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5-4-3 rule A rule that states that a thinnet network can combine as many as five cable segments connected by four repeaters, but only three segments can have stations attached, which leaves two segments untapped.

10Base2 Ethernet topology that transmits at 10 Mbps over a baseband wire, and can carry a signal 185 meters. See also thinnet.

10Base5 See standard Ethernet.

10BaseFL An Ethernet network that typically uses fiber-optic cable to connect computers and repeaters.

10BaseT 10 Mbps Ethernet network topology that typically uses unshielded twisted-pair (UTP) cable to connect computers. The maximum length of a 10BaseT segment is 100 meters (328 feet).

100BaseX Ethernet See fast Ethernet.

100VG (Voice Grade) AnyLAN (100VGAnyLAN) An emerging networking technology that combines elements of both Ethernet and Token Ring.

A

access method The set of rules that defines how a computer puts data onto the network cable and takes data from the cable. When data is moving on the network, access methods help to regulate the flow of network traffic.

access permissions Control access to sharing in Windows NT Server. Permissions can be set for the following access levels:

No Access—Prevents access to the shared directory, its subdirectories, and its files.

Read—Allows viewing of file and subdirectory names, changing to a shared directory's subdirectory, viewing data in files, and running applications.

Change—Allows viewing of file and subdirectory names, changing to a shared directory's subdirectories, viewing data in files and running application files, adding files and subdirectories to shared directory, changing data in files, and deleting subdirectories and files.

Full Control—Includes the same permissions as Change, plus changing permissions (taking ownership of the Windows NT file system [NTFS] files and directories only).

account See user account.

account policy Controls how passwords must be used by all user accounts in a domain or in an individual computer.

Address Resolution Protocol (ARP) Determines hardware addresses (MAC addresses) that correspond to an IP address.

ADSL See Asymmetric Digital Subscriber Line (ADSL).

advanced cable testers Cable testers that work beyond the physical layer of the OSI reference model up into layers 2, 3, and even 4. They can display information about the condition of the physical cable as well as message frame counts, excess collisions, late collisions, error-frame counts, congestion errors, and beaconing. These testers can monitor overall network traffic, certain kinds of error situations, and traffic to and from a particular computer. They indicate if a particular cable or network interface card (NIC) is causing problems.

advanced program-to-program communication (APPC) A specification developed as part of IBM's SNA (Systems Network Architecture) model and designed to enable applications programs running on different computers to communicate and exchange data directly. See also Systems Network Architecture.

AFP See AppleTalk filing protocol (AFP).

agent A program that performs a background task for a user and reports to the user when the task is done or when some expected event has taken place.

American National Standards Institute (ANSI) An organization of American industry and business groups dedicated to the development of trade and communications standards. ANSI is the American representative to the International Organization for Standardization (ISO).

amplifier A device, such as a repeater or bridge, that amplifies or increases the power of electrical signals allowing them to travel on additional cable segments at their original strength. Amplifiers strengthen signals that have been weakened by attenuation.

analog Related to a continuously variable physical property, such as voltage, pressure, or rotation. An analog device can represent an infinite number of values within the range the device can handle. See also analog line, digital.

analog line A communications line, such as a telephone line, that carries information in analog (continuously variable) form. To minimize distortion and noise interference, an analog line uses amplifiers to strengthen the signal periodically during transmission.

ANSI See American National Standards Institute (ANSI).

APPC See advanced program-to-program communication (APPC).

AppleShare AppleShare is the Apple network operating system. Features include file sharing, client software that is included with every copy of the Apple operating system, and the AppleShare print server, a server-based print spooler.

AppleTalk The Apple network architecture that is included in the Macintosh operating system software. It is a collection of protocols that correspond to the OSI reference model. Thus network capabilities are built in to every Macintosh. AppleTalk protocols support LocalTalk, Ethernet (EtherTalk), and Token Ring (TokenTalk).

AppleTalk filing protocol (AFP) Describes how files are stored and accessed on the network. AFP is responsible for the Apple hierarchical filing structure of volumes, folders, and files and provides for file sharing between Macintoshes and MS-DOS-based computers. It provides an interface for communication between AppleTalk and other network operating systems, allowing Macintoshes to be integrated into any network that uses an operating system that recognizes AFP.

application layer The top (seventh) layer of the OSI reference model. This layer serves as the window that application processes use to access network services. It represents the services that directly support user applications, such as software for file transfers, database access, and e-mail.

application programming interface (API) A set of routines that an application program uses to request and carry out lower-level services performed by the operating system.

application protocols Protocols that work at the higher end of the OSI reference model, providing application-to-application interaction and data exchange. Popular application protocols include:

FTAM (file transfer access and management)—
A file access protocol.

SMTP (simple mail transfer protocol)—A TCP/IP protocol for transferring e-mail.

Telnet—A TCP/IP protocol for logging on to remote hosts and processing data locally.

NCP (NetWare core protocol)—The primary protocol used to transmit information between a NetWare server and its clients.

ArcNet (Attached Resource Computer Network) Developed by Datapoint Corporation in 1977, designed as a baseband, token-passing, bus architecture, transmitting at 2.5 Mbps. A successor to the original ArcNet, ArcNetplus supports data transmission rates of 20 Mbps. A simple, inexpensive, flexible network architecture designed for workgroup-sized LANs, ArcNet runs on coaxial, twisted-pair, and fiber-optic cable and supports up to 255 nodes. ArcNet technology predates IEEE Project 802 standards but loosely maps to the 802.4 document. See also Project 802.

ARP (Address Resolution Protocol) See Address Resolution Protocol (ARP).

ARPANET (Advanced Research Projects Agency Network) Acronym for the Department of Defense Advanced Research Projects Agency. A pioneering wide area network (WAN), ARPANET was designed to facilitate the exchange of information between universities and other research organizations. ARPANET, which became operational in the 1960s, is the network from which the Internet evolved.

ASCII (American Standard Code for Information Interchange) A coding scheme that assigns numeric values to letters, numbers, punctuation marks, and certain other characters. By standardizing the values used for these characters, ASCII enables computers and computer programs to exchange information.

Asymmetric Digital Subscriber Line (ADSL) A recent modem technology that converts existing twisted-pair telephone lines into access paths for multimedia and high-speed data communications. These new connections can transmit more than 8 Mbps to the subscriber and up to 1 Mbps from the subscriber. ADSL is recognized as a physical layer transmission protocol for unshielded twisted-pair media.

asynchronous transfer mode (ATM) An advanced implementation of packet switching that provides high-speed data transmission rates to send fixed-size cells over broadband LANs or WANs. Cells are 53 bytes; 48 bytes of data with five additional bytes of address. ATM accommodates voice, data, fax, real-time video, CD-quality audio, imaging, and multimegabit data transmission. ATM uses switches as multiplexers to permit several computers to put data on a network simultaneously. Most commercial ATM boards transmit data at about 155 Mbps, but theoretically a rate of 1.2 gigabits per second is possible.

asynchronous transmission A form of data transmission in which information is sent one character at a time, with variable time intervals between characters. Asynchronous transmission does not rely on a shared timer that would allow the sending and receiving units to separate characters by specific time periods. Therefore, each transmitted character consists of a number of data bits (which compose the character itself) preceded by a start bit and ending in an optional parity bit followed by a 1-, 1.5-, or 2-stop bit.

