating systems and are increasingly difficult to detect on any network.
root directory
root-mean-square
In a computer file system that is organized as a hierarchy or tree, the root directory is the directory that includes all other directories. (Unlike a real tree, a tree file system has only one root!) In UNIX-based as well as in other operating system, the root directory has no name. It is simply represented by the special character that separates directories in a file system .
Also see peak and peak-to-peak.
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In UNIX-based systems, the root directory is represented simply as:
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To determine rms value, three mathematical operations are carried out on the function representing the AC waveform:
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In Windows systems, the root directory is represented as:
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Only a few special users of a shared operating system will be given the authority to access all file directories and files under the root directory.
root server system On the Internet, the root server system is the way that an authoritative master list of all top-level domain name (such as com, net, org, and individual country codes) is maintained and made available. The system consists of 13 file servers. The central or ‘‘A’’ server is operated by Network Solutions, Inc., the company that currently manages domain name registration, and the master list of top-level domain (TLD) names is kept on the A server. On a daily basis, this list is replicated to 12 other geographically dispersed file servers that are maintained by an assortment of agencies. The Internet routing system uses the nearest root server list to update routing tables.
In a direct current (DC) circuit, voltage or current is simple to define, but in an alternating current (AC) circuit, the definition is more complicated, and can be done in several ways. Root-mean-square (rms) refers to the most common mathematical method of defining the effective voltage or current of an AC wave.
(1) The square of the waveform function (usually a sine wave) is determined. (2) The function resulting from step (1) is averaged over time. (3) The square root of the function resulting from step (2) is found. In a circuit whose impedance consists of a pure resistance, the rms value of an AC wave is often called the effective value or DC-equivalent value. For example, if an AC source of 100 volts rms is connected across a resistor, and the resulting current causes 50 watts of heat to be dissipated by the resistor, then 50 watts of heat will also be dissipated if a 100volt DC source is connected to the resistor. For a sine wave, the rms value is 0.707 times the peak value, or 0.354 times the peak-to-peak value. Household utility voltages are expressed in rms terms. A so-called ‘‘117-volt’’ AC circuit carries about 165 volts peak (pk), or 330 volts peak-to-peak (pk-pk).
round robin
RosettaNet RosettaNet is an organization set up by leading information technology companies to define and implement a common set of standards for e-business. RosettaNet is defining a common parts dictionary so that different companies can define the same product the same way. It is also defining up to 100 e-business transaction processes and standardizing them. Because RosettaNet is supported by all or most of the major companies in the IT industry, its standards are expected to be widely adopted. RosettaNet has developed a structured four-part approach for creating what it calls Partner Interface Processes (PIPs). L
Business Process Modeling examines common business procedures and defines the components of the processes.
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Business Process Analysis analyzes the processes and defines a target list of desireable changes to the processes.
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PIP Development establishes guidelines and documentation for the changes. Dictionaries consist of two data dictionary: a technical properties dictionary and a business properties dictionary. Along with the RosettaNet Implementation Framework (which defines an exchange protocol for PIP implementation), the dictionaries form the basis for PIP development.
RosettaNet’s more than 40 members include Microsoft, Netscape, 3Com, Toshiba America, Compaq, CompUSA, Hewlett-Packard, IBM, and Intel. Its name refers to the Rosetta Stone, a stone on which Egyptian hieroglyphics were also written in other languages, making it possible to decipher the hieroglyphics. Rosetta stone has the more general meaning of ‘‘something that provides a key to understanding.’’ The organization’s slogan is ‘‘lingua franca for eBusiness.’’ (A lingua franca is a common second language, such as English for countries in the industrialized world whose first language is not English.)
ROT-13 ROT-13 is the encrypting of a message by exchanging each of the letters on the first half of the alphabet with the corresponding letter in the second half of the alphabet (that is, swapping positions by 13 characters). Thus, A becomes N, B becomes O, and so forth, and conversely, N becomes A, O becomes B, and so forth. Numbers, spaces and punctuation are not changed. ROT-13 is sometimes used to encrypt messages that may be offensive or of questionable taste, or messages that contain spoilers (like movie endings or punch lines). The purpose of the code is not to guarantee security, but simply to make it difficult for anyone to read. ROT-13 is sometimes used on the Internet to encrypt e-mail addresses to discourage spamming. Some browsers or Word editors offer users the ability to convert back and forth
between regular text and ROT-13. Typically, you highlight text you want to encrypt or decrypt and then select the ROT13 encode/decode button. Here’s an example of some regular text: This is a sample of a message encoded using ROT-13 encoding. Because of the simple nature of the encryption, its purpose is not security but to prevent accidental reading.
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that would look like this in ROT-13: Guvf vf n fnzcyr bs n zrffntr rapbqrq hfvat EBG-13 rapbqvat. Orpnhfr bs gur fvzcyr angher bs gur rapelcgvba, vgf checbfr vf abg frphevgl ohg gb cerirag nppvqragny ernqvat.
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ROT-13 is sometimes known as Caesar’s code because the Roman General is said to have used it during the Pelloponesian Wars.
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ROTFL See ‘‘chat abbreviations’’
rotoscoping For either broadcast video or Internet streaming video, rotoscoping is the rotated projection of a sequence of usually photographed action image frames so that the artist can trace from the frame or create an image to superimpose on it. It can be thought of as ‘‘painting on movies’’ efficiently. Prior to computers, an animation stand called a Rotoscope was used to project a sequence of action frames against a surface so that a set of animation frames could be traced or created. The same work can now be done with digital images and special computer software. Tools that provide efficient ways to rotoscope include Digital Magic and Elastic Reality. Rotoscoping is frequently used as a technique for combining (compositing) cartoon figures with realistic settings in television commercials and is also used for special effects in feature-length films. A rotoscoping texture (sometimes called a sequence map) is the use of video within an animation, something like an animation within an animation. For example, in a cartoon animation, the television set could show a program containing another animation. Or in a background to an animation in the foreground, you could include some clouds that slowly changed during the foreground animation. The frame rate for both the main animation and the ‘‘animation within the animation’’ must be the same.
round robin A round robin is an arrangement of choosing all elements in a group equally in some rational order, usually from the top to the bottom of a list and then starting again at the top of the list and so on. A simple way to think of round robin is that it is about ‘‘taking turns.’’ Used as an adjective, round robin becomes ‘‘round-robin.’’
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In computer operation, one method of having different program processes take turns using the resources of the computer is to limit each process to a certain short time period, then suspending that process to give another process a turn (or ‘‘time-slice’’). This is often described as roundrobin process scheduling. In sports tournaments and other games, round-robin scheduling arranges to have all teams or players take turns playing each other, with the winner emerging from the succession of events. A round-robin story is one that is started by one person and then continued sucessively by others in turn. Whether an author can get additional turns, how many lines each person can contribute, and how the story can be ended depend on the rules. Some Web sites have been created for the telling of round robin stories by each person posting the next part of the story as part of an online conference thread.
roundtripping In information processing, roundtripping is the conversion (or, in some usages, the repeated conversion back and forth) of a document in one format such as Microsoft Word to a document in another format such as WordPerfect or HTML and then back again. Since conversion from one format to another sometimes introduces compromises in the appearance or content of the original, conversion back and forth tends to compound the problem. The problem can become a concern for enterprises such as law offices that often exchange documents back and forth a number of times.
router Also see bridge, gateway, hub, and switch. On the Internet, a router is a device or, in some cases, software in a computer, that determines the next network point to which a packet should be forwarded toward its destination. The router is connected to at least two networks and decides which way to send each information packet based on its current understanding of the state of the networks it is connected to. A router is located at any gateway (where one network meets another), including each Internet point-of-presence. A router is often included as part of a network switch. A router may create or maintain a table of the available routes and their conditions and use this information along with distance and cost algorithms to determine the best route for a given packet. Typically, a packet may travel through a number of network points with routers before arriving at its destination. Routing is a function associated with the Network layer (layer 3) in the standard model of network programming, the Open Systems Interconnection (OSI) model. A layer-3 switch is a switch that can perform routing functions.
An edge router is a router that interfaces with an asynchronous transfer mode (ATM) network. A brouter is a network bridge combined with a router.
routine In computer programming, routine and subroutine are general and nearly synonymous terms for any sequence of code that is intended to be called and used repeatedly during the execution of a program. This makes the program shorter and easier to write (and also to read when necessary). The main sequence of logic in a program can branch off to a common routine when necessary. When finished, the routine branches back to the next sequential instruction following the instruction that branched to it. A routine may also be useful in more than one program and save other programmers from having to write code than can be shared. Typically, in assembler languages, a routine that requires some variable input can be encoded into a macro definition with a specified interface called a macro instruction. The programmer can then use a macro instruction instead of having to include and manage the branching to a routine. Macro definitions and instructions also tend to be shared among programmers for use in multiple programs, especially in software development projects. In higher-level computer languages, many commonlyneeded routines are prepackaged as functions, which are routines with specified programming interfaces. Some functions can be compiled in line with other code. Other functions are compiled in as stub that make dynamic calls for system services during program execution. Functions are sometimes called library routines. The compiler and a set of library routines usually come as part of a related software development package. In Windows and some other personal computer operating systems, the system routines for handling specific tasks such as interacting with specific I/O devices are called dynamic link library (DLL) routines. These routines are dynamic because their files are not actually loaded into memory until the first time they are called. A more recent term, procedure, is similar in meaning.
Routing Information Protocol Also see raster image processor (RIP). RIP (Routing Information Protocol) is a widely-used protocol for managing router information within a selfcontained network such as a corporate local area network () or an interconnected group of such LANs. RIP is classified by the Internet Engineering Task Force (IETF) as one of several internal gateway protocols (Interior Gateway Protocol).
RSVP Using RIP, a gateway host (with a router) sends its entire routing table (which lists all the other hosts it knows about) to its closest neighbor host every 30 seconds. The neighbor host in turn will pass the information on to its next neighbor and so on until all hosts within the network have the same knowledge of routing paths, a state known as network convergence. RIP uses a hop count as a way to determine network distance. (Other protocols use more sophisticated algorithms that include timing as well.) Each host with a router in the network uses the routing table information to determine the next host to route a packet to for a specified destination. RIP is considered an effective solution for small homogeneous networks. For larger, more complicated networks, RIP’s transmission of the entire routing table every 30 seconds may put a heavy amount of extra traffic in the network. The major alternative to RIP is the Open Shortest Path First Protocol (OSPF).
routing switch In a network, a routing switch is a device that combines the functions of a switch, which forwards data by looking at a physical device address, and a router, which forwards packets by locating a next hop address.
routing See ‘‘router’’
roving analysis port See ‘‘port mirroring’’
row address strobe See ‘‘RAS’’
RPC See ‘‘Remote Procedure Call’’
RPG See ‘‘Report Program Generator’’
RPR See ‘‘Resilient Packet Ring’’
RS-232C RS-232C is a long-established standard (’’C’’ is the current version) that describes the physical interface and protocol for relatively low-speed serial data communication between computers and related devices. It was defined by an industry trade group, the Electronic Industries Association (EIA), originally for teletypewriter devices. RS-232C is the interface that your computer uses to talk to and exchange data with your modem and other serial devices. Somewhere in your PC, typically on a Universal Asynchronous Receiver/Transmitter chip on your motherboard, the data from your computer is transmitted to an internal or external modem (or other serial device) from its Data Terminal Equipment (Data Terminal Equipment) interface. Since data in your computer flows along parallel circuits and serial devices can handle only one bit at a time, the UART chip converts the groups of bits in parallel to a serial stream of bits. As your PC’s DTE agent, it also communicates with the modem or other serial device, which, in accordance with the RS-232C standard, has a complementary interface called the Data Communications Equipment (DCE) interface.
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See ‘‘Rivest-Shamir-Adleman’’
K RSVP RSVP (Resource Reservation Protocol) is a set of communication rules that allows channels or paths on the Internet to be reserved for the multicast (one source to many receivers) transmission of video and other high-bandwidth messages. RSVP is part of the Internet Integrated Service (IIS) model, which ensures best-effort service, real-time service, and controlled link-sharing. The basic routing philosophy on the Internet is ‘‘best effort,’’ which serves most users well enough but isn’t adequate for the continuous stream transmission required for video and audio programs over the Internet. With RSVP, people who want to receive a particular Internet ‘‘program’’ (think of a television program broadcast over the Internet) can reserve bandwidth through the Internet in advance of the program and be able to receive it at a higher data rate and in a more dependable data flow than usual. When the program starts, it will be multicast to those specific users who have reserved routing priority in advance. RSVP also supports unicast (one source to one destination) and multi-source to one destination transmissions.
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How It Works Let’s assume that a particular video program is to be multicast at a certain time on Monday evening. Expecting to receive it, you send an RSVP request before the broadcast (you’ll need a special client program or perhaps your browser includes one) to allocate sufficient bandwidth and priority of packet scheduling for the program. This request will go to your nearest Internet gateway with an RSVP server. It will determine whether you are eligible to have such a reservation set up and, if so, whether sufficient bandwidth remains to be reserved to you without affecting earlier reservations. Assuming you can make the reservation and it is entered, the gateway then forwards your reservation to the next gateway toward the destination (or source of multicast). In this manner, your reservation is ensured all the way to the destination. (If the reservation can’t be made all the way to the destination, all reservations are removed.)
conventional to use, and as convenient as Perl, but fully object-oriented, which leads to better structured and easierto-maintain programs. To be compliant with the principles of extreme programming (XP), Ruby allows portions of projects to be written in other languages if they are better suited. Ruby has become extremely popular in Japan; it is sometimes said that, at the moment, although there are a huge number of Ruby programmers, most of them don’t speak English. That situation is expected to change, however: Hunt and Thomas predict that Ruby will undergo explosive growth between 2001 and 2002, and overtake Python within four years.
rule base
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When the multicast begins, packets from the source speed through the Internet on a high-priority basis. As packets arrive at a gateway host, they are classified and scheduled out using a set of queues and, in some cases, timers. An RSVP packet is very flexible; it can vary in size and in the number of data types and objects. Where packets need to travel through gateways that don’t support RSVP, they can be ‘‘tunneled’’ through as ordinary packets. RSVP works with both Internet Protocol version 4 and IPv6.
In the context of a computer server acting as a firewall, a rule base is a set of rules that govern what is and what is not allowed through the firewall. A rule base can work in one of two ways: it can either explicitly assume that all traffic is allowed unless there is a rule to prevent it, or, more typically, it can assume that no traffic may flow through it unless there is an explicit rule to allow it. Rule bases usually work on a top-down principle in which the first rule in the list is acted upon first, so that traffic allowed by the first rule, will never be judged by the remainder of the rules. Rule bases typically have the format of SOURCE/DESTINATION/SERVICE/ACTION.
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run book
See ‘‘real-time clock’’
See ‘‘real-time operating system’’
In a computer system or network, a run book is a written set of procedures for the routine and exceptional operation of the system or network by an administrator or operator. Typically, a run book will contain procedures for starting, stopping, and monitoring the system or network; for handling special requests such as the mounting of a storage device containing archived material; and for handling problems that may arise.
Ruby
runbook
Ruby is an open source, interpreted, object-oriented programming language created by Yukihiro Matsumoto, who chose the gemstone’s name to suggest ‘‘a jewel of a language.’’ Ruby is designed to be simple, complete, extensible, and portable. Developed mostly on Linux, Ruby works across most platforms, such as most UNIX-based platforms, DOS, Windows, Macintosh, BeOS, and OS/2, for example. According to proponents, Ruby’s simple syntax (partially inspired by Ada and Eiffel), makes it readable by anyone who is familiar with any modern programming language.
See ‘‘run book’’
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Ruby is considered similar to Smalltalk and Perl. The authors of the book Programming Ruby: The Pragmatic Programmer’s Guide, David Thomas and Andrew Hunt say that it is fully object-oriented, like Smalltalk, although more
runt In networks, a runt is a packet that is too small. For example, the Ethernet protocol requires that each packet be at least 64 bytes long. In Ethernet, which operates on the idea that two parties can attempt to get use of the line at the same time and sometimes do, runts are usually the fragments of packet collisions. Runts can also be the result of bad wiring or electrical interference. Runts are recorded by programs that use the Remote Network Monitoring (RNM) standard information base for network adminstration. RMON calls them ‘‘undersize packets.’’ As you might expect, a giant is a packet that’s oversize.
runtime
runtime Runtime is when a program is running (or being executable). That is, when you start a program running in a computer, it is runtime for that program. In some programming languages, certain reusable programs or ‘‘routines’’ are built and packaged as a ‘‘runtime library.’’ These routines can be linked to and used by any program when it is running. Programmers sometimes distinguish between what gets embedded in a program when it is compiler and what gets embedded or used at runtime. The former is sometimes called ‘‘compile time.’’ For a number of years, technical writers resisted ‘‘runtime’’ as a term, insisting that something like ‘‘when a program is run’’ would obviate the need for a special term. Gradually, the term crept into general usage.
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S interface
S/MIME
In Integrated Services Digital Network (ISDN) service, an S interface is the electrical interface between a network terminating unit 1 (NT1) and up to eight addressable devices such as a computer or a telephone. Like the T interface (which usually connects the signal from the NT1 to a PBX), the S interface has four wires, allowing up to eight devices to be addressed. In Basic Rate Interface ISDN, the bits that flow from the central office through the NT1 are divided into two 64 Kbps channels (known as B, or bearer, channels) and one 16 Kbps channel (the D, for data or delta, channel) for control signals. The control signals allow a specific device to be addressed. The wires to the devices from the NT-1 are sometimes referred to as the S-bus.
S/MIME (Secure Multi-Purpose Internet Mail Extensions) is a secure method of sending e-mail that uses the RivestShamir-Adleman (RSA) encryption system. S/MIME is included in the latest versions of the Web browsers from Microsoft and Netscape and has also been endorsed by other vendors that make messaging products. RSA has proposed S/MIME as a standard to the Internet Engineering Task Force (IETF). An alternative to S/MIME is PGP/MIME, which has also been proposed as a standard.
The following table summarizes the various ISDN electrical interfaces at different demarcation points or places in the traffic flow: Electrical interface
Between what two points
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Central office and NT1 NT1 and NT2 devices (such as a PBX) NT1 or NT2 and ISDN devices (such as a telephone or terminal adapter) Terminal adapter and nonISDN devices (such as a computer) Within the ISDN node at the central office; separates line termination equipment from exchange termination equipment
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MIME itself, described in the IETF standard called Request for Comments 1521, spells out how an electronic message will be organized. S/MIME describes how encryption information and a digital certificate can be included as part of the message body. S/MIME follows the syntax provided in the Public-Key Cryptography Standard format #7.
S/N See ‘‘signal-to-noise ratio’’
S/PDIF S/PDIF (Sony/Philips Digital Interface) is a standard audio transfer file format. It is usually found on digital audio equipment such as a DAT machine or audio processing device. It allows the transfer of audio from one file to another without the conversion to and from an analog format, which could degrade the signal quality. The most common connector used with an S/PDIF interface is the RCA connector, the same one used for consumer audio products. An optical connector is also sometimes used.
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S/390 S/390, recently renamed zSeries 900, is IBM’s large server (or mainframe) line of computer systems, which are marketed to the world’s Fortune 1000 and many mid-size companies as a ‘‘tried-and-true’’ system with a history that stretches back to the System/360 of the early 1960s. The S/ 390 combines a hardware design based on the use of multiple microprocessor packaging with the OS/390 operating system, whose multitasking architecture and long evolution (most of the bugs have already been found) results in a system that seldom crashes. IBM’s VM operating system also runs on the S/390.
Systems Application Architecture (SAA) was IBM’s strategy for enterprise computing in the late 1980s and early 1990s. It was replaced and considerably expanded by IBM’s Open Blueprint, an enterprise strategy for network computing. Both of these are structured views of the types of computing services that an enterprise may need, the relationships between these services, and specification of the standards and products that are envisioned as providing these services. SAA defined three layers of service: L
Common User Access
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Common Programming Interface
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sales automation software By using or creating applications and data that conformed to these three layers (provided by strategic IBM products), an enterprise could be assured of consistency and some amount of program portability among IBM’s range of computer systems. As IBM, along with the rest of the computer industry, discovered the value of industry-wide standardization and open computing, the company revised its strategy. Its new strategy, the Open Blueprint, would recognize that customers wanted consistency and portability of programs and data across different manufacturers’ systems, not just within IBM’s systems. Two other trends not current when SAA was developed have influenced IBM’s Open Blueprint: the client/server model of distributing computing and the concept and implementation of objectoriented programming. The Open Blueprint seems well positioned to absorb another new trend: Network computing, including use of the Internet. Since most of IBM’s large customers have a large investment in legacy applications and data developed during the era of SAA and prior, IBM’s Open Blueprint recognizes the need to support them. Notably, IBM’s Systems Network Architecture (SNA), the keystone of the SAA Common Communications Support, is retained in the Open Blueprint as an alternative to TCP/IP. Other companies have also made investments in SAA, recognizing that IBM customers’ legacy applications will not be going away soon and that in some cases these applications may represent the current best solution. Novell’s Netware for SAA is an example of a widely-installed product that retains the SAA identity.
SACD See ‘‘Super Audio CD’’
Safe Harbor Safe Harbor is the name of a policy agreement established between the United States Department of Commerce and the European Union (E.U.) in November 2000 to regulate the way that U.S. companies export and handle the personal data (such as names and addresses) of European citizens. The agreement is a policy compromise set up in response to a European directive that differed from traditional business procedures for U.S. companies dealing with the E.U. In 1998, the E.U. established the European Commission Directive on Data Protection, which prohibited data transfer to nonEuropean countries that did not adhere to stringent criteria. In effect, because the guidelines were very strict, they made it illegal to transfer most citizens’ personal data outside of Europe. Safe Harbor stipulations require that: Companies collecting personal data must inform people that the data is being gathered, and tell them what will be done with it; they must obtain permission to pass on the information to a third party; they must allow people access to the data gathered; data integrity and security must be assured; and a means of enforcing compliance must be guaranteed.
The agreement establishes a framework for a compromise solution between U.S. and E.U. privacy procedures. All 15 member countries are subject to the agreement, which means that data transfers can proceed without requiring individual authorization. U.S. companies that don’t join Safe Harbor must obtain authorization separately from each European country. E.U. organizations can check a list of U.S. companies that have joined the collective to ensure that the Safe Harbor Privacy Principles will be adhered to.
safe mode Safe mode is an alternate boot method for Windows 95/98 that makes it easier to diagnose problems. The only startup programs loaded are the operating system and drivers for the mouse, keyboard, and display. To find out how to boot Windows 95/98 in safe mode, click Start, then Help, and then type in ‘‘safe mode’’. The procedure varies slightly depending on whether your computer contains a network interface card or not. If a normal startup fails (for example, because the Registry is missing an important key parameter such as SYSTEM), Windows 95/98 will initiate a safe mode boot for you.
sales automation software Sales automation software is a type of program that automates business tasks such as inventory control, sales processing, and tracking of customer interactions, as well as analyzing sales forecasts and performance. Businesses may have a custom version developed specifically for their needs, or choose from among the increasing number of sales automation software products, such as Interact Commerce’s ACT! and GoldMine Software’s GoldMine. Sales automation software is sometimes called sales force automation (SFA) software, and sometimes called customer relations management (CRM) software. Sales automation packages typically include a Web-ready database, an e-mail package, and customizable templates. A three-tiered architecture is typically used to separate the database, server, and application to reduce programming demands on clients. A module-based design is generally used, to allow users to customize the package to suit their needs. In August 2000, Oracle released a free CRM software package, OracleSalesOnline.com which makes information—such as contacts, schedules, and performance tracking—available online through the included database program. The package is designed for medium-to-large enterprises with mobile work forces. All data and storage are based at an Oracle facility, similar to the application service provider (ASP) model, which means that data can be accessed from any Internet connection and that the client doesn’t need special hardware or software. The Oracle package also includes online staff training.
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See ‘‘sales automation software’’
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Samba Samba is a popular freeware program that allows end users to access and use files, printers, and other commonly shared resources on a company’s intranet or on the Internet. Samba is often referred to as a network file system and can be installed on a variety of operating system platforms, including: Linux, most common UNIX platforms, OpenVMS, and OS/2. Samba is based on the common client/server protocol of Server Message Block (SMBP) and Common Internet File System (CIFS). Using client software that also supports SMB/CIFS (for example, most Microsoft Windows products), an end user sends a series of client requests to the Samba server on another computer in order to open that computer’s files, access a shared printer, or access other resources. The Samba server on the other computer responds to each client request, either granting or denying access to its shared files and resources. The Samba SMB/CIFS client is called smbclient.
J sample rate
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In developing an audio sound for computers or telecommunication, the sample rate is the number of samples of a sound that are taken per second to represent the event digitally.
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The more samples taken per second, the more accurate the digital representation of the sound can be. For example, the current sample rate for CD-quality audio is 44,100 samples per second. This sample rate can accurately reproduce the audio frequencies up to 20,500 hertz, covering the full range of human hearing.
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sandbox Using the Java programming language and development environment, the sandbox is the program area and set of rules that programmers need to use when creating Java code (called an applet) that is sent as part of a page. Since a Java applet is sent automatically as part of the page and can be executed as soon as it arrives, the applet can easily do harm, either accidentally or as the result of someone with malicious intent, if it is allowed unlimited access to memory and operating system services. The sandbox restrictions provide strict limitations on what system resources the applet can request or access. Essentially, the programmer must write code that ‘‘plays’’ only within the sandbox, much as children are allowed to make anything they want to within the
confined limits of a real sandbox. The sandbox can be conceived as a small area within your computer where an applet’s code can play freely—but it’s not allowed to play anywhere else. The sandbox is implemented not only by requiring programmers to conform to certain rules but also by providing code checkers. The Java language itself provides features such as automatic memory management, garbage collection, and the checking of address ranges in strings and arrays that inherently help to guarantee safe code. In addition, Java includes a compiled code (Java’s compiled code is known as bytecode) verifier that guarantees adherence to certain limitations. Java also provides for a local name space within which code may be restricted. The Java virtual machine (the layer that interprets the Java bytecode for a given computer platform) also mediates access to system resources and ensures that sandbox code is restricted. In the original sandbox security model, the sandbox code is generally known as untrusted code. In later versions of the Java Development Kit (JDK)—the programmer’s development environment—the sandbox has been made more sophisticated by introducing several levels of trust that the user can specify for sandbox code. The more trust the user allows, the more capability the code has to ‘‘play’’ outside of the sandbox. In the Java Development Kit 1.1 version, the concept of a signed applet was introduced. An applet accompanied by a digital signature can contain trusted code that will be allowed to execute if the signature is recognized by the client browser. In JDK 2.0, Java provides for assigning different levels of trust to all application code, whether loaded locally or arriving from the Internet. A mechanism exists to define a security policy that will be used to screen all code—whether signed or not—as it executes.
SAP SAP, started in 1972 by five former IBM employees in Mannheim, Germany, states that it is the world’s largest inter-enterprise software company and the world’s fourthlargest independent software supplier overall. The original SAP idea was to provide customers with the ability to interact with a common corporate database for a comprehensive range of applications. Gradually, the applications have been assembled and today many corporations, including IBM and Microsoft, are using SAP products to run their own businesses. SAP applications, built around their latest R/3 system, provide the capability to manage financial, asset, and cost accounting, production operations and materials, personnel, plants, and archived documents. The R/3 system runs on a number of platforms including Windows 2000 and uses the client/server model. The latest version of R/3 includes a comprehensive Internet-enabled package.
satellite Internet connection SAP has recently recast its product offerings under a comprehensive Web interface, called mySAP.com, and added new e-business applications, including customer relationship management (CRM) and supply chain management (SCM). In early 2001, SAP, a publicly traded company, had 21,500 employees in over 50 countries, and more than 30,000 installations. SAP is turning its attention to small- andmedium sized businesses. A recent R/3 version was provided for IBM’s AS/400 platform.
SASID See ‘‘Self-scanned Amorphous Silicon Integrated Display’’
satellite A satellite is a specialized wireless receiver/transmitter that is launched by a rocket and placed in orbit around the earth. There are hundreds of satellites currently in operation. They are used for such diverse purposes as weather forecasting, television broadcast, amateur radio communications, Internet communications, and the Global Positioning System, (GPS). The first artificial satellite, launched by Russia (then known as the Soviet Union) in the late 1950s, was about the size of a basketball. It did nothing but transmit a simple Morse code signal over and over. In contrast, modern satellites can receive and re-transmit thousands of signals simultaneously, from simple digital data to the most complex television programming. There are three types of communications satellite systems. They are categorized according to the type of orbit they follow. A geostationary satellite orbits the earth directly over the equator, approximately 22,000 miles up. At this altitude, one complete trip around the earth (relative to the sun) takes 24 hours. Thus, the satellite remains over the same spot on the earth’s surface at all times, and stays fixed in the sky from any point on the surface from which it can be ‘‘seen.’’ Socalled weather satellites are usually of this type. You can view images from some of these satellites on the Internet via the Purdue Weather Processor. A single geostationary satellite can ‘‘see’’ approximately 40 percent of the earth’s surface. Three such satellites, spaced at equal intervals (120 angular degrees apart), can provide coverage of the entire civilized world. A geostationary satellite can be accessed using a dish antenna aimed at the spot in the sky where the satellite hovers. A low-earth-orbit (LEO) satellite system employs a large fleet of ‘‘birds,’’ each in a circular orbit at a constant altitude of a few hundred miles. The orbits take the satellites over, or nearly over, the geographic poles. Each revolution takes approximately 90 minutes to a few hours. The fleet is arranged in such a way that, from any point on the surface at any time, at least one satellite is on a line of sight. The entire
system operates in a manner similar to the way a cellular telephone functions. The main difference is that the transponders, or wireless receiver/transmitters, are moving rather than fixed, and are in space rather than on the earth. A well-designed LEO system makes it possible for anyone to access the Internet via wireless from any point on the planet, using an antenna no more sophisticated than old-fashioned television ‘‘rabbit ears.’’ Some satellites revolve around the earth in elliptical orbits. These satellites move rapidly when they are near perigee, or their lowest altitude; they move slowly when they are near apogee, or their highest altitude. Such ‘‘birds’’ are used by amateur radio operators, and by some commercial and government services. They require directional antennas whose orientation must be constantly adjusted to follow the satellite’s path across the sky.
satellite Internet connection A satellite Internet connection is an arrangement in which the upstream (outgoing) and the downstream (incoming) data are sent from, and arrive at, a computer through a satellite. Each subscriber’s hardware includes a satellite dish antenna and a transceiver (transmitter/receiver) that operates in the microwave portion of the radio spectrum. In a two-way satellite Internet connection, the upstream data is usually sent at a slower speed than the downstream data arrives. Thus, the connection is asymmetric. A dish antenna, measuring about two feet high by three feet wide by three feet deep, transmits and receives signals. Uplink speeds are nominally 50 to 150 Kbps for a subscriber using a single computer. The downlink occurs at speeds ranging from about 150 Kbps to more than 1200 Kbps, depending on factors such as Internet traffic, the capacity of the server, and the sizes of downloaded files. Satellite Internet systems are an excellent, although rather pricey, option for people in rural areas where Digital Subscriber Line (DSL) and cable modem connections are not available. A satellite installation can be used even where the most basic utilities are lacking, if there is a generator or battery power supply that can produce enough electricity to run a desktop computer system. The two-way satellite Internet option offers an always-on connection that bypasses the dial-up process. In this respect, the satellite system resembles a cable modem Internet connection. But this asset can also be a liability, unless a firewall is used to protect the computer against hack attempts. The nature of the satellite connection is good for Web browsing and for downloading of files. Because of long latency compared with purely land-based systems, interactive applications such as online gaming are not compatible with satellite networks. In a two-way geostationary-satellite Internet connection, a transaction requires two round trips between the earth’s surface and transponders orbiting 22,300 miles above the equator. This occurs in addition to land-based data transfer between the
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Whatis?Com’s Encyclopedia of Technology Terms earthbound satellite system hub and the accessed Internet sites. The speed in such a connection is theoretically at least 0.48 second (the time it takes an electromagnetic signal to make two round trips at 186,000 miles per second to and from a geostationary satellite), and in practice is somewhat longer. Satellite systems are also prone to rain fade (degradation during heavy precipitation) and occasional brief periods of solar interference in mid-March and late September, when the sun lines up with the satellite for a few minutes each day. Rain fade and solar interference affect all satellite links from time to time, not just Internet systems. The author of this topic recently had StarBand, a two-way satellite Internet service, installed at his rural home office. Bandwidth tests were conducted with the new system compared with a conventional telephone modem. The telephone connection provided actual bandwidth ranging from 10 to 15 Kbps. StarBand worked at 200 to 1350 Kbps; throughput seemed to depend mainly on the download file size. The fastest speeds were obtained with files of 50 KB (kilobytes) or less, typical of images and text contained in Web sites. Surprisingly, fast downloads were obtained even during times of maximum Internet traffic. Compare satellite return Internet connection.
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satellite return Internet connection A satellite return Internet connection is an arrangement in which incoming data arrives at your computer from a satellite downlink and outgoing data (such as your request for the next Web page) is sent over a regular telephone line. Because most of the traffic on the Internet is toward the user (in the form of text, graphic images, streaming video, and so forth), the kind of high-speed channel that a satellite can provide (direct to the user using wireless radio) is desirable. On the other hand, the user’s outgoing requests, typically for a Web page, are relatively small bursts of text, easily and more economically handled by slower telephone transmission.
POTS. The telephone connection provided actual bandwidth ranging from 10 to 15 Kbps; the AOL Plus link worked at 250 to 770 Kbps.
SAX SAX (Simple API for XML) is an application program interface (API) that allows a programmer to interpret a Web file that uses the Extensible Markup Language (XML)—that is, a Web file that describes a collection of data. SAX is an alternative to using the Document Object Model (DOM) to interpret the XML file. As its name suggests, it’s a simpler interface than DOM and is appropriate where many or very large files are to be processed, but it contains fewer capabilities for manipulating the data content. SAX is an event-driven interface. The programmer specifies an event that may happen and, if it does, SAX gets control and handles the situation. SAX works directly with an XML parser.
SCADA SCADA (supervisory control and data acquisition) is a category of software application program for process control, the gathering of data in real time from remote locations in order to control equipment and conditions. SCADA is used in power plants as well as in oil and gas refining, telecommunications, transportation, and water and waste control. SCADA systems include hardware and software components. The hardware gathers and feeds data into a computer that has SCADA software installed. The computer then processes this data and presents it in a timely manner. SCADA also records and logs all events into a file stored on a hard disk or sends them to a printer. SCADA warns when conditions become hazardous by sounding alarms.
scalability
In a typical satellite return connection, the outgoing data speed for ‘‘plain old telephone service’’ (POTS) is typically 28.8 or 57.6 Kbps (depending on the user’s modem), but in practice is often slower. Incoming data from the satellite to the user’s system arrives at a much higher speed. Thus, the satellite return connection is asymmetric. A medium-sized, elongated dish antenna, measuring about two feet high by three feet wide by three feet deep, receives the signals. When the Internet is heavily used, the downlink occurs at 200 to 300 Kbps. During periods of light usage, the downlink speed can be 400 to 500 Kbps, and sometimes exceeds 700 Kbps.
In information technology, scalability (frequently spelled scaleability) seems to have two usages:
The author of this topic recently had AOL (America Online) Plus, a satellite return Internet service offered jointly by AOL and DirectPC, installed at his rural home office. Bandwidth tests were conducted with the new system compared with
An example: John Young in his book Exploring IBM’s NewAge Mainframes describes the RS/6000 SP operating system as one that delivers scalability (’’the ability to retain performance levels when adding additional processors’’).
1) It is the ability of a computer application or product (hardware or software) to continue to function well as it (or its context) is changed in size or volume in order to meet a user need. Typically, the rescaling is to a larger size or volume. The rescaling can be of the product itself (for example, a line of computer systems of different sizes in terms of storage, RAM, and so forth) or in the scalable object’s movement to a new context (for example, a new operating system).
scalar Another example: In printing, scalable fonts that can be resized smaller or larger using software without losing quality. 2) It is the ability not only to function well in the rescaled situation, but to actually take full advantage of it. For example, an application program would be scalable if it could be moved from a smaller to a larger operating system and take full advantage of the larger operating system in terms of performance (user response time and so forth) and the larger number of users that could be handled. It is usually easier to have scalability upward rather than downward since developers often must make full use of a system’s resources (for example, the amount of disk storage available) when an application is initially coded. Scaling a product downward may mean having to achieve the same results in a more constrained environment.
scalable font A scalable font is a type font that can be resized (enlarged or reduced) without introducing distortion. The outline of each character is stored as a mathematical formula. The set of all such formulas for a complete set of characters is called an outline font. The outline font remains essentially the same regardless of the size of the characters. This eliminates the necessity for storing a gigantic set of characters to encompass all the possible sizes for a particular font. Once the size of the characters has been selected (for example, 12-point), the outlines are filled in with a bitmap for each character. Usually (but not always), the same bitmap is used for all the characters in the font. The bitmap can itself have mathematical formulas incorporated into it. For example, the characters might be italicized and shaded solid blue; rendered as normal and solid black; or a special typeface might be used with shading from blue at the top to red at the bottom.
Scalable Processor Architecture See ‘‘SPARC’’
Scalable Vector Graphics (SVG) is the description of an image as an application of the Extensible Markup Language (XML). Any program such as a Web browser that recognizes XML can display the image using the information provided in the SVG format. Vector graphics is the expression of an image using mathematical statements rather than bit-pattern description. Scalable emphasizes that vector graphic images can easily be made scalable (whereas an image specified in raster graphics is a fixed-size bitmap). Thus, the SVG format enables the viewing of an image on a computer display of any size and resolution, whether a tiny LCD screen in a cell phone or a large CRT display in a workstation. In addition to ease of size reduction and enlargement, SVG allows text within images to be recognized as such, so that the text can be located by a search engine and easily translated into other languages. Vector graphics images also have the potential advantage over the standard Web image formats, the GIF and the JPEG, of size. Compared with a bitmap image, an SVG image may be much smaller and arrive more quickly. GIF and JPG images (and a newer format, the PNG) are expected to continue to predominate. While the bitmaps of these image formats can be resized, dimensional reduction does not necessarily save memory, storage, or bandwidth, and significant enlargement produces irregular edges (’’the jaggies’’). It is expected, however, that bitmaps are, and will likely continue to be, favored for digital transmission of photographs, especially scenes containing complex objects not readily translatable into the formulas used by vector graphics programs.
1) In mathematics, scalar (noun) and scalar (adjective) refer to a quantity consisting of a single real number used to measured magnitude (size). Voltage, mass, and temperature measurements can be described as scalar quantities.
xyz
Some outline-font characters are shown in the first line of the illustration. The same characters, filled in with various bitmaps, are shown in the second line. They have been scaled to different sizes. The mathematical formulas for the bitmaps, as well as for the outline fonts, are pre-determined. So, for example, if the numerals 1 2 3 at lower right were enlarged or reduced, the outline-font formulas could simply be multiplied by a constant. In all other respects, the appearance of the characters would remain the same.
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2) In Perl programming, a scalar is a variable used to represent a block of information. A scalar can refer to an integer, a float, a string, an object, binary information, or just about anything else. It’s a versatile variable that has no length limit and is easily recognizable when looking at Perl code because it is preceded by the symbol ‘‘$’’.
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ScanDisk
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ScanDisk is a Windows utility used to check your hard disk for errors and to correct problems that are found. These errors often occur when Windows locks up and must be restarted.
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To run ScanDisk in Windows 95 and later versions, click the Start button, select Programs, then Accessories, then System Tools, and then Scandisk. It is recommended that you run ScanDisk at least once per month. In OSR2 (a later release of Windows 95) and in Windows 98 and later versions, ScanDisk is initiated for you automatically following system restarts in which shutdown was not complete.
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A scanner captures images from photographic prints, posters, magazine pages, and similar sources for computer editing and display. Scanners come in hand-held, feed-in, and flatbed types and for scanning black-and-white only or color. Very high resolution scanners are used for scanning for high-resolution printing, but lower resolution scanners are adequate for capturing images for computer display. Scanners usually come with software, such as Adobe’s Photoshop product, that lets you resize and otherwise modify a captured image.
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Scanners usually attach to your personal computer with a Small Computer System Interface (SCSI). An application such as PhotoShop uses the TWAIN program to read in the image.
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Some major manufacturers of scanners include: Epson, Hewlett-Packard, Microtek, and Relisys.
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The Scarlet Book is Philips and Sony’s 1999 specification document for Super Audio Compact Disc (SACD), a highresolution audio format that features complex six-channel sound. SACD discs can contain three different versions of the same material. SACD uses Direct Stream Digital (DSD) recording, a proprietary Sony technology that converts an analog waveform to a 1-bit signal for direct recording, instead of the pulse code modulation (PCM) and filtering used by standard CDs. DSD uses lossless compression (so-called because none of the data is lost in the compression process) and a sampling rate of 2.8 MHz to improve the complexity and realism of sound. DSD enables a frequency response of 100 kHz and a dynamic range of 120 dB (the ratio of the softest to the loudest sound—120 db is also the approximate dynamic range of human hearing) on all channels. Scarlet Book specifications include three separate options for disk format: single-layer DSD, dual-layer DSD, or dual-layer hybrid, which includes a Red Book layer that can be played on any existing CD player in addition to the high-density layer that has the capacity to deliver eight channels of DSD.
In addition to DSD and the hybrid disk technology, Scarlet Book specifications include: Super Bit Mapping Direct, a proprietary downconversion method that enables improved audio when the disks are played on an ordinary CD player; Direct Stream Transfer, a type of coding that increases data capacity; and a digital watermark to protect against piracy. According to some, SACD is a hybrid CD/DVD format, since Scarlet Book specifications are identical to those for DVD disks for the file system, sector size, error correction, and modultation. SACD is in competition with a similar product, DVD-Audio, as the format that will replace standard audio CD.
SCART connector A SCART connector is a physical and electrical interconnection between two pieces of audio-visual equipment, such as a television set and a video cassette recorder (VCR). Each device has a female 21-pin connector interface. A cable with a male plug at each end is used to connect the devices. (The 21st pin is actually not a pin but a ground shield contact.) Stereo audio, composite video, and control signals are provided in both directions. RGB video signals are input only. The interface is an industry standard originated by the French company, Peritel. Today, most new television sets and VCRs in the European market and other countries that use the PAL video standard come equipped with a SCART connector. SCART stands for Syndicat francais des Constructeurs d’Appareils Radio et Television. The connector is also known as a Euro-connector. Although it is possible to build your own connecting cable, knowledgable people recommend buying a commercial cable because of the detailed pin-crossovers and close pin connections. Differerent cables serve different purposes. A cable for connecting a VCR or a TV set to a high-fidelity audio system needs to use only three of the 21 pins. It is more difficult to use the SCART connector to interconnect a VCR to a computer monitor because of the difficulty of synchronizing the composite video signal (which is companded into red, green, and blue) from the VCR with the RGB signals needed by the monitor. A user reports that the torsional load of the cable can in time lead to difficulty getting all the connections to align properly.
Schrodinger’s cat Schrodinger’s cat is a famous illustration of the principle in quantum theory of superposition, proposed by Erwin Schrodinger in 1935. Schrodinger’s cat serves to demonstrate the apparent conflict between what quantum theory tells us is true about the nature and behavior of matter on the microscopic level and what we observe to be true about the nature and behavior of matter on the macroscopic level.
scientific notation First, we have a living cat and place it in a thick lead box. At this stage, there is no question that the cat is alive. We then throw in a vial of cyanide and seal the box. We do not know if the cat is alive or if it has broken the cyanide capsule and died. Since we do not know, the cat is both dead and alive, according to quantum law, in a superposition of states. It is only when we break open the box and learn the condition of the cat that the superposition is lost, and the cat becomes one or the other (dead or alive). We know that superposition actually occurs at the subatomic level, because there are observable effects of interference, in which a single particle is demonstrated to be in multiple locations simultaneously. What that fact implies about the nature of reality on the observable level (cats, for example, as opposed to electrons) is one of the stickiest areas of quantum physics. Schrodinger himself said, later in life, that he wished he had never met that cat.
significant figures. Note that an expression can have various degrees of precision; the greater the number of significant figures, the greater the precision. Number in decimal form
Examples in scientific notation
1,222,000.00
1.222 6 106 1.22200000 6 106 0.1222 6 107 0.122200000 6 107 3.45 6 10-5 3.450000 6 10-5 0.345 6 10-4 0.3450000 6 10-4 79.87654 6 109 (approximately) 79.876543210 6 109 (exactly) 70.987654 6 1010 (approximately) 70.9876543210 6 1010 (exactly) 710-8 71.00 6 10-8 70.1 6 10-7 70.100 6 10-7
0.00003450000
79,876,543,210
70.0000000100
scientific notation Scientific notation, also called power-of-10 notation, is a method of writing extremely large and small numbers. There are two forms of this scheme; one is by far more common. In common scientific notation, any nonzero quantity can be expressed in two parts: a coefficient whose absolute value is greater than or equal to 1 but less than 10, and a power of 10 by which the coefficient is multiplied. In some writings, the coefficients are closer to zero by one order of magnitude. In this scheme, any nonzero quantity is expressed in two parts: a coefficient whose absolute value is greater than or equal to 0.1 but less than 1, and a power of 10 by which the coefficient is multiplied. The quantity zero is denoted as 0 unless precision is demanded, in which case the requisite number of signficant digits are written out—for example, 0.00000. For numbers of reasonable magnitude, conventional decimal notation is often used, even in scientific writings. Let s be a number rounded or truncated to a few significant figures. If the absolute value of s is at least 0.001 (10-3) but less than 10,000 (104), then s is usually written out in full. Examples are 21.3389 and -0.002355. However, if the absolute value of s is smaller than 0.001 or if it is 10,000 or larger, scientific notation is usually preferred, because writing such numbers out in decimal form can be confusing and messy. This is especially true when the absolute value of s is very close to zero or is exceedingly large. It is inconvenient, for example, to write out either of the expressions 6.0205 6 1074 or 70.64453 6 10-45 in decimal form. The table shows several examples of numbers written in standard decimal notation (left-hand column) and in scientific notation (right-hand column). For negative numbers, the values are simply mirror-image positive numbers; a minus sign is placed in front of the values. The number of digits in the coefficient is the number of
Scientific notation makes it easy to multiply and divide gigantic and/or minuscule numbers, when the use of decimal notation would give rise to frustration. Consider, for example, the following product:
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2.56 x 1067 6 78.33 6 10-54
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To obtain the product of these two numbers, the coefficients are multiplied, and the powers of 10 are added. This produces the following result:
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2.56 6 (78.33) 6 1067+(-54) = 2.56 6 (78.33) 6 1067-54 = 721.3248 6 1013
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The proper form of common scientific notation requires that the absolute value of the coefficient be larger than 1 and less than 10. Thus, the coefficient in the above expression should be divided by 10 and the power of 10 increased by one, giving:
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72.13248 6 1014 Because both multiplicands in the original product are specified to only three significant figures, a scientist might see fit to round off the final expression to three significant figures as well, yielding:
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72.13 6 10
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as the product. Now consider the quotient of the two numbers multiplied in the previous example: 67
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(2.56 6 10 ) / (78.33 6 10 ) To obtain the quotient, the coefficients are divided, and the powers of 10 are subtracted. This gives the following:
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(2.56 / (78.33)) 6 1067-(-54) = (2.56 / (78.33)) 6 1067+54 = 70.30732 6 10121 The proper form of common scientific notation requires that the absolute value of the coefficient be larger than 1 and less than 10. Thus, the coefficient in the above expression should be multiplied by 10 and the power of 10 decreased by one, giving:
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See ‘‘supply chain management’’
screen
screen shot
In a computer display, the screen is the physical surface on which visual information is presented. This surface is usually made of glass. The screen size is measured from one corner to the opposite corner diagonally. Common screen sizes for desktop display screens are 12, 14, 17, 19, and 21 inches.
A screen shot (sometimes called a screen capture) is a copy of the screen’s contents that can be saved as a graphics file or copied into a document or graphics editor. Various programs are available for creating screen shots, but it is easy to do without any special program.
Because both numbers in the original quotient are specified to only three significant figures, a scientist might see fit to round off the final expression to three significant figures as well, yielding: 73.07 6 10120 as the quotient.
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Also see order of magnitude and significant figures.
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screen scraping Screen scraping is programming that translates between legacy application programs (written to communicate with now generally obsolete input/output devices and user interfaces) and new user interfaces so that the logic and data associated with the legacy programs can continue to be used. Screen scraping is sometimes called advanced terminal emulation. A program that does screen scraping must take the data coming from the legacy program that is formatted for the screen of an older type of terminal such as an IBM 3270 display or a Digital Equipment Corporation VT100 and reformat it for a Windows 2000 user or someone using a Web browser. The program must also reformat user input from the newer user interfaces (such as a Windows graphical user interface or a Web browser) so that the request can be handled by the legacy application as if it came from the user of the older device and user interface.
73.0732 6 10120
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the provided screen savers or one you have added to the Windows screen saver directory. The Mac also lets you set up a screen saver.
This term is not to be confused with panel, which is a representation of what information will appear on the screen in given circumstances.
screen capture See ‘‘screen shot’’
screen saver A screen saver is an animated image that is activated on a personal computer display when no user activity has been sensed for a certain time. The original purpose of a screen saver was to prevent burn-in (the burning of an image into the phosphor inside the cathode ray tube after hours of the same image being rescanned). In fact, today’s CRT display technology makes burn-in unlikely except under extreme conditions. (On larger displays used for room presentations, burn-in is still a possibility.) Windows comes with built-in screen savers that you can select. If you purchase your computer with the operating system already installed, a screen saver may have been selected for you. You can set the screen saver using the task bar by selecting Start, then Settings, then Control Panel, then Display, and then Screen Saver, which lets you select one of
To take a screen shot on a Windows platform, you press the Print Screen button (usually on the top row of the keyboard) to get a shot of the entire screen. To get a shot of the active window only, you press the Alt key and the Print Screen key at the same time. The screen shot is copied to the clipboard (the memory area that temporarily stores information to be copied to another location ) and can then be pasted into a document in a word processing program such as Word or WordPerfect, or into an image editor, such as Adobe Photo Shop, Paint Shop Pro, or Microsoft Paint if you want to alter the image. On a Macintosh platform, you press the Command, Shift, and 3 keys to create an image file (called a PICT file). If you press 4 instead of 3, the cursor changes to allow you to select an area of the screen to save. Command, Shift, and 4 with the Caps Lock pressed allows you to create a circular image. Any of the commands with the Control key pressed copies the image to the clipboard. On a UNIX, there are different methods for the various interfaces. A common one is the command xwd -out filename.xwd, which allows you to click a screen to make an image file.
SCSI
script 1) In computer programming, a script is a program or sequence of instructions that is interpreted or carried out by another program rather than by the computer processor (as a compiler program is). Some languages have been conceived expressly as script languages. Among the most popular are Perl, Rexx (on IBM mainframes), JavaScript, and Tcl/Tk. In the context of the World Wide Web, Perl, VBScript, and similar script languages are often written to handle forms input or other services for a Web site and are processed on the Web server. A JavaScript script in a Web page runs ‘‘client-side’’ on the Web browser. In general, script languages are easier and faster to code in than the more structured and compiled languages such as C and C++ and are ideal for programs of very limited capability or that can reuse and tie together existing compiled programs. However, a script takes longer to run than a compiled program since each instruction is being handled by another program first (requiring additional instructions) rather than directly by the basic instruction processor. 2) A script is sometimes used to mean a list of operating system commands that are prestored in a file and performed sequentially by the operating system’s command interpreter whenever the list name is entered as a single command. 3) Multimedia development programs use ‘‘script’’ to mean the sequence of instructions that you enter to indicate how a multimedia sequence of files will be presented (the sequence of images and sounds, their timing, and the possible results of user interaction).
script kiddie See ‘‘script kiddy’’
script kiddy Script kiddy (sometimes spelled kiddie) is a derogative term, originated by the more sophisticated crackers of computer security systems, for the more immature, but unfortunately often just as dangerous exploiter of security lapses on the Internet. The typical script kiddy uses existing and frequently well-known and easy-to-find techniques and programs or scripts to search for and exploit weaknesses in other computers on the Internet—often randomly and with little regard or perhaps even understanding of the potentially harmful consequences. Hackers view script kiddies with alarm and contempt since they do nothing to advance the ‘‘art’’ of hacking while sometimes unleashing the wrath of authority on the entire hacker community. While a hacker will take pride in the quality of an attack—leaving no trace of an intrusion, for example—a script kiddy may aim at quantity, seeing the number of attacks that can be mounted as a way to obtain attention and
notoriety. Script kiddies are sometimes portrayed in media as bored, lonely teenagers seeking recognition from their peers.
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SCSI SCSI (pronounced SKUH-zee and sometimes colloquially known as ‘‘scuzzy’’), the Small Computer System Interface, is a set of evolving ANSI standard electronic interfaces that allow personal computers to communicate with peripheral hardware such as disk drives, tape drives, CD-ROM drives, printers, and scanners faster and more flexibly than previous interfaces. Developed at Apple Computer and still used in the Macintosh, the present set of SCSIs are parallel interfaces. SCSI ports are built into most personal computers today and are supported by all major operating systems. In addition to faster data rates, SCSI is more flexible than earlier parallel data transfer interfaces. The latest SCSI standard, Ultra-2 SCSI for a 16-bit bus can transfer data at up to 80 megabytes per second (MBps). SCSI allows up to 7 or 15 devices (depending on the bus width) to be connected to a single SCSI port in daisy-chain fashion. This allows one circuit board or card to accommodate all the peripherals, rather than having a separate card for each device, making it an ideal interface for use with portable and notebook computers. A single host adapter, in the form of a PC Card, can serve as a SCSI interface for a ‘‘laptop,’’ freeing up the parallel and serial ports for use with an external modem and printer while allowing other devices to be used in addition. Although not all devices support all levels of SCSI, the evolving SCSI standards are generally backwardscompatible. That is, if you attach an older device to a newer computer with support for a later standard, the older device will work at the older and slower data rate. The original SCSI, now known as SCSI-1, evolved into SCSI2, known as ‘‘plain SCSI.’’ as it became widely supported. SCSI-3 consists of a set of primary commands and additional specialized command sets to meet the needs of specific device types. The collection of SCSI-3 command sets is used not only for the SCSI-3 parallel interface but for additional parallel and serial protocols, including Fibre Channels, Serial Bus Protocol (used with the IEEE 1394 Firewire physical protocol), and the Serial Storage Protocol (SSP). A widely implemented SCSI standard is Ultra-2 (sometimes spelled ‘‘Ultra2’’) which uses a 40 MHz clock rate to get maximum data transfer rates up to 80 MBps. It provides a longer possible cabling distance (up to 12 meters) by using Low Voltage Differential (LVD) signaling. Earlier forms of SCSIs use a single wire that ends in a terminator with a ground. Ultra-2 SCSI sends the signal over two wires with the data represented as the difference in voltage between the two wires. This allows support for longer cables. A low voltage differential reduces power requirements and manufacturing costs.
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The latest SCSI standard is Ultra-3 (sometimes spelled ‘‘Ultra3’’)which increases the maximum burst rate from 80 Mbps to 160 Mbps by being able to operate at the full clock rate rather than the half-clock rate of Ultra-2. The standard is also sometimes referred to as Ultra160/m. New disk drives supporting Ultra160/m will offer much faster data transfer rates. Ultra160/m also includes cyclical redundancy checking (CRC) for ensuring the integrity of transferred data and domain validation for testing the SCSI network. Currently existing SCSI standards table below. Technology Maximum Name Cable Length (meters) SCSI-1 SCSI-2 Fast SCSI-2 Wide SCSI-2 Fast Wide SCSI-2 Ultra SCSI-3, 8-bit Ultra SCSI-3, 16-bit Ultra-2 SCSI Wide Ultra-2 SCSI Ultra-3 (Ultra160/m) SCSI
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The SDII file has also become accepted among personal computer audio application developers. This makes transferring audio from Mac to PC platforms much easier. When used on a PC, the file must use the extension of ‘‘.sd2’’.
are summarized in the
SDH Maximum Speed (MBps)
Maximum Number of Devices
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5 5710 10720 20 20
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0
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SDH uses the following Synchronous Transport Modules (STM) and rates: STM-1 (155 megabits per second), STM-4 (622 Mbps), STM-16 (2.5 gigabits per second), and STM-64 (10 Gbps).
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SDH (Synchronous Digital Hierarchy) is a standard technology for synchronous data transmission on optical media. It is the international equivalent of Synchronous Optical Network. Both technologies provide faster and less expensive network interconnection than traditional PDH (Plesiochronous Digital Hierarchy) equipment. In digital telephone transmission, ‘‘synchronous’’ means the bits from one call are carried within one transmission frame. ‘‘Plesiochronous’’ means ‘‘almost (but not) synchronous,’’ or a call that must be extracted from more than one transmission frame.
See ‘‘software developer’s kit’’
SDLC scuzzy See ‘‘SCSI’’
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The SDII file has become a widely accepted standard for transferring audio files between editing applications. Most Mac CD-ROM writer software, for example, specifies SDII or Audio Interchange File Format as the file format needed when making audio CDs.
See ‘‘modem lights’’
SD2 SDII (Sound Designer II, sometimes seen abbreviated as SD2) is a monophonic/stereophonic audio file format, originally developed by Digidesign for their Macintoshbased recording/editing products. It is the successor to the original monophonic Sound Designer I audio file format. An SDII file can be monophonic or stereophonic. When stereo is used, the tracks are interleaved (sample-001-left, sample-001-right, sample-002-left, sample-002-right, etc.) Files also store sample rate and bit depth information.
SDLC (Synchronous Data Link Control) is a transmission protocol developed by IBM in the 1970s as a replacement for its binary synchronous (BSC) protocol. SDLC is equivalent to layer 2 of the OSI (Open Systems Interconnection) model of network communication. This level of protocol makes sure that data units arrive successfully from one network point to the next and flow at the right pace. SDLC uses the primary station-secondary station model of communication. Typically in IBM mainframe networks, the host mainframe is the primary station and workstations and other devices are secondary stations. Each secondary station has its own address. Typically, multiple devices or secondary stations are attached to a common line in what is known as a multipoint or multidrop arrangement. SDLC can also be used for point-to-point communication. SDLC is primarily for remote communication on corporate wide-area networks (WANs).
search engine SDLC was a basis for the International Organization for Standardization standard data link protocol, HLDLC (HighLevel Data Link Control). SDLC essentially became one of several variations of HDLC, the normal response mode (NRM). While SDLC (and normal response mode) are efficient protocols for closed private networks with dedicated lines, other modes of HDLC serve X.25 and frame relay protocols that manage packets on shared-line switched networks like those used by the Internet. SDLC became part of IBM’s Systems Network Architecture (SNA) and the more comprehensive Systems Application Architecture (SAA) and its more recent Open Blueprint. SDLC is still a commonly encountered and prevalent data link protocol in today’s mainframe environment.
SDM See ‘‘systems development method’’
SDMA See ‘‘spatial division multiple access’’
rather than two joined together. In programming, a seamless user interface often involves developing a back-end program that presents a single user interface that conceals the fact that what is behind the interface are a number of different programs written in different languages, at different times, and by different developers. With programming distributed among different computers in a network, it becomes important to screen the user from the location of the logic and the data behind the user interface. A common objective of any methodology for application integration and systems integration is a seamless user interface. The term is sometimes used loosely to mean that one program works with another one and the user can use both of them. Seamless is somewhat similar to the term transparent. Both mean that the user of something is unburdened by having to see what went into making it.
On the Internet, a search engine has three parts:
See ‘‘software-defined radio’’
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A spider (also called a ‘‘crawler’’ or a ‘‘bot’’) that goes to every page or representative pages on every Web site that wants to be searchable and reads it, using hypertext links on each page to discover and read a site’s other pages
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A program that creates a huge index (sometimes called a ‘‘catalog’’) from the pages that have been read
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A program that receives your search request, compares it to the entries in the index, and returns results to you
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An alternative to using a search engine is to explore a structured directory of topics. Yahoo, which also lets you use its search engine, is the most widely-used directory on the Web. A number of Web portal sites offer both the search engine and directory approaches to finding information.
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Major search engines such as Google, Yahoo, AltaVista, and Lycos index the content of a large portion of the Web and provide results that can run for pages—and consequently overwhelm the user.
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Specialized content search engines are selective about what part of the Web is crawled and indexed. For example, TechTarget sites for products such as the AS/ 400 (http://www.search400.com) and CRM applications (http://www.searchCRM.com) selectively index only the best sites about these products and provide a shorter but more focused list of results.
SDTV See ‘‘standard definition television’’
SDWT See ‘‘self-directed work team’’
seamless interface A seamless interface is the joining of two computer programs so that they appear to be one program with a single user interface. In sewing and other crafts, a seam is the place where two pieces of material are joined together. Often, the seam is visible; you often see the thread where two pieces of cloth are sewn together. In computer program development as in sewing, concealing the seam is often desirable so that the finished work looks like one object
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SDRAM (synchronous DRAM) is a generic name for various kinds of dynamic random access memory (DRAM) that are synchronized with the clock speed that the microprocessor is optimized for. This tends to increase the number of instructions that the processor can perform in a given time. The speed of SDRAM is rated in MHz rather than in nanoseconds (ns). This makes it easier to compare the bus speed and the RAM chip speed. You can convert the RAM clock speed to nanoseconds by dividing the chip speed into 1 billion ns (which is one second). For example, an 83 MHz RAM would be equivalent to 12 ns.
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Ask Jeeves (http://www.askjeeves.com) provides a general search of the Web but allows you to enter a search request in natural language, such as ‘‘What’s the weather in Seattle today?’’
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Special tools and some sites such as Yahoo let you use a number of search engines at the same time and compile results for you in a single list.
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Individual Web sites, especially larger corporate sites, may use a search engine to index and retrieve the content of just their own site. Some of the major search engine companies license or sell their search engines for use on individual sites.
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Where to Search First The last time we looked, the Open Directory Project listed 370 search engines available for Internet users. There are about ten major search engines, each with its own anchor Web site (although some have an arrangement to use another site’s search engine or license their own search engine for use by other Web sites). Some sites, such as Yahoo, search not only using their search engine but also give you the results from simultaneous searches of other search indexes. Sites that let you search multiple indexes simultaneously include: L
Yahoo (http://www.yahoo.com)
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Yahoo first searches its own hierarchically-structured subject directory and gives you those entries. Then, it provides a few entries from the AltaVista search engine. It also launches a concurrent search for entries matching your search argument with six or seven other major search engines. You can link to each of them from Yahoo (at the bottom of the search result page) to see what the results were from each of these search engines. A significant advantage of a Yahoo search is that if you locate an entry in Yahoo, it’s likely to lead you to a Web site or entire categories of sites related to your search argument. A search.com search primarily searches the Infoseek index first but also lets you search the other major search engines as well. EasySearcher lets you choose from either the popular search engines or a very comprehensive list of specialized search engine/databases in a number of fields.
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Yahoo, search.com, and EasySearcher all provide help with entering your search phrase. Most Web portal sites offer a quickly-located search entry box that connects you to the major search engines.
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For ‘‘tips’’ on entering your search argument, see each search engine, including Google’s . It’s good to read the information at least once.
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By ‘‘How to Search,’’ we mean a general approach to searching: what to try first, how many search engines to try, whether to search USENET newsgroups, when to quit. It’s difficult to generalize, but this is the general approach we use at whatis.com: 1. If you know of a specialized search engine such as SearchNetworking that matches your subject (for example, Networking), you’ll save time by using that search engine. You’ll find some specialized databases accessible from Easy Searcher 2. 2. If there isn’t a specialized search engine, try Yahoo. Sometimes you’ll find a matching subject category or two and that’s all you’ll need. 3. If Yahoo doesn’t turn up anything, try AltaVista, Google, Hotbot, Lycos, and perhaps other search engines for their results. Depending on how important the search is, you usually don’t need to go below the first 20 entries on each. 4. For efficiency, consider using a ferret that will use a number of search engines simultaneously for you. We use WebFerret (from a company called SoftFerret). 5. At this point, if you haven’t found what you need, consider using the subject directory approach to searching. Look at Yahoo or someone else’s structured organization of subject categories and see if you can narrow down a category your term or phrase is likely to be in. If nothing else, this may give you ideas for new search phrases. 6. If you feel it’s necessary, also search the Usenet newsgroups as well as the Web. 7. As you continue to search, keep rethinking your search arguments. What new approaches could you use? What are some related subjects to search for that might lead you to the one you really want? 8. Finally, consider whether your subject is so new that not much is available on it yet. If so, you may want to go out and check the very latest computer and Internet magazines or locate companies that you think may be involved in research or development related to the subject.
Search for Extraterrestrial Intelligence SETI (the Search for Extraterrestrial Intelligence) is a scientific effort to discover intelligent life elsewhere in the universe, primarily by attempting to discover radio signals that indicate intelligence. Cornell astronomer Frank Drake is credited with being the first to ‘‘listen’’ for intelligent signals with a radio telescope in 1960. Although NASA has funded some study in the past, current efforts are privately funded, in part by Arthur C. Clarke, Microsoft co-founder Paul Allen, Intel founder Gordon Moore, and Hewlett-Packard cofounders David Packard and William Hewlett.
secret key algorithm The SETI Institute’s Project Phoenix is using computers to search about 1,000 stars within 200 light-years of our solar system for radio signals beamed toward us or any other location. Project Phoenix’s 140-foot radio telescope in Green Bank, West Virginia aims at one star at a time while astronomer-monitored computers search each 1,000 band from 1,000 to 3,000 MHz for a signal limited to a narrowband range. Scientists believe that a signal focused within a narrow frequency band would suggest an intelligent source. About two-thirds of the first 1,000 stars have been searched with no success yet reported. There are, however, over 400 billion stars in our own galaxy so the study may last quite a long time. The directors of the project are soliciting volunteers to help analyze the radio telescope data at their home computers.
search-and-replace Search-and-replace is a capability that allows a computer user to find a given sequence of characters in one or more text files and, if desired, replace the sequence with another sequence of characters. A global search-and-replace means that the replace is to take effect in all files within the context of the search. In UNIX and some other operating systems, a grep command or utility can be used to do search-and-replace. In Windows operating systems, a popular product that provides a graphical user interface is called Search and Replace for Windows 95/NT.
second The second (abbreviation, s or sec) is the International System of Units (SI) unit of time. One second is the time that elapses during 9,192,631,770 (9.192631770 6 109) cycles of the radiation produced by the transition between two levels of the cesium 133 atom. There are other expressions for the second. It is the time required for an electromagnetic field to propagate 299,792,458 meters (2.99792458 6 108 m) through a vacuum. This figure is sometimes rounded to 3.00 6 108 m, or 300,000 kilometers (3.00 6 105 km). One second is equal to 1/86,400 of a mean solar day. This is easy to derive from the fact that there are 60 seconds in a minute, 60 minutes in an hour, and 24 hours in a mean solar day. This definition is, however, subject to limited accuracy because of irregularities in the earth’s orbit around the sun. Engineers and scientists often use smaller units than the second by attaching power-of-10 prefix multipliers. One millisecond is 10-3 s; one microsecond is 10-6 s; one nanosecond is 10-9 s; one picosecond is 10-12 s. During these spans of time, respectively, an EM field propagates through a vacuum over distances of approximately 300 kilometers, 300 meters, 300 millimeters, and 300 micrometers.
The second is sometimes specified as a unit of angular measure, especially in astronomy and global positioning. In these contexts, it is also known as an arc second or a second of arc, and is equal to exactly 1/3600 of an angular degree or 1/1,296,000 of a circle. Sixty arc seconds comprise an arc minute; 60 arc minutes comprise an angular degree. One arc second of latitude at the earth’s surface corresponds to a north-south distance of only about 31 m. Also see International System of Units (SI) and prefix multipliers.
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second generation RISC See ‘‘superscalar’’
secondary storage Secondary storage is all magnetic data storage that is not currently in the computer’s main storage or memory. Synonyms are external storage and auxiliary storage.
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second-hand See ‘‘refurbished’’
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A second-level domain (SLD) is the portion of a Uniform Resource Locator (URL) that identifies the specific and unique administative owner associated with an Internet Protocol address (IP address). The second-level domain name includes the top-level domain (top-level domain) name. For example, in: whatis.com ’’whatis’’ is a second-level domain. ‘‘whatis.com’’ is a second-level domain name (and includes the top-level domain name of ‘‘com’’). Second-level domains can be divided into further domain levels. These subdomains sometimes represent different computer servers within different departments. More than one second-level domain name can be used for the same IP address.
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secret key See ‘‘private key’’
secret key algorithm Also see cryptography. A secret key algorithm (sometimes called a symmetric algorithm) is a cryptographic algorithm that uses the same key to encrypt and decrypt data. The best known algorithm is the U.S. Department of Defense’s Data Encryption Standard (DES). DES, which was developed at IBM in 1977, was thought to be so difficult to break that the U.S. government restricted its exportation.
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A very simple example of how a secret key algorithm might work might be substituting the letter in the alphabet prior to the target letter for each one in a message. The resulting text—‘‘gdkkn,’’ for example—would make no sense to someone who didn’t know the algorithm used (x-1), but would be easily understood by the parties involved in the exchange as ‘‘hello.’’ The problem with secret or symmetric keys is how to securely get the secret keys to each end of the exchange and keep them secure after that. For this reason, an asymmetric key system is now often used that is known as the public key infrastructure (PKI).
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On a computer diskette or hard disk, a sector is one of the ‘‘pies slices’’ the diskette or disk is divided into. Dividing the circular medium into pie slices is a way to organize it so that data can be located by the read/write heads of the drive. The diskette or disk is also divided into a number of concentric circles. Data can be located by knowing the number of the sector and the concentric track that passes through that sector. Each track is divided into a number of clusters that represent the smallest unit of storage that is addressable (can be written to or read). Typically, a cluster is 256 or 512 bytes in length. Sector 0 of the diskette or disk contains a special file, the file allocation table (FAT). The FAT tells where the directory to the files on the medium is located and information about how clusters are used. You can’t look at sector 0 directly. On hard disks, the first sector is called variously the master boot record, the partition sector, or the partition table. This record or table tells how and whether the disk has been divided into logical partitions (for example, you can divide your hard drive into two logical partitions or drives so that you can load different operating systems on to the disk and switch back of forth). When your operating system is being booted or loaded into RAM, a program in this partition sector briefly gets control, determines how your disk is partitioned, and then reads the operating system boot sector and gives that boot sector program control so that the rest of the operating system can be loaded into RAM. The partition sector is the sector that can be ‘‘infected’’ when you leave a diskette in drive A that contains a boot virus. The sectors as well as the rest of the organization of the diskette or disk are set up as a result of the process called formatting. Most diskettes you buy today are already formatted. However, if you’re using an old one, you may need to reformat it. You can do this using a common utility that comes with your operating system.
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SET (Secure Electronic Transaction) is a system for ensuring the security of financial transactions on the Internet. It was supported initially by Mastercard, Visa, Microsoft, Netscape,
and others. With SET, a user is given an electronic wallet (digital certificate) and a transaction is conducted and verified using a combination of digital certificates and digital signatures among the purchaser, a merchant, and the purchaser’s bank in a way that ensures privacy and confidentiality. SET makes use of Netscape’s Secure Sockets Layer (SSL), Microsoft’s Secure Transaction Technology (STT), and Terisa System’s Secure Hypertext Transfer Protocol (S-HTTP). SET uses some but not all aspects of a public key infrastructure (PKI). Here’s how SET works: Assume that a customer has a SET-enabled browser such as Netscape or Microsoft’s Internet Explorer and that the transaction provider (bank, store, etc.) has a SET-enabled server. 1. The customer opens a Mastercard or Visa bank account. Any issuer of a credit card is some kind of bank. 2. The customer receives a digital certificate. This electronic file functions as a credit card for online purchases or other transactions. It includes a public key with an expiration date. It has been through a digital switch to the bank to ensure its validity. 3. Third-party merchants also receive certificates from the bank. These certificates include the merchant’s public key and the bank’s public key. 4. The customer places an order over a Web page, by phone, or some other means. 5. The customer’s browser receives and confirms from the merchant’s certificate that the merchant is valid. 6. The browser sends the order information. This message is encrypted with the merchant’s public key, the payment information, which is encrypted with the bank’s public key (which can’t be read by the merchant), and information that ensures the payment can only be used with this particular order. 7. The merchant verifies the customer by checking the digital signature on the customer’s certificate. This may be done by referring the certificate to the bank or to a third-party verifier. 8. The merchant sends the order message along to the bank. This includes the bank’s public key, the customer’s payment information (which the merchant can’t decode), and the merchant’s certificate. 9. The bank verifies the merchant and the message. The bank uses the digital signature on the certificate with the message and verifies the payment part of the message. 10. The bank digitally signs and sends authorization to the merchant, who can then fill the order.
Secure Hypertext Transfer Protocol See ‘‘HTTPS’’
self-assembly
Secure Shell
Security Identifier
Secure Shell (SSH), sometimes known as Secure Socket Shell, is a UNIX-based command interface and protocol for securely getting access to a remote computer. It is widely used by network administrators to control Web and other kinds of servers remotely. SSH is actually a suite of three utilities—slogin, ssh, and scp—that are secure versions of the earlier UNIX utilities, rlogin, rsh, and rcp. SSH commands are encrypted and secure in several ways. Both ends of the client/server connection are authenticated using a digital certificate, and passwords are protected by being encrypted.
In Windows NT and 2000 operating systems, the Security Identifier (SID) is a unique alphanumeric character string that identifies each operating system and each user in a network of NT/2000 systems.
SSH uses RSA public key cryptography for both connection and authentication. Encryption algorithms include Blowfish, DES, and IDEA. IDEA is the default. SSH2, the latest version, is a proposed set of standards from the Internet Engineering Task Force (IETF).
Secure Socket Shell See ‘‘Secure Shell’’
Secure Sockets Layer The Secure Sockets Layer (SSL) is a commonly used protocol for managing the security of a message transmission on the Internet. SSL has recently been succeeded by Transport Layer Security (TLS), which is based on SSL. SSL uses a program layer located between the Internet’s Hypertext Transfer Protocol (HTTP) and Transport Control Protocol (TCP) layers. SSL is included as part of both the Microsoft and Netscape browsers and most Web server products. Developed by Netscape, SSL also gained the support of Microsoft and other Internet client/server developers as well and became the de facto standard until evolving into Transport Layer Security. The ‘‘sockets’’ part of the term refers to the sockets method of passing data back and forth between a client and a server program in a network or between program layers in the same computer. SSL uses the public-and-private key encryption system from RSA, which also includes the use of a digital certificate. TLS and SSL are an integral part of most Web browsers (clients) and Web servers. If a Web site is on a server that supports SSL, SSL can be enabled and specific Web pages can be identified as requiring SSL access. Any Web server can be enabled by using Netscape’s SSLRef program library which can be downloaded for noncommercial use or licensed for commercial use. TLS and SSL are not interoperable. However, a message sent with TLS can be handled by a client that handles SSL but not TLS.
security policy In business, a security policy is a document that states in writing how a company plans to protect the company’s physical and information technology (IT) assets. A security policy is often considered to be a ‘‘living document’’, meaning that the document is never finished, but is continuously updated as technology and employee requirements change. A company’s security policy may include an acceptable use policy, a description of how the company plans to educate its employees about protecting the company’s assets, an explanation of how security measurements will be carried out and enforced, and a procedure for evaluating the effectiveness of the security policy to ensure that necessary corrections will be made.
self-assembly Self-assembly is a method of constructing devices and machines whose dimensions are measured in nanometers (units of 10-9 meter or millionths of a millimeter). The technique works by simulating the way biological systems build molecules, viruses, cells, plants, and animals. Self-assembly involves a process known as convergent synthesis. This has been taking place for eons in life forms of all kinds. The technology is largely unexploited, but researchers have succeeded in initiating convergent synthesis to produce some primitive nanodevices. Self assembly is of special interest to the manufacturers of integrated circuits (ICs), especially microprocessors. A device called a biochip might be grown like a plant from a seed, giving rise to computers with high-level artificial intelligence (AI). Self-assembly should not be confused with positional assembly. In the latter process, the engineer has a relatively high degree of control over where each individual atom or molecule is placed. In self-assembly, although the positioning and arrangement of atoms or molecules might be just as critical as in positional assembly, the process is carried out largely by nature once the engineer has initiated it. In that sense, self-assembly is a sophisticated chemical reaction. Some nanotechnology researchers are, however, exploring ways to add positional control to chemical synthesis. Also see dendrimer, exponential assembly, nanotechnology, positional assembly, and self-replication.
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self-directed work team
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A self-directed work team (SDWT) is a group of people, usually employees in a company, who combine different skills and talents to work without the usual managerial supervision toward a common purpose or goal. Typically, an SDWT has somewhere between two and 25 members. An optimal SDWT is said to be between five and nine members. An SDWT is similar to a skunkworks except that an SDWT is not temporary and is not limited to a single project.
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SDWT members use their company’s mission statement to develop their purpose, which must be meaningful and beneficial to the company. Purposes might include problem solving, increasing sales and productivity, career training, and product improvement. SDWT members must decide how they want to work together. Because a manager or boss does not lead, they must agree on the rules and deadlines for accomplishing their purpose. Some teams create a charter or set of rules that describe what is expected of each member. If a problem arises during the course of a project, the team members work together to provide a solution. In order for an SDWT to succeed, the company or organization must provide a meaningful mission statement to the team, empower the team to do what it needs to do including making important decisions, support the team, establish and provide the boundaries, rules, and company policies, and train the members with the skills and knowledge needed to accomplish their purpose. But, in the end, the team is held accountable for the success or failure of a project. Saturn and Federal Express are examples of companies that use self-directed work teams.
Self-Monitoring Analysis & Reporting Technology Self-Monitoring Analysis and Reporting Technology (S.M.A.R.T.) is an interface between a computer’s start-up program or basic input/output system and the computer hard disk. It is a feature of the Enhanced Integrated Drive Electronics (EIDE) technology that controls access to the hard drive. If S.M.A.R.T is enabled when a computer is set up, the BIOS can receive analytical information from the hard drive and determine whether to send the user a warning message about possible future failure of the hard drive.
self-replication In nanotechnology, self-replication is a process in which devices whose diameters are of atomic scale, on the order of nanometers, create copies of themselves. A nanometer is 10-9 meter or a millionth of a millimeter. In order for selfreplication to take place in a constructive manner, three conditions must be met.
The first requirement is that each unit be a specialized machine called a nanorobot, one of whose functions is to construct at least one copy of itself during its operational life. (Each unit should also have some other function, unless the intended purpose of the aggregate is to cause havoc by generating the mechanical equivalent of a cancerous tumor.) A hypothetical example of a constructively self-replicating nanorobot is an artificial antibody. In addition to reproducing itself, it seeks and destroys disease-causing organisms. The second requirement is the existence of all the energy and ingredients necessary to build a certain minimum number of complete copies of the nanorobot in question. Ideally, the quantities of each ingredient should be such that they are consumed in the correct proportion. If the process is intended to be finite, then when the desired number of nanorobots has been constructed, there should be no unused quantities of any ingredient remaining. The third requirement is that the environment be controlled so that the replication process can proceed efficiently and without malfunctions. Excessive turbulence, temperature extremes, intense radiation, or other adverse circumstances might prevent the proper functioning of the nanorobots and cause the process to falter or fail. Science-fiction writers have suggested that mutations might occur in a sophisticated selfreplicating nanorobotic community, giving rise to the machine equivalent of natural selection. Such a process might be used to create a self-sustaining, evolving colony of robots on an extraterrestrial planet. Also see exponential assembly, nanotechnology, positional assembly, and self-assembly.
Self-scanned Amorphous Silicon Integrated Display Self-scanned Amorphous Silicon Integrated Display (SASID) is the trade name for an active-matrix liquid crystal display (AMLCD) technology developed by Sarnoff Corporation. The technology is intended primarily for flat-panel displays in notebook and portable computers, and for projection panels. The hallmarks of the SASID are the amorphous silicon (aSi or a-Si) manufacturing process and the self-scanning feature. The horizontal and vertical scanning circuitry, rather than being external to the display, is incorporated directly on the glass substrate of the panel. This reduces the cost of manufacture by up to 35 percent, and enhances reliability by minimizing the number of interconnects. The average useful life of preliminary SASID designs is more than 10,000 hours at 70 degrees centigrade, and the random failure rate is approximately half that of most other AMLCDs. In addition, SASIDs are brighter than most other AMLCDs, offer an image resolution of more than 200 pixels per inch, provide excellent contrast, and consume minimal
semiconductor power (typically less than 200 milliwatts). The SASID is compatible with existing pixel designs, and offers equivalent system interaction to other AMLCDs.
Semantic Web Agreement Group The Semantic Web Agreement Group (SWAG) is an organization of interested parties working together to further the development of the Semantic Web (SW), a concept that might lead to a Web that was self-described in a way that would make it easier for programs to work with it. (Semantic refers to the meaning of something.) Tim Berners-Lee, whose Hypertext Transfer Protocol (HTTP) virtually created the World Wide Web, is also the man behind the Semantic Web. Berners-Lee describes the Semantic Web as ‘‘a web of data that can be processed directly or indirectly by machines’’. SW is a new vision of the Web in which document components describe and comprehend relationships between various aspects of Web content, enabling machines to process data in a more intelligent and autonomous way. Because the Semantic Web would be dependent upon effective data sharing, SWAG is working towards building the support system, such as a standardized glossary to ensure that terms will be processed identically by all computer systems. Computer systems already rely on a standardized vocabulary to communicate; this ensures that instructions (such as load, for example) are interpreted consistently. The Semantic Web would make Web documents and document components much more meaningful to the machines that process them. Markup languages have steadily progressed towards more intelligent versions. Extensible Markup Language (XML), for example, has much more ability to perform tasks autonomously than HTML. New adaptations, such as DARPA Agent Markup Language (DAML) are being developed as potential vehicles that could make it possible for the Semantic Web to function as a comprehensive universal database.
Semantic Web The Semantic Web is an idea of World Wide Web inventor Tim Berners-Lee that the Web as a whole can be made more intelligent and perhaps even intuitive about how to serve a user’s needs. Berners-Lee observes that although search engines index much of the Web’s content, they have little ability to select the pages that a user really wants or needs. He foresees a number of ways in which developers and authors, singly or in collaborations, can use self-descriptions and other techniques so that context-understanding programs can selectively find what users want.
semantics Semantics (pronounced seh-MANT-iks, from Greek semantikos or significant and sema or sign) is the branch of semiotics, the philosophy or study of signs, that deals with
meaning. The other two branches of semiotics are syntactics (the arrangement of signs) and pragmatics (the relationship between the speaker and the signs). In discussing natural and computer languages, the distinction is sometimes made between syntax (for example, the word order in a sentence or the exact computer command notation) and semantics (what the words really say or what functions are requested in the command).
semaphore In programming, especially in UNIX systems, semaphores are a technique for coordinating or synchronizing activities in which multiple processes compete for the same operating system resources. A semaphore is a value in a designated place in operating system (or kernel) storage that each process can check and then change. Depending on the value that is found, the process can use the resource or will find that it is already in use and must wait for some period before trying again. Semaphones can be binary (0 or 1) or can have additional values. Typically, a process using semaphores checks the value and then, if it using the resource, changes the value to reflect this so that subsequent semaphore users will know to wait. Semaphores are commonly use for two purposes: to share a common memory space and to share access to files. Semaphores are one of the techniques for interprocess communication (IPC). The C programming language provides a set of interfaces or ‘‘functions’’ for managing semaphores.
semiconductor See also atom, transistor, bipolar transistor, and field-effect transistor. A semiconductor is a substance, usually a solid chemical element or compound, that can conduct electricity under some conditions but not others, making it a good medium for the control of electrical current. Its conductance varies depending on the current or voltage applied to a control electrode, or on the intensity of irradiation by infrared (IR), visible light, ultraviolet (UV), or X rays. The specific properties of a semiconductor depend on the impurities, or dopants, added to it. An N-type semiconductor carries current mainly in the form of negatively-charged electrons, in a manner similar to the conduction of current in a wire. A P-type semiconductor carries current predominantly as electron deficiencies called holes. A hole has a positive electric charge, equal and opposite to the charge on an electron. In a semiconductor material, the flow of holes occurs in a direction opposite to the flow of electrons. Elemental semiconductors include antimony, arsenic, boron, carbon, germanium, selenium, silicon, sulfur, and tellurium. Silicon is the best-known of these, forming the basis of most integrated circuits (ICs). Common semiconductor com-
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Whatis?Com’s Encyclopedia of Technology Terms pounds include gallium arsenide, indium antimonide, and the oxides of most metals. Of these, gallium arsenide (GaAs) is widely used in low-noise, high-gain, weak-signal amplifying devices. A semiconductor device can perform the function of a vacuum tube having hundreds of times its volume. A single integrated circuit (IC), such as a microprocessor chip, can do the work of a set of vacuum tubes that would fill a large building and require its own electric generating plant.
sendmail On the Internet, sendmail is the most popular UNIX-based implementation of the Simple Mail Transfer Protocol (SMTP) for transmitting e-mail. When a sendmail server receives e-mail, it attempts to deliver the mail to the intended recipient immediately and, if the recipient is not present, it queues messages for later delivery. However, because it does not provide a mailbox facility and for other reasons, other software such as a POP3 or Internet Message Access Protocol server are also needed. Most Internet service providers (ISPs) provide both an SMTP server (such as sendmail) and a POP or IMAP server. A commercial version of sendmail, called Sendmail, includes a POP3 server and other enhancements to the basic open source sendmail. It also comes in a version that can be installed on a Windows NT operating systems platform.
Sequenced Packet Exchange SPX (Sequenced Packet Exchange) is the protocol for handling packet sequencing in a Novell NetWare network. SPX prepares the sequence of packets that a message is divided into and manages the reassembly of received packets, confirming that all have been received and requesting retransmission when they haven’t. SPX works directly with the Internetwork Packet Exchange (IPX), which manages the forwarding of packets in the network.
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IPX/SPX are comparable to the basic Internet protocols, Internet Protocol and Transmission Control Protocol.
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In digital audio recording, a sequencer is a program in a computer or stand-alone keyboard unit that puts together a sound sequence from a series (or sequence) of Musical Instrument Digital Interface (MIDI) events (operations). The MIDI sequencer allows the user to record and edit a musical performance without using an audio-based input source. The performance is recorded as a series of events that would ordinarily be played in from a keyboard instrument. The MIDI sequencer does not record the actual audio, but rather the events related to the performance—what note was played at what time, how hard the key was pressed, when did the sustain pedal get depressed, and so forth. This data is then played back into a MIDI instrument or sound module. Using this method, the performer can select a piano
sound for a musical passage and later decide that the passage would work better as an organ sound. The editor can simply change the sound program on the MIDI keyboard to alter the sound without needing to rerecord the entire performance. Using sound modules and keyboards that can respond on different MIDI channels, the player can layer the sounds of different instruments to create the illusion of an entire band or orchestra. There is sequencing software available in a range from that which would suit a beginner all the way to the most advanced film composer.
sequential consistency In computers, sequential consistency is best defined by its inventor, Leslie Lamport, who declared that a multiprocessing system had sequential consistency if: ‘‘...the results of any execution is the same as if the operations of all the processors were executed in some sequential order, and the operations of each individual processor appear in this sequence in the order specified by its program.’’ This definition was one of the first statements on what constituted ‘‘correct’’ behavior for a symmetric multiprocessing (SMP) system. It has been restated in various forms, among them the following: Sequential consistency requires that a shared memory multiprocessor appear to be a multiprogramming uniprocessor system to any program running on it. Sequential consistency requires that: 1. All instructions are executed in order. 2. Every write operation becomes instantaneously visible throughout the system. Implicit in the definition of sequential consistency was the acknowledgement that SMP systems could fail to be sequentially consistent. Should machines be sequentially consistent? From the vantage point of almost twenty years later, the answers are mixed. Some say ‘‘yes,’’ that machines should conceal implementation complexities in order to make the machines as easy to use as possible. Others say ‘‘no,’’ that violating SC allows machines to increase performance considerably while at the same time inconveniencing only a vanishingly small number of programmers.
Sequential Couleur avec Memoire SECAM (Sequential Couleur avec Memoire) is the television display technology that is standard in France, the countries of the former Soviet Union, and certain other countries. It is one of the three world TV standards together with National Television Standards Committee (United States) and Phase Alternation Line (Europe other than France). Like PAL, SECAM scans the cathode ray tube (CRT) horizontally 625 times to form the video image.
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serendipity In general, serendipity is the act of finding something valuable or delightful when you are not looking for it. In information technology, serendipity often plays a part in the recognition of a new product need or in solving a design problem. Web surfing can be an occasion for serendipity since you sometimes come across a valuable or interesting site when you are looking for something else. The term was coined by English writer Horace Walpole on January 28, 1754, in a letter written to Horace Mann. He credited it to a ‘‘silly fairy tale’’ he once read called The Three Princes of Serendip. According to the fairy tale, three Persian princes sailed off to make their fortunes in the ‘‘land of silk’’, an island called Serendip. (Today, the island is known as Sri Lanka.) Along the way, the princes made all types of wondrous and delightful discoveries about the island, and learned things they never expected. One such learning was purported to be the discovery that a mule blind in its right eye had recently traveled the same road. This was discovered when they noticed that the grass had been eaten only on the left side of the road.
Conventional computers and their programs operate in a serial manner, with the computer reading a program and performing its instructions one after the other. However, some of today’s computers have multiple processors and can perform instructions in parallel.
Serial Line Internet Protocol SLIP (Serial Line Internet Protocol) is a protocol used for communication between two machines that are previously configured for communication with each other. For example, your Internet server provider may provide you with a SLIP connection so that the provider’s server can respond to your requests, pass them on to the Internet, and forward your requested Internet responses back to you. Your dial-up connection to the server is typically on a slower serial line rather than on the parallel or multiplex lines such as a line of the network you are hooking up to. A better service is provided by the Point-to-Point Protocol (PPP).
Serial means one event at a time. It is usually contrasted with parallel, meaning more than one event happening at a time. In data transmission, the techniques of time division and space division are used, where time separates the transmission of individual bits of information sent serially and space (on multiple lines or paths) can be used to have multiple bits sent in parallel. In the context of computer hardware and data transmission: serial connection, operation, and media usually indicate a simpler, slower operation and parallel indicates a faster operation. This indication doesn’t always hold since a serial medium (for example, fiber optic cable) can be much faster than a slower medium that carries multiple signals in parallel.
Before SPD, memory chips were identified with parallel presence detect (PPD). PPD used a separate pin for each bit of information, which meant that only the speed and density of the memory module could be stored because of the limited space for pins. The EEPROM chip on an SPD memory module only requires two pins so more information can be stored.
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On your PC, the printer is usually attached through a parallel interface and cable so that it will print faster. Your keyboard and mouse are one-way devices that only require a serial interface and line. Inside your computer, much of its circuitry supports bits being moved around in parallel. Your computer modem uses one of your PC’s serial connections or COM ports. Serial communication between your PC and the modem and other serial devices adheres to the RS-232C standard.
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serial presence detect When a computer is booted (started), serial presence detect (SPD) is information stored in an electrically erasable programmable read-only memory (EEPROM) chip on a synchronous dynamic random access memory (SDRAM) memory module that tells the basic input/output system (BIOS) the module’s size, data width, speed, and voltage. The BIOS uses this information to configure the memory properly for maximum reliability and performance. If a memory module does not have SPD, the BIOS assumes the memory module’s information. With some memory, this does not cause problems. But SDRAM memory has to have SPD or the computer may not boot at all. If it does boot, the assumed information may cause fatal exception errors
The term is also sometimes used to mean ‘‘the randomness of fate,’’ as in ‘‘whatever happens to cross my desk today.’’
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server 1) In general, a server is a computer program that provides services to other computer programs in the same or other computers. 2) The computer that a server program runs in is also frequently referred to as a server (though it may contain a number of server and client programs). 3) In the client/server programming model, a server is a program that awaits and fulfills requests from client programs in the same or other computers. A given
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Specific to the Web, a Web server is the computer program (housed in a computer) that serves requested HTML pages or files. A Web client is the requesting program associated with the user. The Web browser in your computer is a client that requests HTML files from Web servers.
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A server accelerator card (also known as an SSL card) is a Peripheral Component Interconnect (PCI) card used to generate encryption keys for secure transactions on ecommerce Web sites. When a secure transaction is initiated, the Web site’s server sends its certificate, which has been provided by a certifying authority, to the client machine to verify the Web site’s authenticity. After this exchange, a secret key is used to encrypt all data transferred between sender and receiver so that all personal and credit card information is protected. This process can severely overload a server resulting in fewer transactions processed per second, which means fewer sales. The server accelerator card takes over this process, thus reducing the load on the server. Server accelerator cards support a number of security protocols including Secure Sockets Layer (SSL) and Secure Electronic Transaction (SET).
On the Internet, a Web server farm, or simply Web farm, may refer to a Web site that uses two or more servers to handle user requests. Typically, serving user requests for the files (pages) of a Web site can be handled by a single server. However, larger Web sites may require multiple servers. Web farm is a term that is also simply used to mean a business that performs Web site hosting on multiple servers. Some Web farms allow you to put your own server on their site, a service known as colocation.
