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Glossary of Networking Terms for Visio IT Professionals

Archived content. No warranty is made as to technical accuracy. Content may contain URLs that were valid when originally published, but now link to sites or pages that no longer exist.
Published: November 1, 2002

Microsoft Corporation

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AAL See ATM Adaptation Layer.

access permission Authorization to use a network resource. Several levels of access can be given: read only, read and write (view and change), or read, write, and delete (basically, do whatever one wants presumably responsibly).

access point In a wireless LAN, a transceiver connected to a wired network that links the two network types.

account The recordkeeping mechanism used by networks and multi-user operating systems for keeping track of authorized users. Network accounts are created by network administrators and are used both to validate users and to administer policies for example, access permissions related to each user.

acoustic coupler An archaic device once used in computer communications. The coupler was a cradle-like instrument into which the headset of a telephone was placed. Its function was somewhat similar to the job now done by modems.

active hub A type of hub used on ARCnet networks that both regenerates (boosts) signals and passes them along. Compare passive hub, intelligent hub.

ActiveX A technology developed by Microsoft based on reusable software components that can interact with one another, especially in a networked environment. ActiveX components can be written in any of a number of programming languages. The technology is the basis for creating the ActiveX controls often used to customize and add interactivity to Web pages.

ActiveX control A software component based on Microsoft's ActiveX technology that is used to add interactivity and more functionality, such as animation or a popup menu, to a Web page. An ActiveX control can be written in any of a number of languages, including Java, C++, and Visual Basic.

address As in everyday life, a reference to a location. In terms of networking, an address is the name or number that identifies a particular computer or a site on the Internet. Computing, however, is full of different types of addresses for example, e-mail addresses identify mailboxes for specific individuals, and memory addresses identify specific locations in memory. In all cases, however, addresses are used to pinpoint something or someone.

ad-hoc network A temporary "network" formed by communicating stations in a wireless LAN.

ADSL Stands for Asymmetric Digital Subscriber Line, a form of DSL that transfers information at up to 8 Mbps downstream and up to 640 kbps upstream. See also DSL.

Advanced Program to Program Communication See APPC.

AE Stands for application entity, in the ISO/OSI Reference Model, one of the two software parties involved in a communications session.

algorithm A sequence of steps used to solve a problem or perform a task; essentially, a "recipe" for carrying out some action.

AM See amplitude modulation.

American National Standards Institute See ANSI.

American Standard Code for Information Interchange See ASCII.

amplitude modulation A means of loading information onto a carrier wave by modulating (altering) its height. Compare frequency modulation.

amplitude shift keying A form of amplitude modulation that uses two different wave heights to represent the binary values 1 and 0. See also amplitude modulation.

analog A reference to something, such as a sound wave, that is continuously variable rather than based on discrete units, such as the binary digits 1 and 0. A lighting dimmer switch is an analog device because it is not based on absolute settings. Compare digital.

anchor In HTML, a special tag associated with a hyperlink.

ANSI Stands for American National Standards Institute, an organization of business and industry groups concerned with fostering the development and adoption of standards in the United States. Among its many concerns, ANSI focuses on various networking technologies. It also represents the United States in the ISO.

APPC Stands for Advanced Program to Program Communications, a specification that extends IBM's SNA model to include minicomputers and microcomputers.

applet A small program that can be downloaded over the Internet. Applets are often written in the Java programming language and run within browser software, and they are typically used to customize or add interactive elements to a Web page.

AppleTalk The LAN hardware and software designed by Apple for Macintosh computers.

application entity See AE.

Archie A search program used to find files stored on FTP servers.

architecture In general, the design underlying computer hardware or software. In terms of networks, architecture covers the infrastructure, functionality, protocols, and standards implemented in a particular network design.

ARCnet Stands for Attached Resource Computer Network, a form of token bus network architecture for PC-based LANs developed by Datapoint Corporation. ARCnet relies on a bus or star topology and can support up to 255 nodes. Different versions run at speeds of 1.5 mbps, 20 Mbps (ARCnet Plus), and 100 Mbps.

ASCII Stands for American Standard Code for Information Interchange, a coding scheme that assigns numbers to 256 characters, including text, numbers, punctuation marks, and certain special characters. ASCII is divided into two sets: 128 characters (known as standard ASCII) and an additional 128 known as extended ASCII. Standard ASCII has for years been used as a near-universal "common language" in the microcomputer environment for enabling different programs to exchange information reliably. Compare EBCDIC.

ASK See amplitude shift keying.

Asymmetric Digital Subscriber Line See ADSL.

asynchronous A reference to something that is not dependent on timing for example, asynchronous communications can start and stop at any time instead of having to match the timing governed by a clock.

asynchronous communications Computer-to-computer communications in which the sending and receiving computers do not rely on timing as a means of determining where transmissions begin and end. Compare synchronous communications.

asynchronous transfer mode See ATM.

ATM A high-speed networking technology designed to deliver transmissions consisting of many different kinds of information, including text, voice, audio, and video. By relying on small, fixed-length packets that it multiplexes onto the carrier, ATM achieves speeds as high as 622 Mbps (over fiber optic cable). The basic unit of ATM transmission is known as a cell, a packet consisting of 5 bytes routing information and a 48-byte payload (data).

ATM Adaptation Layer The ATM layer that mediates between higher-level and lower-level services, converting different types of data (such as audio, video, and data frames) to the 48-byte payloads required by ATM.

Attached Resource Computer Network See ARCnet.

attachment unit interface See AUI.

attenuation The weakening of a transmitted signal as it travels farther from the source.

AUI Stands for attachment unit interface, a 15-pin (DB-15) connector commonly used to connect a network interface card to an Ethernet cable.

Authenticode A mechanism devised by Microsoft for identifying the source (author) of code to be downloaded and assuring users that the code has not been changed since it was created. Authenticode does not ensure bug-free or virus-free code, but it does authenticate programs before they are downloaded.

authority On the Internet and World Wide Web, a DNS name server responsible for resolving Internet names and IP addresses at a particular level of authority: top-level domain, second-level domain, or subdomain.


B channel See ISDN.

back end In terms of networking, roughly a reference to a server computer or the processing that takes place on it. Compare front end.

backbone The communications path that carries the majority of traffic within a network. A backbone is often a high-speed transmissions medium and can be used to link network segments, small networks, or (in the case of the Internet) multiple networks spread out over vast geographic distances.

BackOffice A suite of software components developed by Microsoft and designed to work with Windows NT to provide a network with services including e-mail (Exchange), intranet capabilities (Site Server), management (Systems Management Server), high-end database development (SQL Server), and so on.

backward learning A means of contributing to better performance or efficiency by relying on an information store to which new data are added as they become available.

bandwidth In the analog world, the range between the highest and lowest frequencies for a given segment of the electromagnetic spectrum, measured in Hertz (cycles per second)for example, FM radio operates between 88 MHz and 108 MHz, for a bandwidth of 20 MHz. In the digital world, however, bandwidth refers to speed of transmission, measured in bits per second (bps).

Basic Rate Interface See ISDN.

Basic Service Set Communicating stations on a wireless LAN.

beacon On a FDDI network, a special frame generated and passed along when a node detects a problem.

bearer channel See ISDN.

binding The process by which protocols are associated with one another and the network adapter to provide a complete set of protocols needed for handling data from the application layer to the physical layer.

BISDN See broadband ISDN.

bit-oriented protocol A communications protocol in which information is transmitted as a stream of bits, rather than whole characters, and which relies on special bit sequences to represent control information. Compare byte-oriented protocol.

bits per second See bps.

BNC A type of connector used to join segments of coaxial cable. The letters BNC are commonly, though not always, considered an abbreviation for British Naval Connector.

body In general, the main portion of something, for example, the text of a document or the segment of a data packet containing the actual data. In HTML, the body is the section of a Web document that contains the content of the document, along with tags describing the content for example, text format and color and the positioning of elements on the screen.

boost To strengthen a network signal before it is transmitted further.

bps Stands for bits per second, the measure of transmission speed used in relation to networks and communication lines. Although bps represents the basic unit of measure, networks and communications devices, such as modems, are so fast that speeds are usually given in multiples of bpsKbps (kilobits, or thousands of bits, per second), Mbps (megabits, or millions of bits, per second), and Gbps (gigabits, or billions of bits, per second).


bridge A network device used to connect two LANs and allow messages to flow between them.

broadband ISDN Next-generation ISDN based on Asynchronous Transfer Mode technology. Broadband ISDN divides information into two categories: interactive services, which are controlled by the user, and distributed (or distribution) services that can be broadcast to the user.

browser Software that functions either in addition to (as with Netscape Navigator) or as part of (as with Internet Explorer) an operating system to enable use of the Internet, the World Wide Web, and Web technologies implemented in other software, such as word processors and online Help systems.