ATM See asynchronous transfer mode (ATM).

attachment unit interface (AUI) The connector used with standard Ethernet that often includes a cable running off the main, or backbone, coaxial cable. Also known as a DIX connector.

attenuation The weakening or degrading (distorting) of a transmitted signal as it travels farther from its point of origin. This could be a digital signal on a cable or the reduction in amplitude of an electrical signal, without the appreciable modification of the waveform. Attenuation is usually measured in decibels. Attenuation of a signal transmitted over a long cable is corrected by a repeater, which amplifies and cleans up an incoming signal before sending it farther along the cable.

auditing A process that tracks network activities by user accounts and a routine element of network security. Auditing can produce records of list users that have accessed—or attempted to access—specific resources, help administrators identify unauthorized activity, and track activities such as logon attempts, connection and disconnection from designated resources, changes made to files and directories, server events and modifications, password changes, and logon parameter changes.

AUI See attachment unit interface (AUI).

authentication Verification based on user name, passwords, and time and account restrictions.

AWG (American Wire Gauge) A standard that determines wire diameter. The diameter varies inversely to the gauge number.

B

backbone The main cable, also known as the trunk segment, from which transceiver cables connect to computers, repeaters, and bridges.

back end In a client/server application, the part of the program that runs on the server.

backup A duplicate copy of a program, a disk, or data, made to secure valuable files from loss.

backup domain controller (BDC) In a Windows NT Server domain, a computer that receives a copy of the domain's security policy and domain database and authenticates network logons. It provides a backup if the primary domain controller (PDC) becomes unavailable. A domain is not required to have a BDC, but it is recommended to have a BDC to back up the PDC. See also domain, domain controller, primary domain controller.

bandwidth In communications, the difference between the highest and lowest frequencies in a given range. For example, a telephone accommodates a bandwidth of 3000 Hz, or the difference between the lowest (300 Hz) and highest (3300 Hz) frequencies it can carry. In computer networks, greater bandwidth indicates faster or greater data-transfer capability.

barrel connector A component that can connect two pieces of cable to make a longer piece of cable.

baseband A system used to transmit the encoded signals over cable. Baseband uses digital signaling over a single frequency. Signals flow in the form of discrete pulses of electricity or light. With baseband transmission, the entire communication-channel capacity is used to transmit a single data signal.

base I/O port Specifies a channel through which information is transferred between a computer's hardware, such as the network interface card (NIC), and its CPU.

base memory address Defines the address of the location in a computer's memory (RAM) that is used by the NIC. This setting is sometimes called the RAM start address.

baud A measure of data-transmission speed named after the French engineer and telegrapher Jean-Maurice-Emile Baudot. It is a measure of the speed of oscillation of the sound wave on which a bit of data is carried over telephone lines. Because baud was originally used to measure the transmission speed of telegraph equipment, the term sometimes refers to the data-transmission speed of a modem. However, current modems can send at a speed higher than one bit per oscillation, so baud is being replaced by the more accurate bps (bits per second) as a measure of modem speed.

baud rate Refers to the speed at which a modem can transmit data. Often confused with bps (the number of bits per second transmitted), baud rate actually measures the number of events, or signal changes, that occur in one second. Because one event can actually encode more than one bit in high-speed digital communication, baud rate and bps are not always synonymous, and the latter is the more accurate term to apply to modems. For example, the 9600-baud modem that encodes four-bits per event actually operates at 2400 baud, but transmits at 9600 bps (2400 events times 4-bits per event), and thus should be called a 9600-bps modem.

BBS (bulletin board system) See bulletin board system (BBS).

BDC See Backup Domain Controller (BDC).

beaconing The process of signaling computers on a ring system that token passing has been interrupted by a serious error. All computers in an FDDI or Token Ring network are responsible for monitoring the token-passing process. To isolate serious failures in the ring, FDDI and Token Ring use beaconing in which a computer that detects a fault sends a signal, called a beacon, onto the network. The computer continues to send the beacon until it notices a beacon from its upstream neighbor. This process continues until the only computer sending a beacon is the one directly downstream of the failure. When the beaconing computer finally receives its own beacon, it assumes the problem has been fixed and regenerates a token.

bind To associate two pieces of information with one another.

binding A process that establishes the communication channel between a protocol driver and a network interface card driver.

BISDN See broadband ISDN (BISDN).

bisync (binary synchronous communications protocol) A communications protocol developed by IBM. Bisync transmissions are encoded in either ASCII or EBCDIC. Messages can be of any length and are sent in units called frames, optionally preceded by a message header. Because bisync uses synchronous transmission, in which message elements are separated by a specific time interval, each frame is preceded and followed by special characters that enable the sending and receiving machines to synchronize their clocks.

bit Short for binary digit: either 1 or 0 in the binary number system. In processing and storage, a bit is the smallest unit of information handled by a computer. It is represented physically by an element such as a single pulse sent through a circuit or small spot on a magnetic disk capable of storing either a 1 or 0. Eight bits make a byte.

bits per second (bps) A measure of the speed at which a device can transfer data. See also baud rate.

bit time The time it takes for each station to receive and store a bit.

BNC components A family of components that include the BNC cable connector, BNC T connector, BNC barrel connector, and the BNC terminator. The origin of the acronym "BNC" is unclear; names ascribed to these letters range from "British Naval Connector" to "Bayonet Neill-Councelman."

BNC cable connector A connector for coaxial cable that locks when one connector is inserted into another and is rotated 90 degrees.

Boot-sector virus A type of virus that resides in the first sector of a floppy-disk or hard drive. When the computer is booted, the virus executes. In this common method of transmitting viruses from one floppy disk to another, the virus replicates itself onto the new drive each time a new disk is inserted and accessed.

bottleneck A device or program that significantly degrades network performance. Poor network performance results when a device uses noticeably more CPU time, consumes too much of a resource, or lacks the capacity to handle the load. Potential bottlenecks can be found in the CPU, memory, NIC, and other components.

bounce See signal bounce.

bps See bits per second (bps).

bridge A device used to join two LANs. It allows stations on either network to access resources on the other. Bridges can be used to increase the length or number of nodes for a network. The bridge makes connections at the data-link layer of the OSI reference model.

bridged network A network that is connected by bridges.

broadband Integrated Services Digital Network (BISDN) A consultative committee for the CCITT that recommends definitions for voice, data, and video in the megabit-gigabit range. BISDN is also a single ISDN network that can handle voice, data, and video services. BISDN works with an optical cable transport network called Synchronous Optical Network (SONET) and an asynchronous transfer mode (ATM) switching service. SMDS (Switched Multimegabit Data Services) is a BISDN service that offers high bandwidth to WANs.

broadband network A type of LAN on which transmissions travel as analog (radio-frequency) signals over separate inbound and outbound channels. Devices on a broadband network are connected by coaxial or fiber-optic cable, and signals flow across the physical medium in the form of electromagnetic or optical waves. A broadband system uses a large portion of the electromagnetic spectrum with a range of frequencies from 50 Mbps to 600 Mbps. These networks can simultaneously accommodate television, voice, data, and other services over multiple transmission channels.

broadcast A transmission sent simultaneously to more than one recipient. In communications and on networks, a broadcast message is one distributed to all stations or computers on the network.

broadcast storm An event that occurs when there are so many broadcast messages on the network that they approach or surpass the capacity of the network bandwidth. This can happen when one computer on the network transmits a flood of frames saturating the network with traffic so it can no longer carry messages from any other computer. Such a broadcast storm can shut down a network.