Server Message Block Protocol The Server Message Block Protocol (SMB protocol) provides a method for client applications in a computer to read and write to files on and to request services from server programs in a computer network. The SMB protocol can be used over the Internet on top of its TCP/IP protocol or on top of other network protocols such as Internetwork Packet Exchange and NetBEUI. Using the SMB protocol, an application (or the user of an application) can access files at a remote server as well as other resources, including printers, mailslots, and named pipes. Thus, a client application can read, create, and update files on the remote server. It can also communicate with any server program that is set up to receive an SMB client request.
The server accelerator card is installed into the PCI slot of the server. A software driver is loaded, and the server is ready to receive orders. This is much easier and more costeffective than buying additional servers. Additional cards can be installed as the server’s secure transactions increase.
Microsoft Windows operating systems since Windows 95 include client and server SMB protocol support. For UNIX systems, a shareware program, Samba, is available. The SMB protocol originated at Microsoft and has gone through a number of developments. A given client and server may implement different sets of protocol variations which they negotiate before starting a session.
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There are also SSL acceleration appliances. These are external units that have server accelerator cards installed inside them. The unit is then plugged into the server. When a secure transaction is detected, the transaction is routed to the SSL acceleration unit for processing. SSL accelerator appliances can be added as needed by clustering them together.
Microsoft has offered a public or open source version of SMB for the Internet to the Internet Engineering Task Force (IETF). Called the Common Internet File System (CIFS), this new protocol provides more flexibility than existing Internet applications such as the File Transfer Protocol (FTP). CIFS is envisioned as a complement to the Internet’s Hypertext Transfer Protocol for Web browsing.
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A server farm is a group of computers acting as server and housed together in a single location. A server farm is sometimes called a server cluster. A Web server farm is either (1) a Web site that has more than one server, or (2) an Internet service provider (ISP) that provides Web hosting services using multiple servers.
A server space provider is an individual, company, or organization that provides storage space on a server for Web pages, usually for a charge. Some independent access providers and online services provide a limited amount of free space for Web pages (for example, one megabytes of hard disk storage).
In a business network, a server farm or cluster might perform such services as providing centralized access control, file access, printer sharing, and backup for workstation users. The servers may have individual operating systems or a shared operating system and may also be set up to provide load balancing when there are many server requests. In a server farm, if one server fails, another can act as backup.
Note that not all access providers offer server space. And many server space providers do not provide access. Server space and access are two different services.
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Relative to other costs of developing a Web site, the cost of server space is usually small. A new trend on some sites, however, is to charge for the amount of traffic your pages get
service pack rather than the space they use. This is because server space is relatively cheap compared to the administrative costs of managing a busy site for someone.
serverless backup Serverless backup is a method of offloading backup procedures from a server so that the time ordinarily devoted to backup functions can be used to carry out other server tasks. Ordinarily, the amount of time that a server can devote to processing requests from applications is limited by the backup window—the amount of time that must be reserved for data backup. Serverless backup is a storage area network (SAN) solution that is designed to lead to lower hardware costs and improved time-effectiveness, scalability, and fault tolerance. A number of companies, including Legato, Veritas, EMC, and Computer Associates offer or are developing serverless backup products.
A Web file that contains server-side include statements (such as the ‘‘echo’’ statement above) is usually defined by the administrator to be a file with an ‘‘.’’ suffix. You can think of a server-side include as a limited form of common gateway interface application. In fact, the CGI is not used. The server simply searches the server-side include file for CGI environment variables, and inserts the variable information in the places in the file where the ‘‘include’’ statements have been inserted. When creating a Web site, a good idea is to ask your server administrator which environment variables can be used and whether the administrator can arrange to set the server up so that these can be handled. The administrator may be able to help you add the appropriate ‘‘include’’ statements to your HTML file.
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Serverless backup enables disk-to-tape or disk-to-disk backup without depending on server resources or network bandwidth. The Legato, Veritas, and CA products are based on a process known as disk imaging. These applications use an intelligent agent, which makes a snapshot copy of pointers to the data. The snapshot copy consists of an image of the pointers, which indicate the location of the data. The intelligent agent and backup procedures communicate through the Network Data Management Protocol (NDMP). The EMC version is based on something called a Fastrax data movement platform and Hewlett Packard’s OpenView OmniBak II backup application. Proprietary software within the hardware platform is used, first, to mark logical backup objects, and then to back up data on the list.
See ‘‘well-known port numbers’’
server-push
Some metrics that SLAs may specify include:
See ‘‘push’’
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The number of users that can be served simultaneously
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Service Level Agreement A Service Level Agreement (SLA) is a contract between a network service provider and a customer that specifies, usually in measurable terms, what services the network service provider will furnish. Many Internet service providers (ISP)s provide their customers with an SLA. More recently, IS departments in major enterprises have adopted the idea of writing a Service Level Agreement so that services for their customers (users in other departments within the enterprise) can be measured, justified, and perhaps compared with those of outsourcing network providers.
See ‘‘server’’
server-side include A server-side include (SSI) is a variable value (for example, a file ‘‘Last modified’’ date) that a server can include in an HTML file before it sends it to the requestor. If you’re creating a Web page, you can insert an include statement in the HTML file that looks like this:
and the server will obtain the last-modified date for the file and insert it before the HTML file is sent to requestors. LAST_MODIFIED is one of several environment variables that an operating system can keep track of and that can be accessible to a server program. The server administrator can make these environment variables usable when the system is set up.
service pack A service pack is an orderable or downloadable update to a customer’s software that fixes existing problems and, in some cases, delivers product enhancements. IBM and Microsoft are examples of companies that use this term to describe their periodic product updates. When a new product version comes out, it usually incorporates the fixes from the service packs that have been shipped to update the previous product version.
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Service Profile Identifier
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servlet A servlet is a small program that runs on a server. The term was coined in the context of the Java applet, a small program that is sent as a separate file along with a Web (HTML) page. Java applets, usually intended for running on a client, can result in such services as performing a calculation for a user or positioning an image based on user interaction. Some programs, often those that access databases based on user input, need to be on the server. Typically, these have been implemented using a Common Gateway Interface (CGI) application. However, with a Java virtual machine running in the server, such programs can be implemented with the Java programming language. The advantage of a Java servlet on servers with lots of traffic is that they can execute more quickly than CGI applications. Rather than causing a separate program process to be created, each user request is invoked as a thread in a single daemon process, meaning that the amount of system overhead for each request is slight. Instead of a URL that designates the name of a CGI application (in a ‘‘cgi-bin’’ subdirectory), a request in a form on a Web HTML page that results in a Java servlet getting called would call a URL that looks like this: http://www.whatis.com:8080/servlet/gotoUrl?http:// www.someplace.com
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The ‘‘8080’’ port number in the URL means the request is intended directly for the Web server itself. The ‘‘servlet’’ would indicate to the Web server that a servlet was being requested. Add-on modules allow Java servlets to run in Netscape Enterprise, Microsoft Internet Information Server (IIS), and Apache servers.
session Also see user session.
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In telecommunication, a session is a series of interactions between two communication end points that occur during the span of a single connection. Typically, one end point requests a connection with another specified end point and if that end point replies agreeing to the connection, the end points take turns exchanging commands and data (‘‘talking to each other’’). The session begins when the connection is established at both ends and terminates when the connection is ended. In the standard industry communications reference model, Open Systems Interconnection (OSI), the Session layer (sometimes called the ‘‘port layer’’) manages the setting up and taking down of the association between two
communicating end points that is called a connection. A connection is maintained while the two end points are communicating back and forth in a conversation or session of some duration. Some connections and sessions last only long enough to send a message in one direction. However, other sessions may last longer, usually with one or both of the communicating parties able to terminate it. For Internet applications, each session is related to a particular port, a number that is associated with a particular upper layer application. For example, the HTTP program or daemon always has port number 80. The port numbers associated with the main Internet applications are referred to as wellknown port numbers. Most port numbers, however, are available for dynamic assignment to other applications.
session cookie See ‘‘transient cookie’’
Session Initiation Protocol The Session Initiation Protocol (SIP) is an Internet Engineering Task Force (IETF) standard protocol for initiating an interactive user session that involves multimedia elements such as video, voice, chat, gaming, and virtual reality. Like HTTP or SMTP, SIP works in the Application layer of the Open Systems Interconnection (OSI) communications model. The Application layer is the level responsible for ensuring that communication is possible. SIP can establish multimedia sessions or Internet telephony calls, and modify, or terminate them. The protocol can also invite participants to unicast or multicast sessions that do not necessarily involve the initiator. Because the SIP supports name mapping and redirection services, it makes it possible for users to initiate and receive communications and services from any location, and for networks to identify the users whereever they are. SIP is a request-response protocol, dealing with requests from clients and responses from servers. Participants are identified by SIP URLs. Requests can be sent through any transport protocol, such as UDP, SCTP, or TCP. SIP determines the end system to be used for the session, the communication media and media parameters, and the called party’s desire to engage in the communication. Once these are assured, SIP establishes call parameters at either end of the communication, and handles call transfer and termination. The Session Initiation Protocol is specified in IETF Request for Comments [RFC] 2543.
Session layer In the Open Systems Interconnection (OSI) communications model, the Session layer (sometimes called the ‘‘port layer‘‘) manages the setting up and taking down of the association between two communicating end points that is called a
set-top box connection. A connection is maintained while the two end points are communicating back and forth in a conversation or session of some duration. Some connections and sessions last only long enough to send a message in one direction. However, other sessions may last longer, usually with one or both of the communicating parties able to terminate it.
becoming increasingly relevant in software engineering, especially in the fields of artificial intelligence and communications security.
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A set is a group or collection of objects or numbers, considered as an entity unto itself. Sets are usually symbolized by uppercase, italicized, boldface letters such as A, B, S, or Z. Each object or number in a set is called a member or element of the set. Examples include the set of all computers in the world, the set of all apples on a tree, and the set of all irrational numbers between 0 and 1.
A set can have any non-negative quantity of elements, ranging from none (the empty set or null set) to infinitely many. The number of elements in a set is called the cardinality, and can range from zero to denumerably infinite (for the sets of natural numbers, integers, or rational numbers) to non-denumerably infinite for the sets of irrational numbers, real numbers, imaginary numbers, or complex numbers). The most basic relations in set theory can be summarized as follows. L
A set S1 is a subset of set S if and only if every element of S1 is also an element of S.
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A set S1 is a proper subset of set S if and only if every element of S1 is also an element of S, but there are some elements in S that are not elements of S1.
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The intersection of two sets S and T is the set X of all elements x such that x is in S and x is in T.
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The union of two sets S and T is the set Y of all elements y such that y is in S or y is in T, or both.
Relationships between and among sets can be illustrated by means of a special type of drawing called a Venn diagram. The table below denotes common set symbology. Set theory is fundamental to all of mathematics. In its ‘‘pure’’ form, set theory can be esoteric and even bizarre, and is primarily of interest to academics. However, set theory is closely connected with symbolic logic, and these fields are
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For Internet applications, each session is related to a particular port, a number that is associated with a particular upper layer application. For example, the HTTP program or daemon always has port number 80. The port numbers associated with the main Internet applications are referred to as well-known port numbers. Most port numbers, however, are available for dynamic assignment to other applications.
When the elements of a set can be listed or denumerated, it is customary to enclose the list in curly brackets. Thus, for example, we might speak of the set (call it K) of all natural numbers between, and including, 5 and 10 as:
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set theory See ‘‘set’’
SETI See ‘‘Search for Extraterrestrial Intelligence’’
set-top box A set-top box is a device that enables a television set to become a user interface to the Internet and also enables a television set to receive and decode digital television (DTV) broadcasts. DTV set-top boxes are sometimes called receivers. A set-top box is necessary to television viewers who wish to use their current analog television sets to receive digital broadcasts. It is estimated that 35 million homes will use digital set-top boxes by the end of 2006, the estimated year ending the transition to DTV. In the Internet realm, a set-top box is really a specialized computer that can ‘‘talk to’’ the Internet—that is, it contains a Web browser (which is really a Hypertext Transfer Protocol client) and the Internet’s main program, TCP/IP. The service to which the set-top box is attached may be through a telephone line as, for example, with WebTV, or through a cable TV company like TCI. In the DTV realm, a typical digital set-top box contains one or more microprocessors for running the operating system, possibly Linux or Windows CE, and for parsing the MPEG transport stream. A set-top box also includes random access memory, an MPEG decoder chip, and more chips for audio decoding and processing. The contents of a set-top box depend on the DTV standard used. European
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DVB-compliant set-top boxes contain parts to decode COFDM transmissions while ATSC-compliant set-top boxes contain parts to decode VSB transmissions. More sophisticated set-top boxes contain a hard drive for storing recorded television broadcasts, for downloaded software, and for other applications provided by your DTV service provider.
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Digital television set-top boxes are used for satellite, cable, and terrestrial DTV services. They are especially important for terrestrial services because they guarantee viewers free television broadcasting. A set-top box price ranges from $100 for basic features to over $1,000 for a more sophisticated box. It is often leased as part of signing up for a service.
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SGFXML See ‘‘Structured Graph Format’’
SGML SGML (Standard Generalized Markup Language) is a standard for how to specify a document markup language or tag set. Such a specification is itself a document type definition (DTD). SGML is not in itself a document language, but a description of how to specify one. It is metadata. SGML is based on the idea that documents have structural and other semantic elements that can be described without reference to how such elements should be displayed. The actual display of such a document may vary, depending on the output medium and style preferences. Some advantages of documents based on SGML are: L
They can be created by thinking in terms of document structure rather than appearance characteristics (which may change over time).
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Documents originally intended for the print medium can easily be re-adapted for other media, such as the computer display screen.
See ‘‘small form factor’’
SFP See ‘‘small form-factor pluggable’’
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SFT III is a feature providing fault-tolerance in Intel-based PC network server running Novell’s NetWare operating system. SFT III allows two servers to mirror each other so that one server is always available in case the other one fails. (Novell doesn’t say whether SFT is an abbreviation for something. However, ‘‘servers with fault-tolerance’’ seems to fit.) With SFT III, two servers, which can be in different geographical locations up to 40 kilometers apart, are connected by a high-speed data link through network interface cards (NIC). The relationship is one of peers. Either server can be taken offline for maintenance and upgrading while the other continues to run. Up to 1,000 users can be served. Another fault-tolerant feature is that a redundant backup data link can be added. Novell calls the data link a Mirrored Server Link (MSL). A group of administrator can be notified if any system element fails. The NetWare Management System (NMS) provides a graphical map showing the SFT III servers and sends alert messages to a system console in the event of a failure.
SGF See ‘‘Structured Graph Format’’
The language that a Web browser uses, Hypertext Markup Language (HTML), is an example of an SGML-based language. There is a document type definition for HTML (and reading the HTML specification is effectively reading an expanded version of the document type definition). SGML is based somewhat on earlier generalized markup languages developed at IBM, including General Markup Language (GML) and ISIL.
SGRAM See ‘‘Synchronous Graphics RAM’’
shadow password file In the Linux operating system, a shadow password file is a system file in which encryption user passwords are stored so that they aren’t available to people who try to break into the system. Ordinarily, user information, including passwords, is kept in a system file called /etc/passwd. The password for each user is stored in an encrypted form (some would call it an encoded form since it isn’t really encrypted by the usual algorithm) that is created and used as follows:
shareware 1. The original password is encrypted (or encoded) by using a randomly-generated value or encryption key between 1 and 4096 and a one-way hashing function to arrive at the encoded password that is actually stored. Note that the stored result is not something that you can enter as a password itself. 2. The key (referred to as the salt) is stored with the encoded password. Note the key itself can’t be used to decode the encrypted/encoded password because the encoding is one-way. You can’t decode the result back into the original password by using the key. 3. When someone enters a password, their password is then rehashed with the salt value and compared with the encoded password value. If they match, the user is given access to the system. In spite of encoding the password with a randomlygenerated one-way hash function, a cracker could still break the system if they got access to the /etc/passwd file. Using an approach known as the dictionary attack, a cracker could methodically test each encoded password in the file against their dictionary of commonly-used passwords, each encoded 4096 different ways (to cover all the hash possibilities). Assuming that the system was lax in its password creation requirements and some user used one of the many commonly-used passwords, at least one password could be discovered. In Linux, this possibility can be foreclosed by simply moving the passwords in the /etc/passwd file to another file, usually named /etc/shadow and making this file readable only by those who have access to the system root directory. Using a shadow password file requires that the Linux system installer also install the optional Shadow Suite, which, like Linux, is open source software and available from a number of sites on the Web.
shared hosting Shared hosting is Web hosting in which the service provider serves pages for multiple Web sites, each having its own Internet domain name, from a single Web server. Most Web hosting companies provide shared hosting. Although shared hosting is a less expensive way for businesses to create a Web presence, it is usually not sufficient for Web sites with high traffic. These sites need a dedicated Web server, either provided by a Web hosting service or maintained in-house.
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In computer programming, shared memory is a method by which program processes can exchange data more quickly than by reading and writing using the regular operating system services. For example, a client process may have data to pass to a server process that the server process is to modify and return to the client. Ordinarily, this would require the client writing to an output file (using the buffer of the operating system) and the server then reading that file as input from the buffer to its own work space. Using a designated area of shared memory, the data can be made directly accessible to both processes without having to use the system services. To put the data in shared memory, the client gets access to shared memory after checking a semaphore value, writes the data, and then resets the semaphore to signal to the server (which periodically checks shared memory for possible input) that data is waiting. In turn, the server process writes data back to the shared memory area, using the semaphore to indicate that data is ready to be read. Other forms of interprocess communication (IPC) include message queueing, semaphore, and sockets.
shareware
Shadow RAM is a copy of basic input/output operating system (BIOS) routines from read-only memory (ROM) into a special area of random access memory (RAM) so that they can be accessed more quickly. Access in shadow RAM is typically in the 60-100 nanosecond range whereas ROM access is in the 125-250 ns range. In some operating systems such as DOS, certain BIOS routines are not only used during the boot or startup of the system, but also during normal operation, especially to drive the video display terminal. In Windows and OS/2, however, these routines are not used and the use of shadow RAM is not necessary. In some systems, the user can turn the use of shadow RAM off or on.
Shareware is software that is distributed free on a trial basis with the understanding that the user may need or want to pay for it later. Some software developers offer a shareware version of their program with a built-in expiration date (after 30 days, the user can no longer get access to the program). Other shareware (sometimes called liteware) is offered with certain capabilities disabled as an enticement to buy the complete version of the program.
See ‘‘Claude Shannon’’
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Freeware is programming that is offered at no cost. However, it is copyrighted so that you can’t incorporate its programming into anything you may be developing. The least restrictive ‘‘no-cost’’ programs are uncopyrighted programs in the public domain software. These include a number of small UNIX programs. When reusing public domain software in your own programs, it’s good to know the history of the program so that you can be sure it really is in the public domain.
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Among UNIX shell (user interface) users, a shebang is a term for the ‘‘#!’’ characters that must begin the first line of a script. In musical notation, a ‘‘#’’ is called a sharp and an exclamation point—‘‘!’’—is sometimes referred to as a bang. Thus, shebang becomes a shortening of sharp-bang. The term is mentioned in Elizabeth Castro’s Perl and CGI for the World Wide Web.
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Twisted pair is now frequently installed with two pairs to the home, with the extra pair making it possible for you to add another line (perhaps for modem use) when you need it. Twisted pair comes with each pair uniquely color-coded when it is packaged in multiple pairs. Different uses such as analog, digital, and Ethernet require different pair multiples. Although twisted pair is often associated with home use, a higher grade of twisted pair is often used for horizontal wiring in LAN installations because it is less expensive than coaxial cable.
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A shell script is a text file that contains a sequence of commands for a UNIX-based operating system. It’s called a shell script because it combines into a ‘‘script’’ in a single file a sequence of commands that would otherwise have to be presented to the system from a keyboard one at a time. The shell is the operating system’s command interpreter and the set of commands you use to communicate with the system. A shell script is usually created for command sequences for which a user has a repeated need. You initiate the sequence of commands in the shell script by simply entering the name of the shell script on a command line.
The wire you buy at a local hardware store for extensions from your phone or computer modem to a wall jack is not twisted pair. It is a side-by-side wire known as silver satin. The wall jack can have as many five kinds of hole arrangements or pinouts, depending on the kinds of wire the installation expects will be plugged in (for example, digital, analog, or LAN). (That’s why you may sometimes find when you carry your notebook computer to another location that the wall jack connections won’t match your plug.)
In the DOS operating system, a shell script is called a batch file. In IBM’s mainframe VM operating systems, it’s called an EXEC.
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As the outer layer of an operating system, a shell can be contrasted with the kernel, the operating system’s inmost layer or core of services.
Shimmer is the code name for the beta test version of the Lotus iNotes Web Access client. Shimmer is designed to allow users to access their Domino-based mail, calendar, schedule, to-do lists, contact lists, and notebooks from any computer, at any location, as long as they have an Internet connection. Shimmer is being promoted for its ability to allow users to interact with Web-enabled Domino applications by using a feature called ‘‘quicklinks,’’ as well as for its ability to perform ‘‘data islanding,’’ a feature that allows users to download ‘‘islands’’ (segments) of data at one time and minimize server contacts. Lotus iNotes Web Access is integrated with Sametime, Lotus’s instant messaging, and offers offline support through Domino OffLine Services (DOLS).
shielded twisted pair
Shockwave
Shielded twisted pair is a special kind of copper telephone wiring used in some business installations. An outer covering or shield is added to the ordinary twisted pair telephone wires; the shield functions as a ground.
Shockwave, developed by Macromedia, is a family of multimedia players. Web users with Windows and Mac platforms can download the Shockwave players from the Macromedia site and use it to display and hear Shockwave files. Shockwave is especially popular for interactive games. However, Macromedia has identified over 2,000 sites that offer Shockwave files, including sites for General Motors, Nissan, Kodak, Microsoft, Intel and Apple. To create Shockwave files, you use Macromedia Director and several related programs.
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Shell is a UNIX term for the interactive user interface with an operating system. The shell is the layer of programming that understands and executes the commands a user enters. In some systems, the shell is called a command interpreter. A shell usually implies an interface with a command syntax (think of the DOS operating system and its ‘‘C:>’’ prompts and user commands such as ‘‘dir’’ and ‘‘edit’’).
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Twisted pair is the ordinary copper wire that connects home and many business computers to the telephone company. To reduce crosstalk or electromagnetic induction between pairs of wires, two insulated copper wires are twisted around each other. Each signal on twisted pair requires both wires. Since some telephone sets or desktop locations require multiple connections, twisted pair is sometimes installed in two or more pairs, all within a single cable. Shielded twisted pair is often used in business installations. The more common kind of wire that is installed to your home is unshielded twisted pair.
shopping cart software See ‘‘shopping cart’’
S-HTTP
shopping cart On a Web site that sells products or services online, the shopping cart is a common metaphor (from the original grocery store shopping cart) for the catalog or other pages where a user reads and makes selections. Typically, the user checks off any products or services that are being ordered and then, when finished ordering, indicates that and proceeds to a page where the total order is placed and confirmed. The programming that provides a Web site with the ability to build a catalog and its associated database and to integrate pages into its site that provide users the ability to shop is known as shopping cart software. A primary consideration when choosing shopping cart software is whether it will continue to serve a Web site’s needs as its catalog and volume of orders grow.
Short Message Service SMS (Short Message Service) is a service for sending messages of up to 160 characters to mobile phones that use Global System for Mobile (GSM) communication. GSM and SMS service is primarily available in Europe. SMS is similar to paging. However, SMS messages do not require the mobile phone to be active and within range and will be held for a number of days until the phone is active and within range. SMS messages are transmitted within the same cell or to anyone with roaming service capability. They can also be sent to digital phones from a Web site equipped with PC Link or from one digital phone to another. Typical uses of SMS include: L
Notifying a mobile phone owner of a voicemail message
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Notifying a salesperson of an inquiry and contact to call
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Notifying a doctor of a patient with an emergency problem
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Notifying a service person of the time and place of their next call
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Notifying a driver of the address of the next pickup
An SMS gateway is a Web site that lets you enter an SMS message to someone within the cell served by that gateway or that acts as an international gateway for users with roaming capability.
shortcut In general, a shortcut is a path that is shorter than the usual or formal path to something or a method of operation that saves time over the regular operation. In Windows 95 and later operating systems, a shortcut is a computer desktop icon that enables a user to easily see and select a particular program or data object. The operating system comes with some shortcuts already visible on the desktop. A user can remove these or add new ones.
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To create a shortcut in Windows 95 or 98, click on My Computer, then click on your ‘‘C’’ drive (or other drive where your files may be). Find the folder or file that you want to put on your desktop, click on it with the right mouse button, and select Create Shortcut from the pop-up menu. A new icon will now appear in My Computer. Hold the left mouse button down and drag the new icon to your desktop (you may need to minimize the My Computer window to do this). Now your new file or program will always be visible for quick selection on your desktop without having to go through Start menus or the Windows file manager.
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In the Macintosh operating system, a shortcut is called an ‘‘alias.’’
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Shovelware is content taken from any source and put on the Web as fast as possible with little regard for appearance and usability.
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A Web file with the suffix of ‘‘.shtml’’ (rather than the usual ‘‘.htm’’) indicates a file that includes some information that will be added ‘‘on the fly’’ by the server before it is sent to you. A typical use is to include a ‘‘Last modified’’ date at the bottom of the page. This Hypertext Transfer Protocol facility is referred to as a server-side include. (Although rarely done, the server administrator can identify some other file name suffix than ‘‘.shtml’’ as a server-side include file.) You can think of a server-side include as a limited form of common gateway interface application. In fact, the CGI is not used. The server simply searches the server-side include file for CGI environment variables, and inserts the variable information in the places in the file where the ‘‘include’’ statements have been inserted. When creating a Web site, a good idea is to ask your server administrator which environment variables can be used and whether the administrator can arrange to set the server up so that these can be handled. Your server administrator should usually be able to help you insert the necessary include statements in an HTML file.
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S-HTTP
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Secure HTTP (S-HTTP) is an extension to the Hypertext Transfer Protocol (Hypertext Transfer Protocol) that allows the secure exchange of files on the World Wide Web. Each SHTTP file is encrypted and/or contains a digital certificate. For a given document, S-HTTP is an option that is additional to another well-known security protocol, Secure Sockets Layer (SSL). A major difference is that S-HTTP allows the client to send a certificate to authenticate the user whereas, using SSL, only the server can be authenticated. S-HTTP is
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more likely to be used in situations where the server represents a bank and requires authentication from the user that is more secure than a userid and password. S-HTTP does not use any single encryption system, but it does support the Rivest-Shamir-Adleman public-andprivate key encryption system. SSL works at a program layer slightly higher than the Transmission Control Protocol (TCP) level. S-HTTP works at the higher level of the HTTP application. Both security protocols can be used by a browser user, but only one can be used with a given document. Terisa Systems includes both SSL and S-HTTP in their Internet security tool kits. S-HTTP has been submitted to the Internet Engineering Task Force (IETF) for consideration as a standard. Request for Comments (Request for Comments) Internet draft 2660 describes S-HTTP in detail.
SI
In AC and RF circuits, conductive siemens behave the same as they do in DC circuits, provided the root-mean-square (rms) AC voltage is specified. In AC and RF circuits, susceptance exists only when there is a net capacitance or inductance. Capacitive susceptances have positive imaginary number values; inductive susceptances have negative imaginary-number values. The susceptance of a particular capacitor or inductor depends on the frequency. Conductances and susceptances are sometimes expressed in units representing power-of-10 multiples or fractions of 1 S. A kilosiemens (1 kS) is equal to one thousand (103) siemens. A megasiemens (1 MS) is equal to one million (106) siemens. A millisiemens (1 mS) is equal to one-thousandth (10-3) of a siemens. A microsiemens (1 mS) is equal to one-millionth (10-6) of a siemens. Also see conductance, Ohm’s Law, prefix multipliers, resistance, reactance, ohm, and International System of Units (SI).
See ‘‘International System of Units’’
Signal Computing System Architecture
sideband
Signal Computing System Architecture (SCSA) is an industry standard architectural framework for the hardware and software components in a computer-telephony integration system. The framework has three layers:
In electronic signal transmission, a sideband is the portion of a modulated carrier wave that is either above or below the basic (baseband) signal. The portion above the baseband signal is the upper sideband; the portion below is the lower sideband. In regular amplitude modulation (AM) transmission, both sidebands are used to carry a message. In some forms of tranmission, one sideband is removed (single-sideband transmission) or a portion of one sideband is removed.
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Applications, which control the media portion of a call (play, record, voice recognition, and other applications) and the call flow.
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The Software Model, which provides programming interfaces for manipulating the media portion of a call. This portion of the framework includes the SCSA Telephony Applications Objects (TAO) Framework. It also includes the call control framework and possible interfaces such as Telephony Application Program Interface, TSAPI, and JTAPI.
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The Hardware Model, consisting of one of two bus protocols, ANSI/VITA SCBus or ECTF H.100 CT Bus.
siemens (Siemens AG is a German company engaged in electrical engineering and electronics.) The siemens (symbolized S) is the International System of Units (SI) unit of electrical conductance. The archaic term for this unit is the mho (ohm spelled backwards). Siemens are also used, when multiplied by imaginary numbers, to denote susceptance in alternating current (AC) and radio frequency (RF) applications. Reduced to base SI units, 1 S is the equivalent of one second cubed ampere squared per kilogram per meter squared (1 s3 ˙ A2 ˙ kg-1 ˙ m-2). The siemens is also the equivalent of an ampere per volt (A/V). In a direct current (DC) circuit, a component has a conductance of 1 S when a potential difference of one volt (1 V) produces a current of one ampere (1 A) through the component. Thus, one siemens is the equivalent of one ohm. But mathematically, resistance and conductance are reciprocals of each other. If R is the resistance of a component in ohms and G is the conductance in siemens, R=1/G and
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Both the Hardware Model and the Software Model layers can interface with intelligent switch, such as those used in the Advanced Intelligent Network (AIN), and with the Public Switched Telephone Network (PSTN) or the Internet.
shrinkwrapped software Shrinkwrapped software is software that is packaged with a terms-and-conditions statement, ready for installation by the purchaser. Shrinkwrapped software may be sold on retail shelves, through the mail, or by direct selling; the significant point is that (1) it is generic rather than custom software you need to develop yourself or have someone develop for you, and (2) it is designed so that the purchaser can install it with relative ease rather than is depending on on-site assistance or extensive phone communication. The term refers to the plastic covering in which the box and the electronic storage media are enclosed and sealed.
Signaling System 7 The term is sometimes used metaphorically—that is, the software may not actually be shrinkwrapped, but it is sold as an independent, separate product and is designed for purchaser installation. Much of this kind of software is sold over the Web and received by downloading it electronically. Some shrinkwrapped software is usually preinstalled when you buy a computer. Almost all personal computer software and most small-business software is shrinkwrapped software. Shrinkwrapped software is often licensed so that multiple copies can be made and redistributed in enterprises.
band signaling, meaning that signaling (control) information travels on a separate, dedicated 56 or 64 Kbps channel rather than within the same channel as the telephone call. Historically, the signaling for a telephone call has used the same voice circuit that the telephone call traveled on (this is known as in-band signaling). Using SS7, telephone calls can be set up more efficiently and with greater security. Special services such as call forwarding and wireless roaming service are easier to add and manage. SS7 is now an international telecommunications standard.
signal
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Setting up and managing the connection for a call
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Tearing down the connection when the call is complete
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Billing
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Managing call forwarding, calling party name and number display, three-way calling, and other Intelligent Network (IN) services
1) In electronics, a signal is an electric current or electromagnetic field used to convey data from one place to another. The simplest form of signal is a direct current (DC) that is switched on and off; this is the principle by which the early telegraph worked. More complex signals consist of an alternating-current (AC) or electromagnetic carrier that contains one or more data streams. Data is superimposed on a carrier current or wave by means of a process called modulation. Signal modulation can be done in either of two main ways: analog and digital. In recent years, digital modulation has been getting more common, while analog modulation methods have been used less and less. There are still plenty of analog signals around, however, and they will probably never become totally extinct. Except for DC signals such as telegraph and baseband, all signal carriers have a definable frequency or frequencies. Signals also have a property called wavelength, which is inversely proportional to the frequency.
SS7 is used for these and other services:
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Toll-free (800 and 888) and toll (900) calls
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Wireless as well as wireline call service including mobile telephone subscriber authentication, personal communication service (PCS), and roaming
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SS7 messages contain such information as: How should I route a call to 914 331-4985?
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The route to network point 587 is crowded. Use this route only for calls of priority 2 or higher.
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Subscriber so-and-so is a valid wireless subscriber. Continue with setting up the call.
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Because control signals travel in a separate network from the call itself, it is more difficult for anyone to violate the security of the system. (See 2600 and phreak for cracking techniques that are defeated by SS7.)
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3) In telephony, a signal is special data that is used to set up or control communication. See signaling.
The Integrated Services Digital Network (ISDN) also uses out-of-band signaling, extending it all the way to the end user on the ISDN D-channel while voice and data flow on B channels.
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signaling
Briefly How It Works
In telephony, signaling is the exchange of information between involved points in the network that sets up, controls, and terminates each telephone call. In in-band signaling, the signaling is on the same channel as the telephone call. In out-of-band signaling, signaling is on separate channels dedicated for the purpose.
SS7 consists of a set of reserved or dedicated channels known as signaling links and the network points that they interconnect. There are three kinds of network points (which are called signaling points): Service Switching Points (SSPs), Signal Transfer Points (STPs), and Service Control Points (SCPs). SSPs originate or terminate a call and communicate on the SS7 network with SCPs to determine how to route a call or set up and manage some special feature. Traffic on the SS7 network is routed by packet switches called STPs. SCPs and STPs are usually mated so that service can continue if one network point fails.
2) In some information technology contexts, a signal is simply ‘‘that which is sent or received,’’ thus including both the carrier (see 1) and the data together.
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Signaling System 7 On the public switched telephone network (PSTN), Signaling System 7 (SS7) is a system that puts the information required to set up and manage telephone calls in a separate network rather than within the same network that the telephone call is made on. Signaling information is in the form of digital packets. SS7 uses what is called out-of-
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signal-to-noise ratio
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In analog and digital communications, signal-to-noise ratio, often written S/N or SNR, is a measure of signal strength relative to background noise. The ratio is usually measured in decibels (dB).
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If the incoming signal strength in microvolts is Vs, and the noise level, also in microvolts, is Vn, then the signal-to-noise ratio, S/N, in decibels is given by the formula S/N = 20 log10(Vs/Vn) If Vs = Vn, then S/N = 0. In this situation, the signal borders on unreadable, because the noise level severely competes with it. In digital communications, this will probably cause a reduction in data speed because of frequent errors that require the source (transmitting) computer or terminal to resend some packets of data. Ideally, Vs is greater than Vn, so S/N is positive. As an example, suppose that Vs = 10.0 microvolts and Vn = 1.00 microvolt. Then S/N = 20 log10(10.0) = 20.0 dB
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which results in the signal being clearly readable. If the signal is much weaker but still above the noise—say 1.30 microvolts—then S/N = 20 log10(1.30) = 2.28 dB
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which is a marginal situation. There might be some reduction in data speed under these conditions.
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If Vs is less than Vn, then S/N is negative. In this type of situation, reliable communication is generally not possible unless steps are taken to increase the signal level and/or decrease the noise level at the destination (receiving) computer or terminal.
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Communications engineers always strive to maximize the S/ N ratio. Traditionally, this has been done by using the narrowest possible receiving-system bandwidth consistent with the data speed desired. However, there are other methods. In some cases, spread spectrum techniques can improve system performance. The S/N ratio can be increased by providing the source with a higher level of signal output power if necessary. In some high-level systems such as radio telescopes, internal noise is minimized by lowering the temperature of the receiving circuitry to near absolute zero (-273 degrees Celsius or -459 degrees Fahrenheit). In wireless systems, it is always important to optimize the performance of the transmitting and receiving antennas.
signature file A signature file is a short text file you create for use as a standard appendage at the end of your e-mail notes or Usenet messages. For example, you might include your full name, occupation or position, phone number, fax number,
e-mail address, and the address of your Web site if you have one. Many people also include a favorite quote, company motto, or short personal statement. Most e-mail and Usenet news facilities make it possible for you to either create the signature file as part of the application or to specify another file you’ve created with a word processor. Then, you tell the facility (usually in an ‘‘Options’’ menu) the name of your signature file and it automatically adds it to the note or message template it provides you.
significant figures The term significant figures refers to the number of important single digits (0 through 9 inclusive) in the coefficient of an expression in scientific notation. The number of significant figures in an expression indicates the confidence or precision with which an engineer or scientist states a quantity. The table shows several examples of numbers written in standard decimal notation (first column) and in scientific notation (second column). The third column shows the number of signficant figures in the corresponding expression in the second column. Decimal expression
Scientific notation
Sig. figs.
1,222,000.00
1.222 6 106 1.22200000 6 106 3.45 6 10-5 3.450000 6 10-5 79.87654 6 109 79.876543210 6 109 71 6 10-8 71.00 6 10-8
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0.00003450000 79,876,543,210 70.0000000100
9 3 7 6
10 1 3
Significant figures are arrived at by rounding off an expression after a calculation is executed. In any calculation, the number of significant figures in the solution must be equal to, or less than, the number of significant figures in the least precise expression or element. Consider the following product: 2.56 6 1067 6 78.33 6 10-54 To obtain the product of these two numbers, the coefficients are multiplied, and the powers of 10 are added. This produces the following result: 2.56 6 (78.33) 6 1067+(-54) = 2.56 6 (78.33) 6 1067-54 = 721.3248 6 1013 The proper form of common scientific notation requires that the absolute value of the coefficient be larger than 1 and less than 10. Thus, the coefficient in the above expression should be divided by 10 and the power of 10 increased by one, giving: 72.13248 6 1014
SIMD Because both multiplicands in the original product are specified to only three significant figures, a scientist or engineer will round off the final expression to three significant figures as well, yielding: 72.13 6 1014 as the product.
within a few years. Examples include not only smart phones, personal digital assistants (PDAs), and other handheld devices but also ‘‘smart’’ home appliances, computerized clothing, and other less visible sensors. Such devices may communicate locally with Bluetooth RF or with infrared wireless, or at some point be plugged into longer-range wireless or wired networks.
Also see order of magnitude and scientific notation.
silence suppression See ‘‘voice activation detection’’
silicon Silicon is a chemical element (its symbol in chemical formula expressions is ‘‘Si’’) that is present in sand and glass and which is the best known semiconductor material in electronic components. Its atomic number is 14. The most common isotope has atomic weight 28. In its pure state, silicon is a metal-like substance with an appearance and heft somewhat resembling aluminum. In its natural state, silicon appears bound up with other elements in the form of compounds. It is abundant in the crust of the earth. Silicon conducts electricity to an extent that depends on the extent to which impurities are added. The addition of impurities to silicon, or to any semiconductor material, is called doping. Some impurities produce N-type silicon, in which the majority charge carriers are negatively charged electrons. Other impurities result in the production of P-type silicon, in which the majority charge carriers are positively charged holes. Most silicon devices contain both N-type and P-type material. Silicon is used in a wide variety of electronic and computer components, including: L
Diodes
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Light-emitting diodes(LED)
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Power supplies
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Photodetectors
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Field-effect transistors
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Integrated circuits
silicon cockroach Silicon cockroach is a term invented by networking expert John Sidgmore to describe the tiny portable electronic devices that are expected to become popular in the next few years, creating new behavior patterns while putting new demands on network bandwidth capacity. Sidgmore’s pervasive cockroaches are expected to multiply and become a significant driver of Internet growth with the average person carrying as many as five separate devices at a time
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Silicon-On-Insulator (SOI) is a semiconductor fabrication technique developed by IBM that uses pure crystal silicon and silicon oxide for integrated circuits (ICs) and microchips. An SOI microchip processing speed is often 30% faster than today’s complementary metal-oxide semiconductor (CMOS)-based chips and power consumption is reduced 80%, which makes them ideal for mobile devices. SOI chips also reduce the soft error rate, which is data corruption caused by cosmic rays and natural radioactive background signals.
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A CMOS-based chip has impurities added to it, a process called ‘‘doping,’’ that allows the chip to store an electrical charge called capacitance. In order to control the electrical currents needed, the capacitance must be discharged and recharged, which takes time and causes the transistors on the chip to heat up. This production of heat limits the speed at which microchips can operate. For this reason, microchips have poor yield rates above 1 GHz and are not expected to attain future speeds above 5 GHz. SOI microchips are not doped with impurities, which eliminates much of the capacitance and allows an SOI microchip to operate faster and cooler. When IBM began developing SOI chips, they found it difficult to bond the insulator layer of the chip, which was a non-crystal substance, to the pure crystal silicon layer. The SOI technology team tried using several different insulating materials, and found the most promising was sapphire; however, sapphire deteriorated easily. Then they discovered Separation by Implantation of Oxygen (SIMOX). SIMOX involves the direct injection of purified oxygen into the silicon wafer at an extremely high temperature. The oxygen bonds with the silicon and forms thin layers of silicon oxide. This layer of silicon oxide film is perfect enough that it bonds with the pure crystal silicon layer. Currently, SOI chips are used in IBM’s AS/400 iSeries servers. The production of SOI chips requires minor restructuring of current fabrication methods and facilities allowing microchip manufacturers such as Intel or AMD to produce SOI chips with little extra cost. IBM expects SOI chips to replace current CMOS-based chips in consumeroriented devices beginning in 2002.
SIMD
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Simple API for XML
simplicity
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See ‘‘SAX’’
In information technology, simplicity is a quality that is frequently sought by both users and technologists, although, as users frequently attest, it is not always found. In programming, an elegant solution is sometimes one that addresses a need with surprising simplicity and efficiency. In the creation of the user interface to information technology, simplicity is usually one of several important objectives.
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Simple Mail Transfer Protocol See ‘‘SMTP’’
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Simple Network Management Protocol
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Simple Network Management Protocol (SNMP) is the protocol governing network management and the monitoring of network devices and their functions. It is not necessarily limited to TCP/IP networks.
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SNMP is described formally in the Internet Engineering Task Force (IETF) Request for Comment (RFC) 1157 and in a number of other related RFCs.
Simple Object Access Protocol Simple Object Access Protocol (SOAP) is a way for a program running in one kind of operating system (such as Windows 2000) to communicate with a progam in the same or another kind of an operating system (such as Linux) by using the World Wide Web’s Hypertext Transfer Protocol (HTTP) and its Extensible Markup Language (XML) as the mechanisms for information exchange. Since Web protocols are installed and available for use by all major operating system platforms, HTTP and XML provide an already athand solution to the problem of how programs running under different operating systems in a network can communicate with each other. SOAP specifies exactly how to encode an HTTP header and an XML file so that a program in one computer can call a program in another computer and pass it information. It also specifies how the called program can return a response. SOAP was developed by Microsoft, DevelopMentor, and Userland Software and has been proposed as a standard interface to the Internet Engineering Task Force (IETF). It is somewhat similar to the Internet Inter-ORB Protocol (IIOP), a protocol that is part of the Common Object Request Broker Architecture (CORBA). Sun Microsystems’ Remote Method Invocation (RMI) is a similar client/server interprogram protocol between programs written in Java. An advantage of SOAP is that program calls are much more likely to get through firewall servers that screen out requests other than those for known applications (through the designated port mechanism). Since HTTP requests are usually allowed through firewalls, programs using SOAP to communicate can be sure that they can communicate with programs anywhere.
simplex V,W
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See ‘‘duplex’’
The desirability of simplicity is sometimes expressed as the KISS Principle.
Simula Simula, short for ‘‘simulation language,’’ was the first object-oriented programming language. In the late 1960s, the Norwegian developers of Simula, O. J. Dahl and Kristen Nygaard, were looking for a way for programs to simulate or model the world as computer users see it. They conceived the idea that programmers should model programs around objects rather than procedures. They also invented the idea of classes so that objects could be developed that shared similar characteristics. So that objects could communicate and make requests of each other, they conceived the simple notion of the message. Simula led in turn to Smalltalk, C++, and Java.
simultaneous peripheral operations online See ‘‘spool’’
sine wave See ‘‘waveform’’
single in-line memory module A SIMM is a module containing one or several random access memory (RAM) chips on a small circuit board with pins that connect to the computer motherboard. Since the more RAM your computer has, the less frequently it will need to access your secondary storage (for example, hard disk or CD-ROM), PC owners sometimes expand RAM by installing additional SIMMs. SIMMs typically come with a 32 data bit (36 bits counting parity bits) path to the computer that requires a 72-pin connector. SIMMs usually come in memory chip multiples of four megabytes. The memory chips on a SIMM are typically dynamic RAM (DRAM) chips. An improved form of RAM called Synchronous DRAM (SDRAM) can also be used. Since SDRAM provides a 64 data bit path, it requires at least two SIMMs or a dual in-line memory module (DIMM).
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single instruction multiple data As one of the three performance enhancements of MMX, single instruction multiple data (SIMD) lets one microinstruction operate at the same time on multiple data items. This is especially productive for applications in which visual images or audio files are processed. What usually requires a repeated succession of instructions (a loop) can now be performed in one instruction. Intel offers the analogy of a drill sergeant issuing the order ‘‘About face’’ to an entire platoon rather than to each soldier, one at a time.
single mode fiber In optical fiber technology, single mode fiber is optical fiber that is designed for the transmission of a single ray or mode of light as a carrier and is used for long-distance signal transmission. For short distances, multimode fiber is used. Single mode fiber has a much smaller core than multimode fiber.
single signon 1) In any client/server relationship, single signon (pronounced SING-uhl SAIN-awn) is a session/user authentication process that permits a user to enter one name and password in order to access multiple applications. The single signon, which is requested at the initiation of the session, authenticates the user to access all the applications they have been given the rights to on the server, and eliminates future authentication prompts when the user switches applications during that particular session. 2) In e-commerce, the single signon (sometimes referred to as SSO) is designed to centralize consumer financial information on one server—not only for the consumer’s convenience, but also to offer increased security by limiting the number of times the consumer enters credit card numbers or other sensitive information used in billing. Microsoft’s ‘‘Passport’’ single signon service (averaging over 40 million consumers and more than 400 authentications per second) is an example of a growing trend towards the use of Web-based single signons that allow users to register financial information once, shop at multiple Web sites, and feel more confident about security on the Web.
single-electron memory See also phase-state low electron drive memory. Single-electron memory (SEM) refers to experimental hardware technologies in which the position or presence of a single charge carrier, usually an electron, makes the difference between the logical low (0) and high (1) states in a digital system. Primitive forms of SEM have been explored, and some engineers believe it is only a matter of time before it is put into use at the consumer level. When this happens, computer memory chips will have far greater capacity than
anything known today. There are various ways in which SEM might work. Two hypothetical schemes are outlined here: SEM might be used to detect the presence or absence of electrical charges in individual atoms. If the number of electrons in an atom is the same as the number of protons (the normal or usual case), that atom has no electrical charge. If there are fewer electrons than protons, the atom acquires a positive charge; if there are more electrons than protons, the atom acquires a negative charge. Charged atoms are known as ions. The charge (or absence thereof) can be detected, and electronic circuits designed to act in different ways depending on whether a given atom is negative, neutral, or positive. In binary logic, a neutral or negative ion might represent a 0 (low) bit, and a positive ion might represent 1 (high). Ions can also represent the states of trinary logic. A negative ion can represent logic 71; a neutral ion can represent logic 0; a positive ion can represent logic +1. The contents of an ionic SEM chip could be changed by means of precisely directed electrical currents. Another way in which SEM might operate involves the relative energy states of atoms depending on the positions, or orbits, of electrons within. Every atom has several different energy levels, called shells, at which electrons can exist. The shells resemble spheres surrounding the nucleus. The larger the diameter of the shell, the greater the energy contained in an electron orbiting in that shell. Low-energy shells in each atom of a given element might be assigned logic 0; high-energy shells could be assigned logic 1. An electron changes shells when a photon of a certain energy level (represented by a pulse of energy at a specific wavelength) is emitted or absorbed by the atom. When a photon having the correct wavelength is absorbed by the atom, the electron rises to a higher energy state; when the electron falls to a lower energy state, a photon of the same wavelength is emitted by the atom. The contents of such a memory could be changed using tiny electromagnetic pulses directed at specific atoms in a chip.
SIP
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See ‘‘Session Initiation Protocol’’
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A Web site is a collection of Web files on a particular subject that includes a beginning file called a home page. For example, most companies, organizations, or individuals that have Web sites have a single address that they give you. This is their home page address. From the home page, you can get to all the other pages on their site. For example, the Web site for IBM has the home page address of http:// www.ibm.com. (In this case, the actual file name of the home page file doesn’t have to be included because IBM has named this file index.html and told the server that this address really means http://www.ibm.com/index.html.)
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Whatis?Com’s Encyclopedia of Technology Terms Since it sounds like geography is involved, a Web site is rather easily confused with a Web server. A server is a computer that holds the files for one or more sites. A very large Web site may reside on a number of servers that may be in different geographic locations. IBM is a good example; its Web site consists of thousands of files spread out over many servers in world-wide locations. A synonym and less frequently used term for Web site is ‘‘Web presence.’’ That term seems to better express the idea that a site is not tied to specific geographic location, but is ‘‘somewhere in cyberspace.’’ However, ‘‘Web site’’ seems to be used much more frequently.
site map
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A site map is a visual model of a Web site’s content that allows the users to navigate through the site to find the information they are looking for, just as a traditional geographical map helps people find places they are looking for in the real world. An example of information modeling technology, a site map can be considered like an interactive table of contents, in which the listed items link directly to their counterpart sections of the Web site. Site maps perform the same service that the layout maps in large shopping malls perform: Without them, it is possible to explore a complex site by trial and error, but if you want to be sure to find what you’re looking for, the most efficient way to do that is to consult a model of the resources available. If a Web site is small and uncomplicated, a site map may be unnecessary, just as a layout map may not be required for shoppers to find their way through small shopping malls. Typically, site maps are organized hierarchically, breaking down the Web site’s information into increasingly specific subject areas. There are a number of different types of site maps: organizational chart site maps are quite similar in appearance to a traditional table of contents; others, based on a perspective view of the site, are like a three dimensional model with individual pages upright, like index cards, arranged in sections and linked by lines. Structured Graph Format (SGF) site maps use an XML format language to describe Web site content, and a Java SGF viewer to interact with the data. There are a number of companies making site mapping products; generally, these don’t require Web design skills—such as HTML or XML ability—on the part of the user. Popular site mapping products include TheBrain’s SiteBrain, Inxight Software’s Tree Studio, IBM’s Java-based Mappuccino, and Dynamic Diagram’s eponymous product. Site maps can also be created using more general Web site management tools, such as Visual Web, or Microsoft’s Site Analyst.
sitelet A sitelet is a small section of a Web site that has a special purpose and identity. In some cases, users are encouraged to link to a sitelet directly (rather than to come through the main Web site). A related term is minisite. In Web advertising campaigns, a sitelet can be created for the duration of a campaign as the page that is linked to from a banner ad. The sitelet page can develop the campaign message more fully and immediately than if the user were linked to the main corporate or product site. The term is also sometimes used for the temporary Web sites that are created for conventions, award presentations, or other events.
skin On the Internet, a skin is a graphic or audio file used to change the appearance of the user interface to a program or for a game character. Skins can usually be downloaded at no charge from sites that provide them. The skin concept can be compared to the Xpress-On covers that snap to the front of the Nokia 3210 cell phone. These come in various colors and graphic designs such as Cyber Fish and Tribal Sign. The covers alter the appearance of the phone but not the phone itself. When using a skin, the appearance of the user interface changes, but not the functions available with the program. Here are some examples: L
Macintosh enthusiasts can change the skin on their PC at work to make it look and sound like a Mac.
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Neoplanet changes the appearance of Microsoft’s Internet Explorer browser.
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WindowBlinds from StarDock Systems changes the titlebar, the border, the buttons, the taskbar, and other aspects of the Windows 95/98/NT desktop interface.
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Winamp is a media player for Windows 95, 98 and NT for which you can select from over 3,000 skins. A Winamp skin changes the appearance of the cover, the antenna, the display, the fonts, the number pad, the beeping tones, and other aspects.
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Games such as Quake, Hexen, and Heretic use skins to change the appearance of game characters. Using a skin in a game gives you an edge because you are not easily recognized.
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Skins are available for the popular chat program ICQ.
You can download skins from a number of sites that specialize in skins. They come in high-color (for systems that support more than 256 colors) and low-color (256 colors or less). Most skin sites offer templates and instructions for creating your own skin. You need a good graphics editor such as Photoshop or Paint Shop Pro and a rendering
Slashdot Effect program such as Macromedia’s Extreme 3D to create 3dimensional models. If you create a skin, you can share it by uploading it to a skin site.
skin effect Skin effect is a tendency for alternating current (AC) to flow mostly near the outer surface of a solid electrical conductor, such as metal wire, at frequencies above the audio range. The effect becomes more and more apparent as the frequency increases. The main problem with skin effect is that it increases the effective resistance of a wire for AC at moderate to high frequencies, compared with the resistance of the same wire at direct current (DC) and low AC frequencies. The effect is most pronounced in radio-frequency (RF) systems, especially antennas and transmission lines. But it can also affect the performance of high-fidelity sound equipment by causing attenuation in the treble range (the highest-pitched components of the audio). Skin effect can be reduced by using stranded rather than solid wire. This increases the effective surface area of the wire for a given wire gauge. Tinned wire should be avoided because tin has higher resistance than copper. In large RF antenna arrays, hollow tubing can be used in place of solid rods with little or no loss of efficiency; in this respect, skin effect is an asset. It also works in favor of the use of copperclad steel wire for more modest antennas. Such wire is mechanically stronger than solid or stranded copper, because steel has a higher tensile strength than copper. The skin effect causes most of the current to flow through the copper cladding, which is a better electrical conductor than steel. Skin effect occurs with brief pulses of current, for the same reasons it occurs at high AC frequencies. This can save lives. If you are caught in a thundershower, you can take refuge in a car or other metal vehicle and be relatively safe even if you suffer a direct hit. The skin effect causes virtually all of the current to flow on the outside of the vehicle as it passes from cloud to ground.
SKU SKU (stockkeeping unit, sometimes spelled ‘‘Sku’’) is an identification, usually alphanumeric, of a particular product that allows it to be tracked for inventory purposes. Typically, an SKU (pronounced with the individual letters or as SKYEW) is associated with any purchasable item in a store or catalog. For example, a woman’s blouse of a particular style and size might have an SKU of ‘‘3726-8,’’ meaning ‘‘Style 3726, size 8.’’ The SKU identification for a product may or may not be made visible to a customer. SKU numbers can sometimes be seen in online e-commerce sites. An SKU is not the same as a product model number from a manufacturer, although the model number could form all or part of the SKU. The SKU is established by the merchant.
skunkworks A skunkworks is a group of people who, in order to achieve unusual results, work on a project in a way that is outside the usual rules. A skunkworks is often a small team that assumes or is given responsibility for developing something in a short time with minimal management constraints. Typically, a skunkworks has a small number of members in order to reduce communications overhead. A skunkworks is sometimes used to spearhead a product design that thereafter will be developed according to the usual process. A skunkworks project may be secret.
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As for the term’s origin, we like what Harry Newton says in Newton’s Telecom Dictionary:
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‘‘Term for usually-secret high-pressure/high-tech research group in a company or government, often populated by people who don’t see much sunlight or soap. Hence the name, skunkworks.’’
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The name of the animal itself was derived by early American settlers from the Algonquin Indian seganku.
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Slamming is the practice by some U.S. long-distance phone carriers of switching users to their service without the user’s knowledge or authorization. Southwestern Bell, a local carrier, reports that they received calls about slamming from 558,000 customers in 1997. Slamming methods include ‘‘free trials’’ and offers for credit cards that offer prize or giveaway points for each dollar of charges from a given carrier. Phone solicitors often get ‘‘approval’’ from children, babysitters, and domestic employees. Occasionally, your carrier may be switched if your current carrier has been sold to another company. If your carrier is switched, there may be a $5-6 switching charge on your bill. Unless you agreed to the switch, you do not have to pay the charge. You can find out who your current long-distance carrier is by calling 700-555-4141, toll-free.
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Slashdot Effect The Slashdot Effect is the sudden, relatively temporary surge in traffic to a Web site that occurs when a high-traffic Web site or other source posts a story that refers visitors to another Web site. The effect gets its name from the Slashdot Web site, which provides content about Linux and related software, and sometimes features news about other related but less-traveled sites. However, the effect can be perceived when any large Web site posts a high-interest, widely-
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publicized story, about another site. The effect is obviously much more noticeable on smaller sites and the surge in traffic sometimes will slow a site down or make it impossible to reach.
knows that it can send multiple chunks of data before waiting for an acknowledgment. Transmitting multiple data packets between acknowledgments allows the data to be transferred faster and more efficiently than if an acknowledgment must be received prior to each transmission.
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Slashdot, the Web site, is named, according to Slashdot originator Jeff ‘‘Hemos’’ Bates, as ‘‘a play on how terrible it is to say domain names out loud.’’ In this case: h-t-t-p-colonslash-slash-slash-dot-dot-o-r-g
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See ‘‘second-level domain’’
See ‘‘Serial Line Internet Protocol’’
slice and dice
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To slice and dice is to break a body of information down into smaller parts or to examine it from different viewpoints so that you can understand it better. In cooking, you can slice a vegetable or other food or you can dice it (which means to break it down into small cubes). One approach to dicing is to first slice and then cut the slices up into dices. In data analysis, the term generally implies a systematic reduction of a body of data into smaller parts or views that will yield more information. The term is also used to mean the presentation of information in a variety of different and useful ways.
In computers, a slot, or expansion slot, is an engineered technique for adding capability to a computer in the form of connection pinholes (typically, in the range of 16 to 64 closely-spaced holes) and a place to fit an expansion card containing the circuitry that provides some specialized capability, such as video acceleration, sound, or disk drive control.
This term can be compared to drilldown, which is the process of dividing an information area up into finer and finer layers in a hierarchy, but with the purpose of narrowing in to one small area or item.
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You can find out more about TCP by reading Request for Comment (RFC) 793 and 1180.
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sliding windows Sliding windows, a technique also known as windowing, is used by the Internet’s Transmission Control Protocol (TCP) as a method of controlling the flow of packets between two computers or network hosts. TCP requires that all transmitted data be acknowledged by the receiving host. Sliding windows is a method by which multiple packets of data can be affirmed with a single acknowledgment. TCP includes a memory buffer that sits between the upper application layer (for example, Hypertext Transfer Protocol or File Transfer Protocol) and the lower layer that receives data (the Data Link layer in the OSI reference model). The buffer allows TCP to receive and process data independently of the upper application. In other words, TCP can use this buffer space to ‘‘read ahead’’ while the upper application processes the data at its own pace. Window ‘‘announcements’’ are sent by the receiving system to the transmitting system in order to acknowledge data receipt as well as to inform the transmitting system of the current buffer size. For example, if a window size of zero is reported, the transmitting system must wait for an acknowledgment before sending the next chunk of data. If the receiving system reports that the buffer size is larger than the size of a single data packet, the transmitting system
Almost all desktop computers come with a set of expansion slots. These help ensure that you’ll be able to add new hardware capabilities in the future.
Slot 1 and Slot 2 are names for the way Intel P6-based microprocessors connect to a computer motherboard so that it makes contact with the built-in paths called the data bus. Slot 1 and Slot 2 were developed by Intel as a replacement for its Zero Insertion Force (ZIF) sockets, the most familiar of which is Socket 7 for Intel’s Pentium processors. For the Pentium II, which is based on Intel’s P6 micro architecture, Intel switched to the Slot 1 configuration. With the slot approach, the microprocessor is packaged in a cartridge, known as a Single Edge Cartridge, that is easily inserted into a slot designed into the motherboard. The new approach will make connection to the faster and larger number of data paths in future processors more feasible. Officially, Intel now refers to Slot 1 as the 242-contact slot connector and Slot 2 as the 330-contact slot connector. Besides the Pentium II, Intel’s Celeron processor also uses the Slot 1 configuration. Intel’s high-end Pentium II Xeon processor uses the Slot 2 configuration. Intel is said to be developing a Slot M. Since the motherboards are different, computer owners with Socket 7 Pentiums will not be able to upgrade to the newer P6 Pentiums with Slot 1 and Slot 2 configurations. and Cyrix, which offer processors that compete with the Pentium II, say they plan to continue using the Socket 7 motherboard.
small to medium enterprise
slot time In Ethernet and its Carrier Sense Multiple Access/Collision Detect (CSMA/CD) approach to managing which device can use the communication link next, slot time is the amount of time a device waits after a collision before retransmitting. The transmitting device determines the appropriate amount of slot time by adding the amount of time it took for another device to detect a collision, the amount of time it took for the device to notify the original transmitting device of the collision, and the amount of time it took to transmit a jam sequence. Should a second collision occur, the device doubles the slot time in an effort to avoid future collisions; this process is called ‘‘backoff.’’ In half-duplex Ethernet, where data can only travel in one direction at once, slot time becomes an important parameter in determining the how many devices can share a network.
Small Business Extensible Markup Language (SMBXML) SMBXML (Small Business Extensible Markup Language) is an open, XML-based standard, designed especially for use by small and medium-sized businesses, that describes a data exchange format for use in common business applications. The SMBXML package includes elements intended primarily for transactions and recordkeeping. Examples include sales records, purchasing records, payroll, tax records, and general banking. Currency exchange rates can be calculated, and conversions can be made, allowing SMBXML to be used in international business operations. The standard can also be applied to general databases, such as employee shift scheduling, vacation scheduling, sick time, and so forth. SMBXML is compatible with most Internet transport and security protocols. Data can be exchanged between consenting parties provided integration agreements are signed.
Small Computer System Interface See ‘‘SCSI’’
small form factor Small form factor (SFF) refers to any of several physically compact connector designs that have been developed for use in fiber optic systems. They are about half the size of conventional connectors. Currently there are at least three designs: the LC by Lucent, the VF-45 by 3M, and the MT-RJ by Tyco. The main motivator for the development of SFF connectors is an ongoing demand for smaller components in network systems. Using SFF connectors, it is possible to get many more interfaces on a single card. In addition, the use of
connectors, rather than direct soldering, increases the flexibility and versatility of network systems, and makes it easier and less expensive to maintain them. The fact that there are several SFF connector configurations allows the use of products from multiple sources. An argument has been made for standardization, but individual connector makers have so far resisted this idea, preferring instead to develop and promote their own connector designs. SFF connectors are used with transceivers called small-formfactor pluggable (SFP) modules. This design has been called a miniature gigabit interface converter (GBIC). Also see small-form-factor pluggable (SFP) and gigabit interface converter (GBIC).
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small form-factor pluggable Small form-factor pluggable (SFP) is a specification for a new generation of optical modular transceivers. The devices are designed for use with small form factor (SFF) connectors, and offer high speed and physical compactness. They are hot-swappable. SFP transceivers are expected to perform at data speeds of up to five gigabits per second (5 Gbps), and possibly higher. Because SFP modules can be easily interchanged, electrooptical or fiber optic networks can be upgraded and maintained more conveniently than has been the case with traditional soldered-in modules. Rather than replacing an entire circuit board containing several soldered-in modules, a single module can be removed and replaced for repair or upgrading. This can result in a substantial cost savings, both in maintenance and in upgrading efforts. Several companies have formed a consortium supporting the use of SFP transceivers to meet their common objectives of broad bandwidth, small physical size and mass, and ease of removal and replacement. Also see small form factor (SFF) and gigabit interface converter (GBIC).
Small Office Home Office See ‘‘SOHO’’
small to medium enterprise
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1) SME (small-to-medium enterprise) is a convenient term for segmenting businesses and other organizations that are somewhere between the ‘‘small office-home office’’ (SOHO) size and the larger enterprise. The European Union has defined an SME as a legally independent company with no more than 500 employees.
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2) SME also stands for the Society of Manufacturing Engineers.
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Smalltalk is a programming language that was designed expressly to support the concepts of object-oriented programming. In the early 1970s, Alan Kay led a team of researchers at Xerox to invent a language that let programmers envision the data objects they intended to manipulate. Unlike C++, Smalltalk was not built on the syntax of a procedural language; it is a ‘‘pure’’ objectoriented language with more rigorously enforced rules than C++, which permits some of the procedural constructs of the C language.
A smart card is a plastic card about the size of a credit card, with an embedded microchip that can be loaded with data, used for telephone calling, electronic cash payments, and other applications, and then periodically refreshed for additional use. Currently or soon, you may be able to use a smart card to:
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Although Smalltalk may continue to attract a loyal following, Java, a derivative of C++ designed for distributed systems, has become the most prevalent object-oriented language on the Web.
small-unit metric system The small-unit metric system (or more formally, the centimeter-gram-second (cgs) system of units) is based on three fundamental units: The centimeter (cm), which quantifies displacement, the gram (g), which quantifies mass, and the second (s or sec), which quantifies time. The small-unit metric system is so-called because one centimeter is equal to 0.01 meter (10-2 m), and one gram is equal to 0.001 kilogram (10-3 kg). The cgs system, like the metric system, was originally developed by scientists who were frustrated with the English (foot-pound-second) system. The cgs and metric system units of displacement and mass lend themselves neatly to calculations in scientific notation, while English units generally do not. Time remains somewhat messy in all systems; there are 60 seconds in a minute, 60 minutes in an hour, and 24 hours in a mean solar day. The International System of Units (SI) has officially supplanted the small-unit metric system (as well as the metric system). But it is good to have passing familiarity with the small-unit metric system, because that scheme is still used by some astronomers and physicists, and many older scientific books and papers were written with a preference for it. Today’s SI system provides formal definitions for the meter, the kilogram, and the second, and also specifies and defines four additional units: The kelvin for temperature, the ampere for electric current, the candela for luminous intensity, and the mole for material quantity.
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Compare metric system, English system of units, and International System of Units (SI). Also see prefix multipliers.
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smart building
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Dial a connection on a mobile telephone and be charged on a per-call basis
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Establish your identity when logging on to an Internet access provider or to an online bank
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Pay for parking at parking meters or to get on subways, trains, or buses
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Give hospitals or doctors personal data without filling out a form
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Make small purchases at electronic stores on the Web (a kind of cybercash)
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Buy gasoline at a gasoline station
Over a billion smart cards are already in use. Currently, Europe is the region where they are most used. Ovum, a research firm, predicts that 2.7 billion smart cards will be shipped annually by 2003. Another study forecasts a $26.5 billion market for recharging smart cards by 2005. Compaq and Hewlett-Packard are reportedly working on keyboards that include smart card slots that can be read like bank credit cards. The hardware for making the cards and the devices that can read them is currently made principally by Bull, Gemplus, and Schlumberger.
How Smart Cards Work A smart card contains more information than a magnetic stripe card and it can be programmed for different applications. Some cards can contain programming and data to support multiple applications and some can be updated to add new applications after they are issued. Smart cards can be designed to be inserted into a slot and read by a special reader or to be read at a distance, such as at a toll booth. Cards can be disposable (as at a trade-show) or reloadable (for most applications). An industry standard interface between programming and PC hardware in a smart card has been defined by the PC/SC Working Group, representing Microsoft, IBM, Bull, Schlumberger, and other interested companies. Another standard is called OpenCard. There are two leading smart card operating systems: JavaCard and MULTOS.
smart frame See ‘‘digital photo album’’
Smart Tags
smart home or building
smart matter
A smart home or building is a home or building, usually a new one, that is equipped with special structured wiring to enable occupants to remotely control or program an array of automated home electronic devices by entering a single command. For example, a homeowner on vacation can use a Touchtone phone to arm a home security system, control temperature gauges, switch appliances on or off, control lighting, program a home theater or entertainment system, and perform many other tasks.
Smart matter is another term for micro-electromechanical systems (MEMS), a technology that combines computers with tiny mechanical devices such as sensors, valves, gears, mirrors, and actuators imbedded in semiconductor chips. Paul Saffo of the Institute for the Future in Palo Alto, California, believes MEMS or what he calls analog computing will be ‘‘the foundational technology of the next decade.’’
The field of home automation is expanding rapidly as electronic technologies converge. The home network encompasses communications, entertainment, security, convenience, and information systems. A technology known as Powerline Carrier Systems (PCS) is used to send coded signals along a home’s existing electric wiring to programmable switches, or outlets. These signals convey commands that correspond to ‘‘addresses’’ or locations of specific devices, and that control how and when those devices operate. A PCS transmitter, for instance, can send a signal along a home’s wiring, and a receiver plugged into any electric outlet in the home could receive that signal and operate the appliance to which it is attached. One common protocol for PCS is known as X10, a signaling technique for remotely controlling any device plugged into an electrical power line. X10 signals, which involve short radio frequency (RF) bursts that represent digital information, enable communication between transmitters and receivers. In Europe, technology to equip homes with smart devices centers on development of the European Installation Bus, or Instabus. This embedded control protocol for digital communication between smart devices consists of a twowire bus line that is installed along with normal electrical wiring. The Instabus line links all appliances to a decentralized communication system and functions like a telephone line over which appliances can be controlled. The European Installation Bus Association is part of Konnex, an association that aims to standardize home and building networks in Europe. Echelon Corp., the creator of the LonWorks system, is helping drive adoption of an open interoperability standard among vendors in the control networks industry. LonWorks is an open standard for network automation and control for the building, transportation, industrial and home markets. The American National Standards Institute (ANSI) has adopted the protocol underlying LonWorks control networks as an industry standard. The LonMark Interoperability Association is made up of more than 200 controls companies mission working on standard to integrate multi-vendor systems based on LonWorks networks.
MEMS are already used as accelerometers in automobile airbags. They’ve replaced a less reliable device at lower cost and show promise of being able to inflate a bag not only on the basis of sensed deceleration but also on the basis of the size of the person they are protecting. Basically, a MEMS device contains micro-circuitry on a tiny silicon chip into which some mechanical device such as a mirror or a sensor has been manufactured. Potentially, such chips can be built in large quantities at low cost, making them cost-effective for many uses. Among the presently available uses of MEMS or those under study are: L
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Global position system sensors that can be included with courier parcels for constant tracking and that can also sense parcel treatment en route Sensors built into the fabric of an airplane wing so that it can sense and react to air flow by changing the wing surface resistance; effectively creating a myriad of tiny wing flaps
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Optical switching devices that can switch light signals over different paths at 20-nanosecond switching speeds
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Sensor-driven heating and cooling systems that dramatically improve energy savings
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Building supports with imbedded sensors that can alter the flexibility properties of a material based on atmospheric stress sensing
Saffo distinguishes between sensor-effector type microcomputing (which he calls ‘‘MEMS’’) and micro-devices containing gears, mirrors, valves, and other parts (which he calls ‘‘micro-machines’’). Much support for MEMS has come from Defense Advanced Research Projects Agency Research and Development Electronics Technology Office.
smart tag See ‘‘Smart Tags’’
Smart Tags Microsoft’s Smart Tags were a proposed feature of Windows XP that would allow Microsoft and its partners to insert their own links into any Web page viewed through its Internet Explorer browser. These links (which appear as purple dashed underlining, to differentiate them from original
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Whatis?Com’s Encyclopedia of Technology Terms content links) are similar to traditional hyperlinks, but more complex and interactive: when the cursor hovers over a Smart Tagged word, a drop-down list appears with a selection of links related to the word. A number of companies are developing industry or application-specific Smart Tag libraries—for example, for the insurance industry or for medical applications. A Microsoft partner, Keylogix, has an application called ActiveDocs Smart Tags that allows end users to create their own Smart Tags from within Microsoft Word. Although Microsoft may not include Smart Tags as part of XP, they have a version of them available for free download, Smart Tags for the Everyday Web. This Smart Tags application is compatible with Internet Explorer 6 and requires 200 KB of free disk space. How Smart Tags work: Someone reading a sports article moves their cursor over a Smart Tag on the name of a particular baseball team. A drop-down list appears with a selection of links—such as current standings, official web site, and related news, for example; when the user clicks a link in the list they are taken to that Web page. A button on the toolbar turns the Smart Tag option on and off. Although Smart Tag technology has a lot of potential for helping the Web develop its interactive potential, the corporate background has made many Web site developers leery of less benign possibilities. Because the tagged words and associated links are selected by Microsoft, many Web content creators and site owners are very concerned about Smart Tag implementation, which they fear would allow Microsoft to have editorial control of their work. As an example, an anti-Microsoft rant on a Web page might have a lot less impact if links took the reader to pro-Microsoft pages. In response to the outcry against Smart Tags, Microsoft has made them available to Internet Explorer users who want them—but not an integral part of their new operating systems—and created a meta tag that allows developers to disable Internet Explorer-added Smart Tags for their Web pages.
SmartDownload Netscape’s SmartDownload is a free software program that allows a user to pause, stop, and resume a file download without losing any data. If the user choses to stop the download or if a dial-up connection terminates unexpectedly, the SmartDownload client takes advantage of the Multichannel Protocol Plus (MP+) capability of most Web servers to determine that the ‘‘resume call’’ should be added to the existing session. AOL, the owner of Netscape, whose SmartDownload application is compatible with other browsers including Internet Explorer, has been criticized for using the application to gather data and create a user profile without the user’s knowledge. It is speculated that disputes over the legality of these user profiles could have ramifications for
other disputes, including those involving the music industry, whose copyright lawyers would like to be able to locate individual users who are violating the law.
smartphone The term smartphone is sometimes used to characterize a wireless telephone set with special computer-enabled features not previously associated with telephones. In addition to functioning as an ordinary telephone, a smartphone’s features may include: L
Wireless e-mail, Internet, Web browsing, and fax
L
Intercom function
L
Personal information management
L
Online banking
L
LAN connectivity
L
Graffiti style data entry
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Local data transfer between phone set and computers
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Remote data transfer between phone set and computers
L
Remote control of computers
L
Remote control of home or business electronic systems
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Interactivity with unified messaging
SMB protocol See ‘‘Server Message Block Protocol’’
SMBXML See ‘‘Small (SMBXML)’’
Business
Extensible
Markup
Language
SMDS SMDS (Switched Multimegabit Data Service) is a public, packet-switched service aimed at enterprises that need to exchange large amounts of data with other enterprises over the wide-area network on a nonconstant or ‘‘bursty’’ basis. SMDS provides an architecture for this kind of data exchange and a set of services. In general, SMDS extends the performance and efficiencies of a company’s local area network (LANs) over a wide area on a switched, as-needed basis. SMDS is connectionless, meaning that there is no need to set up a connection through the network before sending data. This provides bandwidth on demand for the ‘‘bursty’’ data transmission typically found on LANs. SMDS packets contain up to 7168 bytes of data, which is large enough to accept the most common LAN packets. Each packet includes the source address and the destination address and is sent separately from other packets.
SMS Each enterprise using SMDS is assigned from one to sixteen unique SMDS addresses, depending on needs. An address is a ten digit number that looks like an ordinary telephone number. SMDS also provides for broadcasting packets to multiple SMDS addresses. Each SMDS company is assigned one or more group addresses that can be used to define destination groups. Group addressing is similar to LAN multicasting. It lets routing protocols, such as TCP/IP, use dynamic address resolution and routing updates. Since SMDS is a public service, any SMDS customer can exchange data with any other customer. The SMDS Interest Group, an association of service providers, equipment manufacturers, and users, develops technical specifications, promotes awareness of SMDS, stimulates new applications, and ensures worldwide service interoperability, working with its international affiliates. Their home page provides a list of companies providing SMDS services.
SME See ‘‘small to medium enterprise’’
SMF The SMF (Standard MIDI File) was developed to allow musicians and audio file developers to transfer their sequenced MIDI data from one software application to another. This file format lets users exchange music and data without regard to computer operating system platform. Today, there are many Web sites devoted to the sale and exchange of music data in the Standard MIDI File format. There are two variations of the Standard MIDI File format, called Type 0 and Type 1. Both variations contain the same data, but the arrangement of the data within the file is different. Type 0 is a single track of data; Type 1 is multitrack. Many of the popular sequencing software packages available can import and export either type of file. In the Type 0 Standard MIDI File, all data is saved to a single track without regard to its MIDI channel reference. The file plays normally, but accessing the individual components is a bit more difficult. If editing is needed, the user can open the Standard MIDI File in the application of choice and copy/ paste individual channels of information manually as needed. A Type 1 Standard MIDI File outputs a separate track for each MIDI channel in the original sequence. This format greatly simplifies editing once the sequence has been transferred from one sequencer to another.
SMIL SMIL (Synchronized Multimedia Integration Language), is a language that allows Web site creators to be able to easily define and synchronize multimedia elements (video, sound, still images) for Web presentation and interaction. On
today’s Web, although you can send moving and still images and sound to a Web user, each element is separate from the others and can’t be coordinated with other elements without elaborate programming. SMIL (pronounced ‘‘smile’’) lets site creators send multiple movies, still images, and sound separately but coordinate their timing. Each media object is accessed with a unique Uniform Resource Locator (URL) which means that presentations can be made of objects arriving from more than one place and that objects can easily be reused in multiple presentations. SMIL also lets the ‘‘producer’’ store a media object in multiple versions, each with a different bandwidth so that a lower-bandwidth version of a Web page can be sent to users who need it. SMIL also accommodates multiple language versions of soundtracks. SMIL statements are simple and can be entered with a text editor similar to those used to create Hypertext Markup Language (HTML) pages. A presentation can be described using only three Extensible Markup Language (XML) elements. It’s intended that SMIL will be usable by anyone who can use HTML. SMIL was developed by a group coordinated by the World Wide Web Consortium (W3C) and included representatives from the CD-ROM, interactive television, Web, and audio/ video streaming industries. The first public draft of SMIL was released in November, 1997.
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smiley See ‘‘emoticon’’
SMP SMP (symmetric multiprocessing) is the processing of programs by multiple processors that share a common operating system and memory. In symmetric (or ‘‘tightly coupled’’) multiprocessing, the processors share memory and the I/O bus or data path. A single copy of the operating system is in charge of all the processors. SMP, also known as a ‘‘shared everything’’ system, does not usually exceed 16 processors.
L M N O P
SMP systems are considered better than MPP systems for online transaction processing (OTP) in which many users access the same database in a relatively simple set of transactions. An advantage of SMP for this purpose is the ability to dynamically balance the workload among computers (and as a result serve more users faster).
Q
SMR
T
See ‘‘Specialized Mobile Radio’’
R S
U
SMS See ‘‘Short Message Service’’
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SMTP
snail mail
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SMTP (Simple Mail Transfer Protocol) is a TCP/IP protocol used in sending and receiving e-mail. However, since it’s limited in its ability to queue messages at the receiving end, it’s usually used with one of two other protocols, POP3 or Internet Message Access Protocol, that let the user save messages in a server mailbox and download them periodically from the server. In other words, users typically use a program that uses SMTP for sending e-mail and either POP3 or IMAP for receiving messages that have been received for them at their local server. Most mail programs such as Eudora let you specify both an SMTP server and a POP server. On UNIX-based systems, sendmail is the most widely-used SMTP server for e-mail. A commercial package, Sendmail, includes a POP3 server and also comes in a version for Windows NT.
Snail mail is a slang term for the regular postal service (for example, the U.S. Postal Service) with the implication that it is a lot slower than e-mail. It’s worth noting, perhaps, that in the early days of the Internet, it was proposed that the U.S. Post Office manage e-mail. Electronic messages would come to your local post office and then be delivered to you along with the regular mail. The proposal was not considered for very long.
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SMTP usually is implemented to operate over Transmission Control Protocol port 25. The details of SMTP are in Request for Comments 821 of the Internet Engineering Task Force (IETF). An alternative to SMTP that is widely used in Europe is X.400.
smurfing Smurfing is the attacking of a network by exploiting Internet Protocol (IP) broadcast addressing and certain other aspects of Internet operation. Smurfing uses a program called Smurf and similar programs to cause the attacked part of a network to become inoperable. The exploit of smurfing, as it has come to be known, takes advantage of certain known characteristics of the Internet Protocol (IP) and the Internet Control Message Protocol (ICMP). The ICMP is used by network nodes and their administrators to exchange information about the state of the network. ICMP can be used to ping other nodes to see if they are operational. An operational node returns an echo message in response to a ping message. A smurf program builds a network packet that appears to originate from another address (this is known as spoofing an IP address). The packet contains an ICMP ping message that is addressed to an IP broadcast address, meaning all IP addresses in a given network. The echo responses to the ping message are sent back to the ‘‘victim’’ address. Enough pings and resultant echoes can flood the network making it unusable for real traffic. One way to defeat smurfing is to disable IP broadcast addressing at each network router since it is seldom used. This is one of several suggestions provided by the CERT Coordination Center.
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X,Y,Z
See ‘‘Systems Network Architecture’’
Snail mail or not, one must acknowledge that regular postal services offer a number of valuable services not likely to be available soon on the World Wide Web.
snake oil In security, snake oil is a name for the exaggerated claims made by vendors. Cryptography experts have compared the exaggerated claims made by some vendors to the claims made by medicine show pitchmen in mid-19th century America, who bragged of secret ingredients much as today’s marketers brag of secret proprietary algorithms. Commentators note that both the snake oil pitchman and the snake oil cryptographer may actually have a legitimate product worthy of purchase if the product’s capabilities were not overstated. In mid-19th century America, snake oil was an alternative medicine used by Chinese immigrants working on the trans-continental railroad. The oil seemed to be effective in treating the aches and pains encurred from hard manual labor. European railroad workers learned of snake oil’s healing properties first-hand and began to tell others. Entrepreneurs saw there was a market for snake oil and began to promise consumers that the magic Chinese elixir could cure just about anything that was wrong with them. ‘‘Snake oil’’ rapidly became a synonym for ‘‘fraudulent’’ or ‘‘bogus’’ and people forgot that it had once had a valuable, though limited, use. Matt Curtin, who is credited with applying the term to computer security products, advises buyers to beware of top-secret products that promise unbreakable algorithms and to avoid any vendor who has a ‘‘trust us, we know what we’re doing’’ approach to questions. According to Curtin, public scrutiny of an algorithm by mathematicians and other cryptographers is the best way to ensure an algorithm cannot be broken within a reasonable time frame.
snap-in Snap-in, in general, refers to an object that can be attached to another object and that will then function as part of the whole. Hardware snap-ins usually fit this description. In reference to software applications, a snap-in is a program designed to function as a modular component of another application. Examples of snap-ins include Shym Technology’s PKEnable snap-in security system for use in e-mail and Web applications, and Novell’s ZENworks for
Socket 370 Desktops 3 Cluster Snap-in, which contains the program to support workstation inventory in a cluster environment. iMaximize claims that their snap-in products make it possible for any user who can operate a word processing program to create a feature-rich Web site in an hour.
SNR
Snap-ins are the basic components of Microsoft’s Management Console (MMC). The MMC snap-ins are the actual management tools; the console—sometimes referred to as a ‘‘tools host’’—is simply a framework into which the snap-ins are added. Within the MMC environment—which is similar to Windows explorer—the user selects from a list of all installed snap-ins; these might include, for example, DNS manager or device manager. Multiple copies of a particular snap-in can be added to apply to separate workstations. Snap-ins for MMC can include those from other vendors as well as Microsoft. Many companies, including IBM, Hewlett-Packard, NetIQ, Seagate, Symantec and Compuware, are also making MMC snap-ins; users can develop snap-ins to administer their own systems and custom applications.
See ‘‘Simple Object Access Protocol’’
sneakernet Sneakernet is a jargon term for the method of transmitting electronic information by personally carrying it from one place to another on floppy disk or other removable medium. The idea is that someone is using their shoes (possibly sneakers) to move data around rather than using the telecommunications network.
sniffer A sniffer is a program that monitors and analyzes network traffic, detecting bottlenecks and problems. Using this information, a network manager can keep traffic flowing efficiently. A sniffer can also be used legitimately or illegitimately to capture data being transmitted on a network. A network router reads every packet of data passed to it, determining whether it is intended for a destination within the router’s own network or whether it should be passed further along the Internet. A router with a sniffer, however, may be able to read the data in the packet as well as the source and destination addresses. Sniffers are often used on academic networks to prevent traffic bottlenecks caused by filesharing applications such as Napster or Gnutella. The term ‘‘sniffer’’ is occasionally used for a program that analyzes data other than network traffic. For example, a database could be analyzed for certain kinds of duplication. A number of companies offer products that include ‘‘Sniffer’’ as part of their name.
SNMP See ‘‘Simple Network Management Protocol’’
See ‘‘signal-to-noise ratio’’
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SOAP social engineering In computer security, social engineering is a term that describes a non-technical kind of intrusion that relies heavily on human interaction and often involves tricking other people to break normal security procedures. A social engineer runs what used to be called a ‘‘con game.’’ For example, a person using social engineering to break into a computer network would try to gain the confidence of someone who is authorized to access the network in order to get them to reveal information that compromises the network’s security. They might call the authorized employee with some kind of urgent problem; social engineers often rely on the natural helpfulness of people as well as on their weaknesses. Appeal to vanity, appeal to authority, and oldfashioned eavesdropping are typical social engineering techniques. Another aspect of social engineering relies on people’s inability to keep up with a culture that relies heavily on information technology. Social engineers rely on the fact that people are not aware of the value of the information they possess and are careless about protecting it. Frequently, social engineers will search dumpsters for valuable information, memorize access codes by looking over someone’s shoulder (shoulder surfing), or take advantage of people’s natural inclination to choose passwords that are meaningful to them but can be easily guessed. Security experts propose that as our culture becomes more dependent on information, social engineering will remain the greatest threat to any security system. Prevention includes educating people about the value of information, training them to protect it, and increasing people’s awareness of how social engineers operate.
Socket 370 Socket 370 is the descriptive term for the way certain Intel microprocessors plug into a computer motherboard so that it makes contact with the motherboard’s built-in wires or data bus. The Socket 370 chip is less expensive to manufacture than Slot 1 Pentium II chips. A Socket 370 chip is the same size as a Socket 7 chip but has a different number of pins and a different voltage. Socket 370 is commonly used with Celeron processors Socket 370 uses the Zero Insertion Force (ZIF) socket. As its name implies, the ZIF socket is designed for ease of manufacture and so that the average computer owner will be able to upgrade the microprocessor. The ZIF socket contains
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a lever that opens and closes, securing the microprocessor in place. Different sockets have differing numbers of pins and pin layout arrangements.
Socket 7 Socket 7 is the descriptive term for the way certain Intel Pentium microprocessors plug into a computer motherboard so that it makes contact with the motherboard’s builtin wires or data bus. Socket 7 is the best-known of eight connection variations that use the Zero Insertion Force (ZIF) socket. As its name implies, the ZIF socket is designed for ease of manufacture and so that the average computer owner will be able to upgrade the microprocessor. The ZIF socket contains a lever that opens and closes, securing the microprocessor in place. Various sockets have a differing number of pins and pin layout arrangements. For its Pentium II processor, Intel has moved from the socket configuration to an approach in which the processor is packaged in a cartridge and fits into a slot in the motherboard. The following table summarizes the socket characteristics for different Intel processors. Pins
Layout
Processor(s) Voltage
0 1 2
168 169 238
Inline Inline Inline
L
3
237
Inline
M
4
273
Inline
N
5 6 7 8
320 235 321 387
Staggered Inline Staggered Staggered
486DX5 V 486DX,SX 486DX, SX,DX2 486DX,SX, DX2,DX4 60 or 66 MHz Pentium Pentium 486DX4 Pentium Pentium Pro
K
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| bind() | recvfrom()
Socket
J
socket()
5V 5V 3V or 5V 5V
3 3 3 3
V V V V
sockets Sockets is a method for communication between a client program and a server program in a network. A socket is defined as ‘‘the endpoint in a connection.’’ Sockets are created and used with a set of programming requests or ‘‘function calls’’ sometimes called the sockets application programming interface (API). The most common sockets API is the Berkeley UNIX C interface for sockets. Sockets can also be used for communication between processes within the same computer. This is the typical sequence of sockets requests from a server application in the ‘‘connectionless’’ context of the Internet in which a server handles many client requests and does not maintain a connection longer than the serving of the immediate request:
| (wait for a sendto request from some client) | (process the sendto request) | sendto (in reply to the request from the client...for example, send an HTML file)
A corresponding client sequence of sockets requests would be: socket() | bind() | sendto() | recvfrom()
Sockets can also be used for ‘‘connection-oriented’’ transactions with a somewhat different sequence of C language system calls or functions.
socks Socks (or ‘‘SOCKS’’) is a protocol that a proxy server can use to accept requests from client users in a company’s network so that it can forward them across the Internet. Socks uses sockets to represent and keep track of individual connections. The client side of Socks is built into certain Web browsers and the server side can be added to a proxy server. A socks server handles requests from clients inside a company’s firewall and either allows or rejects connection requests, based on the requested Internet destination or user identification. Once a connection and a subsequent ‘‘bind’’ request have been set up, the flow of information exchange follows the usual protocol (for example, the Web’s Hypertext Transfer Protocol).
soft handoff In cellular telephone communication, soft handoff refers to the overlapping of repeater coverage zones, so that every cell phone set is always well within range of at least one repeater (also called a base station). In some cases, mobile sets transmit signals to, and receive signals from, more than one repeater at a time. Soft handoff technology is used by code-division multiple access (CDMA) systems. Older networks use frequency division multiplex (FDM) or time division multiplex (TDM). In CDMA, all repeaters use the same frequency channel for each mobile phone set, no matter where the set is located. Each set has an identity based on a code, rather than on a frequency (as in FDM) or sequence of time slots (as in TDM).
software-defined radio Because no change in frequency or timing occurs as a mobile set passes from one base station to another, there are practically no dead zones. As a result, connections are almost never interrupted or dropped.
softswitch Softswitch (software switch) is a generic term for any open application program interface (API) software used to bridge a public switched telephone network (PSTN) and voice over Internet protocol (VOiP) by separating the call control functions of a phone call from the media gateway (transport layer).
software Software is a general term for the various kinds of programs used to operate computers and related devices. (The term hardware describes the physical aspects of computers and related devices.) Software can be thought of as the variable part of a computer and hardware the invariable part. Software is often divided into application software (programs that do work users are directly interested in) and system software (which includes operating systems and any program that supports application software). The term middleware is sometimes used to describe programming that mediates between application and system software or between two different kinds of application software (for example, sending a remote work request from an application in a computer that has one kind of operating system to an application in a computer with a different operating system). An additional and difficult-to-classify category of software is the utility, which is a small useful program with limited capability. Some utilities come with operating systems. Like applications, utilities tend to be separately installable and capable of being used independently from the rest of the operating system. Applets are small applications that sometimes come with the operating system as ‘‘accessories.’’ They can also be created independently using the Java or other programming languages. Software can be purchased or acquired as shareware (usually intended for sale after a trial period), liteware (shareware with some capabilities disabled), freeware (free software but with copyright restrictions), public domain software (free with no restrictions), and open source (software where the source code is furnished and users agree not to limit the distribution of improvements).
Software is often packaged on CD-ROMs and diskettes. Today, much purchased software, shareware, and freeware is downloaded over the Internet. A new trend is software that is made available for use at another site known as an application service provider. Some general kinds of application software include: L
Productivity software, which includes word processors, spreadsheets, and tools for use by most computer users
L
Presentation software
L
Graphics software for graphic designers
L
CAD/CAM software
L
Specialized scientific applications
L
Vertical market or industry-specific software (for example, for banking, insurance, retail, and manufacturing environments)
Firmware or microcode is programming that is loaded into a special area on a microprocessor or read-only memory on a one-time or infrequent basis so that thereafter it seems to be part of the hardware.
software developer’s kit A software developer’s kit (SDK) is a set of programs used by a computer programmer to write application programs. Typically, an SDK includes a visual screen builder, an editor, a compiler, a linker, and sometimes other facilities. The term is used by Microsoft, Sun Microsystems, and a number of other companies.
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This term is sometimes seen as software development kit.
software-defined radio Software-defined radio (SDR), sometimes shortened to software radio (SR), refers to wireless communication in which the transmitter modulation is generated or defined by a computer, and the receiver uses a computer to recover the signal intelligence. To select the desired modulation type, the proper programs must be run by microcomputers that control the transmitter and receiver. A typical voice SDR transmitter, such as might be used in mobile two-way radio or cellular telephone communication, consists of the following stages. Items with asterisks represent computer-controlled circuits whose parameters are determined by the programming (software).