BSS See Basic Service Set.

buffer A portion of computer memory set aside as a temporary holding area for data being transferred from one place (or device) to another.

bus A data pathway that connects the different parts of a computer memory, processor, disk drives, and so on. The bus consists of multiple conducting wires (lines) running in parallel. Different lines are used to carry different types of information, including memory locations, data, and control signals.

bus topology A network configuration based on a single main communications line (trunk) to which nodes are attached; also known as linear bus.

byte-oriented protocol A communications protocol in which information is transmitted as a stream of encoded bytes (characters). Compare bit-oriented protocol.


cable modem The device used to connect a PC to a cable television outlet. Cable modems do not operate at the same rate upstream (when sending information) and downstream (when receiving information). Upstream rates vary from about 2 Mbps to 10 Mbps, downstream rates from about 10 Mbps to 36 Mbps.

callback A security feature used to authenticate users calling in to a network. During callback, the network validates the caller's username and password, hangs up, and then returns the call, either to a number requested during the initial call or to a predetermined number.

carrier sense multiple access with collision detection See CSMA/CD.

cascaded star topology A star-wired network in which nodes connect to hubs and hubs connect to other hubs in a hierarchical (cascaded) parent/child relationship. This topology is characteristic of 100BaseVG networks.

CCITT Stands for Comit Consultatif International Tlgraphique et Tlphonique, a standards organization now part of the International Telecommunication Union, standardization sector (ITU-T). Before becoming part of the ITU, CCITT was instrumental in defining numerous communications-related standards, among them the CCITT V series specifications for modems and related technologies and the CCITT X series specifications for networking equipment and protocols.

cell A fixed-length packet, the basic transmission unit on high-speed networks, such as ATM. See also ATM.

cell relay A form of packet switching in which information is multiplexed onto a carrier and transferred in fixed-length packets (cells).

centralized network A network in which nodes connect to and use resources on a single central computer, typically a mainframe.

certification authority An organization that assigns encryption keys.

CGI Stands for Common Gateway Interface, a specification that allows Web developers to customize or add interactivity to Web pages. CGI provides a means for passing data to and from a Web user and a Web server through applications commonly known as scripts. When a Web server receives information from the user (through the browser), it relies on a CGI application to process the information and provide whatever return data the server needs to send back to the user.

Channel Service Unit See DDS.

circuit switching A means of connecting two communicating parties that relies on the creation of a physical link between the two. Circuit switching is characteristic of telephone connections. Unlike other methods of transmission, such as packet switching, it requires the link to be established before any communication can take place. Compare packet switching.

client/server network A LAN built around the division of nodes into client machines (users) and server computers that function as providers of services and resources. Servers on a client/server network commonly perform some of the processing work (called back-end processing) for client machines—for example, sorting through a database before delivering only the record(s) requested by the client. Compare peer-to-peer network.

clustering The grouping of multiple servers in a way that allows them to appear to be a single unit to client computers. Clustering is a means of increasing network capacity and improving data security.

coaxial cable A round, flexible cable consisting of—from the center outwards—a copper wire, a layer of protective insulation, a braided metal mesh sleeve, and an outer shield, or jacket of PVC or fire-resistant material. Coaxial cable is widely used in networks. It is the same type of wiring as that used for cable television. Compare twisted-pair wiring, fiber optic cable.

Common Gateway Interface See CGI.

Compressed Serial Line Internet Protocol See SLIP.

computer Any device capable of processing information to produce a desired result. No matter how large or small they are, computers typically perform their work in three well-defined steps: (1) accepting input, (2) processing the input according to predefined rules (programs), and (3) producing output.

connection A link between two communicating computers.

connectionless Transmission over a flexible path that can include multiple routes between source and destination. Compare connection-oriented.

connection-oriented Transmission over a set path between sender and receiver. Compare connectionless.

connector A piece of hardware used to join devices and cables. Connectors are usually male—plugs with pins—or female—sockets or jacks designed to accept male connectors.

content The "meat" of a document, as opposed to its format, or appearance.

contention A competitive form of gaining network access, in which the first node to seize control of the carrier earns the right to transmit at that time.

crack To gain unauthorized access to a network by breaching its security. Also, to decipher encrypted information.

CRC A form of error checking used in network transmissions in which a value based on the information in the packet is calculated and included in the packet by the sending computer. The receiving computer recalculates the value. If there is no difference, the transmission is assumed to have arrived without error.

crosstalk In communications, interference caused by signal transference from one wire to another.

cryptography The encoding of information so that it is unreadable by anyone other than the person(s) holding the key to the code. See also key.


CSMA/CA Stands for Carrier Sense Multiple Access with Collision Avoidance, a method of controlling network access similar to CSMA/CD, in that nodes listen to the network and transmit only when it is free. But in CSMA/CA, nodes avoid data collisions by signaling their intention with a brief request to send (RTS) signal and then waiting for acknowledgment before actually transmitting.

CSMA/CD Stands for Carrier Sense Multiple Access with Collision Detection. A form of network access in which nodes listen to the line and attempt to transmit only when they sense that the carrier signal is free (not in use). If two nodes attempt to transmit at the same time, thus causing a collision, both nodes back off for random periods before attempting to transmit again.


cut-through switch A network switch that routes packets immediately to the port associated with the packet's recipient.

cyclical redundancy check See CRC.


D channel See ISDN.

DAS See dual attachment station.

data channel See ISDN.

Data Communications Equipment See DCE.

Data Link Connection Identifier A virtual circuit on frame relay networks that permanently identifies the path to a particular destination.

Data Link Control See DLC.

Data Service Unit See DDS.

Data Terminal Equipment See DTE.

datagram A packet of data with addressing information, sent through a packet-switching network.

DB-15 See AUI.

DCE Stands for Data Communications Equipment, the term used in RS-232 and X.25 specifications for a device, such as a modem, that provides another device (known as the DTE) with access to a communications line. Compare DTE.

DDS Stands for digital data service, a dedicated communications line that provides transmission at speeds up to 56 Kbps. DDS lines use a device known as a CSU/DSU rather than a modem for connecting two networks. The CSU, or Channel Service Unit, connects the network to the transmission line; the DSU, or Data Service Unit, converts data for transmission by the CSU and controls data flow.

DECnet The hardware, software, and protocol stack designed by Digital Equipment Corporation for its Digital Network Architecture (DNA).

dedicated connection A permanent, private communications link between two parties. A dedicated connection is leased from the telephone company and is always available for use.

dedicated server A computer—usually quite powerful—that is used solely as a network server. Compare nondedicated server.

deferral time The length of time that nodes on a CSMA/CD network wait before trying to retransmit after a collision.

delta channel See ISDN.

demand priority A network access method in which hubs control network access; a feature of 100BaseVG Ethernet networks. With demand priority, nodes send requests to hubs, and the hubs give permission to transmit based on priority levels assigned to the requests by the nodes.

demodulation The process by which a receiving modem unloads information from a modulated carrier wave and converts the information to the digital form required by the computer to which it is attached. Compare modulation.

dial-up networking Connection to a remote network through use of a modem; typically used in reference to telecommuting, although the term is equally applicable to connecting to the Internet.

dielectric Insulating material, such as rubber or plastic, that does not conduct electricity.

differential phase shift keying A form of phase shift keying in which each phase change is relative to the preceding state of the wave.

digital A reference to something based on digits (numbers) or their representation. PCs are digital computers because they deal with information of all types as different combinations of the binary digits 0 and 1. Compare analog.

digital data service See DDS.

Digital Services See DS.

Digital Signal See DS.

digital signature A security mechanism used on the Internet that relies on two keys, one public and one private, which are used to encrypt messages before transmission and to decrypt them on receipt.

Digital Subscriber Line See DSL.