brouter A network component that combines the best qualities of a bridge and a router. A brouter can act as a router for one protocol and as a bridge for all the others. Brouters can route selected routable protocols, bridge nonroutable protocols, and deliver more cost-effective and manageable internetworking than separate bridges and routers. A brouter is a good choice in an environment that mixes several homogeneous LAN segments with two different segments.

buffer A reserved portion of RAM in which data is held temporarily, pending an opportunity to complete its transfer to or from a storage device or another location in memory.

built-in groups One of four kinds of group accounts used by Microsoft Windows NT and Windows NT Server . Built-in groups, as the name implies, are included with the network operating system. Built-in groups have been granted useful collections of rights and built-in abilities. In most cases, a built-in group provides all the capabilities needed by a particular user. For example, if a domain user account belongs to the built-in Administrators group, logging on with that account gives a user administrative capabilities over the domain and the servers in the domain. See also user account.

bulletin board system (BBS) A computer system equipped with one or more modems or other means of network access that serves as an information and message-passing center for remote users. Many software and hardware companies run proprietary BBSs for customers that include sales information, technical support, and software upgrades and patches.

bus Parallel wires or cabling that connect components in a computer.

bus topology A topology that connects each computer, or station, to a single cable. At each end of the cable is a terminating resistor, or terminator. A transmission is passed back and forth along the cable, past the stations and between the two terminators, carrying a message from one end of the network to the other. As the message passes each station, the station checks the message's destination address. If the address in the message matches the station's address, the station receives the message. If the addresses do not match, the bus carries the message to the next station, and so on.

byte A unit of information consisting of 8 bits. In computer processing or storage, a byte is equivalent to a single character, such as a letter, numeral, or punctuation mark. Because a byte represents only a small amount of information, amounts of computer memory are usually given in kilobytes (1024 bytes or 2 raised to the 10th power), megabytes (1,048,576 bytes or 2 raised to the 20th power), gigabytes (1024 megabytes), terabytes (1024 gigabytes), petabytes (1024 terabytes), or exabytes (1024 petabytes).

C

cable categories The three major groups of cabling that connect the majority of networks: coaxial, twisted-pair (unshielded twisted-pair and shielded twisted-pair) and fiber-optic cabling.

cable tester See advanced cable testers.

cache A special memory subsystem or part of RAM in which frequently used data values are duplicated for quick access. A memory cache stores the contents of frequently accessed RAM locations and the addresses where these data items are stored. When the processor references an address in memory, the cache checks to see whether it holds that address. If it does hold the address, the data is returned to the processor; if it does not, a regular memory access occurs. A cache is useful when RAM accesses are slow as compared to the microprocessor speed.

carrier-sense multiple access with collision avoidance (CSMA/CA) access method An access method by which each computer signals its intent to transmit before it actually transmits data, thus avoiding possible transmission collisions. See also access method.

carrier-sense multiple access with collision detection (CSMA/CD) access method An access method generally used with bus topologies. Using CSMA/CD, a station "listens" to the physical medium to determine whether another station is currently transmitting a data frame. If no other station is transmitting, the station sends its data. A station "listens" to the medium by testing the medium for the presence of a carrier, a specific level of voltage or light, thus the term carrier-sense. The multiple access indicates that there are multiple stations attempting to access or put data on the cable at the same time. The collision detection indicates that the stations are also listening for collisions. If two stations attempt to transmit at the same time and a collision occurs, the stations must wait a random period of time before attempting to transmit.

CCEP (Commercial COMSEC Endorsement Program) See Commercial COMSEC Endorsement Program (CCEP).

CCITT (Comité Consultatif Internationale de Télégraphie et Téléphonie) An organization based in Geneva, Switzerland, and established as part of the United Nations International Telecommunications Union (ITU). The CCITT recommends use of communications standards that are recognized throughout the world. Protocols established by the CCITT are applied to modems, networks, and facsimile transmission.

Cellular Digital Packet Data (CDPD) A communication standard that uses very fast technology, similar to that of cellular telephones, to offer computer data transmissions over existing analog voice networks between voice calls, when the system is not occupied with voice communication.

central file server A network in which specific computers take on the role of server with other computers on the network sharing the resources. See also client/server.

central processing unit (CPU) The computational and control unit of a computer; the device that interprets and carries out instructions. Single-chip CPUs, called microprocessors, made personal computers possible. Examples include the 80286, 80386, 80486, and Pentium processors.

cladding The concentric layer of that which surrounds the extremely thin cylindrical glass core in fiber-optic cable.

client A computer that accesses shared network resources provided by another computer, called a server.

client/server A network architecture designed around the concept of distributed processing in which a task is divided between a back end (server), which stores and distributes data, and a front end (client) which requests specific data from the server. See also central file server.

coaxial cable (coax) A conductive center wire surrounded by an insulating layer, a layer of wire mesh (shielding), and a nonconductive outer layer. Coaxial cable is resistant to interference and signal weakening that other cabling, such as unshielded twisted-pair cable, can experience.

codec (compression/decompression) Compression/decompression technology for digital video and stereo audio.

Commercial COMSEC Endorsement Program (CCEP) A data encryption standard introduced by the National Security Agency. Vendors who have the proper security clearance can join CCEP and be authorized to incorporate classified algorithms into communications systems. See also encryption.

companion virus A companion virus uses the name of a real program, but has a different file extension from that of the program, itself. The virus is activated when its companion program is opened. The companion virus uses a .com file extension, which overrides the .exe file extension and activates the virus.

concentrator Network physical layer device that serves as a central connection for other network devices. See also hub.

contention On a network, competition among stations for the opportunity to use a communications line or network resource . Two or more computers attempt to transmit over the same cable at the same time, thus causing a collision on the cable. Such a system needs regulation to eliminate data collisions on the cable which can destroy data and bring network traffic to a halt. See also carrier-sense multiple access with collision detection (CSMA/CD).

core In coaxial cable, the innermost part of the cable that carries the electronic signals which make up the data. It can be solid (usually copper)or stranded. In fiber-optic cable, digital data signals travel through an extremely thin cylindrical glass core surrounded by cladding.

CPU See central processing unit (CPU).

CRC See cyclical redundancy check (CRC).

crossover cable Used to connect two computers directly with a single patch cable, so that the send wire from one computer is connected to the receive port on the other computer. Crossover cables are useful in troubleshooting network connection problems.

crosstalk Signal overflow from an adjacent wire. When a second faint telephone conversation is heard in the background while one is making a phone call, crosstalk is occurring.

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

cyclical redundancy check (CRC) A form of error checking in transmitting data. The sending packet includes a number produced by a mathematical calculation made at the transmission source. When the packet arrives at its destination, the calculation is redone. If the two figures are the same, this indicates that the data in the packet has remained stable. If the calculation at the destination differs from the calculation at the source, this indicates that the data has changed during the transmission. In that case, the CRC routine signals the source computer to retransmit the data.

D

daisy chain A set of devices, such as Small Computer System Interface (SCSI) and Universal Serial Bus (USB), that are connected in a series. When devices are daisy-chained to a microcomputer, the first device is connected to the computer, the second device is connected to the first, and so on down the line. Signals are passed through the chain from one device to the next. See also Small Computer System Interface (SCSI) and Universal Serial Bus (USB).

database management system (DBMS) A layer of software between the physical database and the user. The DBMS manages all requests for database action from the user, including keeping track of the physical details of file locations and formats, indexing schemes, and so on. In addition, a DBMS permits centralized control of security and data integrity requirements.