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L
Microphone
L
Audio amplifier
L
Analog-to-digital converter (ADC) that converts the voice audio to ASCII data *
L
Modulator that impresses the ASCII intelligence onto a radio-frequency (RF) carrier *
L
Series of amplifiers that boosts the RF carrier to the power level necessary for transmission
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L
Transmitting antenna
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A typical receiver designed to intercept the above-described voice SDR signal would employ the following stages, essentially reversing the transmitter’s action. Again, items followed by asterisks represent programmable circuits. L
Receiving antenna
L
Superheterodyne system that boosts incoming RF signal strength and converts it to a constant frequency
L
Demodulator that separates the ASCII intelligence from the RF carrier *
L
Digital-to-analog converter (DAC) that generates a voice waveform from the ASCII data *
L
Audio amplifier
L
Speaker, earphone, or headset
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The most significant asset of SDR is versatility. Wireless systems employ protocols that vary from one service to another. Even in the same type of service, for example wireless fax, the protocol often differs from country to country. A single SDR set with an all-inclusive software repertoire can be used in any mode, anywhere in the world. Changing the service type, the mode, and/or the modulation protocol involves simply selecting and launching the requisite computer program, and making sure the batteries are adequately charged if portable operation is contemplated. The ultimate goal of SDR engineers is to provide a single radio transceiver capable of playing the roles of cordless telephone, cell phone, wireless fax, wireless e-mail system, pager, wireless videoconferencing unit, wireless Web browser, Global Positioning System (GPS) unit, and other functions still in the realm of science fiction, operable from any location on the surface of the earth, and perhaps in space as well.
SOHO In information technology, SOHO (small office home office) is a term for the small office or home office environment and business culture. A number of organizations, businesses, and publications now exist to support people who work or have businesses in this environment. The term ‘‘virtual office’’ is sometimes used as a synonym.
SOI
S
See ‘‘Silicon-on-Insulator’’
T
solar fade
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X,Y,Z
Solar fade, also called sun interference, is a phenomenon that occurs in satellite communications on certain occasions when the downlink signal is aligned with the sun’s position and it is overcome by signal noise from the sun. The term is used mainly in reference to geostationary (GEO) satellite systems.
The sun is a powerful emitter of electromagnetic energy at all wavelengths, including those in the microwave portion of the radio-frequency (RF) spectrum, where most satellite communication is carried out. Normally, the sun does not affect the reception of microwave signals, because microwave-receiving antennas are rarely pointed right at the sun. But once in awhile, a signal source and the sun line up, and then they compete. At the equinoxes, around March 21 and September 21 of every year, the sun is directly over the earth’s equator. GEO satellites orbit over the equator. Thus, for about a week before and after the equinoxes, the sun lines up almost exactly with any given GEO satellite once a day for users living at the equator. For subscribers in the northern hemisphere, the same thing happens for a couple of weeks before March 21 and after September 21. In the southern hemisphere, the effect is observed just after March 21 and before September 21. Unless the satellite downlink satellite signal is exceptionally strong, RF noise from the sun overpowers it, and reception is degraded or interrupted. After a few minutes, the sun’s course across the sky takes it past the satellite, and normal reception resumes. Solar fade never occurs more than once a day for any GEO satellite, and presents a problem for only a few days out of the year. Nevertheless, it can be frustrating to satellite system users. It is important to realize that solar fade is not caused by a malfunction in system hardware or programming. Compare with rain fade.
Solaris Solaris is the computer operating system that Sun Microsystems provides for its family of SPARC-based processors as well as for Intel-based processors. Sun has historically dominated the large UNIX workstation market. As the Internet grew in the early 1990s, Sun’s SPARC/ Solaris systems became the most widely installed servers for Web sites. Sun emphasizes the system’s availability (meaning it seldom crashes), its large number of features, and its Internet-oriented design. Sun advertises that its latest version, the Solaris 8 Operating Environment, is ‘‘the leading UNIX environment’’ today. Sun emphasizes these features of Solaris: L
Its availability. Special features make it easy to add new capability or to fix problems without having to restart the system. Because it has evolved through a number of versions, it is ‘‘stable’’—that is, like IBM’s well-known mainframe operating system, MVS, Solaris has exercised and fixed almost any code path that might break. It can be upgraded, monitored, and controlled from a remote console.
L
Its scalability. If you move to a larger processor, your applications should not only run, but run faster.
sosofo
L
It is built for network computing. As part of the first and most successful Web server system in history, the latest Solaris systems are built on the company’s experience with early Web sites and network demands.
L
It includes security features. These include support for IPSec, Kerberos, AMI, and smart cards. Sun provides three extensions for its Solaris operating system:
L
The Easy Access Server, which is designed to run in a network that also has Windows NT systems
L
The Enterprise Server, which is aimed at the ‘‘businesscritical’’ environment, and includes support for clustering
L
The Internet Service Provider (ISP) Server
Since Sun originated the platform-independent Java programming language and runtime environment, Solaris systems come with a Java virtual machine and the Java Development Kit (JDK). Solaris replaced SunOS, a system still in use on many Sun machines today.
solenoid Also see toroid. A solenoid is a coil of insulated or enameled wire wound on a rod-shaped form made of solid iron, solid steel, or powdered iron. Devices of this kind can be used as electromagnets, as inductors in electronic circuits, and as miniature wireless receiving antennas. In a solenoid, the core material is ferromagnetic, meaning that it concentrates magnetic lines of flux. This increases the inductance of the coil far beyond the inductance obtainable with an air-core coil of the same dimensions and the same number of turns. When current flows in the coil, most of the resulting magnetic flux exists within the core material. Some flux appears outside the coil near the ends of the core; a small amount of flux also appears outside the coil and off to the side. A solenoid chime is wound on a cylindrical, hollow, plastic or phenolic form with a movable, solid iron or steel core. The core can travel in and out of the coil along its axis. The coil is oriented vertically; the core normally rests somewhat below the coil center. When a current pulse is applied to the coil, the magnetic field pulls the core forcefully upward. Inertia carries the core above the center of the coil, where the core strikes a piece of metal similar to a xylophone bell, causing a loud ‘‘ding.’’
solid-state Solid-state refers to electronic components, devices, and systems based entirely on the semiconductor. The expression was especially prevalent in the late 1950s and early 1960s, during the transition from vacuum tube technology to the semiconductor diode and transistor. More
recently, the integrated circuit (IC), the light-emitting diode (LED), and the liquid-crystal display (LCD) have evolved as further examples of solid-state devices. In a solid-state component, the current is confined to solid elements and compounds engineered specifically to switch and amplify it. Current flows in two forms: As negatively charged electrons, and as positively charged electron deficiencies called holes. In some semiconductors, the current consists mostly of electrons; in other semiconductors, it consists mostly of holes. Both the electron and the hole are called charge carriers. An example of a non-solid-state component is a cathode-ray tube (CRT). In this device, electrons flow freely through a vacuum from an electron gun, through deflecting and focusing fields, and finally to a phosphorescent screen.
soliton A soliton is a special form of light pulse that can be transmitted over a fiber optic channel. Potentially, a soliton retains its shape or wavelength over a longer distance than ordinary light pulses, permitting a higher rate of information bits to be transmitted.
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See ‘‘Synchronous Optical Network’’
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S/PDIF (Sony/Philips Digital Interface) is a standard audio transfer file format. It is usually found on digital audio equipment such as a DAT machine or audio processing device. It allows the transfer of audio from one file to another without the conversion to and from an analog format, which could degrade the signal quality.
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The most common connector used with an S/PDIF interface is the RCA connector, the same one used for consumer audio products. An optical connector is also sometimes used.
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A sosofo (specification of a sequence of flow objects), a term used in the Document Style Semantics and Specification Language (DSSL), is a set of statements that describe how a sequence of document elements (such as a paragraph and headings) fit into a document and how they are to be formatted (for example, in what type font and with what spacing).
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Using a sosofo in DSSSL, a document element can contain other elements (a paragraph can contain ‘‘notes’’ or ‘‘emphasized text’’ in different font sizes).
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In DSSSL, sosofo’s are the main way in which you describe the presentation of language elements. The inventors of DSSSL feel that ‘‘sosofo’’ takes a lot less time to read and say than ‘‘specification of a sequence of flow objects.’’
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sound card
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A sound card (also referred to as an audio card) is a peripheral device that attaches to the ISA or PCI slot on a motherboard to enable the computer to input, process, and deliver sound.
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The sound card’s four main functions are: as a synthesizer (generating sounds), as a MIDI interface, analog-to-digital conversion (used, for example, in recording sound from a microphone), and digital-to-analog conversion (used, for example, to reproduce sound for a speaker). The three methods of sound synthesis are through frequency modulation (FM) technology, wavetable, and physical modeling. FM synthesis is the least expensive and least effective method. Sounds are simulated by using algorithms to create sine waves that are as close to the sound as possible. For example, the sound of a guitar can be simulated, although the result does not really sound very much like a guitar. Wavetable uses actual, digitally recorded sound samples stored on the card for the highest performance. Physical modeling is a new type of synthesizing, in which sounds are simulated through a complex programming procedure. Some sound cards can also have sounds downloaded to them. Creative Lab’s Sound Blaster is the de facto standard sound card, to the extent that some people use the name as a generic term. Most sound cards in the past have been Sound Blaster-compatible, because most programs that use the sound card have been designed that way. Sound cards were once all connected to the ISA slot. However, because connection to the PCI bus offers advantages such as improved signal-to-noise ratio and decreased demand on the CPU, sound cards being produced today are intended for use with a PCI bus. Some sound cards, such as Diamond MX300 and SoundBlaster Live!, have 3-D capabilities enabled by processors on the card that use mathematical formulas to create greater depth, complexity, and realism of sound. High quality audio can be produced through a system that uses the Universal Serial Bus (USB) and does not require a sound card. Processing is left to the CPU, and digital-to-audio conversion to the speakers.
Sound Designer II SDII (Sound Designer II, sometimes seen abbreviated as SD2) is a monophonic/stereophonic audio file format, originally developed by Digidesign for their Macintoshbased recording/editing products. It is the successor to the original monophonic Sound Designer I audio file format. An SDII file can be monophonic or stereophonic. When stereo is used, the tracks are interleaved (sample-001-left, sample-001-right, sample-002-left, sample-002-right, etc.) Files also store sample rate and bit depth information.
The SDII file has become a widely accepted standard for transferring audio files between editing applications. Most Mac CD-ROM writer software, for example, specifies SDII or Audio Interchange File Format as the file format needed when making audio CDs. The SDII file has also become accepted among personal computer audio application developers. This makes transferring audio from Mac to PC platforms much easier. When used on a PC, the file must use the extension of ‘‘.sd2’’.
source code Source code and object code refer to the ‘‘before’’ and ‘‘after’’ versions of a computer program that is compiled (see compiler) before it is ready to run in a computer. The source code consists of the programming statements that are created by a programmer with a text editor or a visual programming tool and then saved in a file. For example, a programmer using the C language types in a desired sequence of C language statements using a text editor and then saves them as a named file. This file is said to contain the source code. It is now ready to be compiled with a C compiler and the resulting output, the compiled file, is often referred to as object code. The object code file contains a sequence of instructions that the processor can understand but that is difficult for a human to read or modify. For this reason and because even debugged programs often need some later enhancement, the source code is the most permanent form of the program. When you purchase or receive operating system or application software, it is usually in the form of compiled object code and the source code is not included. Proprietary software vendors usually don’t want you to try to improve their code since this may created additional service costs for them. Lately, there is a movement to develop software (Linux is an example) that is open to further improvement and here the source code is provided. In large program development environments, there are often management systems that help programmers separate and keep track of different states and levels of code files. For script (noncompiled or interpreted) program languages, such as JavaScript, the terms source code and object code do not apply since there is only one form of the code.
source quench Using the Internet Control Message Protocol (ICMP), a source quench is a message from one host computer to another telling it to reduce the pace at which it is sending packets to that host. The source quench is one of several ways to manage the flow of packets on the Internet. Ideally, a receiving host would detect when packets were stacking up too fast and send a source quench in time to slow the pace down so that no packets were lost. Note that the Internet Protocol (IP) of which ICMP is a part does not itself guarantee the delivery of packets. Higher-level
spam protocols, such as the Transport Control Protocol (TCP), have responsibility for ensuring successful end-to-end communication. IP and ICMP simply report errors or situations as they are detected so that packets can be resent or sent at a different pace. Source quench is not the only way to control flow in a network and not necessarily the most efficient way. In IP Version 4, the most commonly-used IP version, routers are not allowed to originate a source quench and are not obligated to act on a received source quench. Because the source quench message may itself increase network traffic, other approaches to network flow control are preferred.
Source Quench Introduced Delay Also see SQUID, a UNIX-based program for caching Web pages and other Internet content closer to the user. Source Quench Introduced Delay (SQuID) is the use of some algorithms by a computer that is sending messages too fast to a destination computer that will allow the source computer to slow the timing of its transmissions down to a level acceptable to the destination. On the Internet, when packets arrive at a destination host (computer) faster than the host can handle them, they are discarded and a reply may be sent back indicating that they should be resent. Although a destination host may have a message buffer, the buffer may become full before new packets can be read by a higher-layer application. Having a way to slow down the pace at which packets are sent would not only make work more efficient for both sender and receiver but would reduce unnecessary network traffic. SQuID describes a kind of packet that is returned as a source quench message. The source host would use an algorithm to react to one or a sequence of source quench messages. SQuID is described in a Request for Comments paper of the Internet Engineering Task Force (IETF).
source-route transparent bridging Source-route transparent (SRT) bridging is a bridging scheme developed by IBM that combines source-route bridging (SRB) and transparent bridging in the same network. SRT is commonly used with token ring networks. Transparent bridging was first developed by Digital Equipment Corporation in the early 1980s and has become IEEE standard 802.1. A transparent bridge learns the network’s topology from the source address of each incoming frame. It then uses this information to create a table of the paths used by the hosts so that it can use the same paths to forward network traffic. Network hosts do not detect the presence or operation of transparent bridges. When an SRT bridge receives a frame, it immediately checks the frame’s routing information indicator (RII) bit to see if the frame is a transparent bridging frame or an SRB frame. Once it determines the frame’s type, it processes it accordingly.
Southbridge Southbridge is an Intel chipset that manages the basic forms of input/output (I/O) such as Universal Serial Bus (USB), serial, audio, Integrated Drive Electronics (IDE), and Industry Standard Architecture (ISA) I/O in a computer. Southbridge is one of two chipsets that are collectively called Northbridge/Southbridge. Northbridge controls the processor, memory, Peripheral Component Interconnect (PCI) bus, Level 2 cache, and all Accelerated Graphics Port (AGP) activities. Unlike Northbridge, Southbridge consists of one chip, which sits on Northbridge’s PCI bus. The Intel Hub Architecture (IHA) has replaced the Northbridge/Southbridge chipset. The IHA chipset has two parts also, the Graphics and AGP Memory Controller Hub (GMCH) and the I/O Controller Hub (ICH). The IHA architecture is used in Intel’s 800 series chipsets, which is the first chipset architecture to move away from the Northbridge/Southbridge design.
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See ‘‘service pack’’
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spacer GIF See ‘‘transparent GIF’’
spam Spam is unsolicited e-mail on the Internet. From the sender’s point-of-view, it’s a form of bulk mail, often to a list culled from subscribers to a Usenet discussion group or obtained by companies that specialize in creating e-mail distribution lists. To the receiver, it usually seems like junk e-mail. In general, it’s not considered good netiquette to send spam. It’s generally equivalent to unsolicited phone marketing calls except that the user pays for part of the message since everyone shares the cost of maintaining the Internet. Some apparently unsolicited e-mail is, in fact, e-mail people agreed to receive when they registered with a site and checked a box agreeing to receive postings about particular products or interests. This is known as both opt-in e-mail and permission-based e-mail. A first-hand report indicates that the term is derived from a famous Monty Python sketch (’’Well, we have Spam, tomato & Spam, egg & Spam, Egg, bacon & Spam...’’) that was current when spam first began arriving on the Internet. Spam is a trademarked Hormel meat product that was wellknown in the U.S. Armed Forces during World War II.
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spanning tree protocol
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Where two bridges are used to interconnect the same two computer network segments, spanning tree is a protocol that allows the bridges to exchange information so that only one of them will handle a given message that is being sent between two computers within the network. The spanning tree protocol prevents the condition known as a bridge loop.
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In a local area network (LAN) such as an Ethernet or token ring network, computers compete for the ability to use the shared telecommunications path at any given time. If too many computers try to send at the same time, the overall performance of the network can be affected, even to the point of bringing all traffic to a near halt. To make this possibility less likely, the local area network can be divided into two or more network segments with a device called a bridge connecting any two segments. Each message (called a frame) goes through the bridge before being sent to the intended destination. The bridge determines whether the message is for a destination within the same segment as the sender’s or for the other segment, and forwards it accordingly. A bridge does nothing more than look at the destination address and, based on its understanding of the two segments (which computers are on which segments), forwards it on the right path (which means to the correct outgoing port). The benefit of network segmentation (and the bridge) is that the amount of competition for use of the network path is reduced by half (assuming each segment has the same number of computers) and the possibility of the network coming to a halt is significantly reduced. Each bridge learns which computers are on which segment by sending any first-time message to both segments (this is known as flooding) and then noticing and recording the segment from which a computer replied to the message. Gradually, the bridge builds a picture for itself of which computers are in which segments. When a second and subsequent messages are sent, the bridge can use its table to determine which segment to forward it to. The approach of allowing the bridge to learn the network through experience is known as transparent bridging (meaning that bridging does not require setup by an administrator). In order to build into a network, it is typical to add a second bridge between two segments as a backup in case the primary bridge fails. Both bridges need to continually understand the topography of the network, even though only one is actually forwarding messages. And both bridges need to have some way to understand which bridge is the primary one. To do this, they have a separate path connection just between the bridges in which they exchange information, using bridge protocol data units (BPDUs).
The program in each bridge that allows it to determine how to use the protocol is known as the spanning tree algorithm. The algorithm is specifically constructed to avoid bridge loops (multiple paths linking one segment to another, resulting in an infinite loop situation). The algorithm is responsible for a bridge using only the most efficient path when faced with multiple paths. If the best path fails, the algorithm recalculates the network and finds the next best route. The spanning tree algorithm determines the network (which computer hosts are in which segment) and this data is exchanged using Bridge Protocol Data Units (BPDUs). It is broken down into two steps: Step 1: The algorithm determines the best message a bridge can send by evaluating the configuration messages it has received and choosing the best option. Step 2: Once it selects the top message for a particular bridge to send, it compares its choice with possible configuration messages from the non-root-connections it has. If the best option from step 1 isn’t better than what it receives from the non-root-connections, it will prune that port. The spanning tree protocol and algorithm were developed by a committe of the IEEE. Currently, the IEEE is attempting to institute enhancements to the spanning tree algorithm that will reduce network recovery time. The goal is to go from 30 to 60 seconds after a failure or change in link status to less than 10 seconds. The enhancement, called Rapid Reconfiguration or Fast Spanning Tree, would cut down on data loss and session timeouts when large, Ethernet networks recover after a topology change or a device failure.
SPARC SPARC (Scalable Processor Architecture) is a 32- and 64-bit microprocessor architecture from Sun Microsystems that is based on reduced instruction set computing (RISC). SPARC has become a widely-used architecture for hardware used with UNIX-based operating systems, including Sun’s own Solaris systems. Sun has made SPARC an open architecture that is available for licensing to microprocessor manufacturers. In its most recent brand name, UltraSPARC, microprocessors can be built for PC boards (using either Peripheral Component Interconnect or ATX) as well as for SPARC’s original workstation market. As evidence of SPARC’s scalability, Sun says that its UltraSPARC III will be designed to allow up to 1,000 processors to work together. Although the idea of RISC is sometimes attributed to IBM’s John Cocke, Sun Microsystems was the first to provide a microprocessor that exploited it for the workstation market and it’s possible to say that, together with UNIX, SPARC created the workstation market. (IBM has since used it in its successful RISC System/6000 line of workstations.) Since its inception in 1987, the SPARC architecture has included these ideas:
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Reduce the number of instructions that the processor has to perform to a minimal number (one idea of RISC is that a complex instruction in a conventional computer can be reduced to a series of simpler operations, requiring a simpler architecture and a more compact microprocessor)
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Reduce the number of types of memory addresses that the processor needs to handle
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Put as little processor operation as possible in microcode, which requires clock speed-consuming time to access
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Provide language compilers that compile programs that are optimized for a SPARC microprocessor by being arranged in an order that the processor can handle more efficiently
spatial division multiple access Also see frequency division multiple access (FDMA) and time division multiple access (TDMA). Spatial division multiple access (SDMA) is a satellite communications mode that optimizes the use of radio spectrum and minimizes system cost by taking advantage of the directional properties of dish antennas. In SDMA, also known as SDM (spatial-division multiplex), satellite dish antennas transmit signals to numerous zones on the earth’s surface. The antennas are highly directional, allowing duplicate frequencies to be used for multiple surface zones. Consider a scenario in which signals must be transmitted simultaneously by one satellite to mobile or portable wireless receivers in 20 different surface zones. In a conventional system, 20 channels and 20 antennas would be necessary to maintain channel separation. In SDMA, there can be far fewer channels than zones. If duplicate-channel zones are sufficiently separated, the 20 signals can be transmitted to earth using four or five channels. The narrow signal beams from the satellite antennas ensure that interference will not occur between zones using the same frequency. SDMA requires careful choice of zones for each transmitter, and also requires precise antenna alignment. A small error can result in failure of one or more channels, interference among channels, and/or confusion between surface coverage zones.
SPD See ‘‘serial presence detect’’
Specialized Mobile Radio Also see Enhanced Specialized Mobile Radio (ESMR). Specialized Mobile Radio (SMR) is any two-way radio system in which two or more mobile/portable wireless transceivers are linked by a single repeater. The repeater is elevated above average terrain; this maximizes the area of
coverage. Operating frequencies are in the VHF (very-highfrequency) or UHF (ultra-high-frequency) range, that is, between approximately 30 MHz and 3 GHz. In some ways, an SMR system is like a cellular telephone network. But there are important differences. An SMR system is simpler than a cellular telephone network. There is only one repeater in a SMR system, and it links only the mobile/portable units for that system, not to other repeaters. In SMR, the range of each individual mobile/portable transceiver is greater than the range of a cell phone set. But total system coverage is usually far more limited than that of a cellular network, because there is no linking among repeaters. SMR systems use channel pairs. Each transceiver has a transmit frequency and a receive frequency. These frequencies differ by a fixed amount, called the offset. The transmit and receive frequencies are in the same band, that is, relatively close to each other in the radio spectrum. The transmit and receive frequencies of each mobile or portable transceiver in a system are all identical.
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An SMR system uses half-duplex communication and a PTT (push-to-talk) mode. Neither party can hear the other while transmitting. An example of half-duplex operation is a radio conversation between two people using simple walkietalkies. SMR is used by taxi dispatchers, parcel delivery companies, fire departments, paramedic squads, police departments, and amateur radio operators.
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The term specific gravity, symbolized sp gr, refers to the ratio of the density of a solid or liquid to the density of water at 4 degrees Celsius. The term can also refer to the ratio of the density of a gas to the density of dry air at standard temperature and pressure, although this specification is less often used. Specific gravity is a dimensionless quantity; that is, it is not expressed in units. To find the sp gr of a solid or liquid, you must know its density in kilograms per meter cubed (kg/m3) or in grams per centimeter cubed (g/cm3). Then, divide this density by the density of pure water in the same units. If you use kg/ m3, divide by 1000. If you use g/cm3, divide by 1 (that is, leave the number alone). It is important to use the same units in the numerator and denominator. Water has a specific gravity equal to 1. Materials with a specific gravity less than 1 are less dense than water, and will float on the pure liquid; substances with a specific gravity more than 1 are more dense than water, and will sink. An object with a density of 85 kg/m3 has a specific gravity of 0.085, and will float high on the surface of a body of water. An object with a density of 85 g/cm3 has a specific gravity of 85, and will sink rapidly. To find the specific gravity of a gas, you must know its density in kilograms per meter cubed (kg/m3). Then, divide this density by the density of dry air at standard temperature
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and pressure. This value is approximately 1.29 kg/m3. Gases with a specific gravity less than 1 will rise in the atmosphere at sea level; gases with a specific gravity greater than 1 will sink and seek regions of low elevation at the earth’s surface.
memory exchanging bottlenecks or latency by putting all the data into memory in advance of an actual load instruction. Invalid changes or exceptions to the load are delayed and cross-checked until the processor finally resolves them.
Also see kilogram, meter, kilogram per meter cubed, SI (International System of Units), and standard temperature and pressure.
Intel’s IA-64 architecture is based on EPIC and the first processor in the IA-64 line, the Itanium, uses speculation.
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Specific volume is a property of materials, defined as the number of cubic meters occupied by one kilogram of a particular substance. The standard unit is the meter cubed per kilogram (m3/kg or m3 ˙ kg-1). Sometimes specific volume is expressed in terms of the number of cubic centimeters occupied by one gram of a substance. In this case, the unit is the centimeter cubed per gram (cm3/g or cm3 ˙ g-1). To convert m3/kg to cm3/g, multiply by 1000; conversely, multiply by 0.001. Specific volume is inversely proportional to density. If the density of a substance doubles, its specific volume, as expressed in the same base units, is cut in half. If the density drops to 1/10 its former value, the specific volume, as expressed in the same base units, increases by a factor of 10.
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If the chamber is made smaller without allowing gas in or out, the density increases and the specific volume decreases
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If the chamber expands without letting gas in or out, the density decreases and the specific volume increases
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If the size of the chamber remains constant and new atoms of gas are injected, the density increases and the specific volume decreases
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If the size of the chamber remains constant and some atoms are removed, the density decreases and the specific volume increases
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specification of a sequence of flow objects
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spectrum See ‘‘electromagnetic radiation spectrum’’
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Voice Command: Voice Command is a high-level interface that provides command and control speech recognition for applications. Voice Command allows a developer to create a Voice Command menu that contains voice commands, such as ‘‘new file’’ or ‘‘send mail to [email protected]’’ that a user speaks into a microphone or other audio device. The user can control the computer without needing a keyboard or mouse.
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Voice Dictation: Voice Dictation allows the user to dictate into any application that supports speech recognition. An invisible or virtual edit box receives the text the user dictates and displays the text in an application window. Voice Dictation allows text formatting such as capitalization, translation of punctuation words into punctuation symbols, built-in glossary entries, and correction of the last word spoken or a selected word. Applications that use Voice Dictation classify speech by topics that use different language styles. Topics include e-mail speech, formal writing, or programming speech. Voice Dictation stores the information for each topic on your hard drive.
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Voice Text: Voice Text converts text into speech that is played over computer speakers or sent over a telephone line. The speech played has several different modes, each with a different voice.
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Voice Telephony: Voice Telephony uses telephony controls that are similar to Windows controls. Windows controls include buttons, list boxes, sliders and other objects that can be manipulated by a mouse or keyboard. Telephony controls are codes that recognize spoken responses such as Yes or No, your phone number, the date, and the time. Telephony controls create a dialogue between the user and the computer. For example, a user calls a vendor to order an item. The user then answers
Also see density, kilogram, meter cubed, volume, and SI (International System of Units).
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SAPI (Speech Application Program Interface) is an application program interface (API) provided with the Microsoft Windows operating system that allows programmers to write programs that offer text-to-speech and speech recognition capabilities. Interfaces are provided for the C, C++, and Visual Basic programming languages. Using Microsoft’s COM (Component Object Model) architecture, SAPI is the most widely used speech application program interface used today. In the future, Microsoft plans to embed speech technology using SAPI into their operating system.
Imagine a variable-volume, airtight chamber containing a certain number of atoms of oxygen gas. Consider the following four examples:
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Speculation (also known as speculative loading), is a process implemented in Explicitly Parallel Instruction Computing (EPIC) processors and their compilers to reduce processor-
spiral model several questions by speaking into the telephone receiver. The telephony controls recognize these responses and sends them to the application that processes responses. Telephony controls also handle error conditions (these are common with spoken numbers or when the caller does not respond) and variations of answers such as ‘‘January 4th’’ or ‘‘tomorrow.’’ L
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DirectSpeechRecognition: This is a low-level interface similar to Voice Command. The main difference is DirectSpeechRecognition speaks directly to the speech engine. This gives the application more control and speed. DirectTextToSpeech: This is a low-level interface similar to Voice Text that also speaks directly to the speech engine. Audio Objects: An Audio Object tells the speech engine where to get its audio.
The future of speech technology will include products that allow you to do such things as surfing the Internet using speech and asking your television what is showing tonight. Software developers are developing applications that understand concepts. For example, if you tell your computer to print a certain document, your application will know whether to print it on your printer or the network’s printer. Speech technology is important for medical professionals, law enforcement personnel, the physically handicapped, as well as many business and home users.
speech recognition Speech or voice recognition is the ability of a machine or program to recognize and carry out voice commands or take dictation. In general, speech recognition involves the ability to match a voice pattern against a provided or acquired vocabulary. Usually, a limited vocabulary is provided with a product and the user can record additional words. More sophisticated software has the ability to accept natural speech (meaning speech as we usually speak it rather than carefully-spoken speech). Dragon Systems and IBM are two companies that have led in the research and development of products that support speech recognition.
speeds See ‘‘The speed of...’’
SPID A SPID (Service Profile Identifier) is a number assigned by a phone company to a terminal on an Integrated Services Digital Network B-channel. A SPID tells equipment at the phone company’s central office about the capabilities of each terminal (computer or phone) on the B-channels. A Basic Rate home or business user may divide service into two B-
channels with one used for normal phone service and the other for computer data. The SPID tells the phone company whether the terminal accepts voice or data information. Technically, the SPID is a numeric string from 3 to 20 digits in length. A SPID (or more than one, if necessary) is assigned when you order the ISDN Basic Rate Interface (BRI) from the phone company. Beginning in 1998, most phone companies began to use a generic SPID format. In this format, the SPID is a 14-digit number that includes your 10-digit telephone number (which includes your 3-digit Numbering Plan Area ]NPA[ or area code), a 2-digit Sharing Terminal Identifier, and a 2-digit Terminal Identifier (TID). The generic SPID format makes it easier to tell users what to specify when installing an ISDN line and simplifies corporate installation procedures. Beginning in 1998, some ISDN manufacturers began to provide non-initializing terminals (NITs) that do not require the entering of a SPID. Manufacturers also are delivering terminals with automated SPID selection in which the correct SPID is downloaded to the terminal rather than having to be specified by the user.
spider A spider is a program that visits Web sites and reads their pages and other information in order to create entries for a search engine index. The major search engines on the Web all have such a program, which is also known as a ‘‘crawler’’ or a ‘‘bot.’’ Spiders are typically programmed to visit sites that have been submitted by their owners as new or updated. Entire sites or specific pages can be selectively visited and indexed. Spiders are called spiders because they usually visit many sites in parallel at the same time, their ‘‘legs’’ spanning a large area of the ‘‘web.’’ Spiders can crawl through a site’s pages in several ways. One way is to follow all the hypertext links in each page until all the pages have been read. The spider for the AltaVista search engine and its Web site is called Scooter. Scooter adheres to the rules of politeness for Web spiders that are specified in the Standard for Robot Exclusion (SRE). It asks each server which files should be excluded from being indexed. It does not (or can not) go through firewalls. And it uses a special algorithm for waiting between successive server requests so that it doesn’t affect response time for other users.
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spiral model
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The spiral model, also known as the spiral lifecycle model, is a systems development method (SDM) used in information technology (IT). This model of development combines the features of the prototyping model and the systems development life cycle model (SDLC) or waterfall model. The spiral model is favored for large, expensive, and complicated projects.
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Whatis?Com’s Encyclopedia of Technology Terms The steps in the spiral model can be generalized as follows: 1. The new system requirements are defined in as much detail as possible. This usually involves interviewing a number of users representing all the external or internal users and other aspects of the existing system. 2. A preliminary design is created for the new system.
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3. A first prototype of the new system is constructed from the preliminary design. This is usually a scaled-down system, and represents an approximation of the characteristics of the final product. 4. A second prototype is evolved by a fourfold procedure: (1) evaluating the first prototype in terms of its strengths, weaknesses, and risks; (2) defining the requirements of the second prototype; (3) planning and designing the second prototype; (4) constructing and testing the second prototype. 5. At the customer’s option, the entire project can be aborted if the risk is deemed too great. Risk factors might involve development cost overruns, operating-cost miscalculation, or any other factor that could, in the customer’s judgment, result in a less-than-satisfactory final product. 6. The existing prototype is evauated in the same manner as was the previous prototype, and, if necessary, another prototype is developed from it according to the fourfold procedure outlined above. 7. The preceding steps are iterated until the customer is satisfied that the refined prototype represents the final product desired.
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8. The final system is constructed, based on the refined prototype.
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9. The final system is thoroughly evaluated and tested. Routine maintenance is carried out on a continuing basis to prevent large-scale failures and to minimize downtime.
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Also see systems development life cycle (SDLC), and systems development method (SDM).
splash page A splash page (or splash screen) is: 1) An initial Web site page used to capture the user’s attention for a short time as a promotion or lead-in to the site home page or to tell the user what kind of browser and other software they need to view the site. To have the splash page automatically move to the home page after a specified delay, include the HTML tag in the splash page as in this example with a 10-second delay: META http-equiv="refresh" content="10; URL=http:// www.somesitename.com/home.html" >
The advantage of a splash page is that you can create effects or provide information that is only needed once a visit. For example, a user can keep coming back to the home page without having to be bothered with browser requirements. 2) Some sites use ‘‘splash page’’ to mean the home page itself, especially where it contains attention-capturing visual or multimedia effects (creating a ‘‘splash’’).
splay tree A splay tree is a self-adjusting search algorithm for placing and locating files (called records or keys) in a database. The algorithm finds data by repeatedly making choices at decision points called nodes. In a splay tree, as in a binary tree, a node has two branches (also called children). Records are stored in locations called leaves. This name derives from the fact that records always exist at end points; there is nothing beyond them. The starting point is called the root. The number of access operations required to reach the desired record is called the depth. In a practical tree, there can be thousands, millions, or billions of nodes, children, leaves, and records. Not every leaf necessarily contains a record, but more than half do. A leaf that does not contain data is called a null. The splay tree scheme is unique because the tree organization varies depending on which nodes are most frequently accessed. This structural change takes place by means of so-called splaying operations, also called rotations. (In general, to splay is to spread or extend out or apart.) There are several ways in which splaying can be done. It always involves interchanging the root with the node in question. One or more other nodes might change position as well. The purpose of splaying is to minimize the number of access operations required to recover desired data records over a period of time. Also see binary tree, B-tree, and tree structure.
spline In computer graphics, a spline is a curve that connects two or more specific points, or that is defined by two or more points. The term can also refer to the mathematical equation that defines such a curve. Consider the set of points in the illustration below. It is easy to envision a curve that approximately connects the four points. In the old days of mechanical drafting, a flexible metal or wooden strip (called a spline, and the term from which the present term derives) was used to construct approximate graphs such as this. A computer can be used to test various curves, having known formulas, for an optimal ‘‘fit’’ for any finite set of points. In the example shown here, a near-perfect fit exists, and the curve has a relatively simple formula. Not all splines
spool are this straightforward. But in theory, at least one spline curve can be found that approximates a continuous graph for any finite set of points.
splitterless Splitterless refers to a type of Digital Subscriber Line (DSL) telephone service that does not require the installation of a plain old telephone service splitter at the customer location. Splitterless installation means that installation does not require ‘‘a truck roll’’ (that is, the cost of a visit from an installer ).
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spontaneous area network See ‘‘ad-hoc network’’
spoof ’’Spoof’’ was a game involving trickery and nonsense that was invented by an English comedian, Arthur Roberts, prior to 1884, when it is recorded as having been ‘‘revived.’’ Webster’s defines the verb to mean (1) to deceive or hoax, and (2) to make good-natured fun of. On the Internet, ‘‘to spoof’’ can mean: 1) To deceive for the purpose of gaining access to someone else’s resources (for example, to fake an Internet address so that one looks like a certain kind of Internet user)
split Among Internet Relay Chat (IRC) users, a split is a condition where two or more IRC servers are temporarily unable to communicate. When a split occurs between two computers on IRC, each seems to have gone offline to the other. The condition is usually temporary. It can be caused by a packet route timing out, a server rebooting, or a similar circumstance.
splitter In telephony, a splitter, sometimes called a ‘‘plain old telephone service splitter,’’ is a device that divides a telephone signal into two or more signals, each carrying a selected frequency range, and can also reassemble signals from multiple signal sources into a single signal. Users getting connected to the Internet with Asymmetric Digital Subscriber Line (ADSL) service may, in some cases, have a splitter installed at their home or business. Users elsewhere may be able to get splitterless service (which means that a splitter doesn’t need to be installed). For ADSL, the splitter divides the incoming signal into low frequencies to send to voice devices and high frequencies for data to the computer. The telephone company’s central office also uses a POTS splitter to send low-frequency voice signals on to the voice telephone network and to send high-frequency data to a Digital Subscriber Line Access Multiplexor (DSLAM) for transmission to the Internet.
2) To simulate a communications protocol by a program that is interjected into a normal sequence of processes for the purpose of adding some useful function 3) To playfully satirize a Web site.
spool To spool (which stands for ‘‘simultaneous peripheral operations online’’) a computer document or task list (or ‘‘job’’) is to read it in and store it, usually on a hard disk or larger storage medium so that it can be printed or otherwise processed at a more convenient time (for example, when a printer is finished printing its current document). One can envision spooling as reeling a document or task list onto a spool of thread so that it can be unreeled at a more convenient time. The idea of spooling originated in early computer days when input was read in on punched cards for immediate printing (or processing and then immediately printing of the results). Since the computer operates at a much faster rate than input/output devices such as printers, it was more effective to store the read-in lines on a magnetic disk until they could be conveniently printed when the printer was free and the computer was less busy working on other tasks. Actually, a printer has a buffer but frequently the buffer isn’t large enough to hold the entire document, requiring multiple I/O operations with the printer. The spooling of documents for printing and batch job requests still goes on in mainframe computers where many users share a pool of resources. On personal computers, your
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known as the dwell time. A few spread-spectrum circuits employ continuous frequency variation, which is an analog scheme.
spread spectrum
spreadsheet
Spread spectrum is a form of wireless communications in which the frequency of the transmitted signal is deliberately varied. This results in a much greater bandwidth than the signal would have if its frequency were not varied.
A spreadsheet is a sheet of paper that shows accounting or other data in rows and columns; a spreadsheet is also a computer application program that simulates a physical spreadsheet by capturing, displaying, and manipulating data arranged in rows and columns. The spreadsheet is one of the most popular uses of the personal computer.
A conventional wireless signal has a frequency, usually specified in megahertz (MHz) or gigahertz (GHz), that does not change with time (except for small, rapid fluctuations that occur as a result of modulation). When you listen to a signal at 103.1 MHz on an FM stereo receiver, for example, the signal stays at 103.1 MHz. It does not go up to 105.1 MHz or down to 99.1 MHz. The digits on the radio’s frequency dial stay the same at all times. The frequency of a conventional wireless signal is kept as constant as the state of the art will permit, so the bandwidth can be kept within certain limits, and so the signal can be easily located by someone who wants to retrieve the information. There are at least two problems with conventional wireless communications that can occur under certain circumstances. First, a signal whose frequency is constant is subject to catastrophic interference. This occurs when another signal is transmitted on, or very near, the frequency of the desired signal. Catastrophic interference can be accidental (as in amateur-radio communications) or it can be deliberate (as in wartime). Second, a constant-frequency signal is easy to intercept, and is therefore not well suited to applications in which information must be kept confidential between the source (transmitting party) and destination (receiving party). To minimize troubles that can arise from the above mentioned vulnerabilities of conventional communications circuits, the frequency of the transmitted signal can be deliberately varied over a comparatively large segment of the electromagnetic radiation spectrum. This variation is done according to a specific, but complicated mathematical function. In order to intercept the signal, a receiver must be tuned to frequencies that vary precisely according to this function. The receiver must ‘‘know’’ the frequency-versustime function employed by the transmitter, and must also ‘‘know’’ the starting-time point at which the function begins. If someone wants to jam a spread-spectrum signal, that person must have a transmitter that ‘‘knows’’ the function and its starting-time point. The spread-spectrum function must be kept out of the hands of unauthorized people or entities. Most spread-spectrum signals use a digital scheme called frequency hopping. The transmitter frequency changes abruptly, many times each second. Between ‘‘hops,’’ the transmitter frequency is stable. The length of time that the transmitter remains on a given frequency between ‘‘hops’’ is
In a spreadsheet, spaces that hold items of data are called cells. Each cell is labeled according to its placement (for example, A1, A2, A3...) and may have an absolute or relative reference to the cells around it. A spreadsheet is generally designed to hold numerical data and short text strings. Spreadsheets usually provide the ability to portray data relationships graphically. Spreadsheets generally do not offer the ability to structure and label data items as fully as a database and usually do not offer the ability to query the database. In general, a spreadsheet is a much simpler program than a database program. Daniel Bricklin and Bob Frankston created the first spreadsheet application, VisiCalc (for ‘‘visible calculator’’). Lotus 1-2-3 came next, followed by Microsoft Excel. While Lotus 1-2-3 was the first to introduce cell names and macros, Microsoft Excel implemented a graphical user interface and the ability to point and click using a mouse. There are many other spreadsheet applications on the market today; however, Lotus 1-2-3 and Microsoft Excel continue to be the most popular.
spyware In general, spyware is any technology that aids in gathering information about a person or organization without their knowledge. On the Internet, spyware is programming that is put in someone’s computer to secretly gather information about the user and relay it to advertisers or other interested parties. Spyware can get in a computer as a software virus or as the result of installing a new program. Data collecting programs that are installed with the user’s knowledge are not, properly speaking, spyware, if the user fully understands what data is being collected and with whom it is being shared. The cookie is a well-known mechanism for storing information about an Internet user on their own computer. However, the existence of cookies and their use is generally not concealed from users, who can also disallow access to cookie information. Nevertheless, to the extent that a Web site stores information about you in a cookie that you don’t know about, the cookie mechanism could be considered a form of spyware. DoubleClick, a leading banner ad serving company, changed its plans to combine cookie information with database information from other sources to target ad
SQUID campaigns directly to individuals without their permission. DoubleClick’s current policy is not to collect ‘‘personallyidentifiable’’ information about a user without their explicit permission or ‘‘opt-in.’’ Aureate Media, which distributes free software on the Web in exchange for the right to gather user information, is another company that has been criticized for not plainly indicating what data it gathers and for making it difficult to remove its programming. Spyware is part of an overall public concern about privacy on the Internet.
SQL SQL (Structured Query Language) is a standard interactive and programming language for getting information from and updating a database. Although SQL is both an ANSI and an ISO standard, many database products support SQL with proprietary extensions to the standard language. Queries take the form of a command language that lets you select, insert, update, find out the location of data, and so forth. There is also a programming interface.
SQLJ Java See ‘‘SQLJ’’
SQLJ SQLJ is a set of programming extensions that allow a programmer using the Java programming language to embed statements that provide SQL (Structured Query Language) database requests. SQLJ is similar to existing extensions for SQL that are provided for C, Formula Translation, and other programming languages. IBM, Oracle, and several other companies are proposed SQLJ as a standard and as a simpler and easier-to-use alternative to Java Database Connectivity. The SQLJ specifications are in several parts: L
SQLJ: Embedded SQL...Specifications for embedding SQL statements in Java methods.
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times (22) as great; if all linear dimensions are cut to 1/3, the area becomes 1/9 (1/32) as great. An area of 1 m2 is equal to 10,000 centimeters squared (104 cm2) or 1,000,000 millimeters squared (106 mm2). In the opposite sense, 1 m2 is equal to 0.000001 kilometer squared (10-6 km2). When converting between square meters and non-SI units of area such as square inches (sq in) or square miles (sq mi), the linear-unit conversion factor must be squared. For example, one meter is approximately 39.37 inches (39.37 in); therefore 1 m2 = 39.372 = 1550 sq in (approximately). As another example, 1 meter is about 0.0006215 mile (6.215 6 10-4 mi); therefore 1 m2 = (6.215 6 10-4)2 = 3.863 6 10-7 sq mi (approximately). Also see meter, cubic meter, and International System of Units (SI).
Squeak Squeak is an implementation of the Smalltalk programming environment that is designed to allow code to be run on any major operating system, including Windows 95/98/NT, Windows CE, and UNIX. Squeak includes a Smalltalk-80 compiler and a caching just-in-time compiler (JIT) compiler and a large class library. The environment supports realtime sound and music synthesis, Musical Instrument Digital Interface (MIDI) performance and scheduling, text-tospeech synthesis, 2- and 3-D graphics, and Web serving and interaction. Squeak’s easy-to-use graphical interface and its objectoriented approach make it an appealing development environment for educators. A pointer device such as a mouse is required as with all Smalltalk programming environments. The original mouse used with Smalltalk required three buttons each labeled yellow, red, and blue, which is still required today. If a three-button mouse is not available, it must be simulated using another method. For example, the Macintosh-based Squeak environment uses a single-button mouse so the functions are determined by context. This means that if the mouse is placed over a certain area of the screen or another key is pushed at the same time, a menu is activated. The Squeak programming environment is not the same as the Squeak language developed by Rob Pike and Luca Cardelli in 1985.
SQUID
square meter
Also see SQUID (Source Quench Introduced Delay).
The square meter, also called the meter squared, is the International System of Units (SI) unit of area. The symbol for square meter is m2. Less formally, square meter is sometimes abbreviated sq m.
SQUID is a program that caches Web and other Internet content in a UNIX-based proxy server closer to the user than the content-originating site. SQUID is provided as open source software and can be used under the GNU license for Free Software Foundation.
When calculating area, it is important to realize that area is proportional to the square of the linear dimension. Thus, if all linear dimensions are doubled, the area becomes four
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SRAM (static RAM) is random access memory (RAM) that retains data bits in its memory as long as power is being supplied. Unlike dynamic RAM (DRAM), which stores bits in cells consisting of a capacitor and a transistor, SRAM does not have to be periodically refreshed. Static RAM provides faster access to data and is more expensive than DRAM. SRAM is used for a computer’s cache memory and as part of the random access memory digital-to-analog converter on a video card.
For each stage, SSADM sets out a series of techniques and procedures, and conventions for recording and communicating information pertaining to these—both in textual and diagramatic form. SSADM is a very comprehensive model, and a characteristic of the method is that projects may use only those elements of SSADM appropriate to the project. SSADM is supported by a number of CASE tool providers.
SS7
SSH
See ‘‘Signaling System 7’’
See ‘‘Secure Shell’’
SSADM
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SSADM (Structured Systems Analysis & Design Method) is a widely-used computer application development method in the UK, where its use is often specified as a requirement for government computing projects. It is increasingly being adopted by the public sector in Europe. SSADM is in the public domain, and is formally specified in British Standard BS7738.
See ‘‘Server-side include’’
SSADM divides an application development project into modules, stages, steps, and tasks, and provides a framework for describing projects in a fashion suited to managing the project. SSADM’s objectives are to:
See ‘‘Secure Sockets Layer’’
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SSL card See ‘‘server accelerator card’’
SSL SSO See ‘‘single signon’’
SSP See ‘‘storage service provider’’
stack (1) TCP/IP is frequently referred to as a ‘‘stack.’’ This refers to the layers (TCP, IP, and sometimes others) through which all data passes at both client and server ends of a data exchange. A clear picture of layers similar to those of TCP/ IP is provided in our description of OSI, the reference model of the layers involved in any network communication. The term ‘‘stack’’ is sometimes used to include utilities that support the layers of TCP/IP. The Netscape Handbook says (and we quote): ‘‘To make a successful connection to the Internet, your PC needs application software such as Netscape plus a TCP/IP stack consisting of TCP/IP software, sockets software (Winsock.dynamic link library), and hardware driver software (packet drivers). Several popular TCP/IP stacks are available for Windows, including shareware stacks.’’ (2) In programming, a stack is a data area or buffer used for storing requests that need to be handled. The IBM Dictionary of Computing says that a stack is always a push-down list, meaning that as new requests come in, they push down the old ones. Another way of looking at a push-down list—or
standard definition television stack—is that the program always takes its next item to handle from the top of the stack. (This is unlike other arrangements such as ‘‘FIFO’’ or ‘‘first-in first-out.’’)
stackable hub A stackable hub is a hub designed to be connected and stacked or positioned on top of another hub, forming an expanding stack. Since a hub is basically a concentrator of device connections, a set of stackable hubs is just a bigger concentrator. The stackable approach allows equipment to be easily and economically expanded as a grows in size. The stacking feature also reduces clutter. Typically, devices with network interface cards (NICs) are connected to each hub with shielded twisted pair (STP) or unshielded twisted pair (UTP) cable. The set of stackable hubs is interconnected with a very short ‘‘cascading’’ cable in the rear of the stack. A special port, such as an Ethernet Attachment Unit Interface (AUI) port, may be provided to connect the set of stackable hubs to a backbone cable that connects to other sets of stackable hubs or other network devices. Typical stackable hub options include: L
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Hardware and software to let you manage the stackable hubs using the Simple Network Management Protocol (SNMP)
stale data In computer processing, if a processor changes the value of an operand and then, at a subsequent time, fetches the operand and obtains the old rather than the new value of the operand, then it is said to have seen stale data. On a uniprocessor, stale data cannot be tolerated. It would mean that the processor violated fundamental expectations about its own behavior. On shared memory multiprocessors, however, it is considered acceptable for machines to generate stale data on operands shared between processes. For such operands, the expectation is that programs will take precautions (atomic instructions or critical section routines) to prevent stale data from being seen.
stamped multisession CD standard See ‘‘Blue Book’’
standalone dump A standalone dump is the copying of the contents of computer main storage (RAM) to another storage device, usually for the purpose of debugging a programming problem. (An earlier, now out-of-date term is core dump.) The term is primarily used in the mainframe environment. When an IBM mainframe stops operating because of a ‘‘hard wait state’’ or a ‘‘machine check’’ condition, an operator or system programmer can perform a ‘‘Store Status’’ operation that saves the program status word (PSW) and the 16 general purpose registers (GPRs), which tell exactly what the computer was doing when it stopped operating, and then performs a standalone dump to copy real and virtual memory to tape or direct access storage device (disk storage). These operations are performed from a system master console. After an initial program load (IPL) to restart the operating system, the standalone dump can be formatted by a special utility program and printed or displayed. Diagnostic techniques can be used to isolate the failing component so that ultimately the customer can get a fix from IBM or report a new problem. When a mainframe operator encounters ‘‘hard stop’’ errors in the middle of the night and calls the system programmer, the advice is often to ‘‘Take a standalone dump and call me in the morning.’’
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Standard definition television (SDTV) is a digital television (DTV) format that provides a picture quality similar to digital versatile disc (DVD). SDTV and high definition television (HDTV) are the two categories of display formats for digital television (DTV) transmissions, which are becoming the standard.
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Whatis?Com’s Encyclopedia of Technology Terms When the U.S. decided to make the transition from analog television to DTV, the Federal Communications Commission decided to let broadcasters decide whether to broadcast SDTV or HDTV programs. Most have decided to broadcast SDTV programs in the daytime and to broadcast HDTV programs during prime time broadcasting. Both SDTV and HDTV are supported by the Digital Video Broadcasting (DTV) and Advanced Television Systems Committee (ATSC) set of standards.
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Standard MIDI File The SMF (Standard MIDI File) was developed to allow musicians and audio file developers to transfer their sequenced MIDI data from one software application to another. This file format lets users exchange music and data without regard to computer operating system platform. Today, there are many Web sites devoted to the sale and exchange of music data in the Standard MIDI File format. There are two variations of the Standard MIDI File format, called Type 0 and Type 1. Both variations contain the same data, but the arrangement of the data within the file is different. Type 0 is a single track of data; Type 1 is multitrack. Many of the popular sequencing software packages available can import and export either type of file. In the Type 0 Standard MIDI File, all data is saved to a single track without regard to its MIDI channel reference. The file plays normally, but accessing the individual components is a bit more difficult. If editing is needed, the user can open the Standard MIDI File in the application of choice and copy/ paste individual channels of information manually as needed. A Type 1 Standard MIDI File outputs a separate track for each MIDI channel in the original sequence. This format greatly simplifies editing once the sequence has been transferred from one sequencer to another.
standard temperature and pressure Standard temperature and pressure, abbreviated STP, refers to nominal conditions in the atmosphere at sea level. This value is important to physicists, chemists, engineers, and pilots and navigators. Standard temperature is defined as zero degrees Celsius (08C), which translates to 32 degrees Fahrenheit (328F) or 273.15 degrees kelvin (273.158K). This is essentially the freezing point of pure water at sea level, in air at standard pressure. Standard pressure supports 760 millimeters in a mercurial barometer (760 mmHg). This is about 29.9 inches of mercury, and represents approximately 14.7 pounds per inch (14.7 lb/
in2). Imagine a column of air measuring one inch square, extending straight up into space beyond the atmosphere. The air in such a column would weigh about 14.7 pounds. The density of air at STP is approximately 1.29 kilogram per meter cubed (1.29 kg/m3). This fact comes as a surprise to many people; a cubic meter of air weighs nearly three pounds! Also see kilogram, meter, kilogram per meter cubed, SI (International System of Units), and specific gravity.
Star processor IBM’s Star processor is a reduced instruction set computer (RISC) processor that is available in several series versions for IBM’s AS/400 and RS/6000 systems. The three processors in the Star Series are the Northstar, the Pulsar, and the Silicon-On-Insulator (SOI) processor. The first in the Star Series, the Northstar processor, is superscalar, which means it can execute multiple instructions per clock cycle, and includes two 64 Kbps onchip level-one (L1) caches. Its operating frequency is 262 MHz, and it has a 162 mm2 die size. It uses 27 watts of power. The Northstar processor is based on traditional complementary metal-oxide semiconductor (CMOS) fabrication technology. It is often called the A50 processor in AS/400 systems and the RS64-II processor in RS/6000 systems. The Pulsar is the second Star Series processor. It is based on a combined copper and silicon fabrication technology, which affords more operating speed using less power. Its operating frequency is 450 MHz, and it has a 140 mm2 die size. It includes two 128 Kbps on-chip L1 caches and operates on 24 watts of power. The Northstar processor was the basis for the Pulsar. The Pulsar powers the RS/6000 S80 system. It is often called the RS64-III processor. The final Star Series processor is the Silicon-on-Insulatorbased processor used in the AS/400 iSeries server. SOIbased processors are manufactured using pure silicon and silicon oxide. The operating frequency is 550 MHz, and it has a 140 mm2 die size. It includes two 128 Kbps on-chip L1 caches and uses 22 watts of power.
StarBand StarBand is a broadband Internet service provider (ISP) that uses geostationary satellites to provide always-on connection independent of other media. Established in late 2000, StarBand was the first widely-available service for the general public that made use of satellite links for both upstream and downstream data. StarBand connection requires a Universal Serial Bus (USB) port or an Ethernet card, a special modem, a dish antenna measuring approximately 2 feet high by 3 feet wide by 3 feet deep, and two coaxial cables that run between the dish and the modem. Professional installation is recommended.
state machine Upstream data speeds, at the time of writing, were reported by users as 30 to 100 kilobits per second (Kbps), with 50 Kbps being typical. Downstream data speeds depend on the Web site visited, the complexity of downloaded pages, and the time of day. Normal downloads range from 150 Kbps to 500 Kbps. Because StarBand uses a satellite for both the upstream and the downstream links, the latency is considerable. A geostationary satellite orbits 22,300 miles above the Earth’s equator. For a user at a temperate latitude, signals must make four trips through space, each of approximately 23,000 miles, between a mouse click and the appearance of Web site data on the display. This introduces a delay of 0.5 second, because the speed of electromagnetic (EM) radiation is finite. But that is the mathematical minimum. In reality, the latency is usually longer. The upstream data must travel from the user’s dish to the satellite, and then back down to the StarBand hub, where it is transmitted over land-based media. The downstream data must travel from the Web site under observation to the StarBand hub over land-based media, then up to the satellite, and back down to the user. The result is latency that sometimes exceeds 1 second. This is not a system defect, but technical reality. StarBand is not recommended for highly interactive applications such as gaming. But for people in remote regions with no other option, StarBand can provide good broadband Internet service. The author of this definition has had the service since March of 2001, and it has proven invaluable. The only alternative is dial-up through antiquated telephone lines, resulting in real-world browsing speeds averaging 10 Kbps or somewhat higher. StarBand cannot compete with a well-installed DSL or cable modem connection, but it has provided the author with a browsing speed increase of over 1,000 percent, even taking latency into account.
StarOffice StarOffice is a free productivity application suite from Sun Microsystems that includes a word processor (Writer), spreadsheet (Calc), database (Base), presentation maker (Impress), illustrator (Draw), schedule managment (Schedule), e-mail (Mail) and newsgroup (Discussion) component. StarOffice is compatible with Microsoft Office components (for example, you can open and save in Microsoft’s Word, Excel, and PowerPoint file formats) and is available in 11 languages. Fans of StarOffice appreciate its cross-platform capabilities and feel it offers low-budget users a good alternative to Microsoft’s Office. Critics acknowledge that StarOffice is intuitive to use, but find the desktop interface floating palettes awkward. A common complaint about StarOffice is that you must open the entire suite of applications even if you only want to work with an individual component.
Sun Microsystems released an open-source version of StarOffice under the GNU general public license in October of 2000. Sun appointed Openoffice.org to manage the opensource version, using a development model similar to that of Netscape’s Mozilla. Users of the open-source version of StarOffice must not only agree to the GNU general public license terms, they must also agree to Sun Industry Standard Source License (SISSL) which includes control over programming interfaces and compatibility tests. Sun plans to replace their proprietary default file format with Extensible Markup Language (XML), and is promoting the development of a Web-based version of StarOffice. StarOffice is compatible with Sun Solaris SPARC, Sun Solaris Intel, Windows 95/98/NT, Linux, and OS/2. At present, there is no Mac version.
statampere The statampere (abbreviated statA or A-esu) is the unit of electric current in the cgs (centimeter/gram/second) system of electrostatic units (esu). It is the equivalent of one statcoulomb (1 statC), also called one electrostatic unit (1 esu), of charge carriers moving past a specific point in one second. The statampere is a small unit of current, equivalent to approximately 3.3356 6 10-10 ampere (A) or 0.33356 nanoampere (nA). In most applications, the ampere, which is the unit of current in the International System of Units (SI), is preferred. Also see current, cgs or small-unit metric system, and International System of Units (SI).
statcoulomb The statcoulomb (abbreviated statC) is the unit of electric charge quantity in the cgs (centimeter/gram/second) system. It is approximately equal to the charge contained in 2.082 6 109 electrons. When two objects, carrying like electrical charges of 1 statC each, are placed so their charge centers are one centimeter (1 cm) apart, the objects repel each other with a force of one dyne (1 dyn). The statcoulomb is the basis for the system of electrostatic units (esu). It is a comparatively small unit in practical terms, equivalent to approximately 3.3356 6 10-10 coulomb (C). In most applications, the coulomb, which is the standard unit of charge quantity in the International System of Units (SI), is preferred.
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Also see charge quantity, small-unit metric system or cgs system of units, and International System of Units (SI).
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In general, a state machine is any device that stores the status of something at a given time and can operate on input to change the status and/or cause an action or output to take place for any given change. A computer is basically a state
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machine and each machine instruction is input that changes one or more states and may cause other actions to take place. Each computer’s data register stores a state. The read-only memory from which a boot program is loaded stores a state (the boot program itself is an initial state). The operating system is itself a state and each application that runs begins with some initial state that may change as it begins to handle input. Thus, at any moment in time, a computer system can be seen as a very complex set of states and each program in it as a state machine. In practice, however, state machines are used to develop and describe specific device or program interactions. To summarize it, a state machine can be described as: L
An initial state or record of something stored someplace
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A set of new states that may result from the input
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A set of possible actions or output events that result from a new state
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A function that maps states and inputs to states (which is called a state transition function)
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A description of the initial state
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A finite state machine is one that has a limited or finite number of possible states. (An infinite state machine can be conceived but is not practical.) A finite state machine can be used both as a development tool for approaching and solving problems and as a formal way of describing the solution for later developers and system maintainers. There are a number of ways to show state machines, from simple tables through graphically animated illustrations.
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Also see finite state machine. Stateful and stateless are adjectives that describe whether a computer or computer program is designed to note and remember one or more preceding events in a given sequence of interactions with a user, another computer or program, a device, or other outside element. Stateful means the computer or program keeps track of the state of interaction, usually by setting values in a storage field designated for that purpose. Stateless means there is no record of previous interactions and each interaction request has to be handled based entirely on information that comes with it. Stateful and stateless are derived from the usage of state as a set of conditions at a moment in time. (Computers are inherently
stateful in operation, so these terms are used in the context of a particular set of interactions, not of how computers work in general.) The Internet’s basic protocol, the Internet Protocol (IP), is an example of a stateless interaction. Each packet travels entirely on its own without reference to any other packet. When you request a Web page from a Web site, the request travels in one or more packets, each independent of the other as far as the Internet Protocol program itself is concerned. (The upper layer Transmission Control Protocol—TCP—does relate packets to each other, but uses the information within the packet rather than some external information to do this.) The term connectionless is also used to describe communication in which a connection is made and terminated for each message that is sent. IP is connectionless as well as stateless. The Web’s Hypertext Transfer Protocol (HTTP), an application layer above TCP/IP, is also stateless. Each request from a user for a Web page or URL results in the requested pages being served, but without the Web (HTTP) server remembering the request later. In other words, there is no recorded continuity. Each communication is discrete and unrelated to those that precede or follow. In order to have stateful communication, a site developer must furnish a special program that the server can call that can record and retrieve state information. Web browsers such as Netscape Explorer and Microsoft Internet Explorer provide an area in their subdirectories where state information can be stored and accessed. The area and the information that Web browsers and server applications put in this area is called a cookie. In formal protocol specifications, a finite state machine is an abstract desciption of how a stateful system works that describes the action that follows each possible state. The Internet (including the World Wide Web) can be thought of as a stateless system or machine. Most computers and human beings are stateful.
statfarad The statfarad (abbreviated statF) is the unit of capacitance in the cgs (centimeter/gram/second) or small-unit metric system. It is equivalent to 1.1126 6 10-12 farads. The farad (F) is the standard unit of capacitance in the International System of Units (SI). A capacitance as large as 1 F is almost unknown in the real world, but a 1-statF capacitance is common. In fact, two wires only a few centimeters long, placed in the vicinity of and parallel to each other, display a mutual capacitance on the order of 1 statF. In practical applications, the microfarad (mF) and the picofarad (pF) are most often used to quantify capacitance; the statfarad is rarely seen in literature. For comparison, 1 mF = 10-6 F and 1 pF = 10-12 F. Thus, 1 statF = 1.1126 6 10-6 mF = 1.1126 pF.
statohm Also see capacitance, farad, cgs or small-unit metric system, and International System of Units (SI).
stathenry The stathenry (abbreviated statH) is the unit of inductance in the cgs (centimeter/gram/second) or small-unit metric system. It is equivalent to 8.9876 6 1011 henrys. The henry (H) is the standard unit of inductance in the International System of Units (SI). It is rare to see an inductance as large as 1 H, but a 1-statH inductance is unknown in the real world. To produce an inductance that large, an impossibly long span of wire would have to be coiled into a tiny volume around a ferromagnetic core having a permeability higher than that of any known substance. (It might someday be possible, however, to produce inductances of that magnitude by supercooling.) In practical applications, the microhenry (mH) and the nanohenry (nH) are most often used to quantify inductance. For comparison, 1 mH = 10-6 H and 1 nH = 10-9 H. Thus, 1 statH = 8.9876 6 1017 mH = 8.9876 6 1020 nH. Also see inductance, henry, cgs or small-unit metric system, and International System of Units (SI).
static IP address/dynamic IP address Also see IP address. A static IP is a number (in the form of a dotted quad) that is assigned to a computer by an Internet Service Provider (ISP) to be its permanent address on the Internet. Computers use IP addresses to locate and talk to each other on the Internet, much the same way people use phone numbers to locate and talk to one another on the telephone. When you want to visit whatis.com, your computer asks a domain name system (DNS) server (think telephone information operator) for the correct dotted quad number (think phone number) for whatis.com and your computer uses the answer it receives to connect to the whatis.com server. It would be simple if every computer that connects to the Internet could have its own static IP number, but when the Internet was first conceived, the architects didn’t foresee the need for an unlimited number of IP addresses. Consequently, there are not enough IP numbers to go around. To get around that problem, many Internet service providers limit the number of static IP addresses they allocate, and economize on the remaining number of IP addresses they possess by temporarily assigning an IP address to a requesting Dynamic Host Configuration Protocol (DHCP) computer from a pool of IP addresses. The temporary IP address is called a dynamic IP address. Requesting DHCP computers receive a dynamic IP address (think temporary phone number) for the duration of that Internet session or for some other specified amount of time. Once the user disconnects from the Internet, their dynamic
IP address goes back into the IP address pool so it can be assigned to another user. Even if the user reconnects immediately, odds are they will not be assigned the same IP address from the pool. To keep our telephone telephone analogy going, using a dynamic IP address is similar to using a pay phone. Unless there is a reason to receive a call, the user does not care what number he or she is calling from. There are times, however, when users who connect to the Internet using dynamic IP wish to allow other computers to locate them. Perhaps they want to use CU-SeeMe or use a VoIP application to make long distance phone calls using their IP connection. In that case, they would need a static IP address. The user has two choices; they can contact their ISP and request a static IP address, or they can use a dynamic DNS service. Either choice will probably involve an additional monthly fee. Using a dynamic DNS service works as if there was an oldfashioned telephone message service at your computer’s disposal. When a user registers with a DNS service and connects to the Internet with a dynamic IP address, the user’s computer contacts the DNS service and lets them know what IP address it has been assigned from the pool; the service works with the DNS server to forward the correct address to the requesting DHCP computer. (Think of calling the message service and saying ‘‘Hi. I can be reached at 435.44.32.111 right now. Please tell anyone who tries to reach me to call that number.) Using a dynamic DNS service to arrange for computers to find you even though you are using a dynamic IP address is the next-best thing to having a static IP. IPv6, which has also been called ‘‘IPng’’ (IP Next Generation) will lengthen IP addresses from 32 bits to 128 bits and increase the number of available IP addressess significantly, making static IP addresses easier and less expensive to obtain and maintain.
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static random access memory
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statistical time-division multiplexing
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statohm The statohm (symbolized statO) is the unit of resistance in the cgs (centimeter/gram/second) electrostatic system of units. It is equivalent to approximately 8.9876 6 1011 ohms. The ohm (O) is the standard unit of resistance in the International System of Units (SI).
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The statohm is an extremely large unit of resistance. In fact, an object with a resistance of 1 statO would make an excellent insulator or dielectric. In practical applications, the ohm, the kilohm (kO) and the megohm (MO or M) are most often used to quantify resistance. For comparison, 1 statO = 8.9876 6 105 MO = 8.9876 6 108 kO.
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The statsiemens (symbolized statS) is the unit of conductance in the cgs (centimeter/gram/second) electrostatic system of units. It is equivalent to approximately 1.11265 6 10-12 siemens (S) or 1.11265 picosiemens (pS). The siemens is the standard unit of conductance in the International System of Units (SI). The statsiemens is an extremely small unit of conductance. In fact, an object with a conductance of 1 statS would make an excellent insulator or dielectric. In practical applications, the siemens, the millisiemens (mS) and the microsiemens (mS)are most often used to quantify conductance. For comparison, 1 statS = 1.11265 6 10-6 mS = 1.11265 6 10-9 mS. Also see conductance, siemens, cgs or small-unit metric system, and International System of Units (SI).
statvolt The statvolt (symbolized statV) is the unit of electromotive force (EMF) or potential difference in the cgs (centimeter/ gram/second) electrostatic system. When an EMF of 1 statV exists between two points, then one erg of energy is needed to move one statcoulomb (1 statC) of charge carriers between those two points.
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A potential difference of 1 statV will drive a current of one statampere (1 statA) through a resistance of one statohm (statO). Ohm’s Law applies for the electrostatic units, just as it does for the units in the International System (SI). That is:
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E = IR
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where E is the EMF in statvolts, I is the current in statamperes, and R is the resistance in statohms.
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The statvolt is a fairly large unit of EMF, equal to approximately 299.79 volts (V). In most practical applications, the volt, which is the unit of potential difference in SI, is preferred. Also see voltage, cgs or small-unit metric system, and International System of Units (SI).
statwatt The statwatt (symbolized statW) is the unit of power in the cgs (centimeter/gram/second) electrostatic system. In a direct current (DC) circuit, 1 statW is the power dissipated,
radiated, or expended when one statvolt (1 statV) of potential difference drives a current of one statampere (1 statA) through a component. In a DC circuit, or in an alternating current (AC) circuit in which there is no reactance, the following formula holds: P = EI where P is the power in statwatts, E is the potential difference in statvolts, and I is the current in statamperes. The statwatt is a small unit of power, equivalent to 10-7 watt (W) or 0.1 microwatt (mW). In most applications, the watt, which is the unit of power in the International System (SI), is preferred. Also see power, watt, cgs or small-unit metric system, and International System of Units (SI).
STDM STDM, or statistical time division multiplexing, is one method for transmitting several types of data simultaneously across a single transmission cable or line (such as a T1 or T3 line). STDM is often used for managing data being transmitted via a local area network (LAN) or a wide area network (WAN). In these situations, the data is often simultaneously transmitted from any number of input devices attached to the network, including computers, printers, or fax machines. STDM can also be used in telephone switchboard settings to manage the simultaneous calls going to or coming from multiple, internal telephone lines. The concept behind STDM is similar to TDM, or time division multiplexing. TDM allows multiple users or input devices to transmit or receive data simultaneously by assigning each device the same, fixed amount of time on one of many ‘‘channels’’ available on the cable or line. The TDM method works well in many cases, but does not always account for the varying data transmission needs of different devices or users. For example, a busy laser printer shared by many users might need to receive or transmit data 80-90% of the time at a much higher transmission rate than a seldom-used, dataentry computer attached to the same T-1 line. With TDM, even though the printer’s transmission needs are greater, both devices would still be allocated the same duration of time to transmit or receive data. In comparison to TDM, the STDM method analyzes statistics related to the typical workload of each input device (printer, fax, computer) and determines on-the-fly how much time each device should be allocated for data transmission on the cable or line. In the above example, STDM would allocate more time to the group printer, based on its past and current transmission needs and less time to the data-entry computer. Many believe the STDM method is a more efficient use of total bandwidth available than the TDM method.
steradian The main statistics used in STDM are: each input device’s peak data rates (in kbps, or kilobytes per second), and each device’s duty factors (which is the percentage of time the device typically spends either transmitting or receiving data). Besides TDM and STDM, other methods for simultaneously transmitting data on the same channel include Wavelength Division Multiplexing (WDM) and Frequency Division Multiplexing (FDM).
stealth In computing, stealth refers to an event, object, or file that evades methodical attempts to find it. In particular, the term applies to certain computer viruses, and to a state of affairs in which a computer or port is rendered invisible to hacking programs. A stealth virus can pass through an antivirus program unless and until the program is updated to include the virus in its database. Such a virus can infect the boot sector of a diskette, removable disk, or hard drive, thereby copying itself onto other media when files or folders are opened or transferred. If such a virus is written and distributed with malicious intent, it can cause great damage to computers and networks. The problem is especially serious if the virus does its dirty work insidiously, so it is not even discovered until it has spread to thousands of computers and corrupted millions of files. Hackers are constantly scanning the Internet for computers or networks to exploit. An effective firewall can keep hackers out of computers, and is considered a must for computers equipped with broadband, always-on Internet connections. The best firewalls keep hacking programs from detecting the existence of a computer or network port. When a port-scanning program encounters a stealth (invisible) port, no reply is received in response to requests for connection. The hacking program then passes on to the next potential victim.
way, the result will be data that appears to have the ‘‘noise’’ patterns of regular, nonencrypted data. A trademark or other identifying symbol hidden in software code is sometimes known as a watermark. Recently revived, this formerly obsolete term gained currency in its day (1500) from a work by Johannes Trithemius, Steganographia, ostensibly a system of angel magic but also claiming to include a synthesis of how to learn and know things contained within a system of cryptography. The book was privately circulated but never published by the author because those who read it found it rather fearsome.
In modern digital steganography, data is first encrypted by the usual means and then inserted, using a special algorithm, into redundant (that is, provided but unneeded) data that is part of a particular file format such as a JPEG image. Think of all the bits that represent the same color pixels repeated in a row. By applying the encrypted data to this redundant data in some random or nonconspicuous
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steradian The steradian (symbolized sr) is the International System of Units (SI) unit of solid angular measure. There are 4 pi, or approximately 12.5664, steradians in a complete sphere. A steradian is defined as conical in shape, as shown in the illustration. Point P represents the center of the sphere. The solid (conical) angle q, representing one steradian, is such that the area A of the subtended portion of the sphere is equal to r2, where r is the radius of the sphere.
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steganography Steganography (pronounced STEHG-uh-NAH-gruhf-ee, from Greek steganos, or ‘‘covered,’’ and graphie, or ‘‘writing’’) is the hiding of a secret message within an ordinary message and the extraction of it at its destination. Steganography takes cryptography a step farther by hiding an encrypted message so that no one suspects it exists. Ideally, anyone scanning your data will fail to know it contains encrypted data.
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Q R A general sense of the steradian can be envisioned by considering a sphere whose radius is one meter (r = 1m). Imagine a cone with its apex P at the center of the sphere, and that intersects the surface in a circle (shown as an ellipse, the upper half of which is dashed). Suppose the flare angle q of the cone is such that the area A of the spherical segment within the circle is equal to one meter squared (A = 1 m2). Then the flare angle of the cone is equal to 1 steradian (q = 1 sr). The total surface area of the sphere is, in this case, 12.5664 square meters (4 pi times the square of the radius).
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Based on the foregoing example, the geometry of which is independent of scale, it can be said that a solid angle of 1 sr encompasses about 1/12.5664, or 7.9577 percent, of the space surrounding a point. Also see radian and International System of Units (SI).
stickiness Stickiness is anything about a Web site that encourages a visitor to stay longer. A Web site is sticky if a visitor tends to stay for a long time and to return. The two most important measurements of what a Web site has to offer advertisers are ad views and the average time each visitor spends on the site. If you can make your visitor spend more time at your site, they will view more ads and they can be considered more loyal to the site and presumably somewhat more likely to click on the ads and patronize advertisers. Portal sites like Yahoo achieve stickiness by having a great deal of content, and also by finding ways to involve the user with the site. The most common stickiness approaches are:
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Providing content that the user really wants
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Allowing the user to personalize the site (Yahoo, Excite, and MSN Network, for example)
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Building online communities in which users post information or form discussion groups (Yahoo, Geocities)
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Inviting user feedback in response to columnists (ZDNet)
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Adding games to the site (Yahoo)
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Using extensive hypertext cross-references to other parts of the site (whatis.com and many others)
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Stickiness may also be said to apply to a set of related sites such as those of CNET and TechTarget, which have a common design and navigation interface and sell ads across the sites as well as on individual sites.
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1) Generally, stochastic (pronounced stow-KAS-tik, from the Greek stochastikos, or ‘‘skilled at aiming,’’ since stochos is a target) describes an approach to anything that is based on probability. 2) In mathematics, a stochastic approach is one in which values are obtained from a corresponding sequence of jointly distributed random variables. Classic examples of the stochastic process are guessing the length of a queue at a stated time given the random distribution over time of a number of people or objects entering and leaving the queue and guessing the amount of water in a reservoir based on the random distribution of rainfall and water usage.
stockkeeping unit See ‘‘SKU’’
storage Computer storage is the holding of data in an electromagnetic form for access by a computer processor. Primary storage is data in random access memory (RAM) and other ‘‘built-in’’ devices. Secondary storage is data on hard disks, tapes, and other external devices. Primary storage is much faster to access than secondary storage because of the proximity of the storage to the processor or because of the nature of the storage devices. On the other hand, secondary storage can hold much more data than primary storage. In addition to RAM, primary storage includes read-only memory (ROM) and L1 and L2 cache memory. In addition to hard disks, secondary storage includes a range of device types and technologies, including diskettes, Zip drives, RAID systems, and holographic storage. Devices that hold storage are collectively known as storage media.
storage area network A storage area network (SAN) is a high-speed specialpurpose network (or subnetwork) that interconnects different kinds of data storage devices with associated data servers on behalf of a larger network of users. Typically, a storage area network is part of the overall network of computing resources for an enterprise. A storage area network is usually clustered in close proximity to other computing resources such as IBM S/390 mainframes but may also extend to remote locations for backup and archival storage, using wide area network carrier technologies such as asynchronous transfer mode or Synchronous Optical Networks. A storage area network can use existing communication technology such as IBM’s optical fiber ESCON or it may use the newer Fibre Channel technology. Some SAN system integrators liken it to the common storage bus (flow of data) in a personal computer that is shared by different kinds of storage devices such as a hard disk or a CD-ROM player. SANs support disk mirroring, backup and restore, archival and retrieval of archived data, data migration from one storage device to another, and the sharing of data among different servers in a network. SANs can incorporate subnetworks with network-attached storage (NAS) systems.
storage media See ‘‘storage medium’’
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storage medium In computers, a storage medium is any technology (including devices and materials) used to place, keep, and retrieve data on a long-term basis. A medium is an element used in communicating a message; on a storage medium, the ‘‘messages’’—in the form of data—are suspended for use when needed. The plural form of this term is storage media. Storage is sometimes used to mean ‘‘all forms of storage media.’’ Storage is usually distinguished from memory, which is the place in a computer where data is kept on a short-term basis for faster access. Examples of storage media include: L
The diskette
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A redundant array of independent disks (RAID)
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storage service provider Also see two related terms, application service provider and management service provider. On the Internet, a storage service provider (SSP) is a company that provides computer storage space and related management to other companies. In addition to the storage itself, SSPs typically offer periodic backup and archiving and some offer the ability to consolidate data from multiple company locations so that all locations can share the data effectively. Customers may be billed a monthly rate and for each managed terabyte of storage. Two leading SSP companies are StorageNetworks and Managed Storage International. Some companies specialize in providing limited storage service, such as periodic remote backup, to individual computer users and small businesses.
storage-over-IP See ‘‘iSCSI’’
stored procedure In a database management system (DBMS), a stored procedure is a set of Structured Query Language (SQL) statements with an assigned name that’s stored in the database in compiled form so that it can be shared by a number of programs. The use of stored procedures can be helpful in controlling access to data (end-users may enter or change data but do not write procedures), preserving data
integrity (information is entered in a consistent manner), and improving productivity (statements in a stored procedure only need to be written one time).
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STP See ‘‘shielded twisted pair’’
stream cipher A stream cipher is a method of encrypting text (to produce ciphertext) in which a cryptographic key and algorithm are applied to each binary digit in a data stream, one bit at a time. This method is not much used in modern cryptography. The main alternative method is the block cipher in which a key and algorithm are applied to blocks of data rather than individual bits in a stream. See also: encryption
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streaming See ‘‘streaming video’’
streaming media Streaming media is sound (audio) and pictures (video) that are transmitted on the Internet in a streaming or continuous fashion, using data packets. The most effective reception of streaming media requires some form of broadband technology such as cable modem or DSL. Also see streaming sound and streaming video.
streaming sound Streaming sound is sound that is played as it arrives. The alternative is a sound recording (such as a WAV file) that doesn’t start playing until the entire file has arrived. Support for streaming sound may require a plug-in player or come with the browser. Leading providers of streaming sound include Progressive Networks’ RealAudio and Macromedia’s Shockwave for Director (which includes an animation player as well).
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Streaming video is a sequence of ‘‘moving images’’ that are sent in compressed form over the Internet and displayed by the viewer as they arrive. Streaming media is streaming video with sound. With streaming video or streaming media, a Web user does not have to wait to download a large file before seeing the video or hearing the sound. Instead, the media is sent in a continuous stream and is played as it arrives. The user needs a player, which is a special program that uncompresses and sends video data to the display and audio data to speakers. A player can be either an integral part of a browser or downloaded from the software maker’s Web site.
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Major streaming video and streaming media technologies include RealSystem G2 from RealNetwork, Microsoft Windows Media Technologies (including its NetShow Services and Theater Server), and VDO. Microsoft’s approach uses the standard MPEG compression algorithm for video. The other approaches use proprietary algorithms. (The program that does the compression and decompression is sometimes called the codec.) Microsoft’s technology offers streaming audio at up to 96 Kbps and streaming video at up to 8 Mbps (for the NetShow Theater Server). However, for most Web users, the streaming video will be limited to the data rates of the connection (for example, up to 128 Kbps with an ISDN connection). Microsoft’s streaming media files are in its Advanced Streaming Format (ASF). Streaming video is usually sent from prerecorded video files, but can be distributed as part of a live broadcast ‘‘feed.’’ In a live broadcast, the video signal is converted into a compressed digital signal and transmitted from a special Web server that is able to do multicasting, sending the same file to multiple users at the same time.
string In programming, a string is a contiguous (see contiguity) sequence of symbols or values, such as a character string (a sequence of characters) or a bit string (a sequence of binary values).
strobe In computer or memory technology, a strobe is a signal that is sent that validates data or other signals on adjacent parallel lines. In memory technology, the CAS (column address strobe) and RAS (row address strobe) signals are used to tell a dynamic RAM that an address is a column or row address.
strongly-typed A strongly-typed programming language is one in which each type of data (such as integer, character, hexadecimal, packed decimal, and so forth) is predefined as part of the programming language and all constants or variables defined for a given program must be described with one of the data types. Certain operations may be allowable only with certain data types. The language compiler enforces the data typing and use compliance. An advantage of strong data typing is that it imposes a rigorous set of rules on a programmer and thus guarantees a certain consistency of results. A disadvantage is that it prevents the programmer from inventing a data type not anticipated by the developers of the programming language and it limits how ‘‘creative’’ one can be in using a given data type.
Structured Graph Format Structured Graph Format (SGF), an application of the Extensible Markup Language (XML), is used to describe a Web site so that its pages and content can be displayed and accessed in a structured, usually tabular form. Using SGF, a Web site is rendered as a structured graph, which is a set of nodes and links that enable a browser to conveniently interact with the site. A typical use of an SGF file is to describe a site to a browser in the form of a site map, which is created dynamically when requested by the user. If the structure of the site changes, the site map seen by a browser will change accordingly, without a human programmer having to intervene and write new HTML code. Typically, the user can also search by keywords, phrases, or topics. For SGF to be effectively used, the browser must use another client application, such as SFViewer and SGMapper. The client application creates the graphical interface that allows the user to navigate the site according to selected criteria.
Structured Query Language See ‘‘SQL’’
Structured Systems Analysis & Design Method See ‘‘SSADM’’
stub A stub is a small program routine that substitutes for a longer program, possibly to be loaded later or that is located remotely. For example, a program that uses Remote Procedure Calls (RPCs) is compiled with stubs that substitute for the program that provides a requested procedure. The stub accepts the request and then forwards it (through another program) to the remote procedure. When that procedure has completed its service, it returns the results or other status to the stub which passes it back to the program that made the request.
style sheet A term extended from print publishing to online media, a style sheet is a definition of a document’s appearance in terms of such elements as: L
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How all or individual sections should be laid out in terms of space (for example, two newspaper columns, one column with headings having hanging heads, and so forth).
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Line spacing, margin widths on all sides, spacing between headings, and so forth
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How many heading levels should be included in any automatically generated Table of Contents
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Any boilerplate content that is to be included on certain pages (for example, copyright statements)
Typically, a style sheet is specified at the beginning of an electronic document, either by embedding it or linking to it. This style sheet applies to the entire document. As necessary, specific elements of the overall style sheet can be overridden by special coding that applies to a given section of the document. For Web pages, a style sheet performs a similar function, allowing the designer to ensure an underlying consistency across a site’s pages. The style elements can be specified once for the entire document by either imbedding the style rules in the document heading or cross-referring (linking to or importing) a separate style sheet. A browser may allow the user to override some or all of the style sheet attributes. A cascading style sheet is a style sheet that anticipates that other style sheets will either fill in or override the overall style sheet. This provides the designer the advantage of being able to rely on the basic style sheet when desired and overriding it when desired. The filling in or overriding can occur on a succession of ‘‘cascading’’ levels of style sheets. For example, one style sheet could be created and linked to from every Web page of a Web site as the overall style sheet. For any portion of a page that included a certain kind of content such as a catalog of products, another style sheet that amends the basic style sheet could be linked to. And within the span of that style sheet, yet another style sheet could be specified as applying to a particular type of product display. When creating Web pages, the use of style sheets is now recommended by the World Wide Web Consortium. The latest version of the Hypertext Markup Language, HTML 4.0, while continuing to support older tags, indicates which ones should be replaced by the use of style sheet specifications. The Web’s Cascading Style Sheets, level 1 (CSSL1) is a recommendation for cascading style sheets that has been developed by a working group of the World Wide Web Consortium (W3C).
stylus A stylus (pronounced STAI-luhs, from the Latin stilus or ‘‘stake’’) is an instrument for writing and, in computers, an input device used to write text or draw lines on a surface as input to a computer. The term was first used in workstations designed for graphics applications, such as CAD/CAM, where the stylus was attached to an electronically-sensitive tablet or surface on which the stylus user wrote. Later, in handheld computers such as Apple’s Newton and 3Com’s PalmPilot, a stylus was provided as an instrument for
writing text characters and simple pictures. In most of today’s handheld computers that accept written input, the writing instrument is referred to as a stylus or a pen.
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subnet A subnet (short for ‘‘subnetwork’’) is an identifiably separate part of an organization’s network. Typically, a subnet may represent all the machines at one geographic location, in one building, or on the same local area network (LAN). Having an organization’s network divided into subnets allows it to be connected to the Internet with a single shared network address. Without subnets, an organization could get multiple connections to the Internet, one for each of its physically separate subnetworks, but this would require an unnecessary use of the limited number of network numbers the Internet has to assign. It would also require that Internet routing tables on gateways outside the organization would need to know about and have to manage routing that could and should be handled within an organization. The Internet is a collection of networks whose users communicate with each other. Each communication carries the address of the source and destination networks and the particular machine within the network associated with the user or host computer at each end. This address is called the IP address (Internet Protocol address). This 32-bit IP address has two parts: one part identifies the network (with the network number) and the other part identifies the specific machine or host within the network (with the host number). An organization can use some of the bits in the machine or host part of the address to identify a specific subnet. Effectively, the IP address then contains three parts: the network number, the subnet number, and the machine number. The standard procedure for creating and identifying subnets is provided in Internet Request for Comments 950.
The IP Address The 32-bit IP address (we have a separate definition of it with IP address) is often depicted as a dot address (also called dotted quad notation)—that is, four groups (or quad) of decimal digits separated by periods. Here’s an example: 130.5.5.25 Each of the decimal digits represents a string of four binary digits. Thus, the above IP address really is this string of 0s and 1s:
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10000010.00000101.00000101.00011001 As you can see, we inserted periods between each eight-digit sequence just as we did for the decimal version of the IP address. Obviously, the decimal version of the IP address is easier to read and that’s the form most commonly used. Some portion of the IP address represents the network number or address and some portion represents the local machine address (also known as the host number or address).
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IP addresses can be one of several classes, each determining how many bits represent the network number and how many represent the host number. The most common class used by large organizations (Class B) allows 16 bits for the network number and 16 for the host number. Using the above example, here’s how the IP address is divided:
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<–Network address–><–Host address–> 130.5 . 5.25
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If you wanted to add subnetting to this address, then some portion (in this example, eight bits) of the host address could be used for a subnet address. Thus:
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<–Network address–><–Subnet address–><–Host address–> 130.5 . 5 . 25
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To simplify this explanation, we’ve divided the subnet into a neat eight bits but an organization could choose some other scheme using only part of the third quad or even part of the fourth quad.
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The Subnet Mask Once a packet has arrived at an organization’s gateway or connection point with its unique network number, it can be routed within the organization’s internal gateways using the subnet number as well. The router knows which bits to look at (and which not to look at) by looking at a subnet mask. A mask is simply a screen of numbers that tells you which numbers to look at underneath. In a binary mask, a ‘‘1’’ over a number says ‘‘Look at the number underneath’’; a ‘‘0’’ says ‘‘Don’t look.’’ Using a mask saves the router having to handle the entire 32 bit address; it can simply look at the bits selected by the mask. Using the previous example (which is a very typical case), the combined network number and subnet number occupy 24 bits or three of the quads. The appropriate subnet mask carried along with the packet would be: 255.255.255.0 Or a string of all 1’s for the first three quads (telling the router to look at these) and 0’s for the host number (which the router doesn’t need to look at). Subnet masking allows routers to move the packets on more quickly. If you have the job of creating subnets for an organization (an activity called subnetting) and specifying subnet masks, your job may be simple or complicated depending on the size and complexity of your organization and other factors.
way to manage the limited number of Internet Protocol addresses (IP addresses) that are available with Internet Protocol version 4.
sudo Sudo (superuser do) is a utility for UNIX-based systems that provides an efficient way to give specific users permission to use specific system commands at the root (most powerful) level of the system. Sudo also logs all commands and arguments. Using sudo, a system administrator can: L
Give some users (or groups of users) the ability to run some (or all) commands at the root level of system operation
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Control which commands a user can use on each host
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See clearly from a log which users used which commands
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Using timestamp files, control the amount of time a user has to enter commands after they have entered their password and been granted appropriate privileges
The sudo configuration file is easy to create and to refer to.
suffix A suffix is something added at the end of a word that conditions its usage or meaning. In computer system file names, a suffix is a convention for having one or more characters appended to a file name (usually separated from the file name with a dot) so that it can be distinguished from other files or grouped together with similar types of files. For example, all files created with a WordPerfect word processor can be associated with a ‘‘.wpd’’ suffix. That way, a user can easily distinguish it as a file created with that word processor from files created with other tools and selectively process a number of similar files. Different operating systems have different rules about how long the suffix part of a file name can be. DOS, Windows prior to Windows 95, and OS/2 prior to OS/2 Warp limit suffix names to a maximum of three characters. UNIX systems, Windows 95, and Mac systems permit longer suffix names.
sun interference See ‘‘solar fade’’
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A subnetwork is a separately identifiable part of a larger network that typically represents a certain limited number of host computers, the hosts in a building or geographic area, or the hosts on an individual local area network. Companies often create subnetworks (sometimes called subnets) when setting up connection to the Internet as a
Since replaced by Solaris, SunOS was the UNIX-based operating system for the SPARC-based workstations and servers from Sun Microsystems.
superconductivity
Super Audio CD Super Audio Compact Disc (SACD) is a high-resolution audio CD format. Version 1.0 specifications were described by Philips and Sony in March of 1999, in the Scarlet Book. SACD and DVD-Audio (DVD-A) are the two formats competing to replace the standard audio CD. Most of the industry is backing DVD-A, with Philips and Sony being the major exceptions. Like SACD, DVD-A offers 5.1 channel surround sound in addition to 2-channel stereo. Both formats improve the complexity of sound by increasing the bit rate and the sample rate, and can be played on existing CD players, although only at quality levels similar to those of traditional CDs. SACD uses Direct Stream Digital (DSD) recording, a proprietary Sony technology that converts an analog waveform to a 1-bit signal for direct recording, instead of the pulse code modulation (PCM) and filtering used by standard CDs. DSD uses lossless compression (so-called because none of the data is lost in the compression process) and a sampling rate of 2.8MHz to improve the complexity and realism of sound. SACD can also contain extra information, such as text, graphics, and video clips. The first SACD player was released in North America in December of 1999, with an $8000 price tag. In late 2000, Sony released a new model, priced at $1000.
supercomputer A supercomputer is a computer that performs at or near the currently highest operational rate for computers. A supercomputer is typically used for scientific and engineering applications that must handle very large databases or do a great amount of computation (or both). At any given time, there are usually a few well-publicized supercomputers that operate at the very latest and always incredible speeds. The term is also sometimes applied to far slower (but still impressively fast) computers. Most supercomputers are really multiple computers that perform parallel processing. In general, there are two parallel processing approaches: symmetric multiprocessing (SMP) and massively parallel processing (MPP). Perhaps the best-known builder of supercomputers has been Cray Research, now a part of Silicon Graphics. Some supercomputers are at ‘‘supercomputer centers,’’ usually university research centers, some of which, in the United States, are interconnected on an Internet backbone known as vBNS or NSFNet. This network is the foundation for an evolving network infrastructure known as the National Technology Grid. Internet2 is a university-led project that is part of this initiative. At the high end of supercomputing are computers like IBM’s ‘‘Blue Pacific,’’ announced on October 29, 1998. Built in partnership with Lawrence Livermore National Laboratory in California., Blue Pacific is reported to operated at 3.9 teraflop (trillion operations per second), 15,000 times faster
than the average personal computer. It consists of 5,800 processors containing a total of 2.6 trilllion bytes of memory and interconnected with five miles of cable. It was built to simulate the physics of a nuclear explosion. IBM is also building an academic supercomputer for the San Diego Supercomputer Center that will operate at 1 teraflop. It’s based on IBM’s RISC System/6000 and the AIX operating system and will have 1,000 microprocessors with IBM’s own POWER3 chip. At the lower end of supercomputing, a new trend, called clustering, suggests more of a build-it-yourself approach to supercomputing. The Beowulf Project offers guidance on how to ‘‘strap together’’ a number of off-the-shelf personal computer processors, using Linux operating systems, and interconnecting the processors with Fast Ethernet. Applications must be written to manage the parallel processing.