DIP switch Stands for Dual Inline Package switch, one of a set of small (sometimes very small) toggle switches on a circuit board that are used for configuring the device.

directory Roughly, a list, index, or catalog of items. On a network, directories contain names and pertinent information related to authorized users and network resources. On a computer, a directory (or folder) is a device used to organize related files.

directory services A network service, installed on server computers, that handles the information databases (directories) required to identify the users and resources on the network.

distributed (distribution) services See BISDN.

distributed system A non-centralized network consisting of numerous computers that can communicate with one another and that appear to users as parts of a single, large, accessible "storehouse" of shared hardware, software, and data. A distributed system is conceptually the opposite of a centralized, or monolithic, system in which clients connect to a single central computer, such as a mainframe.

DIX Stands for Digital Intel Xerox, the companies that developed the AUI connector for thicknet Ethernet cable. See also AUI.

DLC Stands for Data Link Control, a protocol supported by Microsoft's Windows NT designed to provide access to IBM mainframe computers and to Hewlett-Packard printers connected to the network.

DLCI See Data Link Connection Identifier.

DNS Stands for Domain Name System, the hierarchical system used in naming sites on the Internet. DNS names consist of a top-level domain (such as .com, .org, and .net), a second-level domain (the site name of a business, organization, or individual), and possibly one or more subdomains (servers within a second-level domain).

DNS name server A server computer holding one of the DNS databases through which friendly names are matched to the corresponding IP addresses that computers use in contacting one another on the Internet.

domain In general, a group of some sort that is characterized by a particular attribute or set of features. In reference to the Internet, a domain is a unit in which one or more computers are organized in a particular hierarchy. At the highest level (top-level domain), the computers are grouped by type (commercial, educational, organization, and so on) or by country/region (France, United Kingdom, Germany, United States, and so on). At the next level (second-level domain), computers are grouped by site (Microsoft.com, amazon.com, and so on). At optional lower levels (subdomains within a site), computers are grouped by site administrators into whatever units best suit the needs of the organization.

domain controller In Windows NT, the master server that holds the directory services database that identifies all network users and resources.

Domain Name System See DNS.

downstream Delivery of information from a (Web) server to a client. Compare upstream.

DPSK See differential phase shift keying.

driver A device-specific program that enables a computer to work with a particular piece of hardware, such as a printer, disk drive, or network adapter. Because the driver handles device-specific features, the operating system is freed from the burden of having to understand—and support—the needs of individual hardware devices.

drop cable A cable, also known as a transceiver cable, that is used to connect a network interface card to a Thick Ethernet network.

DS Stands for Digital Services or Digital Signal, a category used in referencing the speed, number of channels, and transmission characteristics of T1, T2, T3, and T4 communications lines. The basic DS unit, or level, is known as DS-0, which corresponds to the 64 Kbps speed of a single T1 channel. Higher levels are made up of multiple DS-0 levels. DS-1 represents a single T1 line that transmits at 1.544 Mbps. For higher rates, T1 lines are multiplexed to create DS-2 (a T2 line consisting of four T1 channels that transmits at 6.312 Mbps), DS-3 (a T3 line consisting of 28 T1 channels that transmits at 44.736 Mbps), and DS-4 (a T4 line consisting of 168 T1 channels that transmits at 274.176 Mbps).

DSL Stands for Digital Subscriber Line, a recently developed (late 1990s) digital communications technology that can provide high-speed transmissions over standard copper telephone wiring. DSL is often referred to as xDSL, where the x stands for one or two characters that define variations of the basic DSL technology. Currently, ADSL (Asymmetric DSL) is the form most likely to be provided, but even it is, as yet, available only to limited groups of subscribers. See also ADSL, DSL Lite, HDSL, SDSL, RADSL, VDSL.

DSL Lite Stands for Digital Subscriber Line Lite, a variation of ADSL currently under development that simplifies installation but transmits more slowly, at 1.544 Mbps. See also DSL.


DTE Stands for Data Terminal Equipment, the term used in RS-232 and X.25 specifications for a device, such as a PC, that transfers information to a mediating device (known as the DCE) for communication. Compare DCE.

dual attachment station A FDDI node with two connections to the network—either through a node and a concentrator or through two concentrators. Compare single attachment station.

dual homing A form of fault tolerance used with critical network devices on FDDI networks, in which such devices are attached to both the primary and secondary (backup) rings through two concentrators to provide the maximum possible security in case the primary ring fails.

Dual Inline Package switch See DIP switch.

dual-ring topology A token-passing ring topology implemented in FDDI networks that consists of two rings in which information travels in opposite directions. One ring, the primary ring, carries information; the second ring is used for backup.

dumb terminal See terminal.

duplex See half duplex, full duplex.


EBCDIC Stands for Extended Binary Coded Decimal Interchange Code, a coding scheme developed by IBM that assigns 256 numeric values to text, numbers, punctuation marks, and transmission-control characters. Although EBCDIC, like ASCII, is used to represent 256 different characters, the characters represented in the two sets are different. EBCDIC is used primarily with mainframes and minicomputers, ASCII with microcomputers. Compare ASCII.

e-commerce The use of the Internet, especially the World Wide Web, as a commercial sales and marketing medium.

EIA Stands for Electronics Industries Association, a standards organization based in the United States that focuses on the development of hardware-related industry standards. EIA is the source of the widely used RS-232 standard that defines the structure of connectors used in serial transmissions.

electronic commerce See e-commerce.

Electronics Industries Association See EIA.

encrypt To encode (scramble) information in such a way that it is unreadable to all but those individuals possessing the key to the code.

encryption Scrambling of information so that it is unreadable by anyone other than those possessing the keys, or codes, required to return the information to readable form.

end-to-end delivery Communications in which packets are delivered and then acknowledged by the receiving system.

enterprise computing A vague term used generally to refer to networks and other computing needs implemented throughout a large, often widely dispersed corporation. Microsoft and IBM, for example, are enterprises; a small business is not.

Ethernet A widely used network that formed the basis for the IEEE 802.3 standard for bus networks that rely on CSMA/CD to control network transmissions. Ethernet is available in various forms, including those known as Thin Ethernet, Thick Ethernet, 10BaseX Ethernet, and 100BaseX Ethernet. Essentially, it is a 10 Mbps or 100 Mbps baseband network, although there is also a recently developed Gigabit Ethernet that operates at 10 times 100 Mbps speed.

Extended Binary Coded Decimal Interchange Code See EBCDIC.

Extensible Markup Language See XML.

extranet A semiprivate network based on Internet technologies; essentially, an intranet that has been extended to allow access to certain authorized outsiders, such as vendors and business partners.


Fast Ethernet See 100BaseX.

fast packet switching Term used in reference to high-speed packet-switching networks, such as frame relay and ATM that perform little or no error checking. The term is often, however, restricted to high-speed networking technologies, such as ATM, that transmit fixed-length cells rather than including those, such as frame relay, that transmit variable-length packets.

fault tolerance In terms of computers and network operating systems, refers to the ability to withstand severe problems that can bring a network to a standstill, damage hardware, or cause loss of data. Although all computers and operating systems should be as fault-tolerant as possible, network hardware and operating systems must strive to be paragons of virtue in this regard.

FDDI Stands for Fiber Distributed Data Interface, a high-speed (100 Mbps) networking technology based on fiber optic cable, token passing, and a ring topology.

female connector See connector.

Fiber Distributed Data Interface See FDDI.

fiber optic cable A form of network cabling that transmits signals optically, rather than electrically as do coaxial and twisted-pair cable. The light-conducting heart of a fiber optic cable is a fine glass or plastic fiber called the core. This core is surrounded by a refractive layer called the cladding that effectively traps the light and keeps it bouncing along the central fiber. Outside both the core and the cladding is a final layer of plastic or plastic-like material called the coat, or jacket. Fiber optic cable can transmit clean signals at speeds as high as 2 Gbps. Because it transmits light, not electricity, it is also immune to eavesdropping.

file sharing The act of making files on one computer accessible to others on a network.