Data Communications Equipment (DCE) One of two types of hardware connected by an RS-232 serial connection, the other being a DTE (data terminal equipment) device. A DCE device takes input from a DTE device and often acts as an intermediary device, transforming the input signal in some way before sending it to the actual recipient. For example, an external modem is a DCE device that accepts data from a microcomputer (DTE), modulates it, then sends the data along a telephone connection. In communications, an RS-232 DCE device receives data over line 2 and transmits over line 3. In contrast, a DTE device receives over line 3 and transmits over line 2. See also Data Terminal Equipment (DTE).

data encryption See encryption.

data encryption standard (DES) A commonly used, highly sophisticated algorithm developed by the U.S. National Bureau of Standards for encrypting and decoding data. See also encryption.

data frame Logical, structured packages in which data can be placed. Data that is being transmitted is segmented into small units and combined with control information such as message start and message end indicators. Each package of information is transmitted as a single unit, called a frame. The data-link layer packages raw bits from the physical layer into data frames. The exact format of the frame used by the network depends on the topology. See also frame.

data link layer The second layer in the OSI reference model. This layer packages raw bits from the physical layer into data frames. See also Open Systems Interconnection (OSI) reference model.

data stream An undifferentiated, byte-by-byte flow of data.

Data Terminal Equipment (DTE) According to the RS-232 hardware standard, a DTE is any device, such as a microcomputer or a terminal, that has the ability to transmit information in digital form over a cable or a communications line. A DTE is one of two types of hardware connected by an RS-232 serial connection, the other being a DCE (data communications equipment) device, such as a modem, that normally connects the DTE to the communication line itself. In communication, an RS-232 DTE device transmits data over line 2 and receives it over line 3. A DCE receives over line 2 and transmits over line 3. See also Data Communications Equipment (DCE).

DB connector A connector that facilitates parallel input and output. The initials DB stand for data bus. The numbers which follow DB indicate the number of wires within the connector. For example, a DB-15 connector has 15 pins and supports up to 15 lines, each of which can connect to a pin on the connector; a DB-25 connector has 25 of each.

DBMS See database management system (DBMS).

DCE See Data Communications Equipment (DCE).

DECnet Digital Equipment Corporation hardware and software products that implement the Digital Network Architecture (DNA). DECnet defines communication networks over Ethernet LANs, Fiber Distributed Data Interface metropolitan area networks (FDDI MANs), and WANs that use private or public data transmission facilities. It can use TCP/IP and OSI protocols as well as Digital's DECnet protocols. See also Fiber Distributed Data Interface, metropolitan area network.

dedicated server A computer on a network that functions only as a server and is not also used as a client. See also server, server based network.

DES See data encryption standard (DES).

device A generic term for a computer subsystem. Printers, serial ports, and disk drives are referred to as devices.

DHCP See Dynamic Host Configuration Protocol (DHCP).

digital A system that encodes information in a binary state, such as 0 and 1. Computers use digital encoding to process data. A digital signal is a discrete binary state, either on or off. See also analog.

digital line A communication line that carries information only in binary-encoded (digital) form. To minimize distortion and noise interference, a digital line uses repeaters to regenerate the signal periodically during transmission. See also analog line.

digital video disc (DVD) An optical storage medium with higher capacity and bandwidth than a compact disc. A DVD can hold a full-length film with up to 133 minutes of high-quality video, in MPEG-2 format, and audio. Also known as digital versatile disc.

digital voltmeter (DVM) A basic, all-purpose electronic measuring tool. In addition to indicating the amount of voltage passing through resistance, in network cable testing, voltmeters measure continuity to determine if a cable is able to carry.

DIP (dual inline package) switch One or more small rocker or sliding type switches that can be set to one of two states—closed or open—to control options on a circuit board.

direct memory access (DMA) Memory access that does not involve the microprocessor, frequently employed for data transfer directly between memory and an "intelligent" peripheral device such as a disk drive.

direct memory access (DMA) channel A channel for direct memory access that does not involve the microprocessor, providing data transfer directly between memory and a disk drive.

disk duplexing See disk mirroring and fault tolerance.

disk duplicating See disk mirroring.

diskless computers Computers that have neither a floppy disk nor a hard disk. Diskless computers depend on special ROM in order to provide users with an interface through which they can log on to the network.

disk mirroring A technique, also known as disk duplicating, in which all or part of a hard disk is duplicated onto one or more hard disks, each of which ideally is attached to its own controller. With disk mirroring, any change made to the original disk is simultaneously made to the other disk(s). Disk mirroring is used in situations in which a backup copy of current data must be maintained at all times. See also disk striping, fault tolerance.

disk striping Divides data into 64K blocks and spreads it equally in a fixed rate and order among all disks in an array. However, disk striping does not provide any fault tolerance because there is no data redundancy. If any partition in the set fails, all data is lost. See also disk mirroring, fault tolerance.

DIX (Digital, Intel, Xerox) connector The connector used with standard Ethernet that often includes a cable running off the main, or backbone, coaxial cable. This is also known as an AUI connector. See also attachment unit interface (AUI).

DMA See direct memory access (DMA).

DMA channel See direct memory access (DMA) channel.

DNS (Domain Name System) See Domain Name System (DNS).

domain For Microsoft networking, a collection of computers and users that share a common database and security policy that is stored on a Windows NT Server domain controller. Each domain has a unique name. See also workgroup.

domain controller For Microsoft networking, the Windows NT Server-based computer that authenticates domain logons and maintains the security policy and the master database for a domain. See also Backup Domain Controller (BDC), Primary Domain Controller (PDC).

Domain Name System (DNS) A general-purpose distributed, replicated, data-query service used primarily on the Internet for translating hostnames into Internet addresses.

downtime The amount of time a computer system or associated hardware remains nonfunctioning. Although downtime can occur because hardware fails unexpectedly, it can also be a scheduled event, such as when a network is shut down to allow time for maintaining the system, changing hardware, or archiving files.

driver A software component that permits a computer system to communicate with a device. For example, a printer driver is a device driver that translates computer data into a form understood by the target printer. In most cases, the driver also manipulates the hardware in order to transmit the data to the device.

DTE See Data Terminal Equipment (DTE).

dual shielded Cable that contains one layer of foil and insulation and one layer of braided metal shielding.

dumb terminal A device used for obtaining or entering data on a network that does not contain any "intelligence" or processing power provided by a CPU.

duplex transmission Also called full-duplex transmission. Communication that takes place simultaneously, in both directions, between the sender and the receiver. Alternative methods of transmission are simplex, which is one-way only, and half-duplex, which is two-way communication that occurs in only one direction at a time.

DVD (digital video disc, also known as digital versatile disc) See digital video disc (DVD).

Dynamic Host Configuration Protocol (DHCP) A protocol for automatic TCP/IP configuration that provides static and dynamic address allocation and management. See also Transport Control Protocol/Internet Protocol (TCP/IP).

E

EBCDIC See Extended Binary Coded Decimal Interchange Code (EBCDIC).

EISA See Extended Industry Standard Architecture (EISA).

encryption The process of making information indecipherable to protect it from unauthorized viewing or use, especially during transmission or when the data is stored on a transportable magnetic medium. A key is required to decode the information. See also CCEP, data encryption standard (DES).

Enhanced Small Device Interface (ESDI) A standard that can be used with high-capacity hard disks and tape drives to enable high-speed communications with a computer. ESDI drivers typically transfer data at about 10 Mbps.

ESDI See Enhanced Small Device Interface (ESDI).