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supercomputer center In general, a supercomputer center is a site with a supercomputer that is shared by a number of other sites, usually research sites. Formerly, in the U.S., there were five supercomputer centers interconnected on the vBNS, a special backbone network financed by the National Science Foundation (NSF) and operated by MCI for the exclusive use of designated research centers. On October 1, 1997, the five supercomputer centers were replaced by two new NSFfunded programs. The new program, Partnerships for Advanced Computational Infrastructure (PACI), named two of the former supercomputer center sites as focuses for a new scientific technology infrastructure (known as the National Technology Grid) that would interconnect 50 university and scientific sites. The first of the two NSF-funded programs is the National Computational Science Alliance (Alliance), being led by the National Center for Supercomputing Applications (NCSA) in Urbana, Illinois. The second program, the National Partnership for an Advanced Computational Infrastructure (NPACI) is led by the San Diego Supercomputer Center at the University of California at San Diego. The earlier supercomputer center program emphasized supercomputer access from researchers’ desktops. The new program aims to exploit the entire World Wide Web as a new ‘‘computermediated center of knowledge.’’ Part of that program is to develop a wired superstructure, referred to as the National Technology Grid, that will enable very high-bandwidth applications such as virtual reality environments. Internet2 is a related university-led project.
superconductivity Superconductivity is the ability of certain materials to conduct electric current with practically zero resistance. This produces interesting and potentially useful effects. For a material to behave as a superconductor, low temperatures are required.
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Superconductivity was first observed in 1911 by H. K. Onnes, a Dutch physicist. His experiment was conducted with elemental mercury at 4 degrees kelvin (approximately—452 degrees Fahrenheit), the temperature of liquid helium. Since then, some substances have been made to act as superconductors at higher temperatures, although the ideal—a material that can superconduct at room temperature—remains elusive.
band, especially if the highest frequency in the band is much different than the lowest frequency. To overcome this inherent difficulty when engineering variable-frequency wireless equipment, the desired data-carrying signal can be combined with the output of an unmodulated, variablefrequency oscillator (VFO) in a circuit called a mixer. When this is done, output is produced at a fixed frequency representing the difference between the input frequencies.
Superconductors have been employed in, or proposed for use in, an enormous variety of applications. Examples include:
When the correct range of frequencies is chosen for the VFO, a receiver can be designed that will intercept incoming signals over a specific band. For example, if the desired input frequency range is 20 MHz to 25 MHz, a VFO can be built that generates an unmodulated carrier at 29 to 34 MHz. When the incoming signal is mixed with the VFO output, the resultant has a constant frequency of 9 MHz, representing the difference between the frequencies of the inputs. The 9MHz output retains the modulation characteristics of the incoming signal. In this case, the receiver is said to have an intermediate frequency (IF) of 9 MHz. This IF signal can be amplified and filtered more easily than signals having frequencies that vary from 20 to 25 MHz.
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Alternative energy storage systems
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Radio-frequency (RF) filters
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Radio-frequency amplifier
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Sensitive visible-light and infrared detectors
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Miniaturized wireless transmitting antennas
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Systems to detect submarines and underwater mines
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Gyroscopes for earth-orbiting satellite
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Superdome is a high-end 64-bit, Hewlett-Packard (HP) PA8600 processor-based UNIX server designed for e-commerce customers using very large databases. Superdome is available in three configurations and comes bundled with HP’s e-service software packages and customer support options. The server is set to run on HP-UX 11i, HP’s version of UNIX, but is intended to be compatible with Windows and Linux operating systems sometime in the future. One of Superdome’s main features is the ability to be split into several partitions, each with its own operating system. Partitions defined in hardware can be further subdivided into software-based virtual partitions which allow resources, such as processor power or communication bandwidth, to be assigned to specific jobs.
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See ‘‘superheterodyne’’
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The term superheterodyne refers to a method of designing and building wireless communications or broadcast equipment, particularly radio receivers. Sometimes a receiver employing this technology is called a ‘‘superheterodyne’’ or ‘‘superhet.‘‘ In many wireless applications, the equipment must function over a range, or band, of frequencies. But it is easier to process a modulated signal at a single frequency than over a
The theory and design of superheterodyne equipment is a rather sophisticated business. Numerous books are devoted to this topic; some engineers make their entire living designing oscillators, mixers, and amplifiers that use this technology. For more information about superheterodyne engineering practice, a professional-level textbook or a formal course is recommended.
superposition Superposition is a principle of quantum theory that describes a challenging concept about the nature and behavior of matter and forces at the atomic level. The principle of superposition claims that while we do not know what the state of any object is, it is actually in all possible states simultaneously, as long as we don’t look to check. It is the measurement itself that causes the object to be limited to a single possibility. In 1935, Erwin Schrodinger proposed an analogy to show how superposition would operate in the every day world: the somewhat cruel analogy of Schrodinger’s cat. First, we have a living cat and place it in a thick lead box. At this stage, there is no question that the cat is alive. We then throw in a vial of cyanide and seal the box. We do not know if the cat is alive or if it has broken the cyanide capsule and died. Since we do not know, the cat is both dead and alive, according to quantum law—in a superposition of states. It is only when we break open the box and see what condition the cat is that the superposition is lost, and the cat must be either alive or dead. Superposition is well illustrated by Thomas Young’s doubleslit experiment, developed in the early nineteenth century to prove that light consisted of waves. In fact, the noted
superstring theory physicist Richard Feynman claimed that the essentials of quantum mechanics could be grasped by an exploration of the implications of Young’s experiment.
execution units to simultaneously carry out two or more independent instructions at a time. Superscalar design is sometimes called ‘‘second generation RISC.’’
The Double-Slit Experiment
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For this experiment, a beam of light is aimed at a barrier with two vertical slits. The light passes through the slits and the resulting pattern is recorded on a photographic plate. If one slit is covered, the pattern is what would be expected: a single line of light, aligned with whichever slit is open. Intuitively, one would expect that if both slits are open, the pattern of light will reflect that fact: two lines of light, aligned with the slits. In fact, however, what happens is that the photographic plate is entirely separated into multiple lines of lightness and darkness in varying degrees. What is being illustrated by this result is that interference is taking place between the waves/particles going through the slits, in what, seemingly, should be two non-crossing trajectories.
A superstitial is a Web ad format developed by Unicast, which calls itself a rich media company. Superstitials combine Flash or other animation technology with Java programming to deliver video-like Web commercials.
We would expect that if the beam of light particles or photons is slowed enough to ensure that individual photons are hitting the plate, there could be no interference and the pattern of light would be two lines of light, aligned with the slits. In fact, however, the resulting pattern still indicates interference, which means that, somehow, the single particles are interfering with themselves. This seems impossible: we expect that a single photon will go through one slit or the other, and will end up in one of two possible light line areas. But that is not what happens. As Feynman concluded, each photon not only goes through both slits, but simultaneously takes every possible trajectory en route to the target, not just in theory, but in fact. In order to see how this might possibly occur, experiments have focused on tracking the paths of individual photons. What happens in this case is that the measurement in some way disrupts the photons’ trajectories (in accordance with the uncertainty principle), and somehow, the results of the experiment become what would be predicted by classical physics: two bright lines on the photographic plate, aligned with the slits in the barrier. Cease the attempt to measure, however, and the pattern will again become multiple lines in varying degrees of lightness and darkness. Each photon moves simultaneously in a superposition of possible trajectories, and, furthermore, measurement of the trajectory causes the superposition of states to collapse to a single position.
superscalar Superscalar describes a microprocessor design that makes it possible for more than one instruction at a time to be executed during a single clock cycle. In a superscalar design, the processor or the instruction compiler is able to determine whether an instruction can be carried out independently of other sequential instructions, or whether it has a dependency on another instruction and must be executed in sequence with it. The processor then uses multiple
A superstitial (trademarked by Unicast as SUPERSTITIAL, but industry usage is generally lower case), like an interstitial, runs in the dead time between Web pages. The superstitial is delivered with what Unicast calls a ‘‘polite’’ cache-and-play delivery system. Designed to load invisibly into the browser’s cache (temporary storage area) within 60 seconds (while the user is viewing the Web site containing the ad), a superstitial doesn’t appear until it is completely downloaded. The ad appears in a variably-sized pop-up window when something the user does (such as a mouse click) triggers a move to another Web page. If the user clicks off the site before a superstitial has finished loading, the ad doesn’t play. A user can click a superstitial ad off at any time, although they can’t stop them from playing altogether. Superstitials are thought to increase stickiness (that is, they tend to hold the viewer’s attention). According to Unicast, they lead to advertising terminology on the Internet rates of up to 40%, and an average of 7.5% compared to the .3% average of banner ads.
superstring theory Superstring theory—known less formally as ‘‘string theory’’—is sometimes called the theory of everything (TOE), because it is a unifying physics theory that reconciles the differences between quantum theory and the theory of relativity to explain the nature of all known forces and matter. According to string theory, at the most microscopic level, everything in the universe is made up of loops of vibrating strings, and apparent particle differences can be attributed to variations of vibration. An object (such as an apple, for example) and a force (such as radiation, for example) can both be broken down into atoms, which can be further broken down into electrons and quarks, which can be, finally, broken down into tiny, vibrating loops of strings. Quantum theory explains the nature and behavior of energy and matter on the microscopic scale, while the theory of relativity explains it on the macroscopic scale. Taken together, they form the basis for modern physics; nevertheless, integral aspects of the two theories conflict with each other in ways that have never been satisfactorily addressed. Although the theories work perfectly separately, equations involving both don’t work at all. Einstein himself, unconvinced that nature would require entirely different
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modes of behavior for differently scaled phenomena, spent thirty years searching for what he called the unified field theory. Superstring theory came to the forefront in the 1980s, when Michael Green at Queen Mary College and John Schwarz at the California Institute of Technology demonstrated that it had the potential to be the unifying theory that Einstein sought: one that could be used to describe gravity as well as electromagnetic forces.
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The reason that general relativity doesn’t work at the subatomic level is because the equation mistakenly includes a figure for point-like particles. Modifying the equation to include a representation of loops of string, instead, makes the equation work.
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String theorists are currently examining the possibility that the strings themselves have multiple dimensions, and are not, as was originally thought, massless (possessing only length, and no width). The idea is that the strings may actually be membranes.
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There are ten dimensions in the natural world (nine spatial dimensions plus time), rather than the four of classical science (the three spatial dimensions plus time). What causes the extra six dimensions to be largely unnoticed is that they are considered to be compacted or curled up.
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All forces and particles in nature are derived from variations in vibrations of strings. As an example, gravity is said to arise from the lowest vibration of a closed string.
supervisor call In computers, especially IBM mainframes, a supervisor call (SVC) is a processor instruction that directs the processor to pass control of the computer to the operating system’s supervisor program. Most SVCs are requests for a specific operating system service from an application program or another part of the operating system. Application program developers usually use a language function or macro instruction to make the request (for example, to get allocated more memory for the program to work with). The language compiler or assembler generates the instruction that includes the specific SVC request. Each service has a preassigned SVC number. When the computer’s processor executes the instruction that contains the SVC, the code representing ‘‘SVC’’ causes a program interrupt to occur, which means that control of the processor is immediately passed to the operating system supervisor program. The supervisor then passes control to programming that performs the service that goes with the specified SVC number.
An SVC routine is a program within the supervisor that performs the service indicated by the specific SVC instruction.
supervisory control and data acquisition See ‘‘SCADA’’
SuperZap SuperZap is an IBM utility program used to install zaps or fixes to MVS operating systems or application program code that is causing a problem. Its file name is IMASPZAP (I M A SuPerZAP).
supplier exchange See ‘‘e-procurement’’
supply chain management Supply chain management (SCM) is the oversight of materials, information, and finances as they move in a process from supplier to manufacturer to wholesaler to retailer to consumer. Supply chain management involves coordinating and integrating these flows both within and among companies. It is said that the ultimate goal of any effective supply chain management system is to reduce inventory (with the assumption that products are available when needed). As a solution for successful supply chain management, sophisticated software systems with Web interfaces are competing with Web-based application service providers (ASP) who promise to provide part or all of the SCM service for companies who rent their service. Supply chain management flows can be divided into three main flows: L
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The finances flow
The product flow includes the movement of goods from a supplier to a customer, as well as any customer returns or service needs. The information flow involves transmitting ordersand updating the status of delivery. The financial flow consists of credit terms, payment schedules, and consignment and title ownership arrangements. There are two main types of SCM software: Planning applications and execution applications. Planning applications use advanced algorithms to determine the best way to fill an order. Execution applications track the physical status of goods, the management of materials, and financial information involving all parties. Some SCM applications are based on open data models that support the sharing of data both inside and outside the enterprise (this is called the extended enterprise, and includes key suppliers, manufacturers, and end customers of
susceptance a specific company). This shared data may reside in diverse database systems, or data warehouses, at several different sites and companies. By sharing this data ‘‘upstream’’ (with a company’s suppliers) and ‘‘downstream’’ (with a company’s clients), SCM applications have the potential to improve the time-tomarket of products, reduce costs, and allow all parties in the supply chain to better manage current resources and plan for future needs. Increasing numbers of companies are turning to Web sites and Web-based applications as part of the SCM solution. A number of major Web sites offer e-procurement marketplaces where manufacturers can trade and even make auction bids with suppliers.
surf In using the World Wide Web, to surf is to either: 1) Explore a sequence of Web sites in a random, unplanned way, or 2) Simply use the Web to look for something in a questing way. As the term is ordinarily used, if you are going to one specific site you already know about, you aren’t surfing. The term suggests an analogy between surfing ocean waves with a surfboard and surfing cyberspace.
surge suppressor See also uninterruptible power supply. A surge suppressor (sometimes optimistically called a ‘‘surge protector’’) is a device inserted in the alternating current (AC) utility line and/or telephone line to prevent damage to electronic equipment from voltage ‘‘spikes’’ called transients. A more accurate term for this type of device is ‘‘transient suppressor.’’ A typical surge suppressor is a small box with several utility outlets, a power switch, and a 3-wire cord for plugging into a wall outlet. In most countries where electronic equipment is used, the effective AC utility voltage is 110 to 120 volts; the peak voltage is on the order of plus-or-minus 160 to 170 volts at a frequency of 60 hertz. But transients, which arise from various causes, commonly reach peak levels of several hundred volts. These pulses are of short duration, measured in microseconds (units of 10-6 second), but in that time, they can cause hardware to malfunction. The worst type of transient occurs when lightning strikes in the vicinity (it is not necessary for a power line to be directly hit). Such a ‘‘spike’’ can peak at thousands of volts and cause permanent damage to equipment. A surge suppressor prevents the peak AC voltage from going above a certain threshold such as plus-or-minus 200 volts. Semiconductor devices are used for this purpose. The power line is effectively short-circuited to electrical ground for transient pulses exceeding the threshold, while the flow
of normal 60-Hz current is unaffected. For the suppressor to work, a 3-wire AC power connection must be used. ‘‘Cheater’’ adapters, which allow 3-wire appliances to be used with 2-wire outlets or extension cords, defeat the electrical ground connection and render most surge suppressors ineffective. Surge suppressors should be used as a matter of habit with all semiconductor-based electronic and computer hardware, including peripherals such as printers, monitors, external disk drives, and modems. But the suppressor should not be relied upon to provide protection against lightning-induced transients. The safest procedure, inconvenient though it be, is to ensure that all susceptible hardware is plugged into the suppressor box, and to unplug the suppressor’s main power cord when the equipment is not in use if you live in a thunderstorm-prone area.
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Susceptance (symbolized B) is an expression of the ease with which alternating current (AC) passes through a capacitance or inductance.
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In some respects, susceptance is like an AC counterpart of direct current (DC) conductance. But the two phenomena are different in important ways, and they can vary independently of each other. Conductance and susceptance combine to form admittance, which is defined in terms of two-dimensional quantities known as complex numbers. When AC passes through a component that contains a finite, nonzero susceptance, energy is alternately stored in, and released from, a magnetic field or an electric field. In the case of a magnetic field, the susceptance is inductive. In the case of an electric field, the susceptance is capacitive. Inductive susceptance is assigned negative imaginary number values, and capacitive susceptance is assigned positive imaginary number values. As the inductance of a component increases, its susceptance becomes smaller negatively (that is, it approaches zero from the negative side) in imaginary terms, assuming the frequency is held constant. As the frequency increases for a given value of inductance, the same thing happens. If L is the inductance in henries (H) and f is the frequency in hertz (Hz), then the susceptance -jBL, in imaginary-number siemens, is given by: -jBL = -j (6.2832fL)
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where 6.2832 is approximately equal to 2 times pi, a constant representing the number of radians in a full AC cycle, and j represents the unit imaginary number (the positive square root of -1).
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As the capacitance of a component increases, its susceptance becomes larger positively in imaginary terms, assuming the frequency is held constant. As the frequency increases for a given value of capacitance, the same thing happens. If C is
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the capacitance in farads (F) and f is the frequency in Hz, then the susceptance +jBC, in imaginary-number ohms, is given by: +jXC = +j (6.2832fC)
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Also see admittance, conductance, reactance, resistance, impedance, ohm, siemens, henry, and farad.
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SuSE (pronounced soo’-sah) is a privately owned German company whose mission is to promote Open Source development and GNU General Public License distribution and to be the most reliable Linux distribution provider. Like Red Hat and Caldera, SuSE assembles open source components for the Linux operating system and related programs into a selection of distribution packages that can be purchased. The advantage of buying the Linux kernel and support programs from a distributor (rather than assembling it at no cost yourself from various sources) is that you save time by purchasing a convenient package that comes with extensive documentation and tech support. SuSE, which even has a Braille edition, has targeted users new to the Linux operating system and hopes to attract Windows expatriots by promoting the Personal edition of SuSE as an easy way for the average desktop user to migrate to Linux. Both the Personal Edition and the Professional Editions come with a large number of applications.
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In general, Windows and UNIX-based operating systems provide a default swap file of a certain size that the user or a system administrator can usually change.
Swing Swing is a set of program components for Java programmers that provide the ability to create graphical user interface (GUI) components, such as buttons and scroll bars, that are independent of the windowing systems for specific operating systems. Swing components are used with the Java Foundation Classes (JFC).
swirl
SuSE is an abbreviation for the German company ‘‘Gesellschaft f|r Software und Systementwicklung mbH’’, whose name loosely translates to mean ‘‘Software and System Development Company’’.
SVC
switch
See ‘‘switched virtual circuit’’
Also see bridge, gateway, hub, and router.
SVG
In telecommunications, a switch is a network device that selects a path or circuit for sending a unit of data to its next destination. A switch may also include the function of the router, a device or program that can determine the route and specifically what adjacent network point the data should be sent to. In general, a switch is a simpler and faster mechanism than a router, which requires knowledge about the network and how to determine the route.
See ‘‘Scalable Vector Graphics’’
See ‘‘display modes’’
SWAG See ‘‘Semantic Web Agreement Group’’
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One advantage of a swap file is that it can be organized as a single contiguous space so that fewer I/O operations are required to read or write a complete file.
Swirl is the term used by some people in the telecommunications industry to describe the background noise that can be present when using a digital cellular telephone phone. It’s the equivalent of static on an analog cellular phone. Digital wireless phones pick up background noise just like analog phones but the background noise is different, something like the sound of air whooshing or swirling through space.
SVGA
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you have more RAM than you actually do. The least recently used files in RAM can be ‘‘swapped out’’ to your hard disk until they are needed later so that new files can be ‘‘swapped in’’ to RAM. In larger operating systems (such as IBM’s OS/ 390), the units that are moved are called pages and the swapping is called paging.
See ‘‘swap file’’
swap file A swap file (or swap space or, in Windows NT, a pagefile) is a space on a hard disk used as the virtual memory extension of a computer’s real memory (RAM). Having a swap file allows your computer’s operating system to pretend that
Relative to the layered Open Systems Interconnection (OSI) communication model, a switch is usually associated with layer 2, the Data-Link layer. However, some newer switches also perform the routing functions of layer 3, the Network layer. Layer 3 switches are also sometimes called IP switches. On larger networks, the trip from one switch point to another in the network is called a hop. The time a switch takes to figure out where to forward a data unit is called its latency. The price paid for having the flexibility that switches provide in a network is this latency. Switches are found at the backbone and gateway levels of a network where one network connects with another and at the
symmetric communications subnetwork level where data is being forwarded close to its destination or origin. The former are often known as core switches and the latter as desktop switches. In the simplest networks, a switch is not required for messages that are sent and received within the network. For example, a local area network may be organized in a token ring or bus arrangement in which each possible destination inspects each message and reads any message with its address.
Switching fabric includes the switching units (individual boxes) in a node, the integrated circuits that they contain, and the programming that allows switching paths to be controlled. The switching fabric is independent of the bus technology and infrastructure used to move data between nodes and also separate from the router. The term is sometimes used to mean collectively all switching hardware and software in a network.
Circuit-Switching version Packet-Switching
The term apparently uses the woven material metaphor to suggest the possible complexity and web-like structure of switching paths and ports within a node.
A network’s paths can be used exclusively for a certain duration by two or more parties and then switched for use to another set of parties. This type of ‘‘switching’’ is known as circuit-switching and is really a dedicated and continuously connected path for its duration. Today, an ordinary voice phone call generally uses circuit-switching.
The Signal Computing System Architecture (SCSA), which provides a model framework for computer telephony, uses the term. In the SCSA framework, part of the hardware model includes a Switch Fabric Controller. Asynchronous transfer mode (ATM) and frame relay are sometimes described as switching fabric technologies.
Most data today is sent, using digital signals, over networks that use packet-switching. Using packet-switching, all network users can share the same paths at the same time and the particular route a data unit travels can be varied as conditions change. In packet-switching, a message is divided into packets, which are units of a certain number of bytes. The network addresses of the sender and of the destination are added to the packet. Each network point looks at the packet to see where to send it next. Packets in the same message may travel different routes and may not arrive in the same order that they were sent. At the destination, the packets in a message are collected and reassembled into the original message.
The switching fabric typically includes data buffers and the use of shared memory.
Switched Multimegabit Data Service See ‘‘SMDS’’
switched virtual circuit SVC also stands for supervisor call. In a network, a switched virtual circuit (SVC) is a temporary virtual circuit that is established and maintained only for the duration of a data transfer session. A permanent virtual circuit (PVC) is a continuously dedicated virtual circuit. A virtual circuit is one that appears to be a discrete, physical circuit available only to the user but that is actually a shared pool of circuit resources used to support multiple users as they require the connections. Switched virtual circuits are part of an X.25 network. Conceptually, they can also be implemented as part of a frame relay network.
switching fabric Switching fabric is the combination of hardware and software that moves data coming in to a network node out by the correct port (door) to the next node in the network. The term suggests that the near synonym, switch, tends to make switching seem like a simple hardware function.
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Sybase Sybase is a computer software company that develops and sells database management system (DBMS) and middleware products. The company was founded in 1984, and the headquarters offices are in Emeryville, CA. Sybase products have found extensive application, particularly in commercial, industrial, and military communications systems. Spanish telecommunications operator Airtel uses Sybase Adaptive Server IQ Multiplex (ASIQ) to standardize its business information. Telstra, an Australian telecommunications provider, uses datavisualization software powered by Sybase ASIQ Multiplex to optimize its mobile phone network management capacity. Primark uses Sybase MQSeries Integrator to maximize transaction rates, optimize reliability, and allow customization of inputs and outputs. Sybase claims that its MQSeries Integrator makes it possible to add new data formats to meet the needs of clients, and to accommodate diverse system architectures at client locations. Sybase SQL Anywhere, embedded in Geodyn FxView, is used in two-way communications between military command centers and soldiers in the field. Also see Oracle and IBM’s DB2.
symmetric communications Compare asymmetric communications. In telecommunications, the term symmetric (also symmetrical) refers to any system in which data speed or quantity is the same in both directions, averaged over time. Examples include two-way radio, standard twisted-pair telephone Internet connections, cable modem Internet connections in which the cable is used for transmission as well as for reception, and full-motion videoconferencing.
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Symmetric communications is not necessarily the most efficient mode in a given application. Consider casual Web browsing, in which most of the bytes come downstream (from Internet servers to the user) as relatively large graphics, sound, multimedia, and HTML files, while upstream data (from user to a server) consists mainly of new link (URL) requests by the user, which, in comparison, contain few bytes. In this environment, it often makes the best use of available resources to supply the user with a more broadbanded ‘‘pipeline’’ in the downstream direction, as compared with the upstream direction.
standard is necessary for future development of the wireless industry, which has been held back by the existing proprietary technologies.
The ultimate Internet connection is broadband symmetrical, such as is provided by true cable modem connections and optical fiber systems. At the time of this writing, symmetric broadband is not generally available outside of metropolitan areas.
Synchronized Multimedia Integration Language
symmetric multiprocessing See ‘‘SMP’’
SynchBurst SRAM See ‘‘burst SRAM’’
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SynchML SyncML is an Extensible Markup Language (XML) protocol under development as an open standard for the universal synchronization of data between devices, one of the most important building blocks in the development of third generation (3G) wireless. The SyncML Initiative was founded in February of 2000, with a stated goal of developing and promoting an open and portable standard for consistent synchronization of remote data across networks, platforms, and devices. SyncML leverages existing standards such as MIME, the vCard, and the iCalendar, in addition to XML. Synchronization of data allows changes made to data on one device (such as a smartphone or a laptop computer) to be instantly reflected in data on another device (such as a networked computer). For example, if a file is edited on one device, the updates can be automatically transfered to the other device. With automatic data synchronization, a mobile worker doesn’t have to worry about the coordination of data between networked computers and devices used while out of the office. This lessens the need for sneakernet operations; the user doesn’t have to recopy data, manually transfer it from one device to another, or deal with uncoordinated information in various versions of single files. The problem with existing data synchronization protocols is that they are only compatible with some standards and some devices. Meanwhile, with a growing number of people using wireless devices, the need to synchronize data will only increase. Given the variety of hardware and technology, a universal
Founders of the initiative (Ericsson, IBM, Lotus, Motorola, Nokia, Psion, Palm Inc. and Starfish Software) showcased devices using the protocol in September 2000 in Dublin. Initiative members report that SyncML-compliant products may be released by early 2001. SyncML Version 1.0 Alpha is currently available to the more than 470 supporting companies.
See ‘‘SMIL’’
Synchronous Data Link Control See ‘‘SDLC’’
Synchronous Digital Hierarchy See ‘‘SDH’’
synchronous DRAM See ‘‘SDRAM’’
Synchronous Graphics RAM Synchronous Graphics RAM (SGRAM) is clocksynchronized random access memory that is used for video memory. It is relatively low-cost video memory. It uses masked write, which enables selected data to be modified in a single operation rather as a sequence of read, update, and write operations. It also uses block write, which allows data for background or foreground image fills to be handled more efficiently. SGRAM is single-ported. Its special features are what make it a moderately fast form of video memory. The Matrox Mystique is an example of a video card that uses SGRAM.
Synchronous Optical Network SONET is the American National Standards Institute standard for synchronous data transmission on optical media. The international equivalent of SONET is synchronous digital hierarchy (SDH). Together, they ensure standards so that digital networks can interconnect internationally and that existing conventional transmission systems can take advantage of optical media through tributary attachments. SONET provides standards for a number of line rates up to the maximum line rate of 9.953 gigabits per second (Gbps). Actual line rates approaching 20 gigabits per second are possible. SONET is considered to be the foundation for the physical layer of the broadband ISDN (BISDN).
syntax Asynchronous transfer mode runs as a layer on top of SONET as well as on top of other technologies. SONET defines a base rate of 51.84 Mbps and a set of multiples of the base rate known as ‘‘Optical Carrier levels (OCx).’’
synchronous In general, synchronous (pronounced SIHN-kro-nuhs, from Greek syn-, meaning ‘‘with,’’ and chronos, meaning ‘‘time’’) is an adjective describing objects or events that are coordinated in time. In information technology, the term has several different usages. 1) In telecommunication signaling within a network or between networks, synchronous signals are those that occur at the same clock rate when all clocks are based on a single reference clock. (Plesiochronous signals are almost but not quite in synchronization and asynchronous signals are those that run from different clocks or at a different transition rate.) 2) In program-to-program communication, synchronous communication requires that each end of an exchange of communication respond in turn without initiating a new communication. A typical activity that might use a synchronous protocol would be a transmission of files from one point to another. As each transmission is received, a response is returned indicating success or the need to resend. Each successive transmission of data requires a response to the previous transmission before a new one can be initiated. Synchronous program communication is contrasted with asynchronous program communication.
Synclink Dynamic RAM SyncLink SDRAM, along with Direct Rambus DRAM, is a protocol-based approach where all signals to random access memory (RAM) are on the same line (rather than having separate column address strobe, row address strobe, address, and data lines). Since access time does not depend on synchronizing operations on multiple lines, SLDRAM promises RAM speed of up to 800 MHz. Like Double Data Rate SDRAM, SLDRAM can operate at twice the system clock rate. SyncLink is an open industry standard that is expected to compete with Direct Rambus DRAM.
syndication In general, syndication is the supply of material for reuse and integration with other material, often through a paid service subscription. The most common example of syndication is in newspapers, where such content as wireservice news, comics, columns, horoscopes, and crossword puzzles are usually syndicated content. Newspapers receive the content from the content providers, reformat it as required, integrate it with other copy, print it, and publish it.
For many years mainly a feature of print media, today content syndication is the way a great deal of information is disseminated across the Web. Reuters, for example, provides online news content to over 900 Web sites and portals, such as Yahoo and America Online. Online content syndication is a growing industry sector, in terms of both content syndication and hardware and software development. In the early days, online content syndication was a cumbersome manual process: after an agreement had been reached between the supplier and their customers, the customers would simply copy the desired content from the supplier’s Web site and paste it into their own. All related activities—such as content updates, for example—were also manual, and dependent upon the user to track and perform necessary tasks. Electronic content syndication can potentially automate every aspect of online content syndication. Two crucial components for the further development of the industry built on online syndication have been cited as: the need for a standardized data exchange mechanism, and the need for a standardized metadata vocabulary (since suppliers and subscribers are often using different and incompatible technologies). The Information & Content Exchange (ICE) protocol was developed as an open standard for data exchange that enables the automation of all processes involved. An organization made up of content providers and vendors, the Publishing Requirements for Industry Standard Metadata (PRISM) working group is collaborating to develop a standard vocabulary.
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synergy In general, synergy (pronounced SIHN-uhr-djee, from Greek sunergia, meaning ‘‘cooperation,’’ and also sunergos, meaning ‘‘working together’’) is the combined working together of two or more parts of a system so that the combined effect is greater than the sum of the efforts of the parts. In business and technology, the term describes a hoped-for or real effect resulting from different individuals, departments, or companies working together and stimulating new ideas that result in greater productivity. The process of synergy as a way of originating new ideas or making new discoveries can be contrasted to serendipity, in which ideas and discoveries emerge seemingly by accident.
syntax Syntax is the grammar, structure, or order of the elements in a language statement. (Semantics is the meaning of these elements.) Syntax applies to computer languages as well as to natural languages. Usually, we think of syntax as ‘‘word order.’’ However, syntax is also achieved in some languages such as Latin by inflectional case endings. In computer languages, syntax can be extremely rigid as in the case of
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most assembler languages or less rigid in languages that make use of ‘‘keyword’’ parameters that can be stated in any order. C.W. Morris in his Foundations of the Theory of Signs (1938) organizes semiotics, the study of signs, into three areas: syntax (the study of the interrelation of the signs); semantics (the study of the relation between the signs and the objects to which they apply); and pragmatics (the relationship between the sign system and the user).
sysop
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See ‘‘system operator’’
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sysplex and Parallel Sysplex
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A sysplex is IBM’s systems complex (the word sysplex comes from the first part of the word system and the last part of the word complex), introduced in 1990 as a platform for the MVS/ESA operating system for IBM mainframe servers. An enhanced version, Parallel Sysplex, was subsequently introduced for the newer operating system, OS/390. The sysplex consists of the multiple computers (the systems) that make up the complex. A sysplex is designed to be a solution for business needs involving any or all of the following: parallel processing; online transaction processing (OLTP); very high transaction volumes; very numerous small work units—online transactions, for example (or large work units that can be broken up into multiple small work units); or applications running simultaneously on separate systems that must be able to update to a single database without compromising data integrity. According to IBM, the Parallel Sysplex is the end result of IBM large systems’ developments over the years, from the single system uniprocessor, to tightly-coupled multiprocessors, to loosely-coupled configurations, to the sysplex, and finally to the Parallel Sysplex. A single system uniprocessor consists of a single central processor complex (CPC)—which consists of a single central processor (CP) and all associated system hardware and software, controlled by a single copy of the operating system. Tightly coupled multiprocessors consist of a number of CPs added to a CPC that share central storage and a single copy of the operating system. Work is assigned to an available CP by the operating system and can be rerouted to another if the first CP fails. A loosely coupled configuration has multiple CPCs (which may be tightly coupled multiprocessors) with separate storage areas, managed by more than one copy of the operating system and connected by channel-to-channel communications. A sysplex is similar to a loosely coupled configuration, but differs in that it has a standard communication mechanism (the cross-system coupling facility, or XCF) for MVS system applications that enables communication between application programs on one or multiple computers. The sysplex is made up of number of CPCs that collaborate, through
specialized hardware and software, to process a work load. This is what a large computer system does in general; a sysplex, through XCF, increases the number of processing units and operating systems that can be connected. The Parallel Sysplex, IBM’s latest method of configuration for CPCs, is a clustering architecture that has improved communication capabilities and supports more connected CPCs and more copies of the operating system. There are several areas of improvement over the base sysplex. The Parallel Sysplex Coupling Facility is a new processor that stores crucial system information, usually configured on a separate device. Use of the coupling facility increases the capacity for data sharing among systems and subsystems. Because it is used through both systems and subsystems, it also ensures data integrity and consistency throughout the sysplex. Another feature of the new technology is the Workload Manager (WLM), part of OS/390 that is in each system in a Parallel Sysplex configuration. WLM manages resources more responsively than the earlier schedule-based methods through dynamic workload balancing and prioritization according to user-set criteria. The datasharing capability enables simultaneous, multiple-system access to data.
system A system is a collection of elements or components that are organized for a common purpose. The word sometimes describes the organization or plan itself (and is similar in meaning to method, as in ‘‘I have my own little system’’) and sometimes describes the parts in the system (as in ‘‘computer system’’). A computer system consists of hardware components that have been carefully chosen so that they work well together and software components or programs that run in the computer. The main software component is itself an operating system that manages and provides services to other programs that can be run in the computer. A filing system is a group of files organized with a plan (for example, alphabetical by customer). All of nature and the universe can be said to be a system. We’ve coined a word, ecosystem, for the systems on Earth that affect life systems. The term can be very useful because so many things can be described as systems. It can also be very unuseful when a more specific term is needed.
system operator A sysop is the person who runs a computer server. In general, a sysop or system operator is one who runs the dayto-day operation of a server and the term suggests a person who is available when the system is. A related term is administrator. In larger computer systems, the administrator
systems development method manages security and user access while a system operator monitors and performs routine operations at the computer. In smaller computer systems (for example, UNIX systems), the administrator and the system operator tend to be the same person.
system tray The system tray (or ‘‘systray’’) is a section of the taskbars in the Microsoft Windows desktop user interface that is used to display the clock and the icons of certain programs so that a user is continually reminded that they are there and can easily click one of them. The system tray is a section located at the one end of the taskbar in Windows 95, 98, and NT. Icons that are often installed in the system tray include the volume control for sound, anti-virus software, scanner software, a channel viewer, player, and a system resources indicator. When more icons are installed in the system tray than can fit in the space allotted, the system tray becomes horizontally scrollable or expandable. To interact with a program in the system tray, you select an icon with your mouse and double-click or right-click the icon. When you minimize the program after using it, it shrinks back into the system tray instead of into the main part of the taskbar. What makes a system tray icon special is that, when installed, it can have a customized message appear when the mouse is hovering over it; the menu that appears when you right-click it can be customized; and the action that occurs when you double-click it can also be customized. For example, the double-click action for Internet Explorer could be to launch (start, or open a window for) the program, but the double-click action for a desktop icon could be to ‘‘View desktop,’’ simply remove the active windows and show the underlying desktop. An icon can be placed in the system tray when a program is installed by making an entry in the system registry or later by creating a shortcut to a program and dragging the shortcut icon to the system tray. An icon can be removed by deleting the registry entry or by right-clicking it and selecting Delete on the popup menu. The clock is removed by deselecting the show clock option in the taskbar Properties.
Systems Application Architecture See ‘‘SAA’’
systems development life cycle model (The systems development life cycle (SDLC) model is sometimes referred to as the waterfall model. SDLC is also an abbreviation for Synchronous Data Link Control.) The systems development life cycle (SDLC) model is an approach to developing an information system or software product that is characterized by a linear sequence of steps that progress from start to finish without revisiting any
previous step. The SDLC model is one of the oldest systems development models and is still probably the most commonly used. In general, these are the steps: 1. The existing system is evaluated. Deficiencies are identified. This can be done by interviewing users of the system and consulting with support personnel. 2. The new system requirements are defined. In particular, the deficiencies in the existing system must be addressed with specific proposals for improvement.
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3. The proposed system is designed. Plans are be laid out concerning the physical construction, hardware, operating systems, programming, communications, and security issues.
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4. The new system is developed. The new components and programs must be obtained and installed. Users of the system must be trained in its use, and all aspects of performance must be tested. If necessary, adjustments must be made at this stage.
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5. The system is put into use. This can be done in various ways. The new system can phased in, according to application or location, and the old system gradually replaced. In some cases, it may be more cost-effective to shut down the old system and implement the new system all at once. 6. Once the new system is up and running for awhile, it should be exhaustively evaluated. Maintenance must be kept up rigorously at all times. Users of the system should be kept up-to-date concerning the latest modifications and procedures. Critics of the SDLC model say that it doesn’t allow sufficiently for error discovery and redesign.
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systems development method A systems development method (SDM) is a work discipline that is chosen by the developers of a computer system or product as a way to ensure successful results. Typically, a systems development method specifies a series of stages that encompass requirements gathering, design, development, testing, delivery, maintenance, and enhancement of a system or product. The term systems development model is not used universally. Similar terms such as process model or development guidelines are sometimes used instead.
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Before systems development methods came into being, the development of new systems or products was often carried out by using the experience and intuition of management and technical personnel. However, the complexity of modern systems and computer products long ago made the need clear for some kind of orderly development process.
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Some of the approaches to a systems development method include:
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The systems development life cycle model (also known as the waterfall model)
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The prototyping model
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The exploratory model
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The spiral model
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The reuse model
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Rapid application development (RAD)
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Frequently, several models are combined into some sort of hybrid process. Record-keeping is important regardless of the type of SDM chosen or devised for any application, and is usually done in parallel with the development process.
systems management Systems management is the management of the information technology systems in an enterprise. This includes gathering requirements, purchasing equipment and software, distributing it to where it is to be used, configuring it, maintaining it with enhancement and service updates, setting up problem-handling processes, and determining whether objectives are being met. Systems management is usually under the overall responsibility of an enterprise’s Chief Information Officer (CIO). The department that performs systems management is sometimes known as management information systems (MIS) or simply information systems (IS). Network management and database management can be viewed as part of systems management or they may be viewed as coequal parts of a total information system. Trends and issues in systems management include: L
The total cost of ownership, which emphasizes that updating and servicing equipment is likely to be a major cost
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The right balance of resources and control between centrally-managed and network-distributed systems
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The outsourcing of all or part of information systems and systems management
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Tactical versus strategic purchasing decisions
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The choices between proprietary, compatible, and Open Source software
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Exploitation of the Internet and Web interfaces
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Graphical user interfaces for controlling the information system
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Security management, including security for mobile device users
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Systems Network Architecture SNA is a proprietary IBM architecture and set of implementing products for network computing within an enterprise. It existed prior to and became part of IBM’s Systems Application Architecture (SAA) and it is currently part of IBM’s Open Blueprint. With the advent of multi-
enterprise network computing, the Internet, and the de facto open network architecture of TCP/IP, IBM is finding ways to combine its own SNA within the enterprise with TCP/IP for applications in the larger network. SNA itself contains several functional layers and includes an application program interface called the Virtual Telecommunications Access Method (VTAM), a communications protocol for the exchange of control information and data, and a data link layer, Synchronous Data Link Control (SDLC). SNA includes the concepts of nodes that can contain both physical units that provide certain setup functions and logical units, each associated with a particular network transaction.
TACACS and TACACS+
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See ‘‘tesla’’
See ‘‘T1’’
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T interface
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In Integrated Services Digital Network (ISDN) service, a T interface is the electrical interface between a network terminating unit 1 (NT1) and a network terminating unit 2 (NT2) device, which typically is a private branch exchange (PBE). Like the S interface, the T interface has four wires, allowing up to eight devices to be addressed. In Basic Rate Interface (BRI) ISDN, the bits that flow from the central office through the NT1 are divided into two 64 Kbps channels (known as B, or bearer, channels) and one 16 Kbps channel (the D, for data or delta, channel) for control signals. The control signals allow a specific device to be addressed.
See ‘‘T-carrier system’’
The following table summarizes the various ISDN electrical interfaces at different demarcation points or places in the traffic flow: Electrical interface
Between what two points
U interface T interface
Central office and NT1 NT1 and NT2 devices (such as a PBX) NT1 or NT2 and ISDN devices (such as a telephone or terminal adapter) Terminal adapter and nonISDN devices (such as a computer) Within the ISDN node at the central office; separates line termination equipment from exchange termination equipment
S interface
R interface
V interface
T1 Also see the T-carrier system, of which the T1 is a part. The T1 (or T-1) carrier is the most commonly used digital line in the U.S., Canada, and Japan. In these countries, it carries 24 pulse code modulation (PCM) signals using timedivision multiplexing (TDM) at an overall rate of 1.544 million bits per second (Mbps). T1 lines span distances within and between major metropolitan areas. A T1 Outstate System has been developed for longer distances between cities. It’s probable that your Internet access provider is connected to the Internet as a point-of-presence (POP) on a T1 line owned by a major telephone network.
D
tablet computer See ‘‘Tablet PC’’
Tablet PC Microsoft’s Tablet PC is a design for a fully-equipped personal computer that allows a user to take notes using natural handwriting on a stylus- or digital pen-sensitive touch screen instead of requiring the use of a keyboard. The Tablet PC is similar in size and thickness to a yellow paper notepad. It will run existing Windows-based applications and is intended to be the user’s primary personal computer as well as a note-taking device. Microsoft’s next operating system, Windows XP, (code-named Whistler) is designed to support the Tablet PC. The first Tablet PCs will be geared toward business users. The Tablet PC uses what is called digital ink. Digital ink allows the user to write a note on the screen in freehand just as the user would write on a piece of paper. Writing can be natural rather than being required to match a pattern. The note can be edited and revised using the stylus and even indexed and searched. The note stays in ‘‘ink’’ or handwritten form until the user translates the note into a text document. Handwritten notes can be shared via e-mail. The recognition of handwritten notes and instructions is designed to be as reliable as input from a keyboard or mouse is today. The idea of tablet computing is generally credited to Alan Kay of Xerox who sketched out the idea in 1971. The bestknown and first widely-sold tablet computer was Apple Computer’s Newton, which was not a commercial success. With today’s extended battery life, better display resolution, handwriting recognition software, larger memory, and wireless Internet access, the tablet computers of the early 2000’s are deemed to have a better chance of being accepted as a viable computing option. Microsoft’s goal is to make its tablet computers available by 2002.
TACACS and TACACS+ TACACS (Terminal Access Controller Access Control System) is an older authentication protocol common to UNIX networks that allows a remote access server to forward a user’s logon password to an authentication server to determine whether access can be allowed to a given
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system. TACACS is an encryption protocol and therefore less secure than the later TACACS+ and Remote Authentication Dial-In User Service protocols. A later version of TACACS is XTACACS (Extended TACACS). Both are described in Request for Comments 1492.
without interfering with the recording. Another use is to announce the title or other relevant information at the beginning of a recording (called a ‘‘slate’’).
In spite of its name, TACACS+ is an entirely new protocol. TACACS+ and RADIUS have generally replaced the earlier protocols in more recently built or updated networks. TACACS+ uses the Transmission Control Protocol (TCP) and RADIUS uses the User Datagram Protocol (UDP). Some administrators recommend using TACACS+ because TCP is seen as a more reliable protocol. Whereas RADIUS combines authentication and authorization in a user profile, TACACS+ separates the two operations.
On the Internet, especially in the United Kingdom, a talker is a Web site that hosts online chatting or conversations entered at the keyboard. In the U.S., the term chat site is more common. Talkers and chat sites tend to be ‘‘middle-of-theroad’’ in terms of interests and participants. Technically, MUDs have conversations but these tend to be much more focused in purpose and in the intensity and dedication of participants. A special kind of talker is one that is part of an Internet Relay Chat (IRC).
TACACS and XTACACS are still running on many older systems.
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tag A tag is a generic term for a language element descriptor. The set of tags for a document or other unit of information is sometimes referred to as markup, a term that dates to precomputer days when writers and copy editors marked up document elements with copy editing symbols or shorthand.
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TIFF (Tag Image File Format) is a common format for exchanging raster graphics (bitmap) images between application programs, including those used for scanner images. A TIFF file can be identified as a file with a ‘‘.tiff’’ or ‘‘.tif’’ file name suffix. The TIFF format was developed in 1986 by an industry committee chaired by the Aldus Corporation (now part of Adobe Software). Microsoft and Hewlett-Packard were among the contributors to the format. One of the most common graphic image formats, TIFF files are commonly used in desktop publishing, faxing, 3-D applications, and medical imaging applications.
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TIFF files can be in any of several classes, including gray scale, color palette, or RGB full color, and can include files with JPEG, LZW, or CCITT Group 4 standard run-length image compression.
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A talkback is a microphone-and-receiver system installed in a recording/mixing console for communication between people in the control room and performers in the recording studio. Most semi-professional and professional consoles include such a system. The typical setup includes an internal microphone built directly into the console, and a series of switches. The switches allow the recording engineer to route the microphone signal to a variety of audio paths in the studio, such as the performer’s headphones, a set of speakers in the recording area, or directly to a tape recorder. Using this tool, the engineer can communicate with a performer with headphones while they are performing in the studio
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In computers, tape is an external storage medium, usually both readable and writable, consisting of a loop of flexible celluloid-like material that can store data in the form of electromagnetic charges that can be read and also erased. A tape drive is the device that positions, writes to, and reads from the tape. A tape cartridge is a protectively-encased tape that is portable. In early business computers, tape was a primary storage medium and computer system operators spent a lot of time mounting and unmounting tapes for different jobs at different times of the day. With the development of the magnetic disk, tape became a medium for backing up (see tape backup) the large amounts of data on mainframes. Apart from any other consideration, one drawback of tape is that it can only be accessed by starting at the beginning and rolling through the tape until the desired data is located. For this reason, its main application use has been for batch processing of large amounts of data (payroll is the classic example). Today, tape is still widely used on mainframes for archiving and backup. Software is provided that allows a company to easily manage automatic backup. On personal computers, tape is also used for backup.
tape backup In computers, tape backup is the ability to periodically copy the contents of all or a designated amount of data from its usual storage device to a tape cartridge device so that, in the event of a hard disk crash or comparable failure, the data will not be lost. Tape backup can be done manually or, with appropriate software, be programmed to happen automatically. Tape backup systems exist for needs ranging from backing up the hard disk on a personal computer to backing up large amounts of storage for archiving and disaster recovery purposes in a large enterprise as part of a storage area network (SAN), usually combining a hardware and software package. For personal computer tape backup, the Onstream
taxonomy USB tape drive is popular. For enterprise tape backup, Linear Tape-Open (LTO) is an industry open standard from Hewlett-Packard, IBM, and Seagate. Tape backup also includes the ability to restore data that has been backed up back to hard disk storage devices when needed.
TAPI TAPI (Telephony Application Program Interface) is a standard program interface that lets you and your computer ‘‘talk’’ over telephones or video phones to people or phoneconnected resources elsewhere in the world. Assuming your computer is equipped with TAPI and your setup includes the right application and hardware, you may be able:
tar Tar (for ‘‘Tape ARchive’’) is a UNIX shell command that creates a single file called an ‘‘archive’’ from a number of specified files or extracts the files from such an archive. A tar archive has the file suffix ‘‘.tar’’. The files in a tar archive are not compressed, just gathered together in one file.
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The name is derived from a time when files were commonly backed up on and occasionally retrieved from magnetic tape as a permanent storage device. (They still are in some data centers.) A tar archive is perhaps more frequently used today to transfer files among UNIX systems. A popular archive handler for Windows systems, WinZIP, can be used to extract the files from a tar archive.
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Call someone by clicking on their picture or other image
Tarball is a jargon term for a tar archive, suggesting ‘‘a bunch of files stuck together in a ball of tar.’’
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Use a similar graphical user interface (GUI) to set up a conference call and then attend the call at the scheduled time
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In computer programming, a task is a basic unit of programming that an operating system controls. Depending on how the operating system defines a task in its design, this unit of programming may be an entire program or each successive invocation of a program. Since one program may make requests of other utility programs, the utility programs may also be considered tasks (or subtasks). All of today’s widely-used operating systems support multitasking, which allows multiple tasks to run concurrently, taking turns using the resources of the computer.
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In preemptive multitasking, each task may be assigned a priority depending on its relative importance, the amount of resources it is consuming, and other factors. The operating system then preempts (cuts short) a task having a lower priority value so that a higher priority task is given a turn. Windows 2000, OS/390, Linux, and Amiga are examples of operating systems that use preemptive multitasking. Cooperative multitasking is the ability for an operating system to manage multiple tasks such as application programs at the same time, but without the ability to necessarily preempt them.
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See who you’re talking to individually or at a conference call
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Add a voice note to an e-mail note you send or listen to a voice note attached to an e-mail note you receive
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Program your computer to automatically receive phone calls from certain numbers (but not from others)
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Send and receive faxes
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Do these things from a portable wireless cellular telephone telephone/computer as well as from a desktop computer
Developed jointly by Intel and Microsoft, TAPI is included with the Windows 95/98 and Windows NT operating system. Using TAPI, programmers can take advantage of different telephone systems, including ordinary public switched telephone network, ISDN, and private branch exchange without having to understand all their details. Each phone system hardware provider (for example, the modem maker or ISDN card maker) provides a specific software driver that interfaces directly with the hardware. TAPI provides a high-level interface for dialing and disconnecting. Instead of having to encode an ATDT dial string and the ATH disconnect string, the programmer codes a much simpler ‘‘function call.’’ In addition to the interface for applications, TAPI includes a Service Provider Interface (SPI) for hardware vendors who are writing the driver software. The TAPI Dynamic Link Library (DLL) maps the API to the SPI and coordinates input/output traffic.
Multithreading and multitasking are similar and are often confused. Multithreading is the management of multiple concurrent uses of the same program. Most operating systems and modern computer languages also support multithreading.
taxonomy Taxonomy (from Greek taxis meaning arrangement or division and nomos meaning law) is the science of classification according to a pre-determined system, with the resulting catalog used to provide a conceptual framework for discussion, analysis, or information retrieval. In theory, the development of a good taxonomy takes into account the importance of separating elements of a group (taxon) into subgroups (taxa) that are mutually exclusive, unambiguous,
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and taken together, include all possibilities. In practice, a good taxonomy should be simple, easy to remember, and easy to use. One of the best known taxonomies is the one devised by the Swedish scientist, Carl Linnaeus, whose classification for biology is still widely used (with modifications). In Web portal design, taxonomies are often created to describe categories and subcategories of topics found on the Web site. The categorization of words on whatis.com is similar to any Web portal taxonomy.
T-carrier system To see the relationship between T-carrier, E-carrier, and DS0 multiples, see digital signal X. The T-carrier system, introduced by the Bell System in the U.S. in the 1960s, was the first successful system that supported digitized voice transmission. The original transmission rate (1.544 Mbps) in the T-1 line is in common use today in Internet service provider (ISP) connections to the Internet. Another level, the T-3 line, providing 44.736 Mbps, is also commonly used by Internet service providers. Another commonly installed service is a fractional T-1, which is the rental of some portion of the 24 channels in a T1 line, with the other channels going unused. The T-carrier system is entirely digital, using pulse code modulation and time-division multiplexing. The system uses four wires and provides duplex capability (two wires for receiving and two for sending at the same time). The T-1 digital stream consists of 24 64-Kbps channels that are multiplexed. (The standardized 64 Kbps channel is based on the bandwidth required for a voice conversation.) The four wires were originally a pair of twisted pair copper wires, but can now also include coaxial cable, optical fiber, digital microwave, and other media. A number of variations on the number and use of channels are possible. In the T-1 system, voice signals are sampled 8,000 times a second and each sample is digitized into an 8-bit word. With 24 channels being digitized at the same time, a 192-bit frame (24 channels each with an 8-bit word) is thus being transmitted 8,000 times a second. Each frame is separated from the next by a single bit, making a 193-bit block. The 192 bit frame multiplied by 8,000 and the additional 8,000 framing bits make up the T-1’s 1.544 Mbps data rate. The signaling bits are the least significant bits per frame.
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Tcl is an interpreted script language developed by Dr. John Ousterhout at the University of California, Berkeley, and now developed and maintained by Sun Laboratories. Tcl is comparable to: L
Netscape JavaScript
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In general, script languages are easier and faster to code in than the more structured, compiled languages such as C and C++. Script languages are sometimes considered good ‘‘glue’’ languages for tying several programs together. TclBlend is a version of Tcl that can access certain Java language facilities. Tcl has a companion program, Tool Kit (Tk), to help create a graphical user interface with Tcl. Both Tcl and Tk can be downloaded or ordered from Sun’s Web site.
TCO TCO (total cost of ownership) is a type of calculation designed to help consumers and enterprise managers assess both direct and indirect costs and benefits related to the purchase of any IT component. The intention is to arrive at a final figure that will reflect the effective cost of purchase, all things considered. When you decide to buy a computer you may go through a TCO analysis: for example, the greater cost price of a high-end computer might be one consideration, but one that would have to be balanced by adding likely repair costs and earlier replacement to the purchase cost of the bargain brand. TCO analysis originated with the Gartner Group several years ago and has since been developed in a number of different methodologies and software tools. TCO analysis performs calculations on extended costs for any purchase—these are called fully burdened costs. For the consumer’s purchase of a computer, the fully burdened cost may include costs of purchase, repairs, maintenance, and upgrades. For the business purchase of a computer, the fully burdened costs can also include such things as service and support, networking, security, user training, and software licensing. The TCO has to be compared to the total benefits of ownership (TBO) to determine the viability of the purchase.
TCP TCP (Transmission Control Protocol) is a set of rules (protocol) used along with the Internet Protocol (IP) to send data in the form of message units between computers over the Internet. While IP takes care of handling the actual delivery of the data, TCP takes care of keeping track of the individual units of data (called packets) that a message is divided into for efficient routing through the Internet. For example, when an HTML file is sent to you from a Web server, the Transmission Control Protocol (TCP) program layer in that server divides the file into one or more packets, numbers the packets, and then forwards them individually to the IP program layer. Although each packet has the same destination IP address, it may get routed differently through the network. At the other end (the client program in your
TCPMAN computer), TCP reassembles the individual packets and waits until they have arrived to forward them to you as a single file. TCP is known as a connection-oriented protocol, which means that a connection is established and maintained until such time as the message or messages to be exchanged by the application programs at each end have been exchanged. TCP is responsible for ensuring that a message is divided into the packets that IP manages and for reassembling the packets back into the complete message at the other end. In the Open Systems Interconnection (OSI) communication model, TCP is in layer 4, the Transport layer.
TCP port numbers See ‘‘well-known port numbers’’
TCP Wrapper TCP Wrapper is a public domain computer program that provides firewall services for UNIX servers. The program was developed by Wietse Venema. When an unprotected UNIX computer is connected to a network, the computer’s system is exposed to other computer users connected to the network. For example, by using the finger utility, a hacker may be able to determine which users are logged on to a given server. It is also possible to find out the identities of individual computers, and various details about their users’ recent Internet behavior. A hacker can determine when a workstation is likely to be idle, and then access and use that workstation when it is unattended. TCP Wrapper can act as a firewall to prevent this. TCP Wrapper monitors incoming packets. If an external computer or host attempts to connect, TCP Wrapper checks to see if that external entity is authorized to connect. If it is authorized, then access is permitted; if not, access is denied. The program can be tailored to suit individual user or network needs.
TCP/IP Transmission Control Protocol/Internet Protocol (TCP/IP) is the basic communication language or protocol of the Internet. It can also be used as a communications protocol in a private network (either an intranet or an extranet). When you are set up with direct access to the Internet, your computer is provided with a copy of the TCP/IP program just as every other computer that you may send messages to or get information from also has a copy of TCP/IP. TCP/IP is a two-layer program. The higher layer, Transmission Control Protocol, manages the assembling of a message or file into smaller packets (see packet) that are transmitted over the Internet and received by a TCP layer that reassembles the packets into the original message. The lower layer, Internet Protocol, handles the address part of
each packet so that it gets to the right destination. Each gateway computer on the network checks this address to see where to forward the message. Even though some packets from the same message are routed differently than others, they’ll be reassembled at the destination. TCP/IP uses the client/server model of communication in which a computer user (a client) requests and is provided a service (such as sending a Web page) by another computer (a server) in the network. TCP/IP communication is primarily point-to-point, meaning each communication is from one point (or host computer) in the network to another point or host computer. TCP/IP and the higher-level applications that use it are collectively said to be ‘‘stateless’’ because each client request is considered a new request unrelated to any previous one (unlike ordinary phone conversations that require a dedicated connection for the call duration). Being stateless frees network paths so that everyone can use them continuously. (Note that the TCP layer itself is not stateless as far as any one message is concerned. Its connection remains in place until all packets in a message have been received.) Many Internet users are familiar with the even higher layer application protocols that use TCP/IP to get to the Internet. These include the World Wide Web’s Hypertext Transfer Protocol (HTTP), the File Transfer Protocol (FTP), Telnet (Telnet) which lets you logon to remote computers, and the Simple Mail Transfer Protocol (SMTP). These and other protocols are often packaged together with TCP/IP as a ‘‘suite.’’ Personal computer users usually get to the Internet through the Serial Line Internet Protocol (SLIP) or the Point-to-Point Protocol (PPP). These protocols encapsulate the IP packets so that they can be sent over a dial-up phone connection to an access provider’s modem. Protocols related to TCP/IP include the User Datagram Protocol (UDP), which is used instead of TCP for special purposes. Other protocols are used by network host computers for exchanging router information. These include the Internet Control Message Protocol (ICMP), the Interior Gateway Protocol (IGP), the Exterior Gateway Protocol (EGP), and the Border Gateway Protocol (BGP).
TCPMAN TCPMAN is a utility for manual or script modem dialing that is used when accessing the Internet. TCPMAN comes with the shareware program, Trumpet Winsock. Winsock is an interface between an application program in a Windows operating system and the Internet’s Transmission Control Protocol program. (There are other Winsock programs than Trumpet’s, which was one of the first and is among the most widely installed today.) TCPMAN is the ‘‘tcpman.exe’’ file in Trumpet Winsock. Although the ‘‘MAN’’ stands for ‘‘manual,’’ most users set up or are provided with a script of modem AT commands
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that is processed automatically each time you start TCPMAN. This is viewed as ‘‘scripted manual dialing.’’ Using the information about your modem that you provide during setup, TCPMAN negotiates the actual line speed and other characteristics of the connection between your modem and the modem that is being dialed. Typically, you will know about TCPMAN if you are with a service provider that furnishes or requires you to have the Trumpet Winsock. If not, your dialing capability is furnished by some other program.
TDM See ‘‘time-division multiplexing’’
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TDMA TDMA (time division multiple access) is a technology used in digital cellular telephone communication that divides each cellular channel into three time slots in order to increase the amount of data that can be carried. TDMA is used by Digital-American Mobile Phone Service (D-AMPS), Global System for Mobile communications (GSM), and Personal Digital Cellular (PDC). However, each of these systems implements TDMA in a somewhat different and incompatible way. An alternative multiplexing scheme to FDMA with TDMA is CDMA (code division multiple access), which takes the entire allocated frequency range for a given service and multiplexes information for all users across the spectrum range at the same time. TDMA was first specified as a standard in EIA/TIA Interim Standard 54 (IS-54). IS-136, an evolved version of IS-54, is the United States standard for TDMA for both the cellular (850 MHz) and personal communications services (1.9 GHz) spectrums. TDMA is also used for Digital Enhanced Cordless Telecommunications (DECT).
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A teach box is a device that registers and memorizes mechanical motions or processes for later recall and execution by an electronic or computer system. The term especially applies to programmable robots. Robot programming can be divided into levels, starting with the least complex and progressing to the equivalent of human intelligence. The table shows a four-level scheme. Level 3, just below human-equivalent artificial intelligence (AI), is called task-level programming. Programs at level 3 can instruct a robot to execute complete operations, such as shoveling the snow from a driveway or flying an aircraft on a reconnaissance mission.
Programming Level
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Human intelligence Complete tasks Sequences of motions or operations Single motions or operations
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An example of a level-1 teach box is a remote-control for the doors of a car. When the wireless receiver gets the signal from the remote unit, it locks or unlocks the doors. Another example of a level-1 teach box is the remote box that controls a hi-fi sound system or videocassette recorder (VCR). An example of a level-2 teach box is the microcomputer that controls a telephone answering machine. When a call comes in, the sequence of operations is recalled from memory. The machine answers the phone, makes an announcement, takes the message, and resets for the next incoming call. Level-3 teach boxes are used in aerospace, miliatary, and industrial applications. The intended movements of a robot are entered into memory by pressing buttons or guiding a joystick or other three-dimensional control device. The robot’s path, variations in speed, rotations, and gripping/ grasping movements are all programmed. Then, when the memory is recalled, the robot reproduces these movements in the exact sequence, and to the exact extent and speed in all dimensions. Also see artificial intelligence and robotics.
technobabble In information technology and other specialized areas, technobabble is the use of technical or ‘‘insider’’ terms that, to the uninitiated, have no meaning. Technobabble can be divided into (1) technical terms with some formal standing in language such as new transmission or computer communication protocols, especially in their abbreviated or acronym forms, (2) marketing terms in which terms with prior meaning are give new missions (for example, industrial strength), and (3) informal, colloquial, or jargon terms (of which technobabble itself would seem to be an example). Although this term primarily connotes words that discourage understanding, it is not always used in a negative sense, but often in the sense that ‘‘here is some technical information expressed in the terms that have been invented for it.’’ Closely related terms include: neologism, technospeak, and geekspeak.
teleconference
techno-fiend In information technology, a techno-fiend is someone who is addicted to finding out and knowing how things work in one or more aspects of cyberspace. Techno-fiends frequently know about and consult the places where you can find out. Some techno-fiends also frequent Usenet or other online discussions. Techno-fiends usually suspect that there’s some place or someone with information that they should know about but don’t. Subjects that compel the attention of techno-fiends include: Web site design and browser behavior, Web server installation and management, any new emerging standard (a techno-fiend will read the main standard and even some of the ancillary standards), and any new technology, especially hardware technologies. In general (with some exceptions), techno-fiends tend to be lay people rather than experts (whose motivation for understanding how things work is professional and somewhat economically motivated). A techno-fiend is less dedicated to a subject or a technology than a geek or a hacker, who both tend to be among the professionals. However, you can be an expert in one area and a technofiend in another.
telco In the United States and possibly other countries, ‘‘telco’’ is a short form for telephone company. Sometimes it means a local telephone company, such as a Bell operating company or an independent local telephone company. Sometimes it means any telephone company, including one offering longdistance services.
telecenter A telecenter (U.S. spelling) or telecentre (UK spelling) is a work location usually in a different place than the organization’s main office that provides convenient occasional access for telecommuting to work equipment that they don’t have at home or on the road. For example, a home telecommuter might need to print and reproduce printed copies of a document occasionally using a highspeed printer not available at home. Or a mobile worker might occasionally check the telecenter for fax mail or to send a fax. A telecenter could also have teleconference facilities.
telecentre See ‘‘telecenter’’
Telecommunications Act of 1996 The Telecommunications Act of 1996, enacted by the U.S. Congress on February 1, 1996, and signed into law by President Bill Clinton on February 8, 1996, provided major
changes in laws affecting cable TV, telecommunications, and the Internet. The law’s main purpose was to stimulate competition in telecommunication services. The law specifies: L
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The deregulation of cable TV rates
Included with the Act was the former Communications Decency Act, which, among other provisions, makes it a crime to convey pornography over the Internet in a way that is easily accessible to children.
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Telecommunications Management Network
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telecommuting Telecommuting and telework are synonyms for the use of telecommunication to work outside the traditional office or workplace, usually at home (SOHO) or in a mobile situation. According to one study, telecommuting has been growing at 15% a year since 1990 in North America. 80% of Fortune 1000 companies are likely to introduce it within the next two to three years. Although work at the company premises is not likely to disappear, new forms of telecommunication such as voice and picture communication and groupware are likely to make telecommuting more social in the future. Factors that will continue to affect the future of telecommuting include the availability of bandwidth and fast Internet connections in a given country; social methodologies for balancing work control and work freedom; the perceived values and economies in telecommuting; and the opportunities and need for working collaboratively across large distances, including globally. With the arrival of the Internet and the Web as a kind of ‘‘standard’’ for groupware, one can join a virtual organization to access resources developed for members who work almost entirely through telecommunication with an occasional face-to-face meeting.
teleconference A teleconference is a telephone meeting among two or more participants involving technology more sophisticated than a simple two-way phone connection. At its simplest, a teleconference can be an audio conference with one or both ends of the conference sharing a speaker phone. With considerably more equipment and special arrangements, a teleconference can be a conference, called a videoconference, in which the participants can see still or motion video images of each other. Because of the high bandwidth of video and the opportunity for larger and multiple display screens, a videoconference requires special telecommunica-
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Whatis?Com’s Encyclopedia of Technology Terms tion arrangements and a special room at each end. As equipment and high-bandwidth cabling become more commonplace, it’s possible that videoconferences can be held from your own computer or even in a mobile setting. One of the special projects of Internet2 is to explore the possibility of having teleconferences in which all participants actually appear to be in the same room together. Today’s audio teleconferences are sometimes arranged over dial-up phone lines using bridging services that provide the necessary equipment for the call.
teledactyl A teledactyl (pronounced TEHL-eh-DAK-til) is a specialized voice recognition system designed to produce ASCII text from human speech. The teledactyl can be used in conjunction with telephone lines, allowing voice-operated directory assistance, voice-operated remote control, and dictation over the Internet. We’re not familiar with this term’s origin, but Greek tele means ‘‘far off or distant’’ and daktylos, Greek for ‘‘finger,’’ is the source of dactyl, a term used in English verse for a poetic meter consisting of one accented syllable followed by two weaker ones. Dactylography is the analysis of fingerprints for identification. Our guess is that the term’s orginators wanted to convey the idea of ‘‘writing at a distance by speaking over the telephone.’’
telematics Telematics is the blending of computers and wireless telecommunications technologies, ostensibly with the goal of efficiently conveying information over vast networks to improve a host of business functions or government-related public services. The most notable example of telematics may be the Internet itself, since it depends on a number of computer networks connected globally through telecommunication backbones. The term has evolved to refer to automobile systems that combine global positioning satellite (GPS) tracking and other wireless communications for automatic roadside assistance and remote diagnostics. General Motors Corp. first popularized automotive telematics with its OnStar system. Major automakers are equipping new prototype vehicles with wireless-based services controlled by voice commands. This kind of telematics could enable motorists to perform a variety of wireless functions such as accessing the Internet, receiving or sending e-mail, downloading digital audio and video files, or obtaining ‘‘smart’’ transportation information. The telematics industry is not limited to automotive applications. Other applications are being studied or developed for monitoring water and air pollution, for medical informatics and health care, and for distance
learning. Many European countries are developing uniform policies to integrate telematics applications into government, business and education.
telemetrics Telemetrics is the technology involving automatic measurement and transmission of data from remote sources. The process of measuring data at the source and transmitting it automatically is called telemetry. The two terms, telemetry and telemetrics, are often used interchangeably. Originally, data was transmitted over wires, but now telemetrics frequently refers to wireless communication. Telemetrics applications include the monitoring of space flights, meteorological data transmission, videoconferencing, the Global Positioning System (GPS), wildlife tracking, camera control robotics, and oceanography. In the first telemetrics application, in Chicago in 1912, telephone lines were used to transmit operational data from power plants to a central office. Because telemetry was used originally in projects like this, the first telemetry systems were called supervisory systems. In 1960, the interrogationreply principle was developed, which led to selective transmission of data, to be sent only upon request. In general, telemetrics works in the following way: Sensors at the source measure either electrical data (such as voltage or current) or physical data (such as temperature or pressure). These are converted to specific electrical voltages and a multiplexer combines the voltages, along with timing data, into a single data stream for transmission to the distant receiver. Upon reception, the data stream is separated into its original components and the data is displayed and processed according to user specifications.
telephone jacks In the U.S., telephone jacks are also known as registered jacks, sometimes described as RJ-XX, and are a series of telephone connection interfaces (receptacle and plug) that are registered with the U.S. Federal Communications Commission (FCC). They derive from interfaces that were part of AT&T’s Universal Service Order Codes (USOC) and were adopted as part of FCC regulations (specifically Part 68, Subpart F. Section 68.502). The term jack sometimes means both receptacle and plug and sometimes just the receptacle.
RJ-11 The most common telephone jack is the RJ-11 jack, which can have six conductors but usually is implemented with four. The RJ-11 jack is likely to be the jack that your household or office phones are plugged into from the ordinary ‘‘untwisted’’ wire (sometimes called ‘‘gray satin’’ or ‘‘flat wire’’) people are most familiar with. In turn, the jacks
teleportation connect to the ‘‘outside’’ longer wires known as twisted pair that connect to the telephone company central office or to a private branch exchange (PBX).
local phone connection rates, an international or other longdistance call will be much less expensive than through the traditional call arrangement.
The four wires are usually characterized as a red and green pair and a black and white pair. The red and green pair typically carry voice or data. On an outside phone company connection, the black and white pair may be used for lowvoltage signals such as phone lights. On a PBX system, they may be used for other kinds of signaling.
On the Internet, three new services are now or will soon be available:
Your computer modem is usually connected to an RJ-11 jack.
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The ability to send fax transmissions at very low cost (at local call prices) through a gateway point on the Internet in major cities
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RJ-14 The RJ-14 is similar to the RJ-11, but the four wires are used for two phone lines. Typically, one set of wires (for one line) contains a red wire and a green wire. The other set contains a yellow and black wire. Each set carries one analog ‘‘conversation’’ (voice or data).
RJ-45 The RJ-45 is a single-line jack for digital transmission over ordinary phone wire, either untwisted or twisted. The interface has eight pins or positions. For connecting a modem, printer, or a data PBX at a data rate up to 19.2 Kbps, you can use untwisted wire. For faster transmissions in which you’re connecting to an Ethernet 10BaseT network, you need to use twisted pair wire. (Untwisted is usually a flat wire like common household phone extension wire. Twisted is often round.) There are two varieties of RJ-45: keyed and unkeyed. Keyed has a small bump on its end and the female complements it. Both jack and plug must match.
telephone network standards See ‘‘V.xx’’
Telephony Application Program Interface See ‘‘TAPI’’
telephony Telephony is the technology associated with the electronic transmission of voice, fax, or other information between distant parties using systems historically associated with the telephone, a handheld device containing both a speaker or transmitter and a receiver. With the arrival of computers and the transmittal of digital information over telephone systems and the use of radio to transmit telephone signals, the distinction between telephony and telecommunication has become difficult to make. Internet telephony is the use of the Internet rather than the traditional telephone company infrastructure and rate structure to exchange spoken or other telephone information. Since access to the Internet is available at
The ability to make a normal voice phone call (whether or not the person called is immediately available; that is, the phone will ring at the location of the person called) through the Internet at the price of a local call
Some companies that make products that provide or plan to provide these capabilities include: IDT Corporation (Net2Phone), Netspeak, NetXchange, Rockwell International, VocalTec, and Voxspeak. Among uses planned for Internet phone services are phone calls to customer service people while viewing a product catalog online at a Web site. You can now add telephone capabilities to your computer by adding a telephony board, available for under $300, that combines the functions of modem, sound board, speakerphone, and voicemail system. A telephony board is often integrated into new machines targeted for small business and home office users. A Telephony API (TAPI) is available from Microsoft and Intel that allows Windows client applications to access voice services on a server and that interconnects PC and phone systems. Both Microsoft and Netscape provide or plan to provide support for voice e-mail.
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teleportation Teleportation is the duplication or re-creation of physical objects or their properties using light beams, according to researchers at the California Institute of Technology. Also calling it quantum teleportation, the researchers have successfully transmitted information about the properties of an object at the speed of light so that the object could theoretically be duplicated or reconstructed at the destination. The experiment takes advantage of the atomic particle property in which two particles at a great distance are in some mysterious way intertwined. Thus, an effect on one particle is almost simultaneously felt in the other particle as well. In physics, this characteristic is called entanglement. The CalTech researchers believe the characteristic may one day have practical applications, one of which would be a quantum computer in which information is moved with light using the entanglement principle rather than wires. In their experiment, the researchers created two entangled light beams. (A light beam is a stream of photons, and photons, which have both wave and particle characteristics, are the basic units of light.) The entangled light beams carried information about the quantum state of a third light
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beam over the distance of a yard (about one meter) at the speed of light. The researchers believe the concept can be applied to transmitting the physical property attributes of solid objects at the speed of light. Is there any difference between a particular solid object and the collection of its attributes reproduced at a great distance at the speed of light? The researchers say no one knows for sure.
teletypewriter A teletypewriter (TTY) is an input device that allows alphanumeric characters to be typed in and sent, usually one at a time as they are typed, to a computer or a printer. The Teletype Corporation developed the teletypewriter, which was an early interface to computers. Teletype mode is the capability of a keyboard, computer, application, printer, display, or modem to handle teletypewriter input and output. Basically, this is a one-character-at-a-time mode of sending, receiving, or handling data, although it is often modified to handle a line of characters at a time. Since this mode requires little programming logic, it is often used where memory is limited. The basic input/output operating system (BIOS) sends messages to a PC display using teletype mode. Most printers offer a teletype mode. The simplest video display output format is text in teletype mode. Many modems today continue to include support for a TTY interface.
telework
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See ‘‘telecommuting’’
Telnet Telnet is the way you can access someone else’s computer, assuming they have given you permission. (Such a computer is frequently called a host computer.) More technically, Telnet is a user command and an underlying TCP/IP protocol for accessing remote computers. On the Web, HTTP and FTP protocols allow you to request specific files from remote computers, but not to actually be logged on as a user of that computer. With Telnet, you log on as a regular user with whatever privileges you may have been granted to the specific application and data on that computer. A Telnet command request looks like this (the computer name is made-up): telnet the.libraryat.whatis.edu
The result of this request would be an invitation to log on with a userid and a prompt for a password. If accepted, you would be logged on like any user who used this computer every day. Telnet is most likely to be used by program developers and anyone who has a need to use specific applications or data located at a particular host computer.
temperature Temperature (symbolized T) is an expression of heat energy. Temperature can mean different things in different situations. Thermodynamic temperature is a measure of the kinetic energy in molecules or atoms of a substance. The greater this energy, the faster the particles are moving, and the higher the reading an instrument will render. This is the method lay people most often use. Spectral temperature is defined according to the wavelength at which the electromagnetic (EM) energy that an object emits is greatest. The shorter the wavelength, the higher the frequency of maximum EM energy, and the higher the spectral temperature. This is the temperature scheme that astronomers use to measure the heat in distant objects such as the sun’s corona or the gas and dust between stars. There are three temperature scales in common use today: kelvin (K), centigrade or Celsius (C), and Fahrenheit (F). A fourth scale, known as the Rankine (R) temperature scale, is less often used. The kelvin is the unit of temperature in the International System of Units (SI). One kelvin is formally defined as 1/273.16 (3.6609 6 10-3) of the thermodynamic temperature of the triple point of pure water (H2O). A temperature of 0 K represents absolute zero, the absence of all heat. In the centigrade or Celsius temperature scale, the freezing point of pure water at one atmosphere is assigned the value zero; the boiling point is +100 C. One-degree increments in the centigrade scale are the same size as those in the kelvin scale. At standard Earth-atmospheric sea-level pressure, water freezes at 0 C or +273.15 K, and boils at +100 C or +373.15 K. A temperature of 0 K thus corresponds to -273.15 C. To convert a kelvin temperature figure to Celsius, subtract 273.15. To convert a Celsius temperature figure to kelvin, add 273.15. The Fahrenheit scale is used mostly by non-scientists. Pure water at one atmosphere (the average sea-level pressure) freezes at +32 degrees Fahrenheit (F); pure water at one atmosphere boils at +212 degrees F. Absolute zero is 7459.67 degrees F. One Fahrenheit degree increment is 5/9 (0.55555) times the size of a kelvin or centigrade degree. If C is the centigrade temperature, the Fahrenheit temperature F is given by F = (1.8 6 C) + 32. The Fahrenheit and centigrade scales agree at 740. The Rankine scale is not often used. The degree increments in this temperature scheme are the same size as those of the Fahrenheit scale, but 0 R corresponds to absolute zero or 0 K. Degrees Rankine can be obtained from degrees kelvin by multiplying the kelvin temperature by 1.8. Degrees Fahrenheit are obtained from Rankine readings by subtracting 459.67. Also see kelvin, and International System of Units (SI).
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temperature, standard See ‘‘standard temperature and pressure’’
Tempest Tempest was the name of a classified (secret) U.S. government project to study (probably for the purpose of both exploiting and guarding against) the susceptibility of some computer and telecommunications devices to emit electromagnetic radiation (EMR) in a manner that can be used to reconstruct intelligible data. Tempest’s name is believed to have been a code name used during development by the U. S. government in the late 1960s, but at a somewhat later stage, it became an acronym for Telecommunications Electronics Material Protected from Emanating Spurious Transmissions. Today, in military circles, the term has been officially supplanted by Emsec (for Emissions Security); however, the term Tempest is still widely used in the civilian arena. Exactly when it was first noticed that certain devices can emanate decipherable data is unclear. (The practice of monitoring and capturing such data is known as Van Eck phreaking after the author of a formal paper on the subject.) Today, cathode ray tube (CRT) and to a lesser extent liquid crystal display (LCD) monitors, microchips, and composite devices such as printers and PCs all emit EMR into space or into some conductive medium (such as power lines, communications wires, or even water piping). The EMR that is emitted contains, to varying degrees, the information that the device is displaying, creating, storing, or transmitting. With the correct equipment and techniques, it is possible to reconstruct all or a substantial portion of that data. Some equipment is far more susceptible than others. For example, some US Robotics data/fax modems generate incredibly strong EMR when active, which can be read even by comparatively crude equipment. Wireless handsets and office speakerphones are other devices that generate extremely strong EMR signals. The range in which an eavesdropper can monitor emanations varies tremendously according to conditions. In most cases, the emanations can be picked up with proper equipment from a distance of around 200-300 meters. However, in some cases where a signal has been captured by a conductive medium (such as a power line), monitoring can occur over a distance of many kilometers. Monitoring devices include various kinds of sensitive receivers, which can monitor a wide range of frequencies, and a combination of hardware and software that is capable of processing the received signals into the original data. The data that is picked up is often corrupted by such things as external EMR interference, signal weakness over distances, and partial transmission. Advanced algorithms can help provide a more complete picture of the original information.
Shielding of devices from EMR is achieved by a number of methods. The most sophisticated devices use advanced micro-components that have been designed from scratch to minimize Tempest emanations. Generally, shielding involves encompassing the device in a Faraday cage that does not permit stray emanations, along with special modifications to the power source. This usually involves a heavy metal case around an object. Tempest shielding also involves such issues as the design of a room and placement of equipment within it, to ensure that no information can escape. For individuals who wish to be more secure against Van Eck phreaking but cannot invest in this level of equipment, some software products recommend special displays that limit the effectiveness of monitoring of emanations from a CRT monitor. National Communications Security Committee Directive 4 sets U.S. Tempest (shielding) standards. The requirements are set out in document NACSIM 5100A, which is classified. Tempest certification for private sector usage is extremely expensive and, as a result, it has led to a newer standard, called ZONE, which is more cost effective, though somewhat less secure. Approved Tempest-shielded devices are classed into 3 categories. Type 1 is extremely secure and available only to the U.S. government and approved contractors, who must undergo strict vetting. Type 2 is somewhat less secure, but still requires government approval to use. Type 3 is for general commercial use. In the private arena, there are few individuals who are competent in Tempest technology. With only a handful of exceptions, the only qualifications of significance come from individuals who have served in either the military or intelligence communities and who have attended courses run and approved by the NSA, namely the ones held at Lackland Air Force Base, just outside San Antonio, Texas or at National Cryptologic School at Linthicum, Maryland. As a note of warning, there is nothing illegal in the U.S. in attempting to procure equipment that is Tempest-shielded, though some devices are classified and only obtainable to authorized entities. You are free (subject to patent law and FCC guidelines for spurious admissions) to make any modifications to equipment to shield them from attack. However, securing or attempting to secure surveillance devices is illegal and can subject all individuals involved to severe penalties. Even ‘‘scam’’ technology that is completely ineffective will get individuals into trouble, as the law relates also to intention. Full details are available at US Code / Title 18—Crimes and Criminal Procedures / Part 1—Crimes / Chapter 119, 2510 to 2521). The only exceptions to this are related to the military, intelligence agencies, and law enforcement bodies with court orders.
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A template (from French templet, diminutive of temple, a part of a weaving loom for keeping it stretched transversely) is a form, mold, or pattern used as a guide to making something. Here are some examples:
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A ruler is a template when used to draw a straight line.
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A document in which the standard opening and closing parts are already filled in is a template that you can copy and then fill in the variable parts.
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Flowcharting templates (not used much now) help programmers draw flowcharts or logic sequences in preparation for writing the code.
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In programming, a template is a generic class or other unit of source code that can be used as the basis for unique units of code. In C++, an object-oriented computing language, there are Standard Template Libraries from which programmers can choose individual template classes to modify. The Microsoft Foundation Class Library (MFCL) is an example.
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An overlay that you put on your computer keyboard telling you special key combinations for a particular application is a template for selecting the right keys to press.
terabit In measuring data transmission speed, a terabit is one trillion binary digits, or 1,000,000,000,000 (that is, 1012) bits. A terabit is used for measuring the amount of data that is transferred in a second between two telecommunication points or within network devices. For example, several companies are building a network switch that passes incoming packets through the device and out again at a terabits-per-second speed. Terabits per second is usually shortened to Tbps. Although the bit is a unit of the binary number system, bits in data communications have historically been counted using the decimal number system. For example, 28.8 kilobits per second (Kbps) is 28,800 bits per second. Because of computer architecture and memory address boundaries, bytes are always some multiple or exponent of two.
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A terabyte is a measure of computer storage capacity and is 2 to the 40th power or approximately a thousand billion bytes (that is, a thousand gigabytes). Also see byte, gigabyte, teraflop, and petabyte.
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teraflop A teraflop is a measure of a computer’s speed and can be expressed as:
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A trillion floating point operations per second
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10 to the 12th power floating-point operations per second
Today’s fastest parallel computing operations are capable of teraflop speeds. Scientists have begun to envision computers operating at petaflop speeds.
terahertz The terahertz, abbreviated THz, is a unit of electromagnetic (EM) wave frequency equal to one trillion hertz (1012 Hz). The terahertz is used as an indicator of the frequency of infrared (IR), visible, and ultraviolet (UV) radiation. An EM wave having a frequency of 1 THz has a wavelength of 0.3 millimeters (mm), or 300 micrometers (mm). An EM wave of 540 THz is in the middle of the visible-light spectrum. Wireless transmissions and computer clock speeds are at frequencies far below 1 THz. The terahertz is not commonly used in computer and wireless technology, although it is possible that a microprocessor with a clock speed of 1 THz might someday be developed. At present, the terahertz is of interest primarily to physicists and astronomers. More commonlyused units of frequency are the kilohertz (kHz), equal to 1,000 Hz or 10-9 THz, the megahertz (MHz), equal to 106 Hz or 10-6 THz, and the gigahertz (GHz), equal to 109 Hz or 0.001 THz.
Teraplex Teraplex (short for Teraplex Integration Center) is the name IBM has given to its ‘‘real life’’ testing centers whose purpose is to allow IBM customers and business partners to test very large database (VLDB) applications on a full-scale basis by using actual workloads. IBM makes its Teraplex Integration Centers freely available to its customers and business partners, but sets strict criteria for what tests can be run. The tests must involve a terabyte or more of data (hence the name Teraplex) and they must test business intelligence applications (as opposed to online transaction processing applications). The tests must evaluate scalability, and they must test the feasibility of a potential business solution in a realistic time frame. Each center can test applications and products from multiple IBM divisions and from non-IBM vendors. Currently, IBM designates individual centers for each IBM platform and provides IBM and third-party developers on site to assist with the testing.
terbo The suffix terbo appears in the V.32terbo modem protocol and indicates the third version of the V.32 protocol. Terbo is an invented word based on the Old Latin ter meaning ‘‘three times’’ and the word turbo (Latin for ‘‘whirling top’’ or ‘‘whirlwind’’) meaning ‘‘speed.’’
Terminal Server product Also see bis, a suffix used in several modern protocols that means ‘‘second version.’’
terminal 1) In data communications, a terminal is any device that terminates one end (sender or receiver) of a communicated signal. In practice, it is usually applied only to the extended end points in a network, not central or intermediate devices. In this usage, if you can send signals to it, it’s a terminal. 2) In telephony, the term Data Terminal Equipment (DTE) is used to describe the computer end of the DTE-to-DCE (Data Communications Equipment) communication between a computer and a modem. 3) In computers, a terminal (sometimes qualified as a ‘‘dumb’’ terminal) is an end-use device (usually with display monitor and keyboard) with little or no software of its own that relies on a mainframe or another computer (such as a PC server) for its ‘‘intelligence.’’ IBM’s 3270 Information Display System was a widely-installed system of such terminals in corporations. Many applications designed for the 3270 or other ‘‘dumb’’ terminals are still in use at PCs that emulate or act like a 3270. The VT-100 from Digital Equipment Corporation is another example of a widelyused so-called ‘‘dumb’’ terminal. A variation of this kind of terminal is being revived in the idea of the thin client or network computer. 4) The term is sometimes used to mean any personal computer or user workstation that is hooked up to a network.
terminal adapter A terminal adapter (TA) is a hardware interface between a computer and an Integrated Services Digital Network line. It’s what replaces a modem when you are using an ISDN connection. Unlike ‘‘plain old telephone service,’’ which carries signals in analog (voice) form between your computer and the telephone company’s office, ISDN carries signals in digital form so there is no need to modulate and demodulate between analog and digital signals. The terminal adapter is what you have to install on a computer so that data can be fed directly into the ISDN line in digital form. Since ISDN service is not available from telephone companies in all areas, the terminal adapter is not usually built into a computer. You purchase and install it when you sign up for ISDN service. Some manufacturers and telephone companies use the term ISDN modem, instead.
terminal emulation Terminal emulation is the ability to make one computer terminal, typically a PC, appear to look like another, usually older type of terminal so that a user can access programs originally written to communicate with the other terminal
type. Terminal emulation is often used to give PC users the ability to log on and get direct access to legacy programs in a mainframe operating system. Terminal emulation requires installing a special program in the PC or on a local area network (LAN) server to which it is connected. Typically, an enterprise with mainframe computers installs a terminal emulation program in all its workstations (or LAN servers). Workers can work locally with Windows or other PC or workstation applications and also open a window and work directly with mainframe applications. The terminal emulation program runs like any other workstation application as a separate program task providing its own window to the user. However, instead of content with a graphical user interface (GUI), the terminal emulation window presents some particular mainframe operating system or application interface that is text-only. Different terminal emulation is required for specific types of terminal—for example, the IBM 3270 display terminal, the AS/400’s 5250 display terminal, or DEC’s VT100 terminal. The program performing the terminal emulation must understand the data stream from the mainframe at several communication levels, including data link control and session control.
Terminal Server product The Microsoft Windows Terminal Server (WTS) is a server program running on its Windows NT 4.0 (or higher) operating system that provides the graphical user interface (GUI) of the Windows desktop to user terminals that don’t have this capability themselves. The latter include the relatively low-cost NetPC or ‘‘thin client’’ that some companies are purchasing as alternatives to the autonomous and more expensive PC with its own operating system and applications. The Windows Terminal Server was codenamed ‘‘Hydra’’ during development. The Windows Terminal Server has three parts: the multiuser core server itself, the Remote Desktop Protocol that enables the Windows desktop interface to be sent to the terminals by the server, and the Terminal Server Client that goes in each terminal. Users will have access to 32-bit Windows-based applications. The new terminal devices are being made by a number of vendors, including Network Computing Devices and Wyse Technologies. In addition, users of existing PCs running Windows 95 and Windows 3.11 operating systems can also access the Server and its applications. The Terminal Server can also serve terminals and workstations that run UNIX, Macintosh, or DOS operating systems that can’t be upgraded to 32-bit Windows. Co-developed with Citrix, Microsoft’s Windows NT 4.0 Terminal Server Edition together with Citrix’s MetaFrame product replace Citrix’s WinFrame product.
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Also see Terminal Server product, Microsoft software that serves the Windows desktop and applications to terminals.
The tesla (symbolized T) is the standard unit of magnetic flux density. It is equivalent to one weber per meter squared (1 Wb ˙ m-2). Reduced to base units in the International System of Units (SI), 1 T represents one kilogram per second squared per ampere (kg ˙ s-2 ˙ A-1). The tesla was named for Nikola Tesla, the Croatian-born scientist who contributed greatly toward the development of modern-day electrical power systems.
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Generally in information technology, a terminal server is a hardware device or server that provides terminals (PCs, printers, and other devices) with a common connection point to a local or wide area network. The terminals connect to the terminal server from their RS-232C or RS-423 serial port. The other side of the terminal server connects through network interface cards (NIC) to a local area network (LAN) (usually an Ethernet or token ring LAN) through modems to the dial-in/out wide area network, or to an X.25 network or a 3270 gateway. (Different makes of terminal server offer different kinds of interconnection. Some can be ordered in different configurations based on customer need.) The use of a terminal server means that each terminal doesn’t need its own network interface card or modem. The connection resources inside the terminal server are usually shared dynamically by all attached terminals. Some terminal servers can be shared by up to 128 terminals. The terminals can be PCs, terminals that emulate 3270s, printers, or other devices with the RS-232/423 interface. In some terminal servers, the terminals can use TCP/IP for a Telnet connection to a host, LAT to a Digital Equipment Corporation host, or TN3270 for a Telnet connection to an IBM host with 3270 applications. With some terminal servers, a given terminal user can have multiple host connections to different kinds of host operating systems (UNIX, IBM, DEC).
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A terminate and stay resident (TSR) program is one that is set up to be loaded and then remain in computer memory so that it is quickly accessible when a user presses a certain keyboard combination. TSR programs are used in Disk Operating Systems and perhaps other non-task operating systems. They aren’t needed in multitasking systems such as Windows and OS/2. TSR programs typically include calculators, clocks, and notepads. When running another program in DOS, you can press the preset keyboard key or combination of keys and the TSR program will ‘‘pop up’’ into view.
Terrestrial Trunked Radio See ‘‘TETRA’’
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In practice, the tesla is a large unit, and is used primarily in industrial electromagnetics. When dealing with practical magnets of the sort encountered in consumer products, a smaller unit of flux density called the gauss (symbolized G) is often used. There are ten thousand gauss in one tesla (1 T = 104 G). Also see magnetic field, gauss, and International System of Units.
testing In general, testing is finding out how well something works. In terms of human beings, testing tells what level of knowledge or skill has been acquired. In computer hardware and software development, testing is used at key checkpoints in the overall process to determine whether objectives are being met. For example, in software development, product objectives are sometimes tested by product user representatives. When the design is complete, coding follows and the finished code is then tested at the unit or module level by each programmer; at the component level by the group of programmers involved; and at the system level when all components are combined together. At early or late stages, a product or service may also be tested for usability. At the system level, the manufacturer or independent reviewer may subject a product or service to one or more performance tests, possibly using one or more benchmarks. Whether viewed as a product or a service or both, a Web site can also be tested in various ways—by observing user experiences, by asking questions of users, by timing the flow through specific usage scenarios, and by comparing it with other sites.
testing your bandwidth See ‘‘bandwidth test’’
TETRA TETRA (Terrestrial Trunked Radio) is a set of standards developed by the European Telecommunications Standardisation Institute (ETSI) that describes a common mobile radio communications infrastructure throughout Europe. This infrastructure is targeted primarily at the mobile radio needs
text editor of public safety groups (such as police and fire departments), utility companies, and other enterprises that provide voice and data communications services.
TETRA standards are currently being considered along with other standards for inclusion in a later phase of the APCO 25 Project.)
All of these groups have been high-end users of private/ professional mobile radio (PMR) or public access mobile radio (PAMR) technology. This is especially true in the areas of law enforcement and public safety, where fast and accurate field communications to and from a central office or dispatcher are often critical. TETRA is a standard solution for groups that use both PMR and PAMR.
ExpressNet is purported to be the first commercial development of a TETRA-based system in Europe, which integrates both data and voice mobile communications into a single network. It is planned for initial deployment throughout the UK, France, and Germany.