File Transfer Protocol See FTP.

firewall A protective mechanism, usually a combination of hardware and software, designed to act as a barrier, keeping external networks, such as the Internet, completely separate from an internal network. Firewalls forbid access to anyone but individuals authorized to use the network.

fractional T1 A form of T1 service in which part, rather than all, of a T1 line can be leased.

frame Often, a term used to refer to data packets at lower networking layers. Also, a single unit of information in a synchronous transmission.

frame relay A digital packet switching technology that transmits variable-length data packets at speeds up to 2 Mbps over predetermined, set paths known as permanent virtual circuits, or PVCs.

frequency modulation A means of loading information onto a carrier wave by modulating (altering) its timing. Compare amplitude modulation.

frequency shift keying A form of frequency modulation that uses two different frequencies to represent the binary values 1 and 0. See also frequency modulation.

front end In terms of networking, roughly a reference to a client computer or the processing that takes place on it. Compare back end.

FSK See frequency shift keying.

FTP Stands for File Transfer Protocol, a fast, application-level TCP/IP protocol widely used for transferring both text-based and binary files to and from remote systems, especially over the Internet.

full duplex Modem operation in which transmission can occur in both directions at the same time. Compare half duplex, simplex.


gateway On a network, a device often a specialized computer that enables communication between networks based on different architectures and using different protocols. A gateway converts information being transmitted to a form in which it can be understood by the receiving network.

gender changer A type of connector used to enable male-to-male or female-to-female connections.

Gigabit Ethernet A recently developed form of Ethernet that operates at 1000 Mbps (1 gigabit per second). Gigabit Ethernet is expected to be used primarily as a high-speed LAN backbone.

Gopher A search program used to find documents on the Internet. Gopher searches all available Gopher servers, a virtual library known as Gopherspace, and it presents the user with menus and submenus with which to narrow a search and, eventually, choose desired documents. Gopher is assisted by two programs: Veronica, which narrows searches by combing the menus on all Gopher servers for information matching search criteria entered by the user, and Jughead, which searches for keywords on the top-level menus on specific Gopher servers.

Gopherspace See Gopher.

groupware A relatively vague term used to describe various types of software applications that enable network users to interact or work together. Groupware typically includes such applications as e-mail and scheduling software.


half duplex Modem operation in which transmission occurs in both directions, but not at the same time. Compare full duplex, simplex.

handshake A transmission that occurs at the beginning of a session between communicating computers. The handshake ensures that the two computers agree on how the transmission will proceed.

Hayes command set The set of transmission signals commands used originally by Hayes modems for exchanging status information, such as readiness to send or receive. The command set now represents a de facto standard for modem-to-modem communications.

HDSL Stands for High bit-rate Digital Subscriber Line, a form of DSL that transmits at 1.544 Mbps in both directions. See also DSL.

head In relation to software or documents, the top or beginning of something. In HTML, the head is the section of coding that precedes the body of a document and is used to describe the document itself (title, author, and so on) rather than the elements within the document. In terms of hardware, of course, head typically refers to the read/write device that transfers information to and from storage. Compare body.

High bit-rate Digital Subscriber Line See HDSL.

home page On the World Wide Web, the starting page for a Web site.

hop One leg of the journey taken by a packet from router to router in order to reach its destination.

host The main computer in a mainframe or minicomputer environment, that is, the computer to which terminals are connected. In PC-based networks, a host is a computer that provides access to other computers.

HTML Stands for HyperText Markup Language, the coding scheme used to tag the elements of Web documents (text format, font, color, image identifier, and so on) so that browser software can display the documents correctly on screen.

HTTP Stands for HyperText Transfer Protocol, the protocol used to carry requests from a browser to a Web server and to transport pages from Web servers back to the requesting browser. Although HTTP is almost universally used on the Web, it is not an especially secure protocol. One variation of HTTP known as SHTTP (Secure HTTP) adds encryption and other security features. Another, known as HTTPS (HTTP Secure) provides for encryption and transmission through a secure port. HTTPS was devised by Netscape and allows HTTP to run over a security technology known as SSL (Secure Sockets Layer). See also SSL.


hub A hardware device to which nodes connect on star-wired networks. Hubs can be passive, active, or intelligent, depending on how they operate. A passive hub simply acts as a connection point; an active hub both acts as a connection point and has the ability to regenerate signals; an intelligent hub is one with additional capabilities, such as the ability to configure the network.

hyperlink A connection that enables a user to jump from one element in an HTML document to another document or Web site. A hyperlink is associated with a normally invisible tag, coded in a markup language such as HTML that enables a Web browser to find and display the linked document. See also HTML.

hypermedia The combination of text, video, graphic images, sound, hyperlinks, and other elements in the form typical of Web documents. Essentially, hypermedia is the modern extension of hypertext, the hyperlinked, text-based documents of the original Internet.

HyperText Markup Language See HTML.

HyperText Transfer Protocol See HTTP.


IAB See Internet Architecture Board.

ICANN Stands for Internet Corporation for Assigned Names and Numbers, the organization approved in 1998 as the successor to NSI and IANA as the controlling body responsible for domain names and IP addresses.

IEEE Stands for Institute of Electrical and Electronics Engineers, a society of technical professionals based in the United States but boasting worldwide membership. The IEEE (pronounced "eye triple ee") focuses on electrical, electronics, computer engineering, and science-related matters. In the networking world, it is especially well known for its development of the IEEE 802.x specifications dealing with networking standards.

IEEE 802.x A series of specifications developed by the IEEE that defines numerous networking standards. The x following 802 is a placeholder for individual specifications, including 802.3 (Ethernet), 802.4 (Token Bus), and 802.5 (Token Ring).

IETF Stands for Internet Engineering Task Force, a worldwide organization of individuals interested in networking and the Internet. The work of the IETF is carried out by various Working Groups that concentrate on specific topics, such as routing and security. The IETF is the publisher of the specifications that led to the TCP/IP protocol standard.

Institute of Electrical and Electronics Engineers See IEEE.

Integrated Services Digital Network See ISDN.

Integrated Services LAN See isochronous network.

intelligent hub A type of hub that, in addition to transmitting signals, has built-in capability for other network chores, such as monitoring or reporting on network status. Intelligent hubs are used in different types of networks, including ARCnet and 10BaseT Ethernet.

interactive services See BISDN.

interface The connection between two elements that enables them to work together or exchange information. A user interface, for example, is the connecting point between a computer user and the computer's software.

International Organization for Standardization A federation of standards bodies representing 130 countries/regions throughout the world. Although the ISO is concerned with numerous standards unrelated to computing, it is justly famed in networking references as the originator of the seven-layer ISO/OSI Reference Model. See also ISO/OSI Reference Model.

International Telecommunication Union See ITU.

Internet Architecture Board The advisory arm of the Internet Society focused on technical matters.

Internet Corporation for Assigned Names and Numbers See ICANN.

Internet Engineering Task Force See IETF.

Internet Protocol See IP.

Internet Protocol address See IP address.

Internet Reference Model See TCP/IP Reference Model.

Internet Relay Chat See IRC.

Internet Research Task Force An arm of the Internet Society focused on long-term, Internet-related research projects.

Internet Service Provider See ISP.

Internet Society A nonprofit organization based in Reston, Virginia, with a worldwide membership of individuals concerned with Internet standards, education, and policy. See also Internet Architecture Board, Internet Engineering Task Force, Internet Research Task Force.

internetwork A network made up of smaller, interconnected networks.

Internetwork Packet Exchange/Sequenced Packet Exchange See IPX/SPX.

interrupt A signal from a device to a computer's processor requesting the processor's attention. Interrupts are generated for many reasons, including software errors and the signaling of the presence of data to be processed.

intranet A private network based on Internet technologies but confined to use within an organization, such as a corporation. Compare extranet.

IP Stands for Internet Protocol, the TCP/IP protocol responsible for routing packets. IP runs at the internetwork layer in the TCP/IP model—equivalent to the network layer in the ISO/OSI Reference Model.

IP address The 32-bit numeric address that identifies an Internet host to other computers on the Internet.

IPX/SPX Stands for Internetwork Packet Exchange/Sequenced Packet Exchange, a set of two protocols designed by Novell for NetWare networks. IPX is a connectionless protocol that handles addressing and routing of packets. SPX, which runs above IPX, ensures correct delivery. IPX/SPX is comparable to the TCP and IP protocols in the TCP/IP suite.

IRC Stands for Internet Relay Chat, an Internet service that enables participants to chat in real time.

IRTF See Internet Research Task Force.