Ethernet A LAN developed by Xerox in 1976. Ethernet became a widely implemented network from which the IEEE 802.3 standard for contention networks was developed. It uses a bus topology and the original Ethernet relies on CSMA/CD to regulate traffic on the main communication line.

EtherTalk Allows the AppleTalk network protocols to run on Ethernet coaxial cable. The EtherTalk card allows a Macintosh computer to connect to an 802.3 Ethernet network. See also AppleTalk.

event An action or occurrence to which a program might respond. Examples of events are mouse clicks, key presses, and mouse movements. Also, any significant occurrence in the system or in a program that requires users to be notified or an entry to be added to a log.

exabyte See byte.

Extended Binary Coded Decimal Interchange code (EBCDIC) A coding scheme developed by IBM for use with IBM mainframe and personal computers as a standard method of assigning binary (numeric) values to alphabetic, numeric, punctuation, and transmission-control characters.

Extended Industry Standard Architecture (EISA) A 32-bit bus design for x86-based computers introduced in 1988. EISA was specified by an industry consortium of nine computer-industry companies (AST Research, Compaq, Epson, Hewlett-Packard, NEC, Olivetti, Tandy, Wyse, and Zenith). An EISA device uses cards that are upwardly compatible from ISA. See also Industry Standard Architecture (ISA).

F

fast Ethernet Also called 100BaseX Ethernet. An extension to the existing Ethernet standard, it runs on UTP Category 5 data-grade cable and uses CSMA/CD in a star-wired bus topology, similar to 10BaseT in which all cables are attached to a hub.

fault tolerance The ability of a computer or an operating system to respond to an event such as a power outage or a hardware failure in such a way that no data is lost and any work in progress is not corrupted.

Fiber Distributed Data Interface (FDDI) A standard developed by the ANSI for high-speed fiber-optic local area networks. FDDI provides specifications for transmission rates of 100 Mbps on networks based on the Token Ring standard.

fiber-optic cable Cable that uses optical fibers to carry digital data signals in the form of modulated pulses of light.

file infector A type of virus that attaches itself to a file or program and activates any time the file is used. Many subcategories of file infectors exist. See also companion virus, macro virus, polymorphic virus, stealth virus.

File Transfer Protocol (FTP) A process that provides file transfers between local and remote computers. FTP supports several commands that allow bidirectional transfer of binary and ASCII files between computers. The FTP client is installed with the TCP/IP connectivity utilities. See also ASCII, Transport Control Protocol/Internet Protocol (TCP/IP).

firewall A security system, usually a combination of hardware and software, intended to protect a network against external threats coming from another network, including the Internet. Firewalls prevent an organization's networked computers from communicating directly with computers that are external to the network, and vice versa. Instead, all incoming and outgoing communication is routed through a proxy server outside the organization's network. Firewalls also audit network activity, recording the volume of traffic and information about unauthorized attempts to gain access. See also proxy server.

firmware Software routines stored in ROM. Unlike RAM, ROM stays intact even in the absence of electrical power. Startup routines and low-level I/O instructions are stored in firmware.

flow control Regulating the flow of data through routers to ensure that no segment becomes overloaded with transmissions.

frame A package of information transmitted on a network as a single unit. Frame is a term most often used with Ethernet networks. A frame is similar to the packet used in other networks. See also data frame, packet.

frame preamble Header information, added to the beginning of a data frame in the physical layer of the OSI reference model.

frame relay An advanced fast-packet variable-length, digital, packet-switching technology. It is a point-to-point system that uses a private virtual circuit (PVC) to transmit variable length frames at the data-link layer of the OSI reference model. Frame relay networks can also provide subscribers with bandwidth as needed that allows users to make nearly any type of transmission.

front end In a client/server application, front end refers to the part of the program carried out on the client computer.

FTP See File Transfer Protocol (FTP).

full-duplex transmission Also called duplex transmission. Communication that takes place simultaneously, in both directions. See also duplex transmission.

G

gateway A device used to connect networks using different protocols so that information can be passed from one system to the other. Gateways functions at the network layer of the OSI reference model.

Gb See gigabit.

GB See gigabyte.

gigabit 1,073,741,824 bits. Also referred to as 1 billion bits.

gigabyte Commonly, a thousand megabytes. However, the precise meaning often varies with the context. A gigabyte is 1 billion bytes. In the context of computing, bytes are often expressed in multiples of powers of two. Therefore, a gigabyte can also be either 1000 megabytes or 1024 megabytes, where a megabyte is considered to be 1,048,576 bytes (2 raised to the 20th power).

global group One of four kinds of group accounts used by Microsoft Windows NT and Windows NT Server. Used across an entire domain, global groups are created on a primary domain controller (PDC) in the domain in which the user accounts reside. Global groups can contain only user accounts from the domain in which the global group is created. Members of global groups obtain resource permissions when the global group is added to a local group. See also group.

group In networking, an account containing other accounts that are called members. The permissions and rights granted to a group are also provided to its members; thus, groups offer a convenient way to grant common capabilities to collections of user accounts. For Windows NT, groups are managed with User Manager. For Windows NT Server, groups are managed with User Manager for Domains.

H

half-duplex transmission Two-way communication occurring in only one direction at a time.

handshaking A term sometimes applied to modem-to-modem communication. Refers to the process by which information is transmitted between the sending and receiving devices to maintain and coordinate data flow between them. Proper handshaking ensures that the receiving device will be ready to accept data before the sending device transmits.

hard disk One or more inflexible platters coated with material that allows the magnetic recording of computer data. A typical hard disk rotates at up to 7200 revolutions per minute (RPM), and the read/write heads ride over the surface of the disk on a cushion of air 10 to 25 millionths of an inch deep. A hard disk is sealed to prevent contaminants from interfering with the close head-to-disk tolerances. Hard disks provide faster access to data than floppy disks and are capable of storing much more information. Because platters are rigid, they can be stacked so that one hard-disk drive can access more than one platter. Most hard disks have between two and eight platters.

hardware The physical components of a computer system, including any peripheral equipment such as printers, modems, and mouse devices.

hardware compatibility list (HCL) A list of computers and peripherals that have been tested and have passed compatibility testing with the product for which the HCL is being developed. For example, the Windows NT 3.51 HCL lists the products which have been tested and found to be compatible with Window NT 3.51.

hardware loopback A connector on a computer that is useful for troubleshooting hardware problems, allowing data to be transmitted to a line, then returned as received data. If the transmitted data does not return, the hardware loopback detects a hardware malfunction.

HCL See hardware compatibility list (HCL).

HDLC See High-Level Data Link Control (HDLC).

header In network data transmission, one of the three sections of a packet component. Includes an alert signal to indicate that the packet is being transmitted, the source address, the destination address, and clock information to synchronize transmission.

hermaphroditic connector A connector that is neither male nor female, such as IBM cable connectors, in which any two can be connected together, as opposed to BNC connectors that require both a male part and female part before a connection can be made.

hertz (Hz) The unit of frequency measurement. Frequency measures how often a periodic event occurs, such as the manner in which a wave's amplitude changes with time. One hertz equals one cycle per second. Frequency is often measured in kilohertz (KHz, 1000 Hz), megahertz (MHz), gigahertz (GHz, 1000 MHz), or terahertz (THz, 10,000 GHz).