In recent years, when European disasters have struck, emergency response teams from several European nations had a difficult time communicating with each other, due in part to the lack of standardization in their mobile radio equipment. The TETRA standards evolved to answer this communication challenge as well as others faced or anticipated by the European Commission (EC) in its efforts to unify European countries.
Also see a similar term, voxel.
Based on digital, trunked radio technology, TETRA is believed to be the next-generation architecture and standard for current, analog PMR and PAMR markets. TETRA actually takes its features from several different technological areas: Mobile radio, digital cellular telephone, paging, and wireless data. TETRA relies on digital trunking. TETRA-based products come with built-in encryption features to ensure the privacy and confidentiality of sensitive data/voice communications. These products are also designed with the ability to transfer data at faster rates than seen before in mobile communications. TETRA, which originally stood for Trans-European Trunked Radio, was renamed Terrestrial Trunked Radio after ETSI found widespread interest in the TETRA standards beyond Europe’s geographic borders. ETSI has had a successful history of developing communications standards that later become accepted industry-wide, such as the Global System for Mobile Communications (GSMC) standard.
texel In computer graphics, a texel (texture element) represents the smallest graphical element in two-dimensional (2-D) texture mapping to ‘‘wallpaper’’ the rendition of a threedimensional (3-D) object to create the impression of a textured surface. A texel is similar to a pixel (picture element) because it represents an elementary unit in a graphic. But there are differences between the texels in a texture map and the pixels in an image display. In special instances, there might be a one-to-one correspondence between texels and pixels in some parts of the rendition of a 3-D object. But for most, if not all, of a 3-D rendition, the texels and pixels cannot be paired off in such a simple way. When a 3-D texture-mapped object appears close to the viewer so that the texture elements appear relatively large, there may be several pixels in each texel and the pattern of the texture map is easy to see. When the same 3-D object is removed to increasing distances, the texture-map pattern appears smaller and smaller. Eventually, each texel can become smaller than a pixel. Then an averaging process must be used; several texels are combined to form each pixel. If the object becomes distant enough, or if one of its facets appears at a sharp angle with respect to the viewer, the texels may become so small that the essence of the pattern is lost in the observed image.
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The TETRA Memorandum of Understanding (MoU) was created in December 1994 as a forum to represent the needs of various members of the mobile communications industry (including product manufacturers, telecommunications agencies, end users of mobile devices). Ole M. Lauridsen, chairman of the TETRA MoU, anticipates that the introduction of TETRA across Europe will not only standardize and improve mobile radio communications for current PMR/PAMR users, but it will also significantly decrease the cost of equipment and terminals by as much as 30-40% over analog equipment currently on the market.
In information technology, text is a human-readable sequence of characters and the words they form that can be encoded into computer-readable formats such as ASCII. Text is usually distinguished from non-character encoded data, such as graphic images in the form of bitmaps and program code, which is sometimes referred to as being in ‘‘binary’’ (but is actually in its own computer-readable format).
Efforts are currently underway to introduce TETRA standards in other parts of the world, including Denmark, China and the U.S. (In the United States, another standard called APCO 25 is currently in use by public safety agencies.
A text editor is a computer program that lets a user enter, change, store, and usually print text (characters and numbers, each encoded by the computer and its input and output devices, arranged to have meaning to users or to other programs). Typically, a text editor provides an ‘‘empty’’ display screen (or ‘‘scrollable page’’) with a fixedline length and visible line numbers. You can then fill the
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Whatis?Com’s Encyclopedia of Technology Terms lines in with text, line by line. A special command line lets you move to a new page, scroll forward or backward, make global changes in the document, save the document, and perform other actions. After saving a document, you can then print it or display it. Before printing or displaying it, you may be able to format it for some specific output device or class of output device. Text editors can be used to enter program language source statements or to create documents such as technical manuals.
A complex pattern can, in some cases, be seamlessly wedded to the surface of a 3-D object using a sophisticated graphics program. The pattern is generated directly on the 3-D rendition, rather than using a texture map. For example, a sphere can be given a wood-grain finish. The squares-on-asphere problem cannot be solved, but it is possible to fit a pattern of triangles onto a sphere by adjusting the sizes of the triangles.
A popular text editor in IBM’s large or mainframe computers is called XEDIT. In UNIX systems, the two most commonly used text editors are Emacs and vi. In personal computer systems, word processors are more common than text editors. However, there are variations of mainframe and UNIX text editors that are provided for use on personal computers. An example is KEDIT, which is basically XEDIT for Windows.
text on nine keys T9 (text on nine keys) is a system that lets fixed and mobile phone users send text messages by pressing a number key for each letter in the message—effectively making a keyboard out of the nine numeric phone entry keys. The system is already somewhat familiar to anyone who enters the letters of a name when checking a bank balance or looking up someone in a company’s phone directory over the phone. Originated by Tegic Communications, the T9 system has been licensed for use by over 20 mobile phone manufacturers. T9 will often be used in combination with the Short Message Service (SMS) and the Wireless Application Protocol (WAP).
texture mapping Texture mapping is a graphic design process in which a twodimensional (2-D) surface, called a texture map, is ‘‘wrapped around’’ a three-dimensional (3-D) object. Thus, the 3-D object acquires a surface texture similar to that of the 2-D surface. Texture mapping is the electronic equivalent of applying wallpaper, paint, or veneer to a real object. The simplest texture mappings involve processes such as that shown below. Three identical squares, each covered randomly with dots, are directly mapped onto the three visible facets of a 3-D cube. This distorts the sizes and shapes of the dots on the top and right-hand facets. In this mapping, the texture map covers the cube with no apparent discontinuities because of the way the dots are arranged on the squares. In some mappings, the correspondence between the 2-D texture map and the 3-D object’s surface becomes ‘‘messy.’’ An example is the application of a pattern of squares to the surface of a sphere. It is impossible to paste checkered wallpaper onto a sphere without cutting the paper in such a way as to create discontinuities in the pattern. This problem occurs with many texture mappings.
Mapping a 2-D surface onto a 3-D rendition alters the sizes and shapes of the image elements
texture sensing Texture sensing is the ability of a robot end effector to determine whether a surface is smooth or rough. There are several ways to do this; the simplest method involves reflected light. A laser beam and photoreceptors can be used to tell the difference between a smooth surface and a rough or matte surface with high albedo (low light absorbtion). A shiny surface, such as a sheet of polished metal or a pane of unfrosted glass, behaves according to the principle that the angle of reflection equals the angle of incidence relative to a plane tangent to the point of incidence. A matte surface, such as a sheet of printer paper, scatters incident light rays. If several photoreceptors are set up to capture reflected light, a shiny surface will reflect the beam back only to a receptor that is positioned in the path of the beam whose reflection angle equals its incidence angle. The matte surface will reflect the beam back to all the receptors. A laser-based texture sensor cannot accurately measure the degree of roughness of a surface that is not shiny. A piece of printer paper reflects visible light in much the same way as a layer of flour, sand, or gravel. In addition, a visible-lightbased texture sensor does not work well against rough or matte surfaces with low albedo. Such a surface, such as carbon black, produces a negative response at the photoreceptors. For surfaces of this type, high-speed
The speed of... electrons, infrared (IR) rays, or ultraviolet (UV) rays sometimes work. But efficient emitters and receptors of these forms of energy are expensive. Also see proximity sensing, pressure sensing, and robotics.
TFT See ‘‘thin-film transistor’’
TFTP See ‘‘Trivial File Transfer Protocol’’
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The Open Group is a software standards organization that is sponsored by a number of major software vendors. The Open Group develops and fosters industry standards for software interfaces, often using technologies developed by one of the sponsoring companies. The Open Group originated by combining two previous organizations, X/ Open and the Open Software Foundation (OSF). Standards that the Open Group maintains include the standard UNIX program interfaces and Structured Query Language (the database query language).
This table shows the stated data rates for the most important end-user and backbone transmission technologies. Technology
Speed
Physical Medium
Application
GSM mobile telephone service
9.6 to 14.4 Kbps
RF in space (wireless)
High-speed circuitswitched data service (HSCSD) Regular telephone service (POTS) Dedicated 56Kbps on frame relay
Up to 56 Kbps
RF in space (wireless)
Mobile telephone for business and personal use Mobile telephone for business and personal use Home and small business access Business e-mail with fairly large file attachments The base signal on a channel in the set of Digital Signal levels Mobile telephone for business and personal use BRI: Faster home and small business access PRI: Medium and large enterprise access Faster home and small business access Local area network for Apple devices; several networks can be bridged; non-Apple devices can also be connected Mobile telephone for business and personal use Faster home and small enterprise access Large company backbone for LANs to ISP ISP to Internet infrastructure
Twisted pair
56 Kbps
Various
DS0
64 Kbps
All
General Packet Radio System (GPRS)
56 to 114 Kbps
RF in space (wireless)
ISDN
BRI: 64 Kbps to 128 Kbps PRI: 23 (T-1) or 30 (E1) assignable 64-Kbps channels plus control channel; up to 1.544 Mbps (T-1) or 2.048 (E1)
BRI: Twisted-pair PRI: T-1 or E1 line 128 Kbps Twisted-pair
IDSL AppleTalk
230.4 Kbps
Twisted pair
Enhanced Data GSM Environment (EDGE)
384 Kbps
RF in space (wireless)
Satellite
400 Kbps (DirecPC and others)
RF in space (wireless)
Frame relay
56 Kbps to 1.544 Mbps
Twisted-pair or coaxial cable
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The speed of...
Up to 56 Kbps
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Technology
Speed
Physical Medium
Application
DS1/T-1
1.544 Mbps
Twisted-pair, coaxial cable, or optical fiber
Up to 2 Mbps
RF in space (wireless)
C
Universal Mobile Telecommunications Service (UMTS) E-carrier
2.048 Mbps
D
T-1C (DS1C)
3.152 Mbps
Twisted-pair, coaxial cable, or optical fiber Twisted-pair, coaxial cable, or optical fiber
E
IBM Token Ring/802.5
4 Mbps (also 16 Mbps)
Twisted-pair, coaxial cable, or optical fiber
DS2/T-2
6.312 Mbps
Twisted-pair, coaxial cable, or optical fiber
Digital Subscriber Line (DSL)
512 Kbps to 8 Mbps
Twisted-pair (used as a digital, broadband medium)
E-2
8.448 Mbps
Cable modem
512 Kbps to 52 Mbps (see ‘‘Key and explanation’’ below)
Ethernet
10 Mbps
IBM Token Ring/802.5
16 Mbps (also 4 Mbps)
Twisted-pair, coaxial cable, or optical fiber Coaxial cable (usually uses Ethernet); in some systems, telephone used for upstream requests 10BASE-T (twisted-pair); 10BASE-2 or -5 (coaxial cable); 10BASE-F (optical fiber) Twisted-pair, coaxial cable, or optical fiber
Large company to ISP ISP to Internet infrastructure Mobile telephone for business and personal use (available in 2002) 32-channel European equivalent of T-1 Large company to ISP ISP to Internet infrastructure Second most commonly-used local area network after Ethernet Large company to ISP ISP to Internet infrastructure Home, small business, and enterprise access using existing copper lines Carries four multiplexed E-1 signals Home, business, school access
E-3
34.368 Mbps
DS3/T-3
44.736 Mbps
Twisted-pair or optical fiber Coaxial cable
OC-1
51.84 Mbps
Optical fiber
High-Speed Serial Interface (HSSI)
Up to 53 Mbps
HSSI cable
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Most popular business local area network (LAN) Second most commonly-used local area network after Ethernet Carries 16 E-l signals ISP to Internet infrastructure Smaller links within Internet infrastructure ISP to Internet infrastructure Smaller links within Internet infrastructure Between router hardware and WAN lines Short-range (50 feet) interconnection between slower LAN devices and faster WAN lines
The speed of... Technology
Speed
Physical Medium
Application
Fast Ethernet
100 Mbps
100BASE-T (twisted pair); 100BASE-T (twisted pair); 100BASE-T (optical fiber) Optical fiber
Workstations with 10 Mbps Ethernet cards can plug into a Fast Ethernet LAN Large, wide-range LAN usually in a large company or a larger ISP ISP to Internet infrastructure Smaller links within Internet infrastructure Carries 4 E3 channels Up to 1,920 simultaneous voice conversations Large company backbone Internet backbone Carries 4 E4 channels Up to 7,680 simultaneous voice conversations Internet backbone Workstations/ networks with 10/100 Mbps Ethernet will plug into Gigabit Ethernet switches Internet backbone Part of the vBNS backbone Internet backbone Backbone Backbone
Fiber DistributedData Interface (FDDI)
T-3D (DS3D)
E–4
100 Mbps
135 Mbps
139.264 Mbps
Optical fiber
Optical fiber
OC-3/SDH
155.52 Mbps
Optical fiber
E-5
565.148 Mbps
Optical fiber
OC-12/STM-4 Gigabit Ethernet
622.08 Mbps 1 Gbps
Optical fiber Optical fiber (and ‘‘copper’’ up to 100 meters)
OC-24 SciNet
1.244 Gbps 2.325 Gbps (15 OC-3 lines)
Optical fiber Optical fiber
OC-48/STM-16 OC-192/STM-64 OC-256
2.488 Gbps 10 Gbps 13.271 Gbps
Optical fiber Optical fiber Optical fiber
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Key and Explanation We use the U.S. English ‘‘Kbps’’ as the abbreviation for ‘‘thousands of bits per second.’’ In international English outside the U.S., the equivalent usage is ‘‘kbits s-1’’ or ‘‘kbits/s.’’ Engineers use data rate rather than speed, but speed (as in ‘‘Why isn’t my Web page getting here faster?’’) seems more meaningful for the less technically inclined. Many of us tend to think that the number of bits getting somewhere over a period of time is their speed of travel.
Relative to data transmission, a related term, bandwidth or ‘‘capacity,’’ means how wide the pipe is and how quickly the bits can be sent down the channels in the pipe. (The analogy of multiple lanes on a superhighway with cars containing speed governors may help. One reason why digital traffic flows faster than voice traffic on the same copper line is because digital has managed to convert a one-lane or narrowband highway into a many-lane or broadband highway.) These ‘‘speeds’’ are aggregate speeds. That is, the data on the multiple signal channels within the carrier is usually allocated by channel for different uses or among different users.
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Whatis?Com’s Encyclopedia of Technology Terms Key: ‘‘T’’ = T-carrier system in U.S., Canada, and Japan....‘‘DS’’= digital signal (that travels on the T-carrier or E-carrier)...‘‘E’’ = Equivalent of ‘‘T’’ that uses all 8 bits per channel; used in countries other than U.S. Canada, and Japan....‘‘OC’’ = optical carrier (Synchronous Optical Network)....‘‘STM’’ = Synchronous Transport Modules (see Synchronous Digital Hierarchy) Only the most common technologies are shown. ‘‘Physical medium’’ is stated generally and doesn’t specify the classes or numbers of pairs of twisted pair or whether optical fiber is single-mode or multimode. The effective distance of a technology is not shown. There are published standards for many of these technologies. Some of these are indicated on pages linked to from the table.
I
Cable modem note: The upper limit of 52 Mbps on a cable is to an ISP, not currently to an individual PC. Most of today’s PCs are limited to an internal design that can accomodate no more than 10 Mbps (although the PCI bus itself carries data at a faster speed). The 52 Mbps cable channel is subdivided among individual users. Obviously, the faster the channel, the fewer channels an ISP will require and the lower the cost to support an individual user.
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theory of everything
G H
See ‘‘superstring theory’’
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theory of relativity Albert Einstein’s theory of relativity is actually two separate theories: his special theory of relativity, postulated in the 1905 paper, The Electrodynamics of Moving Bodies and his theory of general relativity, an expansion of the earlier theory, published as The Foundation of the General Theory of Relativity in 1916. Einstein sought to explain situations in which Newtonian physics might fail to deal successfully with phenomena, and in so doing proposed revolutionary changes in human concepts of time, space, and gravity. The special theory of relativity was based on two main postulates: first, that the speed of light is constant for all observers; and second, that observers moving at constant speeds should be subject to the same physical laws. Following this logic, Einstein theorized that time must change according to the speed of a moving object relative to the frame of reference of an observer. Scientists have tested this theory through experimentation—proving, for example, that an atomic clock ticks more slowly when traveling at a high speed than it does when it is not moving. The essence of Einstein’s paper was that both space and time are relative (rather than absolute), which was said to hold true in a special case, the absence of a gravitational field. Relativity was a stunning concept at the time; scientists all over the world debated the veracity of Einstein’s famous equation, E=mc2, which implied that matter and energy were equivalent and, more specifically, that a single particle of matter could be converted into a huge quantity of energy.
However, since the special theory of relativity only held true in the absence of a gravitational field, Einstein strove for 11 more years to work gravity into his equations and discover how relativity might work generally as well. According to the Theory of General Relativity, matter causes space to curve. It is posited that gravitation is not a force, as understood by Newtonian physics, but a curved field (an area of space under the influence of a force) in the space-time continuum that is actually created by the presence of mass. According to Einstein, that theory could be tested by measuring the deflection of starlight traveling near the sun; he correctly asserted that light deflection would be twice that expected by Newton’s laws. This theory also explained why the light from stars in a strong gravitational field was closer to the red end of the spectrum than those in a weaker one. For the final thirty years of his life, Einstein attempted to find a unified field theory, in which the properties of all matter and energy could be expressed in a single equation. His search was confounded by quantum theory’s uncertainty principle, which stated that the movement of a single particle could never be accurately measured, because speed and position could not be simultaneously assessed with any degree of assurance. Although he was unable to find the comprehensive theory that he sought, Einstein’s pioneering work has allowed countless other scientists to carry on the quest for what some have called ‘‘the holy grail of physicists.’’
thermal transfer printer A thermal transfer printer is a non-impact printer that uses heat to register an impression on paper. A thermal transfer printer has a printhead containing many small resistive heating pins that on contact, depending on the type of thermal transfer printer, melt wax-based ink onto ordinary paper or burn dots onto special coated paper. A microprocessor determines which individual heating pins are heated to produce the printed image. The printhead spans the entire width of the paper or medium to be printed on. Thermal transfer printers are popular for printing bar codes, labels, price tags, and other specialty print jobs. There are two types of thermal transfer printers: direct thermal and thermal wax transfer. Direct thermal: The direct thermal printer prints the image by burning dots onto coated paper as it passes over the heated printhead. Direct thermal printers do not use ribbons. Early fax machines used direct thermal printing. Thermal wax transfer: This type of printer uses a thermal transfer ribbon that contains wax-based ink. Heat is applied to the ribbon using a thermal printhead that melts the ink transferring it to the paper where it is permanent after it cools. A typical thermal transfer ribbon consists of three layers: the base material, the heat melting ink, and the coating on the print side of the base material. The coating and base material help keep ink from adhering to the printhead which can cause poor print quality. Monochrome
thin-film transistor and color thermal transfer ribbons are available. It is recommended that the printhead be cleaned between each ribbon change with a cotton swab and isopropyl alcohol.
through a Web browser user interface. The hardware processor sometimes uses reduced instruction set computer processing.
The cost of a thermal transfer printer varies depending on the size of the printer and the features included. The cost of consumables such as paper, ribbons, and printheads is about the same for both types of printers. The print quality depends on the printer, the ribbon, the paper, and the environment such as where the printer is stored, the temperature, and the humidity.
Some thin servers are designed and marketed for use on local area networks in businesses. A newer development is a thin server intended for home use. Data General makes a thin server for the home or small office that performs the single function of providing access to the Internet. Other desktop and notebook computers and possibly other electronic appliances can be hooked up as clients and share the thin server’s connection or connections to the Internet.
Thicknet See ‘‘Thinnet’’
Thickwire See ‘‘Thinnet’’
thin client A thin client is a low-cost, centrally-managed computer devoid of CD-ROM players, diskette drives, and expansion slots. The term derives from the fact that small computers in networks tend to be clients and not servers. Since the idea is to limit the capabilities of these computers to only essential applications, they tend to be purchased and remain ‘‘thin’’ in terms of the client applications they include. The term ‘‘thin client’’ seems to be used as a synonym for both the NetPC and the network computer (NC), which are somewhat different concepts. The Net PC is based on Intel microprocessors and Windows software (Intel was a leader in defining the Net PC specification). The network computer (NC) is a concept backed by Oracle and Sun Microsystems that may or may not use Intel microprocessors and uses a Java-based operating system. The increased numbers of thin clients in today’s workplace and educational facilites reflects a corporate and institutional need for low-cost computers dedicated to Internet use.
The thin server and the thin client concepts arise from the same idea: why pay for the function in a computer that you don’t need? Whereas the thin client is a constrained personal computer that gets applications and data the user needs from a shared and usually full-function server computer, the thin server serves the client requests of other computers and their users by doing one thing well without needing to provide any other service. A thin server is similar to a thin client in that both may be thought of as single application, special-purpose computers, almost always with a very limited storage capability and with ‘‘trimmed-down’’ operating systems.
thin-film disk A thin-film disk is a hard disk storage medium with a very thin coating of a metallic alloy or glass instead of the usual carbon-based material found on other types of hard disks. The thin film lets the read/write head get closer to the surface, allowing more data to be stored in a given space. Thin-film technology is another approach to packing more bits of data onto a disk. As the number of data bits on a disk increase, the disk drive’s head must become more sensitive, getting closer to the disk to read or write the bits. Researchers are experimenting with thin films of various materials in order to help protect the disk from the head and to help the disk have a longer life.
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thin server
thin-film transistor
In the computer industry, a thin server is a PC that contains just enough hardware and software to support a particular function that users can share in a network, such as access to files on a storage device, access to CD-ROM drives, printing, or Internet access. According to the first companies who have used the term, a thin server can be quickly added to a network and costs less than providing the same service through a more general-purpose computer server. Usually, a thin server contains an abbreviated version of one or more operating systems, such as Windows 2000, Macintosh, or UNIX, and necessary network protocols, such as TCP/IP and NetBEUI. Typically, it also includes Hypertext Transfer Protocol so that it can be configured, administered, and used
A display screen made with TFT (thin-film transistor) technology is a liquid crystal display (LCD), common in notebook and laptop computers, that has a transistor for each pixel (that is, for each of the tiny elements that control the illumination of your display). Having a transistor at each pixel means that the current that triggers pixel illumination can be smaller and therefore can be switched on and off more quickly. TFT is also known as active matrix display technology (and contrasts with ‘‘passive matrix’’ which does not have a transistor at each pixel). A TFT or active matrix display is more responsive to change. For example, when you move your mouse across the screen, a TFT display is fast enough to
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reflect the movement of the mouse cursor. (With a passive matrix display, the cursor temporarily disappears until the display can ‘‘catch up.’’) A more recent development is organic thin-film transistor technology, which makes it possible to have flexible display surfaces.
Thinnet Thicknet and Thinnet (sometimes called ThickWire and ThinWire) are commonly used terms for the larger and smaller size of coaxial cable used in Ethernet local area networks. Thicknet, also known as Thickwire, is 0.4 inches in diameter and has 50 ohms of electromagnetic impedance. Thinnet, also known as Thinwire and Cheapernet, is 0.2 inches in diameter with the same impedance as Thickwire. Thicknet was the original Ethernet wiring, but Thinnet, which is cheaper and can be installed more easily, is the more commonly installed Ethernet wire. Thicknet continues to be used for backbone wiring. An alternative to Thinnet on an Ethernet network is twisted pair.
Thinwire See ‘‘Thinnet’’
thought recognition Thought recognition is the ability of a computer to recognize neural patterns in the brain as a form of input and control of computer processes. Computer programming and the necessary sensing devices already exist to detect other forms of biophysical activity, such as muscular activity and eye movement. Using these forms of input, paralyzed and other handicapped people have been made able to communicate with a computer. Brain waves suggest the possibility of an even more direct form of computer input—the thought command. However, thus far it has been difficult to interpret the brain waves with sufficient delineation to afford any kind of discriminatory input—for example, to sense a letter or word that a person might be thinking of. Brain waves can be discerned in these patterns: L
Alpha waves, with large modulations and moderate frequencies, indicate unfocused brain activity
L
Beta waves, with higher frequencies and small modulations, indicate focused brain activity
L
Theta waves, with moderately low frequencies, indicate emotional stress
L
Delta waves, with extremely low frequencies, indicate a condition of sleep
L
Mu waves, have a croquet-shaped pattern of frequencies and modulation, are associated with physical activity
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Scientists feel that a number of more refined distinctions about brain patterns must be made before they can be used as computer input.
thrashing Thrashing is computer activity that makes little or no progress, usually because memory or other resources have become exhausted or too limited to perform needed operations. When this happens, a pattern typically develops in which a request is made of the operating system by a process or program, the operating system tries to find resources by taking them from some other process, which in turn makes new requests that can’t be satisfied. In a virtual storage system (an operating system that manages its logical storage or memory in units called pages), thrashing is a condition in which excessive paging operations are taking place. A system that is thrashing can be perceived as either a very slow system or one that has come to a halt.
thread 1) On the Internet in Usenet newsgroups and similar forums, a thread is a sequence of responses to an initial message posting. This enables you to follow or join an individual discussion in a newsgroup from among the many that may be there. A thread is usually shown graphically as an inital message and successive messages ‘‘hung off’’ the original message. As a newsgroup user, you contribute to a thread by specifying a ‘‘Reference’’ topic as part of your message. 2) In computer programming, a thread is placeholder information associated with a single use of a program that can handle multiple concurrent users. From the program’s point-of-view, a thread is the information needed to serve one individual user or a particular service request. If multiple users are using the program or concurrent requests from other programs occur, a thread is created and maintained for each of them. The thread allows a program to know which user is being served as the program alternately gets re-entered on behalf of different users. (One way thread information is kept by storing it in a special data area and putting the address of that data area in a register. The operating system always saves the contents of the register when the program is interrupted and restores it when it gives the program control again.) Multithreading and multitasking are similar and are often confused. Some computers can only execute one program instruction at a time, but because they operate so fast, they appear to run many programs and serve many users simultaneously. The computer operating system (for example, Windows 2000) gives each program a ‘‘turn’’ at running, then requires it to wait while another program gets a turn. Each of these programs is viewed by the operating system as a ‘‘task’’ for which certain resources are identified and kept track of. The operating system manages each application program in your PC system (spreadsheet, word processor, Web browser) as a separate task and lets you look at and control items on a ‘‘task list.’’ If the program initiates
tier an I/O request, such as reading a file or writing to a printer, it creates a thread so that the program will be reentered at the right place when the I/O operation completes. Meanwhile, other concurrent uses of the program are maintained on other threads. Most of today’s operating systems provide support for both multitasking and multithreading. They also allow multithreading within program processes so that the system is saved the overhead of creating a new process for each thread. The Portable Operating System Interface.4a C specification provides a set of application program interfaces that allow a programmer to include thread support in the program. Higher-level program development tools and application subsystems and ‘‘middleware’’ also offer thread management facilities. Object-oriented programming languages also accommodate and encourage multithreading in several ways. Java supports multithreading by including synchronization modifiers in the language syntax, by providing class developed for multithreading that can be inherited by other classes, and by doing background ‘‘garbage collection’’ (recovering data areas that are no longer being used) for multiple threads.
thread-safe Thread-safe is a term used to describe a routine that can be called from multiple programming threads without unwanted interaction between the threads. Thread safety is of particular importance to Java programmers, since Java is a programming language that provides built-in support for threads. By using thread-safe routines, the risk that one thread will interfere and modify data elements of another thread is eliminated by circumventing potential data race situations with coordinated access to shared data. It is possible to ensure that a routine is thread-safe by: 1. Making sure that concurrent threads use synchronized algorithms that cooperate with each other. 2. Confining the address of a shared object to one thread whenever an unsynchronized algorithm is active.
Three Laws of Robotics (Isaac Asimov) See ‘‘Asimov’s Three Laws of Robotics’’
throughput 1) In computer technology, throughput is the amount of work that a computer can do in a given time period. Historically, throughput has been a measure of the comparative effectiveness of large commercial computers that run many programs concurrently. An early throughput measure was the number of batch jobs completed in a day. More recent measures assume a more complicated mixture of work or focus on some particular aspect of computer operation. While ‘‘cost per million instructions per second (MIPS)’’ provides a basis for comparing the cost of raw
computing over time or by manufacturer, throughput theoretically tells you how much useful work the MIPS are producing. Another measure of computer productivity is performance, the speed with which one or a set of batch programs run with a certain workload or how many interactive user requests are being handled with what responsiveness. The amount of time between a single interactive user request being entered and receiving the application’s response is known as response time. A benchmark can be used to measure throughput. 2) In data transmission, throughput is the amount of data moved successfully from one place to another in a given time period.
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thumbnail Thumbnail is a term used by graphic designers and photographers for a small image representation of a larger image, usually intended to make it easier and faster to look at or manage a group of larger images. For example, software that lets you manage a number of images often provides a miniaturized version of each image so that you don’t have to remember the file name of each image. A thumbnail is also used to mean a small and approximate version of an image or a brochure layout as a preliminary design step. Adobe’s Acrobat viewer lets you show a sequence of thumbnails of viewable pages as a way to navigate among the pages in a document. Adobe’s Photoshop lets you view a thumbnail version of certain kinds of images.
G
Web sites with many pictures, such as online stores with visual catalogs, often provide thumbnail images instead of larger images to make the page download faster. The user controls which images need to be seen in full size.
M
The term probably springs from the idea of ‘‘a picture the size of your thumbnail.’’
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thunking
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Thunking is the transformation between 16-bit and 32-bit instruction formats in an operating system. For example, Windows 95 can run application programs written in both 16-bit and 32-bit instruction sets.
THz
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N
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See ‘‘terahertz’’
tier In general, a tier (pronounced TEE-er; from the medieval French tire meaning rank, as in a line of soldiers) is a row or layer in a series of similarly arranged objects. In computer programming, the parts of a program can be distributed
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among several tiers, each located in a different computer in a network. Such a program is said to be tiered, multitier, or multitiered.
instant cross-referencing. The introduction of Tim BernersLee’s hypertext system revolutionized the way the Internet was used.
The 3-tier application model is probably the most common way of organizing a program in a network. N-tier applications (programs) are those that are tiered but the number of tiers isn’t specified or may vary.
In 1989, Tim Berners-Lee began work at CERN on the first World Wide Web server, which he called ‘‘httpd,’’ and the first client, which he called ‘‘WWW.’’ In the beginning, WWW was a WYSIWYG hypertext browser/editor that ran in the NeXTStep environment. It is interesting to note that before choosing the name ‘‘World Wide Web’’ for his global hypertext system, Berners-Lee considered the name ‘‘Mine of Information’’ (MOI) but thought it might be ‘‘un peu egoiste’’ (a little egotistical). He abandoned the name ‘‘The Information Mine’’ (TIM) for the same reason, and didn’t use ‘‘Information Mesh’’ because he thought it sounded too much like ‘‘mess.’’
C TIFF
D
See ‘‘Tag Image File Format’’
E
tiger team
F G H I J
In the computer industry, a tiger team is a group of programmers or users who volunteer or are hired to expose errors or security holes in new software or to find out why a computer network’s security is being broken. In the U.S. military, a tiger team is a group that is given the job of trying to break through security around a military base or special restricted area. In hiring or recruiting volunteers for a tiger team, some software developers advise others to be sure that tiger team members don’t include crackers, who might use their special knowledge of the software to disable or compromise it in the future.
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tilde
L
A tilde (pronounced TILL-duh or TILL-day) looks like this: ~. It’s a special typographic character found on most keyboards. In some operating systems, including UNIX, the tilde is used to represent the current user’s home directory. On Web server systems, the tilde is frequently used by convention as the first character for any user’s home directory in the file system. Since users often keep personal or business Web pages on a server under their personal home directory, you will often see the tilde as part of Web addresses.
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The tilde is one of the 128 alphanumeric and special characters in ASCII, the most common standard for electronic text exchange. The tilde happens to be ASCII character 126. It’s sometimes called a ‘‘twiddle’’ or a ‘‘squiggle.’’
Tim Berners-Lee Tim Berners-Lee is the creator of the World Wide Web and director of the coordinating body for Web development, the W3C. Berners-Lee credits Vint Cerf and Bob Khan (who defined the Internet protocol (IP) that allows computers to exchange packets of information) with the invention of the Internet, but it was Tim Berners-Lee who created the most widely used part of the Internet, the part that we call the World Wide Web (often abbreviated ‘‘WWW’’ or called ‘‘the Web’’). Its outstanding feature is hypertext, a method of
By 1990, the World Wide Web was available within CERN and by 1991, it was available to anyone using the Internet. Tim Berners-Lee has never profited personally by creating the World Wide Web. Instead, he gave away his proprietary rights in the belief that the Web needed to be an open system if it was to grow and become universal. Berners-Lee currently occupies the 3Com Founders chair at the MIT Laboratory for Computer Science and has been the recipient of numerous awards, including a spot in Time magazine’s list of the 100 greatest minds of the century. When asked what surprised him most about how the World Wide Web has turned out thus far, Tim Berners-Lee said that he was surprised at how many ordinary people were willing to learn how to write code using the hypertext markup language he developed.
time Time is an observed phenomenon, by means of which human beings sense and record changes in the environment and in the universe. A literal definition is elusive. Time has been called an illusion, a dimension, a smooth-flowing continuum, and an expression of separation among events that occur in the same physical location. Time is a practical convenience in modern life. Numerous standards have been set up, allowing people to coordinate events and, in general, keep their lives running smoothly. The earth has been divided into so-called time zones that reflect the fact that high noon occurs at different times at different places on the planet. All of these time zones are referenced to the time at the longitude of Greenwich, England. A universal standard, coinciding almost exactly with the time at Greenwich, is known as Coordinated Universal Time (UTC). There are various other time standards. The fundamental unit of time in the International System of Units (SI) is the second (symbolized s or sec). One second elapses during the occurrence of exactly 9,192,631,770 (9.192631770 6 109) cycles of the radiation produced by the transition between two levels of the cesium 133 atom. Other
time-to-live common units of time include the hour, the mean solar day, and the synodic year (sun-based earth year). The table below shows the relationship among the second, the hour, the mean solar day, and the synodic year. Unit To convert to Conversely, (and seconds, multiply by: symbol) multiply by: Hour (hr) Mean solar day (dy) Synodic year (yr)
3600 8.6400 6 104
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Isaac Newton believed that time is continuous, and that it flows at an unchanging rate everywhere in the universe. This was accepted by most scientists until the MichelsonMorley experiment around the end of the 19th century, from which it was discovered that the speed of light is the same regardless of the direction of propagation, and regardless of the motion of the source. Albert Einstein considered this result an axiom, from which he derived the special and general theories of relativity. According to relativistic physics, the rate at which time passes depends on the relative motion between observers, and also on the strength of a gravitational or acceleration field. Time is one of three primary phenomena in the meter/ kilogram/second (mks) and centimeter/gram/second (cgs) systems of measurement. It is one of seven fundamental quantities or phenomena in the International System of Units. Also see mks system of units, cgs system of units or smallunit metric system, the International System of Units (SI), and our Table of Physical Units.
time division multiple access See ‘‘TDMA’’
time-division multiplexing TDM (time-division multiplexing) is a scheme in which numerous signals are combined for transmission on a single communications line or channel. Each signal is broken up into many segments, each having very short duration. The circuit that combines signals at the source (transmitting) end of a communications link is known as a multiplexer. It accepts the input from each individual end user, breaks each signal into segments, and assigns the segments to the composite signal in a rotating, repeating sequence. The composite signal thus contains data from all the end users. At the other end of the long-distance cable, the individual signals are separated out by means of a circuit called a demultiplexer, and routed to the proper end users. A twoway communications circuit requires a multiplexer/ demultiplexer at each end of the long-distance, highbandwidth cable.
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If many signals must be sent along a single long-distance line, careful engineering is required to ensure that the system will perform properly. An asset of TDM is its flexibility. The scheme allows for variation in the number of signals being sent along the line, and constantly adjusts the time intervals to make optimum use of the available bandwidth. The Internet is a classic example of a communications network in which the volume of traffic can change drastically from hour to hour. In some systems, a different scheme, known as frequency-division multiplexing (FDM), is preferred.
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TTL (time-to-live) is a value in an Internet Protocol (IP) packet that tells a network router whether or not the packet has been in the network too long and should be discarded. For a number of reasons, packets may not get delivered to their destination in a reasonable length of time. For example, a combination of incorrect routing tables could cause a packet to loop endlessly. A solution is to discard the packet after a certain time and send a message to the originator, who can decide whether to resend the packet. The initial TTL value is set, usually by a system default, in an 8-binary digit field of the packet header. The original idea of TTL was that it would specify a certain time span in seconds that, when exhausted, would cause the packet to be discarded. Since each router is required to subtract at least one count from the TTL field, the count is usually used to mean the number of router hops the packet is allowed before it must be discarded. Each router that receives a packet subtracts one from the count in the TTL field. When the count reaches zero, the router detecting it discards the packet and sends an Internet Control Message Protocol (ICMP) message back to the originating host. The default Windows 95/98 TTL value is 32 hops. Some users recommend changing this to 128 if you have difficulty reaching certain sites. The ping and the traceroute utilities both make use of the TTL value to attempt to reach a given host computer or to trace a route to that host. Traceroute intentionally sends a packet with a low TTL value so that it will be discarded by each successive router in the destination path. The time between sending the packet and receiving back the ICMP message that it was discarded is used to calculate each successive hop travel time. Using the multicast IP protocol, the TTL value indicates the scope or range that a packet may be forwarded. By convention: L
0 is restricted to the same host
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See ‘‘Topic Map Query Language’’
Tivoli Tivoli Systems is an IBM-owned company that develops software that allows a business to manage its computing environment. In a large organization, keeping track of and upgrading computer equipment and various levels of operating systems and applications is a formidable task. Tivoli offers products and solutions in these areas: L
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Tk See ‘‘Tool Kit’’
TLD See ‘‘top-level domain’’
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TMQL token ring A token ring network is a local area network (LAN) in which all computers are connected in a ring or star topology and a bit- or token-passing scheme is used in order to prevent the collision of data between two computers that want to send messages at the same time. The token ring protocol is the second most widely-used protocol on local area networks after Ethernet. The IBM Token Ring protocol led to a standard version, specified as IEEE 802.5. Both protocols are used and are very similar. The IEEE 802.5 token ring technology provides for data transfer rates of either 4 or 16 megabits per second. Very briefly, here is how it works: 1. Empty information frames are continuously circulated on the ring. 2. When a computer has a message to send, it inserts a token in an empty frame (this may consist of simply changing a 0 to a 1 in the token bit part of the frame) and inserts a message and a destination identifier in the frame. 3. The frame is then examined by each successive workstation. If the workstation sees that it is the destination for the message, it copies the message from the frame and changes the token back to 0. 4. When the frame gets back to the originator, it sees that the token has been changed to 0 and that the message has been copied and received. It removes the message from the frame. 5. The frame continues to circulate as an ‘‘empty’’ frame, ready to be taken by a workstation when it has a message to send.
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The token scheme can also be used with bus topology LANs.
See ‘‘Transport Layer Security’’
See ‘‘triplecast’’
The standard for the token ring protocol is Institute of Electrical and Electronics Engineers (IEEE) 802.5. The Fiber Distributed-Data Interface (FDDI) also uses a token ring protocol.
TMN
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TMN (Telecommunications Management Network) originated formally in 1988 under the auspices of the International Telecommunication Union (ITU-TS) as a strategic goal to create or identify standard interfaces that would allow a network to be managed consistently across all network element suppliers. The concept has fostered and tracked a series of interrelated efforts at developing standard ways to define and address network elements. TMN uses the OSI Management Standards as its framework. TMN applies to wireless communications and cable TV as well as to private and public wired networks.
See ‘‘toolbar’’
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Tool Command Language See ‘‘Tcl’’
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Tool Kit Tool Kit (Tk) is a companion program to Tool Command Language (Tcl) for creating graphical user interfaces. Together with Tcl, Tk is a rapid program development tool from Scriptics. Both can be ordered from the Scriptics Web site. Tk provides a collection of standard buttons, menus, listboxes, and scrollbars and two special features, the text widget and the canvas widget, that the Tcl programmer can use to speed up program development.
toolbar In the graphical user interface (GUI) for a computer, a toolbar is a horizontal row or vertical column of selectable image ‘‘buttons’’ that give the user a constantly visible reminder of and an easy way to select certain desktop or other application functions, such as saving or printing a document or moving pages forwards or backwards within a Web browser. Word processing, spreadsheet, and many other types of application programs come with one or more toolbars as part of their user interface. In addition to the toolbars that come with a desktop or an application, some software developers or third parties provide supplementary toolbars that can be downloaded and installed. Supplementary toolbar functions may include quick access to news, sports, and weather headlines; instant form completion; instant access to favorite sites; and faster searches.
Topic Map A Topic Map (ISO/IEC Standard 13250:2000) is an SGML or XML document used for navigation—by people or by machine—within an information set. A single information set can include any number of types of source data: for example, HTML files, PDF files, and databases could all be part of one information set. A Topic Map (TM) can be either embedded within a document, or may be, itself, a separate document. A Topic Map provides a particular view of topics within the information set. There could be, potentially, an infinite number of different TMs for any information set: for example, an information set dealing with the plays of Shakespeare could have a TM for navigation within the perspective of Elizabethan culture, or the perspective of themes throughout the plays. Each TM explicitly declares topics and provides links to relevant information.
Anne Hathaway would all be topics. Topics are broken down into types: ‘‘Avon’’ would be defined as a town, and ‘‘Romeo and Juliet’’ as a play, for example. The word topic refers to both the topic link and the topic itself. Generally, topics have three types of characteristics: Names, which explicitly identify topics; occurrences, which are topicrelevant information resources; and roles in associations, which are details of how topics are related. TMs are being heralded as an important information management tool for the development of the Semantic Web, because they can enable very precise data access—they are sometimes called ‘‘the Global Positioning System (GPS) of the Web’’—within a vast, complex, and intricately interconnected global information repository. One way of looking at the way a TM works is to think of it as similar to a book’s index, and to compare seeking specific information within an indexed and an unindexed book. Although the information available on the Web now is linked, it is not linked in a systematic, standardized, and autonomously intelligent way (which explains why Web searches can often turn up so many irrelevant documents). Eventually, Topic Maps may be part of a comprehensive, user-friendly index to the body of human knowledge.
Topic Map Query Language Topic Map Query Language (TMQL) is an XML-based extension of Structured Query Language (SQL), a query language developed for use in meeting the specialized data access requirements of Topic Maps (TMs). Two types of data access for Topic Maps are: information retrieval (IR), which is focused on separate search instances of a single user looking for specific information; and information filtering, which is a query process that builds up a sort of user profile, filtering information to construct a selection of data relevant to a particular user.
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TMQL is intended to be easier to learn by developers, most of whom are likely to already be familiar with SQL. However, SQL was created to be used on the data in a relational database, which has a well-defined pre-existing structure; TMQL must be able to retrieve information from a vast and constantly changing body of information. A relational database may be expressed in terms of a simplified Topic Map, but TMQL will need to work with a much more complex data repository. A typical SQL select query (used to retrieve data from a table in a database) could be rewritten to retrieve data from a Topic Map.
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The TAO of Topic Maps
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Literally, Tao means the way; the term is expanded to mean the underlying unifying principle of the universe. In the case of TM, the unifying principle underlies the informational universe of the Web. The TAO of TMs stands for topics, associations, and occurrences. A topic is a subject: in terms of Shakespeare, for example, Avon, Romeo and Juliet, and
On the Internet, a top-level domain (TLD) identifies the most general part of the domain name in an Internet address. A TLD is either a generic top-level domain (gTLD), such as ‘‘com’’ for ‘‘commercial,’’ ‘‘edu’’ for ‘‘educational,’’ and so forth, or a country code top-level domain (ccTLD), such as ‘‘fr’’ for France or ‘‘is’’ for Iceland.
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A topology (from Greek topos: place) is a description of any kind of locality in terms of its physical layout. In the context of communication networks, a topology describes pictorially the configuration or arrangement of a (usually conceptual) network, including its nodes and connecting lines.
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toroid A toroid is a coil of insulated or enameled wire wound on a donut-shaped form made of powdered iron. A toroids is used as an inductor in electronic circuits, especially at low frequencies where comparatively large inductances are necessary. A toroid has more inductance, for a given number of turns, than a solenoid with a core of the same material and similar size. This makes it possible to construct high-inductance coils of reasonable physical size and mass. Toroidal coils of a given inductance can carry more current than solenoidal coils of similar size, because larger-diameter wires can be used, and the total amount of wire is less, reducing the resistance. In a toroid, all the magnetic flux is contained in the core material. This is because the core has no ends from which flux might leak off. The confinement of the flux prevents external magnetic fields from affecting the behavior of the toroid, and also prevents the magnetic field in the toroid from affecting other components in a circuit.
total cost of ownership
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See ‘‘TCO’’
touch pad A touch pad is a device for pointing (controlling input positioning) on a computer display screen. It is an alternative to the mouse. Originally incorporated in laptop computers, touch pads are also being made for use with desktop computers. A touch pad works by sensing the user’s finger movement and downward pressure.
connected. The layers with electrodes are charged with a constant alternating current (AC). As the finger approaches the electrode grid, the current is interrupted and the interruption is detected by the circuit board. The initial location where the finger touches the pad is registered so that subsequent finger movement will be related to that initial point. Some touch pads contain two special places where applied pressure corresponds to clicking a left or right mouse button. Other touch pads sense single or double taps of the finger at any point on the touch pad.
touch screen A touch screen is a computer display screen that is sensitive to human touch, allowing a user to interact with the computer by touching pictures or words on the screen. Touch screens are used with information kiosks, computerbased training devices, and systems designed to help individuals who have difficulty manipulating a mouse or keyboard. Touch screen technology can be used as an alternative user interface with applications that normally require a mouse, such as a Web browser. Some applications are designed specifically for touch screen technology, often having larger icons and links than the typical PC application. Monitors are available with built-in touch screen technology or individuals can purchase a touch screen kit. A touch screen kit includes a touch screen panel, a controller, and a software driver. The touch screen panel is a clear panel attached externally to the monitor that plugs into a serial or Universal Serial Bus (USB) port or a bus card installed inside the computer. The touch screen panel registers touch events and passes these signals to the controller. The controller then processes the signals and sends the data to the processor. The software driver translates touch events into mouse events. Drivers can be provided for both Windows and Macintosh operating systems. Internal touch screen kits are available but require professional installation because they must be installed inside the monitor. There are three types of touch screen technology: L
Resistive: A resistive touch screen panel is coated with a thin metallic electrically conductive and resistive layer that causes a change in the electrical current which is registered as a touch event and sent to the controller for processing. Resistive touch screen panels are generally more affordable but offer only 75% clarity and the layer can be damaged by sharp objects. Resistive touch screen panels are not affected by outside elements such as dust or water.
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Surface wave: Surface wave technology uses ultrasonic waves that pass over the touch screen panel. When the panel is touched, a portion of the wave is absorbed. This change in the ultrasonic waves registers the position of the touch event and sends this information to the
The first touch pad was invented by George E. Gerpheide in 1988. Apple Computer was the first to license and use the touch pad in its Powerbook laptops in 1994. The touch pad has since become the leading cursor-controlling device in laptops. Many laptops use a trackball. IBM ThinkPad laptops use a ‘‘pointing stick’’ (called a TrackPoint) that is set into the keyboard.
How the Touch Pad Works The touch pad contains several layers of material. The top layer is the pad that you touch. Beneath it are layers (separated by very thin insulation) containing horizontal and vertical rows of electrodes that form a grid. Beneath these layers is a circuit board to which the electrode layers are
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controller for processing. Surface wave touch screen panels are the most advanced of the three types, but they can be damaged by outside elements.
If you have a Windows operating system, try traceroute out by clicking on Start–>Programs–>MS-DOS Prompt, and then at the C:WINDOWS prompt, enter:
Capacitive: A capacitive touch screen panel is coated with a material that stores electrical charges. When the panel is touched, a small amount of charge is drawn to the point of contact. Circuits located at each corner of the panel measure the charge and send the information to the controller for processing. Capacitive touch screen panels must be touched with a finger unlike resistive and surface wave panels that can use fingers and stylus. Capacitive touch screens are not affected by outside elements and have high clarity.
tracert www.whatis.com
trace route See ‘‘traceroute’’
traceroute Traceroute is a utility that records the route (the specific gateway computers at each hop) through the Internet between your computer and a specified destination computer. It also calculates and displays the amount of time each hop took. Traceroute is a handy tool both for understanding where problems are in the Internet network and for getting a detailed sense of the Internet itself. Another utility, ping, is often used prior to using traceroute to see whether a host is present on the network. The traceroute utility comes included with a number of operating systems, including Windows and UNIX-based operating systems (such as IBM’s AIX/6000) or as part of a TCP/IP package. If your system doesn’t include the utility, you can install it. There are freeware versions that you can download.
How It Works When you enter the traceroute command, the utility initiates the sending of a packet (using the Internet Control Message Protocol or ICMP), including in the packet a time limit value (known as the ‘‘time to live’’ (TTL) that is designed to be exceeded by the first router that receives it, which will return a Time Exceeded message. This enables traceroute to determine the time required for the hop to the first router. Increasing the time limit value, it resends the packet so that it will reach the second router in the path to the destination, which returns another Time Exceeded message, and so forth. Traceroute determines when the packet has reached the destination by including a port number that is outside the normal range. When it’s received, a Port Unreachable message is returned, enabling traceroute to measure the time length of the final hop. As the tracerouting progresses, the records are displayed for you hop by hop. Actually, each hop is measured three times. (If you see an asterisk (*), this indicates a hop that exceeded some limit.)
or whatever domain name for a destination host computer you want to enter. You can also enter the equivalent numeric form of the IP address.
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trackball A trackball is a computer cursor control device used in many notebook and laptop computers. The trackball is usually located in front of the keyboard toward the user. Essentially, the trackball is an upside-down mouse that rotates in place within a socket. The user rolls the ball to direct the cursor to the desired place on the screen and can click one of two buttons (identical to mouse buttons) near the trackball to select desktop objects or position the cursor for text entry. IBM’s ThinkPad series of notebook computers uses a ‘‘pointing stick,’’ called a TrackPoint, that is integrated into the middle of the keyboard keys.
transaction In computer programming, a transaction usually means a sequence of information exchange and related work (such as database updating) that is treated as a unit for the purposes of satisfying a request and for ensuring database integrity. For a transaction to be completed and database changes to made permanent, a transaction has to be completed in its entirety. A typical transaction is a catalog merchandise order phoned in by a customer and entered into a computer by a customer representative. The order transaction involves checking an inventory database, confirming that the item is available, placing the order, and confirming that the order has been placed and the expected time of shipment. If we view this as a single transaction, then all of the steps must be completed before the transaction is successful and the database is actually changed to reflect the new order. If something happens before the transaction is successfully completed, any changes to the database must be kept track of so that they can be undone. A program that manages or oversees the sequence of events that are part of a transaction is sometimes called a transaction monitor. Transactions are supported by Structured Query Language, the standard database user and programming interface. When a transaction completes successfully, database changes are said to be committed; when a transaction does not complete, changes are rolled back. In IBM’s Customer Information Control System product, a transaction is a unit of application data processing that results from a particular type of transaction request. In CICS, an instance of a particular transaction request by a computer operator or user is called a task.
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Whatis?Com’s Encyclopedia of Technology Terms Less frequently and in other computer contexts, a transaction may have a different meaning. For example, in IBM mainframe operating system batch processing, a transaction is a job or a job step.
Transaction Server The Microsoft Transaction Server (MTS), called ‘‘Viper’’ while it was being developed, is a program that runs on an Internet or other network server with a Windows NT system and manages application and database transaction requests on behalf of a client computer user. The Transaction Server screens the user and client computer from having to formulate requests for unfamiliar databases and, if necessary, forwards the requests to database servers. It also manages security, connection to other servers, and transaction integrity. The Transaction Server is Microsoft’s bid to make distributed applications and data in a network relatively easy to create. It’s one of a category of programs sometimes known as middleware or multi-tier programming for the enterprise market that IBM has traditionally controlled with its CICS and similar transaction management products. Microsoft designed the Transaction Server to fit in with its overall object-oriented programming strategy. Using the Transaction Server, you can use a drag-and-drop interface to create a transaction model for a single user, then allow the Transaction Server to manage the model for multiple users, including the creation and management of user and task threads and processes. MTS runs on Windows NT Server 4.0 or higher.
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Transactional e-mail is a type of Web-based marketing in which e-mail recipients can buy goods and services directly from an e-mail message, without being redirected to the retailer’s Web site. According to proponents, transactional email leads to significantly higher conversion rates—the ratio of shoppers to buyers, which goes up when the former is converted to the latter—than regular e-mail marketing approaches. A number of different transactional e-mail products are available, from Cybuy, Radical Communication, and EActive, among others. The transactional e-mail retail experience is easier, and minimizes the tasks involved in online shopping. Within the body of the message, an e-mail recipient can view merchandise, select items, and submit an order. Typically, the e-mail messages contain windows that change to display different products when the recipient clicks listed items. A message assuring the customer of the security of the transaction, and an order form are displayed when the customer clicks the appropriate buttons. When the order form is completed, the customer clicks the ‘‘submit’’ button, and resumes reading their other e-mail messages.
Zagat’s, a well-known distributor of restaurant guides, tried transactional e-mail sales for the 2000 holiday season, allowing customers to buy restaurant guides directly from the body of the e-mail message, rather than just including a link to their Web site. The campaign attained conversion rates five times as high as those of regular e-mail marketing. Besides making shopping simpler for the consumer, transactional e-mail also takes into account the fact that someone who is reading e-mail is doing just that: reading their e-mail. Marketing messages, even for interesting products, may be set aside because they interfere with the task at hand. Even if the consumer wants to buy a product, they may not respond to traditional e-mail marketing messages, because the process involved (going to the retailer’s Web site, going to separate pages to order, and so on) is too cumbersome. Transactional e-mail also makes impulse purchases more likely, because a transaction can be completed before the customer grows weary of the procedures involved.
transceiver Also see repeater and transponder. A transceiver is a combination transmitter/receiver in a single package. The term applies to wireless communications devices such as cellular telephones, cordless telephone sets, handheld two-way radios, and mobile two-way radios. Occasionally the term is used in reference to transmitter/ receiver devices in cable or optical fiber systems. In a radio transceiver, the receiver is silenced while transmitting. An electronic switch allows the transmitter and receiver to be connected to the same antenna, and prevents the transmitter output from damaging the receiver. With a transceiver of this kind, it is impossible to receive signals while transmitting. This mode is called half duplex. Transmission and reception often, but not always, are done on the same frequency. Some transceivers are designed to allow reception of signals during transmission periods. This mode is known as full duplex, and requires that the transmitter and receiver operate on substantially different frequencies so the transmitted signal does not interfere with reception. Cellular and cordless telephone sets use this mode. Satellite communications networks often employ full-duplex transceivers at the surface-based subscriber points. The transmitted signal (transceiver-to-satellite) is called the uplink, and the received signal (satellite-to-transceiver) is called the downlink.
transcendental number A transcendental number is a real number that is not the solution of any single-variable polynomial equation whose coefficients are all integers. All transcendental numbers are irrational numbers. But the converse is not true; there are some irrational numbers that are not transcendental.
transconductance Examples of transcendental numbers include pi, the ratio of a circle’s circumference to its diameter in a plane, and e, the base of the natural logarithm. The case of pi has historical significance. The fact that pi is transcendental means that it is impossible to draw to perfection, using a compass and straightedge and following the ancient Greek rules for geometric constructions, a square with the same area as a given circle. This ancient puzzle, known as squaring the circle, was, for centuries, one of the most baffling challenges in geometry. Schemes have been devised that provide amazingly close approximations to squaring the circle. But in theoretical mathematics (unlike physics and engineering), approximations are never good enough; a solution, scheme, or method is either valid, or else it is not. It can be difficult, and perhaps impossible, to determine whether or not a certain irrational number is transcendental. Some numbers defy classification (algebraic, irrational, or transcendental) to this day. Two examples are the product of pi and e (call this quantity Ppie) and the sum of pi and e (call this Spie). It has been proved that pi and e are both transcendental. It has also been shown that at least one of the two quantities Ppie and Spie are transcendental. But as of this writing, no one has rigorously proven that Ppie is transcendental, and no one has rigorously proved that Spie is transcendental.
transcoding Transcoding is a technology used to adapt computer application displays and Web content so that they can be viewed on any of the increasingly diverse devices on the market. Transcoding servers and services reformat material that would otherwise have to be developed separately for display on different platforms. Working like an interpreter, the technology translates content to suitable formats for various platforms, regardless of protocol, application, screen size, and language used. A variant of transcoding has been used for some time in Web applications such as AltaVista’s Babel Fish language translation program. Although wireless connectivity is steadily increasing, the question of how to enable content display has been problematic. Handheld wireless devices—such as the smartphone and the personal digital assistant (PDA)—tend to have limitations in terms of power, memory, resolution, and screen size; this means that they have problems displaying Web content suitable for a full-sized computer. Typically, the mobile user wants to be able to access the crucial data (such as a stock quote, for example) without any extra detail or graphic display. Without transcoding, content must be written specifically (and separately) to meet the requirements and constraints of each device, and the process must be repeated every time the content is updated. Manual reauthoring of content can cause problems for a mobile workforce that depends on getting reliable information. Each
time content is rewritten, possibilities for errors exist, and updates may not always be available simultaneously for all devices. There are two main options for those who want to automate the reformatting of content: Using a transcoding server product, such as Aether’s Scout Web, AvantGo’s Enterprise Server, or IBM’s WebSphere Transcoding Publisher, or using a transcoding service, such as Everypath. Although the server products offer the most control over the final content, they also require in-house expertise, as developers must work directly with the original HTML and eXtensible Markup Language (XML) content. Transcoding services are application service providers (ASPs) that take responsibility for the entire process and deliver the reformatted content from legacy material. There are a number of different ways that transcoding can take place. In one example, the original material (an HTML or XML document, for example) is analyzed by a program that then creates a separate version (rather than changing the source) that contains annotations. The annotations include information that will instruct the reformatting process, such as importance ratings of document elements, for example, so that when space is limited, non-essential elements will not be displayed. When a request for the document is sent to the hosting server, the server submits the annotated version to an authoring application. The material is reformatted there, and sent on to a transcoding proxy server. The proxy server accesses information about device preferences, and may adapt the material further before delivering it to the end user. The device user may also have specific display preferences, either previously set in the device, or chosen at the time that they request the document, so that the document is reformatted dynamically.
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Transconductance is an expression of the performance of a bipolar transistor or field-effect transistor (FET). In general, the larger the transconductance figure for a device, the greater the gain (amplification) it is capable of delivering, when all other factors are held constant.
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Formally, for a bipolar device, transconductance is defined as the ratio of the change in collector current to the change in base voltage over a defined, arbitrarily small interval on the collector-current-versus-base-voltage curve. For an FET, transconductance is the ratio of the change in drain current to the change in gate voltage over a defined, arbitrarily small interval on the drain-current-versus-gate-voltage curve.
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The symbol for transconductance is gm. The unit is the siemens, the same unit that is used for direct-current (DC) conductance.
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If dI represents a change in collector or drain current caused by a small change in base or gate voltage dE, then the transconductance is approximately:
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gm = dI / dE
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As the size of the interval approaches zero—that is, the change in base or gate voltage becomes smaller and smaller—the value of dI / dE approaches the slope of a line tangent to the curve at a specific point. The slope of this line represents the theoretical transconductance of a bipolar transistor for a given base voltage and collector current, or the theoretical transconductance of an FET for a given gate voltage and drain current.
transducer A transducer is an electronic device that converts energy from one form to another. Common examples include microphones, loudspeakers, thermometers, position and pressure sensors, and antennas. Although not generally thought of as transducers, photocells, LEDs (light-emitting diodes), and even common light bulbs are transducers. Efficiency is an important consideration in any transducer. Transducer efficiency is defined as the ratio of the power output in the desired form to the total power input. Mathematically, if P represents the total power input and Q represents the power output in the desired form, then the efficiency E, as a ratio between 0 and 1, is given by: E = Q/P If E% represents the efficiency as a percentage, then: E% = 100Q/P No transducer is 100-percent efficient; some power is always lost in the conversion process. Usually this loss is manifested in the form of heat. Some antennas approach 100-percent efficiency. A well-designed antenna supplied with 100 watts of radio frequency (RF) power radiates 80 or 90 watts in the form of an electromagnetic field. A few watts are dissipated as heat in the antenna conductors, the feed line conductors and dielectric, and in objects near the antenna. Among the worst transducers, in terms of efficiency, are incandescent lamps. A 100-watt bulb radiates only a few watts in the form of visible light. Most of the power is dissipated as heat; a small amount is radiated in the UV (ultraviolet) spectrum.
transient cookie On the Web, a transient cookie, sometimes called a session cookie, is a small file that contains information about a user that disappears when the user’s browser is closed. Unlike a persistent cookie, a transient cookie is not stored on your hard drive but is only stored in temporary memory that is erased when the browser is closed. A transient cookie is created by simply not setting a date in the Set-Cookie option when an application creates the cookie. (For a persistent cookie, an expiration date is set and the cookie is stored on the user’s hard drive until the expiration date or until the user deletes it.)
Transient cookies are often used to enable a site to be able to track the pages that a user has visited during a visit so that information can be customized for the user in some way. Some sites use Secure Sockets Layer (SSL) to encrypt the information contained in a cookie.
transistor The transistor, invented by three scientists at the Bell Laboratories in 1947, rapidly replaced the vacuum tube as an electronic signal regulator. A transistor regulates current or voltage flow and acts as a switch or gate for electronic signals. A transistor consists of three layers of a semiconductor material, each capable of carrying a current. A semiconductor is a material such as germanium and silicon that conducts electricity in a ‘‘semi-enthusiastic’’ way. It’s somewhere between a real conductor such as copper and an insulator (like the plastic wrapped around wires). The semiconductor material is given special properties by a chemical process called doping. The doping results in a material that either adds extra electrons to the material (which is then called N-type for the extra negative charge carriers) or creates ‘‘holes’’ in the material’s crystal structure (which is then called P-type because it results in more positive charge carriers). The transistor’s three-layer structure contains an N-type semiconductor layer sandwiched between P-type layers (a PNP configuration) or a P-type layer between N-type layers (an NPN configuration). As the current or voltage is changed in one of the outer semiconductor layers, it affects a larger current or voltage in the inner layer resulting in the opening or closing of an electronic gate. Today’s computers use circuitry made with complementary metal oxide semiconductor (CMOS) technology. CMOS uses two complementary transistors per gate (one with N-type material; the other with P-type material). When one transistor is maintaining a logic state, it requires almost no power. Transistors are the basic elements in integrated circuits (ICs), which consist of very large numbers of transistors interconnected with circuitry and baked into a single silicon microchip or ‘‘chip.’’
transit Transit is the connection to and use of a telecommunication path provided by a vendor. Transit may be billed separately or, where peering is also provided, may be billed as part of the peering charge.
Transmeta Transmeta is a Silicon Valley start-up company known for its recruitment of high profile talent and its Crusoe chip, designed for mobile Internet computing. David Ditzel (Sun UltraSparc) founded Transmeta in 1995 and recruited Linus
Transport Layer Security Torvalds, the creator of Linux, to be a member of Transmeta’s software team. (Paul Allen, co-founder of Microsoft is a major investor.) For four and a half years, the company operated in a shroud of secrecy, causing a lot of speculation about what Transmeta actually did. In November of 2000, Transmeta went public and revealed it had developed a low-power microprocessing chip called Crusoe (named after Daniel Defoe’s shipwrecked character, Robinson Crusoe). Crusoe is the first of what Transmeta hopes will be a family of smart microprocessors for mobile Internet devices.
Transmission Control Protocol See ‘‘TCP’’
Transmission Control Protocol/Internet Protocol See ‘‘TCP/IP’’
transparent GIF A transparent GIF (Graphics Interchange Format) is an image file that has one color assigned to be ‘‘transparent’’ so that the assigned color will be replaced by the browser’s background color, whatever it may be. Pretend, for example, that you have created a rectangular GIF image of a large red star on a white background. If you are only interested in having the red star appear on your Web page, and don’t want to see the white background, you can transparentize the white background color so that it changes to whatever the Web page’s background color is (yellow, for example). Then, when you view the Web page, you will only see a red star on a yellow background. A single color transparent GIF can also be used as a place holder in a table cell on a Web page. Another name for this use of a transparent GIF is ‘‘spacer GIF.’’
transponder A transponder is a wireless communications, monitoring, or control device that picks up and responds to an incoming signal. The term is a contraction of the words transmitter and responder. Transponders can be either passive or active. A passive transponder allows a computer or robot to identify an object. Magnetic labels, such as those on credit cards and store items, are common examples. A passive transponder must be used with an active sensor that decodes and transcribes the data the transponder contains. The transponder unit can be physically tiny, and its information can be sensed up to several feet away. Simple active transponders are employed in location, identification, and navigation systems. An example is an RFID (radio-frequency identification) device that transmits a coded signal when it receives a request from a monitoring or control point. The transponder output signal is tracked, so
the position of the transponder can be constantly monitored. The input (receiver) and output (transmitter) frequency are preassigned. Transponders of this type can operate over distances of thousands of miles. Sophisticated active transponders are used in communications satellites and on board space vehicles. They receive incoming signals over a range, or band, of frequencies, and retransmit the signals on a different band at the same time. The device is similar to a repeater of the sort used in landbased cellular telephone networks. The incoming signal, usually originating from a point on the earth’s surface, is called the uplink. The outgoing signal, usually sent to a point or region on the surface, is the downlink. These transponders sometimes operate on an interplanetary scale.
Transport layer In the Open Systems Interconnection (OSI) communications model, the Transport layer ensures the reliable arrival of messages and provides error checking mechanisms and data flow controls. The Transport layer provides services for both ‘‘connection-mode’’ transmissions and for ‘‘connectionlessmode’’ transmissions. For connection-mode transmissions, a transmission may be sent or arrive in the form of packets that need to be reconstructed into a complete message at the other end.
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The Transmission Control Protocol portion of TCP/IP is a program that can be mapped to the Transport layer.
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Transport Layer Security (TLS) is a protocol that ensures privacy between communicating applications and their users on the Internet. When a server and client communicate, TLS ensures that no third party may eavesdrop or tamper with any message. TLS is the successor to the Secure Sockets Layer (SSL). TLS is composed of two layers: the TLS Record Protocol and the TLS Handshake Protocol. The TLS Record Protocol provides connection security with some encryption method such as the Data Encryption Standard (DES). The TLS Record Protocol can also be used without encryption. The TLS Handshake Protocol allows the server and client to authenticate each other and to negotiate an encryption algorithm and cryptographic keys before data is exchanged. The TLS protocol is based on Netscape’s SSL 3.0 protocol; however, TLS and SSL are not interoperable. The TLS protocol does contain a mechanism that allows TLS implementation to back down to SSL 3.0. The most recent browser versions support TLS. The TLS Working Group, established in 1996, continues to work on the TLS protocol and related applications.
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1) In a Web site, a trap is a page that does not allow the reader to back up a previous page (the Back button on the toolbar is inoperable). A few Web site creators apparently use this technique to hold the reader and force them to read the page or to encourage them to visit other pages on their site. To exit a trap, the reader must either close the browser and open it again or enter a URL on the address line. Traps are highly unpopular among Web users.
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2) In assembler language programming, a trap is a place in a program for handling unexpected or unallowable conditions—for example, by sending an error message to a log or to a program user. If a return code from another program were being checked by a calling program, a return code value that was unexpected and unplanned for could cause a branch to a trap that recorded the situation and took other appropriate action.
traveling-wave tube A traveling-wave tube (TWT) is a specialized vacuum tube used in wireless communications, especially in satellite systems. The TWT can amplify or generate microwave signals. Two common types of TWT include the Klystron and the magnetron.
internal noise. This makes it possible to obtain excellent sensitivity in receiving systems, minimizing data-transfer errors.
tree structure A tree structure is an algorithm for placing and locating files (called records or keys) in a database. The algorithm finds data by repeatedly making choices at decision points called nodes. A node can have as few as two branches (also called children), or as many as several dozen. The structure is straightforward, but in terms of the number of nodes and children, a tree can be gigantic. In a tree, records are stored in locations called leaves. This name derives from the fact that records always exist at end points; there is nothing beyond them. The starting point is called the root. The maximum number of children per node is called the order of the tree. The maximum number of access operations required to reach the desired record is called the depth. In some trees, the order is the same at every node and the depth is the same for every record. This type of structure is said to be balanced. Other trees have varying numbers of children per node, and different records might lie at different depths. In that case, the tree is said to have an unbalanced or asymmetrical structure.
In the Klystron, a negatively charged cathode emits a beam of high-speed, high-energy electrons that travel through the cylindrical tube in straight lines to a positively charged anode. A coil is wound around the tube. When the coil is energized with a radio-frequency (RF) signal, the electrons in the beam alternately bunch up and spread out. In the magnetron, the electrons move in circles rather than in straight lines. The circular motion, produced by magnets at either end of the tube, allows the electrons to pick up energy over a greater distance.
Order = 2 Depth = 4 Leaves = 8
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Inside the TWT, the regions of high and low electron concentration move along or around the tube in waves. When the tube is properly operating, some of the energy from the electrons is imparted to the signal in the coil. The result is amplification of the signal. A TWT can be made to function as an oscillator by coupling some of the output back into the input. This configuration is called a backward-wave oscillator, because the feedback is applied opposite to the direction of movement of the electrons inside the tube. Such an oscillator can generate up to approximately 0.1 watt of signal power in the microwave range. A parametric amplifier is a TWT amplifier that operates from a high-frequency alternating current (AC) power source, rather than the usual direct current (DC) source. Some characteristic of the circuit, such as its impedance, is made to vary with time at the power-supply frequency. Parametric amplifiers are useful because they generate very little
Order = 3 Depth = 3 Leaves = 9
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Order = 5 Depth = 3 Leaves = 13
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C D E F Cell phone The illustration shows three examples of tree structures. (Note that the portrayals are upside-down compared to real tree plants.) Structures A and B are balanced, and structure C is unbalanced. Roots are at the top, and are represented by arrows. Nodes are shown as gray dots. Children are solid black lines. Leaves are at the bottom. As the process moves toward the leaves and away from the root, children can branch out from a node, but children never merge into a node. In a practical tree, there can be thousands, millions, or billions of nodes, children, leaves, and records. Not every leaf necessarily contains a record, but more than half do. A leaf that does not contain data is called a null. The trees shown here are simple enough to be rendered in two dimensions, but with some large databases, three dimensions are needed to clearly depict the structure. Also see binary tree, B-tree, M-tree, quad tree, splay tree, and X-tree.
triangulation Triangulation is a process by which the location of a radio transmitter can be determined by measuring either the radial distance, or the direction, of the received signal from two or three different points. Triangulation is sometimes used in cellular communications to pinpoint the geographic position of a user. The drawings below illustrate the basic principle of triangulation. In the scenario shown by the top drawing, the distance to the cell phone is determined by measuring the relative time delays in the signal from the phone set to three different base stations. In the scenario shown by the bottom drawing, directional antennas at two base stations can be used to pinpoint the location of the cell phone. Triangulation is difficult to carry out unless the person using the cell phone wants to be located. This might be the case, for example, in an emergency situation. Triangulation is the method by which the so-called 911 cell phones work.
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H Triangulation apparatus can be confused by the reflection of signals from objects such as large steel-frame buildings, water towers, communications towers, and other obstructions. For this reason, at least two independent triangulation determinations should be made to confirm the position of a cell phone or other radio transmitter. A more sophisticated form of triangulation is used by the Global Positioning System (GPS).
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trigger A trigger (from the Dutch trekken, meaning to pull) is a lever which, when pulled by the finger, releases the hammer on a firearm. In a database, a trigger is a set of Structured Query Language (SQL) statements that automatically ‘‘fires off’’ an action when a specific operation, such as changing data in a table, occurs. A trigger consists of an event (an INSERT, DELETE, or UPDATE statement issued against an associated table) and an action (the related procedure). Triggers are used to preserve data integrity by checking on or changing data in a consistent manner.
triplecast A triplecast (TM) is the simultaneous broadcast of a program on television, radio, and an Internet site or channel. NetTalk Live! originated the term to describe its weekly program about the Internet. In addition to listening to or viewing the show on radio or television in the Dallas/Fort Worth area, you can ‘‘tune in’’ from a Web site, Audionet, from which you can download a RealAudio player and listen to streaming sound as the show progresses.
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Trivial File Transfer Protocol (TFTP) is an Internet software utility for transferring files that is simpler to use than the File Transfer Protocol (FTP) but less capable. It is used where user authentication and directory visibility are not required. TFTP uses the User Datagram Protocol (UDP) rather than the Transmission Control Protocol (TCP). TFTP is described formally in Request for Comments (RFC) 1350.
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In computers, a Trojan horse is a program in which malicious or harmful code is contained inside apparently harmless programming or data in such a way that it can get control and do its chosen form of damage, such as ruining the file allocation table on your hard disk. In one celebrated case, a Trojan horse was a program that was supposed to find and destroy computer viruses. A Trojan horse may be widely redistributed as part of a computer virus.
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The term comes from Homer’s Iliad. In the Trojan War, the Greeks presented the citizens of Troy with a large wooden horse in which they had secretly hidden their warriors. During the night, the warriors emerged from the wooden horse and overran the city.
tropo See ‘‘tropospheric propogation’’
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tropospheric propogation Radio waves can propagate over the horizon when the lower atmosphere of the earth bends, scatters, and/or reflects the electromagnetic fields. These effects are collectively known as tropospheric propagation, or tropo for short. Tropospheric propagation can affect wireless communications, sometimes enhancing the usable range, but also compounding interference problems. The most well-known form of tropo is called bending. Air reduces radio-wave propagation speed compared with the speed in a vacuum. The greater the air density, the more the air slows the waves, and thus the greater is the index of refraction. The density and index of refraction are highest near the surface, and steadily decrease with altitude. This produces a tendency for radio waves at very-high frequencies (VHF, 30 to 300 MHz) and ultra-high frequencies (UHF, 300 MHz to 3 GHz) to be refracted toward the surface. A wave beamed horizontally can follow the curvature of the earth for hundreds of miles. The lower atmosphere scatters electromagnetic radiation over a vast range, including radio wavelengths. This effect is known as tropospheric scatter, or troposcatter. In general, troposcatter is most pronounced at UHF and microwave radio frequencies (300 MHz and above). A radio wave beamed slightly above the horizon can be scattered at altitudes up to several miles, making over-the-horizon
communication possible. The greatest communications range can be realized over flat land or over water. Scattered waves are weak, so high-power transmitters and sensitive receivers are necessary. A less common, but often dramatic, form of tropo is called ducting or duct effect. This occurs when there is a defined, horizontal boundary between air masses having different densities. When a cool air mass is overlain by a warm air mass, as is the case along and near warm fronts and cold fronts, radio waves at VHF and UHF are reflected at the boundary if they strike it at a near-grazing angle from beneath (within the cooler air mass). Because radio waves are also reflected from the earth’s surface, the result can be efficient propagation for hundreds or, in some cases, upwards of 1,000 miles, as the waves alternately bounce off the frontal boundary and the surface. Ducting can allow long-distance radio reception in the frequency-modulation (FM) broadcast band between 88 and 108 MHz. It can also affect the lower VHF television channels if receiving antennas (rather than cable networks) are used.
true color True color is the specification of the color of a pixel on a display screen using a 24-bit value, which allows the possibility of up to 16,777,216 possible colors. Many displays today support only an 8-bit color value, allowing up to 256 possible colors. The number of bits used to define a pixel’s color shade is its bit-depth. True color is sometimes known as 24-bit color. Some new color display systems offer a 32-bit color mode. The extra byte, called the alpha channel, is used for control and special effects information.
true power True power is the power manifested in tangible form such as electromagnetic radiation, acoustic waves, or mechanical phenomena. In a direct current (DC) circuit, or in an alternating current (AC) circuit whose impedance is a pure resistance, the voltage and current are in phase, and the following formula holds: P = ErmsIrms where P is the power in watts, Erms is the root-mean-square voltage in volts, and Irms is the rms current in amperes. But in an AC circuit whose impedance consists of reactance as well as resistance, the voltage and current are not in phase. This complicates the determination of power. In the absence of reactance, this voltage-current product represents true power. But when there is reactance in an AC circuit, the product ErmsIrms is larger than the true power, and is known as apparent power. The vector difference between the apparent and true power is called reactive power, and represents energy alternately stored and released by inductor and/or capacitor.
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TrueType Created at Apple Computer, TrueType was designed to fill the need for an optimized, scalable font format. The format uses hinting, a technique that preserves a font’s design, even at a small scale or on a display with low resolution. It was initially developed in response to the technical limitations of Adobe’s Postscript and Type 1 font formats. TrueType was first introduced in the Macintosh System 7 operating system in 1990. A year later, Apple licensed TrueType to Microsoft, and it was introduced in their Windows 3.1 operating system. After making some performance improvements to the format, they released version 1.5 in Windows NT 3.1, and have continued to update the format. In order to view and print TrueType fonts on a computer, two components are necessary: the actual TrueType font file (with the extension TTF) and the TrueType raster graphics. The rasterizer is built into Windows and Mac operating systems. If you use TrueType fonts on a Web page or in Word document, it is possible to embed them into the file, so that your fonts will still be viewable by people who do not have that font installed on their machine. Also keep in mind that the Macintosh and Windows versions of TrueType fonts are not compatible. When downloading TrueType fonts from the Internet (such as from a free fonts Web site), you must select the Mac or PC version. TrueType Font converters are available if a TrueType font is only available for one platform. Microsoft’s most recent font development is called OpenType. This font format ensures that fonts are compatible across the Windows and Apple Macintosh platforms and encompasses Adobe’s Type 1 and PostScript technologies.
truncate To truncate is to shorten by cutting off. In computer terms, when information is truncated, it is ended abruptly at a certain spot. For example, if a program truncates a field containing the value of pi (3.14159265...) at four decimal places, the field would show 3.1415 as an answer. This is different from rounding, which would result in the answer of 3.1416. For another example, consider a text entry field in an electronic form. If a program limits the size of the field to 255 characters, it may allow you to continue typing past the maxiumum number of characters. However, when the information is submitted or saved, the program truncates the data to the first 255 characters and any additional characters are disregarded.
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Several operating systems or programming languages use truncate as a command or function for limiting the size of a field, data stream, or file. Linux includes a truncate command for rounding integers.
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In telephone systems, a trunk is a line that carries multiple voice or data channels between two telephone exchange switching systems. In digital communications, a trunk is often a T-carrier system.
trusted PC The trusted PC is an industry ideal of a PC with built-in security mechanisms that place minimal reliance on the user or administrator to keep a PC and its peripheral devices secure. Trusted personal computing devices are being developed that maximize the security of individual computers through hardware and operating system-based mechanisms rather than through add-in programs and policies. To that end, security mechanisms are being built into chips, chipsets, and motherboards, among other things, because industry consensus is that hardware-based mechanisms are inherently more trustworthy than those created with software. The Trusted Computing Platform Alliance (TCPA) was formed in October 1999 by Compaq, HP, IBM, Intel and Microsoft. TCPA, whose membership now includes more than 140 companies, states as its goals: ‘‘To develop a specification, based on the collaboration of PC industry platform, operating system, application, and technology vendors, that delivers a set of hardware and operating system security capabilities that customers can use to enhance the trust and security in their computing environments.’’ Once effective mechanisms are built into the actual computer hardware, security will not be as dependent upon the vigilance of individual administrators as it has been in the past. If the trusted PC works the way it should, an inexperienced or lackadaisical administrator won’t be able to unwittingly compromise system or network security through inefficient policies. The idea of a ‘‘trusted PC’’ is similar to the trust relationships that can be set up between networked computers to allow the terminals involved varying degrees of access to each other’s data, depending on their requirements and level of trust. The new trusted PC uses the word in a different sense, in that its ‘‘trust’’ is based on hard-wired security—somewhat like trusting children to stay out of the cookie jar because it is locked in a safe.
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1) In programming languages, such as Lisp, Python, Linda, and others, a tuple (pronounced TUH-pul) is an ordered set of values. The separator for each value is often a comma (depending on the rules of the particular language). Common uses for the tuple as a data type are (1) for passing a string of parameters from one program to another, and (2) representing a set of value attributes in a relational database. In some languages, tuples can be nested within other tuples within parentheses or brackets or other delimiters. Tuples can contain a mixture of other data types.
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Here’s an example of a tuple that emphasizes the different data types that may exist within a tuple data type:
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The above example is sometimes referred to as a 4-tuple, since it contains four values. An n-tuple would be one with an indeterminate or unspecified number of values.
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2) A tuple is analogous to a record in nonrelational databases.
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The term originated as an abstraction of the sequence: single, double, triple, quadruple, quintuple, ... n-tuple. Tuple is used in abstract mathematics to denote a multidimensional coordinate system.
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Tux Tux, the penguin, is the generally accepted official logo for the Linux operating system. Linus Torvalds, the creator of Linux, chose Larry Ewing’s design simply because he likes penguins. It is said that the name Tux was the winning entry in the ‘‘Let’s Name The Penguin While Linus Is Away Contest’’ with the ‘‘T’’ standing for Torvald and the ‘‘U’’ and ‘‘X’’ for Linux.
Tuxedo Tuxedo (which stands for Transactions for UNIX, Enhanced for Distributed Operation) is a middleware product that uses a message-based communications system to distribute applications across various operating system platforms and databases. Tuxedo operates as an extension of the operating system: as a platform for execution as well as development, it is designed for the creation and administration of e-commerce online transaction processing (OLTP) systems. Originally developed at AT&T in the 1970s, Tuxedo was subsequently owned by Unix System Laboratories (USL), and then Novell before it was taken over by BEA Systems, the current owners. Tuxedo’s three main functions are: as middleware, to relay request and response communications between servers and clients; as a transaction processing (TP) monitor, to initiate, monitor, and terminate transactions; and, as a distributed TP monitor, to enable interaction between transaction participants on different machines and associated with
different databases. The Gap, E*TRADE, and Hong Kong International Terminals are some organizations that use Tuxedo for large scale transaction processing.
TWAIN TWAIN is a widely-used program that lets you scan an image (using a scanner) directly into the application (such as PhotoShop) where you want to work with the image. Without TWAIN, you would have to close an application that was open, open a special application to receive the image, and then move the image to the application where you wanted to work with it. The TWAIN driver runs between an application and the scanner hardware. TWAIN usually comes as part of the software package you get when you buy a scanner. It’s also integrated into PhotoShop and similar image manipulation programs. The software was developed by a work group from major scanner manufacturers and scanning software developers and is now an industry standard. In several accounts, TWAIN was an acronym developed playfully from ‘‘technology without an important name.’’ However, the TWAIN Working Group says that after the name chosen originally turned out to be already trademarked, an 11th hour meeting of the group came up with TWAIN, deriving it from the saying ‘‘Ne’er the twain shall meet,’’ because the program sits between the driver and the application. The name is not intended to be an acronym.
tweak freak A tweak freak is a person who tinkers with hardware and/ or programming to a point approaching obsession. Sometimes the intent is to enhance system performance beyond the norm. But many tweak freaks engage in the activity largely because it is interesting, fun, and can be a learning experience. It can also be frustrating and at its worst, dangerous. The term ‘‘tweak’’ may have originated in the early days of electronics, when tweezers were used to adjust the position of a wire on a crystal of galena in order to detect amplitudemodulated (AM) radio broadcast signals. Nowadays, hardware and programs of all kinds can (and often should) be aligned or debugged for optimum performance; technicians and programmers call the process tweaking. The tweaking of computers, especially software and operating systems, is not for the faint-of-heart. Excessive or improper tweaking can cause computers and Web browsers to become unstable. Vulnerability to virus or Trojan horse infection may be increased. Excessive downloading and installation of tweaks and patches can increase the probability of program conflicts and crashes. Some tweaks facilitate illegal or questionable use of the Internet, and should be avoided by people who respect the law and the rights of others (and who wish to stay out of court). Let the tweaker beware.
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TweakUI TweakUI is a software utility that lets Windows 9x, Windows NT, and Windows 2000 users modify the desktop user interface and other system characteristics to their liking. Using TweakUI, you can change the menu speed, how windows are animated, and a number of features about Microsoft Internet Explorer. TweakUI allows the user to change settings that are not easy to locate, including some in the system registry. For example, a user can speed up system startup by reducing the frequency with which the ScanDisk utility is run. Microsoft includes TweakUI as one of several ‘‘Power Toys’’ that can be optionally installed. Users are advised to always back up their registry (for example, copy it to a new file with another name) before doing anything that might modify it.
twinaxial cable Twinaxial cable is coaxial cable that contains two inner conducting wires rather than one. IBM’s AS/400 and System/3x midrange computer systems use twinaxial cable.
twip A twip (twentieth of a point) is a measure used in laying out space or defining objects on a page or other area that is to be printed or displayed on a computer screen. A twip is 1/ 1440th of an inch or 1/567th of a centimeter. That is, there are 1440 twips to an inch or 567 twips to a centimeter. The twip is 1/20th of a point, a traditional measure in printing. A point is approximately 1/72nd of an inch. A number of computer software programs, notably Microsoft’s Visual Basic, as well as its rich text file format (RTF), require programmers to specify screen positions and image and icon sizes in twips rather than in another common measure, the pixel. Like the pixel, the twip can be adjusted in size as screen resolution is changed, but, unlike the pixel, expresses an absolute value for printing. Since not all software development tools work with twips, a programmer may sometimes need to convert between twips and pixels, and the reverse.
twisted pair (Also see categories of twisted pair cabling systems and registered jack.) Twisted pair is the ordinary copper wire that connects home and many business computers to the telephone company. To reduce crosstalk or electromagnetic induction between pairs of wires, two insulated copper wires are twisted around each other. Each connection on twisted pair requires both wires. Since some telephone sets or desktop locations require multiple connections, twisted pair is sometimes installed in two or more pairs, all within a single cable. For some business locations, twisted pair is enclosed in a shield that
functions as a ground. This is known as shielded twisted pair (STP). Ordinary wire to the home is unshielded twisted pair (UTP). Twisted pair is now frequently installed with two pairs to the home, with the extra pair making it possible for you to add another line (perhaps for modem use) when you need it. Twisted pair comes with each pair uniquely color coded when it is packaged in multiple pairs. Different uses such as analog, digital, and Ethernet require different pair multiples. Although twisted pair is often associated with home use, a higher grade of twisted pair is often used for horizontal wiring in LAN installations because it is less expensive than coaxial cable. The wire you buy at a local hardware store for extensions from your phone or computer modem to a wall jack is not twisted pair. It is a side-by-side wire known as silver satin. The wall jack can have as many five kinds of hole arrangements or pinout, depending on the kinds of wire the installation expects will be plugged in (for example, digital, analog, or LAN) . (That’s why you may sometimes find when you carry your notebook computer to another location that the wall jack connections won’t match your plug.)
two-way pager A two-way pager is a pager that allows you to send data as well as receive it. In some cases, a two-way pager can serve as an alternative to a cellular telephone. At least one manufacturer, Paging Network, lets you record a message and have an answering service on the machine. Apple Computer, Hewlett-Packard, MobileMedia Corporation, Motorola, AirTouch Communications, Casio Computer, Mobile Telecommunication Technologies (Mtel), and Sharp are among companies that manufacture two-pagers or offer a two-way paging service.
typeface A typeface is a design for a set of printer or display fonts, each for a set of characters, in a number of specific sizes. Since outline fonts such as TrueType and Type 1 are scalable, a computer typeface designer must anticipate the possibility of the design being scaled through a range of sizes.
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Typefaces often come as a family of typefaces, with individual typefaces for italic, bold, and other variations in the main design.
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typosquatting Typosquatting is a form of Internet cybersquatting, based on the probability that a certain number of Internet users will mistype the name of a Web site (or actually its URL) when surfing the Web. Typically, a typosquatter will register several possible input errors for a ‘‘brand name’’ Web site
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Whatis?Com’s Encyclopedia of Technology Terms known for its high traffic, and then monitor to see how many clicks a day each of their ‘‘typo’’ domain names receives, and use the information to sell advertising for the sites that receive a high volume of accidental traffic. Advertising revenue might come from selling ads to the original site’s competitors or by providing redirect pages to related products or services.
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Electrical interface
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A U is a standard unit of measure for designating the height in computer enclosures and rack cabinets. A U equals 1.75 inches. For example, a 4U chassis is 7 inches high. A 40U rack cabinet is 70 inches high.
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Within the ISDN node at the central office; separates line termination equipment from exchange termination equipment
U interface In Integrated Services Digital Network (ISDN) Basic Rate Interface service, a U interface is the electrical interface for the single twisted pair wire connection from a local phone company (the central office) to a home or business. Unlike a regular 64 Kbps analog phone connection, however, the twisted-pair using ISDN carries two 64 Kbps channels (known as B, or bearer, channels) and an additional 16 Kbps channel (the D, for data or delta, channel) for control signals. The U interface twisted-pair is usually connected at the home or business to a network terminator 1 (NT1) box, sometimes called a network terminating unit. (In the UK and some other countries, the NT1 is located at the central office.) The other side of the NT1 has plugs for four wires, which can be connected on a loop configuration known as an S-bus or S interface to up to eight devices (for example, two computers and six phones) or to a T interface. An NT1 can also be integrated into a modem or other device, in which case the ISDN connection can only serve that device. The U-loop or U-V loop, as it is sometimes called, uses the 2B1Q line code protocol, meaning that two binary digits are used to represent one quadratude—that is, four possible variations of signal level (amplitude and polarity). Communication is full-duplex, meaning that data can be arriving at the same time you are sending data. The U-V loop replaces the traditional local loop. The maximum distance for the ISDN loop is 6,500 meters (about 18,000 feet). The following table summarizes the various ISDN electrical interfaces at different demarcation points or places in the traffic flow: Electrical interface Between what two points U interface T interface S interface
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Central office and NT1 NT1 and NT2 devices (such as a PBX) NT1 or NT2 and ISDN devices (such as a telephone or terminal adapter) Terminal adapter and nonISDN devices (such as a computer)
UART A UART (Universal Asynchronous Receiver/Transmitter) is the microchip with programming that controls a computer’s interface to its attached serial devices. Specifically, it provides the computer with the RS-232C Data Terminal Equipment (DTE) interface so that it can ‘‘talk’’ to and exchange data with modems and other serial devices. As part of this interface, the UART also:
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Converts the bytes it receives from the computer along parallel circuits into a single serial bit stream for outbound transmission
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On inbound transmission, converts the serial bit stream into the bytes that the computer handles
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Adds a parity bit (if it’s been selected) on outbound transmissions and checks the parity of incoming bytes (if selected) and discards the parity bit
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Adds start and stop delineators on outbound and strips them from inbound transmissions
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Handles interrupts from the keyboard and mouse (which are serial devices with special ports)
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May handle other kinds of interrupt and device management that require coordinating the computer’s speed of operation with device speeds
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More advanced UARTs provide some amount of buffering of data so that the computer and serial devices data streams remain coordinated. The most recent UART, the 16550, has a 16-byte buffer that can get filled before the computer’s processor needs to handle the data. The original UART was the 8250. If you purchase an internal modem today, it probably includes a 16550 UART (although you should ask when you buy it). According to modem manufacturer US Robotics, external modems do not include a UART. If you have an older computer, you may want to add an internal 16550 to get the most out of your external modem.
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UCM See ‘‘configuration management’’
UDA
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UDDI (Universal Description, Discovery, and Integration) is an XML-based registry for businesses worldwide to list themselves on the Internet. Its ultimate goal is to streamline online transactions by enabling companies to find one another on the Web and make their systems interoperable for e-commerce. UDDI is often compared to a telephone book’s white, yellow, and green pages. The project allows businesses to list themselves by name, product, location, or the Web services they offer.
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Microsoft, IBM, and Ariba spearheaded UDDI. The project now includes 130 companies, including some of the biggest names in the corporate world. Compaq, American Express, SAP AG, and Ford Motor Company are all committed to UDDI, as is Hewlett-Packard, whose own XML-based directory approach, called e-speak, is now being integrated with UDDI. While the group does not refer to itself as a standards body, it does offer a framework for Web services integration. The UDDI specification utilizes World Wide Web Consortium (W3C) and Internet Engineering Task Force (IETF) standards such as XML, HTTP, and Domain Name System (DNS) protocols. It has also adopted early versions of the proposed Simple Object Access Protocol (SOAP) messaging guidelines for cross platform programming. In November 2000, UDDI entered its public beta-testing phase. Each of its three founders—Microsoft, IBM, and Ariba—now operates a registry server that is interoperable with servers from other members. As information goes into a registry server, it is shared by servers in the other businesses. In the future, other companies will act as operators of the UDDI Business Registry. UDDI registration is open to companies worldwide, regardless of their size.