ISDN Stands for Integrated Services Digital Network, a high-speed communications network developed to provide all-digital service over the existing telephone network. ISDN was designed to carry not only voice but data, images, and video. ISDN is available in two forms, known as BRI (Basic Rate Interface) and PRI (Primary Rate Interface). BRI consists of two B (bearer) channels that carry data at 64 Kbps and one D (data) channel that carries control and signal information at 16 Kbps. In North America and Japan, PRI consists of 23 B channels and 1 D channel, all operating at 64 Kbps; elsewhere in the world, PRI consists of 30 B channels and 1 D channel.

ISLAN See isochronous network.

ISO See International Organization for Standardization.

ISO/OSI Reference Model A well-known standard that defines networks in terms of seven protocol layers, each concerned with a different level of service that contributes to preparing data for transmission over a network. The ISO/OSI Reference Model covers all aspects of a network, from the physical medium to the ways in which applications access the network. It is a fundamental blueprint designed to help guide the creation of networking hardware and software.

ISOC See Internet Society.

isochronous network Also known as Integrated Services LAN, or ISLAN, a type of network defined in the IEEE 802.9 specification that combines ISDN and LAN technologies to enable networks to carry multimedia.

ISP A business that provides Internet access to individuals and businesses.

ITU Stands for International Telecommunication Union, an international organization based in Geneva, Switzerland. The ITU focuses on telecommunications and is divided into three sectors dealing with radio communications, standardization, and development. The standardization sector is the new (and current) home of the organization formerly known as the CCITT. See also CCITT.


Java An object-oriented programming language developed by Sun Microsystems and designed around the idea that a program, once written, can run without modification on more than one computing platform, such as Windows and Macintosh. Unlike programs created in other languages, such as C++, Java programs are compiled (translated for execution) into a form known as bytecode. This bytecode, which is not refined to the point of relying on platform-specific instructions, runs on a computer in a special software environment known as a virtual machine. Java is difficult for non-programmers to understand, so don't be appalled if it doesn't make much sense to you. Its primary impact on end users is its use in creating Web applets and, increasingly, distributed network applications.

Jughead See Gopher.


key In encryption and digital signatures, a string of bits used for encrypting and decrypting information to be transmitted. Encryption commonly relies on two different types of keys, a public key known to more than one person (say, both the sender and the receiver) and a private key known only to one person (typically, the sender).

keyword In search programs, a word or phrase used as the key for searching databases to locate information.


LAN See local area network.

LANtastic A network operating system from Artisoft designed to support both peer-to-peer and client/server networks.

layered architecture The division of a network model into multiple discrete layers, or levels, through which messages pass as they are prepared for transmission. In a layered architecture, protocols at each layer provide specific services or functions and rely on protocols in the layers above and below them for other needed services.

LCP See Point-to-Point Protocol.

line A segment in a SONET network that runs between two multiplexers. See also path, section.

linear bus See bus topology.

link See hyperlink.

Link Control Protocol See Point-to-Point Protocol.

Linux A UNIX-like operating system developed by Linus Torvalds and numerous programmers throughout the world. Linux is distributed free, and its source code is open to modification by anyone who chooses to work on it. It is often used as an operating system for network servers.

LLC Stands for Logical Link Control. In the IEEE 802.x specifications, the higher of two sublayers that make up the ISO/OSI data link layer. The LLC is responsible for managing communications links and handling frame traffic. See also MAC.

local area network A network that is relatively limited in scope for example, one that connects computers in a single department or building.

local loop The (end) portion of a telephone connection running between the subscriber and the local telephone exchange.

log on To gain access to a network by identifying oneself with a username and a password.

logical Conceptually true to a particular design or idea for example, transmissions travel in a circle around a logical ring, even though the ring shape itself is not physically apparent. Compare physical.

Logical Link Control See LCC.

LU Stands for Logical Unit, an entity that enables applications and devices to communicate on an SNA network.


MAC Stands for Media Access Control. In the IEEE 802.x specifications, the lower of two sublayers that make up the ISO/OSI data link layer. The MAC manages access to the physical network, delimits frames, and handles error control. See also LLC.

male connector See connector.

MAU Stands for Multistation Access Unit, a hub to which nodes connect on a Token Ring network.

maximum transmission unit See MTU.

Media Access Control See MAC.

microcomputer A computer based on a microprocessor. Essentially, in today's terms, a desktop PC.

MIME Stands for Multipurpose Internet Mail Extensions, a protocol widely used on the Internet for enabling e-mail to include multiple types of information, including text, graphics, sound, and video. MIME uses the message header for describing media types included in the document. This information is then used by software on the destination machine to determine whether the particular data types can be "replayed," and if so, by what programs.

mirroring A means of protecting data on a network by duplicating it, in its entirety, on a second disk. Mirroring is one strategy implemented in RAID security. See also RAID.

modem The device that enables computer-to-computer communication over a telephone line. Modems work by modulating and demodulating computer signals. When transmitting, the modem transforms (modulates) signals from the digital form required by the computer to the analog form required by the phone line. When receiving, the modem reverses the action, demodulating the signal from analog back to digital form.

modulation The process of modifying a carrier wave in such a way that the modification(s) to the wave represent information. Transmitting modems use modulation to load information onto a telephone signal (carrier). Compare demodulation.

MOO Stands for MUD, object oriented, a type of virtual environment on the Internet, similar to a game-oriented MUD but based on an object-oriented language and generally focused more on programming than on games.

MTU Stands for maximum transmission unit, the largest packet size that can be supported by a particular network implementation.

MUD Stands for multi-user dungeon, an interactive environment on the Internet where users can get together to play multi-user role-playing, dungeons and dragons types of games.

multiplexing The process of weaving multiple signals onto a single channel or communications line. In multiplexing, segments of information from each signal are interleaved and generally separated by time, frequency, or space.

multiport repeater See active hub.

multiprogramming A form of processing in which a computer holds more than one program in memory and works on them in round-robin fashion that is, by sharing out the processor's time so that each program receives some attention some of the time. This way of working is in contrast to using the processor to run one program at a time.

Multipurpose Internet Mail Extensions See MIME.

Multistation Access Unit See MAU.

multitasking A form of processing supported by most current operating systems in which a computer works on multiple tasks roughly, separate "pieces" of work seemingly at the same time by parceling out the processor's time among the different tasks. Multitasking can be either cooperative or preemptive. In the former, the operating system relies on the task to voluntarily cede control to another task; in the latter, the operating system decides which task receives priority.


NAP See network access point.

narrowband ISDN Name used to distinguish current ISDN lines from the developing broadband ISDN technology.

NC See network computer.

NCP See Point-to-Point Protocol.

NDIS Stands for Network Device Interface Specification, a software-interface specification developed by Microsoft and 3Com that mediates between network adapters and higher-level protocols to enable the adapter to support multiple protocol stacks.

NetBEUI Stands for NetBIOS Extended User Interface, a transport-layer protocol provided with Microsoft networking products. NetBEUI is capable of rapid data transfer but is limited to LANs because it is a nonroutable protocol (meaning it does not support routing, which relies on network addresses to move transmissions from one network to another).

NetBIOS Extended User Interface See NetBEUI.

NetWare A network operating system from Novell, developed in both peer-to-peer and client server versions, the latter available for both servers and workstations.

network access point Also called national attachment point, one of the four locations in the United States through which Internet Service Providers connect to the Internet backbone.

network computer A computer designed for use on a network in which programs and storage are provided by servers. Network computers, unlike dumb terminals, have their own processing power, but their design does not include local storage, and they depend on network servers for applications.

Network Control Protocol See Point-to-Point Protocol.

Network Device Interface Specification See NDIS.

network interface card The circuit board needed to provide network access to a computer or other device, such as a printer. Network interface cards, or NICs, mediate between the computer and the physical media, such as cabling, over which transmissions travel.

Network News Transfer Protocol See NNTP.

network operating system An operating system specifically designed to support networking. A server-based network operating system, or NOS, provides networking support for multiple simultaneous users as well as administrative, security, and management functions. On the desktop, a network-aware operating system provides users with the ability to access network resources.

newsfeed A collection of news articles delivered through the NNTP protocol by news servers on the Internet.

newsgroup A group of individuals on the Internet with a common interest in a particular subject or set of related subjects. Newsgroups receive and post articles and discussions bearing on their area of interest.

NIC See network interface card.