High-Level Data Link Control (HDLC) HDLC is a widely accepted international protocol, developed by the International Organization for Standardization (ISO), that governs information transfer. HDLC is a bit-oriented, synchronous protocol that applies to the data-link (message packaging) layer of the OSI reference model. Under the HDLC protocol, data is transmitted in frames, each of which can contain a variable amount of data, but which must be organized in a particular way. See also data frame, frame.

hop In routing through a mesh environment, the transmission of a data packet through a router.

host See server.

hot fixing See sector sparing.

HTML See Hypertext Markup Language (HTML).

hub A connectivity component that provides a common connection among computers in a star-configured network. Active hubs require electrical power, but are able to regenerate and retransmit network data. Passive hubs simply organize the wiring.

hybrid hub An advanced hub that can accommodate several different types of cables.

hybrid network A network made up of mixed components.

Hypertext Markup Language (HTML) A language developed for writing pages for the World Wide Web. HTML allows text to include codes that define fonts, layout, embedded graphics, and hypertext links. Hypertext provides a method for presenting text, images, sound, and videos that are linked together in a nonsequential web of associations.

Hypertext Transport Protocol (HTTP) The method by which World Wide Web pages are transferred over the network.

I

IAB (Internet Architecture Board) See Internet Architecture Board (IAB).

IBM cabling system Used in a Token Ring environment. Introduced by IBM in 1984 to define cable connectors, face plates, distribution panels, and cable types. Many parameters are similar to non-IBM specifications. Uniquely shaped, the IBM connector is hermaphroditic. See also hermaphroditic connector.

ICMP (Internet Control Message Protocol) See Internet Control Message Protocol (ICMP).

IDE See Integrated Device Electronics (IDE).

IEEE See Institute of Electrical and Electronics Engineers, Inc. (IEEE).

IEEE Project 802 A networking model developed by the IEEE. Named for the year and month it began (February 1980), Project 802 defines LAN standards for the physical and data-link layers of the OSI reference model. Project 802 divides the data-link layer into two sublayers: Media Access Control (MAC) and Logical Link Control (LLC).

impedance Impedance had two aspects. The first is resistance, which impedes both direct and alternating current. Resistance is always greater than zero. The second is reactance, which impedes alternating current only. Reactance varies with frequency, and can be positive or negative.

Industry Standard Architecture (ISA) An unofficial designation for the bus design of the IBM PC/XT. It allows various adapters to be added to the system by inserting plug-in cards into expansion slots. Commonly, ISA refers to the expansion slots themselves; such slots are called 8-bit slots or 16-bit slots. See also Enhanced Industry Standard Architecture (EISA), Micro Channel Architecture.

infrared transmission Electromagnetic radiation with frequencies in the electromagnetic spectrum in the range just below that of visible red light. In network communications, infrared technology offers extremely high transmission rates and wide bandwidth in line-of-sight communications.

Institute of Electrical and Electronics Engineers (IEEE) An organization of engineering and electronics professionals; noted in networking for developing the IEEE 802.x standards for the physical and data-link layers of the OSI reference model, applied in a variety of network configurations.

Integrated Device Electronics (IDE) A type of disk-drive interface in which the controller electronics reside on the drive itself, eliminating the need for a separate network interface card. The IDE interface is compatible with the Western Digital ST-506 controller.

Integrated Services Digital Network (ISDN) A worldwide digital communication network that evolved from existing telephone services. The goal of the ISDN is to replace current telephone lines, which require digital-to-analog conversions, with completely digital switching and transmission facilities capable of carrying data ranging from voice to computer transmissions, music, and video. The ISDN is built on two main types of communications channels: B channels, that carry voice, data, or images at a rate of 64 Kbps (kilobits per second), and a D channel, that carries control information, signaling, and link management data at 16 Kbps. Standard ISDN Basic Rate desktop service is called 2B+D. Computers and other devices connect to ISDN lines through simple, standardized interfaces.

interfaces In the OSI reference model, each layer provides some service or action that prepares the data for delivery over the network to another computer. Interfaces are boundaries that separate the layers from each other.

intermediate systems Equipment that provides a network communication link, such as bridges, routers, and gateways.

International Organization for Standardization (ISO) An organization made up of standards-setting groups from various countries. For example, the United States member is the ANSI, the American National Standards Institute. The ISO works to establish global standards for communications and information exchange. Primary among its accomplishments is development of the widely accepted OSI reference model.

International Telecommunications Union (ITU) The organization responsible for setting the standards for international telecommunications.

Internet Architecture Board (IAB) A body that develops and maintains Internet architectural standards as part of the Internet Society (ISOC). It also serves to adjudicate disputes in the standards process.

Internet Control Message Protocol (ICMP) Used by IP and higher-level protocols to send and receive status reports about information being transmitted.

Internet Protocol (IP) The TCP/IP protocol for packet forwarding. See also Transport Control Protocol/Internet Protocol (TCP/IP).

Internetworking The intercommunication in a network that is made up of smaller networks.

Internetwork Packet Exchange/Sequenced Packet Exchange (IPX/SPX) A protocol stack that is used in Novell networks. IPX is the NetWare protocol for packet forwarding and routing. It is a relatively small and fast protocol on a LAN, is a derivative of Xerox Network System (XNS), and supports routing. SPX is a connection-oriented protocol used to guarantee the delivery of the data being sent. NWLink is the Microsoft implementation of the IPX/SPX protocol.

Interoperability The ability of components in one system to work with components in other systems.

interrupt request (IRQ) An electronic signal sent to a computer's CPU to indicate that an event has taken place which requires the processor's attention.

IP See Internet Protocol (IP). See also Transport Control Protocol/Internet Protocol (TCP/IP).

ipconfig A diagnostic command that displays all current TCP/IP network configuration values. It is of particular use on systems running DHCP because it allows users to determine which TCP/IP configuration values have been configured by the DHCP server. See also winipcfg.

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

IRQ See interrupt request (IRQ).

ISA See Industry Standard Architecture (ISA).

ISDN See Integrated Services Digital Network (ISDN).

ISO See International Organization for Standardization (ISO).

ITU (International Telecommunications Union) See International Telecommunications Union (ITU).

J

jitter Instability in a signal wave form over time that can be caused by signal interference or an unbalanced ring in FDDI or Token Ring environments.

jumper A small plastic-and-metal plug or wire for connecting different points in an electronic circuit. Jumpers are used to select a particular circuit or option from several possible configurations. Jumpers can be used on network interface cards to select the type of connection through which the card will transmit, either DIX or BNC.

K

KEVLAR A brand name of the DuPont Corporation for the fibers in the reinforcing layer of plastic that surrounds each glass strand of a fiber-optic connector. The name is sometimes used generically.

key 1. In database management, an identifier for a record or group of records in a data file. Most often, the key is defined as the contents of a single field, called the key field in some database management programs and the index field in others. Keys are maintained in tables and are indexed to speed record retrieval. 2. Keys also refer to code that deciphers encrypted data.

kilo (K) One thousand (1000) in the metric system. In computing terminology, because computing is based on powers of 2, kilo is most often used to mean 1024 (2 raised to the 10th power). To distinguish between the two contexts, a lower case k is often used to indicate 1000, an uppercase K for 1024. A kilobyte is 1024 bytes.

kilobit (Kbit) One thousand twenty-four bits. See also bit, kilo.

kilobyte (KB) One thousand twenty-four bytes. See also byte, kilo.

LAN See local area network (LAN).

LAN requester See requester (LAN requester).

laser transmission Wireless network that uses a laser beam to carry data between devices.