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UDP (User Datagram Protocol) is a communications protocol that offers a limited amount of service when messages are exchanged between computers in a network that uses the Internet Protocol (IP). UDP is an alternative to the Transmission Control Protocol (TCP) and, together with IP, is sometimes referred to as UDP/IP. Like the Transmission Control Protocol, UDP uses the Internet Protocol to actually get a data unit (called a datagram) from one computer to another. Unlike TCP, however, UDP does not provide the service of dividing a message into packets (datagrams) and reassembling it at the other end. Specifically, UDP doesn’t provide sequencing of the packets that the data arrives in. This means that the application program that uses UDP must be able to make sure that the
entire message has arrived and is in the right order. Network applications that want to save processing time because they have very small data units to exchange (and therefore very little message reassembling to do) may prefer UDP to TCP. The Trivial File Transfer Protocol (TFTP) uses UDP instead of TCP. UDP provides two services not provided by the IP layer. It provides port numbers to help distinguish different user requests and, optionally, a checksum capability to verify that the data arrived intact. In the Open Systems Interconnection (OSI) communication model, UDP, like TCP, is in layer 4, the Transport layer.
UHF The UHF (ultrahigh frequency) range of the radio spectrum is the band extending from 300 MHz to 3 GHz. The wavelengths corresponding to these limit frequencies are 1 meter and 10 centimeters. In the UHF band, signals from earth-based transmitters are not returned by the ionosphere to the surface; they always pass into space. Conversely, signals from space always penetrate the ionosphere and reach the surface. The global ‘‘shortwave’’ propagation familiar to users of lower frequencies is unknown at UHF. The troposphere can cause bending, ducting, and scattering at UHF, extending the range of communication significantly beyond the visual horizon. Auroral, meteor-scatter, and EME (earth-moonearth, also called moonbounce) propagation are sometimes observed, but these modes do not offer reliable communication and are of interest primarily to amateur radio operators. In the upper portion of the band, waves can be focused or collimated by dish antennas of modest size. The UHF band is extensively used for satellite communication and broadcasting, in cellular telephone and paging systems, and by third-generation (3G) wireless services. Because the frequency is high and the band is vast (a span of 2.7 gigahertz from the low end to the high end), wideband modulation and spread spectrum modes are practical. Channels and subbands within the UHF portion of the radio spectrum are allocated by the International Telecommunication Union (ITU).
UIML UIML (User Interface Markup Language) is a descriptive language that lets you create a Web page that can be sent to any kind of interface device—for example, to a PC with a large display and a keyboard or to a ‘‘smart phone’’ with a tiny display and no keyboard. The advantage of UIML is that Web content can be created once without understanding the existing and future device types to which the content will be sent. A content developer uses the markup language (sometimes referred to as a set of tags) to describe user interface elements such as input boxes, text messages, menus, and buttons. A programmer can then write
UMTS applications that use the UIML page to generate a page appropriate to each device type, such as a PC, smart phone, or voice output device, to which the content may need to be sent. UIML is an application of the Extensible Markup Language (XML). One can think of it as the XML description that describes the data structure (names of fields or elements) of a user interface. A given UIML file also describes the specific content (text, names of images, and so forth) in those fields or elements. UIML also allows you to describe possible user input events and resulting actions. The markup is described in the UIML Specification, which also includes the formal XML document type definition (DTD) for UIML. Since UIML requires the specification of user interface elements (often called widgets) in terms of names used by specific language development toolkits (for example, Java AWT or Microsoft Foundation Classes—MFCL), you need to identify the specific tookit and know the names for various elements and their properties that are used by the toolkit. Harmonia, the software company where UIML was developed, has published it as an open source language and plans to submit it to a standards organization after comments have been received on version 2.0.
Ultra DMA Ultra DMA (UDMA, or, more accurately, Ultra DMA/33) is a protocol for transferring data between a hard disk drive through the computer’s data paths (or bus) to the computer’s random access memory (RAM). The Ultra DMA/33 protocol transfers data in burst mode at a rate of 33.3 MBps (megabytes per second), twice as fast as the previous Direct Memory Access (DMA) interface. Ultra DMA was developed as a proposed industry standard by the Quantum Corporation, makers of hard disk drives, and Intel, makers of chipsets that support computer bus technology. Ultra DMA support in your computer means that it will boot (start) and open new applications more quickly. It will also help users of graphics-intensive and other applications that require large amounts of access to data on the hard drive. Ultra DMA uses Cyclical Redundancy Checking (CRC), offering a new level of data protection. Because the Ultra DMA protocol is designed to work with legacy application PIO and DMA protocols, it can be added to many existing computers by installing an Ultra DMA/33 Peripheral Component Interconnect adapter card. Ultra DMA uses the same 40-pin Integrated Drive Electronics interface cable as PIO and DMA.
ultra wideband radio Ultra wideband radio (also known as digital pulse wireless) is a revolutionary wireless technology for transmitting large amounts of digital data over a wide spectrum of frequency bands with very low power. Ultra wideband radio not only
can carry a huge amount of data over a short distance (up to 230 feet) at very low power (less than 0.5 milliwatts), but has the ability to carry signals through doors and other obstacles that tend to reflect signals at more limited bandwidths and a higher power. Ultra wideband radio can be compared with another short-distance wireless technology, Bluetooth, which is a proposed standard for connecting handheld wireless devices with other similar devices and with desktop computers. Ultra wideband radio broadcasts digital pulses that are timed very precisely on a signal across a very wide spectrum (number of frequency channels) at the same time. Transmitter and receiver must be coordinated to send and receive pulses with an accuracy of trillionths of a second. On any given frequency band that may already be in use, the ultra wideband signal has less power than the normal and anticipated background noise so theoretically no interference is possible. Time Domain, a company applying to use the technology, uses a microchip manufactured by IBM to transmit 1.25 million bits per second, but says there is the potential for a data rate in the billions of bits per second.
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Ultra wideband has two main types of application:
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1) Radar-type applications, in which the signal penetrates nearby surfaces but reflects surfaces that are farther away, allowing objects to be detected behind walls or other coverings.
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2) Voice and data transmission using digital pulses, allowing a very low powered and relatively low cost signal to carry information at very high rates within a restricted range.
UML
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UMTS UMTS (Universal Mobile Telecommunications Service) is a so-called ‘‘third-generation (3G),’’ broadband, packet-based transmission of text, digitized voice, video, and multimedia at data rates up to 2 megabits per second (Mbps) that will offer a consistent set of services to mobile computer and phone users no matter where they are located in the world. Based on the Global System for Mobile (GSM) communication standard, UMTS, endorsed by major standards bodies and manufacturers, is the planned standard for mobile users around the world by 2002. Once UTMS is fully implemented, computer and phone users can be constantly attached to the Internet as they travel and, as they roaming service, have the same set of capabilities no matter where they travel to. Users will have access through a combination of terrestrial wireless and satellite transmissions. Until UMTS is fully implemented, users can have multi-mode devices that switch to the currently available technology (such as GSM 900 and 1800) where UMTS is not yet available.
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Today’s cellular telephone systems are mainly circuitswitched, with connections always dependent on circuit availability. Packet-switched connections, using the Internet Protocol (IP), means that a virtual connection is always available to any other end point in the network. It will also make it possible to provide new services, such as alternative billing methods (pay-per-bit, pay-per-session, flat rate, asymmetric bandwidth, and others). The higher bandwidth of UMTS also promises new services, such as video conferencing. UMTS promises to realize the Virtual Home Environment (VHE) in which a roaming user can have the same services to which the user is accustomed when at home or in the office, through a combination of transparent terrestrial and satellite connections. Trials of UMTS technology, using advanced mobile phone/ computing device prototypes, have been conducted by Nortel Networks and BT (British Telecommunications). The electromagnetic radiation spectrum for UMTS has been identified as frequency bands 1885-2025 MHz) for future IMT-2000 systems, and 1980-2010 MHz and 2170-2200 MHz for the satellite portion of UMTS systems.
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In a network, the UNC (Universal Naming Convention) is a way to identify a shared file without having to specify or know the network storage drive it is on. In Windows operating systems, Novell NetWare, and possibly other operating systems, the UNC can be used instead of the local naming system (such as the DOS naming system in Windows). In Windows operating systems, the UNC name format is: \\servername\sharename\path\filename
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The share name is sometimes said to logically identify the volume or storage device that the file is on, but the idea is to free the user from having to know this. The path is zero or more folder or subfolder names (in other words, the file name may exist directly under the sharename). For example: \\corp1\lawdeptforms\patentap.html
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might specify on a server in the corporate main office a shared file (patentap.html) kept with other legal forms that members of a corporation’s legal department might download and read or print and use. Printers and other devices can also be addressed using UNC.
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Uncompressing (or decompressing) is the act of expanding a compression file back into its original form. Software that you download from the Internet often comes in a compressed package that can uncompress itself when you click on it. You can also uncompress files using popular tools such as PKZIP in the DOS operating system, WinZip in Windows, and MacZip in Macintosh.
Undernet For terms frequently used in online keyboard chatting, see chat abbreviations/IRC/BBS. According to its home page, the Undernet is the largest network of Internet Relay Channels (IRCs) on the Internet. The Undernet interconnects users from over 35 countries for Internet chatting on about 45 servers located on three continents. At any one time, as many as 20,000 people may be chatting on one of several hundred Undernet channels. Users must download free IRC client software from one of a number of possible Web sites and then register as an Undernet organization member. They will also furnish personal Web page space for members.
unicast Unicast is communication between a single sender and a single receiver over a network. The term exists in contradistinction to multicast, communication between a single sender and multiple receivers, and anycast, communication between any sender and the nearest of a group of receivers in a network. An earlier term, point-topoint communication, is similar in meaning to unicast. The new Internet Protocol version 6 (IPv6) supports unicast as well as anycast and multicast.
Unicode Unicode is an entirely new idea in setting up binary codes for text or script characters. Officially called the Unicode Worldwide Character Standard, it is a system for ‘‘the interchange, processing, and display of the written texts of the diverse languages of the modern world.’’ It also supports many classical and historical texts in a number of languages. Currently, the Unicode standard contains 34,168 distinct coded characters derived from 24 supported language scripts. These characters cover the principal written languages of the world. Additional work is underway to add the few modern languages not yet included. Also see the currently most prevalent script or text codes, ASCII and extended binary-coded decimal interchange code (EBCDIC).
unified field theory Unified field theory is sometimes called the Theory of Everything (TOE, for short): The long-sought means of tying together all known phenomena to explain the nature and behavior of all matter and energy in existence. In physics, a field refers to an area under the influence of some force, such as gravity or electricity, for example. A unified field theory would reconcile seemingly incompatible aspects of various field theories to create a single comprehensive set of equations. Such a theory could potentially unlock all the
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secrets of nature and make a myriad of wonders possible, including such benefits as time travel and an inexhaustible source of clean energy, among many others. According to Michio Katu, a theoretical physicist at City College, City University of New York, those in pursuit of a unified field theory seek ‘‘an equation an inch long that would allow us to read the mind of God.’’
Unified Modeling Language
James Clerk Maxwell proposed the first field theory, for electromagnetism, in the middle of the 1800s. Early in the 20th century, Albert Einstein’s general theory of relativity—dealing with gravitation—became the second field theory. The term unified field theory was coined by Einstein, who was attempting to prove that electromagnetism and gravity were different manifestations of a single fundamental field. When quantum theory entered the picture, the puzzle became more complex. The theory of relativity explains the nature and behavior of all phenomena on the macroscopic level (things that are visible to the naked eye); quantum theory explains the nature and behavior of all phenomena on the microscopic (atomic and subatomic) level. Perplexingly, however, the two theories are incompatible. Unconvinced that nature would prescribe totally different modes of behavior for phenomena that were simply scaled differently, Einstein sought a theory that would reconcile the two apparently irreconcilable theories that form the basis of modern physics.
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Grady Booch’s methodology for describing a set of objects and their relationships
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James Rumbaugh’s Object-Modeling Technique (OMT)
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Ivar Jacobson’s approach which includes a use case methodology
Although electromagnetism and the strong and weak nuclear forces have long been explained by a single theory known as the standard model, gravitation does not fit into the equation. The current quest for a unified field theory (sometimes called the holy grail of physicists) is largely focused on superstring theory and, in particular, on an adaptation known as M-theory.
unified messaging Unified messaging (sometimes referred to as the unified messaging system or UMS) is the handling of voice, fax, and regular text messages as objects in a single mailbox that a user can access either with a regular e-mail client or by telephone. The PC user can open and play back voice messages, assuming their PC has multimedia capabilities. Fax images can be saved or printed.
UML (Unified Modeling Language) is a standard notation for the modeling of real-world objects as a first step in developing an object-oriented design methodology. Its notation is derived from and unifies the notations of three object-oriented design and analysis methodologies:
Other ideas also contributed to UML, which was the result of a work effort by Booch, Rumbaugh, Jacobson, and others to combine their ideas, working under the sponsorship of Rational Software. UML has been fostered and now is an accepted standard of the Object Management Group (OMG), which is also the home of Common Object Request Broker Architecture, the leading industry standard for distributed object programming. Vendors of computeraided software engineering products are now supporting UML and it has been endorsed by almost every maker of software development products , including IBM and Microsoft (for its Visual Basic environment). Martin Fowler, in his book UML Distilled, observes that, although UML is a notation system so that everyone can communicate about a model, it’s developed from methodologies that also describe the processes in developing and using the model. While there is no one accepted process, the contributors to UML all describe somewhat similar approaches and these are usually described along with tutorials about UML itself. Among the concepts of modeling that UML specifies how to describe are: Class (of objects), object, association, responsibility, activity, interface, use case, package, sequence, collaboration, and state. Fowler’s book provides a good introduction to UML. Booch, Rumbaugh, and Jacobson all have or soon will have published the ‘‘offficial’’ set of books on UML.
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A user can access the same mailbox by telephone. In this case, ordinary e-mail notes in text are converted into audio files and played back.
See ‘‘URI’’
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Unified messaging is particularly convenient for mobile business users because it allows them to reach colleagues and customers through a PC or telephone, whichever happens to be available. Some services offer worldwide telephone access.
See ‘‘URL’’
Uniform Resource Locator Uniform Resource Name A URN (Uniform Resource Name) is an Internet resource with a name that has persistent significance—that is, the user of the URN can expect that someone else (or a program) will be able to find the resource. A URN looks something like a Web page address or Uniform Resource Locator (URL). For example, here’s a hypothetical URN:
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Whatis?Com’s Encyclopedia of Technology Terms urn:def://blue_laser
where ‘‘def://’’ might indicate an agency or an accessible directory of all dictionaries, glossaries, and encyclopedias on the Internet and ‘‘blue laser’’ was the name of a term. The result of using the agency could be the ‘‘best definition,’’ the ‘‘longest definition,’’ or even all definitions that the agency could find of ‘‘blue laser.’’
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A comparable URL would need to specify one specific location for a definition such as:
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http://www.whatis.com/bluelase.htm
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In this case, the user has to know where the resource is located as well as how to spell the file name and suffix. With a URN, the user only needs to know the name of a resource. One or more agencies will presumably be able to locate the nearest copy of the resource and the user is freed from understanding where resources are located or relocated to.
The illustration shows a unipolar signal as it might appear on the screen of an oscilloscope. Each horizontal division represents one bit (binary digit). The logic 0 state is approximately 0 volts and logic 1 is approximately +5 volts. (There is some room for error.) This is positive logic. Alternatively, logic 0 might be approximately +5 volts, and logic 1 might be approximately 0 volts; this would be negative logic. The bandwidth of a unipolar signal is inversely proportional to the duration of each data bit. Typical data speeds in baseband are several megabits per second (Mbps); hence the duration of each bit is a fraction of a microsecond. Voltage +6
Both URN and URL are types of a concept called the Uniform Resource Identifier (URI). A URN is associated with another concept called Uniform Resource Characteristics (URC), which allows descriptive information to be associated with a URN, such as author, date, length, and so forth.
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uninterruptible power supply
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An uninterruptible power supply (UPS) is a device that allows your computer to keep running for at least a short time when the primary power source is lost. It also provides protection from power surges. A UPS contains a battery that ‘‘kicks in’’ when the device senses a loss of power from the primary source. If you are using the computer when the UPS notifies you of the power loss, you have time to save any data you are working on and exit gracefully before the secondary power source (the battery) runs out. When all power runs out, any data in your computer’s random access memory (RAM) is erased. When power surges occur, a UPS intercepts the surge so that it doesn’t damage your computer.
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unipolar transmission See ‘‘unipolar signaling’’
universal access server See ‘‘network access server’’
Universal ADSL See ‘‘G.lite’’
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Software is available that automatically backs up (saves) any data that is being worked on when the UPS becomes activated.
Universal Asynchronous Receiver/ Transmitter
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See ‘‘UART’’
See also bipolar signaling.
Universal Coordinated Time
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Unipolar signaling, also called unipolar transmission, is a baseband method of sending binary data over wire or cable. There are two logic states, low and high, represented by the digits 0 and 1 respectively.
See ‘‘Coordinated Universal Time’’
Universal Serial Bus
Universal Data Access Universal Data Access (UDA) is Microsoft’s model or framework for a single uniform application program interface to different software makers’ databases, both relational and nonrelational. UDA is the database access part of Microsoft’s Component Object Model (COM), an overall framework for creating and distributing objectoriented programming programs in a network. UDA consists mainly of the high-level interface, ActiveX Data Objects (ADO) and the lower-level services called OLE DB. IBM, Oracle, and other companies have provided database bridges that interface with OLE DB.
Universal Description, Discovery, and Integration See ‘‘UDDI’’
Universal Disk Format Universal Disk Format (UDF) is a CD-ROM and DVD file system standard developed as a means of ensuring consistency among data written to various optical media, by facilitating both data interchange and the implementation of the ISO/IEC 13346 standard. UDF is required for DVDROMs, and is used by DVD to contain MPEG audio/video streams. Originally developed as a replacement for the file system specifications in the original CD-ROM standard, ISO 9660, UDF is used by CD-R and CD-RW in a process called packet writing that makes CD writing more efficient in terms of the time and disk space required. The Optical Storage Technology Association (OSTA) defines the UDF specification as a subset of ISO/IEC 13346, which it promotes as a single file system that overcomes limitations of ISO 9660 and redirectors such as CDFS. UDF is used to ensure compatibility across platforms, as well as among various CD and DVD applications.
Universal Mobile Telecommunications Service
switching (although the same network also supports connectionless traffic and packet-switching for data). The video broadcast and cable TV networks deliver mainly the higher-bandwidth continous-flow traffic of streaming video and sound.
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Universal Plug and Play
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See also Plug and Play, an earlier, proprietary Microsoft approach.
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Universal Plug and Play (UPnP) is a standard that uses Internet and Web protocols to enable devices such as PCs, peripherals, intelligent appliances, and wireless devices to be plugged into a network and automatically know about each other. With UPnP, when a user plugs a device into the network, the device will configure itself, acquire a TCP/IP address, and use a discovery protocol based on the Internet’s Hypertext Transfer Protocol (HTTP) to announce its presence on the network to other devices. For instance, if you had a camera and a printer connected to the network and needed to print out a photograph, you could press a button on the camera and have the camera send a ‘‘discover’’ request asking if there were any printers on the network. The printer would identify itself and send its location in the form of a universal resource locator (URL). The camera and printer would use Extensible Markup Language (XML) to establish a common language, or ‘‘protocol negotiation,’’ to talk to each other and determine capabilities. Once a common language was established, the camera would control the printer and print the photograph you selected. Microsoft, one of 29 companies sponsoring UPnP, hopes that UPnP will make it as easy to plug a device or appliance into a home or small business data network as it is to plug a lamp into an electrical outlet.
See ‘‘UNC’’
universal network
Universal Serial Bus
The ‘‘universal network’’ is the idea of a single network that integrates the existing voice and public telecommunications network (including the Internet), cable TV, data networks, and video broadcast networks so that they work together well. Currently, each has a different kind of traffic and the older networks bear the burden of an out-of-date infrastructure. For example, the public voice network supports connections of phone-call duration and circuit-
Universal Serial Bus (USB) is a plug-and-play interface between a computer and add-on devices (such as audio players, joysticks, keyboards, telephones, scanners, and printers). With USB, a new device can be added to your computer without having to add an adapter card or even having to turn the computer off. The USB peripheral bus standard was developed by Compaq, IBM, DEC, Intel,
Universal Naming Convention
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Juniper Networks planned something close to a universal network in a fiber-optic network that included very fast (multigigabit) switches using microchips customized for Internet traffic. The microchips included application-specific (ASIC) circuits made by IBM. The switches used wavedivision multiplexing (WDM).
Universal Plug and Play is an open industry standard that Microsoft, a leading promoter of the standard, describes as ‘‘seamless proximity networking’’ that provides ‘‘standardization on the wire rather than in the devices,’’ using existing Internet standards.
See ‘‘UMTS’’
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Microsoft, NEC, and Northern Telecom and the technology is available without charge for all computer and device vendors. USB supports a data speed of 12 megabits per second. This speed will accommodate a wide range of devices, including MPEG video devices, data gloves, and digitizers. It is anticipated that USB will easily accommodate plug-in telephones that use ISDN and digital PBX. Since October, 1996, the Windows operating systems have been equipped with USB drivers or special software designed to work with specific I/O device types. USB is integrated into Windows 98 and later versions. Today, most new computers and peripheral devices are equipped with USB. A different plug-and-play standard, IEEE 1394, supports much higher data rates and devices such as video camcorders and digital video disk (DVD) players. However, USB and IEEE 1394 serve different device types.
Universal Time Coordinated
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UNIX (sometimes spelled ‘‘Unix’’) is an operating system that originated at Bell Labs in 1969 as an interactive timesharing system. Ken Thompson and Dennis Ritchie are considered the inventors of UNIX. The name (pronounced YEW-nihks) was a pun based on an earlier system, Multics. In 1974, UNIX became the first operating system written in the C language. UNIX has evolved as a kind of large freeware product, with many extensions and new ideas provided in a variety of versions of UNIX by different companies, universities, and individuals. Partly because it was not a proprietary operating system owned by any one of the leading computer companies and partly because it is written in a standard language and embraced many popular ideas, UNIX became the first open or standard operating system that could be improved or enhanced by anyone. A composite of the C language and shell (user command) interfaces from different versions of UNIX were standardized under the auspices of the Institute of Electrical and Electronics Engineers as the Portable Operating System Interface (POSIX). In turn, the POSIX interfaces were specified in the X/Open Programming Guide 4.2 (also known as the ‘‘Single UNIX Specification’’ and ‘‘UNIX 95’’). Version 2 of the Single UNIX Specification is also known as UNIX 98. The ‘‘official’’ trademarked UNIX is now owned by the The Open Group, an industry standards organization, which certifies and brands UNIX implementations. UNIX operating systems are used in widely-sold workstation products from Sun Microsystems, Silicon Graphics, IBM, and a number of other companies. The UNIX environment and the client/server program model were
important elements in the development of the Internet and the reshaping of computing as centered in networks rather than in individual computers. Linux, a UNIX derivative available in both ‘‘free software’’ and commercial versions, is increasing in popularity as an alternative to proprietary operating systems.
unshielded twisted pair Unshielded twisted pair is the most common kind of copper telephone wiring. Twisted pair is the ordinary copper wire that connects home and many business computers to the telephone company. To reduce crosstalk or electromagnetic induction between pairs of wires, two insulated copper wires are twisted around each other. Each signal on twisted pair requires both wires. Since some telephone sets or desktop locations require multiple connections, twisted pair is sometimes installed in two or more pairs, all within a single cable. For some business locations, twisted pair is enclosed in a shield that functions as a ground. This is known as shielded twisted pair (STP). Twisted pair is now frequently installed with two pairs to the home, with the extra pair making it possible for you to add another line (perhaps for modem use) when you need it. Twisted pair comes with each pair uniquely color coded when it is packaged in multiple pairs. Different uses such as analog, digital, and Ethernet require different pair multiples. Although twisted pair is often associated with home use, a higher grade of twisted pair is often used for horizontal wiring in LAN installations because it is less expensive than coaxial cable. The wire you buy at a local hardware store for extensions from your phone or computer modem to a wall jack is not twisted pair. It is a side-by-side wire known as silver satin. The wall jack can have as many five kinds of hole arrangements or pinouts, depending on the kinds of wire the installation expects will be plugged in (for example, digital, analog, or LAN). (That’s why you may sometimes find when you carry your notebook computer to another location that the wall jack connections won’t match your plug.)
unzipping Unzipping is the act of extracting the files from a zipped single file or similar file archive. If the files in the package were also compressed (as they usually are), unzipping also uncompresses them. When you download programs from shareware or freeware companies on the Web, they almost always send you a zip file (the file name has a suffix of ‘‘.zip’’). You can usually extract the files from a zipped file just by double-clicking on it since many archives include a ‘‘self-extracting’’ program. Several popular tools exist for zipping and unzipping: PKZIP in the DOS operating system, WinZip in Windows, and MacZip in Macintosh.
URL
uplink See ‘‘downlink and uplink’’
uploading Uploading is the transmission of a file from one computer system to another, usually larger computer system. From a network user’s point-of-view, to upload a file is to send it to another computer that is set up to receive it. People who share images with others on bulletin board services (BBS) upload files to the BBS. Downloading is transmission in the other direction: from one, usually larger computer to another, usually smaller computer. From an Internet user’s point-of-view, downloading is receiving a file from another computer. The File Transfer Protocol (FTP) is the Internet facility for downloading and uploading files. (If you are uploading a file to another site, you must usually have permission in advance to access the site and the directory where the file is to be placed.) When you send or receive an attached file with an e-mail note, this is just an attachment, not a download or an upload. However, in practice, many people use ‘‘upload’’ to mean ‘‘send’’ and ‘‘download’’ to mean receive. The term is used loosely in practice and if someone says to you ‘‘Download (or upload) such–and-such a file to me’’ via e-mail, they simply mean ‘‘Send it to me.’’ In short, from the ordinary workstation or small computer user’s point-of-view, to upload is to send a file and to download is to receive a file.
UPnP See ‘‘Universal Plug and Play’’
UPS See ‘‘uninterruptible power supply’’
upstream See ‘‘downstream’’
urban legend An urban legend is a tale of contemporary folklore that purports to be true and is often designed to elicit an emotional response from the audience. On the Internet, urban legends are spread and survive over time through repeated e-mail forwardings and Web site postings. Like traditional legends, urban legends can seldom be traced to a single author. An urban legend often revolves around a strong central character or social issue of contemporary interest. Sociologists suggest that the Internet has proven to be a rich environment for urban legends because the Web allows people to share stories with each other faster than any
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other medium in history. Occasionally urban legends turn out to be true, but the majority are simply Internet misinformation, sometimes entertaining, but more often annoying, as in the case of the rather common virus hoax.
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URI
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To paraphrase the World Wide Web Consortium, Internet space is inhabited by many points of content. A URI (Uniform Resource Identifier; pronounced YEW-AHR-EYE) is the way you identify any of those points of content, whether it be a page of text, a video or sound clip, a still or animated image, or a program. The most common form of URI is the Web page address, which is a particular form or subset of URI called a Uniform Resource Locator (URL). A URI typically describes:
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The mechanism used to access the resource
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The specific computer that the resource is housed in
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The specific name of the resource (a file name) on the computer
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For example, this URI: http://www.w3.org/Icons/WWW/w3c_main.gif
identifies a file that can be accessed using the Web protocol application, Hypertext Transfer Protocol, (’’http://’’) that is housed on a computer named ‘‘www.w3.org’’ (which can be mapped to a unique Internet address). In the computer’s directory structure, the file is located at ‘‘/Icons/WWW/ w3c_main.gif.’’ Character strings that identify File Transfer Protocol FTP addresses and e-mail addresses are also URIs (and, like the HTTP address, are also the specific subset of URI called a URL). Another kind of URI is the Uniform Resource Name (URN). A URN is a form of URI that has ‘‘institutional persistence,’’ which means that its exact location may change from time to time, but some agency will be able to find it.
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The URI rules of syntax, set forth in the Internet Engineering Task Force (IETF) Request for Comments 1630, apply for all Internet addresses. In Tim Berner-Lee’s original working document, URI stood for Universal Resource Identifier.
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A URL (Uniform Resource Locator) (pronounced YU-AHREHL or, in some quarters, UHRL) is the address of a file (resource) accessible on the Internet. The type of resource depends on the Internet application protocol. Using the World Wide Web’s protocol, the Hypertext Transfer Protocol (HTTP) , the resource can be an HTML page, an image file, a program such as a common gateway interface application or Java applet, or any other file supported by HTTP. The URL contains the name of the protocol required to access the resource, a domain name that identifies a specific computer on the Internet, and a hierarchical description of a file location on the computer.
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On the Web (which uses the Hypertext Transfer Protocol), an example of a URL is:
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http://www.mhrcc.org/kingston
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which describes a Web page to be accessed with an HTTP (Web browser) application that is located on a computer named www.mhrcc.org. The specific file is in the directory named /kingston and is the default page in that directory (which, on this computer, happens to be named index.html).
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An HTTP URL can be for any Web page, not just a home page, or any individual file.
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A URL for a program such as a forms-handling common gateway interface script written in PERL might look like this: http://whatis.com/cgi-bin/comments.pl
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A URL for a file meant to be downloaded would require that the ‘‘ftp’’ protocol be specified like this one: ftp://www.somecompany.com/whitepapers/widgets.ps
conjunction with user-centered design, a method by which a product is designed according to the needs and specifications of users. Within the last several years, the usability of Web sites has become a hot topic for Web developers. Many major Web sites employ usability engineers to ensure that they have an easy-to-use, friendly site that provides a positive customer experience. Major sites such as Drugstore.com and Altavista.com have recently been re-designed based on recommendations made during usability tests and evaluations.
USB See ‘‘Universal Serial Bus’’
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URL-minder is an agent or robot program (bot) that notifies you when a particular Web page has changed. At a URLminder site, you specify the Uniform Resource Locator (URL) for the Web page and the URL-minder periodically checks the page, notices whether it has been updated, and then sends you an e-mail message when it has.
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A use case is a methodology used in system analysis to identify, clarify, and organize system requirements. The use case is made up of a set of possible sequences of interactions between systems and users in a particular environment and related to a particular goal. It consists of a group of elements (for example, classes and interfaces) that can be used together in a way that will have an effect larger than the sum of the separate elements combined. The use case should contain all system activities that have significance to the users. A use case can be thought of as a collection of possible scenarios related to a particular goal, indeed, the use case and goal are sometimes considered to be synonymous.
See ‘‘Uniform Resource Name’’
A use case (or set of use cases) has these characteristics:
A URL is a type of URI (Uniform Resource Identifier).
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Organizes functional requirements
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Models the goals of system/actor (user) interactions
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Also see human-computer interaction and graphical user interface.
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Records paths (called scenarios) from trigger events to goals
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Usability is the measure of a product’s potential to accomplish the goals of the user. In information technology, the term is often used in relation to software applications and Web sites, but it can be used in relation to any product that is employed to accomplish a task (for example, a toaster, a car dashboard, or an alarm clock). Some factors used in determining product usability are ease-of-use, visual consistency, and a clear, defined process for evolution.
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Describes one main flow of events (also called a basic course of action), and possibly other ones, called exceptional flows of events (also called alternate courses of action)
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Is multi-level, so that one use case can use the functionality of another one.
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Usability testing is a method by which users of a product are asked to perform certain tasks in an effort to measure the product’s ease-of-use, task time, and the user’s perception of the experience. Usability testing can be done formally, in a usability lab with video cameras, or informally, with paper mock-ups of an application or Web site. Changes are made to the application or site based on the findings of the usability tests. Whether the test is formal or informal, usability test participants are encouraged to think aloud and voice their every opinion. Usability testing is best used in
Use cases can be employed during several stages of software development, such as planning system requirements, validating design, testing software, and creating an outline for online help and user manuals.
Usenet Usenet is a collection of user-submitted notes or messages on various subjects that are posted to servers on a worldwide network. Each subject collection of posted notes is known as a newsgroup. There are thousands of newsgroups and it is possible for you to form a new one. Most newsgroups are hosted on Internet-connected servers, but they can also be
UTP hosted from servers that are not part of the Internet. Usenet’s original protocol was UNIX-to-UNIX Copy (UUCP), but today the Network News Transfer Protocol (NNTP) is used. Most browsers, such as those from Netscape and Microsoft, provide Usenet support and access to any newsgroups that you select. On the Web, Google and other sites provide a subject-oriented directory as well as a search approach to newsgroups and help you register to participate in them. In addition, there are other newsgroup readers, such as Knews, that run as separate programs.
Roaming profiles are user profiles that are stored in the server. Each time the user logs on, their profile is requested and sent to whatever machine makes the request. This allows the user to move from machine to machine and still maintain a consistent personal working environment. Network administrators find roaming profiles to be especially beneficial in a work or learning environment when more than one user shares the same computer, or when a user moves from place-to-place during the course of a workday.
User Datagram Protocol
user session
See ‘‘UDP’’
Also see session, a different meaning.
User Interface Markup Language See ‘‘UIML’’
user interface Also see human-computer interaction.
In tabulating statistics for Web site usage, a user session (sometime referred to as a visit) is the presence of a user with a specific IP address who has not visited the site recently (typically, anytime within the past 30 minutes). The number of user sessions per day is one measure of how much traffic a Web site has. A user who visits a site at noon and then again at 3:30 pm would count as two user visits.
In information technology, the user interface (UI) is everything designed into an information device with which a human being may interact—including display screen, keyboard, mouse, light pen, the appearance of a desktop, illuminated characters, help messages, and how an application program or a Web site invites interaction and responds to it. In early computers, there was very little user interface except for a few buttons at an operator’s console. The user interface was largely in the form of punched card input and report output. Later, a user was provided the ability to interact with a computer online and the user interface was a nearly blank display screen with a command line, a keyboard, and a set of commands and computer responses that were exchanged. This command line interface led to one in which menus (list of choices written in text) predominated. And, finally, the graphical user interface (GUI) arrived, originating mainly in Xerox’s Palo Alto Research Center, adopted and enhanced by Apple Computer, and finally effectively standardized by Microsoft in its Windows operating systems.
In computers, a utility is a small program that provides an addition to the capabilities provided by the operating system. In some usages, a utility is a special and nonessential part of the operating system. The print ‘‘utility’’ that comes with the operating system is an example. It’s not absolutely required to run programs and, if it didn’t come with the operating system, you could perhaps add it. In other usages, a utility is an application that is very specialized and relatively limited in capability. A good example is a search-and-replace utility. Some operating systems provide a limited capability to do a search-andreplace for given character strings. You can add a much more capable search-and-replace utility that runs as an application program. However, compared to a word processor, a search-and-replace utility has limited capability.
user profile In Windows NT, a user profile is a record of user-specific data that define the user’s working environment. The record can include display settings, application settings, and network connections. What the user sees on his or her computer screen, as well as what files, applications and directories they have access to, is determined by how the network administrator has set up the user’s profile.
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Other measures of Web site traffic in a given time period are the number of hits (the number of individual files served), the number of pages served, the number of ad views, and the number of unique visitors.
The user interface can arguably include the total ‘‘user experience,’’ which may include the aesthetic appearance of the device, response time, and the content that is presented to the user within the context of the user interface.
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See ‘‘Coordinated Universal Time’’
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UTP See ‘‘unshielded twisted pair’’
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UUCP (UNIX-to-UNIX Copy Protocol) is a set of UNIX programs for copying (sending) files between different UNIX systems and for sending commands to be executed on another system. The main UUCP commands (each supported by a UUCP program) are:
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uucp, which requests the copying of a specific file to another specified system
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uux, which sends a UNIX command to another system where it is queued for execution
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uucico, which runs on a UNIX system as the program that carries out the copying and initiates execution of the commands that have been sent. Typically, this program is run at various times of day; meanwhile, the copy (uucp) and command (uux) requests are queued until the uucico program is run.
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uuxqt, which executes the commands sent by uux, usually after being started by the uucico program
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The uucico programs are the programs that actually communicate across a network. There are several network protocols (variations on packet size and error-checking) that can be used by uucico programs, depending on the kinds of carrier networks being used.
Uuencode Uuencode (also called Uuencode/Uudecode) is a popular utility for encoding and decoding files exchanged between users or systems in a network. It originated for use between users of UNIX systems (its name stood for ‘‘UNIX-to-UNIX encoding’’). However, it’s available or easily obtainable for use in all operating systems and most e-mail applications provide it as an encoding alternative, especially for e-mail attachments. If you’re sending e-mail with an attachment and you suspect your recipient(s) may not have a MIMEcompliant system (for example, an older PC or UNIX system), you may want to specify ‘‘Uuencode’’ for the attachment to an e-mail note. (In Eudora, when writing a note, look for the little box set to a default of ‘‘MIME’’ and change it to ‘‘Uuencode.’’) Basically, what Uuencode does is to translate or convert a file or e-mail attachment (it can be an image, a text file, or a program) from its binary or bit-stream representation into the 7-bit ASCII set of text characters. Text can be handled by older systems that may not handle binary files well and larger files can be more easily divided into multi-part transmissions. If your recipent is likely to be on an older Macintosh system, you may want to try sending the attachment in another format called BinHex. A popular program for encoding and decoding files in Uuencode format is WinZip, which is shareware that you can download from the Winzip Web site.
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V.22 – V.22bis – V.32 – V.32bis – V.32terbo – V.34 – V.34bis – V.35 – V.42 See ‘‘V.xx’’
V.90 V.90 is a standard, approved by the International Telecommunications Union (ITU-TS), for transmitting data downstream to modems at 56 Kbps (thousand bits per second). The V.90 standard was arrived at by combining the x2 technology from US Robotics (now part of 3Com) and the K56flex technology from Rockwell. Transmission upstream from a computer modem is slower than downstream (about 33 Kbps) since it requires digital-to-analog conversion. 56 Kbps transmission technologies exploit the fact that most telephone company offices are interconnected with digital lines. Assuming your Internet connection provider has a digital connection to its telephone company office, the downstream traffic from your local Internet access provider can use a new transmission technique on your regular twisted pair phone line that bypasses the usual digital-toanalog conversion. A V.90 modem doesn’t need to demodulate the downstream data. Instead, it decodes a stream of multi-bit voltage pulses generated as though the line was equipped for digital information. (Upstream data still requires digital-to-analog modulation.) Unlike Integrated Services Digital Network, the 56 Kbps technologies do not require any additional installation or extra charges from your local phone company. On the other hand, the maximum transmission speed of ISDN is twice that of V.90 at 128 Kbps. You also have the flexibility of combining digital and voice transmission on the same line.
V.xx The V Series Recommendations from the ITU-TS are summarized in the table below. They include the most commonly used modem standards and other telephone network standards. Prior to the ITU-T standards, the American Telephone and Telegraph Company and the Bell System offered its own standards (Bell 103 and Bell 212A) at very low transfer rates. Another set of standards, the Microcom Networking Protocol, or MNP Class 1 through Class 10 (there is no Class 8), has gained some currency, but the development of an international set of standards means these will most likely prevail and continue to be extended. (Some modems offer both MNP and ITU-T standards.)
Beginning with V.22bis, ITU-T transfer rates increase in 2400 bps multiples. (Bis refers to a ‘‘second version.’’ Terbo refers to a ‘‘third version.’’) Standard
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Provides 1200 bits per second at 600 baud (state changes per second) The first true world standard, it allows 2400 bits per second at 600 baud Provides 4800 and 9600 bits per second at 2400 baud Provides 14,400 bits per second or fallback to 12,000, 9600, 7200, and 4800 bits per second Provides 19,200 bits per second or fallback to 12,000, 9600, 7200, and 4800 bits per second; can operate at higher data rates with compression; was not a CCITT/ITU standard Provides 28,800 bits per second or fallback to 24,000 and 19,200 bits per second and backwards compatility with V.32 and V.32bis Provides up to 33,600 bits per second or fallback to 31,200 or V.34 transfer rates The trunk interface between a network access device and a packet network at data rates greater than 19.2 Kbps. V.35 may use the bandwidths of several telephone circuits as a group. There are V.35 Gender Changers and Adapters. Same transfer rate as V.32, V.32bis, and other standards but with better error correction and therefore more reliable Provides up to 56,000 bits per second downstream (but in practice somewhat less). Derived from the x2 technology of 3Com (US Robotics) and Rockwell’s K56flex technology.
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vacuum tube
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Also see cathode ray tube (CRT), the specialized kind of vacuum tube that is in most desktop display monitors.
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A vacuum tube (also called an electron tube or, in the UK, a valve) is a device sometimes used to amplify electronic signals. In most applications, the vacuum tube is obsolete, having been replaced decades ago by the bipolar transistor and, more recently, by the field-effect transistor. However, tubes are still used in some high-power amplifiers, especially at microwave radio frequencies and in some hi-fi audio systems. Tubes operate at higher voltages than transistors. A typical transistorized amplifier needs 6 to 12 volts to function; an equivalent tube type amplifier needs 200 to 400 volts. At the highest power levels, some tube circuits have power supplies delivering several kilovolts. Vacuum tubes are making a comeback among audiophiles who insist that tubes deliver better audio quality than transistors. These old-fashioned components are more electrically rugged than their solid-state counterparts; a tube can often withstand temporary overload conditions and power-line transients that would instantly destroy a transistor.
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The major disadvantages of tubes include the fact that they require bulky power supplies, and the high voltages can present an electric shock hazard.
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See ‘‘voice activation detection’’
value chain According to John Del Vecchio writing for Fool.com, a value chain is ‘‘a string of companies working together to satisfy market demands.’’ The value chain typically consists of one or a few primary value (product or service) suppliers and many other suppliers that add on to the value that is ultimately presented to the buying public. Microsoft and its Windows operating systems, the nucleus of the personal computer desktop for which much business software is developed, is often cited as a prime example of a company and product that drives a value chain. The businesses who buy personal computer software may spend far more on the add-on software than on the essential operating system that is the de facto standard for running the software. To the extent that companies standardize on Windows, Microsoft is said to control a value chain. This particular value chain was reported in a McKinsey study to be worth $383 billion in 1998. Although Microsoft’s share of the value chain was reported to be only 4% of the total, that was still $15.3 billion.
A company that develops a product or service that engenders a value chain by providing a platform for other companies is considered more likely to increase its market share than a company that tries to provide the entire value chain on its own.
value-added network VAN is also an acronym for virtual area network. A value-added network (VAN) is a private network provider (sometimes called a turnkey communications line) that is hired by a company to facilitate electronic data interchange (EDI) or provide other network services. Before the arrival of the World Wide Web, some companies hired value-added networks to move data from their company to other companies. With the arrival of the World Wide Web, many companies found it more cost-efficient to move their data over the Internet instead of paying the minimum monthly fees and per-character charges found in typical VAN contracts. In response, contemporary value-added network providers now focus on offering EDI translation, encryption, secure e-mail, management reporting, and other extra services for their customers.
value-added reseller In the computer and other industries, a value-added reseller (VAR) is a company that takes an existing product, adds its own ‘‘value’’ usually in the form of a specific application for the product (for example, a special computer application), and resells it as a new product or ‘‘package.’’ For example, a VAR might take an operating system such as IBM’s OS/390 with UNIX services and, adding its own proprietary UNIX application designed for architects, resell the package to architectural firms. Depending on sales and installation requirements, the VAR could choose whether or not to identify OS/390 as part of the package. Also see original equipment manufacturer (OEM), a company that includes hardware components from other companies in its own product.
vampire tap A vampire tap is a connection to a coaxial cable in which a hole is drilled through the outer shield of the cable so that a clamp can be connected to the inner conductor of the cable. A vampire tap is used to connect each device to Thicknet coaxial cable in the bus topology of an Ethernet 10BASE-T local area network. A different connection approach, the BNC, is used for the thinner coaxial cable known as Thinnet.
VAN See ‘‘virtual area network’’
Vannevar Bush
van Eck phreaking Van Eck phreaking is a form of eavesdropping in which special equipment is used to pick up telecommunication signals or data within a computer device by monitoring and picking up the electromagnetic fields (EM-fields) that are produced by the signals or movement of the data. This electromagnetic radiation is present in, and with the proper equipment, can be captured from computer displays that use cathode ray tubes (CRTs), from printers, and from other devices. Here is an example: The image on a CRT is created by electron beams that scan across the screen in a series of horizontal lines from left-to-right and top-to-bottom, in the same way you read a page of text (except much faster). This occurs at a specific frequency for each individual monitor; there are only a few standard frequencies in existence, and every monitor uses one of them. The intensity of the electron beams determines the relative red, blue, and green brightness for each pixel (picture element) on the screen. As a result, the CRT produces a modulated EM field that contains all the information in the image displayed on the screen at any moment. This information looks like a meaningless, irregular waveform if viewed directly on an oscilloscope. But, like a television (TV) signal, it can be demodulated with special equipment, and the image on the screen thereby retrieved, from some distance away. This term combines the name of Wim van Eck, who in 1985 authored an academic paper that described this form of electronic eavesdropping, with the term phreaking, the earlier practice of using special equipment to make phone calls without paying. Van Eck phreaking is identified in the U.S. government project known as Tempest and, although some information remains classified, has probably been used to spy on suspected criminals and in espionage. The Tempest project has also led to advice and some standards development for how to shield devices so that eavesdropping is not possible. However, the cost of shielding means that many commercial devices are still vulnerable and, for this and other reasons, some of the details about what equipment is required to do van Eck phreaking remains classified. Susceptibility to eavesdropping can also be minimized by designing equipment that generates little EM energy. Depending on the type of CRT used, the sensitivity of the detection equipment, and the general level of EM energy in the area, Van Eck phreaking can be done over distances ranging from a few meters up to several hundred meters. Also see ELF.
vandal A vandal is an executable file, usually an applet or an ActiveX control, associated with a Web page that is designed to be harmful, malicious, or at the very least inconvenient to the user. Such applets or little application
programs can be embedded in any HTML file. They can also arrive as an e-mail attachment or automatically as the result of being pushed to the user. Vandals can be viewed as viruses that can arrive over the Internet stuck to a Web page. Vandals are sometimes referred to as ‘‘hostile applets.’’ Vandals can be harmful in two general ways: L
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They can get access to sensitive information within the computer system where they execute, such as passwords and encryption keys. They can cause loss or denial of service within the local computer system. For example, they can flood the system with data so that it runs out of memory, or they can slow down Internet connections.
The best way to protect yourself against a hostile applet is to know who you are downloading a Web page from or who has sent you an HTML page as an e-mail attachment. Major corporate Web sites or major Web publishers are unlikely to be the source of a vandal (but it can happen).
vanilla In information technology, vanilla (pronounced vah-NIHLuh) is an adjective meaning plain or basic. The unfeatured version of a product is sometimes referred to as the vanilla version. The term is based on the fact that vanilla is the most popular or at least the most commonly served flavor of ice cream. Or, as Eric Raymond, editor of The New Hacker’s Dictionary, puts it, the default ice cream. IBM’s BookMaster product, a text publishing system used in mainframe environments, provides a default way, called vanilla, to specify which parts of the book to publish, and another fancier way to specify it, called mocha.
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Some Web sites with frames call the simpler version of their site the vanilla version.
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An electrical engineer by training, Vannevar Bush is credited with having the idea of hypertext or ‘‘instant crossreferencing,’’ decades before the term itself was conceived by Ted Nelson and before the concept was widely implemented on the World Wide Web. In an article in The Atlantic Monthly in 1945, Bush described his proposed ‘‘memex’’ device, an electronic stored-information system, an aide-de-memoire for mankind. Humans, Bush argued, have associative memories where information is accessed by following a series of mental links and pathways. His hypertext system incorporated this idea, allowing users to tie information together logically as well as to annotate it and link the annotations to the body of data already present. The bulk of Bush’s career was spent at MIT’s Department of Electrical Engineering where he became Dean. His mathematical work for the military during World War Two led to his invention of the first analog computer, the Rockefeller Differential Analyser, soon rendered obsolete by
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the first digital computers (whose invention was spurred by the work of one of the RDA’s engineers—Claude Shannon). The Differential Analyser was notable for its use of decimal rather than the binary number system. As an advisor to several presidents, Bush was arguably the most powerful scientist in the post-war U.S. He was the driving force behind the founding of the National Science Foundation. In 1980, the National Science Board established the Vannevar Bush award to honor those scientists who make outstanding contributions to the public service. Bush’s primary legacy remains that of the hypertext concept—he is remembered as a forefather of the World Wide Web.
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Vaporware is software or hardware that is either (1) announced or mentioned publicly in order to influence customers to defer buying competitors’ products or (2) late being delivered for whatever reason. Most computer companies have from time to time delivered vaporware, either by calculation or unintentionally.
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Vaporware is also the name of a company that makes games and other products for Amiga users.
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In programming, a variable is a value that can change, depending on conditions or on information passed to the program. Typically, a program consists of instructions that tell the computer what to do and data that the program uses when it is running. The data consists of constants or fixed values that never change and variable values (which are usually initialized to ‘‘0’’ or some default value because the actual values will be supplied by a program’s user). Usually, both constants and variables are defined as certain data types. Each data type prescribes and limits the form of the data. Examples of data types include: An integer expressed as a decimal number, or a string of text characters, usually limited in length. In object-oriented programming, each object contains the data variables of the class it is an instance of. The object’s methods are designed to handle the actual values that are supplied to the object when the object is being used.
VAX VAX is an established line of mid-range server computers from the Digital Equipment Corporation (DEC), which is now part of Compaq. It followed DEC’s PDP-11 in 1978 and also introduced a new operating system, VMS. VAX included a 32-bit processor and virtual memory. Historically, VAX has competed with a number of Hewlett-Packard and IBM computers in the small enterprise and university-scientific marketplace. In earlier times, this size and price range of computer was known as the minicomputer.
DEC indicates that over 250,000 VAX systems are in use. Based on VMS, its operating system is called OpenVMS. Among other features, OpenVMS can be used with special software that facilitates its use with Windows NT servers. OpenVMS also supports the open standard Portable Operating System Interface set of commands and C language programming interface, effectively making it both a VMS system and a UNIX system. Eric S. Raymond notes that the VAX running a BSD UNIX system was a favorite of hackers for its especially large and programmer-friendly set of assembler instructions.
VBI See ‘‘vertical blanking interval’’
vBNS The vBNS (very high-speed Backbone Network Service) is a network that interconnects a number of supercomputer centers in the United States and is reserved for science applications requiring the massive computing that supercomputers can provide. Scientists at the supercomputer centers and other locations apply for time on the supercomputers and use of the vBNS by describing their projects to a committee that apportions computer time and vBNS resources. The vBNS and the supercomputer centers were initiated and are maintained by the National Science Foundation (NSF). The vBNS began operation in April, 1995, as the successor to the NSFNet. The NSFNet itself succeeded DARPANET, the original Internet network. The vBNS is the scientific portion of the Internet that NSF continues to fund. The physical infrastructure for the original Internet is now owned and maintained by the national commercial backbone companies in the United States and worldwide. Currently, MCI provides the backbone infrastructure for the vBNS under contract from the National Science Foundation. The backbone consists mainly of interconnected Optical Carrier levels (OCx) lines (operating at 155 Mbps or higher). The vBNS provides connections to the four national network access points (NAPs). The vBNS infrastructure itself is not shared with commercial companies and ordinary users. As part of the evolution toward a commercially selfsustained Internet, the National Science Foundation continues to operate the routing arbiter, a service that the NAPs and other routers use to route and reroute packets and optimize traffic flow on the Internet. The routing arbiter service is managed by Merit under a contract from the NSF that expires in July, 1999. The vBNS has recently become part of the infrastructure of Internet2. A new NFS-funded initiative is developing an advanced network infrastructure referred to as the National Technology Grid.
VCD
VBScript
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VBScript is an interpreted script language from Microsoft that is a subset of its Visual Basic programming language. VBScript can be compared to other script languages designed for the Web, including:
A vCard is an electronic business (or personal) card and also the name of an industry specification for the kind of communication exchange that is done on business or personal cards. You may have seen a vCard attached to an email note someone has sent you. Because vCard is a published industry specification, software application developers can create programs that process vCards by letting you view them, or drag-and-drop them to an address book or some other application. vCards can include images and sound as well as text.
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Sun Microsystem’s Tcl
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The UNIX-derived Perl
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In general, script languages are easier and faster to code in than the more structured, compiled languages such as C and C++ and are ideal for smaller programs of limited capability or that can reuse and tie together existing compiled programs. VBScript is Microsoft’s answer to Netscape’s popular JavaScript. Both are designed to work with an interpreter that comes with a Web browser—that is, at the user or client end of the Web client/server session. VBScript is designed for use with Microsoft’s Internet Explorer browser together with other programming that can be run at the client, including ActiveX controls, automation servers, and Java applets. Although Microsoft does support Netscape’s JavaScript (it converts it into its own JScript), Netscape does not support VBScript. For this reason, VBScript is best used for intranet Web sites that use the Internet Explorer browser only.
VC See ‘‘venture capital’’
vCalendar vCalendar is an industry standard format for exchanging scheduling and activity-recording information electronically. If someone sends you their week’s schedule in a vCalendar attachment to an e-mail note, you can drag-and drop it (or otherwise move it) to a personal information manager (PIM) type of application program and integrate with or relate it to your own schedule. vCalendar was developed along with the vCard electronic business card specification. Like vCard, vCalendar was developed by a consortium founded by Apple, AT&T, IBM, and Siemens. The specification was turned over to an industry group, the Internet Mail Consortium (IMC), in 1996. Like vCard, vCalendar requires that you have an application program that supports its use. In time, it’s expected that most personal information manager programs on all computer software platforms will support vCard and vCalendar as a standard exchange format.
vCard was developed by a consortium founded by Apple, AT&T, IBM, and Siemens, which turned the specification over to an industry group, the Internet Mail Consortium (IMC) in 1996. The vCard specification makes use of the ‘‘person’’ object defined by the CCITT X.500 Series Recommendation for Directory Services and can be considered an extension of it. A vCard contains a name, address information, date and time, and optionally photographs, company logos, sound clips, and geopositioning information. To open (look at) a vCard that someone has attached to an email note, your e-mail program needs to support vCards and not all such programs do yet. However, if you have an online address book or personal information manager that supports vCards, you can move it to that program for viewing or for addition to that program’s database. (If you can’t open a vCard you’ve received, remember that its information may be repeated elsewhere in the note. It’s basically just a business card.) A promising future use of a vCard will be as a way to quickly fill in application forms on the Web. Just drag-anddrop your own vCard to the form and you won’t have so many blanks to fill in. For software developers, there is a Personal Data Interchange (PDI) Software Development Kit (SDK). The specification is located at the Internet Mail Consortium’s Web site where you can also find out about vCalendar, a similar exchange standard for personal time scheduling.
VCD VCD (also called video CD or video compact disc) is a compact disc format based on CD-ROM XA that is specifically designed to hold MPEG-1 video data and to include interactive capabilities. VCD has a resolution similar to that of VHS, which is far short of the resolution of DVD. Each VCD disc holds 72774 minutes of video and has a data transfer rate of 1.44 Mbps. VCDs can be played on a VCD player connected to a television set (in the same way that video cassettes can on a VCR) or computer, on a CD-i player, on some CD-ROM drives, and some DVD players.
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VCD was introduced in 1993 by JVC, Philips, SONY and Matsushita and is described in detail in the White Book specifications. Video data is demanding in terms of storage capacity; it requires approximately 5 MB of storage per second of video, which would translate to about two minutes of video on a 680 MB CD. In order to store video information on a CD in a practical fashion, the data must be compressed for storage and then decompressed for replay in real time. MPEG-1 compresses data at ratios of up to 200:1. MPEG is an international standard, and can be used by any manufacturer to create hardware for use with MPEG video. MPEG video can also be recorded on any CD. VCD formatting removes unnecessary information from MPEG-1 data, and adds specialized video authoring capabilities through inclusion of a CD-i (CD- Interactive) runtime application. VCD variations include: VCD 2.0, which was introduced in 1995 and adds hi-resolution stills, fast-forward, and rewind functions to the original specifications; VCD-ROM, which was introduced in 1997 and enables the creation of hybrid VCD/CD-ROM disck; VCD-Internet, which was introduced in 1997 and is a standardized means of linking video and Internet data; and SuperVCD, which uses either high bit rate MPEG-1 or variable bit rate MPEG-2 for the use of CD-R drives instead of DVD drives. VCD is more common in Asia than it is in North America, where the VCR had already cornered the home video market by the time that VCD was introduced. Because of the ease with which VCDs can be pirated, they are creating the same kind of problem for the movie industry that MP3 caused for the music industry. Thieves may record movies from a movie screen with a camcorder, or may copy them from laserdiscs or DVDs. With the advent of recordable CDs (such as the CD-R and CD-RW), it became possible for the home user to create VCDs on some CD recorders.
VDT VDT (video display terminal, or sometimes visual display terminal) is a term used, especially in ergonomic studies, for the computer display. A display is a computer output surface and projecting mechanism that shows text and often graphic images to the computer user, using a cathode ray tube (CRT), liquid crystal display (LCD), light-emitting diode, gas plasma, or other image projection technology. The display is usually considered to include the screen or projection surface and the device that produces the information on the screen. In some computers, the display is packaged in a separate unit called a monitor. Major issues related to the VDT include the readability of content relative to other media such as paper; the effect of prolonged visual interaction with display content in terms of eye and other muscular fatigue and deterioration; proper viewing distances; and the amount and effect of exposure on users to the extremely low-frequency (ELF) radiation that emanates from VDTs.
vector graphics Vector graphics is the creation of digital images through a sequence of commands or mathematical statements that place lines and shapes in a given two-dimensional or threedimensional space. In physics, a vector is a representation of both a quantity and a direction at the same time. In vector graphics, the file that results from a graphic artist’s work is created and saved as a sequence of vector statements. For example, instead of containing a bit in the file for each bit of a line drawing, a vector graphic file describes a series of points to be connected. One result is a much smaller file. At some point, a vector image is converted into a raster graphics image, which maps bits directly to a display space (and is sometimes called a bitmap). The vector image can be converted to a raster image file prior to its display so that it can be ported between systems. A vector file is sometimes called a geometric file. Most images created with tools such as Adobe Illustrator and CorelDraw are in the form of vector image files. Vector image files are easier to modify than raster image files (which can, however, sometimes be reconverted to vector files for further refinement). Animation images are also usually created as vector files. For example, Shockwave’s Flash product lets you create 2-D and 3-D animations that are sent to a requestor as a vector file and then rasterized ‘‘on the fly’’ as they arrive.
vector A vector is a quantity or phenomenon that has two independent properties: magnitude and direction. The term also denotes the mathematical or geometrical representation of such a quantity. Examples of vectors in nature are velocity, momentum, force, electromagnetic fields, and weight. (Weight is the force produced by the acceleration of gravity acting on a mass.) A quantity or phenomenon that exhibits magnitude only, with no specific direction, is called a scalar. Examples of scalars include speed, mass, electrical resistance, and hard-drive storage capacity. Vectors can be depicted graphically in two or three dimensions. Magnitude is shown as the length of a line segment. Direction is shown by the orientation of the line segment, and by an arrow at one end. The illustration shows three vectors in two-dimensional rectangular coordinates (the Cartesian plane) and their equivalents in polar coordinates. Compare vector graphics. Also see scalar.
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velocity Velocity is a vector expression of the displacement that an object or particle undergoes with respect to time. The standard unit of velocity magnitude (also known as speed) is the meter per second (m/s or m ˙ s-1). Alternatively, the centimeter per second (cm/s or m ˙ s-1) can be used to express velocity magnitude. The direction of a velocity vector can be expressed in various ways, depending on the number of dimensions involved. Velocity is relative. Consider a car moving at 20 m/s with respect to the surface of a highway, traveling northward. If you are driving the car, the velocity of the car relative to your body is zero. If you stand by the side of the road, the velocity of the car relative to you is 20 m/s northward. If you are driving a car at 15 m/s with respect to the road and are traveling northward, and another car moving 20 m/s with respect to the road passes you in the same direction, that other car’s velocity relative to you is 5 m/s northward. But if that other car passes you going the opposite way on the road, its velocity relative to you is 35 m/s southward. Circular motion illustrates the fundamental difference between speed and velocity. Think of yourself whirling a ball, tied to a string, around your body so the ball’s tangential speed is 10 m/s. Even though the ball’s speed is constant relative to your body, its velocity relative to your body constantly changes as the direction vector describes circles. Velocity can be expressed either as an average over a period of time, or as an instantaneous value at a single moment in time. Suppose you are in a car that is not moving relative to
the road surface, and then you hit the accelerator and increase velocity uniformly from zero to 30 m/s northward in a few seconds. Your average velocity over that time frame is 15 m/s northward. However, the instantaneous velocity depends on time, and might be anything between zero and 30 m/s northward, depending on the exact moment at which it is measured.
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A vendor (pronounced VEHN-duhr, from French vendre, meaning to sell) is any person or company that sells goods or services to someone else in the economic production chain. Parts manufacturers are vendors of parts to other manufacturers that assemble the parts into something sold to wholesalers or retailers. Retailers are vendors of products to consumers. (When you have a street fair, the people who set up booths and tables are often referred to as vendors.) In information technology as well as in other industries, the term is commonly applied to suppliers of goods and services to other companies.
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A machine that takes your money and gives you a product is known as a vending machine.
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A Venn diagram is an illustration of the relationships between and among sets, groups of objects that share something in common. Usually, Venn diagrams are used to depict set intersections (denoted by an upside-down letter U). This type of diagram is used in scientific and engineering presentations, in theoretical mathematics, in computer applications, and in statistics. The drawing is an example of a Venn diagram that shows the relationship among three overlapping sets X, Y, and Z. The intersection relation is defined as the equivalent of the logic AND. An element is a member of the intersection of two sets if and only if that element is a member of both sets. Venn diagrams are generally drawn within a large rectangle that denotes the universe, the set of all elements under consideration. In this example, points that belong to none of the sets X, Y, or Z are white. Points belonging only to set X are a somewhat darker shade of gray in color; points belonging only to set Y are an even darker gray; points belonging only to set Z are very light gray. Points contained in all three sets are black. Here is a practical example of how a Venn diagram can illustrate a situation. Let the universe be the set of all computers in the world. Let X represent the set of all notebook computers in the world. Let Y represent the set of all computers in the world that are connected to the Internet. Let Z represent the set of all computers in the world that have anti-virus software installed. If you have a notebook computer and surf the Net, but you are not worried about viruses, your computer is probably represented by a point in
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the X and Y region. If you get concerned about computer viruses and install an anti-virus program, the point representing your computer will move into the black area.
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venture capital Venture capital (VC) is funding invested, or available for investment, in an enterprise that offers the probability of profit along with the possibility of loss. Indeed, venture capital was once known also as risk capital, but that term has fallen out of usage, probably because investors don’t like to see the words ‘‘risk’’ and ‘‘capital’’ in close conjunction. Venture capitalists often don’t tend to think that their investments involve an element of risk, but are assured a successful return by virtue of the investor’s knowledge and business sense. DataMerge, a financial information provider, says that VC investments in an enterprise are usually between $500,000 and $5 million, and that the investor is likely to expect an annual return of 20% to 50%. Venture capitalists were instrumental in the enormous increase in the number of dot-com startups of recent years. Because the Internet was a new and untried business venue with enormous potential, many analysts feel that standard business rules were too frequently suspended in what was a very optimistic market. Internet-based enterprises were expected to enjoy unprecedented success; many venture capitalists were said to have encouraged dot-coms to focus on scaling upward rather than on realizing early profits. According to VentureWire, U.S. venture capital funding for 2000 was $105 billion, more than the total funding available in all the 15 years before that. However, in April of that same year, severe market corrections brought about a radical change in the financial climate, and since then online businesses have been failing at rates similar to the rates of startups in the early days of the dot-com boom. Vulture capitalist, a term coined in the volatile financial environment
of the 1980s, has been revived to refer to the venture capitalists that have recently begun to buy up failing dotcom enterprises at rock-bottom prices. Venture capital is the second or third stage of a traditional startup financing sequence, which starts with the entrepreneurs putting their own available funding into a shoestring operation. Next, an angel investor may be convinced to contribute funding. Generally an angel investor is someone with spare funds and some personal or industryrelated interest—angels are sometimes said to invest ‘‘emotional money,’’ while venture capitalists are said to invest ‘‘logical money’’—that is willing to help give the new enterprise a more solid footing. First-round venture capital funding involves a significant cash outlay and managerial assistance. Second-round venture capital involves a larger cash outlay and instructions to a stock or initial public offering (IPO) underwriter, who will sell stock in exchange for a percentage of what is sold. Finally, in the IPO stage, an investment bank is commissioned to sell shares to the public. In the currently sober economic climate, a return to traditional business wisdom has meant that enterprises are generally expected to show a clear path to profitability if they want to attract investment funds.
VersaModular Eurocard bus See ‘‘VMEbus’’
vertical blanking interval The vertical blanking interval (VBI) is a portion of a television signal that can carry information other than video or audio, such as closed-caption text and stock market data. The interval in sending a video signal is required for the time it takes the electron gun in a television monitor’s cathode ray tube (CRT) to move back up to the top of the tube. VBI data can be inserted by a cable TV provider and transmitted to a special receiver that connects to a computer’s RS-232C port.
vertical market A vertical market is a particular industry or group of enterprises in which similar products or services are developed and marketed using similar methods (and to whom goods and services can be sold). Broad examples of vertical markets are: Insurance, real estate, banking, heavy manufacturing, retail, transportation, hospitals, and government. Vertical market software is software aimed at a particular vertical market and can be contrasted with horizontal market software (such as word processors and spreadsheet programs) that can be used in a cross-section of industries.
vertical portal See ‘‘vortal’’
VHF
vertical solutions provider On the Internet, a vertical solutions provider (VSP) is a company that focuses on delivering industry-specific application hosting services to customers in a specific vertical market such as manufacturing, health care, retailing, or financial service. A VSP is a type of application service provider (ASP) that enables a company in a particular vertical market to take advantage of Internet technologies and applications without having to acquire the know-how and other resources on its own. For example, a retailer could hire a VSP to configure and administer its shopping cart, inventory management and credit-card processing applications on a continuing basis, saving the company the need to hire, train, and pay its own administrators. Companies seeking VSP services may be independent dotcoms or other start-ups or they may be companies that are launching an e-commerce branch of their existing business. Examples of VSPs include Virtual Financial Services (ViFi) and OneChem.
very high-speed Backbone Network Service See ‘‘vBNS’’
very high frequency See ‘‘VHF’’
Very Large Scale Integration VLSI, the current level of computer microchip miniaturization, refers to microchips containing hundreds of thousands of transistors. LSI (large-scale integration) refers to microchips containing thousands of transistors. Earlier, MSI (medium scale integration) meant a microchip containing hundreds of transistors and SSI (small-scale integration) meant transistors in the tens.
and supports 32-bit data flow at 50 MHz. A VESA Local Bus is implemented by adding a supplemental slot and card that aligns with and augments an Industry Standard Architecture (ISA) expansion card. (ISA is the most common expansion slot in today’s computers.)
vestigial sideband Vestigial sideband (VSB) is a type of amplitude modulation (AM) technique (sometimes called VSB-AM) that encodes data by varying the amplitude of a single carrier frequency. Portions of one of the redundant sidebands are removed to form a vestigial sideband signal—so-called because a vestige of the sideband remains. In AM, the carrier itself does not fluctuate in amplitude. Instead, the modulating data appears in the form of signal components at frequencies slightly higher and lower than that of the carrier. These components are called sidebands. The lower sideband (LSB) appears at frequencies below the carrier frequency; the upper sideband (USB) appears at frequencies above the carrier frequency. The actual information is transmitted in the sidebands, rather than the carrier; both sidebands carry the same information. Because LSB and USB are essentially mirror images of each other, one can be discarded or used for a second channel or for diagnostic purposes. VSB transmission is similar to single-sideband (SSB) transmission, in which one of the sidebands is completely removed. In VSB transmission, however, the second sideband is not completely removed, but is filtered to remove all but the desired range of frequencies. Eight-level VSB (8-VSB) was developed by Zenith for inclusion in the Advanced Television Systems Committee (ATSC) set of digital television (DTV) standards.
VFAT See ‘‘Virtual File Allocation Table’’
very long instruction word
VGA
See ‘‘VLIW’’
See ‘‘display modes’’
Very Small Aperture Terminal
VHE
See ‘‘VSAT’’
See ‘‘Virtual Home Environment’’
VESA Local Bus
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VESA Local Bus (sometimes called the VESA VL bus) is a standard interface between your computer and its expansion slot that provides faster data flow between the devices controlled by the expansion cards and your computer’s microprocessor. A ‘‘local bus’’ is a physical path on which data flows at almost the speed of the microprocessor, increasing total system performance. VESA Local Bus is particularly effective in systems with advanced video cards
The VHF (very high frequency) range of the radio spectrum is the band extending from 30 MHz to 300 MHz. The wavelengths corresponding to these limit frequencies are 10 meters and 1 meter. In the VHF band, electromagnetic fields are affected by the earth’s ionosphere and troposphere. Ionospheric propagation occurs regularly in the lower part of the VHF spectrum, mostly at frequencies below 70 MHz. In this mode, the
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communication range can sometimes extend over the entire surface of the earth. The troposphere can cause bending, ducting, and scattering, extending the range of communication significantly beyond the visual horizon. Auroral, meteor-scatter, and EME (earth-moon-earth, also called moonbounce) propagation take place on occasion, but these modes do not offer reliable communication and are of interest primarily to amateur radio operators.
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vi, pronounced by using each letter (vee-aye), is a widelyused and popular UNIX-based text editor. Like most UNIX system interfaces and other text editors, it lets you control the system by using the keyboard rather than a combination of mouse selections and keystrokes. The succinctness of the interface makes it highly useful for people who work at a computer all day, especially programmers entering or manipulating language statements. vi was invented before and is a popular alternative to Emacs, which is said to be more capable, more complicated, and somewhat slower. Like Emacs, vi is most commonly used for entering and editing program language statements and for writing short notes including e-mail. vi usually comes as part of any UNIX-based system.
video adapter A video adapter (also called a display adapter or video board) is an integrated circuit card in a computer or, in some cases, a monitor that provides digital-to-analog conversion, video RAM, and a video controller so that data can be sent to a computer’s display. Today, almost all displays and video adapters adhere to a common denominator de facto standard, Video Graphics Array (VGA). VGA describes how data—essentially red, green, blue data streams—is passed between the computer and the display. It also describes the frame refresh rates in hertz. It also specifies the number and width of horizontal lines, which essentially amounts to specifying the resolution of the pixels that are created. VGA supports four different resolution settings and two related image refresh rates. In addition to VGA, most displays today adhere to one or more standards set by the Video Electronics Standards Association (VESA). VESA defines how software can determine what capabilities a display has. It also identifies resolutions setting beyond those of VGA. These resolutions include 800 by 600, 1024 by 768, 1280 by 1024, and 1600 by 1200 pixels.
Video CD See ‘‘VCD’’
video display unit A video display unit (VDU) consists of: L
A computer output device that uses a cathode ray tube or other technology to present visual images
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One or more input devices, such as a keyboard, a mouse, or both.
A synonym sometimes used in the United States is video display terminal (VDT). VDU seems to predominate elsewhere. The term is widely used in ergonomic discussions, guidelines, and standards related to worker safety and efficiency. The ‘‘classic’’ VDU is the desktop display and keyboard. However, as personal computers and workstations become more portable and smaller in size, the safety and design guidelines for VDUs call for continuing reconsideration.
video or visual display terminal See ‘‘VDT’’
video RAM Also see RAM types. Video RAM (VRAM) means in general all forms of random access memory (RAM) used to store image data for a computer display. All types of video RAM are special arrangements of dynamic RAM (DRAM). Video RAM is really a buffer between the computer processor and the display and is often called the frame buffer. When images are to be sent to the display, they are first read by the processor as data from some form of main (non-video) RAM and then written to video RAM. From video RAM (the frame buffer), the data is converted by a RAM digital-to-analog converter (RAMDAC) into analog signals that are sent to the display presentation mechanism such as a cathode ray tube (CRT). Usually, video RAM comes in a 1 or 2 megabyte package and is located on the video adapter card in the computer. Most forms of video RAM are dual-ported, which means that while the processor is writing a new image to video RAM, the display is reading from video RAM to refresh its current display content. The dual-port design is the main difference between main RAM and video RAM. Somewhat confusingly, the most common type of video RAM is called Video RAM (VRAM). Video RAM is the vanilla flavor of video RAM. It is dual-ported, allowing the processor to write to it at the same time that it is refreshing the image on the display monitor. Other forms of video RAM include:
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Synchronous Graphics RAM (SGRAM) is clocksynchronized RAM that is used for video memory. It is relatively low-cost video memory. It uses masked write, which enables selected data to be modified in a single operation rather than as a sequence of read, update, and write operations. It also uses block write, which allows data for background or foreground image fills to be handled more efficiently. SGRAM is single-ported. Its special features are what make it a moderately fast form of video memory. The Matrox Mystique is an example of a video card that uses SGRAM. Window RAM (WRAM), unrelated to Microsoft Windows, is very high-performance video RAM that is dual-ported and has about 25% more bandwidth than VRAM but costs less. It has features that make it more efficient to read data for use in block fills and text drawing. It can be used for very high resolution (such as 1600 by 1200 pixels) using true color. It’s used in the Matrox Millenium video card. Multibank Dynamic RAM (MDRAM) is a highperformance RAM, developed by MoSys, that divides memory into multiple 32 kilobytes parts or ‘‘banks’’ that can be accessed individually. Traditional video RAM is monolithic; the entire frame buffer is accessed at one time. Having individual memory banks allows accesses to be interleaved concurrently, increasing overall performance. It’s also cheaper since, unlike other forms of video RAM, cards can be manufactured with just the right amount of RAM for a given resolution capability instead of requiring it to be in multiples of megabytes. Rambus Dynamic RAM (DRAM) is a video RAM designed by Rambus that includes a proprietary bus that speeds up the data flow between video RAM and the frame buffer. It’s optimized for video streaming.
videoconference A videoconference is a live connection between people in separate locations for the purpose of communication, usually involving audio and often text as well as video. At its simplest, videoconferencing provides transmission of static images and text between two locations. At its most sophisticated, it provides transmission of full-motion video images and high-quality audio between multiple locations. Videoconferencing software is quickly becoming standard computer equipment. For example, Microsoft’s NetMeeting is included in Windows 2000 and is also available for free download from the NetMeeting homepage. For personal use, free or inexpensive videoconference software and a digital camera afford the user easy—and cheap—live connections to distant friends and family. Although the audio and video quality of such a minimal setup is not high, the combined benefits of a video link and long-distance savings may be quite persuasive.
The tangible benefits for businesses using videoconferencing include lower travel costs and profits gained from offering videoconferencing as an aspect of customer service. The intangible benefits include the facilitation of group work among geographically distant teammates and a stronger sense of community among business contacts, both within and between companies. In terms of group work, users can chat, transfer files, share programs, send and receive graphic data, and operate computers from remote locations. On a more personal level, the face-to-face connection adds nonverbal communication to the exchange and allows participants to develop a stronger sense of familiarity with individuals they may never actually meet in the same place.
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A videoconference can be thought of as a phone call with pictures—Microsoft refers to that aspect of its NetMeeting package as a ‘‘web phone’’—and indications suggest that videoconferencing will some day become the primary mode of distance communication.
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On the Internet, viral marketing is any marketing technique that induces Web sites or users to pass on a marketing message to other sites or users, creating a potentially exponential growth in the message’s visibility and effect. One example of successful viral marketing is Hotmail, a company, now owned by Microsoft, that promotes its service and its own advertisers’ messages in every user’s email notes. Sites that serve an immediate need when they are first launched seem to get the kind of viral marketing known as buzz marketing. Everyone simply tells everyone else. ICQ, a service that tells you when selected friends or colleagues are also online, is an example of a service that is viral by its nature. Its users want to tell their friends about it so that it will be more useful for them. Some marketing people prefer terms other than viral marketing. In his popular e-mail newsletter about selling on the Web, John Audette asked readers to suggest alternatives, including other terms in current use. Among those suggested have been: avalanche marketing buzz marketing cascading style marketing centrifugal marketing exponential marketing fission marketing grass roots marketing organic marketing propogation marketing referral marketing (borrowing a term long used in marketing prior to the Web) ripple marketing
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self-perpetuation marketing self-propogation marketing wildfire marketing One reader suggested word of mouse.
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virtual In general, virtual means the quality of effecting something without actually being that something. In information technology, there seems to be a virtual version of (virtually) everything. ’’Virtual’’ derives from the Latin ‘‘vir’’ (’’man’’ in an idealized sense), from which developed the Latin ‘‘virtus’’ (strength, manliness, virtue). In Middle English, the adjective meant ‘‘possessed of certain physical virtues.’’ By modern times, it had come to mean, as defined in Webster’s, ‘‘being in essence or effect but not in fact.’’
virtual area network
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A virtual area network (VAN) is a network on which users are enabled to share a more visual sense of community through high band-width connections. As conceived by PennWell Media Online, an online directory for specialized networking products, a virtual area network is something like a metropolitan area network (MAN) or extended local area network (LAN) in which all users can meet over highbandwidth connections, enabling ‘‘face-to-face’’ online ‘‘coffeehouses,’’ remote medical diagnosis and legal consultation, and online corporate or extracorporate workgroups, focus groups, and conferences. A VAN requires multi-megabyte data flow and can be implemented through the use of Asymmetric Digital Subscriber Line but more likely through the installation of cable modems. Since the high-bandwidth connections imply a common infrastructure, the first VANs are likely to be local or regional. However, a VAN can also be national or international in geographic scope, assuming all users share similar capabilities.
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A virtual circuit is a circuit or path between points in a network that appears to be a discrete, physical path but is actually a managed pool of circuit resources from which specific circuits are allocated as needed to meet traffic requirements. A permanent virtual circuit (PVC) is a virtual circuit that is permanently available to the user just as though it were a dedicated or leased line continuously reserved for that user. A switched virtual circuit (SVC) is a virtual circuit in which a connection session is set up for a user only for the duration of a connection. PVCs are an important feature of frame relay networks and SVCs are proposed for later inclusion.
virtual community A virtual community is a community of people sharing common interests, ideas, and feelings over the Internet or other collaborative networks. A possible inventor of this term and one of its first proponents was Howard Rheingold, who created one of the first major Internet communities, called ‘‘The Well.’’ In his book, The Virtual Community, Rheingold defines virtual communities as social aggregations that emerge from the Internet when enough people carry on public discussions long enough and with sufficient human feeling to form webs of personal relationships in cyberspace. Virtual communities might be thought of as subgroups within Marshall McLuhan’s notion of cyberspace as a ‘‘global village.’’ Before the Web, virtual communities existed on bulletin board services (BBS) and many still do. Some virtual communities or facilitators of them use the metaphor of a coffee house or something similar to help users visualize the community. In general, there are two kinds of communication among virtual community members: message postings and real-time chat. Usenet newsgroups are an example of the former. Many Web sites, such as Geocities, foster subject information exchanges. For real-time chat, Internet Relay Chat (IRC) is a system used by many Web sites that foster virtual communities.
virtual device driver In Windows, a virtual device driver is a program for each of the computer’s main hardware devices, including the hard disk drive controller, keyboard, and serial and parallel ports. They’re used to maintain the status of a hardware device that has changeable settings. Virtual device drivers handle software interrupts from the operating system rather than hardware interrupts. A virtual device driver usually has a file name suffix of VXD.
Virtual File Allocation Table Virtual File Allocation Table (VFAT) is the part of the Windows 95 and later operating systems that handles long file names, which otherwise could not be handled by the original file allocation table (FAT) programming. A file allocation table is the means by which the operating system keeps track of where the pieces of a file are stored on a hard disk. Since the original FAT for the Disk Operating System (DOS) assumed file names were limited to a length of eight characters, a program extension was needed to handle the longer names allowed in Windows 95. Microsoft refers to this extension as a driver since other operating systems may need to install and use it in order to access FAT partitions written by Windows 95 and later Windows systems. The VFAT extension runs in protected mode, uses 32-bit code, and uses VCACHE for disk cache.
virtual machine
Virtual Home Environment In mobile computing, the Virtual Home Environment (VHE) is the concept that a network supporting mobile users should provide them the same computing environment on the road that they have in their home or corporate computing environment. VHE is part of IMT-2000 and the Universal Mobile Telecommunications System (UMTS). With VHE, a network (referred to as a foreign network) emulates the behavior of the user’s home network and the user has the same services that they are accustomed to at home.
virtual hosting On the Internet, virtual hosting is the provision of Web server hosting services so that a company (or individual) doesn’t have to purchase and maintain its own Web server and connections to the Internet. A virtual hosting provider is sometimes called a Web or Internet ‘‘space provider.’’ Some companies providing this service simply call it ‘‘hosting.’’ Typically, virtual hosting provides a customer who wants a Web site with: Domain name registration assistance, multiple domain names that map to the registered domain name, an allocation of file storage and directory setup for the Web site files (HTML and graphic image files), e-mail addresses, and, optionally, Web site creation services. The virtual hosting user (the Web site owner) needs only to have a File Transfer Protocol (FTP) program for exchanging files with the virtual host. Some virtual hosting providers make it possible for customers to have more control of their Web site file system, e-mail names, passwords, and other resources and say that they are providing each customer a virtual server—that is, a server that appears to be entirely their own server. When a customer does indeed want to have its very own server, some hosting providers allow the customer to rent a dedicated server at the hosting provider’s location. If a customer is allowed to place their own purchased equipment at the provider’s location, this is known as colocation.
Virtual Instrument file vi (or ‘‘.vi‘‘) is the Virtual Instrument file format. It is used by National Instruments’ LabVIEW development software. LABView lets you develop applications for automating and controlling processes in manufacturing and other environments. National Instruments uses a concept of ‘‘virtual ‘‘instrumentation’’ to generalize and accelerate the development of this kind of application.
virtual LAN A virtual (or logical) LAN is a local area network with a definition that maps workstations on some other basis than geographic location (for example, by department, type of user, or primary application). The virtual LAN controller can
change or add workstations and manage loadbalancing and bandwidth allocation more easily than with a physical picture of the LAN. Network management software keeps track of relating the virtual picture of the local area network with the actual physical picture.
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VLANs are considered likely to be used with campus environment networks. Among companies likely to provide products with VLAN support are Cisco, Bay Networks, and 3Com.
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There is a proposed VLAN standard, Institute of Electrical and Electronics Engineers 802.10.
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1) Virtual machine is a term used by Sun Microsystems, developers of the Java programming language and runtime environment, to describe software that acts as an interface between compiler Java binary code and the microprocessor (or ‘‘hardware platform’’) that actually performs the program’s instructions. Once a Java virtual machine has been provided for a platform, any Java program (which, after compilation, is called bytecode) can run on that platform. Java was designed to allow application programs to be built that could be run on any platform without having to be rewritten or recompiled by the programmer for each separate platform. Java’s virtual machine makes this possible. The Java virtual machine specification defines an abstract rather than a real ‘‘machine’’ (or processor) and specifies an instruction set, a set of registers, a stack, a ‘‘garbage heap,’’ and a method area. The real implementation of this abstract or logically defined processor can be in other code that is recognized by the real processor or be built into the microchip processor itself. The output of ‘‘compiling’’ a Java source program (a set of Java language statements) is called bytecode. A Java virtual machine can either interpret the bytecode one instruction at a time (mapping it to a real microprocessor instruction) or the bytecode can be compiled further for the real microprocessor using what is called a just-in-time compiler. 2) At IBM, a virtual machine is any multi-user sharedresource operating system that gives each user the appearance of having sole control of all the resources of the system. 3) At IBM, a virtual machine is also used to mean an operating system that is in turn managed by an underlying control progam. Thus, IBM’s VM/ESA can control multiple virtual machines on an IBM S/390 system. 4) Elsewhere, virtual machine has been used to mean either an operating system or any program that runs a computer. We quote: ‘‘A running program is often referred to as a virtual machine—a machine that doesn’t exist as a matter of actual physical reality. The virtual machine idea is
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Whatis?Com’s Encyclopedia of Technology Terms itself one of the most elegant in the history of technology and is a crucial step in the evolution of ideas about software. To come up with it, scientists and technologists had to recognize that a computer running a program isn’t merely a washer doing laundry. A washer is a washer whatever clothes you put inside, but when you put a new program in a computer, it becomes a new machine.... The virtual machine: A way of understanding software that frees us to think of software design as machine design.’’ From David Gelernter’s ‘‘Truth, Beauty, and the Virtual Machine,’’ Discover Magazine, September 1997, p. 72.
Virtual Memory System
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Virtual Memory System (VMS) is an operating system from the Digital Equipment Corporation (DEC) that runs in its computers. VMS originated in 1979 as a new operating system for DEC’s new VAX computer, the successor to DEC’s PDP-11. VMS is a 32-bit system that exploits the concept of virtual memory. VMS was renamed OpenVMS when it was redeveloped for the Alpha processor. (OpenVMS is also the name now used on the VAX computer.) The ‘‘Open’’ suggests the added support for the UNIX-like interfaces of the Portable Operating System Interface (POSIX) standard. Programs written to the POSIX standard, which includes a set of standard C language programming functions, can be ported to any POSIX-supporting computer platform. Among other features, OpenVMS can be used with special software that facilitates its use with Windows NT servers.
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Virtual (or logical) memory is a concept that, when implemented by a computer and its operating system, allows programmers to use a very large range of memory or storage addresses for stored data. The computing system maps the programmer’s virtual addresses to real hardware storage addresses. Usually, the programmer is freed from having to be concerned about the availability of data storage. In addition to managing the mapping of virtual storage addresses to real storage addresses, a computer implementing virtual memory or storage also manages storage swapping between active storage (RAM) and hard disk or other high volume storage devices. Data is read in units called ‘‘pages’’ of sizes ranging from a thousand bytes (actually 1,024 decimal bytes) up to several megabyes in size. This reduces the amount of physical storage access that is required and speeds up overall system performance.
virtual microscopy Virtual microscopy is a method of posting microscope images on, and transmitting them over, computer networks. This allows independent viewing of images by large numbers of people in diverse locations. Classroom viewing of microscope slides has traditionally been a cumbersome business. It is possible to set up an optical microscope with projection apparatus, but this is an inflexible scheme at best. It requires operation by a professor or instructor, and students do not have individual control over what they see. It also requires that all the students be physically present in the same classroom at the same time. These limitations also apply to the sharing of images among teams of doctors. Although closed-circuit television networks have been used for this purpose, image resolution in these systems is notoriously poor. With the advent of computers and broadband Internet connections, microscope slide images can be digitized and posted online. Each student can use a computer to independently look at any image in a database that can contain thousands of slides. Zooming provides the equivalent of variable magnification. Using UP/DOWN/ RIGHT/LEFT arrow buttons, the viewer can move the center of the field of view at will. Students can view images when and where they choose. For example, if there are 50 students in a virtual class all looking at the same image at the same time, the students can independently choose 50 different placements and magnifications of the image. Reports by users indicate that this can actually be easier than using an optical microscope in a laboratory setting. One program used for virtual microscopy is ‘‘Virtual Slice’’ developed by MicroBrightField, Inc. of Colchester, Vermont. If recent tests are any indication, virtual microscopy will largely replace older instructional methods. In addition, the technology has applications in videoconferencing, and in the exchange of medical information among doctors and hospitals worldwide. This will improve the quality of patient care by increasing the quality and quantity of medical information that can be shared.
virtual organization A virtual organization or company is one whose members are geographically apart, usually working by computer email and groupware while appearing to others to be a single, unified organization with a real physical location.
virtual server
Virtual PC
Virtual Reality Modeling Language
Virtual PC is a program that emulates Windows 95, Windows 98, Windows NT, as well as IBM OS/2, or Linux on a Macintosh personal computer, assuming it’s equipped with a sufficiently fast microprocessor. With Virtual PC installed, a Mac can show the desktop for the emulated operating system on one part of the display or it can take up the entire screen. You can run any program that will run under the other operating systems on ‘‘regular’’ (Intel microprocessor-based) PCs. Virtual PC works by converting Intel x86 instructions into PowerPC instructions on the fly.
See ‘‘VRML’’
To install and use Virtual PC, the Mac must have the PowerPC 604e microprocessor, or a 603e version that runs at a minimum of 180 MHz. An extra 150 to 300 megabytes of hard disk storage is needed and you should have at least 24 megabytes of random access memory (RAM) with 32 megabytes recommended. Versions of Virtual PC are available that will emulate Windows 3.1 and DOS; these can run with a slower microprocessor and less storage. Virtual PC may be helpful for Mac users who need to run a Windows program occasionally or in families where some members want to run Mac applications and other members want to run Windows applications. According to a Wall Street Journal reviewer, the emulation appears to be complete, except that the Mac mouse, which has only one button, requires the user to press an additional key when a right-hand click is required.
virtual private network A virtual private network (VPN) is a private data network that makes use of the public telecommunication infrastructure, maintaining privacy through the use of a tunneling protocol and security procedures. A virtual private network can be contrasted with a system of owned or leased lines that can only be used by one company. The idea of the VPN is to give the company the same capabilities at much lower cost by using the shared public infrastructure rather than a private one. Phone companies have provided secure shared resources for voice messages. A virtual private network makes it possible to have the same secure sharing of public resources for data. Companies today are looking at using a private virtual network for both extranets and widearea intranets. Using a virtual private network involves encrypting data before sending it through the public network and decrypting it at the receiving end. An additional level of security involves encrypting not only the data but also the originating and receiving network addresses. Microsoft, 3Com, and several other companies have developed the Point-to-Point Tunneling Protocol (PPTP) and Microsoft has extended Windows NT to support it. VPN software is typically installed as part of a company’s firewall server.
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virtual reality Virtual reality is the simulation of a real or imagined environment that can be experienced visually in the three dimensions of width, height, and depth and that may additionally provide an interactive experience visually in full real-time motion with sound and possibly with tactile and other forms of feedback. The simplest form of virtual reality is a 3-D image that can be explored interactively at a personal computer, usually by manipulating keys or the mouse so that the content of the image moves in some direction or zooms in or out. Most of these images require installing a plug-in for your browser. As the images become larger and interactive controls more complex, the perception of ‘‘reality’’ increases. More sophisticated efforts involve such approaches as wrap-around display screens, actual rooms augmented with wearable computers, and haptics joystick devices that let you feel the display images. Virtual reality can be divided into: L
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The simulation of real environments such as the interior of a building or a spaceship often with the purpose of training or education
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The development of an imagined environment, typically for a game or educational adventure
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Popular products for creating virtual reality effects on personal computers include Bryce, Extreme 3D, Ray Dream Studio, trueSpace, 3D Studio MAX, and Visual Reality. The Virtual Reality Modelling Language (VRML) allows the creator to specify images and the rules for their display and interaction using textual language statements.
Virtual Router Redundancy
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See ‘‘VRRP’’
virtual server On the Internet, a virtual server is a server (computer and various server programs) at someone else’s location that is shared by multiple Web site owners so that each owner can use and administer it as though they had complete control of the server. Some Internet service providers (ISPs) offer a virtual server service instead of, or in addition to, virtual hosting. Using a virtual server, a company or individual with a Web site can not only have their own domain name and IP address, but can administer their own file directories, add e-mail accounts and address assignments, assign multiple domain names that resolve to a basic domain name without involvement from the ISP, manage their own logs and statistics analysis, and maintain passwords. Users of a
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Whatis?Com’s Encyclopedia of Technology Terms virtual server, however, do not have to manage the hardware aspects of running a server and effectively share the cost of expensive line connections to the Internet.
Virtual Storage Access Method
IBM and Storage Technology are well-established vendors of virtual tape systems. Sutmyn Storage sells a product that provides a virtual interface to existing IBM and other systems.
See ‘‘VSAM’’
Virtual Telecommunications Access Method
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Virtual Storage Portal
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Virtual Storage Portal (VSP), a service mark of StorageNetworks, is a software application with a Web browser interface that lets a customer see how much storage is being used, predict future needs, grant access to storage administrators, view performance, and otherwise manage the storage that they have rented from StorageNetworks. As a leading storage service provider, StorageNetworks operates a network of what it calls ‘‘storage point-ofpresence’’ data centers. An enterprise or a ‘‘dotcom’’ company with significant storage needs but a lack of expertise in buying and managing storage might choose to outsource its storage from a company like StorageNetworks. VSP allows this particular company’s customers to look at its storage infrastructure and otherwise manage and forecast its rented storage resources. VSP is offered free to its customers. StorageNetworks has no immediate plans to market it independently. Like similar companies, StorageNetworks offers ‘‘plain old storage space’’ as well as more expensive backup service and a ‘‘zero loss’’ service.
virtualization
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Virtualization is the pooling of physical storage from multiple network storage devices into what appears to be a single storage device that is managed from a central console. Virtualization is often used as part of a storage area network (SAN). A virtual storage device appears as one storage device to the operating system, regardless of the types of storage devices pooled. Hard disk drives are the most common storage device used. Virtualization is similar to a redundant array of independent disks (RAID) except that the stored data is not copied redundantly on each storage device (although this capability is sometimes provided). The benefits of virtualization vary with each software package, but usually enable a customer to: L
Add a storage device without requiring network downtime
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Assign storage volumes from a downed server or storage device to another server or storage device within the network
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Mix hard disk drives of different sizes, speeds, and vendors
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Easily create, delete, or expand a virtual drive. For example, if a server requires more storage that its storage device allows, virtualization allows unused space from another storage device to be used. If a server needs less storage, the unused storage space is recycled or pooled back into the drive pool.
virtual tape Virtual tape is the use of a special storage device that manages less-frequently needed data so that it appears to be stored entirely on tape cartridges when some parts of it may actually be located in faster, hard disk storage. The programming for a virtual tape system is sometimes called a virtual tape server (VTS). Virtual tape can be used with a hierarchical storage management (HSM) system in which data is moved as it falls through various usage thresholds to slower but less costly forms of storage media. Virtual tape may also be used as part of a storage area network (SAN) where less-frequently used or archived data can be managed by a single virtual tape server for a number of networked computers. A virtual tape system offloads from the main computer the processing involved in deciding whether data should be available in the faster disk cache or written onto a tape cartridge. The virtual tape system also can manage data so that more of the space on a tape cartridge is actually used.