NNTP Stands for Network News Transfer Protocol, a de facto protocol standard on the Internet used to distribute news articles and query news servers.

node A device on a network for example, a client computer, a server, or a shared printer.

noise Electromagnetic interference that distorts or degrades a communications signal.

nondedicated server A computer on a network that can function as both a client and a server; typically, a desktop machine on a peer-to-peer network. Compare dedicated server.


ODI Stands for Open Data-Link Interface, a software-interface specification developed by Novell and Apple for providing a common boundary between network adapters and higher-level protocols so that the adapters can support more than one protocol stack.

Open Data-Link Interface See ODI.

open source A movement in the programming community for making source code (program instructions) free and freely available to anyone interested in using or working with it.

Open Systems Interconnection Reference Model See ISO/OSI Reference Model.

optical fiber See fiber optic cable.

OS Stands for operating system, the foundation software that interacts with the computer, runs applications, provides needed services to software and attached devices, and presents the user with a keyboard-based or mouse-based user interface for controlling the computer.

OSI protocol stack The set of protocols based on and corresponding to the ISO/OSI Reference Model.

OSI Reference Model See ISO/OSI Reference Model.


packet The fundamental message unit transmitted over a network. A packet consists of a header that contains addressing information, a block of data (the largest portion of the packet), and a trailer that often contains error-checking information. Packet size and makeup vary with the network and protocols in use.

packet assembler and disassembler See PAD.

packet header The portion of a data packet that precedes the body (data). The header contains data, such as source and destination addresses and control and timing information that is needed for successful transmission.

packet switching A transmission method by which information is broken into same-sized units called packets that are addressed and routed over a network from source to destination. Although the packets forming a single transmission can be sent over different routes and can arrive at their destination at different times, the receiving computer reassembles them in the correct order. Packet switching does not require a physical link between communicating parties, nor does it require that a connection be made before transmission can occur. Compare circuit switching.

Packet Switching Exchange An intermediary switching station in a packet-switching network.

packet trailer The portion of a data packet that follows the body (data). The trailer typically contains information related to error checking and correction.

PAD Stands for packet assembler/disassembler, a device on a packet-switching network that assembles packets for transmission and reassembles packets in the correct order on receipt.

page On the World Wide Web, a document called up by, and viewed through, browser software. A Web page can be a multimedia document containing text, graphics, sound, and video. It is also characterized by the inclusion of hyperlinks that enable users to move directly from one document (page) to another.

parallel transmission A form of transmission in which groups of bits typically the 8 bits forming 1 byte are sent at the same time over multiple wires. Compare serial transmission.

parity bit A bit in asynchronous communications used to indicate the type of error checking used in a transmission.

passive hub A type of hub used on ARCnet networks that passes signals along but has no additional capability. Compare active hub, intelligent hub.

passive node A network node that "listens" for transmissions but is not actively involved in passing them along the network; typical of a node on a bus network.

password The private string of characters entered by a user to verify his or her identity to the network. Ideally a password is a combination of text, numbers, and punctuation or other characters that cannot be guessed at or easily cracked by intruders.

path The complete route from source to destination multiplexer on a SONET network. See also line, section.

PCT Stands for Private Communication Technology, a protocol standard drafted by Microsoft and submitted to the IETF for consideration. PCT, like the Netscape-designed SSL (Secure Sockets Layer), supports authentication and encryption for securing privacy in Internet communications.

peer-to-peer network A network in which computers function as equals, with each one being able to function as a client, a server, or both, depending on the resources being used and/or shared out. In LANs, typically a small network of desktop PCs. Compare client/server network.

permanent virtual circuit A connection established by a connection-oriented service in which a permanent link exists between the communicating parties.

phase shift keying A means of loading information onto a carrier wave by modulating (altering) the phase of the wave.

physical Of or relating to something real, visible, or tangible for example, a physical ring would be one that forms a visible circle. Compare logical.

physical address In networking, a unique numeric address (value) assigned to a network interface card.

Plain Old Telephone Service See POTS.

platform In general, a reference to a particular type of computer or operating system for example, the Windows platform or the Macintosh platform. More specifically, a platform refers to the technology the details that differentiate one computer or operating system from another.

plug See connector.

Point to Point Tunneling Protocol See PPTP.

Point-to-Point Protocol See PPP.

point-to-point tunneling A means of setting up secure communications over an open, public network such as the Internet. See PPTP.

port A channel, or conduit, through which information flows to or from a computer and attached input or output devices.

portal A site designed to act as an entryway to the World Wide Web.

post office The server and associated storage and mail handling services that provide the centralized location for collection and distribution of e-mail over a network.

POTS Stands for Plain Old Telephone Service, that is, the network of telephone lines and switching facilities that forms the basis of everyday telephone service.

PPP Stands for Point-to-Point Protocol, a widely used Internet protocol for transmitting TCP/IP packets. PPP supports dynamic allocation of IP addresses and so is easier to use than the older SLIP. The PPP protocol itself is based on a Link Control Protocol (LCP) responsible for setting up a computer-to-computer link over telephone lines and a Network Control Protocol (NCP) responsible for negotiating network-layer details related to the transmission. See also PPTP.

PPTP Stands for Point to Point Tunneling Protocol, an extension of the Point-to-Point Protocol used for communications on the Internet. PPTP was developed by Microsoft to support virtual private networks (VPNs), which allow individuals and organizations to use the Internet as a secure means of communication. PPTP supports encapsulation of encrypted packets in secure wrappers that can be transmitted over a TCP/IP connection.


Primary Rate Interface See ISDN.

Private Communication Technology See PCT.

private key See key.

Project 802 The IEEE project to define networking standards that resulted in the 802.x specifications. See also IEEE, IEEE 802.x.

promiscuous mode On a network, a method of operation in which a device monitors all network traffic.

protocol A set of rules governing the way in which computers communicate, either with one another or with attached devices. Networking is filled with a bewildering variety and number of protocols, many of which perform the same, or similar, services. All, however, are designed to enable reliable communication.

protocol stack A complete set of protocols that work together to enable communication on a network. Compare protocol suite.

protocol suite A set of protocols designed, usually by one vendor, as complementary parts of a protocol stack. Compare protocol stack.

proxy A computer (or the software that runs on it) that acts as a barrier between a network and the Internet by presenting only a single network address to external sites. By acting as a go-between representing all internal computers, the proxy protects network identities while still providing access to the Internet.

PSE See Packet Switching Exchange.

PSK See phase shift keying.

PSTN See Public Switched Telephone Network.

public key See key.

Public Switched Telephone Network The public telephone system.

push A technology developed in relation to the World Wide Web, designed to provide end users with personalized Web access by having a site actively "push" requested information to the user's desktop, either automatically or at specified intervals. Push was developed as a means of relieving users from having to actively retrieve ("pull") information from the Web. It is not, as yet, especially popular.

PVC See permanent virtual circuit.


QAM See quadrature amplitude modulation.

QOS Stands for quality of service, a term used generally to refer to performance at or above a certain standard. More specifically, QOS refers to the maximum amount of delay and data loss considered acceptable for transmissions on an ATM network.

quadrature amplitude modulation A means of loading information onto a carrier wave by modulating both its phase and amplitude. Quadrature amplitude modulation creates a constellation of points that allow a single signal change to represent multiple bits of information.

quality of service See QOS.


RADSL Stands for Rate Adaptive Digital Subscriber Line, a form of DSL that adjusts transmissions based on line quality and distance traveled. RADSL operates at 2.2 Mbps downstream and up to 1.088 Mbps upstream.

RAID Stands for Redundant Array of Inexpensive Disks, a means of improving network performance and ensuring data integrity and security by storing information on multiple disk drives. Different levels of RAID security can be implemented, depending on the levels of reliability and performance desired.

Rate Adaptive Digital Subscriber Line See RADSL.

real time Occurring in the same time frame experienced by people.

redirector Software on a client computer that intercepts requests for information and, when appropriate, directs them to the network. Redirectors can either be built into the client operating system or be part of an added networking package.

Reduced Instruction Set Computing See RISC.

repeater A network device that cleans and boosts a signal before sending it on its way.

resource sharing The act of making files, printers, and other network resources available for use by others.

ring topology A network configuration in which nodes are attached to the main communications line in a logical or physical ring shape.