LAT See local area transport (LAT).

layering The coordination of various protocols in a specific architecture that allows the protocols to work together to ensure that the data is prepared, transferred, received, and acted upon as intended.

link The communication system that connects two LANs. Equipment that provides the link, including bridges, routers, and gateways.

local area network (LAN) Computers connected in a geographically confined network, such as in the same building, campus, or office park.

local area transport (LAT) A nonroutable protocol from Digital Equipment Corporation.

local group One of four kinds of group accounts used by Microsoft Windows NT and Windows NT Server. Implemented in each local computer's account database, local groups contain user accounts and other global groups that need to have access, rights, and permissions assigned to a resource on a local computer. Local groups cannot contain other local groups.

LocalTalk Cabling components used in an AppleTalk network, including include cables, connector modules, and cable extenders. These components are normally used in a bus or tree topology. A LocalTalk segment supports a maximum of 32 devices. Because of LocalTalk's limitations, clients often turn to vendors other than Apple for AppleTalk cabling. Farallon PhoneNet, for example, can accommodate 254 devices.

local user The user at the computer.

Logical Link Control (LLC) sublayer One of two sublayers created by the IEEE 802 project out of the data-link layer of the OSI reference model. The Logical Link Control (LLC) is the upper sublayer that manages data-link communication and defines the use of logical interface points, called service access points (SAPs).used by computers to transfer information from the LLC sublayer to the upper OSI layers. See also Media Access Control (MAC) sublayer, service access point (SAP).

lost token Refers to an error on a Token Ring network that causes an errant station to halt the token, leaving the ring without a token.

M

macro virus A file infector virus named because it is written as a macro for a specific application. Macro viruses are difficult to detect and becoming more common, often infecting widely used applications, such as word-processing programs. When an infected file is opened, the virus attaches itself to the application, then infects any files accessed by that application. See also file infector.

magneto-optical (MO) A plastic or glass disk, coated with a compound containing special properties, that is read by bouncing a low-intensity laser off the disk.

MAN (Metropolitan Area Network) See Metropolitan Area Network (MAN).

MAU See Multistation Access Unit (MSAU or MAU).

Mb (Mbit) See megabit.

MB See megabyte.

Mbps See millions of bits per second (Mbps).

media The vast majority of LANs today are connected by some sort of wire or cabling which acts as the LAN transmission medium carrying data between computers. The cabling is often referred to as the media.

Media Access Control (MAC) driver The device driver located at the Media Access Control sublayer of the OSI reference model. This driver is also known as the network interface card driver (NIC driver). It provides low-level access to NICs by providing data transmission support and some basic NIC management functions. These drivers also pass data from the physical layer to transport protocols at the network and transport layers.

Media Access Control (MAC) sublayer One of two sublayers created by the IEEE 802 project out of the data-link layer of the OSI reference model. The Media Access Control (MAC) sublayer communicates directly with the network interface card (NIC) and is responsible for delivering error-free data between two computers on the network. See also Logical Link Control (LLC) sublayer.

megabit (Mb) Usually, 1,048,576 bits; sometimes interpreted as 1 million bits. See also bit.

megabyte 1,048,576 bytes (2 raised to the 20th power). See also byte.

mesh network topology Connects remote sites over telecommunication links. Common in wide area networks (WANs), meshes use routers to search among multiple active paths (the mesh) and determine the best path for that particular moment.

Metropolitan Area Network (MAN) A data network designed for a town or city. In geographic breadth, MANs are larger than local area networks but smaller than wide area networks. MANs are usually characterized by very high-speed connections using fiber-optic cable or other digital media.

Micro Channel Architecture The design of the bus in IBM PS/2 computers (except Models 25 and 30). The Micro Channel is electrically and physically incompatible with the IBM PC/AT bus. Unlike the PC/AT bus, the Micro Channel functions as either a 16-bit or 32-bit bus. The Micro Channel also can be driven independently by multiple bus master processors. See also Enhanced Industry Standard Architecture (EISA), Industry Standard Architecture (ISA).

Microcom Network Protocol (MNP) The standard for asynchronous data-error control developed by Microcom Systems. The method works so well that other companies have adopted not only the initial version of the protocol, but later versions as well. Currently, several modem vendors incorporate MNP Classes 2, 3, 4, and 5.

Microsoft Technical Information Network (TechNet) Provides informational support for all aspects of networking, with an emphasis on Microsoft products.

millions of bits per second (Mbps) The unit of measure of supported transmission rates on the following physical media: coaxial cable, twisted-pair cable, and fiber-optic cable. See also bit.

MNP See Microcom Network Protocol (MNP).

MO (magneto-optical) See magneto-optical (MO).

mobile computing Incorporates wireless adapters using cellular telephone technology to connect portable computers with the cabled network.

modem A communication device that enables a computer to transmit information over a standard telephone line. Because a computer is digital, it works with discrete electrical signals representing binary 1 and binary 0. A telephone is analog and carries a signal that can have many variations. Modems are needed to convert digital signals to analog and back. When transmitting, modems impose (modulate) a computer's digital signals onto a continuous carrier frequency on the telephone line. When receiving, modems sift out (demodulate) the information from the carrier and transfer it in digital form to the computer.

MSAU See Multistation Access Unit (MAU).

multiplexer (mux) A device used to divide a transmission facility into two or more channels. It can be a program stored in a computer. Also, a device for connecting a number of communication lines to a computer.

Multistation Access Unit (MSAU or MAU) The name for a Token Ring wiring concentrator. Also referred to as a hub.

multitasking A mode of operation offered by an operating system in which a computer works on more than one task at a time. There are two primary types of multitasking: preemptive and nonpreemptive. In preemptive multitasking, the operating system can take control of the processor without the task's cooperation. In nonpreemptive multitasking, the processor is never taken from a task. The task itself decides when to give up the processor.

A true multitasking operating system can run as many tasks as it has processors. When there are more tasks than processors, the computer must "time slice" so that the available processors devote a certain amount of time to one task and then move on to the next task, alternating between tasks until all the tasks are completed.

mux See multiplexer (mux).

N

name binding protocol (NBP) An Apple protocol responsible for keeping track of entities on the network and matching names with Internet addresses. It works at the transport layer of the OSI reference model.

narrowband (single-frequency) transmission High frequency radio transmission similar to broadcasting. The user tunes both the transmitter and the receiver to a certain frequency to send and receive data.

nbtstat A diagnostic command that displays protocol statistics and current TCP/IP connections using NBT (NetBIOS over TCP/IP). This command is available only if the TCP/IP protocol has been installed. See also netstat.

NDIS See Network Device Interface Specification (NDIS).

NetBEUI (NetBIOS extended user interface) A protocol supplied with all Microsoft network products. NetBEUI advantages include small stack size (important for MS-DOS-based computers), speed of data transfer on the network medium, and compatibility with all Microsoft-based networks. The major drawback of NetBEUI is that it is a LAN transport protocol and therefore does not support routing. It is also limited to Microsoft-based networks.

NetBIOS (network basic input/output system) An application programming interface (API) that can be used by application programs on a LAN consisting of IBM-compatible microcomputers running MS-DOS, OS/2, or some version of UNIX. Primarily of interest to programmers, NetBIOS provides application programs with a uniform set of commands for requesting the lower-level network services required to conduct sessions between nodes on a network, and to transmit information between them.

netstat A diagnostic command that displays protocol statistics and current TCP/IP network connections. This command is available only if the TCP/IP protocol has been installed. See also nbtstat.