Companies that offer virtualization software include Compaq, Veritas, and XIOtech.
virus hoax A virus hoax is a false warning about a computer virus. Typically, the warning arrives in an e-mail note or is distributed through a note in a company’s internal network. These notes are usually forwarded using distribution lists and they will typically suggest that the recipient forward the note to other distribution lists. If you get a message about a new virus, you can check it out by going to one of the leading Web sites that keep up with viruses and virus hoaxes. If someone sends you a note about a virus that you learn is a virus hoax, reply to the sender that the virus warning is a hoax.
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A virus is a piece of programming code usually disguised as something else that causes some unexpected and usually undesirable event. A virus is often designed so that it is automatically spread to other computer users. Viruses can be transmitted as attachments to an e-mail note, as downloads, or be present on a diskette or CD. The source of the e-mail note, downloaded file, or diskette you’ve received is often unaware of the virus. Some viruses wreak their effect as soon as their code is executed; other viruses lie dormant until circumstances cause their code to be executed by the computer. Some viruses are playful in intent and effect (’’Happy Birthday, Ludwig!’’) and some can be quite harmful, erasing data or causing your hard disk to require reformatting.
A viseme is a generic facial image that can be used to describe a particular sound. A viseme is the visual equivalent of a phoneme or unit of sound in spoken language. Using visemes, the hearing-impaired can view sounds visually—effectively, ‘‘lip-reading’’ the entire human face.
Generally, there are three main classes of viruses: File infectors. Some file infector viruses attach themselves to program files, usually selected .COM or .EXE files. Some can infect any program for which execution is requested, including .SYS, .OVL, .PRG, and .MNU files. When the program is loaded, the virus is loaded as well. Other file infector viruses arrive as wholly-contained programs or scripts sent as an attachment to an e-mail note. System or boot-record infectors. These viruses infect executable code found in certain system areas on a disk. They attach to the DOS boot sector on diskettes or the Master Boot Record on hard disks. A typical scenario (familiar to the author) is to receive a diskette from an innocent source that contains a boot disk virus. When your operating system is running, files on the diskette can be read without triggering the boot disk virus. However, if you leave the diskette in the drive, and then turn the computer off or reload the operating system, the computer will look first in your A drive, find the diskette with its boot disk virus, load it, and make it temporarily impossible to use your hard disk. (Allow several days for recovery.) This is why you should make sure you have a bootable floppy. Macro viruses. These are among the most common viruses, and they tend to do the least damage. Macro viruses infect your Microsoft Word application and typically insert unwanted words or phrases. The best protection against a virus is to know the origin of each program or file you load into your computer or open from your e-mail program. Since this is difficult, you can buy anti-virus software that can screen e-mail attachments and also check all of your files periodically and remove any viruses that are found. From time to time, you may get an email message warning of a new virus. Unless the warning is from a source you recognize, chances are good that the warning is a virus hoax.
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Visitor-based networking (VBN) is the provision of highspeed Internet access for mobile PC users in need of temporary service in public places. A visitor-based network is most commonly established in a hotel, airport, convention center, press area, mall, university, sales center, or corporate meeting room. For years, mobile users were limited to analog dial-up connections through a PBX system or other proprietary network. Visitor-based networking goes a step further in creating a more efficient and effective work environment for them. It aims to give the on-the-go worker a productive way to temporarily connect PCs to local LANs and broadband Internet connections.
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A visitor-based network usually includes hardware (such as servers, hubs, and routers), telecommunications (an Internet connection), software (a browser), and service (telephone support). Virtually any Internet-based Ethernet LAN can become a visitor-based network by adding a server. The server provides the necessary layer of management between public users and the gateway router to enable a seamless connection for visitors. A successful visitor-based network usually also features additional services like printing and customer support. Many ISPs are building broadband access networks in public places to enable more efficient visitorbased networks. Technology vendors have developed ways to package their network offerings with billing and management applications that make offering temporary Internet access more than just a convenience, but also a viable business. The key commercial player in the visitor-based networking arena is 3Com; smaller vendors include Elastic Networks and Tut Systems. IT analyst Gartner Group projects the remote Internet access market will hit $31 billion by 2003. It also forecasts that by 2005 half of all remote business users will have remote access connections that are faster than analog modems.
Visor Visor is the trade name of a handheld computer manufactured by Handspring. The Visor is similar to the Palm computer and uses the Palm operating system (Palm OS). Data can be easily transferred between the Visor and other computers, including Palm handhelds, using HotSync technology. Interfacing is possible with Macintosh as well as with IBM-compatible desktop and notebook computers.
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The Visor can be tailored to meet the needs of the individual user. It contains an expansion slot called Springboard that allows the addition of modules, each of which is designed for a specific function. The modules are plug-and-play and are hot-swappable (they are automatically recognized by the Visor, and they can be exchanged without powering-down or rebooting). Data can be stored and backed up with an optional flash memory module.
Visual Basic Visual Basic is a programming environment from Microsoft in which a programmer uses a graphical user interface to choose and modify preselected sections of code written in the BASIC programming language. Since Visual Basic is easy to learn and fast to write code with, it’s sometimes used to prototype an application that will later be written in a more difficult but efficient language. Visual Basic is also widely used to write working programs. Microsoft says that there are at least 3 million developers using Visual Basic.
visual computing Visual computing is computing that lets you interact with and control work by manipulating visual images either as direct work objects or as objects representing other objects that are not necessarily visual themselves. The visual images can be photographs, 3-D scenes, video sequences, block diagrams, or simple icons. The term is generally used to describe either (or both) of these:
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1. Any computer environment in which a visual paradigm rather than a conventional (text) paradigm is used
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2. Applications that deal with large or numerous image files, such as video sequences and 3-D scenes
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Intel’s 64-bit Merced microprocessor has been promoted to portend a new era of visual computing at your workstation. High-bandwidth networks will help support visual computing, especially the shipping around of large amounts of video.
visual display See ‘‘VDT’’
Visual FoxPro Visual FoxPro is a relational database with an objectoriented programming environment from Microsoft that comes with prewritten classes. Visual FoxPro is part of Microsoft’s Visual Studio suite of products. Visual FoxPro is relatively easy to learn and fast for developing reusable code. A programmer can write code to access a FoxPro database or as well as to connect to such databases as SQL Server and Oracle. Microsoft says that there are at least 500,000 developers using FoxPro.
Visual InterDev Visual InterDev is Microsoft’s development tool for building a dynamic, data-driven Web site. Whereas Microsoft’s FrontPage is an HTML editor aimed at letting nonprogrammers build the pages for a Web site, Visual InterDev provides the tools for programmers to build a Web site. (FrontPage and Visual InterDev are said to be compatible.) Visual InterDev offers a user interface similar to those for Visual Basic, Visual J++, and Visual Studio. Using Visual InterDev, one can assemble pages that use Microsoft’s ActiveX technologies, including Active Server Page (ASP) technology. The developer can build and insert ActiveX control or Java applets. Visual InterDev includes an HTML editor and support for dynamic HTML. The Web site can be integrated with server programs written in any language and access to almost any database using Microsoft’s Universal Data Access, including ActiveX Data Objects, Open Database Connectivity, and OLE DB.
visualization Visualization is the process of representing abstract business or scientific data as images that can aid in understanding the meaning of the data.
Vital Product Data See ‘‘VPD’’
VLAN See ‘‘virtual LAN’’
VLIW Very long instruction word (VLIW) describes a computer processing architecture in which a language compiler or preprocessor breaks program instructions down into basic operations that can be performed by the processor in parallel (that is, at the same time). These operations are put into a very long instruction word which the processor can then take apart without further analysis, handing each operation to an appropriate functional unit. VLIW is sometimes viewed as the next step beyond the reduced instruction set computing (RISC) architecture, which also works with a limited set of relatively basic instructions and can usually execute more than one instruction at a time (a characteristic referred to as superscalar). The main advantage of VLIW processors is that complexity is moved from the hardware to the software, which means that the hardware can be smaller, cheaper, and require less power to operate. The challenge is to design a compiler or pre-processor that is intelligent enough to decide how to build the very long instruction words. If dynamic pre-processing is done as the program is run, performance may be a concern.
VMEbus The Crusoe family of processors from Transmeta uses very long instruction words that are assembled by a preprocessor that is located in a flash memory chip. Because the processor does not need to have the ability to discover and schedule parallel operations, the processor contains only about a fourth of the transistors of a regular processor. The lower power requirement enables computers based on Crusoe technology to be operated by battery almost all day without a recharge. The Crusoe processors emulate Intel’s x86 processor instruction set. Theoretically, pre-processors could be designed to emulate other processor architectures.
VLSI VLSI (very large-scale integration), the current level of computer microchip miniaturization, refers to microchips containing in the hundreds of thousands of transistors. LSI (large-scale integration) meant microchips containing thousands of transistors. Earlier, MSI (medium scale integration) meant a microchip containing hundreds of transistors and SSI (small-scale integration) meant transistors in the tens.
VM VM (Virtual Machine), currently embodied in a product called VM/ESA, is a widely-installed operating system for mainframes from IBM that can host other operating systems, including MVS and Conversational Monitor System (CMS), so that each of the operating systems seems to have its own complete system of software and hardware resources (data storage, telecommunications, processor, and so forth). That is, VM gives each of these operating systems its own ‘‘virtual machine.’’ VM, together with CMS, is popular in many large corporations as a system that can let a large number of interactive users communicate or develop and run applications at the same time. VM also provides a good test environment for MVS programs that can be run on an MVS virtual machine. Many corporations use MVS for business applications and VM as an application development and user communication environment.
VMEbus VMEbus (VersaModular Eurocard bus) is a bus (computer data path) system, designed by Motorola, Signetics, Mostek, and Thompson CSF, that is used in industrial, commercial, and military applications worldwide. VMEbuses are used in traffic control systems, weapons control systems, telecommunication switching systems, data acquisition, video imaging, and robots. VMEbus systems withstand shock, vibration, and extended temperatures better than the buses used in desktop computers, making them ideal for harsh environments.
A VMEbus system is based on the VME standard. The VME standard defines the mechanical specifications such as board dimensions, connector specifications, and enclosure characteristics, as well as the electronic specifications for sub-bus structures, signal functions, timing, signal voltage levels, and master/slave configurations. The most recent VME standard is the VME64 standard. The VME64 standard specifies a 64-bit data path for 6U cards, a 32-bit data path for 3U cards, twice the bandwidth for data transmission, lower noise, and plug and play features. Since the VME64 standard, an extension called the VME64x was added that supports hot swap. VME64 cards can be used on older VME bus systems and older VMEbus cards can be used on VME64 systems. In 1997, a modified VME bus architecture called the VME320 was released by Arizona Digital. This architecture is designed to increase data transfer to 320 Mbps and bandwidth to 500 Mbps. The backplane design is different from the original VMEbus backplane. The VMEbus system uses Eurocards. A Eurocard is a European designed circuit board that uses a 96-pin plug instead of an edge connector making it more durable. There are three sizes: 3U which is 4 6 6 inches, 6U which is 6 6 12 inches, and 9U which is 14 6 18 inches. 3U cards support 8and 16-bit data paths and 6U cards support 32-bit data paths. The VME standard does not support 9U cards. Each card is plugged into a backplane. A backplane can have up to 21 slots for cards. A VMEbus system is scalable and modular, which means a card can be added when needed without having to make any other changes to the system. A real-time operating system (RTOS) is included when a VMEbus system is purchased. An RTOS is better for VMEbus systems because of their ability to do a task within a certain time limit. Non real-time operating systems can be used but are not recommended.
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A VMEbus system uses a master/slave architecture. A master is a device that controls another device. For example, a computer sends data to a printer. The computer is the master, and the printer is the slave because the printer cannot control the computer. A VMEbus system may have several master devices, which is why it is called a multiprocessing bus.
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A VMEbus system has four sub-buses: the arbitration bus, the data transfer bus, the priority interrupt bus, and the utility bus.
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The arbitration bus controls the requests from various devices using an arbiter module. It gives permission to each device to use the bus and notifies requesting devices when the bus is busy. Requests are based on priority. Requests that are the same in priority are daisychained. The arbiter module resides in slot 1 of the backplane.
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The priority interrupt bus handles interrupts and monitors the interrupt request lines, which range from interrupt request1 to IRQ7. IRQ7 has the highest priority.
illusion of a constant transmission stream during silence suppression, the listener would be likely to think the line had gone dead.
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The utility bus supports a system clock.
Voice Markup Language
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See ‘‘VoxML’’
See ‘‘Virtual Memory System’’
Voice over IP
vocoder
See ‘‘VoIP’’
A vocoder is an audio processor that captures the characteristic elements of an an audio signal and then uses this characteristic signal to affect other audio signals. The technology behind the vocoder effect was initially used in attempts to synthesize speech. The effect called vocoding can be recognized on records as a ‘‘talking synthesizer’’, made popular by artists such as Stevie Wonder. The basic component extracted during the vocoder analysis is called the formant. The formant describes the fundamental frequency of a sound and its associated noise components.
voice portal
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The vocoder works like this: The input signal (your voice saying ‘‘Hello, my name is Fred’’) is fed into the vocoder’s input. This audio signal is sent through a series of parallel signal filters that create a signature of the input signal, based on the frequency content and level of the frequency components. The signal to be processed (a synthesized string sound, for example) is fed into another input on the vocoder. The filter signature created above during the analysis of your voice is used to filter the synthesized sound. The audio output of the vocoder contains the synthesized sound modulated by the filter created by your voice. You hear a synthesized sound that pulses to the tempo of your voice input with the tonal characteristics of your voice added to it.
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In Voice over IP (VOiP), voice activation detection (VAD) is a software application that allows a data network carrying voice traffic over the Internet to detect the absence of audio and conserve bandwidth by preventing the transmission of ‘‘silent packets’’ over the network. Most conversations include about 50% silence; VAD (also called ‘‘silence suppression’’) can be enabled to monitor signals for voice activity so that when silence is detected for a specified amount of time, the application informs the Packet Voice Protocol and prevents the encoder output from being transported across the network. Voice activation detection can also be used to forward idle noise characteristics (sometimes called ambient or comfort noise) to a remote IP telephone or gateway. The universal standard for digitized voice, 64 Kbps, is a constant bit rate whether the speaker is actively speaking, is pausing between thoughts, or is totally silent. Without idle noise giving the
A voice portal is a Web site or other service that a user can reach by telephone for information such as weather, sport scores, or stock quotes. For example, a mobile user with a cellular telephone might dial in to a voice portal Web site and request information using voice or Touchtone keys and receive the requested information from a special voiceproducing program at the Web site. Whereas a user with a smartphone can connect to the Internet and get information on a small visual display (perhaps with a WAP interface), the user of a voice portal needs only a regular cellular phone. After requesting information by speaking or pressing keys, the voice portal responds with voice information or, in some cases, with an e-mail message. Two companies that act as voice portals are BeVocal and Tellme.
voice recognition Voice or speech recognition is the ability of a machine or program to receive and interpret dictation, or to understand and carry out spoken commands. For use with computers, analog audio must be converted into digital signals. This requires analog-to-digital conversion. For a computer to decipher the signal, it must have a digital database, or vocabulary, of words or syllables, and a speedy means of comparing this data with signals. The speech patterns are stored on the hard drive and loaded into memory when the program is run. A comparator checks these stored patterns against the output of the A/D converter. In practice, the size of a voice-recognition program’s effective vocabulary is directly related to the random access memory capacity of the computer in which it is installed. A voice-recognition program runs many times faster if the entire vocabulary can be loaded into RAM, as compared with searching the hard drive for some of the matches. Processing speed is critical as well, because it affects how fast the computer can search the RAM for matches. All voice-recognition systems or programs make errors. Screaming children, barking dogs, and loud external conversations can produce false input. Much of this can be avoided only by using the system in a quiet room. There is also a problem with words that sound alike but are spelled
volt differently and have different meanings—for example, ‘‘hear’’ and ‘‘here.’’ This problem might someday be largely overcome using stored contextual information. However, this will require more RAM and faster processors than are currently available in personal computers.
standards so that users can locate other users and the use of touch-tone signals for automatic call distribution and voice mail.
VoiceXML
In addition to IP, VoIP uses the real-time protocol (RTP) to help ensure that packets get delivered in a timely way. Using public networks, it is currently difficult to guarantee quality of service (QoS). Better service is possible with private networks managed by an enterprise or by an Internet telephony service provider (ITSP).
VoiceXML is an application of the Extensible Markup Language (XML) which, when combined with voice recognition technology, enables interactive access to the Web through the telephone or a voice-driven browser. An individual session works through a combination of voice recognition and keypad entry.
A technique used by at least one equipment manufacturer, Netspeak, to help ensure faster packet delivery is to Ping all possible network gateway computers that have access to the public network and choose the fastest path before establishing a Transmission Control Protocol (TCP) sockets connection with the other end.
VoiceXML 1.0 was created through a collaboration of AT&T, IBM, Lucent Technologies, and Motorola, who were each working on their own approach but joined forces to create an open standard. Using XML, a programmer can enable voice recognition through the addition of a few simple tags.
Using VoIP, an enterprise positions a ‘‘VoIP device’’ (such as Cisco’s AS5300 access server with the VoIP feature) at a gateway. The gateway receives packetized voice transmissions from users within the company and then routes them to other parts of its intranet (local area or wide area network) or, using a T-carrier system or E-carrier interface, sends them over the public switched telephone network.
Industry leaders in voice recognition include IBM and Dragon Systems.
VXML allows people with an ordinary voice telephone to access the Internet to get and send email, check sports scores, make reservations, and so on. VXML also can support natural language, which means that the user is not locked into a limited script, but can speak naturally. In what is called a ‘‘modeless’’ or ‘‘conversational’’ mode, the user can even interrupt the system with an out-of-context question and thus redirect the session. The goal is to make the exchange as natural as possible, as if two humans were interacting. Here’s an example of what VXML would enable a user to do: Pick up a phone, dial the number of the weather Web site, and request, for example, the weekend forecast. The voice request activates an XML query and then the query result is converted back to a voice message to give the user the information requested.
volatile In general, volatile (from the Latin ‘‘volatilis’’ meaning ‘‘to fly’’) is an adjective used to describe something unstable or changeable. In computers, volatile is used to describe memory content that is lost when the power is interrupted or switched off. Your computer’s ordinary memory (or RAM) is volatile memory. Volatile memory contrasts with nonvolatile memory, memory that does not lose content when power is lost, that has a continuous source of power, or does not need to have its memory content periodically refreshed.
VoIP
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VoIP (voice over IP—that is, voice delivered using the Internet Protocol) is a term used in IP telephony for a set of facilities for managing the delivery of voice information using the Internet Protocol (IP). In general, this means sending voice information in digital form in discrete packets rather than in the traditional circuit-committed protocols of the public switched telephone network (PSTN). A major advantage of VoIP and Internet telephony is that it avoids the tolls charged by ordinary telephone service.
The volt (symbolized V) is the International System of Units (SI) unit of electric potential or electromotive force. A potential of one volt appears across a resistance of one ohm when a current of one ampere flows through that resistance. Reduced to SI base units, 1 V = 1 kg times m2 times s-3 times A-1 (kilogram meter squared per second cubed per ampere).
VoIP, now used somewhat generally, derives from the VoIP Forum, an effort by major equipment providers, including Cisco, VocalTec, 3Com, and Netspeak to promote the use of ITU-T H.323, the standard for sending voice (audio) and video using IP on the public Internet and within an intranet. The Forum also promotes the user of directory service
Voltage can be expressed as an average value over a given time interval, as an instantaneous value at a specific moment in time, or as an effective or root-mean-square (rms) value. Average and instantaneous voltages are assigned a polarity either negative (7) or positive (+) with respect to a zero, or ground, reference potential. The rms voltage is a dimensionless quantity, always represented by a nonnegative real number.
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Whatis?Com’s Encyclopedia of Technology Terms For a steady source of direct-current (DC) electric potential, such as that from a zinc-carbon or alkaline electrochemical cell, the average and instantaneous voltages are both approximately +1.5 V if the negative terminal is considered the common ground; the rms voltage is 1.5 V. For standard utility alternating current (AC), the average voltage is zero (the polarity constantly reverses); the instantaneous voltage ranges between approximately 7165 V and +165 V; the rms voltage is nominally 117 V. Voltages are sometimes expressed in units representing power-of-10 multiples or fractions of one volt. A kilovolt (symbolized kV) is equal to one thousand volts (1 kV = 103 V). A megavolt (symbolized MV) is equal to one million volts (1 MV = 106 V). A millivolt (symbolized mV) is equal to one-thousandth of a volt (1 mV = 10-3 V). A microvolt (symbolized mV) is equal to one-millionth of a volt (1 mV = 10-6 V).
volt per meter The standard unit of electric field (E-field) strength is the volt per meter (V/m). An E field of 1 V/m is represented by a potential difference of 1 V existing between two points that are 1 m apart. Reduced to base SI units, 1 V/m is the equivalent of one meter kilogram per second cubed per ampere (m ˙ kg ˙ s-3 ˙ A-1). The volt per meter, or some fractional unit based on it, is used as a means of specifying the intensity of the electromagnetic field (EM field) produced by a radio transmitter. Although an EM field contains a magnetic (M) component as well as an electric (E) component, the relative field strength of radio signals is easier to measure in free space by sampling only the E component. The magnitude of the E component from a distant radio transmitter is often much less than 1 V/m, and in such cases, fractional units are preferred. One millivolt per meter (mV/m) is equal to 10-3 V/m; one microvolt per meter (m V/m) is equal to 10-6 V/m; one nanovolt per meter (nV/m) is equal to 10-9 V/m; one picovolt per meter (pV/m) is equal to 10-12 V/m. The magnitude of the E component of a radio wave varies inversely with the distance from the transmitter in a freespace, line-of-sight link. If the distance is doubled, the E-field intensity is cut in half; if the distance increases by a factor of 10, the E-field intensity becomes 1/10 (0.1 times) as great. The E component of an EM field is measured in a single dimension, so the intensity-versus-distance relation is a straight inverse rule, not the inverse-square law. When expressing the intensity of EM fields at infrared (IR), visible, ultraviolet (UV), X-ray, and gamma-ray wavelengths, the watt per meter squared, or one of the fractional units based on it, is more commonly used. Also see electromagnetic field.
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voltage Also see current, resistance, power, and Ohm’s Law. Voltage, also called electromotive force (EMF), is an expression for electric potential or potential difference. If a conductive or semiconductive path is provided between the two points having a relative potential difference, an electric current flows. The common symbol for voltage is the uppercase letter V or E. The standard unit is the volt, symbolized by V. One volt is the EMF required to drive one coulomb of electrical charge (6.24 6 1018 charge carriers) past a specific point in one second. Voltage can be either direct or alternating. A direct voltage maintains the same polarity at all times. In an alternating voltage, the polarity reverses direction periodically. The number of complete cycles per second is the frequency, which is measured in hertz. An example of pure direct voltage is the EMF between the terminals of an electrochemical cell. The output of a power-supply rectifier, prior to filtering, is an example of pulsating direct voltage. The voltage that appears at the terminals of common utility outlets is alternating. A potential difference produces an electrostatic field, even if no current flows. As the voltage increases between two points separated by a specific distance, the electrostatic field becomes more intense. As the separation increases between two points having a given potential difference, the electrostatic flux density diminishes in the region between them. A single charged object is surrounded by an electrostatic field whose intensity is directly proportional to the voltage of the object relative to other objects in its vicinity.
voltage regulator module A voltage regulator module (VRM) is an installable module that senses a computer’s microprocessor voltage requirements and ensures that the correct voltage is maintained. If you are changing your computer’s microprocessor (for example, changing from a Pentium to a Pentium Pro or a Pentium with MMX), you need to add a voltage regulator module to the existing voltage regulator in the motherboard so that the new voltage requirements can be detected and accommodated. In the case of the Pentium, the original Pentium has the same voltage requirement for its core or basic operation as for its I/ O operation. Both use 2.8 volts. However, the Pentium Pro and the Pentium with MMX have different voltage requirements for core (2.8 volts) and I/O (3.3 volts). Adding a VRM allows it to regulate the voltage for I/O while the original regulator built into the motherboard continues to regulate core voltage.
Voodoo
volt-ampere Volt-ampere (VA) is a measurement of power in a direct current (DC) electrical circuit. The VA specification is also used in alternating current (AC) circuits, but it is less precise in this application, because it represents apparent power, which often differs from true power. In a DC circuit, 1 VA is the equivalent of one watt (1 W). The power, P (in watts) in a DC circuit is equal to the product of the voltage V (in volts) and the current I (in amperes): P = VI In an AC circuit, power and VA mean the same thing only when there is no reactance. Reactance is introduced when a circuit contains an inductor or capacitor. Because most AC circuits contain reactance, the VA figure is greater than the actual dissipated or delivered power in watts. This can cause confusion in specifications for power supplies. For example, a supply might be rated at 600 VA. This does not mean it can deliver 600 watts, unless the equipment is reactance-free. In real life, the true wattage rating of a power supply is 1/2 to 2 /3 of the VA rating. When purchasing a power source such as an uninterruptible power supply (UPS) for use with electronic equipment (including computers, monitors, and other peripherals), be sure the VA specifications for the equipment are used when determining the minimum ratings for the power supply. The VA figure is nominally 1.67 times (167 percent of) the power consumption in watts. Alternatively, you can multiply the VA rating of the power supply by 0.6 (60 percent) to get a good idea of its power-delivering capability in watts.
volume Also see volume (amount of physical bulk). 1) In computers, a volume is an identifiable unit of data storage that is sometimes (but not always) physically removable from the computer or storage system. In tape storage systems, a volume may be a tape cartridge (or, in older systems, a tape reel). In mainframe storage systems, a volume may be a removable hard disk. Each volume has a system-unique name or number that allows it to be specified by a user. In some systems, the physical unit may be divided into several separately identifiable volumes. 2) In audio, volume is the loudness of the signal.
volume (amount of physical bulk) Volume (symbolized V) is a three-dimensional quantity representing amount or extent of physical bulk. The standard unit of volume in the International System of Units (SI) is the meter cubed (m3). Volume is usually measured or defined in Euclidean (flat) three-space in Newtonian (non-relativistic) physics. We might speak of the volume of a particle, the volume of a
sample of liquid or gas, or the volume of the earth. In relativistic physics, volume is determined for non-Euclidean (curved) three-space. In the case of certain complex or esoteric objects, volume might be impossible to define or measure. One example is a mathematical object having a fractional number of dimensions. Another example is an object without a well-defined surface. When expressing large or small volumes, prefix multipliers are attached to the meter cubed. The table below shows the most common alternative volume units and their relationship to the meter cubed. Unit To convert to Conversely, (and meters cubed, multiply by: symbol) multiply by: Kilometer cubed (km3) Centimeter cubed (cm3) Millimeter cubed (mm3) Foot cubed (ft3) Micrometer cubed or micron cubed (m3) Nanometer cubed (nm3) Angstro¨m squared ¨ 3) (A
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Compare displacement, and area. Also see meter, meter squared, meter cubed, and International System of Units (SI).
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von Neumann, John
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Voodoo Voodoo is a graphics accelerator chipset that, depending on the version, is used either with or instead of a computer’s video adapter for a more realistic graphics display and improved interactivity, especially for games. Voodoo, originally produced by 3Dfx, is considered to be the groundbreaking 3-D graphics accelerator, providing better graphics quality and faster frame rates than competitors.
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The original version of Voodoo was 2-D/3-D; subsequent versions are 3-D. Voodoo 3 and later versions are complete replacements for the computer’s video card. Earlier cards are add-ons that don’t replace the video card, but work in conjunction with it. A cable from the computer’s video adapter connects to the Voodoo card and then to the monitor, blocking the signal of the original adapter when 3D acceleration is called for, but allowing the system card to function otherwise. Voodoo is considered to be particularly helpful for playing games on older and slower computers, allowing the graphics display to reflect user input more swiftly. However, the original version of Voodoo is not considered to offer the same degree of benefit to faster computers.
vortal On the Web, a vortal (vertical industry portal) is a Web site that provides a gateway or portal to information related to a particular industry such as health care, insurance, automobiles, or food manufacturing. (A vertical industry is one that is focused on a relatively narrow range of goods and services, whereas a horizontal industry is one that aims to produce a wide range of goods and services. Because most industry tends to specialize, most industry tends to be vertical.) A term that might also be used is interest community Web site since any vertical industry brings together people sharing an interest in buying, selling, or exchanging information about that particular industry. Vortals are also seen as likely business-to-business communities—for example, small business people with home offices might be attracted to a comprehensive vortal that provided ideas and product information related to setting up and maintaining the home office.
Medical practitioners and researchers are now using images defined by voxels and 3-D software to view X-rays, cathode tube scans, and magnetic resonance imaging (MRI) scans from different angles, effectively to see the inside of the body from outside. Geologists can create 3-D views of earth profiles based on sound echoes. Engineers can view complex machinery and material structures to look for weaknesses. 2) Voxel is also a company that makes a scanning camera that will produce 3-D images of the inside of the human body by using a patented digital holography system.
VoxML VoxML (Voice Markup Language) is a technology from Motorola for creating a voice dialog with a Web site in which a user can call a Web site by phone and interact with it through speech recognition and Web site responses. VoxML (TM) allows a developer to create a script of the conversation a user can have with an application program run by a Web server. The user calling in is connected to a client program called a voice browser. The voice browser in turn passes requests on to the Web server. The markup defined in VoxML is consistent with the Extensible Markup Language (XML), the strategic data definition language for the Internet. Using VoxML is intended to be no more difficult than writing a Web page using the Hypertext Markup Language (HTML). Motorola is providing VoxML as an open interface and potential developers can download the development kit from Motorola’s Web site. Motorola has already provided the technology to some early product developers, including The Weather Channel. Typical applications might include delivering weather information, flight information, and sports scores, as well as online product and service ordering.
Related terms are infomediary and vertical market.
How the User Uses VoxML
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voting online See ‘‘e-voting’’
voxel
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1) A voxel is a unit of graphic information that defines a point in three-dimensional space. Since a pixel (picture element) defines a point in two dimensional space with its x and y coordinates, a third z coordinate is needed. In 3-D space, each of the coordinates is defined in terms of its position, color, and density. Think of a cube where any point on an outer side is expressed with an x, y coordinate and the third, z coordinate defines a location into the cube from that side, its density, and its color. With this information and 3-D rendering software, a two-dimensional view from various angles of an image can be obtained and viewed at your computer.
1. The user would identify a Web site that offered phone interaction. For example, some e-tailing sites might allow users to order from the Web site by phone instead of by filling out a form. 2. The company with the Web site would publish a phone number to call for their Web site. 3. The user would call the number on a regular phone. (Initially—that is, making the phone connection through the Internet itself—is not supported.) 4. The user would get connected to a voice browser at the Web site. 5. When connected, the user would hear a recorded voice that invited the user to interact with the Web site—for example, to place an order or choose some other option.
VSAM 6. As the user responded, the selected responses could trigger requests from the voice browser to a Web server application. The Web server might also send back new Web pages to the user’s computer—for example, pictures of catalog items and order confirmations.
VRAM
7. The user and the VoxML application would interact until the transaction was completed. Then the user would hang up, ending the session.
See ‘‘voltage regulator module’’
How the Developer Develops a VoxML Application
VRML (Virtual Reality Modeling Language) is a language for describing three-dimensional (3-D) image sequences and possible user interactions to go with them. Using VRML, you can build a sequence of visual images into Web settings with which a user can interact by viewing, moving, rotating, and otherwise interacting with an apparently 3-D scene. For example, you can view a room and use controls to move the room as you would experience it if you were walking through it in real space.
1. First, the developer designs and specifies the application, writing down the possible voice dialogs that will be possible. 2. Working at a regular PC that has the development software installed, the developer uses VoxML, the user dialog statements, and the names of recorded audio response files to create a file that formally defines the dialogs.
See ‘‘video RAM’’
VRML
3. The developer (or someone else) records each possible voice inquiry, creating a speech file against which users’ voice entries can be matched. The developer arranges for any predefined voice responses, which may be available from another application. 4. The developer has the voice client on the PC so that the VoxML script can be tested.
VRRP
6. The developer modifies the Web site to invite users to request the voice application. 7. Once the VoxML application is set up, users can interact with the Web site through their mouse and keyboard as usual, and also, for invited purposes, over a telephone.
VPD 1) VPD (vital product data) is information about a device that is stored on a computer’s hard disk (or the device itself) that allows the device to be administered at a system or network level. Typical VPD information includes a product model number, a unique serial number, product release level, maintenance level, and other information specific to the device type. Vital product data can also include userdefined information, such as the building and department location of the device. The collection and use of vital product data allows the status of a network or computer system to be understood and service provided more quickly. 2) VPD (Visual Parts Database) is a service from TEC Engineering in which a photographic database is created of a company’s products and product parts. 3) VPD is also an abbreviation for ‘‘virtual printer device.’’
VPN See ‘‘virtual private network’’
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To view a VRML file, you need a VRML viewer or browser, which can be a plug-in for a Web browser you already have. Among viewers you can download for the Windows platforms are blaxxun’s CC Pro, Platinum’s Cosmo Player, WebFX, WorldView, and Fountain. Whurlwind and Voyager are two viewers for the Mac.
5. After testing, all files are put on the Web site server.
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VRRP (Virtual Router Redundancy Protocol) is an Internet protocol that provides a way to have one or more backup routers when using a statically configured router on a local area network (LAN). Although there are other alternatives, the most common arrangement is to specify one router to serve as the router for forwarding packets from a group of hosts on a LAN. If that router fails, however, there is no way to use another router as a backup. Using VRRP, a virtual IP address can be specified manually or with Dynamic Host Configuration Protocol (DHCP) as a default. A virtual IP address is shared among the routers, with one designated as the master router and the others as backups. In case, the master fails, the virtual IP address is mapped to a backup router’s IP address. (This backup becomes the master router.) VRRP can also be used for load balancing. VRRP is part of both IPv4 (the version of IP that most networks currently use) and IPv6.
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VSAM (Virtual Storage Access Method) is a file management system for IBM’s larger operating systems, including its primary mainframe operating system, MVS, now called OS/390. Using VSAM, an enterprise can create and access records in a file in the sequential order that they were entered. It can also save and access each record with a key (for example, the name of an employee).
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Many corporations that developed programs for IBM’s mainframes still run programs that access VSAM files (also called data sets). VSAM succeeded earlier IBM file access methods, SAM (Sequential Access Method) and ISAM (Indexed Sequential Access Method). Although VSAM is still supported for legacy applications, IBM now promotes (DB2), a relational database management system.
VSAT VSAT (Very Small Aperture Terminal) is a satellite communications system that serves home and business users. A VSAT end user needs a box that interfaces between the user’s computer and an outside antenna with a transceiver. The tranceiver receives or sends a signal to a satellite transponder in the sky. The satellite sends and receives signals from an earth station computer that acts as a hub for the system. Each end user is interconnected with the hub station via the satellite in a star topology. For one end user to communicate with another, each transmission has to first go to the hub station which retransmits it via the satellite to the other end user’s VSAT. VSAT handles data, voice, and video signals. VSAT is used both by home users who sign up with a large service such as DirecPC and by private companies that operate or lease their own VSAT systems. VSAT offers a number of advantages over terrestrial alternatives. For private applications, companies can have total control of their own communication system without dependence on other companies. Business and home users also get higher speed reception than if using ordinary telephone service or ISDN.
VSP See ‘‘Virtual Storage Portal’’
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VTAM VTAM (Virtual Telecommunications Access Method) is an IBM application program interface (API) for communicating with telecommunication devices and their users. VTAM was the first IBM program to allow programmers to deal with devices as ‘‘logical units’’ without having to understand the details of line protocol and device operation. Prior to VTAM, programmers used IBM’s Basic Telecommunications Access Method (BTAM) to communicate with devices that used the binary synchronous (BSC) and start-stop line protocols. VTAM became part of IBM’s strategic Systems Network Architecture (SNA) which in turn became part of the more comprehensive Systems Application Architecture (SAA). As the computer industry turned to open standard architectures, IBM began to deemphasize its proprietary architectures in favor of becoming a participant and leader in developing open standard architectures. However, most
of its customer base retains a large investment in legacy applications and system skills based on VTAM, SNA, and SAA. VTAM’s interface consists of ‘‘macro instructions’’ that set up connection control blocks and then do SENDs to devices and either synchronous or asynchronous READs from them. Typically, programs written in COBOL, PL/1, and assembler language use VTAM to communicate with interactive devices and their users. Programs that use VTAM macro instructions are generally exchanging text strings (for example, online forms and the user’s form input) and the most common interactive device used with VTAM programs was the 3270 Information Display System.
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WAIS
W2K stands for Windows 2000, which is also sometimes shortened to Win2000.
No longer much used, wide-area information servers (WAIS) is an Internet system in which specialized subject databases are created at multiple server locations, kept track of by a directory of servers at one location, and made accessible for searching by users with WAIS client programs. The user of WAIS is provided with or obtains a list of distributed databases. The user enters a search argument for a selected database and the client then accesses all the servers on which the database is distributed. The results provide a description of each text that meets the search requirements. The user can then retrieve the full text.
W3C The World Wide Web Consortium (W3C) describes itself as follows: ‘‘The World Wide Web Consortium exists to realize the full potential of the Web. The W3C is an industry consortium which seeks to promote standards for the evolution of the Web and interoperability between WWW products by producing specifications and reference software. Although W3C is funded by industrial members, it is vendor-neutral, and its products are freely available to all. The Consortium is international; jointly hosted by the MIT Laboratory for Computer Science in the U.S. and in Europe by INRIA who provide both local support and performing core development. The W3C was initially established in collaboration with CERN, where the Web originated, and with support from DARPA and the European Commission.’’ Organizations may apply for membership to the Consortium; individual membership isn’t offered. The W3C has taken over what was formerly called the CERN Hypertext Transfer Protocol daemon or Web server.
WAD A WAD (pronounced WAHD) is a file that comes with Doom and possibly other Id Software games, containing information about game levels, game objects, creatures, graphic images, and sound. Typically, these are files in the megabyte range. Many game editors allow users to create additional WAD files with their own game levels, backgrounds, creatures, and weapons. The new WAD file is loaded with the game’s original WAD file and can be selected from the game’s menu. The WAD file is unique to Id Software products, but there are comparable files for other games, such as PIG and HOG files for LucasArts’ Dark Forces game, MAP for Duke Nukem 3D, and PAK for Quake. WAD is not an acronym or abbreviation. It’s just ‘‘WAD.’’
WAIS (pronounced ‘‘ways’’) uses its own Internet protocol, an extension of the Z39.50 standard (Information Retrieval Service Definition and Protocol Specification for Library Applications) of the National Information Standards Organization. Web users can use WAIS by either downloading a WAIS client and a ‘‘gateway’’ to the Web browser or by using Telnet to connect to a public WAIS client.
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wait state A wait state is a situation in which a computer program or processor is waiting for the completion of some event before resuming activity. A program or process in a wait state is inactive for the duration of the wait state. For example, an application program that communicated with one other program might send that program a message and then go into a wait state until it was ‘‘reawakened’’ by a message back from the other program. When a computer processor works at a faster clock speed (expressed in MHz or millions of cycles per second) than the random access memory (RAM) that sends it instructions, it is set to go into a wait state for one or more clock cycles so that it is synchronized with RAM speed. In general, the more time a processor spends in wait states, the slower the performance of that processor.
Wake on LAN Wake on LAN is a technology that allows a network professional to remotely power on a computer or to wake it up from sleep mode. Wake on LAN (local area network) is a part of a joint Intel-IBM Wired for Management technology. Wired for Management is designed to help network professionals save time and money on automated tasks such as software installation and upgrades, backups, and virus scans by scheduling these tasks during hours when network activity is at a minimum. By remotely triggering the computer to wake up and start these scheduled tasks, the
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technician does not have to physically visit each computer to turn them on first. Wake on LAN is for use with Ethernet and token ring networks. Wake on LAN works by sending a wake-up frame or packet to a client machine from a server machine that has remote network management software installed. The Wake on LAN network adapter installed in the client receives the wake-up frame and turns on. The scheduled tasks then begin. To use Wake on LAN technology you need a Wake on LAN network adapter, Wake on LAN-enabled motherboard, and remote management software: L
The Wake on LAN network adapter continually monitors the network looking for wake-up frames. The adapter must have a constant power source in order to boot up, which is usually from a special power supply that delivers a certain amount of power continually. The Wake on LAN adapter also decodes the wake-up frame to determine if it is a wake-up. The key to determining a wake-up frame is if the media access control (MAC address) address is repeated 16 times without breaks or interruptions.
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The motherboard must contain a complementary metaloxide semiconductor (CMOS) that is designed to use Wake on LAN technology.
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The remote management software sends the wake-up frames. This software also enables a professional to disable Wake on LAN technology. The remote management software allows the scheduling of tasks that are needed and tells the computer to shut down or go into sleep mode when done.
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Because wireless devices such as smartphones are often limited to the content provided by their carriers, the portion of the Web that is available to wireless users is frequently referred to as a walled garden. Speaking of the Web as a whole, AOL is generally considered the major—and most successful—practitioner of the walled garden approach. According to a spokesperson from Disney (arguing against the recent AOL—Time Warner merger), 85% of AOL users never leave AOL territory; according to The Economist, almost 40% of the time Americans spend on the Web is within the confines of AOL’s walled garden. The term’s creation is attributed to John Malone, former owner of Tele-Communications Inc. AT&T, who purchased Malone’s company, compares the walled garden to a magazine, in which a compilation of various types of content is made available to the reader. The walled garden concept is unpopular with many consumers. Although it offers an easy-to-navigate selection of services and content, that selection includes only a very small part of what the Web has to offer. Alternate names, such as ‘‘walled prison’’ and ‘‘walled desert’’ have been proposed by some as more reflective of the confinement and lack of diversity of the walled garden.
wallet A wallet is a small software program used for online purchase transactions. Many payment solution companies, such as CyberCash, offer free wallet software that allows several methods of payment to be defined within the wallet (for example, several different credit cards).
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wallpaper
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See ‘‘handie talkie’’
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walled garden
On a computer that is provided with a desktop kind of user interface, wallpaper is the background pattern or picture against which desktop menus, icons, and other elements are displayed and moved around. A wallpaper image can be in a JPEG or a GIF file format. Wallpaper is commonly used in Microsoft Windows, Macintosh Mac OS, Linux, and in other operating systems as well. Each operating system provides several pre-installed wallpaper images for the user to choose from. A user can also choose to download and install third-party wallpapers and use one of these instead.
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On the Internet, a walled garden is an environment that controls the user’s access to Web content and services. In effect, the walled garden directs the user’s navigation within particular areas, to allow access to a selection of material, or prevent access to other material. An Internet service provider (ISP) may or may not allow users to select some of the Web sites contained or barred from the garden. Although the walled garden does not actually prevent users from navigating outside the walls, it makes it more difficult than staying within the environment. ISPs want to fence in users for a number of reasons. In 1999, for example, America Online (AOL) UK’s Kid Channel established a walled garden to prevent access to inappropriate Web sites. However, a common reason for the construction of walled gardens is for the profits they generate: vendors collaborate to direct consumer’s Internet navigation to each others’ Web sites and to try to keep them from accessing the Web sites of competitors.
Typically, a wallpaper image may be centered, stretched, or tiled. When an image is centered, it is placed in the middle of the desktop and is surrounded by a solid color. When an image is stretched, it is stretched to cover all of the desktop. Only certain images can be stretched or they look distorted. An image that is tiled is placed on the desktop much like tiles are placed in a shower or a tiled floor. Tiling is commonly used for patterns instead of photos because a pattern is one square image that repeats itself across and down the screen, effectively forming a single image.
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WAN See ‘‘wide area network’’
WAP WAP (Wireless Application Protocol) is a specification for a set of communication protocols to standardize the way that wireless devices, such as cellular telephones and radio transceivers, can be used for Internet access, including email, the World Wide Web, newsgroups, and Internet Relay Chat (IRC). While Internet access has been possible in the past, different manufacturers have used different technologies. In the future, devices and service systems that use WAP will be able to interoperate. The WAP layers are: L
Wireless Application Environment (WAE)
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Wireless Session Layer (WSL)
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Wireless Transport Layer Security (WTLS)
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Wireless Transport Layer (WTP)
The WAP was conceived by four companies: Ericsson, Motorola, Nokia, and Unwired Planet (now Phone.com). The Wireless Markup Language (WML) is used to create pages that can be delivered using WAP. There are other approaches to an industry standard besides WAP, including i-Mode.
war dialer A war dialer is a computer program used to identify the phone numbers that can successfully make a connection with a computer modem. The program automatically dials a defined range of phone numbers and logs and enters in a database those numbers that successfully connect to the modem. Some programs can also identify the particular operating system running in the computer and may also conduct automated penetration testing. In such cases, the war dialer runs through a predetermined list of common user names and passwords in an attempt to gain access to the system. A war dialer, usually obtained as freeware, is typically used by a hacker to identify potential targets. If the program does not provide automated penetration testing, the intruder attempts to hack a modem with unprotected log-ins or easily cracked passwords. Commercial war dialers, also known as modem scanners, are also used by system administrators, to identify unauthorized modems on an enterprise network. Such modems can provide easy access to a company’s intranet.
warez Warez (pronounced as though spelled ‘‘wares’’ or possibly by some pronounced like the city of ‘‘Juarez’’) is a term used by software ‘‘pirates’’ to describe software that has been
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stripped of its copy-protection and made available on the Internet for downloading. People who create warez sites sometimes call them ‘‘warez sitez’’ and use ‘‘z’’ in other pluralizations.
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See ‘‘wireless application service provider’’
C waterfall model (The waterfall model is also referred to as the systems development life cycle model.)
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In software engineering, the waterfall model describes a development method that is linear and sequential. Waterfall development has distinct goals for each phase of development. Imagine a waterfall on the cliff of a steep mountain. Once the water has flowed over the edge of the cliff and has begun its journey down the side of the mountain, it cannot turn back. It is the same with waterfall development. Once a phase of development is completed, the development proceeds to the next phase and there is no turning back.
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The advantage of waterfall development is that it allows for departmentalization and managerial control. A schedule can be set with deadlines for each stage of development and a product can proceed through the development process like a car in a carwash, and theoretically, be delivered on time. Development moves from concept, through design, implementation, testing, installation, troubleshooting, and ends up at operation and maintenance. Each phase of development proceeds in strict order, without any overlapping or iterative steps. The disadvantage of waterfall development is that it does not allow for much reflection or revision. Once an application is in the testing stage, it is very difficult to go back and change something that was not well-thought out in the concept stage.
watermark See ‘‘steganography’’
watt The watt (abbreviated W) is the standard unit of power (or energy per unit time) and is the equivalent of one joule per second. The watt is used to specify the rate at which electrical energy is dissipated, or the rate at which electromagnetic energy is radiated, absorbed, or dissipated. In DC (direct current) and low-frequency AC (alternating current) electrical circuits and systems, power is the product of the current and the voltage. Power is also proportional to the ratio of the square of the voltage to the resistance, and to the product of the resistance and the square of the current. Consider a circuit in which the current, voltage, and resistance are all constant. If the current in amperes is
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represented by I, the voltage (or potential difference) in volts is represented by E, and the resistance in ohms is represented by R, then the following equations hold for power in watts, represented by P: P = EI P = E2/R P = I2R In radio-frequency (RF) circuits and systems, the calculation of power becomes more complex. This is because, at high frequencies, AC is affected not only by resistance, but by reactance. In these situations, no simple formulas exist for the calculation of power. However, dissipated or radiated RF power in can be determined by direct measurement using an instrument called an RF wattmeter. In situations involving very high or very low power, prefix multipliers are commonly used to obtain power units. As the rate of dissipated or radiated power increases, one kilowatt (kW) is equal to 1000 W; one megawatt (MW) is equal to 106 W; one gigawatt (GW) is equal to 109 W. As power decreases, one milliwatt (mW) is equal to 0.001 W; one microwatt (W) is equal to 10-6 W; one nanowatt (nW) is equal to 10-9 W.
watt per steradian The watt per steradian (W ˙ sr-1) is the standard unit of radiant intensity. Reduced to base units in the International System of Units (SI), it is the equivalent of a kilogram meter squared per second cubed per steradian (kg ˙ m2 ˙ s-3 ˙ sr-1). A point source of electromagnetic (EM) power that radiates equally well in all directions, and whose output is 1 W ˙ sr-1, has a total output power of 4p (approximately 12.5664) watts (W). This is because there are 4p steradians in threedimensional space with respect to a point of reference. The watt per steradian can be used to define the radiant intensity at any EM wavelength, from low-frequency radio waves through the gamma-ray spectrum. For visible light, in the wavelength range of approximately 390 to 770 nanometers (nm), the lumen is the preferred unit. Suppose an electromagnetic field having P watts of total power is radiated from an isotropic emitter (that is, a point source that radiates equally well in all directions). Then the radiant intensity, P’, in watts per steradian is given by the following formula:
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P’ = P / (4p)
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Also see electromagnetic field, lumen, power, International System of Units (SI), steradian, and watt.
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wav See ‘‘Wave file’’
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Wave file A Wave file is an audio file format, created by Microsoft, that has become a standard PC audio file format for everything from system and game sounds to CD-quality audio. A Wave file is identified by a file name extension of WAV (.wav). Used primarily in PCs, the Wave file format has been accepted as a viable interchange medium for other computer platforms, such as Macintosh. This allows content developers to freely move audio files between platforms for processing, for example. In addition to the uncompressed raw audio data, the Wave file format stores information about the file’s number of tracks (mono or stereo), sample rate, and bit depth.
wave number The term wave number refers to the number of complete wave cycles of an electromagnetic field (EM field) that exist in one meter (1 m) of linear space. Wave number is expressed in reciprocal meters (m-1). The wave number for an EM disturbance is simply the reciprocal of the wavelength in meters. For example, if a microwave radio signal has a wavelength of one centimeter (1 cm or 0.01 m), then the wave number of that signal is 1/ (0.01 m) = 100 m-1. As the wavelength grows shorter, the wave number becomes larger. Wave number is usually specified for EM disturbances in a vacuum, also called free space. In most situations, the air is equivalent to a vacuum. In media other than free space, the wave number for a given disturbance may increase. When a ray of light passes from air into water or glass, or a radio signal propagates through a polyethylene dielectric rather than air, the wavelength is shortened because the speed of propagation decreases. This causes the wave number to increase. In free space, the wave number w (in reciprocal meters) is related to the frequency f (in hertz) according to the following formula: w = f/c where c is the speed of EM propagation in free space, approximately equal to 2.99792 6 108 meters per second. In media other than free space, c must be multiplied by a velocity factor v. The velocity factor for a particular medium is the ratio of the speed of EM propagation in that medium to the speed of EM propagation in free space. As such, the velocity factor is always greater than 0 and less than or equal to 1. Taking velocity factor into account, the above formula becomes: w = f/(vc) See also EM field, frequency, and wavelength.
wavelength
waveform A waveform is a representation of how alternating current (AC) varies with time. The most familiar AC waveform is the sine wave, which derives its name from the fact that the current or voltage varies with the sine of the elapsed time. Other common AC waveforms are the square wave, the ramp, the sawtooth wave, and the triangular wave. Their general shapes are shown below.
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cylindrical metal tube or pipe. The electromagnetic field propagates lengthwise. Waveguides are most often used with horn antennas and dish antennas. An electromagnetic field can propagate along a waveguide in various ways. Two common modes are known as transverse-magnetic (TM) and transverse-electric (TE). In TM mode, the magnetic lines of flux are perpendicular to the axis of the waveguide. In TE mode, the electric lines of flux are perpendicular to the axis of the waveguide. Either mode can provide low loss and high efficiency as long as the interior of the waveguide is kept clean and dry. To function properly, a waveguide must have a certain minimum diameter relative to the wavelength of the signal. If the waveguide is too narrow or the frequency is too low (the wavelength is too long), the electromagnetic fields cannot propagate. At any frequency above the cutoff (the lowest frequency at which the waveguide is large enough), the feed line will work well, although certain operating characteristics vary depending on the number of wavelengths in the cross section.
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Wavelength is the distance between identical points in the adjacent cycles of a waveform signal propogated in space or along a wire, as shown in the illustration. In wireless systems, this length is usually specified in meters, centimeters, or millimeters. In the case of infrared, visible light, ultraviolet, and gamma radiation, the wavelength is more often specified in nanometers (units of 10-9 meter) or Angstrom units (units of 10-10 meter).
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N Some AC waveforms are irregular or complicated. Square or sawtooth waves are produced by certain types of electronic oscillators, and by a low-end UPS (uninterruptible power supply) when it is operating from its battery. Irregular AC waves are produced by audio amplifiers that deal with analog voice signals and/or music. The sine wave is unique in that it represents energy entirely concentrated at a single frequency. An ideal, unmodulated wireless signal has a sine waveform, with a frequency usually measured in megahertz (MHz) or gigahertz (GHz). Household utility current has a sine waveform with a frequency of 60 Hz in most countries including the United States, although in some countries it is 50 Hz.
waveguide A waveguide is an electromagnetic feed line used in microwave communications, broadcasting, and radar installations. A waveguide consists of a rectangular or
O P Q Wavelength is inversely related to frequency. The higher the frequency of the signal, the shorter the wavelength. If f is the frequency of the signal as measured in megahertz, and w is the wavelength as measured in meters, then w = 300/f
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and conversely f = 300/w
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Wavelength is sometimes represented by the Greek letter lambda.
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wavelength-division multiplexing In fiber optic communications, a single laser beam can carry millions of independent signals. A common way to get a large number of signals onto a visible-light or infrared (infrared transmission) laser is to assign each signal a separate radio frequency. At the fiber optic transmitter, the laser is modulated at all signal frequencies simultaneously. At the receiver, the signals are separated by a device similar to a radio receiver. This process is frequency-division multiplexing (FDM). A modulated laser has a frequency far higher than that of any of the signals it carries. For visible light and IR, the wavelength is more often specified than the frequency. A visible-red laser might have a wavelength of 735 nanometers (nm), where 1 nm = 10-9 meter. A blue laser might produce a beam of 440 nm; an IR laser might generate energy at 1500 nm. Most fiber optic systems use lasers having wavelengths between 1300 nm and 1600 nm, in the near-IR region. In a simple fiber optic system, the receiver is not sensitive to the wavelength of the laser. But wavelength-sensitive filters, the IR analog of visible-light color filters, can be used at the receiving end of a fiber optic system. Then, lasers of various wavelengths can be transmitted along a single fiber, and each laser can be modulated by its own set of radiofrequency signals. This is known as wavelength-division multiplexing (WDM).
method. Thus, for example, a photograph that requires 200 KB and takes a minute to download in JPEG format might require only 50 KB and take 15 seconds to download in wavelet-compressed format. Wavelet compression works by analyzing an image and converting it into a set of mathematical expressions that can then be decoded by the receiver. A wavelet-compressed image file is often given a name suffix of ‘‘WIF.’’ Either your browser must support these files or it will require a plug-in program to read the fles. Wavelet compression is not yet widely used on the Web. The most common compressed image formats remain the Graphics Interchange Format (GIF), used mainly for drawings, and JPEG, used mainly for photographs.
wavetable In computer technology, a wavetable is a table of stored sound waves that are digitized samples of actual recorded sound. A wavetable is stored in read-only memory (ROM) on a sound card chip but it can also be supplemented with software. Originally, computer sounds (digital versions of analog waveforms) were generated through frequency modulation (FM). Prestoring sound waveforms in a lookup table improved quality and throughput.
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Today’s more advanced sound cards include wavetables with 32 ‘‘voices’’ or instruments (that are combined during creation and playback). Some sound cards work with software that provides additional voices. Wavetables are used as part of music or sound synthesizers that use the musical instrument digital interface (MIDI). MIDI lets you capture sound and play it back based on the commands in files that are essentially little ‘‘scripts’’ to the ‘‘orchestra’’ (one might think of it as a written description of what the conductor is doing and which instruments are being pointed to with the baton). A wavetable sound can be enhanced or modified using reverberation or other effects before it is saved in the table. Some wavetable chips include a special section for drum sounds to support rhythmic effects.
A wavelet is a mathematical function useful in digital signal processing and image compression. The use of wavelets for these purposes is a recent development, although the theory is not new. The principles are similar to those of Fourier analysis, which was first developed in the early part of the 19th century.
Many sound cards take advantage of Direct Memory Access (DMA). Many also include an FM synthesizer in order to play back sounds from older applications or files. A fullduplex sound card lets you record and playback at the same time or, if you’re using Internet telephony, talk and hear at the same time.
In signal processing, wavelets make it possible to recover weak signals from noise. This has proven useful especially in the processing of X-ray and magnetic-resonance images in medical applications. Images processed in this way can be ‘‘cleaned up’’ without blurring or muddling the details.
Wavetable sound cards use digital signal processor (DSP) chips.
The use of WDM can multiply the effective bandwidth of a fiber optic communications system. But its cost must be weighed against the alternative of using several separate fibers bundled into a cable. An emerging fiber optic repeater device called the erbium amplifier promises to make WDM a cost-effective technology.
In Internet communications, wavelets have been used to compress images to a greater extent than is generally possible with other methods. In some cases, a waveletcompressed image can be as small as about 25 percent the size of a similar-quality image using the more familiar JPEG
WAX Wireless Abstract XML (WAX) is an abstract markup language and associated tools that facilitate wireless application development. WAX comes as an integral part of Morphis, an open source transcoding platform from Kargo, Inc. Because WAX tags perform at a higher level of abstraction than those of earlier wireless markup languages,
Web bug WAX translates to common languages, such as Hypertext Markup Language (HTML), Wireless Markup Language (WML), and Handheld Device Markup Language (HDML) through Extensible Stylesheet Language (XSL) style sheets and XSL Transformations (XSLT). The major features of WAX include: the WAX language itself; translation stylesheets, which are used to translate the WAX language into the most suitable language for the requesting device; the device registry, which includes an XML database of device particulars; dynamic image and text selection, which allows content to be written a single time for multiple transformations; and the application foundation, a WAX servlet that creates a foundation for WAX applications. Kargo claims that WAX transforms content for various wireless graphical user interfaces (GUIs) more easily and seamlessly than other wireless languages. WAX tags have complex and variable functionality: for example, a single tag can create different display options on separate devices. Because WAX is an extensible language, elements of other languages can easily be incorporated into applications.
scaled down for body-wear. Others employ brand new technology. Both general and special purposes are envisioned. A number of wearables have been designed for the disabled. Among the challenges of wearable computers are: how to minimize their weight and bulkiness, how and where to locate the display, and what kind of data entry device to provide. Some of the applications envisioned for wearable computers include: L
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WBT See ‘‘Web-based training’’
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WCDMA Also see CDMA, CDMA One, and CDMA2000. WCDMA (wideband code-division multiple access), an ITU standard derived from code-division multiple access (CDMA), is officially known as IMT-2000 direct spread. WCDMA is a third-generation (3G) mobile wireless technology offering much higher data speeds to mobile and portable wireless devices than commonly offered in today’s market. WCDMA can support mobile/portable voice, images, data, and video communications at up to 2 Mbps (local area access) or 384 Kbps (wide area access). The input signals are digitized and transmitted in coded, spread-spectrum mode over a broad range of frequencies. A 5 MHz-wide carrier is used, compared with 200 KHz-wide carrier for narrowband CDMA.
WDM See ‘‘dense wavelength division multiplexing’’
wearable computer Some inventors and other theorists not only believe you could wear a computer; they believe there’s no reason why you shouldn’t. Assuming you remembered to wear it, a wearable computer is always available. Currently, several companies sell wearables and there is a considerable literature on the subject. Some wearable computers are basically desktop or notebook computers that have been
Augmented memory, a concept originated by Thad Starner and being developed by Bradley Rhodes at the MIT Media Lab, in which as you enter a room, your wearable computer could sense the people present and remind you of their names or personal history, or a scheduler could whisper the time of an important meeting in your ear, or a ‘‘remembrance agent’’ could look for related documents by observing the words you were typing Immediate access to important data for anyone whose occupation requires mobility, such as real estate agents, rural doctors, fire and police professionals, lawyers in courtrooms, horse bettors, military personnel, stock brokers, and many others The ability to take notes immediately. For example, for reporters, geologists, botanists, vendor show representatives, field service repair personnel.
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Web bug A Web bug is a file object, usually a graphic image such as a transparent one pixel-by-one pixel GIF, that is placed on a Web page or in an e-mail message to monitor user behavior, functioning as a kind of spyware. Unlike a cookie, which can be accepted or declined by a browser user, a Web bug arrives as just another GIF on the Web page. A Web bug is typically invisible to the user because it is transparent (matches the color of the page background) and takes up only a tiny amount of space. It can usually only be detected if the user looks at the source version of the page to find a an IMG tag that loads from a different Web server than the rest of the page. Although proponents of Internet privacy object to the use of Web bugs in general, they also concede that Web bugs can be put to positive use, for example to track copyright violations on the Web.
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According to Richard M. Smith, a Web bug can gather the following statistics:
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The IP address of the computer that fetched the Web bug
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The URL of the page that the Web bug is located on
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The URL of the Web bug image
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The time the Web bug was viewed
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The type of browser that fetched the Web bug image
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A previously set cookie value
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Web farm
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See ‘‘server farm’’
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Web hosting Web hosting (also known as Webhosting, Web site hosting, and hosting) is the business of housing, serving, and maintaining files for one or more Web sites.
Web Proxy Autodiscovery Web Proxy Autodiscovery (WPAD) is a proposed Internet protocol that allows a client, such as a Web browser or a streaming media application, to automatically locate and interface with cache services in a network so that information can be delivered more quickly to the user. A cache service stores copies of popular Web pages at a location closer to the users, usually on the outer edges of a network, for faster access. When a particular page is requested, the browser is directed to the cache service for that page instead of having to travel through the network to the originating site. Cache services are maintained and offered by Internet service providers (ISPs) and special cache-providing services such as Akamai. The first WPAD-enabled browser was Internet Explorer 5.0. WPAD is a joint effort of Inktomi, Microsoft, Real Networks, and Sun Microsystems. When an Internet session is started, the WPAD-enabled client automatically uses Dynamic Host Configuration Protocol (DHCP) to locate a cache service. If unsuccessful, WPAD then uses the Service Location Protocol (SLP). SLP is a protocol that allows network applications to discover the location and configuration of network services in an enterprise. If still unsuccessful, WPAD then searches through domain name system (DNS) records. Once a nearby cache service is located, WPAD automatically connects to that location for related page requests.
Web ring A Web ring (or Webring) is a way of interlinking related Web sites so that you can visit each site one after the other, eventually (if you keep going) returning to the first Web site. Typically, users can also elect to go backwards through the ring of sites, skip a certain number at a time, visit sites randomly, or see a list of all the sites on the ring. A ring is managed from one site which includes a common gateway interface (CGI) application that can select random sites and bypass sites that have dropped out or aren’t reachable. The ring idea seems to have caught on as a more dynamic alternative to the list of ‘‘favorite sites’’ that many Web sites offer. The originator of the idea, Sage Weil (now 19 and in college), started the first ring in May, 1995. With several collaborators, Sage has created WebRing, a Web ring management system. As of April, 1998, there were over 40,000 Web rings using the system.
There are Web rings on acrobatics, quilting, mermaids, the macabre, Spanish hotels, the Chevrolet, Dixieland, medieval studies, native American sites, and Winnie the Pooh. The largest number are on computer- and game-related subjects.
Web server A Web server is a program that, using the client/server model and the World Wide Web’s Hypertext Transfer Protocol (HTTP), serves the files that form Web pages to Web users (whose computers contain HTTP clients that forward their requests). Every computer on the Internet that contains a Web site must have a Web server program. The most popular Web servers are Microsoft’s Internet Information Server (IIS), which comes with the Windows NT server; Netscape FastTrack and Enterprise servers; and Apache, a Web server for UNIX-based operating systems. Other Web servers include Novell’s Web Server for users of its NetWare operating system and IBM’s family of Lotus Domino servers, primarily for IBM’s OS/390 and AS/400 customers. Web servers often come as part of a larger package of Internet- and intranet-related programs for serving e-mail, downloading requests for File Transfer Protocol (FTP) files, and building and publishing Web pages. Considerations in choosing a Web server include how well it works with the operating system and other servers, its ability to handle server-side programming, and publishing, search engine, and site building tools that may come with it.
Web server farm See ‘‘server farm’’
Web Services Description Language The Web Services Description Language (WSDL) is an XMLbased language used to describe the services a business offers and to provide a way for individuals and other businesses to access those services electronically. WSDL is the cornerstone of the Universal Description, Discovery, and Integration (UDDI) initiative spearheaded by Microsoft, IBM, and Ariba. UDDI is an XML-based registry for businesses worldwide, which enables businesses to list themselves and their services on the Internet. WSDL is the language used to do this. WSDL is derived from Microsoft’s Simple Object Access Protocol (SOAP) and IBM’s Network Accessible Service Specification Language (NASSL). WSDL replaces both NASSL and SOAP as the means of expressing business services in the UDDI registry.
webcam
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Web Standards Project
This definition is also listed under presence, site and Website.
The Web Standards Project (WaSP) is a grass roots effort to encourage the main browser makers to create a standard implementation of the Hypertext Markup Language (HTML) and other Web standards and recommendations of the World Wide Consortium (W3C). WaSP maintains that the adoption of these standards by both manufacturers can reduce development time and budgets required for Webbased applications by up to 25%.
A Web site is a related collection of World Wide Web (WWW) files that includes a beginning file called a home page. A company or an individual tells you how to get to their Web site by giving you the address of their home page. From the home page, you can get to all the other pages on their site. For example, the Web site for IBM has the home page address of http://www.ibm.com. (The home page address actually includes a specific file name like index.html but, as in IBM’s case, when a standard default name is set up, users don’t have to enter the file name.) IBM’s home page address leads to thousands of pages. (But a Web site can also be just a few pages.) Since site implies a geographic place, a Web site can be confused with a Web server. A server is a computer that holds the files for one or more sites. A very large Web site may be spread over a number of servers in different geographic locations. IBM is a good example; its Web site consists of thousands of files spread out over many servers in world-wide locations. But a more typical example is probably the site you are looking at, whatis.com. We reside on a commercial space provider’s server with a number of other sites that have nothing to do with Internet glossaries. A synonym and less frequently used term for Web site is ‘‘Web presence.’’ That term seems to better express the idea that a site is not tied to specific geographic location, but is ‘‘somewhere in cyberspace.’’ However, ‘‘Web site’’ seems to be used much more frequently. You can have multiple Web sites that cross-link to files on each others’ sites or even share the same files.
Web site hosting Web site hosting (also known as hosting, Website hosting, Web hosting, and Webhosting) is the business of housing, serving, and maintaining files for one or more Web sites.
Web slate A Web slate is a wireless Internet appliance that consists of a liquid crystal display (LCD) with a touch screen that allows the user to view and interact with Web pages.
Web Solution Platform Web Solution Platform is a framework created by Microsoft for fitting Windows and the PC into the 3-tier application concept. In addition to the Windows operating system (OS) and the Internet Explorer (IE) browser, Microsoft identifies a number of concepts, services, and products as part of its Web Solution Platform. Microsoft’s Web Solution Platform was formerly known as Distributed interNet Applications Architecture (DNA).
In spite of the existence of the W3C’s recommendations almost since the beginning of Web browsers, both Microsoft’s Internet Explorer and Netscape’s Navigator have insisted on promoting and supporting non-standard extensions and in implementing existing recommendations differently, forcing public Web site developers to create different versions of their site, one for each browser (and often additional versions to accommodate changes in evolving browser versions). For future versions of today’s browsers, the WaSP is pushing for full support for W3C recommendations related to the Document Object Model (DOM), Cascading Style Sheets (CSS), and Extensible Markup Language (XML).
Web-based training Web-based training (sometimes called e-learning) is anywhere, any-time instruction delivered over the Internet or a corporate intranet to browser-equipped learners. There are two primary models of Web-based instruction: synchronous (instructor-facilitated) and asynchronous (selfdirected, self-paced). Instruction can be delivered by a combination of static methods (learning portals, hyperlinked pages, screen cam tutorials, streaming audio/video, and live Web broadcasts) and interactive methods (threaded discussions, chats, and desk-top video conferencing). The ASTD (American Society for Training and Development) estimates that 75% of the U.S. workforce will need some kind of retraining within the next five years in order to keep pace with industry needs and increasingly global competition. Hewlett-Packard estimates the half-life of an bachelor’s degree in engineering can be as short as 18 months. Enthusiasts feel that Web-based instruction is the perfect solution to meeting the needs of life-long learners because it is available on demand, does not require travel, and is cost-efficient. Critics point out that Web-based training is a good alternative for independent, selfmotivated students but that technical issues and the need for human contact limit its usefulness for students with other learning styles.
webcam See ‘‘cam’’
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Webcasting
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Also see push technology, another usage. In both usages, netcasting is a synonym.
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The term ‘‘Webcasting’’ is used to describe the ability to use the Web to deliver live or delayed versions of sound or video broadcasts. NetTalk Live! is an example of the former. They use an Internet site to deliver a RealAudio sound version of a live radio and television program at 11 pm (CST) each Sunday night. (They call this a triplecast.) CNet and some other Web sites use the term ‘‘Webcast’’ to describe delayed or preview versions of movies, music videos, or regular radio and television broadcasts as a way to promote the live broadcasts. Each sample is known as a Webisode. Viewing Webcasts requires having an appropriate video viewing application such as the NetShow, RealVideo, or VXtreme streaming video players; these can usually be downloaded from any site offering a Webcast.
WebDAV (World Wide Web Distributed Authoring and Versioning) is the Internet Engineering Task Force (IETF) standard for collaborative authoring on the Web: a set of extensions to the Hypertext Transfer Protocol (HTTP) that facilitates collaborative editing and file management between users located remotely from each other on the Internet. WebDAV is expected to have an impact on the development of virtual enterprises, by enabling remote groups to work together in new ways. For example, WebDAV-conforming tools could be used by a virtual organization to develop business plans, create software, or write libraries of information. The WebDAV work group is part of the applications section of the World Wide Web Consortium (W3C); their charter states its goal as to ‘‘define the HTTP extensions necessary to enable distributed Web authoring tools to be broadly interoperable, while supporting user needs.’’ WebDAV is expected to fulfill early expectations of the Web’s collaborative potential, by adding write access to the read access afforded by HTTP. WebDAV will enable users to collaborate over the Web in the same way as they might over a corporate intranet.
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Locking (also known as concurrency control), which prevents accidental overwriting of files;
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XML properties, which facilitate operations (such as storage and retrieval) on metadata, so that data about other data can be organized;
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The DASL (DAV Searching and Locating) protocol, which enables searches based on property values for locating resources on the Web;
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Namespace manipulation, which supports copy and move operations. Collections, which are similar to file system directories, may be created and listed.
Webification 1) Webification (sometimes seen with a lower case w) is the act of converting content from its original format into a format capable of being displayed on the World Wide Web. Some conversion examples are: L
A Postscript source file or ASCII text to a Hypertext Markup Language (HTML) file
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A Microsoft Word document to HTML (sometimes referred to as ‘‘DOC to HTML’’). More recent versions of Microsoft Word include this capability.
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Hard-copy print publication pages into files in the Portable Document Format (PDF) for viewing on the Web with Adobe’s downloadable Acrobat viewer
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A Lotus Notes database to HTML files
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An image in a scanned-in or other format to a Web-ready image, either a GIF or a JPEG file
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A speech or interview into a file in the RealAudio format for playing as streaming sound on the Web
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A video tape recording into a streaming video file
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WebDAV features include:
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The DAV protocol, which enables property setting, deleting, and retrieving;
Using the File Transfer Protocol (FTP) from the Web browser, text pages (with files in the ASCII TXT format) can also be ‘‘Webified’’ for display by Web users. Many Internet Request for Comment (RFC) documents are available on the Web in the text format. The only Webification these files need is to simply make them available in a directory accessible to the FTP server. 2) Webification is the name of a Web site firm in Hollywood, Florida, that specializes in internetworking and in Web site design.
Webify See ‘‘Webification’’
Webinar A Webinar is a live or replayed interactive multimedia presentation conducted from a Web site. The term combines Web with seminar. A Webinar typically uses some combination of: L
The presenter speaking (usually with streaming audio)
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A panel of presenters
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A chat session that shows typed-in questions and answers as a live Webinar progresses or the entire session when it is played from an archive
of collection of daily highlights from other Web sites. Jessamyn West’s librarian.net is a daily log of items interesting to librarians and possibly others, too.
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A slide presentation that can be viewed simultaneously
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For a small group, a whiteboard that allows the presenter and auditors to draw pictures
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For a small group, PC cameras and microphone that allow some of the auditors to talk to the presenter
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Conference telephone connections to the presenter
As a format and content approach for a Web site, the Weblog seems popular because the viewer knows that something changes every day, there is a personal (rather than bland commercial) point-of-view, and, on some sites, there is an opportunity to collaborate or respond with the Web site and its participants.
One type of Webinar is a chat session with an expert who delivers answers visually and aurally rather than by replying with typed-in responses. A Webinar often specifies that the auditor use or download RealPlayer or Microsoft Media Player and requires site registration. The presentation aspect of a Webinar is sometimes referred to as a Webcast.
Webisode A Webisode is a single push technology episode. A Webisode can be a preview or promotion of a particular TV show, music video, or other show presented from a Web site using streaming video or other techniques. Some sites are dedicated to presenting Webisodes.
Weblog 1) A Weblog (which is sometimes written as ‘‘web log’’ or ‘‘weblog’’) is a Web site of personal or non-commercial origin that uses a dated log format that is updated on a daily or very frequent basis with new information about a particular subject or range of subjects. The information can be written by the site owner, gleaned from other Web sites or other sources, or contributed by users. A Web log often has the quality of being a kind of ‘‘log of our times’’ from a particular point-of-view. Generally, Weblogs are devoted to one or several subjects or themes, usually of topical interest, and, in general, can be thought of as developing commentaries, individual or collective on their particular themes. A Weblog may consist of the recorded ideas of an individual (a sort of diary) or be a complex collaboration open to anyone. Most of the latter are moderated discussions. Since there are a number of variations on this idea and new variations can easily be invented, the meaning of this term is apt to gather additional connotations with time. Currently, the most popular Weblog is Slashdot.org, the product of programmer and graphic artist Rob Malden and several colleagues. Slashdot.org carries discussion thread on many subjects including: Money, Quake (the game), Netscape, Sun Microsystems, Hardware, and Linux. Slashdot.org solicits and posts interesting stories reported by contributors, includes a link to the story, and manages the threads of the ensuing discussion by other users. Another well-known Weblog is Jorn Barger’s Robot Wisdom Log, which is more
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2) Weblog is the name of a software product from South Korea that analyzes a Web site’s access access log and reports the number of visitors, views, hits, most frequently visited pages, and so forth.
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BEA Systems’ WebLogic is a server software application that runs on a middle tier, between back-end databases and related applications and browser-based thin clients. WebLogic is a leading e-commerce online transaction processing (OLTP) platform, developed to connect users in a distributed computing environment and to facilitate the integration of mainframe applications with distributed corporate data and applications. WebLogic server is based on Java 2 Platform, Enterprise Edition (J2EE), the standard platform used to create Javabased multi-tier enterprise applications. J2EE platform technologies were developed through the efforts of BEA Systems and other vendors in collaboration with the main developer, Sun Microsystems. Because J2EE applications are standardized modules, WebLogic can automate many system-level tasks that would otherwise have demanded programming time. The main features of WebLogic server include connectors that make it possible for any legacy application on any client to interoperate with server applications, Enterprise JavaBean (EJB) components, resource pooling, and connection sharing that make applications very scalable. An administration console with a user interface makes management tasks more efficient and features such as Secure Sockets Layer (SSL) support for the encryption of data transmissions, as well as authentication and authorization mechanisms, make applications and transactions secure.
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Webmaster A Webmaster is a person who either:
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Creates and manages the information content (words and pictures) and organization of a Web site
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Or does both.
Companies advertising for a Webmaster vary in their use of the term. In a smaller company, a Webmaster typically ‘‘does it all.’’ In a larger company, a Webmaster tends to be
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Whatis?Com’s Encyclopedia of Technology Terms someone with either a writing and/or graphics design background who has acquired Web site creation skills (mainly knowledge and experience with HTML) or a more technical person with some programming skills. The ‘‘technical’’ Webmaster runs the server (for example, by managing the creation and authorization associated with file systems) and writes programs or Perl scripts required by the Web site. In a very large corporation, there may be a Webmaster team of people at the top of the corporation who establish the overall corporate Web design and policies, arrange the necessary technical resources (working with the people who provide the corporation its network infrastructure), and supervise the design of the corporation’s Web site (which is often done by an outside firm). At division and product levels, there may be additional Webmasters who organize and develop the Web content and programming for their division or product. In addition, there is likely to be an interrelated effort to create a Web design, organization, and content for the corporation’s intranet. At a small corporation, the Webmaster may be in charge of creating the site and putting it on a separate company’s server or setting up one within the company. The Web design and creation may be done initially by an outside Web design firm that turns the finished site over to the company’s in-house Webmaster to maintain and perhaps add content within the established design. And if you are a firm that specializes in creating Web sites, you may refer to the overall producer or art director as the Webmaster for a site. Obviously, this term (and job) is is still defining itself. A Webmaster is what a company says one is. In general, almost any Webmaster would be expected to know the Hypertext Markup Language (HTML) and have a good understanding of why a company should want a Web site.
WebNFS WebNFS is a product and proposed standard protocol from Sun Microsystems that extends its Network File System (NFS) to the Internet. Sun believes WebNFS offers considerable performance advantages over the current Internet protocols, the Hypertext Transport Protocol (HTTP) and the File Transfer Protocol (FTP). Netscape, Oracle, IBM, Apple, and Novell have announced support for WebNFS. To use WebNFS, your Web browser needs a client and the Web or FTP server needs to have a WebNFS server. When requesting a file with WebNFS, your Internet address or Uniform Resource Locator (URL) would look something like this: nfs://computer.site.com/filedirectory/file
WebNFS has several advantages over HTTP and FTP. The WebNFS protocol does not require the opening and closing of a connection for each requested file. Very large file downloads are supported and, because they are
downloaded in sections, they can be resent more quickly in case the download is interrupted. A WebSoft client is included with Sun’s thin-client Network Computer (NC). Sun Microsystems suggests that WebNFS may be a technology proposal to replace or be part of the next generation of HTTP, currently being worked on by members of the World Wide Web Consortium (W3C).
Website See ‘‘Web site’’
WebSphere WebSphere is a set of Java-based tools from IBM that allow customers to create and manage sophisticated business Web sites. The central WebSphere tool is the WebSphere Application Server (WAS), an application server that a customer can use to connect Web site users with Java applications or servlets. Servlets are Java programs that run on the server rather than on the user’s computer as Java applets do. Servlets can be developed to replace traditional common gateway interface (CGI) scripts, usually written in C or Perl, and run much faster because all user requests run in the same process space. In addition to Java, WebSphere supports open standard interfaces such as the Common Object Request Broker Architecture (CORBA) and Java Database Connectivity (JDC) and is designed for use across different operating system platforms. One edition of WebSphere is offered for small-to-medium size businesses and another edition for larger businesses with a higher number of transactions. WebSphere also includes Studio, a developer’s environment with additional components that allow a Web site’s pages to be created and managed. Both editions support Solaris, Windows NT, OS/2, OS/390, and AIX operating systems. WebSphere Studio includes a copy of the Apache Web server so that developers can test Web pages and Java applications immediately.
webtone Webtone is immediate and continuous access to the Internet in the same way that we think of dialtone when we pick up a phone receiver. To have webtone in the same way that we have dialtone, most users of the term believe that not only access is required, but also sufficient bandwidth to meet user demands as well as the same quality of service we expect today from the telephone system. Since the telephone system and the Internet are tending to converge, some believe that eventually webtone will include dialtone. Webtone also implies Internet access from mobile devices, supercomputers, kitchen appliances, and perhaps eventually the very walls that surround us.
whip antenna
WebTV WebTV, now owned by Microsoft, was one of the first entries in the much publicized convergence of the World Wide Web with television. You buy a set-top box similar to a cable TV box, then sign up with the WebTV access service and browse Web pages using a WebTV’s browser and a hand-held control. A keyboard is provided optionally. WebTV uses your television set as an output device; the signals arrive, however, through a modem and a telephone line at 33.6 Kbps (Classic model) or 56 Kbps (Plus model). The most recent WebTV service includes support for frames (with certain limitations), JavaScript, RealAudio, Internet Relay Chat (IRC), and other Web technology.
weenie 1) On bullet board systems (BBS) and in Internet chatting groups, a weenie is an avid but immature participant who disrupts orderly conversation. According to cyberlorist Eric Raymond, a weenie is ‘‘typically, a teenage boy with poor social skills traveling under a grandiose handle derived from fantasy or heavy-metal rock lyrics’’ whose contributions are liable to consist of ‘‘marginally literate and profanity-laden flaming...’’ 2) In the context of program development and among the ‘‘hackerdom’’ that Raymond chronicles, the term weenie can be ascribed respectfully to someone who is highly knowledgeable, intensely committed to, or even just employed on a particular endeavor or in a particular operating system culture. For example, a ‘‘UNIX weenie’’ may mean someone who is an expert at using or modifying UNIX. But, depending on the context, it could also mean a ‘‘UNIX bigot.’’ 3) In the popular game, Quake (and its multiplayer Internet version, QuakeWorld), a weenie is an embarrassingly new player who is usually quickly ‘‘destroyed’’ in some horrible way. All new Quake players apparently qualify as weenies.
weight Weight (symbolized w) is a dimensionless quantity representing the force exerted on a particle or object by an acceleration field, particularly the gravitational field of the earth at the surface. The standard unit of weight in the International System of Units (SI) is the kilogram (kg), which is the force produced by gravitation on a 1-kg mass at the earth’s surface. In the English system, the standard unit of weight is the pound (lb), which is the force produced by gravitational acceleration on approximately 0.454 kg of mass at the earth’s surface. Weight is not the same thing as mass. Mass is a literal representation of the amount of matter in a particle or object, and is independent of external factors such as speed, acceleration, or applied force (as long as relativistic effects are small enough to be neglected). Weight has meaning only
when an object having a specific mass is placed in an acceleration field. At the earth’s surface, a kilogram mass weighs about 2.2 pounds, for example. But on Mars, the same kilogram mass would weigh only about 0.8 pounds, and on Jupiter it would weigh roughly 5.5 pounds. Also see kilogram, mass, and International System of Units (SI).
well-known port numbers The well-known port numbers are the port numbers that are reserved for assignment by the Internet Corporation for Assigned Names and Numbers (ICANN) for use by the application end points that communicate using the Internet’s Transmission Control Protocol (TCP) or the User Datagram Protocol (UDP). Each kind of application has a designated (and thus ‘‘well-known’’) port number. For example, a remote job entry application has the port number of 5; the Hypertext Transfer Protocol (HTTP) application has the port number of 80; and the Post Office Protocol Version 3 (POP3) application, commonly used for e-mail delivery, has the port number of 110. When one application communicates with another application at another host computer on the Internet, it specifies that application in each data transmission by using its port number. The well-known ports cover the range of possible port numbers from 0 through 1023. The registered ports are numbered from 1024 through 49151. The remaining ports, referred to as dynamic ports or private ports, are numbered from 49152 through 65535. On most systems, a well-known port number can only be used by a system (root) process or by a program run by a privileged user. The well-known port numbers are administered by the Internet Internet Assigned Numbers Authority (IANA) on behalf of ICANN.
WetPC WetPC (TM) is a wearable computer for underwater use. Developed by the Australian Institute of Marine Science, it allows a diver to gather and enter data immediately into a computer worn as part of the diver’s gear. Instead of a keyboard or mouse, the diver interacts with the computer using a multi-button device worn at chest level and requiring only one hand. The display is attached to the diver’s mask. The developers invented a new, easy-to-intuit human/computer interface that lets the user provide information in the form of multiple finger position combinations that are always visible on the display.
whip antenna A whip antenna is a single-element antenna that can be used with an unbalanced feed line such as coaxial cable, or attached directly to a wireless transmitter, receiver, or transceiver. The whip resembles a ground-plane antenna without the radial system. The short, flexible ‘‘rubber duck’’
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Whatis?Com’s Encyclopedia of Technology Terms antennas found on handheld two-way radios and cell phones are examples of whip antennas. So are the long, flexible, stainless-steel antennas used in Citizens Band mobile installations. Some portable whip antennas can be telescoped down to a length of only few inches for transport and storage, and extended to several feet for operation. Assets of the whip antenna include electrical and mechanical simplicity. Little or no installation is necessary. But, because most whip antennas are operated with a poor electrical ground system, they are usually inefficient. A whip directly connected to a transmitter radiates radio-frequency (RF) energy into the immediate operating environment. This may cause nearby electronic apparatus (including medical devices) to malfunction. Whip antennas used with handheld radios and cell phones subject the human body, especially the head and hands, to strong electromagnetic fields. This has given rise to uncertainty about the biological safety of whip antennas directly connected to handheld transceivers. In high-powered or long-range wireless communications, substantial outdoor antennas, used with well-engineered feed systems, work much better than whip antennas. In addition, when a transmitting antenna is placed at a distance from humans and electronic equipment, the potential electromagnetic hazard is minimized.
Whistler See ‘‘Windows XP’’
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White Book The White Book, which was released in 1993 by Sony, Philips, Matsushita, and JVC, is the specification document for Video CD (VCD). Like other compact disc (CD) standards documents, the White Book is based on the original CD specification document, Philips and Sony’s Red Book. VCD is defined in the White Book as a particular adaptation of CD-ROM XA (extended architecture) that is designed to hold MPEG-1 video data. The CD-ROM XA sector structure (as described in the Yellow Book and ISO 9660) is used to define the physical and logical blocks, and MPEG-1 is used to compress data so that full-screen, fullmotion video data can be contained on the disc—without compression, the disc could only hold about 2 minutes worth of video. VCD resolution is similar to that of VHS. White Book specifications include the disc format (such as the use of tracks, for example), a data retrieval structure compatible with ISO 9660, data fields to enable fast forward and reverse, and closed captioning. VCD, Photo CD, and Karaoke CD are defined as bridge discs, a format based on CD-ROM XA to enable the discs to work in compatible CDROM and CD-i (CD-Interactive) drives. Following the original specifications, VCD 2.0 was released in 1995, VCDInternet in 1997, and SuperVCD in 1998, all from extensions to the White Book. Video CD is expected to be one of the first
of all the CD formats to be completely supplanted by digital versatile disc (DVD) technology, which is of much higher quality.
white hat White hat describes a hacker (or, if you prefer, cracker) who identifies a security weakness in a computer system or network but, instead of taking malicious advantage of it, exposes the weakness in a way that will allow the system’s owners to fix the breach before it is can be taken advantage by others (such as black hat hackers.) Methods of telling the owners about it range from a simple phone call through sending an e-mail note to a Webmaster or administrator all the way to leaving an electronic ‘‘calling card’’ in the system that makes it obvious that security has been breached. While white hat hacking is a hobby for some, others provide their services for a fee. Thus, a white hat hacker may work as a consultant or be a permanent employee on a company’s payroll. A good many white hat hackers are former black hat hackers. The term comes from old Western movies, where heros often wore white hats and the ‘‘bad guys’’ wore black hats.
white noise White noise is a sound that contains every frequency within the range of human hearing (generally from 20 hertz to 20 kHz) in equal amounts. Most people perceive this sound as having more high-frequency content than low, but this is not the case. This perception occurs because each successive octave has twice as many frequencies as the one preceding it. For example, from 100 Hz to 200 Hz, there are one hundred discrete frequencies. In the next octave (from 200 Hz to 400 Hz), there are two hundred frequencies. White noise can be generated on a sound synthesizer. Sound designers can use this sound, with some processing and filtering, to create a multitude of effects such as wind, surf, space whooshes, and rumbles. Pink noise is a variant of white noise. Pink noise is white noise that has been filtered to reduce the volume at each octave. This is done to compensate for the increase in the number of frequencies per octave. Each octave is reduced by 6 decibels, resulting in a noise sound wave that has equal energy at every octave.
white paper A white paper is an article that states an organization’s position or philosophy about a social, political, or other subject, or a not-too-detailed technical explanation of an architecture, framework, or product technology. Typically, a white paper explains the results, conclusions, or construction resulting from some organized committee or research collaboration or design and development effort.
widget Several versions of Webster’s indicate that the term arose within the past few decades in England to distinguish short government reports from longer, more detailed ones that were bound in blue covers and referred to as ‘‘blue books’’ (not to be confused with the blue books used when taking college exams). A shorter government publication providing a report or position about something was bound in the same white paper as the text—hence, ‘‘a white paper.’’ In information technology, a white paper is often a paper written by a lead product designer to explain the philosophy and operation of a product in a marketplace or technology context. Many if not most Web sites for software products include a white paper in addition to a frequently-asked questions (FAQ) page and more detailed product specifications. In government, a white paper is often a policy or position paper. The U.S. Government’s June, 1998 policy statement on the Management of Internet Names and Addresses (known generally as ‘‘The White Paper’’) is an example of great interest to many Internet users.
wide area network A wide area network (WAN) is a geographically dispersed telecommunications network. The term distinguishes a broader telecommunication structure from a local area network (LAN). A wide area network may be privately owned or rented, but the term usually connotes the inclusion of public (shared user) networks. An intermediate form of network in terms of geography is a metropolitan area network (MAN).
whole number See ‘‘integer’’
WHQL See ‘‘Windows Hardware Quality Labs’’
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wideband CDMA
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See ‘‘WCDMA’’
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See ‘‘WCDMA’’
Recently, the Internet Corporation for Assigned Names and Numbers (ICANN) has opened up domain name registration to a number of other companies. To search all of these companies at the same time for registration information, you can use BetterWhois.
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Wideband is a transmission medium or channel that has a wider bandwidth than one voice channel (with a carrier wave of a certain modulated frequency). This term is usually contrasted with narrowband.
whois is a program that will tell you the owner of any second-level domain name who has registered it with Network Solutions (formerly the only and still the most widely used of the Internet registrars of the com, net, and org domain names). If a Web site obtained its domain name from Network Solutions, you can look up the name of the owner of the Web site by entering (for example):
whois can also be used to find out whether a domain name is available or has already been taken. If you enter a domain name you are considering and the search result is ‘‘No match,’’ the domain name is likely to be available and you can apply to register it.
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wideband code-division multiple access
and whois will tell you the owner of that second-level domain name.
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widget 1) In general, widget (pronounced WIH-jit) is a term used to refer to any discrete object, usually of some mechanical nature and relatively small size, when it doesn’t have a name, when you can’t remember the name, or when you’re talking about a class of certain unknown objects in general. (According to Eric Raymond, ‘‘legend has it that the original widgets were holders for buggy whips,’’ but this was possibly written with tongue-in-cheek.) 2) In computers, a widget is an element of a graphical user interface (GUI) that displays information or provides a specific way for a user to interact with the operating system and application. Widgets include icons, pull-down menus, buttons, selection boxes, progress indicators, on-off checkmarks, scroll bars, windows, window edges (that let you resize the window), toggle buttons, forms, and many other devices for displaying information and for inviting, accepting, and responding to user actions. In programming, a widget also means the small program that is written in order to describe what a particular widget looks like, how it behaves, and how it interacts in response to user actions. Most operating systems include a set of ready-to-tailor widgets that a programmer can incorporate in an application, specifying how it is to behave. New widgets can be created. The term was apparently applied first in UNIX-based operating systems and the X Window System. In object-oriented programming (OOP), each type of widget is defined as a class (or a subclass under a broad
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generic widget class) and is always associated with a particular window. In the AIX Enhanced X-Window Toolkit, a widget is the fundamental data type.
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Most if not all application development languages today, such as Java and Tcl, come with a ready-made library of widgets that a programmer can incorporate and modify. Using Microsoft’s Visual Basic, a widget can be implemented as or part of an ActiveX control.
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wildcard character
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A wildcard character is a special character that represents one or more other characters. The most commonly used wildcard characters are the asterisk (*), which typically represents zero or more characters in a string of characters, and the questionmark (?), which typically represents any one character. For example, in searching:
Wildcard characters are used in regular expressions (a form of programming in which input data is modified based on specified patterns) and in searching through file directories for similar file names (for example, if all the work files on a project start with the characters ‘‘P5,’’ you could easily locate all the project files by simply searching for ‘‘P5*‘‘).