RISC Pronounced "risk." An acronym for Reduced Instruction Set Computing, a description applied to microprocessors optimized for carrying out a relatively small number of instructions extremely quickly. In terms of the simple instructions most often carried out by computers, RISC chips are faster than the general-purpose CISC (Complex Instruction Set Computing) chips typical of many desktop computers. For complex instructions that must be broken down into numerous simpler ones, RISC chips are slower than their CISC brethren.

RJ-45 A type of connector used with twisted-pair wiring; similar to, but larger than, a telephone jack.

robot See spider.

root server A DNS name server that is the authority for resolving Internet names and IP addresses at the highest domain level, the top-level domain.

routable protocol A network protocol that can be used to route packets from one network to another.

router A network device that transmits message packets, routing them over the best route available at the time. Routers are used to connect multiple network segments, including those based on differing architectures and protocols.

routing table A table of network addresses used by routers and bridges in determining where to forward transmissions. Although both routers and bridges use routing tables, the tables are not the same for both devices. Tables for routers, for example, include information about different paths between other routers, whereas tables for bridges include network addresses only.


S/MIME Stands for Secure/Multipurpose Internet Mail Extensions, a security-oriented protocol that adds encryption and support for digital signatures to the widely used MIME e-mail protocol.

SAS See single attachment station.

scalability In relation to network hardware and software, the ability to support larger or smaller numbers of computers as needs and circumstances dictate. Essentially, the ability to grow or shrink gracefully.

script A program that interacts with an application or a utility program. On the World Wide Web, scripts are commonly used to customize or add interactivity to Web pages.

SDH See Synchronous Digital Hierarchy.

SDSL Stands for Single-line Digital Subscriber Line, a variation of HDSL that uses one pair rather than two pairs of wires and transmits at 1.544 Mbps.

search engine On the Internet, a document-finding program used to find and retrieve information based on keywords indexed in databases.

second-level domain The level immediately beneath the top-level domain in the Internet's DNS hierarchy.

section A length of fiber optic cable in a SONET network. See also line, path.

Secure HTTP See HTTP.

Secure Sockets Layer See SSL.

Secure/Multipurpose Internet Mail Extensions See S/MIME.

Serial Line Internet Protocol See SLIP.

serial transmission A form of transmission in which bits are sent sequentially, one at a time, over a single line. Compare parallel transmission.

service Specialized, software-based functionality provided by network servers for example, directory services that provide the network equivalent of "phone books" needed for locating users and resources.

session The time during which two computers are communicating.

shared medium The communications medium shared by network nodes; essentially, the network bandwidth.

shielded twisted-pair wiring See twisted-pair wiring.


Simple Mail Transfer Protocol See SMTP.

Simple Network Management Protocol See SNMP.

simplex Modem operation in which transmission occurs in one direction only. Compare full duplex, half duplex.

single attachment station A FDDI node that connects to the primary ring through a concentrator. Compare dual attachment station.

Single-line Digital Subscriber Line See SDSL.

site On the World Wide Web, a collection of documents (pages), as well as files and databases, belonging to a particular individual or organization.

SLIP Stands for Serial Line Internet Protocol, a protocol used for Internet connections. SLIP enables computers to transmit TCP/IP packets over serial lines. It is an older, less secure protocol than the PPP (Point-to-Point Protocol) and does not support dynamic allocation of IP addresses. A newer form of SLIP, known as CSLIP (Compressed SLIP), optimizes transmission of long documents by compressing header information. See also PPP.

SMDS Stands for Switched Multimegabit Data Service, a connectionless, packet-switched technology used by businesses to connect LANs in different locations.

SMP See symmetric multiprocessing.

SMS See Systems Management Server.

SMTP Stands for Simple Mail Transfer Protocol, a protocol in the TCP/IP suite used to transfer e-mail over the Internet.

SNA Stands for Systems Network Architecture, a network model devised by IBM to enable IBM products, including mainframes, terminals, and peripherals, to communicate and exchange data. SNA started out as a five-layer model and was later extended with two additional layers to correspond more closely to the ISO/OSI Reference Model. More recently, the SNA model was modified to include minicomputers and microcomputers in a specification known as APPC (Advanced Program to Program Communications). See also APPC. Compare ISO/OSI Reference Model.

snail mail Semi-sarcastic jargon used by e-mail fans to refer to non-electronic (paper-based) mail sent and delivered through traditional postal services.

SNMP Stands for Simple Network Management Protocol, a protocol in the TCP/IP suite used for network monitoring and management.

socket In networking, a means of enabling client and server applications to communicate over a network. A socket is essentially an end point in a communications session. In reference to hardware, a socket is a receptacle, a female connector. See also connector.

SONET Stands for Synchronous Optical NETwork, a high-speed network that provides a standard interface for communications carriers to connect networks based on fiber optic cable. SONET is designed to handle multiple data types (voice, video, and so on). It transmits at a base rate of 51.84 Mbps, but multiples of this base rate go as high as 2.488 Gbps (gigabits per second).

spider A program that roams the World Wide Web, seeking documents to be indexed in a database that can then be examined for matches by search engines. Also called a crawler, robot, or bot.

spoofing A means of masquerading on a network by making a transmission appear to come from an authorized computer.

SSL Stands for Secure Sockets Layer, a protocol developed by Netscape Communications Corporation for ensuring security and privacy in Internet communications. SSL supports authentication of client, server, or both, as well as encryption during a communications session. See also PCT.

star bus A network topology in which nodes connect to hubs in a star pattern, but the hubs are connected by a bus trunk. Star bus is a combination of star and bus topologies.

star topology A network configuration based on a central hub, from which nodes radiate in a star-shaped pattern.

start bit A bit (actually a timing signal) that represents the beginning of a character in asynchronous communications. Compare parity bit, stop bit.

star-wired ring A network topology in which hubs and nodes connect to a central hub in typical star fashion, but the connections within the central hub form a ring. Star-wired ring is a combination of star and ring topologies.

station In the IEEE 802.11 wireless LAN specification, a single, often mobile, node.

stop bit A bit (actually a timing signal) that represents the end of a character in asynchronous communications. Compare parity bit, start bit.

store and forward A method of delivering transmissions in which messages are held temporarily by an intermediary before being sent on to their destination. Store and forward is used by some switches in delivering packets to their destinations. Compare cut-through switch.

storm On a network, a sudden, excessive burst of traffic. Storms are often responsible for network outages.

STP See twisted-pair wiring.

striping A means of protecting data on a network by spreading it across multiple disks. In the most commonly used approach, striping is combined with parity (error-correcting information) to ensure that if some portion of the data is lost, it can be reconstructed. Striping is implemented in RAID security. See also RAID.

subdomain A domain, often representing an administrative or other organizational subgroup within a second-level domain.

subnet In general, a network that forms part of a larger network. In terms of the ISO/OSI Reference Model, the subnet comprises the layers below the transport layer that is, the network, data link, and physical layers.

subnetwork A network that is part of another, larger network.

SVC See switched virtual circuit.

switch A network device capable of forwarding packets directly to the ports associated with particular network addresses.

Switched Multimegabit Data Service See SMDS.

Switched T1 A circuit-switched form of T1 communications.

switched virtual circuit A connection established by a connection-oriented service in which a temporary link is created between the communicating parties.

symmetric multiprocessing A method of processing in which multiple processors work together on the same tasks; a means of improving speed and increasing a computer's ability to handle large workloads.

synchronous communications Computer-to-computer communications in which transmissions are synchronized by timing between the sending and receiving machines.

Synchronous Digital Hierarchy An ITU recommendation implemented in Europe and similar in most respects to the SONET standard used in North America and Japan. See also SONET.

Synchronous Optical NETwork See SONET.

Systems Management Server A Microsoft BackOffice component that provides services for centralized network management.

Systems Network Architecture See SNA.


T1 A high-speed communications line, sometimes called T-1 carrier, originally developed to carry multiple conversations over standard twisted-pair telephone wiring. Currently, T1 provides digital communications and Internet access at the rate of 1.54 Mbps. T1 speed is attained through multiplexing 24 separate 64 kbps channels into a single data stream.

T-1 carrier See T1.

TA See terminal adapter.

tag In general, a marker of some sort. In the HTML markup language of the World Wide Web, tags are embedded codes used to identify and define the properties of the element within a document images, formatted text, hyperlinks, and so on. These tags are used by browser software in determining how documents are displayed on screen.