WinChip (also known as the WinChip C6) is a microprocessor designed for use in business computers that run Microsoft operating systems Made by IDT, the microprocessor reportedly provides comparable performance to Intel’s more expensive Pentium microprocessors as well as to the more competitively-priced K6 and the Cyrix 6X86MX. IDT designed a smaller die area for the microprocessor, making it easier and cheaper to manufacture. WinChip cites Winstone tests that show the C6 has the highest rating per square millimeter of die area among Socket 7 microprocessors. The C6 does not include branch prediction and other features of some of the other microprocessors. It is optimized for running the most frequently used instructions and aims to be the most costeffective choice for business applications such as word processing, spreadsheets, and database management.
A wildcard character is a type of meta character. In various games of playing cards, a wild card is a designated card in the deck of cards (for example, the two of spades) that can be used as though it were any possible card.
Willamette WIMP
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The NCSA Mosaic browser was an example of a 32-bit application that would run on Windows 3.1 if you downloaded Win32s.
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Win32s is a Microsoft upgrade to the Windows 3.1 and Windows for Workgroups 3.1 operating systems, which run 16-bit applications, that allows them to run some 32-bit applications. Users of Windows 3.1 or Windows for Workgroups 3.1 who did not want to install Windows 95 (which was designed for 32-bit applications) could download Win32s and install it so that certain 32-bit applications would run.
run*
See ‘‘Pentium 4’’
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Win32s
would mean ‘‘any word that starts with ’run’ and has any kind of ending.’’ If you entered ‘‘run*’’ at a search engine that offered a wildcard character capability, you would get results for run, runs, running, runner, runners—in short, any possible word that might begin with the three letters.
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may include speech recognition and voice command interfaces, haptics devices, eye-movement detection, and new 2-D and 3-D visual models.
WIMP is an acronym describing the desktop user interface familiar to Windows and Mac computer users, significant features of which are windows, icons, a mouse, and pulldown menus. Other explanations of what the letters stand for include: window-icon-mouse-pointer and window-iconmenu-pointing device. This combination of computer-user interface ideas originated at Xerox’s Palo Alto Research Laboratory, was incorporated in early Apple computers, and adopted by Microsoft in its Windows operating system, in UNIX’s X Window System, in IBM’s OS/2, and in other operating systems. The WIMP interface is now so familiar to most of us that it may be difficult to understand that other models for a user interface are also possible. Since learning a user interface is a personal as well as a business investment, it is likely that future user interfaces will continue to include all or parts of the WIMP environment. New user interfaces
See ‘‘Windows 9x’’
WinChip
window A window is a separate viewing area on a computer display screen in a system that allows multiple viewing areas as part of a graphical user interface (GUI). Windows are managed by a windows manager as part of a windowing system. A window can usually be resized by the user. For example, it can be stretched on any side, minimized, maximized, and closed. On today’s multitasking operating systems, you can have a number of windows on your screen at the same time, interacting with each whenever you choose. The window first came into general use as part of the Apple Macintosh. Later, Microsoft made the idea the foundation of its Windows operating system (which was actually a graphical user interface for the Disk Operating System (DOS) operating system on IBM-compatible PCs). The X
Windows 98 Window System was developed as an open cross-platform windowing system for use in networks. It allows a client application in one computer to request windowing services at a user’s workstation computer.
The Windows 2000 product line consists of four products:
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Window RAM (WRAM), unrelated to Microsoft Windows, is very high-performance video RAM that is dual-ported and has about 25% more bandwidth than VRAM but costs less. It has features that make it more efficient to read data for use in block fills and text drawing. It can be used for very high resolution (such as 1600 by 1200 pixels) projection using true color. WRAM is used in the Matrox Millenium video card.
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windowing system A windowing system is a system for sharing a computer’s graphical display presentation resources among multiple applications at the same time. In a computer that has a graphical user interface (GUI), you may want to use a number of applications at the same time (this is called multitasking). Using a separate window for each application, you can interact with each application and go from one application to another without having to reinitiate it. Having different information or activities in multiple windows may also make it easier for you to do your work. A windowing system uses a window manager to keep track of where each window is located on the display screen and its size and status. A windowing system doesn’t just manage the windows but also other forms of graphical user interface entities. The X Window System is a cross-platform windowing system that uses the client/server model to distribute services in a network so that applications can run in a remote computer. Users of workstations or terminals using the X Window System don’t need to know where the application is located. Apple’s Macintosh and Microsoft’s Windows operating systems have their own windowing systems built into the operating system.
Windows 2000 Previously called Windows NT 5.0, Windows 2000 is described by Microsoft as evolutionary and ‘‘Built on NT Technology.’’ Windows 2000 is designed to appeal to small business and professional users as well as to the more technical and larger business market for which the NT was designed.
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Windows 2000 Professional, aimed at individuals and businesses of all sizes. It includes security and mobile use enhancements. It is the most economical choice. Windows 2000 Server, aimed at small-to-medium size businesses. It can function as a Web server and/or a workgroup (or branch office) server. It can be part of a two-way symmetric multiprocessing system. NT 4.0 servers can be upgraded to this server. Windows 2000 Advanced Server, aimed at being a network operating system server and/or an application server, including those involving large databases. This server facilitates clustering and load-balancing. NT 4.0 servers with up to eight-way SMP can upgrade to this product. Windows 2000 Datacenter Server, designed for large data warehouses, online transaction processing (OLTP), econometric analysis, and other applications requiring high-speed computation and large databases. The Datacenter Server supports up to 16-way SMP and up to 64 gigabytes of physical memory.
Windows 2000 is reported to be more stable (less apt to crash) than Windows 98/NT systems. A significant new feature is Microsoft’s Active Directory, which, among other capabilities, enables a company to set up virtual private networks, to encrypt data locally or on the network, and to give users access to shared files in a consistent way from any network computer.
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Windows 98 Windows 98 (called ‘‘Memphis’’ during development and previously called ‘‘Windows 97’’ based on an earlier schedule) is a widely-installed product in Microsoft’s evolution of the Windows operating system for personal computers. Windows 98 expresses Microsoft’s belief that users want and should have a global view of their potential resources and that Web technology should be an important part of the user interface. Although building Microsoft’s own Web browser into the user desktop has been an issue in the U.S. Justice Department’s suit, Windows 98 was released as planned with its tightly integrated browser.
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In Windows 98, Microsoft’s Internet Explorer is an integral part of the operating system. Using the Active Desktop of Windows 98, you can view and access desktop objects that reside on the World Wide Web as well as local files and applications. The Windows 98 desktop is, in fact, a Web page with HTML links and features that exploit Microsoft’s ActiveX control.
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With Windows 98 (or with Internet Explorer 4.0 in Windows 95), you can set up news and other content to be push technology to you from specified Web sites.
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Windows 98 also provides a 32-bit file allocation table (FAT) that allows you to have a single-partition disk drive larger than 2 Gbytes. Other features in Windows 98 include: L
Support for Universal Serial Bus (USB), which makes it easy to plug in new devices
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Support for Digital Versatile Disc (DVD)
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Support for a new industry-standard form of power management called Advanced Configuration and Power Interface (ACPI)
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Windows 98 is gradually being replaced by Windows 2000, an evolution of the Windows OS, that is designed for personal or small-office professional or business use.
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Windows 9x Windows 9x is short for ‘‘Windows 95 or Windows 98.’’
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Windows CE is based on the Microsoft Windows operating system but is designed for including or embedding in mobile and other space-constrained devices. Although Microsoft does not explain the ‘‘CE,’’ it is reported to have originally stood for ‘‘Consumer Electronics.’’ Windows CE is used in several brands of handheld computers and as part of cable TV set-top boxes built for TCI. It competes with EPOC and also with similar operating systems from 3Com (for its PalmPilot) and other companies. Like the full-scale Windows systems, Windows CE is a 32-bit multitasking, multithreading operating system. Microsoft emphasizes that the system was ‘‘built from scratch’’ while taking advantage of Windows architectural concepts and interfaces. Microsoft argues that Windows desktop system users will find that products with Windows CE provide a familiar user interface. In addition to handheld computers and cable TV boxes, Windows CE is also offered as the operating system for the Auto PC, Microsoft’s concept of controlling applications (such as selecting radio channels) while driving, using interactive speech technology.
Windows Hardware Quality Labs Windows Hardware Quality Labs (WHQL) is a Microsoft procedure for certifying that the hardware for peripherals and other components is compatible (works as expected) with Microsoft Windows operating systems. WHQL provides test kits to third-party developers so that they can test their product’s compatibility. Products that are submitted to and meet the tests at Microsoft are allowed to display the Microsoft Windows logo on their marketing materials and are included in Microsoft’s Hardware Compatibility List (HCL). A hardware developer downloads a WHQL test kit for a specific product from Microsoft’s Web site. The test kit includes a test tool, test procedures, and errata and is used
throughout the development process of the hardware component. The errata shows which errors encountered during testing are not considered a failure of the component and are overlooked by WHQL. When the component is finished, the latest WHQL test kit is downloaded and used to test the final product. If the component passes, the developer submits his test to WHQL. After WHQL receives the test submission, the developer is notified by e-mail within three days that the submission was received. WHQL processes the submission, which can take up to 30 days. WHQL then sends the developer a report via e-mail. If the component passed, the developer receives a signed Logo License Agreement with a logo kit containing the camera-ready logo artwork, an inclusion of the hardware component in Microsoft’s Hardware Compatibility List (HCL), and a distribution of the hardware driver, if needed, on Microsoft’s Windows Driver Library. If the component didn’t pass, the developer must make the necessary changes to the component and retest.
Windows Internet Naming Service Windows Internet Naming Service (WINS), part of the Microsoft Windows NT and 2000 Servers, manages the association of workstation names and locations with Internet Protocol addresses (IP addresses) without the user or an administrator having to be involved in each configuration change. WINS automatically creates a computer name-IP address mapping entry in a table, ensuring that the name is unique and not a duplicate of someone else’s computer name. When a computer is moved to another geographic location, the subnet part of the IP address is likely to change. Using WINS, the new subnet information will be updated automatically in the WINS table. WINS complements the NT Server’s Dynamic Host Configuration Protocol (DHCP), which negotiates an IP address for any computer (such as your workstation) when it is first defined to the network. If you’re a computer user on a network connected to a Windows NT/2000 Server, you may find WINS mentioned in some of your network-related programs or system messages. Based on Microsoft’s paper, DHCP and WINS have been submitted to the Internet Engineering Task Force (IETF) as proposed open standards in Request for Comments 1533, 1534, 1541, and 1542. New features are included in Windows 2000.
Windows Me Windows Me (Millenium Edition) is an operating system from Microsoft, released commercially September 14, 2000. Windows Me was developed specifically for the home user, just as their Windows 2000 version was developed specifically for the business user. Microsoft claims that this Windows 98 update (which has a very similar look-and-feel
Windows NT to the earlier version) combines a more intuitive interface for the new user with added functionality for the experienced user. Designed to be backwards compatible with Windows 98, Windows Me works with the earlier version’s applications and drivers. Improvements over Windows 98 include functions to enhance system stability and error correction (what Microsoft is calling ‘‘PC health’’), as well as features designed for digital media, home networking, and online experience:
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System restore, which allows users to return to a prior (functioning) configuration when encountering problems
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System file protection, which prevents accidental overwriting of critical system files
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Auto-update, which automatically downloads Microsoft updates
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Image acquisition, which facilitates transfer of images from digital equipment
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Movie Maker, which enables digital editing, saving and sharing of videos
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Media Player 7, which coordinates and organizes digital media operations DirectPlay voice chat, which enables chat between users playing games over the Web
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Home Networking Wizard, which walks the user through steps to connect multiple computers in a home
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Improved technology for Internet connection-sharing
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Simplified Universal Serial Bus (USB) networking
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Universal plug-and-play technology
Windows NT Windows NT is a Microsoft Windows personal computer operating system designed for users and businesses needing advanced capability. NT’s technology is the base for the Microsoft successor operating system, Windows 2000. Windows NT (which may originally have stood for ‘‘New Technology,’’ although Microsoft doesn’t say) is actually two products: Microsoft NT Workstation and Microsoft NT Server. The Workstation is designed for users, especially business users, who need faster performance and a system a little more fail-safe than Windows 95 and Windows 98. The Server is designed for business machines that need to provide services for network-attached computers. The Server is required, together with an Internet server such as Microsoft’s Internet Information Server (IIS), for a Windows system that plans to serve Web pages. Windows NT Workstation: Microsoft says that 32-bit applications run 20% faster on this system than on Windows 95 (assuming both have 32 megabytes of RAM). Since older 16-bit applications run in a separate address space, one can crash without crashing other applications or the operating system. Security and management features not available on Windows 95 are provided. The Workstation has the same desktop user interface as Windows 95. Windows NT Server: The NT Server is probably the second most installed network server operating system after Novell’s NetWare operating system. Microsoft claims that its NT servers are beginning to replace both NetWare and the various UNIX-based systems such as those of Sun Microsystems and Hewlett-Packard. NT Server 5.0. essentially became what was renamed Windows 2000. Notable features of the Windows 2000 products are:
Internet Explorer 5.5
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NetMeeting 3.0 for conference capability
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Enhanced interoperability of Outlook and MSN Messenger Service
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Easier Web publishing
Although this is the first time that many of these features have been combined into a package, it’s worthwhile to note that many—including Media Player 7.0, Internet Explorer 5.5, DirectX 7.0a and NetMeeting 3.0—are already available for free download from Microsoft’s Web site. Windows Me is to be the last operating system based on the Windows 95 kernel.
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A fully-customizable administrative console that can be based on tasks rather than files, applications, or users:
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A new file directory approach called Active Directory that lets the administrator and other users view every file and application in the network from a single point-ofview.
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Dynamic Domain Name Server (DNS), which replicates changes in the network using the Active Directory Services, the Dynamic Host Configuration Protocol (DHCP), and the Windows Internet Naming Service (WINS) whenever a client is reconfigured.
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The ability to create, extend, or mirror a disk volume without having to shut down the system and to back up data to a variety of magnetic and optical storage media.
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A Distributed File System (DFS) that lets users see a distributed set of files in a single file structure across departments, divisions, or an entire enterprise.
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Close integration with and support for Microsoft’s Message Queue Server, Microsoft Transaction Server, and Internet Information Server (IIS).
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Windows XP (code named Whistler by Microsoft) is the latest version of the Windows desktop operating system for the PC. Microsoft and trade publication writers view Windows XP as the most important version of Windows since Windows 95. Windows XP is built on the Windows 2000 kernel but brings a new, more personalized look to the desktop that will also make it easier for users to scan or import images and to acquire music files on the Web and transfer them to portable devices. The new Windows will allow different family members to use their own desktop and personal sets of files. In addition to the ‘‘My Computer’’ and ‘‘My Documents’’ views provided in Windows 2000, Windows XP users see ‘‘My Music’’ and ‘‘My Pictures.’’ The Start Menu has been redesigned to make the most-used programs easiest to find. Windows XP will come in a Professional version and a Home Edition version.
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windows, icons, mouse, and pull-down menus
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WinFrame is a software product from Citrix that, together with a Windows NT operating system, allows a computer server to provide Windows applications and data for attached computer workstations. With Winframe, a company can install all applications and data at the server, simplifying administration, and possibly saving on application software cost. The low-cost workstations are known as thin clients because they are minimally-equipped and because they contain client software that interacts with the server. To the user, the applications and data appear to be running on the workstation. WinFrame can be compared to two other thin client approaches, the Network Computer (NC) and the NetPC. Up to 15 workstations can be supported by a server with Windows NT and WinFrame. The NT server computer needs 32 megabytes of random access memory (RAM) and an additional 6 to 8 MB for each attached workstation. Each user is allocated 32 MB of data storage on the server’s hard disk. A significant advantage of WinFrame is that it can make newer Windows applications available to older PCs. WinFrame can also handle users who dial in to the server (for example, mobile users and those working at home or in branch offices). There are two key components in the WinFrame approach: 1) The Intelligent Console Architecture (ICA) Windows Presentation Services, a protocol that manages the user (client) input and the server output so that the data that travels back and forth is reduced to a minimum. Data travels in compression packet.
2) The MultiWin Multi-User Architecture, which allows NT to manage a virtual session for each user of the same application. Microsoft and Citrix co-developed Microsoft’s Terminal Server product Edition of Windows NT 4.0, using WinFrame’s multi-user technology. In addition, Citrix offers MetaFrame, a product that adds the thin-client/server interface to the Terminal Server. Together, the Terminal Server and MetaFrame replace WinFrame in Windows NT 4.0 and forthcoming versions.
Wingate Wingate is a product that allows people on a small home network or a larger business network to share and control access to the Internet through a single computer connection. The Wingate program can be installed in a computer hooked up to the Internet with a dial-up, Integrated Services Digital Network (ISDN), Digital Subscriber Line (DSL), cable modem, or dedicated T-carrier system connection. The computer with Wingate acts as a proxy server and firewall for the computer users inside the home or business. All users share a common Internet connection through one computer, which does not have to be dedicated to its gateway role. Wingate requires that all computers use a Windows 95 or later operating system and include TCP/IP. Wingate has a server component and a client component. The server component is installed on the computer that has the Internet connection capability. The client component is installed on the other computers in the network. The server component then routes all Internet requests from the other computers through the Internet connection. Wingate allows users to connect or disconnect to the Internet using their own computers. The firewall component in Wingate prevents outsiders from accessing network computers. When Wingate receives a request for access into the local network, it compares the IP address of the computer requesting access with a list of those eligible. If the IP address is not recognized, the request is denied. Wingate also uses TCP port binding as a method of controlling access. Network administrators can assign access rights to individuals or groups. For example, in a home network in which children use the Internet, a parent can prohibit access to sites that are not suitable. A history also keeps track of the Web sites visited, time spent online, and other information of each individual. This helps the administrator determine whether an individual is abusing his access rights. Wingate also provides Web page caching.
WinNuke See ‘‘blue bomb’’
WinZip
WINS
Winsock 2
See ‘‘Windows Internet Naming Service’’
Like Winsock, Winsock 2 is a programming interface and the supporting program that handles input/output requests for Internet applications in a Windows operating system. It’s called Winsock because it’s an adaptation for Windows of the Berkeley UNIX sockets interface. Sockets is a particular convention for connecting with and exchanging data between two program processes. Winsock 2 is a 32-bit version of Winsock.
Winsock Winsock is a programming interface and the supporting program that handles input/output requests for Internet applications in a Windows operating system. It’s called Winsock because it’s an adaptation for Windows of the Berkeley UNIX sockets interface. Sockets is a particular convention for connecting with and exchanging data between two program processes within the same computer or across a network. Winsock runs between an application program such as a Netscape browser and the Internet program in your computer that uses TCP/IP. A request flows in the following order: Web browser or other application | winsock.dll | TCP/IP layers | Modem or network card | The Internet and destination Winsock provides this interface for different versions of the Windows operating system. A comparable interface exists for Mac computers. Beginning with Windows 95, Winsock came as part of the operating system, but in earlier systems, a Winsock program had to be installed. UNIX systems do not require a Winsock equivalent because TCP/IP and its use of sockets was designed to run directly with UNIX application programs. A number of companies offer a Winsock program, sometimes along with a suite of Internet protocol programs and applications. For example, Chameleon offers a suite that includes a Web browser, an FTP utility, a mail utility, and others. The Winsock program is included. The Trumpet Winsock is another popular stand-alone version. Winsock runs as a Windows dynamic link library (DLL) file. That is, it is loaded into the computer when an application needs it but doesn’t need to be included as part of the application. If you have an older computer, when you initially get set up with Internet access, you may need to make sure you have the right version of Winsock for your operating system and the applications provided by the access provider. If your operating system provides one version and the application suite provided by the access provider provides another, one version of Winsock may need to be removed.
Winsock 2 runs between an application program such as a Web browser and the program in your computer that handles TCP/IP.
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In addition to TCP/IP, it provides an interface to a variety of protocols, including IPX/SPX, ISDN and wireless protocols.
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An application can request multicast and other protocol services not supported by Winsock 1.1.
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It offers access to multiple name spaces.
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It offers an application the possibility of choosing services based on cost.
Winsock 2 is included in the latest versions of the Windows operating systems.
Wintel Wintel is a computer trade industry term for personal computers based on the Intel microprocessor and one of the Windows operating systems from Microsoft. The term ‘‘PC’’ has often been used for this purpose. That is, the IBM ‘‘PC’’ (with the Intel chip and Windows operating system) became the prevalent personal computer in the business world and has usually been distinguished from personal computers from Apple (with a Motorola microchip and a proprietary operating system) and sometimes from UNIX workstations. In most trade publication articles, Wintel implies a Pentiumlevel microprocessor and the Windows 95 or NT operating systems.
WinZip WinZip is a Windows program that lets you archive and compress files so that you can store or distribute them more efficiently. WinZip is a more capable and easier-to-use Windows equivalent of two earlier programs commonly used in the DOS operating system, PKZIP and PKUNZIP. WinZip has a simple drag-and-drop interface that allows you to view individual files in a zip file without unzipping the file. WinZip will also launch installation programs from a zip file and automatically clean up after the installation. When creating a zip file (or archive), you can choose from five levels of compression, including ‘‘None,’’ for each added file. With a disk-spanning add-on, you can also create a zip file that will span multiple diskettes.
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WinZip also supports other popular Internet file formats, including tar, gzip, Unix compress, Uuencode, BinHex, and MIME. ARJ, LZH, and ARC files are supported through other programs. WinZip provides an interface to most virus scanner programs and is available in 16-bit and 32-bit versions.
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Wire speed is whatever rate of data transfer a given telecommunication technology provides at the physical wire level. Wire-speed, an adjective, describes any hardware box or function that tends to support this data transfer rate without slowing it down. It’s common to refer to functions embedded in microchips rather than in software programming as working at wire speed. Switches,, routers, and other devices are sometimes described by their manufacturers as operating at wire speed. Data encryption and decryption and hardware emulation are software functions that might run at wire speed (or close to it) when embedded in a microchip.
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Wired Equivalent Privacy (WEP) is a security mechanism, specified in the IEEE Wireless Fidelity (Wi-Fi) standard, 802.11b. The WEP protocol is designed to provide a wireless local area network (WLAN) with a similar level of privacy to what is possible for a wired LAN. A wired local area network (LAN is generally protected by physical security mechanisms (controlled access to a building, for example) that are effective for a controlled physical environment, but may be ineffective for WLANs because radio waves are not necessarily bound by the walls containing the network. WEP seeks to establish similar protection to that offered by the wired network’s physical security measures by encrypting data transmitted over the WLAN. Data encryption protects the vulnerable wireless link between clients and access points; once this measure has been taken, other typical LAN security mechanisms such as password protection, end-toend encryption, virtual private networks (VPNs), and authentication can be put in place to ensure privacy. A research group from the University of California at Berkeley recently published a report citing ‘‘major security flaws’’ in WEP that left WLANs using the protocol vulnerable to attacks (called wireless equivalent privacy attacks). In the course of the group’s examination of the technology, they were able to intercept and modify transmissions and gain access to restricted networks. The Wireless Ethernet Compatibility Alliance (WECA) claims that WEP—which is included in many networking products—was never intended to be the sole security mechanism for a WLAN, and that, in conjunction with
traditional security practices, it is very effective. Nevertheless, IEEE enhancements to 802.11 security are expected to be announced by the end of 2001.
wireless Wireless is a term used to describe telecommunications in which electromagnetic waves (rather than some form of wire) carry the signal over part or all of the communication path. Some monitoring devices, such as intrusion alarms, employ acoustic waves at frequencies above the range of human hearing; these are also sometimes classified as wireless. The first wireless transmitters went on the air in the early 20th century using radiotelegraphy (Morse code). Later, as modulation made it possible to transmit voices and music via wireless, the medium came to be called ‘‘radio.’’ With the advent of television, fax, data communication, and the effective use of a larger portion of the spectrum, the term ‘‘wireless’’ has been resurrected. Common examples of wireless equipment in use today include: L
Cellular phones and pagers—provide connectivity for portable and mobile applications, both personal and business
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Global Positioning System (GPS)—allows drivers of cars and trucks, captains of boats and ships, and pilots of aircraft to ascertain their location anywhere on earth
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Cordless computer peripherals—the cordless mouse is a common example; keyboards and printers can also be linked to a computer via wireless
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Cordless telephone sets—these are limited-range devices, not to be confused with cell phones
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Home-entertainment-system control boxes—the VCR control and the TV channel control are the most common examples; some hi-fi sound systems and FM broadcast receivers also use this technology
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Remote garage-door openers—one of the oldest wireless devices in common use by consumers; usually operates at radio frequencies
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Two-way radios—this includes Amateur and Citizens Radio Service, as well as business, marine, and military communications
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Baby monitors—these devices are simplified radio transmitter/receiver units with limited range
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Satellite television—allows viewers in almost any location to select from hundreds of channels
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Wireless LANs or local area networks—provide flexibility and reliability for business computer users Wireless technology is rapidly evolving, and is playing an increasing role in the lives of people throughout the world. In addition, ever-larger numbers of people are relying on the technology directly or indirectly. (It has
Wireless Bitmap been suggested that wireless is overused in some situations, creating a social nuisance.) More specialized and exotic examples of wireless communications and control include: L
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Global System for Mobile Communication (GSM)—a digital mobile telephone system used in Europe and other parts of the world; the de facto wireless telephone standard in Europe General Packet Radio Service (GPRS)—a packet-based wireless communication service that provides continuous connection to the Internet for mobile phone and computer users Enhanced Data GSM Environment (EDGE)—a faster version of the Global System for Mobile (GSM) wireless service Universal Mobile Telecommunications System (UMTS)—a broadband, packet-based system offering a consistent set of services to mobile computer and phone users no matter where they are located in the world Wireless Application Protocol (WAP)—a set of communication protocols to standardize the way that wireless devices, such as cellular telephones and radio transceivers, can be used for Internet access i-Mode—the world’s first ‘‘smart phone’’ for Web browsing, first introduced in Japan; provides color and video over telephone sets Wireless can be divided into: Fixed wireless—the operation of wireless devices or systems in homes and offices, and in particular, equipment connected to the Internet via specialized modems Mobile wireless—the use of wireless devices or systems aboard motorized, moving vehicles; examples include the automotive cell phone and PCS (personal communications services) Portable wireless—the operation of autonomous, battery-powered wireless devices or systems outside the office, home, or vehicle; examples include handheld cell phones and PCS units IR wireless—the use of devices that convey data via IR (infrared) radiation; employed in certain limited-range communications and control systems
wireless application service provider WASP is also an acronym for the Web Standards Project. A wireless application service provider (WASP) is part of a growing industry sector resulting from the convergence of two trends: wireless communications and the outsourcing of services. A WASP performs the same service for wireless clients as a regular application service provider (ASP) does for wired clients: it provides Web-based access to applications and services that would otherwise have to be stored locally. The main difference with WASP is that it enables customers to access the service from a variety of wireless devices, such as a smartphone or personal digital assistant (PDA). Although the business world is increasingly mobile, many corporations are resisting the idea of wireless communication, because of concerns about set-up and maintenance costs and the need for in-house expertise. WASPs offer businesses the advantages of wireless service with less expense and fewer risks. Because mobile applications are subscribed to, rather than purchased, up-front costs are lower; because the WASP provides support, staffing and training costs are lower. WASP services may include: L
Constant system monitoring
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Text formatting for various devices
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Problem detection and reporting
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wireless ASP
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See ‘‘wireless application service provider’’
Wireless Bitmap
See ‘‘WAX’’
A Wireless Bitmap (WBMP) is a graphic image format for use when sending Web content to handheld wireless devices. The format is defined as part of the Wireless Application Protocol (WAP), Wireless Application Environment (WAE) Specification. If you are creating Web content that is directed to handheld phones or personal digital assistants (PDA) that have Web access, you use the Wireless Markup Language (WML) to encode the page and its text. An image can be included in the form of a WBMP
See ‘‘WAP’’
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There are still issues to be resolved. Coverage areas remain limited, for example, and data synchronization among devices can be problematic. Nevertheless, WASPs provide an easier, safer, and cheaper way for organizations to add mobile components, and a number of major companies are opting for them. UPS, Sprint, and eBay are among the early subscribers to WASP services. Interestingly, some ASPs have begun to offer WASP services, while others are purchasing them.
Wireless Abstract XML Wireless Application Protocol
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Whatis?Com’s Encyclopedia of Technology Terms file—initially, supported only in black-and-white— that you can convert from a Graphics Interchange Format (GUI), Tag Image File Format (TIFF), or other graphic formats. The initial WAP WAE specification supports only WBMP type 0, which is an compression image in monochrome. Later, as the bandwidth for wireless transmission increases, richer images will be supported.
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A wireless LAN is one in which a mobile user can connect to a local area network (LAN) through a wireless (radio) connection. A standard, IEEE 802.11, specifies the technologies for wireless LANs. The standard includes an encryption method, the Wired Equivalent Privacy algorithm.
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High-bandwidth allocation for wireless will make possible a relatively low-cost wiring of classrooms in the U.S. A similar frequency allocation has been made in Europe. Hospitals and businesses are also expected to install wireless LAN systems where existing LANs are not already in place.
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A wireless LAN adapter can be made to fit on a PC card for a laptop or notebook computer.
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wireless local area network See ‘‘wireless LAN’’
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Wireless Markup Language WML (Wireless Markup Language), formerly called HDML (Handheld Devices Markup Languages), is a language that allows the text portions of Web pages to be presented on cellular telephones and personal digital assistants (PDAs) via wireless access. WML is part of the Wireless Application Protocol (WAP) that is being proposed by several vendors to standards bodies. The Wireless Application Protocol works on top of standard data link protocols, such as Global System for Mobile communication, code-division multiple access, and time division multiple access, and provides a complete set of network communication programs comparable to and supportive of the Internet set of protocols. WML is an open language offered royalty-free. Specifications are available at Phone.com’s Web site. According to Phone.com, any programmer with working knowledge of HTML, CGI, and Structured Query Language should be able to write a presentation layer using WML. A filter program can be written or may be available from a vendor that will translate HTML pages into WML pages.
wireless service provider A wireless service provider (WSP) is a company that offers transmission services to users of wireless devices (handheld computers and telephones) through radio frequency (RF) signals rather than through end-to-end wire communication. Generally, a WSP offers either cellular telephone telephone service, personal communication service (PCS) service, or both. The term also seems applicable to satellite television and Internet access providers.
Wireless Transport Layer Security Wireless Transport Layer Security (WTLS) is the security level for Wireless Application Protocol (WAP) applications. Based on Transport Layer Security (TLS) v1.0 (a security layer used in the Internet, equivalent to Secure Socket Layer 3.1), WTLS was developed to address the problematic issues surrounding mobile network devices—such as limited processing power and memory capacity, and low bandwidth—and to provide adequate authentication, data integrity, and privacy protection mechanisms. Wireless transactions, such as those between a user and their bank, require stringent authentication and encryption to ensure security to protect the communication from attack during data transmission. Because mobile networks do not provide end-to-end security, TLS had to be modified to address the special needs of wireless users. Designed to support datagrams in a high latency, low bandwidth environment, WTLS provides an optimized handshake through dynamic key refreshing, which allows encryption keys to be regularly updated during a secure session.
wireless Web The wireless Web refers to use of the World Wide Web through a wireless device, such as a cellular telephone or personal digital assistant (PDA). Wireless Web connection provides anytime/anywhere connection to e-mail, mobile banking, instant messaging, weather and travel information, and other services. In general, sites aiming to accommodate wireless users must provide services in a format displayable on typically small wireless devices. It is estimated that 95% of wireless Internet devices being manufactured today use the Wireless Application Protocol (WAP) developed by Ericsson, Motorola, Nokia, and Unwired Planet (now Phone.com) for presenting content. The wireless Web is not gaining in popularity as quickly as some have predicted. The low bandwidth of today’s wireless service, relatively high usage charges, and small and difficult-to-use input and output devices contribute to impeding growth, a condition that has been referred to as ‘‘wapathy’’ (WAP apathy).
workflow
See ‘‘wireless LAN’’
used to express work. One erg reduces to one gramcentimeter squared per second squared (g ˙ cm2/s2 or g ˙ cm2 -2 ˙ s ) in base SI units.
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To convert from joules to ergs, mulitiply by 10,000,000 (107). Conversely, multiply by 0.0000001 (10-7).
WLAN
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word In computer architecture, a word is a unit of data that can be moved in a single operation from storage to a processor register. In the most familiar architectures of the past few decades, a word has been four eight-bit bytes in length, or 32 bits. Both IBM’s mainframe processors and Intel’s processors, used in standard PCs, have used a 32-bit word. Recent processor architectures from Intel and others provide for a 64-bit word (which may be treated as though two 32-bit words can be loaded at the same time into two different registers). In a 32-bit word system, a 16-bit unit of data is sometimes referred to as a half-word. A word can contain a computer instruction, a storage address, or application data that is to be manipulated (for example, added to the the data in another word space).
word processor A word processor is a computer program that provides special capabilities beyond that of a text editor. The term originated to distinguish editors that were ‘‘easy to use’’ from conventional text editors, and to suggest that the program was more than just an ‘‘editor.’’ An early user of this term was Wang, which made a popular workstation system designed especially for secretaries and anyone else who created business letters and other documents. In general, word processors screen the user from structural or printer-formatting markup (although WordPerfect and other word processors optionally let you see the markup they insert in your text). Without visible markup, it’s possible to describe a word processor as having a WYSIWYG (what you see is what you get) user interface. The most popular word processor is Microsoft Word. Many people still use a former favorite, WordPerfect.
work Work is force applied over distance. Examples of work include lifting an object against the Earth’s gravitation, driving a car up a hill, and pulling down a captive helium balloon. Work is a mechanical manifestation of energy. The standard unit of work is the joule (J), equivalent to a newton-meter (N ˙ m). This reduces to one kilogram-meter squared per second squared (kg ˙ m2/s2 or kg ˙ m2 ˙ s-2) in base International System of Units (SI) units. Alternatively, the erg, equivalent to a dyne-centimeter (dyn ˙ cm), can be
For a given force F (in newtons) applied over a displacement d (in meters), the work w (in joules) is given by: w = Fd If, in practice, force is not applied in the same direction as the displacement that results in actual work (this is the case for a car ascending a hill, for example), the vector form of the above formula must be used. For a given force vector F (in newtons, in a specified direction) applied to an object that undergoes displacement d (in meters), the work w (in joules) is given by the dot product of the force vector and the displacement vector: w = F ˙ d = Fd cos q where q is the angle between the applied force vector and the direction of the displacement that results in actual work. The above formulas also apply for work in ergs, force magnitude in dynes, and displacement magnitude in centimeters. Although work is usually defined in mechanical terms, it can result from the action of electric fields, magnetic fields, thermal heating, particle bombardment, and various other phenomena. Also see energy, force, mass, newton, second, SI, and Table of Physical Units.
workflow Workflow is a term used to describe the tasks, procedural steps, organizations or people involved, required input and output information, and tools needed for each step in a business process. A workflow approach to analyzing and managing a business process can be combined with an object-oriented programming approach, which tends to focus on documents and data. In general, workflow management focuses on processes rather than documents. A number of companies make workflow automation products that allow a company to create a workflow model and components such as online forms and then to use this product as a way to manage and enforce the consistent handling of work. For example, an insurance company could use a workflow automation application to ensure that a claim was handled consistently from initial call to final settlement. The workflow application would ensure that each person handling the claim used the correct online form and successfully completed their step before allowing the process to proceed to the next person and procedural step. A workflow engine is the component in a workflow automation program that knows all the procedures, steps in a procedure, and rules for each step. The workflow engine determines whether the process is ready to move to the next
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step. Some vendors sell workflow automation products for particular industries such as insurance and banking or for commonly-used processes such as handling computer service calls. Proponents of the workflow approach believe that task analysis and workflow modeling in themselves are likely to improve business operations.
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A working draft (sometimes called a draft document) is a type of technical report that is a work in progress, a preliminary form of a possible future document. A working draft indicates a commitment on the part of the issuing organization to do further work in the area outlined in the document. Several revisions of the working draft may be issued before the final document is written, or the document may be made obsolete by future developments. The World Wide Web Consortium (W3C) provides extensive guidelines for their own Working Drafts, which may be followed by other groups. W3C Working Drafts must include a provision in the status section of the document specifying that it is a work-in-progress and, as such, may be updated, replaced, or made obsolete by subsequent documents and, furthermore, that it does not represent W3C or member consensus. Readers are cautioned not to use the document as a reference or, if they do cite it, to refer to it as a work in progress. According to W3C specifications for their own technical reports, the Working Draft is the first of a minimum of four documents which may lead to a Recommendation. The Working Draft is a document put out by an associated Working Group. As long as the Working Group is together, the Working Draft can be maintained as such, but the group is responsible for ensuring that the document is updated every three months. If all the issues in a Working Draft have been dealt with, it receives Last Call Working Draft status, a (largely internal) review process which typically lasts three weeks. When the Last Call Working Draft is approved, it attains Candidate Recommendation status, which is a call for implementation and review from outside the organization. The next stage in the process is Proposed Recommendation, upon review of which, the document may achieve Recommendation status, which represents a consensus among the W3C members that the technology or ideas in the document should be widely implemented. At any point in the process, if the document fails to meet criteria, it reverts to Working Draft status. Standards groups, such as the Internet Engineering Task Force (IETF), have similar processes for the advancement of proposed standards. IETF working drafts are called Internet Drafts.
workstation 1) A workstation is a computer intended for individual use that is faster and more capable than a personal computer. It’s intended for business or professional use (rather than home or recreational use). Workstations and applications designed for them are used by small engineering companies, architects, graphic designers, and any organization, department, or individual that requires a faster microprocessor, a large amount of random access memory (RAM), and special features such as high-speed graphics adapters. Historically, the workstation developed technologically about the same time and for the same audience as the UNIX operating system, which is often used as the workstation operating system. Among the most successful makers of this kind of workstation are Sun Microsystems, Hewlett-Packard, DEC, and IBM. 2) In IBM and other corporations, the term ‘‘workstation’’ is sometimes used to mean ‘‘any individual personal computer location hooked up to a mainframe computer.’’ In today’s corporate environments, many workers have such workstations. They’re simply personal computers attached to a local area network (LAN) that in turn shares the resources of one or more large computers. Since they are PCs, they can also be used independently of the mainframe assuming they have their own applications installed and their own hard disk storage. This use of the term ‘‘workstation’’ (in IBM, sometimes called a ‘‘programmable workstation’’) made a distinction between the earlier ‘‘terminal’’ or ‘‘display terminal’’ (or ‘‘dumb terminal’’) of which the 3270 Information Display System is an example.
World Wide Web A technical definition of the World Wide Web is: all the resources and users on the Internet that are using the Hypertext Transfer Protocol (HTTP). A broader definition comes from the organization that Web inventor Tim Berners-Lee helped found, the World Wide Web Consortium (W3C): ’’The World Wide Web is the universe of network-accessible information, an embodiment of human knowledge.’’
World Wide Web Distributed Authoring and Versioning See ‘‘WebDAV’’
worm 1) WORM (for write once, read many) is a data storage technology that allows information to be written to a an optical disc a single time and prevents the drive from erasing the data. The discs are intentionally not rewritable, because they are especially intended to store data that the user does not want to erase accidentally. Because of this feature, WORM devices have long been used for the archival
WYSIWYG purposes of organizations such as government agencies or large enterprises. A type of optical media, WORM devices were developed in the late 1970s and have been adapted to a number of different media. The discs have varied in size from 5.25 to 14 inches wide, in varying formats ranging from 140MB to more than 3 GB per side of the (usually) doublesided medium. Data is written to a WORM disc with a lowpowered laser that makes permanent marks on the surface. Because of a lack of standardization, WORM discs have typically been only readable by the drive on which they were written, and hardware and software incompatibility has hampered their marketplace acceptance. Other optical media, such as CDs and DVDs that can be recorded once and read an unlimited number of times are sometimes considered WORM devices, although there is some argument over whether formats that can be written in more than one session (such as the multisession CD) qualify as such. CD-R has gradually been replacing traditional WORM devices, and it is expected that some newer technology, such as DVD-R or HD-ROM will eventually replace both WORM and CD-R devices. 2) A worm is a self-replicating virus that does not alter files but resides in active memory and duplicates itself. Worms use parts of an operating system that are automatic and usually invisible to the user. It is common for worms to be noticed only when their uncontrolled replication consumes system resources, slowing or halting other tasks.
3) In data transmission, a wrapper is the data that is put in front of or around a transmission that provides information about it and may also encapsulate it from view to anyone other than the intended recipient. A wrapper often consists of a header that precedes the encapsulated data and the trailer that follows it.
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writing tablet See ‘‘Big Chief tablet’’
In information technology, a wrapper is data that precedes or frames the main data or a program that sets up another program so that it can run successfully. 1) On the Internet, ‘‘http://’’ and ‘‘ftp://’’ are sometimes described as wrappers for the Internet addresses or Uniform Resource Locator URL that follow. A set of bracketing symbols (such as < and >, used here to wrap the word ‘‘and’’) are also sometimes referred to as wrappers . 2) In programming, a wrapper is a program or script that sets the stage and makes possible the running of another, more important program.
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WTS See ‘‘Terminal Server product’’
A WYSIWYG (pronounced ‘‘wiz-ee-wig’’) editor or program is one that allows an interface or content developer to create a graphical user interface (GUI) or page of text so that the developer can see what the end result will look like while the interface or document is being created. WYSIWYG is an acronym for ‘‘what you see is what you get.’’ A WYSIWYG editor can be contrasted with more traditional editors that require the developer to enter descriptive codes (or markup) and do not permit an immediate way to see the results of the markup.
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See ‘‘Web Proxy Autodiscovery’’
Window RAM (WRAM), unrelated to Microsoft Windows, is very high-performance video RAM that is dual-ported and has about 25% more bandwidth than VRAM but costs less. It has features that make it more efficient to read data for use in block fills and text drawing. It can be used for very high resolution (such as 1600 by 1200 pixels) projection using true color. It’s used in the Matrox Millenium video card.
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4) In database technology, a wrapper can be used to determine who has access to look at or change the data that is wrapped.
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A true WYSIWYG editor, such as Microsoft’s FrontPage or Adobe’s PageMill or Go Live (formerly CyberStudio), conceals the markup and allows the developer to think entirely in terms of how the content should appear. (One of the trade-offs, however, is that a WYSIWYG editor does not always make it easy to fine-tune its results.)
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X X is a shortened name for the X Window System. X originated in the early 1980s as the result of a research collaboration between Stanford University and MIT (aided by IBM) to develop a cross-platform windowing system.
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In compact disc (CD) and digital versatile disc (DVD) technology, X is a base multiplier that expresses the time it took to read data from the compact disc in its original version, which was 150 kilobytes (KB) per second. As successively faster CDs arrived, they adopted the convention of indicating the read time in terms of the original speed. Thus, a 2X CD had a read access time of 300 KB and so forth. Our table relates each common drive speed to its read access time. It also shows the range of revolutions per minute (RPM) used to make the read access time possible. CD/DVD Drive Speed
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There are only minor increases in speed as one moves from the 24X, 32X, and 40X drives. The high rotation speeds produced can create noise and vibrations, and performance may vary from drive to drive.
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x, y coordinates are respectively the horizontal and vertical addresses of any pixel or addressable point on a computer display screen. The x coordinate is a given number of pixels along the horizontal axis of a display starting from the pixel (pixel 0) on the extreme left of the screen. The y coordinate is
a given number of pixels along the vertical axis of a display starting from the pixel (pixel 0) at the top of the screen. Together, the x and y coordinates locate any specific pixel location on the screen. x and y coordinates can also be specified as values relative to any starting point on the screen or any subset of the screen such as an image. On the Web, each clickable area of an image map is specified as a pair of x and y coordinates relative to the upper left-hand corner of the image.
x coordinate See ‘‘x and y coordinates’’
X server An X server is a server of connections to X terminals in a distributed network that uses the X Window System. From the terminal user’s point-of-view, the X server may seem like a server of applications in multiple windows. Actually, the applications in the remote computer with the X server are making client requests for the services of a windows manager that runs in each terminal. X servers (as part of the X Window System) typically are installed in a UNIX-based operating system in a mainframe, minicomputer, or workstation. The X server may be compared to Microsoft’s Terminal Server product except that the latter is running on a Windows-based operating system. However, there are X servers designed to run on a Microsoft Windows-based operating system.
X terminal An X terminal is typically a diskless computer especially designed to provide a low-cost user interface for applications that run in a network X server as part of a distributed X Window System. Typically, X terminals are connected to a server running a UNIX-based operating system on a mainframe, minicomputer, or workstation. X terminals (and the X Window System) appear to have been the forerunner for what is now generally called ‘‘network computers’’ or thin clients. The X Window System and X terminals continue to offer an alternative to Microsoft’s Terminal Server product and their NetPCs.
X Window System The X Window System (sometimes referred to as ‘‘X’’ or as ‘‘XWindows’’) is an open, cross-platform, client/server system for managing a windowed graphical user interface in a distributed network. In general, such systems are known as windowing systems. In X Window, the client-server
x2 relationship is reversed from the usual. Remote computers contain applications that make client requests for display management services in each PC or workstation. X Window is primarily used in networks of interconnected mainframes, minicomputers, and workstations. It is also used on the X terminal, which is essentially a workstation with display management capabilities but without its own applications. (The X terminal can be seen as a predecessor of the network PC or thin client computer.) The X Window System was the result of research efforts in the early 1980s at Stanford University and MIT, aided by IBM, to develop a platform-independent graphics protocol. The X Window System is an open standard that is managed by the X.Org consortium. Although Microsoft has its own platform-dependent windowing system (an integral part of the Windows 95/98/NT operating systems), there are vendor-supplied X Windows products that can be installed to run on these systems.
X.25 The X.25 protocol, adopted as a standard by the Consultative Committee for International Telegraph and Telephone (CCITT), is a commonly-used network protocol. The X.25 protocol allows computers on different public networks (such as CompuServe, Tymnet, or a TCP/IP network) to communicate through an intermediary computer at the network layer level. X.25’s protocols correspond closely to the data-link and physical-layer protocols defined in the Open Systems Interconnection (OSI) communication model.
X.400 X.400 is the messaging (notably e-mail) standard specified by the ITU-TS (International Telecommunications Union—Telecommunication Standard Sector). It’s an alternative to the more prevalent e-mail protocol, Simple Mail Transfer Protocol (SMTP). X.400 is common in Europe and Canada. It’s actually a set of standards, each in the 400-number range. Because X.400 stipulates a number of possible address characteristics that SMTP does not, an X.400 address can be long and cumbersome. On the other hand, X.400 adherents note that it is an official standard whereas SMTP is a ‘‘de facto’’ standard. Thus, products with X.400 implementations can be tested more rigorously than products with SMTP implementations can. X.400 offers more capabilities than SMTP does. However, many of these capabilities are seldom used. An SMTP e-mail address that looks like this hypothetical address: [email protected]
might look like this in an X.400 e-mail message: G=Georg; S=Hansen; O=sintef; OU=delab; PRMD=uninett; ADMD=uninett; C=no
X.400 is a complex standard that is difficult to summarize here. The arguments in favor of and against each of the opposing e-mail standards are also complicated.
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X.500 X.500 Directory Service is a standard way to develop an electronic directory of people in an organization so that it can be part of a global directory available to anyone in the world with Internet access. Such a directory is sometimes called a global White Pages directory. The idea is to be able to look up people in a user-friendly way by name, department, or organization. Many enterprises and institutions have created an X.500 directory. Because these directories are organized as part of a single global directory, you can search for hundreds of thousands of people from a single place on the World Wide Web. The X.500 directory is organized under a common ‘‘root’’ directory in a ‘‘tree’’ hierarchy of: country,organization, organizational unit, and person. An entry at each of these levels must have certain attributes; some can have optional ones established locally. Each organization can implement a directory in its own way as long as it adheres to the basic schema or plan. The distributed global directory works through a registration process and one or more central places that manage many directories. Providing an X.500 directory allows an organization to make itself and selected members known on the Internet. Two of the largest directory service providers are InterNIC, the organization that supervises domain name registration in the U.S., and ESnet, which maintains X.500 data for all the U.S. national laboratories. ESNet and similar providers also provide access to looking up names in the global directory, using a number of different user interfaces including designated Web sites, whois, and finger. These organizations also provide assistance to organizations that are creating their own Directory Information Tree (DIT). In X.500, each local directory is called a Directory System Agent (DSA). A DSA can represent one organization or a group of organizations. The DSAs are interconnected from the Directory Information Tree (DIT). The user interface program for access to one or more DSAs is a Directory User Agent (DUA). DUAs include whois, finger, and programs that offer a graphical user interface. X.500 is implemented as part of the Distributed Computing Environment (DCE) in its Global Directory Service (GDS). The University of Michigan is one of a number of universities that use X.500 as a way to route e-mail as well as to provide name lookup, using the Lightweight Directory Access Protocol (LDAP).
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x2 is a technology from US Robotics (now 3Com) for the downstream transmission of data over ordinary phone lines at 56 Kbps (thousands of bits per second). The 56 Kbps speed is achieved in the downstream direction only (to your
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home or business). Upstream speed is at the regular maximum speed of 33.6 Kbps. (The actual achieved downstream speed is reported by users to be about 53 Kbps.) x2 provided input to and has been replaced by the V.90 ITUTS standard. 56 Kbps technologies exploit the fact that most telephone company offices are interconnected with digital lines. Assuming your Internet connection provider has a digital connection to its telephone company office, the downstream traffic from your local Internet access provider can use a new transmission technique on your regular twisted pair phone line that bypasses the usual digital-to-analog conversion. A V.90-equipped modem doesn’t need to demodulate the downstream data. Instead, it decodes a stream of multi-bit voltage pulses generated as though the line was equipped for digital information. (Upstream data still requires digitalto-analog modulation.) Unlike Integrated Services Digital Network, the V.90 technology does not require any additional installation or extra charges from your local phone company. On the other hand, the maximum transmission speed of ISDN is twice that of V.90 at 128 Kbps. You also have the flexibility of combining digital and voice transmission on the same line.
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x86 is a generic name for the series of Intel microprocessor families that began with the 80286 microprocessor. This series has been the provider of computing for personal computers since the 80286 was introduced in 1982. x86 microprocessors include the 386DX/SX/SL family, the 486DX/SX//DX2/SL/DX4 family, and the Pentium 3 family. The x86 line replaced Intel’s previous series, the 8086/8088. Adopted by IBM for its first PCs, the 8086/8088 and the continuing x86 series have made Intel the predominant force in microprocessor design and manufacture.
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Xalan is a specification for transforming Extensible Markup Language (XML) documents into Hypertext Markup Language (HTML) or other XML document types. XalanJava version 1.2 is based on World Wide Web Consortium (W3C) recommendations for Extensible Stylesheet Language Transformations (XSLT) and XML Path Language (XPL). Xalan normally uses the Xerces XML parser, but it can use any parser that conforms to either Document Object Model (DOM) level 2 or Simple API for XML (SAX) level 1. Xalan-Java version 2 recasts Xalan as an implementation of Transformations for XML (TraX) interfaces. TRaX provides a modular framework and a standard application programming interface for performing an open-ended range of XML transformations. In conjunction with TRaX, Xalan-Java 2 relies on system properties to configure its operational settings. The default settings point to the Xalan
stylesheet processor, the serializers shipped with Xalan, and the Xerces SAX parser. Xalan-Java version 2 builds on SAX level 2, DOM level 2, and the Java API for XML Parsing. Xalan-C++ version 1.0 is an implementation of W3C recommendations for XSLT and XPath. It uses the XercesC++ version 1.3.0 XML parser. LotusXSL, which used to be a specification in its own right, has been recast as a sub-specification of Xalan.
Xanadu Xanadu (TM) is a set of ideas and a software design project for a universal system of electronic information storage and access. Its inventor, Ted Nelson, is credited with inventing the term hypertext, an idea that is a central part of Xanadu. Conceived in the early 1980’s or perhaps slightly earlier, Xanadu in some ways seems to have anticipated the Web and such ideas as groupware, group writing, virtual organization, and information. Nelson has described Xanadu as ‘‘an instantaneous electronic literature’’ and ‘‘perhaps the ultimate’’ hypertext system.
X-Box X-Box is a game console being developed by Microsoft that is intended to provide players with a more realistic and immediate interactive experience than current consoles and to gain Microsoft a leadership position in the game console market. The X-Box will include a 600 MHz Intel processor, a custom-designed graphics processor referred to as the XChip, 64 megabytes of memory, and a built-in digital versatile disc movie player. Microsoft claims that the X-Box will deliver 300 million polygons per second (imagery in games is developed in uniformly-sized polygonal units) and they compare this performance with the 66 million polygons per second delivered by Sony’s PlayStation 2 console. A modem for Internet access will be an option. Microsoft expects to develop about 30% of the games offered for the X-Box and offers its DirectX graphics development interface to companies that will develop the remaining 70%. Activision is one company planning to develop or modify games for the X-Box. Existing makers of game consoles include Atari, Nintendo, Sega, and Sony. Although Microsoft is new to the console player business, it currently is a leader in software for the PC game market. According to Microsoft, there are about 29 million console players, 11 million PC game players, and 7 million people who play both. Console games emphasize extremely fast interactive response and are described as more ‘‘visceral,’’ whereas PC games tend to take longer to play and are more ‘‘cerebral.’’
Xerces At least one industry observer suggests that the X-Box may be part of a Microsoft strategy to bring a computer under another name into many living rooms that currently don’t include a computer. In this view, the X-Box could later be enhanced to provide capabilities additional to game playing.
Microsystem’s Network File System (NFS). Because it is built on international standards such as TCP/IP and MPEG, XDMA is easily integrated into existing infrastructures.
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Xeon (pronounced ZEE-ahn) is a Pentium microprocessor from Intel for use in ‘‘mid-range’’ enterprise servers and workstations. On a server motherboard from Intel, up to eight (and later even more) Xeon processors will be able to do multiprocessing sharing the same 100 Mhz bus. Xeon is replacing the Pentium Pro as Intel’s main enterprise microchip. Xeon is designed for Internet and large transactional database servers as well as for engineering, graphics, and multimedia applications that require moving a lot of data around quickly. Xeon is the high end of the Pentium line (Celeron is the low end).
XBRL (Extensible Business Reporting Language) is an XMLbased language being developed specifically for the automation of business information requirements, such as the preparation, sharing, and analysis of financial reports, statements, and audit schedules. XBRL steering committee members include financial, accounting, software, and governmental communities from around the world: member organizations include Microsoft, ACCPAC, Oracle, Deloitte & Touche, IBM, Hyperion, and the American Institute of Certified Public Accountants (AICPA). The goal of the steering committee is to create a standard specification for industry-wide use. The use of XBRL for creating financial statements will facilitate what has been a manual, repetitive, and errorprone endeavor. Statements must frequently be duplicated separately—and altered—for different uses (a Web publication and printed document, for example), and the extraction of data can be very time-consuming. XBRL will automate these processes. In the future, XBRL is intended, among other things, to provide a standardized framework and integrated methodology for the preparation and publication of reports, as well as a consistent, automated process for the reliable extraction and exchange of financial statements.
XDMA was developed by Xing Technology, producers of the first MPEG software encoders and decoders. Xing was purchased by RealNetworks in 1999; the Seattle-based company’s RealPlayer Plus and Microsoft’s Windows Media Player are the two leading players of streaming media. The XDMA architecture supports live, on-demand multimedia transmission to multiple users over local area networks (LANs) or wide area networks (WANs)—such as the Internet—without requiring the use of file system applications such as Novell’s NetWare or Sun
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Xeon is based on the Pentium microprocessor’s P6 architecture. It’s designed to work with a new and faster Peripheral Component Interconnect bus and Accelerated Graphics Port. Xeon features: L
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A faster L1 and L2 cache, either 512 Kbytes or 1 Mbyte, that runs at the same 400 Mhz clock speed of the processor. A faster bus to carry data between the processor, RAM, and I/O devices. The 450NX PCIset is a chipset that works at a 100 Mhz clock speed and supports up to 8 GB of extended data output RAM memory.
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A larger Accelerated Graphics Port (AGP) chip set called the 440GX AGPset that also runs at 100 Mhz. It supports 2 GB of 100 Mhz SDRAM.
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An extended server memory architecture that provide for 36-bit addresses, allowing up to 64 GB of physical memory to be addressed.
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Everything premounted in a motherboard package for faster manufacturing
XDMA XDMA (Xing Distributed Media Architecture) is a network architecture for multicast streaming media transmissions. Streaming media enables live viewing of a transmission as compared to processes that require that files be completely transferred to the user’s computer before their data can be viewed. Unlike prior network architectures, XDMA enables low-cost multiple access. In general, multimedia network architectures have required point-to-point connections and complex server programming, which made the multicasting of streaming media impractical, if not impossible.
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Typically, a computer with a Xeon microprocessor would use a Windows NT, NetWare, or UNIX operating system. Xeon-based systems are expected to offer competition to Sun Microsystems, Silicon Graphics, and others in the workstation market, but its primary market is expected to be the mid-range server.
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Xerces Xerces (the name comes from the Xerces blue butterfly) is a set of parsers compatible with Extensible Markup Language (XML). (A parser is a program that analyzes and organizes formal language statements into a usable form for a given purpose.) Xerces parsers are available for Java and C++, implementing World Wide Web Consortium (W3C) XML, Document Object Model (DOM), and Simple API for XML (SAX) standards.
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All of the Xerces parsers are modular and configurable. A Perl wrapper is provided for the C++ version; this allows access to a fully-validating XML parser from Perl. It also provides access to Unicode strings.
XGA-2 is a display mode that provides 1,024 by 768 resolution in high color and higher-refresh rates than XGA. XGA was introduced by IBM in 1990 as an improvement on an earlier IBM display standard, the 8514/A.
The Xerces Java Parser 1.2.0 supports XML 1.0, and can be used for building XML-compatible Web servers; building the next generation of XML-based vertical applications; creating XML editors; ensuring the integrity of XML data; and building global XML applications.
The XGA standard is used in desktop and laptop computers as well as in projection systems.
Xerces-C is an XML parser written in a portable subset of C++. Xerces-C allows applications to read and write XML data. A shared library facilitates parsing, generating, manipulating, and validating XML documents. Other features include conformity to XML 1.0; tracking of latest DOM and SAX specifications; programmatic generation and validation of XML; pluggable catalogs, validators and encodings; and customizable error handling. XML4P includes a collection of Perl wrapper objects that internally use their XML4C counterparts for highperformance, scalable and localizable DOM parsing. Features include programmatic generation and validation of XML; conformity to DOM specifications; and customizable error handling.
XFDL Extensible Forms Description Language (XFDL) is a use of the Extensible Markup Language (XML) that provides a standard way to define the data fields and layout for a complex business or government form for digital storage and display. An XFDL form can be sent to a Web user as an XML page. XFDL makes it possible to sign an XFDL page with one or more digital certificates. XFDL also describes a syntax for inline mathematical and conditional expressions. XFDL combines the Universal Forms Definition Language (UFDL), developed earlier, with XML. PureEdge, an e-commerce standards firm, formerly called UWI.com, and World Wide Web Consortium (W3C) editor Tim Bray introduced XFDL in 1998 as the world’s first open standard for secure, legally-binding XML documents. Since then, vendors such as GTE, CommerceOne, and Verisign have moved to support or endorse XDFL in their products.
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XGA and XGA-2 (Extended Graphics Array)
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Also see display modes. XGA (Extended Graphics Array) is a high-resolution video display mode that provides screen pixel resolution of 1,024 by 768 in 256 colors or 640 by 480 in high (16-bit) color. XGA monitors can be interlaced displays.
XHTML As the World Wide Web Consortium (W3C) describes it, XHTML (Extensible Hypertext Markup Language) is ‘‘a reformulation of HTML 4.0 as an application of the Extensible Markup Language (XML).’’ For readers unacquainted with either term, HTML is the set of codes (that’s the ‘‘markup language’’) that a writer puts into a document to make it displayable on the World Wide Web. HTML 4 is the current version of it. XML is a structured set of rules for how one might define any kind of data to be shared on the Web. It’s called an ‘‘extensible’’ markup language because anyone can invent a particular set of markup for a particular purpose and as long as everyone uses it (the writer and an application program at the receiver’s end), it can be adapted and used for many purposes—including, as it happens, describing the appearance of a Web page. That being the case, it seemed desirable to reframe HTML in terms of XML. The result is XHTML, a particular application of XML for ‘‘expressing’’ Web pages. XHTML is, in fact, the follow-on version of HTML 4. You could think of it as HTML 5, except that it is called XHTML 1.0. In XHTML, all HTML 4 markup elements and attributes (the language of HTML) will continue to be supported. Unlike HTML, however, XHTML can be extended by anyone that uses it. New elements and attributes can be defined and added to those that already exist, making possible new ways to embed content and programming in a Web page. In appearance, an XHTML file looks like a somewhat more elaborate HTML file.
Advantages To quote the W3C again, the advantages are ‘‘extensibility and portability.’’ Extensibility means that as new ideas for Web communication and presentation emerge, they can be implemented without having to wait for the next major version of HTML and browser support. New tags or attributes can be defined to express the new possibilities and, assuming some program at the receiving end can understand and act on them, new things may happen on your Web page that never happened before. Specific sets of extensions for XHTML are planned for mathematical expressions, vector graphics, and multimedia applications. If extensibility is likely to lead to more complicated pages and larger programs, the portability advantage means that Web pages can now be made simpler than they were before
XML so that small devices can handle them. This is important for mobile devices and possibly household devices that contain microprocessors with embedded programming and smaller memories. XHTML defines several levels of possible markup complexity and each document states its level of complexity at the beginning. Programs in microdevices might expect XHTML-coded files that state the simplest level of complexity so that they could be handled by a small program and memory.
Differences and Distinctive Features You can find out more by reading the specification and tutorials, but here are some distinctive features of XHTML and differences between HTML 4: L
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XHTML requires strict adherence to coding rules. Notably, it requires that you use closing as well as opening elements (this is known as well-formed syntax) and that all elements be in lower case. HTML was much less rigorous about notation and browsers tended to be even more forgiving. This means that XHTML files will tend to be ‘‘busier’’ than HTML. However, they won’t necessarily be harder to read because rigor may force more order in coding. In addition, the major editing and file creation tools will probably lay out pages for easier readability.
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XHTML would seem to encourage a more structured and conceptual way of thinking about content and, combined with the style sheet, a more creative way of displaying it.
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XHTML will make it easier for people to dream up and add new elements (and develop browsers or other applications that support them).
XIPC XIPC is a middleware product that manages interprocess communication across programs in a network. A process is a unit of work associated with a particular user request (for example). Completing that work usually involves creating other processes or communicating between processes. This interprocess communication is performed using: L
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XIPC allows a programmer to use these interprocess communication methods across processes that are distributed in different locations in a network so that the programmer need not be aware of the network or individual process locations. To do this, XIPC adds peer-to-peer messaging to the IPC methods. The developer of XIPC, Level 8 Software, calls this ‘‘message-oriented middleware (MOM).’’
XJACK XJACK is a type of connector for notebook computer modems that allows a standard telephone connector to snap into the modem. Internal modems for notebook computers come on a slim PC Card. Many cards provide a connection that requires a special plug that in turn has to be plugged into a relay attachment. This plug can easily become loose in the PC Card socket, losing your connection. XJACK is a connector on the PC Card that pulls out and allows you to snap a regular wall jack connector into the modem directly. This connection is more secure. XJACK is manufactured by Megahertz, a subsidiary of 3Com. However, a number of other modem manufacturers now build the XJACK into their mobile-size modems.
XMI XMI (XML Metadata Interchange) is a proposed use of the Extensible Markup Language (XML) that is intended to provide a standard way for programmers and other users to exchange information about metadata (essentially, information about what a set of data consists of and how it is organized). Specifically, XMI is intended to help programmers using the Unified Modeling Language (UML) with different languages and development tools to exchange their data models with each other. In addition, XMI can also be used to exchange information about data warehouses. Effectively, the XMI format standardizes how any set of metadata is described and requires users across many industries and operating environments to see data the same way. XMI is a proposal from the Object Management Group (OMG) that builds on and extends these industry standards or recommendations:
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Meta Object Facility (MOF), another standard from the OMG for a metamodeling and metadata repository
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Ideally, XMI will allow different cooperating companies a way to use each other’s data repositories. XMI is described as similar to, yet competing with, Microsoft’s Open Information Model.
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XML XML (Extensible Markup Language) is a flexible way to create common information formats and share both the format and the data on the World Wide Web, intranets, and elsewhere. For example, computer makers might agree on a standard or common way to describe the information about a computer product (processor speed, memory size, and so forth) and then describe the product information format
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with XML. Such a standard way of describing data would enable a user to send an intelligent agent (a program) to each computer maker’s Web site, gather data, and then make a valid comparison. XML can be used by any individual or group of individuals or companies that wants to share information in a consistent way. XML, a formal recommendation from the World Wide Web Consortium (W3C), is similar to the language of today’s Web pages, the Hypertext Markup Language (HTML). Both XML and HTML contain markup symbols to describe the contents of a page or file. HTML, however, describes the content of a Web page (mainly text and graphic images) only in terms of how it is to be displayed and interacted with. For example, the letter ‘‘p’’ placed within markup tags starts a new paragraph. XML describes the content in terms of what data is being described. For example, the word ‘‘phonenum’’ placed within markup tags could indicate that the data that followed was a phone number. This means that an XML file can be processed purely as data by a program or it can be stored with similar data on another computer or, like an HTML file, that it can be displayed. For example, depending on how the application in the receiving computer wanted to handle the phone number, it could be stored, displayed, or dialed. XML is ‘‘extensible’’ because, unlike HTML, the markup symbols are unlimited and self-defining. XML is actually a simpler and easier-to-use subset of the Standard Generalized Markup Language (SGML), the standard for how to create a document structure. It is expected that HTML and XML will be used together in many Web applications. XML markup, for example, may appear within an HTML page.
XMLNews-Story is an XML document type for text-based news and information. It defines the format of a news story’s content and is a subset of News Industry Text Format (NITF), the XML document type definition (DTD) designed to mark up and deliver news content in a variety of ways, including print, wireless devices, and the Web. XMLNewsMeta defines the format of any metadata associated with a story (or any other kind of news object) and is based on the World Wide Web Consortium’s Resource Description Framework (RDF).
Xmodem Xmodem is an error-correcting protocol for modem that was created in 1978 by Ward Christensen and became a de facto standard. Modems that agree on using the Xmodem protocol send data in 128-byte blocks. If a block is received successfully, a positive (ACK) acknowledgement is returned. If an error is detected, a negative (NAK) acknowledgement is returned and the block is resent. Xmodem uses the checksum method of error checking.
X-modem See ‘‘modem error-correcting protocols’’
Xon/Xoff
Early applications of XML include Microsoft’s Channel Definition Format (CDF), which describes a channel, a portion of a Web site that has been downloaded to your hard disk and is then is updated periodically as information changes. A specific CDF file contains data that specifies an initial Web page and how frequently it is updated. Another early application is ChartWare, which uses XML as a way to describe medical charts so that they can be shared by doctors. Applications of XML have also been created for banking, e-commerce ordering, personal preference profiles, purchase orders, litigation documents, part lists, and many others.
Xon/Xoff (sometimes written ‘‘X-on/X-off’’ or ‘‘XON/ XOFF’’ and pronounced eks-AWN eks-AWF) is a protocol for controlling the flow of data between computers and other devices on an asynchronous serial connection. For example, a computer typically sends data to a printer faster than the printer can print. The printer contains a buffer where data is stored until the printer catches up with the computer. If the buffer becomes full before the printer catches up, a small microprocessor in the printer sends back an X/off signal to stop sending data. When enough data is printed and buffer storage becomes free, the printer sends an X/on signal telling the computer to resume sending data.
XML Pointer Language
The ‘‘X’’ stands for ‘‘transmitter’’ so the X/on and X/off are signals to turn a transmitter on or off. The actual signal for X/on is the same bit configuration as the ASCII Ctrl-Q keyboard combination. The X/off signal is the Ctrl-S character.
See ‘‘XPointer’’
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XMLNews uses Extensible Markup Language (XML) and industry standards developed by the International Press Telecommunications Council and the Newspaper Association of America. XMLNews has two parts: XMLNews-Story and XMLNews-Meta.
XMLNews is a set of specifications for exchanging news objects such as stories, images, or audio clips in a standard format across different applications and operating systems.
When you define your modem to your computer’s operating system, you may need to specify the use of flow control with X/on/Xoff or with CTS/RTS (Clear to Send/Ready to Send). When sending binary data, Xon/Xoff may not be recognized because it is character-encoded.
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See ‘‘logic gate’’
XQL (XML Query Language) is a way to locate and filter the elements (data fields) and text in an Extensible Markup Language (XML) document. XML files are used to transmit collections of data between computers on the Web. XQL provides a tool for finding and/or selecting out specific items in the data collection in an XML file or set of files. It is based on the pattern syntax used in the Extensible Stylesheet Language (XSL) and is proposed as an extension to it.
XPath XPath is a language that describes a way to locate and process items in Extensible Markup Language (XML) documents by using an addressing syntax based on a path through the document’s logical structure or hierarchy. This makes writing programming expressions easier than if each expression had to understand typical XML markup and its sequence in a document. XPath also allows the programmer to deal with the document at a higher level of abstraction. XPath is a language that is used by and specified as part of both the Extensible Stylesheet Language Transformations (XSLT) and by XPointer (SML Pointer Language). It uses the information abstraction defined in the XML Information Set (Infoset). Since XPath does not use XML syntax itself, it could be used in contexts other than those of XML. XPath uses a syntax something like an informal set of directions for finding a particular geographic location. When telling someone how to find the Minneapolis campus of the University of Minnesota within the United States, for example, you might write: US/MN/Mpls/SE/WashingtonAve/bridge
which would put the user in the middle of the campus.
The XSL pattern language is a declarative way to indicate specific elements for processing. It uses simple directory notation. For example, book/author means: Select all author elements in all book elements in a particular context (for example, within an XML file or a set of files). XQL adds to this directory pattern notation the ability to use boolean logic, to filter out elements, to index into a collection of elements, and to do some other things. Using XQL, a program could be written to search repositories of XML files, to provide hypertext links to specific elements, and for other applications. For more information, we recommend reading the XQL proposal at the World Wide Web Consortium (W3C) Web site.
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XPointer XPointer is a language for locating data within an Extensible Markup Language (XML) document based on properties such as location within the document, character content, and attribute values. XPointer consists of a description that comes after the # symbol in a Uniform Resource Locator (URL). XPointer can be used alone or together with XPath, another language for locating data within an XML document.
Think of an XML page as similar to an HTML page (like the one you are reading now), but containing data in identified fields rather than text and graphics. XSL gives a developer the tools to describe exactly which data fields in an XML file to display and exactly where and how to display them. Like any style sheet language, XSL can be used to create a style definition for one XML document or reused for many other XML documents.
In Hypertext Markup Language (HTML), the # symbol enables linking to a specific marked point within an HTML page. XPointer allows linking to a point based on content as well. In this way, a reader can, for example, be enabled to link to the next instance of a certain word, phrase, or sequence of characters within an XML document.
XSL is being developed under the auspices of the World Wide Web Consortium (W3C) and is currently in the working draft stage.
XPath uses the concepts of the concept node (the point from which the path address begins), the logical tree that is inherent in any XML document, and the concepts expressing logical relationships that are defined in the XML Information Set, such as ancestor, attribute, child, parent, and self. XPath includes a small set of expressions for specifying mathematics functions and the ability to be extended with other functions.
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XSL XSL (Extensible Stylesheet Language), formerly called Extensible Style Language, is a language for creating a style sheet that describes how data sent over the Web using the Extensible Markup Language (XML) is to be presented to the user. For example, in an XML page that describes the characteristics of one or more automobiles for an insurance company, a set of open and close tags might contain the name of an auto manufacturer. Using XSL, you could tell the Web browser that the auto manufacturer name should be displayed, where to display it on a page, and that it should be displayed in a bold font. XSL is based on and extends the Document Style Semantics and Specification Language (DSSSL) and the Cascading Style Sheet, level 1 (CSS1) standards.
The key difference between XPath and earlier languages is that XPath specifies a route, rather than pointing to a specific set or sequence of characters, words, or other elements.
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XSL Transformations
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XSL Transformations (XSLT) is a standard way to describe how to transform (change) the structure of an XML (Extensible Markup Language) document into an XML document with a different structure. XSLT is a Recommendation of the World Wide Web Consortium (W3C).
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XSLT can be thought of as an extension of the Extensible Stylesheet Language (XSL). XSL is a language for formatting an XML document (for example, showing how the data described in the XML document should be presented in a Web page). XSLT shows how the XML document should be reorganized into another data structure (which could then be presented by following an XSL style sheet). XSLT is used to describe how to transform the source tree or data structure of an XML document into the result tree for a new XML document, which can be completely different in structure. The coding for the XSLT is also referred to as a style sheet and can be combined with an XSL style sheet or be used independently.
I XSLT
J K
See ‘‘XSL Transformations’’
xSP
N
xSP is a generic term for any kind of service provider on the Internet. The two main kinds of service provider are the Internet service provider (ISP), which provides users with connection to the Internet and sometimes offers hosting and other services, and the application service provider (ASP), which provides remote access to one or more computer applications.
O
An application service provider may be one of these kinds of xSP:
L M
P Q R S T
L
A storage service provider (SSP)
L
A management service provider (MSP)
L
A business service provider (BSP)
L
A security application service provider (SASP)
L
A wireless application service provider (WASP)
L
And so forth
XTACACS See ‘‘TACACS’’
XUL
U V,W
X,Y,Z
XUL (Extensible User-interface Language) is a standard way to exchange data that describes a program’s user interface, or at least the portion of it that can be controlled by programming. Historically, user interfaces have been difficult to customize so they would work across various
operating platforms such as Windows and Macintosh. Netscape and Mozilla, an open group that develops Netscape, have jointly developed XUL to solve this problem. (XUL developers say that XUL is pronounced ZOOL, rhyming with ‘‘rule.’’) One of the main assets of XUL is simplicity. XUL is easier to work with than the interface programming tools of the past. A few lines of XUL can accomplish what previously required many lines of code. One need not be a professional programmer to build a basic Web browser interface using XUL. This fact may ultimately translate into lower cost for the development of user interfaces. Flexibility is another asset of XUL. It is an application of Extensible Markup Language (XML), Cascading Style Sheets (CSS), the Document Object Model (DOM), and the Hypertext Markup Language (HTML). Portions of some XUL documents must be platform-specific, because platforms have different default preferences concerning the arrangement of ‘‘widgets’’ such as mail-inbox icons and dialog buttons.
Yellow Book
Y
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Y
Yahoo
See ‘‘admittance’’
Yahoo! is a directory of World Wide Web sites organized in a hierarchy of topic categories. As a directory, it provides both new and seasoned Web users the reassurance of a structured view of hundreds of thousands of Web sites and millions of Web pages. It also provides one of the best ways to search the Web for a given topic. Since Yahoo is associated with the most popular Web search sites, if a search argument doesn’t lead to a Yahoo topic page, it will still lead to results from Google or the six or seven other popular search engine sites Yahoo links to.
y coordinate See ‘‘x and y coordinates’’
Y2K See ‘‘2000’’
Yagi antenna A Yagi antenna, also known as a Yagi-Uda array or simply a Yagi, is a unidirectional antenna commonly used in communications when a frequency is above 10 MHz. This type of antenna is popular among Amateur Radio and Citizens Band radio operators. It is used at some surface installations in satellite communications systems. A basic Yagi consists of two or three straight elements, each measuring approximately 1/2 electrical wavelengths. The antenna can be balanced or unbalanced. The Yagi is inherently a balanced antenna, but it can be fed with coaxial cable and a device called a balun at the point where the feed line joins the driven element. The driven element of a Yagi is the equivalent of a centerfed, half-wave dipole antenna. Parallel to the driven element, and approximately 0.2 to 0.5 wavelength on either side of it, are straight rods or wires called reflectors and directors. A reflector is placed behind the driven element and is slightly longer than 1/2 wavelength; a director is placed in front of the driven element and is slightly shorter than 1/2 wavelength. A typical Yagi has one reflector and one or more directors. The antenna propagates electromagnetic field energy in the direction running from the driven element toward the director(s), and is most sensitive to incoming electromagnetic field energy in this same direction. The Yagi antenna not only has a unidirectional radiation and response pattern, but it concentrates the radiation and response. The more directors a Yagi has, the greater the socalled forward gain. As more directors are added to a Yagi, it becomes longer. Some Yagi antennas have as many as 10 or even 12 directors in addition to the driven element and one reflector. Long Yagis are rarely used below 50 MHz, because at these frequencies the structure becomes physically unwieldy.
Yahoo! began as the bookmark lists of two Stanford University graduate students, David Filo and Jerry Yang. After putting their combined bookmark lists organized by categories on a college site, the list began to grow into an Internet phenomenon. It became the first such directory with a large following. Filo and Yang postponed their graduate work and became part of a public offering for a multimillion dollar corporation.
Yellow Book The Yellow Book is the informal name for Philips and Sony’s ECMA-130 standard specification for compact disc, readonly-memory (CD-ROM). Published by the two companies in 1988, the Yellow Book is an extension of the Red Book that enables the CD to contain data other than audio data. In 1989, the Yellow Book was issued by the International Organization for Standardization (ISO) as ISO/IEC 10149, Data Interchange on Read-Only 120mm Optical Discs (CDROM). Because the Yellow Book only defines the physical arrangement of the data on the disc, other standards are used in conjunction with it to define directory and file structures. They include ISO 9660, HFS (Hierarchical File System, for Macintosh computers), and Hybrid HFS-ISO. In addition to the disc specification, optical stylus parameters, the control/display system, and sector structure, the Yellow Book includes modulation and error correction data. Definitions include two data modes, mode 1 and mode 2. CD-ROM, Mode 1 is the standard data storage mode used by almost all standard data CDs (CD-ROMs). Of the 2,352 bytes of data in each block, 2048 are allocated for the data that the user sees. The remaining 304 bytes are used for added error detection and correction code. CD-ROM, Mode 2 can contain 2336 bytes of user data. It is the same as Mode 1, except that the error detection and code correction bytes are not included. The Mode 2 format offers a flexible method for storing graphics and video. It allows different kinds of data to be mixed together, and became the
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Whatis?Com’s Encyclopedia of Technology Terms basis for another standard known as CD-ROM XA (Extended Architecture). The specification for CD-ROM XA was published as an extension to the Yellow Book in 1991.
Ymodem Ymodem is an error-correcting protocol for a modem that uses larger data blocks for greater efficiency. Modems that agree on using the Ymodem protocol send data in 1024-byte blocks. Blocks received successfully are not acknowledged. Blocks with errors are acknowledged (with a NAK or negative acknowledgement) and the blocks are retransmitted. Ymodem is similar to Xmodem-1K except that a batch mode is provided. In batch mode, a number of files can be sent with a single command. Ymodem uses cyclic redundancy checking as the method of error detection.
Zero Administration
Z
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z coordinate A z coordinate is the third-dimensional coordinate in a volume pixel, or voxel. Together with x and y coordinates, the z coordinate defines a location in a three-dimensional space. See voxel for more information.
Z Object Publishing Environment See ‘‘Zope’’
z/OS z/OS is the computer operating system for IBM’s zSeries 900 (z900) line of large (mainframe) servers. z/OS is a renamed and upgraded version of OS/390, which in turn evolved from the MVS operating system. IBM’s renamed servers and operating systems reflect a strategy to realign its products more closely with the Internet and its own ebusiness initiatives. z/OS is described as an extremely scalable and secure highperformance operating system based on the 64-bit z/ Architecture. Like its predecessor, OS/390, z/OS lays claim to being highly reliable for running mission-critical applications. The operating system supports Web- and Javabased applications.
Z39.50 Z39.50 is a standard communications protocol for the search and retrieval of bibliographic data in online databases. Z39.50 is used on the Internet to search the Online Public Access Catalogues (OPAC) of library holdings. It is also sometimes used to link disparate OPACs into a single ‘‘union’’ OPAC. Z39.50 is an American National Standards Institute (ANSI/NISO) standard.
zap In general usage, zap (noun) is spiciness, kick, or a powerful force. Dishes with chili pepper have zap. This term may be traceable to a comic book convention in which the hurling of an electronic beam or bolt of electricity by a hero (or antihero) using a ray gun is often accompanied by a sound balloon that says ‘‘ZAP!!!’’ The term is also used in Paintball and in some computer games. In information technology, zap has several meanings: 1) In programming, a zap (noun) is a precise and immediate correction for a computer code problem. Most proprietary software programs are distributed to customers as compiler code in the form of an unreadable string of computer binary digits. When a bug is detected after the software is released,
the only way to fix the already compiled code is to overlay the bad code with a sequence of good code. This overlaying is known as zapping and the fix itself is a zap. IBM provides its mainframe software customers with a special program for applying zaps that is called SuperZap. 2) In computers, to zap (verb) can also mean to erase or get rid of something. On a Macintosh computer, to ‘‘zap the PRAM’’ is to erase PRAM so that the system can rebuild its contents. (See parameter RAM for this procedure.) 3) Relative to computer hardware, to zap (verb) can mean to ruin something electrically. Thus, a power surge when you don’t have a surge protector can zap a computer’s electronic components.
z-buffering Z-buffering is an algorithm used in 3-D graphics to ensure that perspective works the same way in the virtual world as it does in the real one: a solid object in the foreground will block the view of one behind it. You’ve seen this illustrated in the real world when someone stands between you and the television screen. Z-buffering is a type of algorithm known as a Visual Surface Determination (VSD) algorithm. Z-buffering works by testing pixel depth and comparing the current position (z coordinate) with stored data in a buffer (called a z-buffer) that holds information about each pixel’s last postion. The pixel in the closer position to the viewer is the one that will be displayed, just as the person in front of the television is what the viewer sees rather than the screen. Z-buffering is one of three VSD algorithms commonly used for this purpose. The other two, BSP trees and depth sorting, work with polygons and consequently are less effective for portrayal of movement and overlap. Since it works at the pixel level, z-buffering can be demanding in terms of memory and processing time. Nevertheless, its more complex and life-like simulation of real-world object dynamics ensures its continuing popularity as a 3-D graphics development tool.
Zero Administration Zero Administration for Windows is Microsoft’s initiative to help make its operating system easier to install and manage. The goal is to reduce the ownership costs of PCs, particularly in large corporations with many thousands of desktop computers to support. Microsoft introduced Zero Administration in November 1996, at a time when Oracle and Sun were touting their own approach of ‘‘network computers’’ as a new way of doing corporate applications with less overhead than traditional PCs. Microsoft responded with its concept of Zero Administration. The idea was to include in all of Microsoft’s
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Windows operating systems features that would simplify the time it took to install and maintain the system on thousands of machines—and at the same time reduce the costs of keeping PCs in corporate America. Microsoft provides Zero Administration packs for Windows NT and Windows 98. These packs include features to automatically send out operating-system and application software updates throughout the company from one workstation, and having these updates roll back to the previous version if something goes wrong during the process.
A ZIF (Zero Insertion Force) socket is the physical way that Intel’s 486 and Pentium microprocessors up to Pentium II connect on the computer motherboard to the data bus. As its name implies, the ZIF socket is designed for ease of manufacture and so that the average computer owner will be able to upgrade the microprocessor. The ZIF socket contains a lever that opens and closes, securing the microprocessor in place.
Socket
Pins
Layout
Processor(s) Voltage
0 1 2
168 169 238
Inline Inline Inline
3
237
Inline
U
4
273
Inline
V,W
5 6
320 235
Staggered Inline
486DX5 V 486DX,SX 486DX, SX,DX2 486DX, SX,DX2, DX4 60 or 66 MHz Pentium Pentium 486DX4
S T
X,Y,Z
Pentium Pentium Pro
3V 3V
zine See ‘‘ezine’’
ZIP codes
Also see zipping.
The following table summarizes the Zero Insertion Force (ZIF) socket characteristics for different Intel processors.
R
Processor(s) Voltage
Staggered Staggered
ZIF
Q
O
Layout
321 387
ZIP (Zoning Improvement Plan) codes are postal codes in the United States that identify preassigned geographic boundaries and make mail sorting and delivery more efficient. Other countries have similar designations, usually referred to as post codes or postal codes.
P
N
Pins
7 8
For Windows 2000, Zero Administration’s Group Policy feature lets administrators set rights and permissions for network and application access on the group level instead of having to do it for each individual user. If everyone in the legal department needs the same permissions and rights to applications, administrators need set this up only once for all the employees in the department.
The ZIF interface evolved through eight variations, each with a differing number of pins and pin layout arrangements. Currently, the best-known is Socket 7, the configuration used in the Pentium microprocessor. (However, the Pentium Pro uses Socket 8.) With the Pentium II microprocessor, which is based on Intel’s new P6 micro architecture, Intel has changed to a new connection configuration called Slot 1. In this configuration, the microprocessor comes packaged in a cartridge that fits into a 242-contact or 330-contact slot in the motherboard.
M
Socket
Zip drive A Zip drive is a small, portable disk drive used primarily for backing up and archiving personal computer files. The trademarked Zip drive was developed and is sold by Iomega Corporation. Zip drives and disks come in two sizes. The 100 megabyte size actually holds 100,431,872 bytes of data or the equivalent of 70 floppy diskettes. There is also a 250 megabyte drive and disk. The Iomega Zip drive comes with a software utility that lets you copy the entire contents of your hard drive to one or more Zip disks. In addition to data backup, Iomega suggests these additional uses: L
Archiving old e-mail or other files you don’t use any more but may want to access someday
L
Storing unusually large files, such as graphic images that you need infrequently
L
Exchanging large files with someone
L
Putting your system on another computer, perhaps a portable computer
L
Keeping certain files separate from files on your hard disk (for example, personal finance files)
5V 5V
The Zip drive can be purchased in either a parallel or a Small Computer System Interface (SCSI) version. In the parallel version, a printer can be chained off the Zip drive so that both can be plugged into your computer’s parallel port.
3V or 5 V
SyQuest makes a similar product and also makes larger and more expensive removable (portable) disk drives that are worth considering if you have a more professional system.
5V
zipping A Zip drive is something different.
3V 3V
Zipping is the act of packaging a set of files into a single file or archive that is called a zip file. Usually, the files in a zip file are compressed so that they take up less space in storage
Zoopraxiscope or take less time to send to someone. There are several popular tools that can be used for zipping files: PKZIP for the DOS operating system, WinZip and NetZIP for Windows, MacZip for Macintosh users, and Zip and UnZip for UNIX systems. The result of zipping is a single file with a ‘‘.zip’’ suffix. After you receive a zip file, you may have to extract and decompress the file by using the same kind of tool that was used to zip the original file. Most software that you download from the Internet will arrive as a self-extracting zip file. Typically, by doubleclicking on a self-extracting zip file, it will automatically extract, decompress, and store the individual files. One of these files is usually called the ‘‘setup.exe’’ file. Doubleclicking on this file will cause the software to be installed as a selectable program in your operating system.
Zmodem protocol Zmodem is an error-correcting protocol for modems. Modems that agree on using the Zmodem protocol send data in 512-byte blocks. If a block arrives and an error is detected, a ‘‘NAK’’ (negative acknowledgement) is returned and the block is resent.
Z-modem
zombie In the West Indies, a zombie is a will-less, automaton-like person who is said to have been revived from the dead and must now do the will of the living. There are at least three usages of the term related to computers and the Internet. 1) In the UNIX operating system world, developers sometimes use the term to refer to a program process that has died but hasn’t yet given its process table entry back to the system. 2) On the World Wide Web, a zombie is an abandoned and sadly out-of-date Web site that for some reason has been moved to another Web address. It’s a ghost site that appears to have moved. Zombies contribute to linkrot. 3) In at least one form of denial of service, one or more insecure Web servers are compromised by hackers who place code in each Web server that, when triggered, will launch an overwhelming number of requests toward an attacked Web site, which will soon be unable to service legitimate requests from its users. A compromised Web site that is used as an attack launch point is known as a zombie.
zoo
See ‘‘modem error-correcting protocols’’
A zoo is a Web site that holds collections of Internet viruses. These sites may be illegal in certain countries.
Zoetrope
Zoopraxiscope
The Zoetrope (pronounced ZOH-uh-trohp), invented in 1834 by William George Horner, was an early form of motion picture projector that consisted of a drum containing a set of still images, that was turned in a circular fashion in order to create the illusion of motion. Horner originally called it the Daedatelum, but Pierre Desvignes, a French inventor, renamed his version of it the Zoetrope (from Greek word root zoo for animal life and trope for ‘‘things that turn.’’)
The Zoopraxiscope (pronounced ZOH-uh-PRACKS-uhscohp), invented by British photographer Eadweard Muybridge and first shown in 1879, was a primitive motion picture device that worked by showing a sequence of still photographs in rapid succession. Muybridge, perhaps best known today for his sequence of photographs of a race horse in motion (which proved for the first time that at top speed all feet leave the ground), studied photography in the early 1860s with daguerrotypist Silas Selleck and later achieved recognition for his photographs of the Yosemite Valley and other scenes of the American Far West. The Zoopraxiscope emerged out of his studies of motion as shown in sequences of still photographs. His 11-volume work, Animal Locomotion, published in 1887, contained over 100,000 photographs. In 1893, he lectured at ‘‘Zoopraxigraphical Hall’’ at the World’s Columbian Exposition in Chicago.
A Zoetrope is relatively easy to build. It can be turned at a variable rate to create slow-motion or speeded-up effects. Like other motion simulation devices, the Zoetrope depends on the fact that the human retina retains an image for about a tenth-of-a-second so that if a new image appears in that time, the sequence was seem to be uninterrupted and continuous. It also depends on what is referred to as the Phi phenomenon, which observes that we try to make sense out of any sequence of impressions, continuously relating them to each other. The visual effect created by a Zoetrope (or Zoopraxiscope) is still used today to create animated GIFs and video display technologies such as streaming video, which essentially create an effect of motion by presenting discrete but closelyrelated images one after the other.
The Zoetrope, along with the Zoopraxiscope and the Thaumatrope, are forerunners of the animated GIF and video display technologies such as streaming video, which essentially create an effect of motion by presenting discrete but closely-related images one after the other.
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Zope
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Zope (Z Object Publishing Environment) is a Web site builder and application server that uses the idea that it is serving (or ‘‘publishing’’) objects rather than merely providing content that will be added to a Web page. Zope’s proponents believe that it is competitive with site builders and application servers such as ColdFusion and the Netscape Application Server. Zope software is free and uses Open Source code.
The Zoomed Video port (ZV port) is a technology that supports the delivery of full-screen motion video and multimedia to notebook computers. The ZV port allows special software and a version of the PC Card called a ZV Port Card to provide a separate dedicated, point-to-point bus or path from continuously arriving video signals directly to the display controller so that they do not need to be handled by the main bus or the central processing unit. ZV ports are provided in IBM, Toshiba, and other manufacturers’ notebook computers.
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Zope programmers describe Zope as ‘‘object publishing software.’’ They propose that users are interacting directly with ‘‘real objects’’ rather than with dynamically updated files that are being served. Zope consists of a ‘‘publisher’’ that publishes the objects using Zope’s Persistent CGI protocol; a framework for the folders, files, and images that Zope views as ‘‘built-in objects’’; an object database; a template for dynamic Web page generation; and Structured Query Language methods and database adapters so that Zope can interact with data in popular database servers, such as Microsoft’s SQL Server. By default, Zope’s object database uses the operating system’s file system to manage data. However, it can also work with relational database management systems. Specifically, Oracle, Sybase, MySQL, and Open Database Connectivity are supported. Zope was created by Digital Creations and has been used by the U.S. government, U.S. newspapers, and many company Web sites. The software is written in Python, an interpreted, interactive, object-oriented programming language similar to Java, with small pieces written in C for better performance. Web site developers using Zope do not need to use Python, however. Zope runs on all major operating system platforms.
Zulu Zulu (short for ‘‘Zulu time’’) is used in the military and in navigation generally as a term for Universal Coordinated Time (UCT), sometimes called Universal Time Coordinated (UTC) or Coordinated Universal Time (but abbreviated UTC), and formerly called Greenwich Mean Time. In military shorthand, the letter Z follows a time expressed in Greenwich Time. Greenwich Time, now called Universal Coordinated Time, is the time at longitude 0 degrees 0 minutes—the prime meridian or longitudinal line that separates East from West in the world geographical coordinate system. This line of longitude is based on the location of the British Naval Observatory in Greenwich, England, near London. ‘‘Zulu’’ is the radio transmission articulation for the letter Z. Traditionally, ship and airplane navigation is conducted using Zulu time. Zulu time is usually expressed in terms of a 24-hour clock using the Gregorian calendar time divisions of hours and minutes.
With the ZV port technology, video signals are sent in compressed files using the MPEG standard. The ZV Port Card decompresses the files and sends the data directly to the video frame buffer managed by the display controller. The technology can also be used for capturing images sent to the display from a video camera and storing them on a hard disk. Combining user input from the computer’s regular PCI bus with the video from the ZV port, notebook users can play interactive MPEG-based games. The specification for the ZV port is an industry standard sponsored by the Personal Computer Memory Card International Association.