TCP Stands for Transmission Control Protocol, the TCP/IP protocol responsible for creating and reassembling packets and ensuring that information is delivered correctly. TCP runs at the transport layer and relies on IP (Internet Protocol) for delivery. It is a connection-oriented, reliable protocol (reliable in the sense of ensuring error-free delivery). Compare UDP.

TCP/IP Stands for Transmission Control Protocol/Internet Protocol, a protocol suite (set of protocols) designed for enabling communications over interconnected, sometimes dissimilar, networks. TCP/IP is supported by almost all networks. It lies at the heart of Internet communications.

TCP/IP Reference Model A networking model designed around the concept of internetworking the exchange of information among different networks, often built on different architectures. The TCP/IP Reference Model, often called the Internet Reference Model, consists of four layers, the most distinctive of which is the internetwork that deals with routing messages and that has no equivalent in the ISO/OSI Reference Model or the SNA model. Compare ISO/OSI Reference Model, SNA.

telnet A protocol in the TCP/IP suite that enables individuals to log on to and use a remote computer as if they were sitting at a terminal directly connected to the machine.

terminal In terms of networking, a device with a keyboard and a monitor that is connected to a computer, such as a mainframe, through a communications link or cable and that relies on the larger machine for processing and data.

terminal adapter The correct name for an ISDN "modem," which connects a PC to an ISDN line but does not modulate or demodulate signals as a typical modem does.

terminal emulation The use of software to help a computer behave as if it were a particular kind of terminal attached to a larger computer, such as a mainframe.

terminator A hardware device used to "cap" the end of a cable in a bus network in order to keep signals from bouncing back along the line.

Thick Ethernet See 10Base5.

thin client A network device that has the ability to process information independently but relies on servers for applications, data storage, and administration. See also network computer, Windows terminal.

Thin Ethernet See 10Base2.

thread On the Internet and in e-mail discussions, a series of comments related to a particular topic.

throughput The data transfer rate of a network, measured as the number of bits per second transmitted.

timesharing A method, used primarily in the 60s and 70s, for sharing the capabilities (and cost) of a computer, such as a mainframe. Timesharing allowed different clients to "rent" time on a large computer and pay for only the portion of time they used.

token The special packet circulated by nodes in a token-passing network. Possession of the token gives a node the right to transmit.

token bus The IEEE 802.4 specification for token-passing networks based on a bus or tree topology. Token bus networks were designed primarily for manufacturing, but the specification also corresponds to the ARCnet architecture used for LANs.

token passing A means of controlling network access through the use of a small packet, the token, which is circulated through the network from node to node. A node can transmit only when it holds the token.

token ring Spelled with lowercase t and r, the IEEE specification 802.5 for token ring networks. See also Token Ring.

Token Ring Spelled with uppercase T and R, the token ring architecture developed by IBM and widely implemented in token-passing networks. Token Ring networks are usually star-wired rings in which nodes connect to hubs known as Multistation Access Units, or MAUs. These networks typically operate at 4 Mbps or 16 Mbps, although both 100 Mbps and 1 Gbps forms are under development.

top-level domain The highest-level domain in the Internet's DNS hierarchy.

topology The layout of a network; its configuration.

transceiver From transmitter/receiver, a device that both transmits and receives signals. On a network, a transceiver is the device that connects a computer to the network and that converts signals to and from parallel and serial form.

transceiver cable See drop cable.

Transmission Control Protocol See TCP.

transparent A term used to describe computer hardware or software that works so well and is so easy to use that its underlying complexity, or the means by which it performs its work, is invisible to the user.

trellis-coded modulation A variation of quadrature amplitude modulation that allows encoding of both data and error-checking information.

trunk The cable forming the main communications path on a network; on a bus network, the single cable to which all nodes connect. See also backbone.

tunneling A method of transmission over internetworks based on differing protocols. In tunneling, a packet based on one protocol is wrapped, or encapsulated, in a second packet based on whatever differing protocol is needed in order for it to travel over an intermediary network. In effect, the second wrapper "insulates" the original packet and creates the illusion of a tunnel through which the wrapped packet travels across the intermediary network. In real-life terms, tunneling is comparable to "encapsulating" a present (the original packet) in a box (the secondary wrapper) for delivery through the postal system.

twisted-pair wiring Wiring consisting of two insulated strands of copper twisted around one another to form a cable. Twisted-pair wiring comes in two forms, unshielded twisted pair (UTP) and shielded twisted pair (STP), the latter named for an extra protective sheath wrapped around each insulated pair of wires. Twisted-pair wiring can consist of a single pair of wires or, in thicker cables, two, four, or more pairs of wires. Twisted-pair wiring is typical of telephone cabling. Compare coaxial cable, fiber optic cable.


UDP Stands for User Datagram Protocol, a transport-level TCP/IP protocol that breaks messages into packets for delivery by the IP protocol. Unlike TCP, UDP is a connectionless, unreliable protocol, meaning that it does not establish a path between sender and receiver before transmitting and does not verify that packets arrive correctly.

Universal Resource Locator See URL.

UNIX A powerful multitasking operating system developed in 1969 for use in a minicomputer environment; still a widely used network operating system.

unreliable protocol A communications protocol that makes a "best effort" attempt to deliver a transmission but does not provide for verifying that the transmission arrives without error.

unshielded twisted-pair wiring See twisted-pair wiring.

upstream Delivery of information from a client to a (Web) server. Compare downstream.

URL Stands for Uniform Resource Locator or, sometimes, Universal Resource Locator, the address for a resource (document) on the Internet.

User Datagram Protocol See UDP.

username The "friendly" name by which a user is known and addressed on a network.

UTP See twisted-pair wiring.


V series The series of recommendations devised by the CCITT (now part of the ITU) relating to modems and modem communications over the public phone system.

vampire tap A type of transceiver used on Ethernet networks that is equipped with sharp metal prongs that pierce the insulation on thicknet cable to make contact with the copper core over which signals travel.

VDSL Stands for Very high rate Digital Subscriber Line, a developing form of DSL that can transmit up to 52 Mbps downstream and up to 2.3 Mbps upstream, though only over short distances (up to 4500 feet).

Veronica See Gopher.

Very high rate Digital Subscriber Line See VDSL.

VINES A UNIX-based networking operating system from Banyan Systems.

virtual circuit A connection between communicating computers that provides the computers with what appears to be a direct link but can actually involve routing data over a defined, but longer path.

virtual private network A network that uses encryption and other technologies, including tunneling, to provide secure communications over the Internet. A virtual private network essentially provides users with an inexpensive, Internet-based equivalent of a network connected by private communications lines.

VPN See virtual private network.


W3C Abbreviation for the World Wide Web Consortium, a standards body based in the United States, Europe, and Japan. The W3C ("double-yew-three-see") is dedicated (in part) to encouraging the development of open Web standards, such as the HTML and XML document markup languages.

WAIS Stands for Wide Area Information Service, a UNIX-based document-search system that searches collections known as WAIS libraries for files matching keywords entered by the user.

WAN See wide area network.

Wide Area Information Service See WAIS.

wide area network A geographically widespread network, one that relies on communications capabilities to link the various network segments. A WAN can be one large network, or it can consist of a number of linked LANs.

Windows The name for any of the graphical, mouse-oriented operating systems from Microsoft, available in different forms and versions for desktop computers (Windows 95 and 98), workstations (Windows NT Workstation), and network servers (Windows NT/2000 Server).

Windows terminal A thin-client solution from Microsoft, designed to enable terminals and minimally configured computers to display Windows applications even if they are not, in themselves, capable of running Windows software. Windows terminals work in conjunction with Windows NT Server, Terminal Server edition. See also thin client.

Wireless LAN A local area network based either fully or in part on wireless transmission technologies, such as infrared light and radio signals.

WLAN See wireless LAN.

World Wide Web Consortium See W3C.



Xerox Network System See XNS.

XML Stands for Extensible Markup Language, an evolving Web markup language designed to extend the capabilities of HTML by giving page designers a means of describing not only the appearance of a document, but also its content.

XNS Stands for Xerox Network System, a set of protocols assigned to five numbered layers (0 through 4) that form a suite designed to (as usual) handle packaging and delivery of network transmissions.


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