NetWare Core Protocol (NCP) Defines the connection control and service request encoding that make it possible for clients and servers to interact. This is the protocol that provides transport and session services. NetWare security is also provided within this protocol.

network In the context of computers, a system in which a number of independent computers are linked together to share data and peripherals, such as hard disks and printers

network adapter card See network interface card (NIC).

network analyzers Network troubleshooting tools, sometimes called protocol analyzers. They perform a number of functions in real-time network traffic analysis, and carry out packet capture, decoding, and transmission. They can also generate statistics based on the network traffic to help create a picture of the network's cabling, software, file server, clients, and NICs. Most analyzers have a built-in TDR. See also time-domain reflectometer (TDR).

Network Device Interface Specification (NDIS) A standard that defines an interface for communication between the Media Access Control (MAC) sublayer and protocol drivers. NDIS allows for a flexible environment of data exchange. It defines the software interface, called the NDIS interface, which is used by protocol drivers to communicate with the network interface card. The advantage of NDIS is that it offers protocol multiplexing so that multiple protocol stacks can be used at the same time. See also Open Data-Link Interface (ODI).

network interface card (NIC) An expansion card installed in each computer and server on the network, the NIC acts as the physical interface or connection between the computer and the network cable.

network layer The third layer in the OSI reference model. This layer is responsible for addressing messages and translating logical addresses and names into physical addresses. This layer also determines the route from the source to the destination computer. It determines which path the data should take based on network conditions, priority of service, and other factors. It also manages traffic problems such as switching, routing, and controlling the congestion of data packets on the network. See also Open Systems Interconnection (OSI) reference model.

network monitors Network monitors track all or a selected part of network traffic. They examine frame-level packets and gather information about packet types, errors, and packet traffic to and from each computer.

NIC (Network Interface Card) See Network Interface Card (NIC).

Network News Transfer Protocol (NNTP) A protocol defined in RFC 977. It is a de facto protocol standard on the Internet used for the distribution, inquiry, retrieval, and posting of Usenet news articles over the Internet.

NNTP (Network News Transfer Protocol) See Network News Transfer Protocol (NNTP)

node On a LAN, a device that is connected to the network and is capable of communicating with other network devices. For example, clients, servers, and repeaters are called nodes.

noise Random electrical signals that can get onto the cable and degrade or distort the data. Noise is generated by power lines, elevators, air conditioners, or any device with an electric motor, relays, and radio transmitters. See also shielding.

nonpreemptive multitasking A form of multitasking in which the processor is never taken from a task. The task itself decides when to give up the processor. Programs written for nonpreemptive multitasking systems must include provisions for yielding control of the processor. No other program can run until the non-preemptive program gives up control of the processor. See also multitasking.

Novell's NetWare One of the leading network architectures.

O

ODI See Open Data-Link Interface (ODI).

ohm The unit of measure for electrical resistance. A resistance of 1 ohm will pass 1 ampere of current when a voltage of 1 volt is applied. A 100-watt incandescent bulb has a resistance of approximately 130 ohms.

Open Data-Link Interface (ODI) A specification defined by Novell and Apple to simplify driver development and to provide support for multiple protocols on a single network interface card. Similar to NDIS in many respects, ODI allows Novell NetWare drivers to be written without concern for the protocol that will be used on top of them.

Open Shortest Path First (OSPF) A routing protocol for IP networks, such as the Internet, that allows a router to calculate the shortest path to each node for sending messages.

Open Systems Interconnection (OSI) reference model A seven-layer architecture that standardizes levels of service and types of interaction for computers exchanging information through a network. It is used to describe the flow of data between the physical connection to the network and the end-user application. This model is the best known and most widely used model for describing networking environments.

OSI layer	Focus
7. application layer	Program-to-program transfer of information
6. presentation layer	Text formatting and display code conversion
5. session layer	Establishing, maintaining, and coordinating communication
4. transport layer	Accurate delivery, service quality
3. network layer	Transport routes, message handling and transfer
2. data-link layer	Coding, addressing, and transmitting information
1. physical layer	Hardware connections

optical drive A drive that accommodates optical disks.

optical fiber Medium that carries digital data signals in the form of modulated pulses of light. An optical fiber consists of an extremely thin cylinder of glass, called the core, surrounded by a concentric layer of glass, known as the cladding.

oscilloscope An electronic instrument that measures the amount of signal voltage per unit of time and displays the results on a monitor.

OSI See Open Systems Interconnection (OSI) reference model.

OSPF See Open Shortest Path First (OSPF).

P

packet A unit of information transmitted as a whole from one device to another on a network. In packet-switching networks, a packet is defined more specifically as a transmission unit of fixed maximum size that consists of binary digits representing data, a header containing an identification number, source, and destination addresses, and sometimes error-control data. See also frame.

packet assembler/disassembler (PAD) A device that breaks large chunks of data into packets, usually for transmission over an X.25 network, and reassembles them at the other end. See also packet switching.

Packet Internet Groper (ping) A simple utility that tests if a network connection is complete, from the server to the workstation, by sending a message to the remote computer. If the remote computer receives the message, it responds with a reply message. The reply consists of the remote workstation's IP address, the number of bytes in the message, how long it took to reply—given in milliseconds (ms)—and the length of time-to-live (TTL) in seconds. Ping works at the IP level and will often respond even when higher level TCP-based services cannot.

packet switching A message delivery technique in which small units of information (packets) are relayed through stations in a computer network along the best route available between the source and the destination. Data is broken into smaller units and then repacked in a process called packet assembly and disassembly (PAD). Although each packet can travel along a different path, and the packets composing a message can arrive at different times or out of sequence, the receiving computer reassembles the original message. Packet-switching networks are considered fast and efficient. Standards for packet switching on networks are documented in the CCITT recommendation X.25.

PAD See packet assembler/disassembler (PAD).

page-description language (PDL) A language that communicates to a printer how printed output should appear. The printer uses the PDL to construct text and graphics to create the page image. PDLs are like blueprints in that they set parameters and features such as type sizes and fonts, but leave the drawing to the printer.

parity An error-checking procedure in which the number of 1s must always be the same—either odd or even—for each group of bits transmitted without error. Parity is used for checking data transferred within a computer or between computers.

partition A portion of a physical disk that functions as if it were physically a separate unit.

password-protected share Access to a shared resource that is granted when a user enters the appropriate password.

PBX Private Branch Exchange (PABX Private Automated Branch Exchange) A switching telephone network that allows callers within an organization to place intraorganizational calls without going through the public telephone system.

PDA See Personal Digital Assistant (PDA).

PDC See Primary Domain Controller (PDC).

PDL See page-description language (PDL).

PDN See public data network (PDN).

peer-to-peer network A network in which there are no dedicated servers or hierarchy among the computers. All computers are equal and, therefore, known as peers. Generally, each computer functions as both client and server.

performance monitor A tool for monitoring network performance that can display statistics, such as the number of packets sent and received, server processor utilization, and the amount of data going into and out of the server.

peripheral A term used for devices such as disk drives, printers, modems, mouse devices, and joysticks that are connected to a computer and controlled by its microprocessor.

Peripheral Component Interconnect (PCI) 32-bit local-bus used in most Pentium computers and in the Apple Power Macintosh. Meets most of the requirements for providing Plug and Play functionality.

permanent virtual circuit (PVC) A permanent logical connection between two notes on a packet-switching network; similar to leased lines that are permanent and virtual, except that with PVC, the customer pays only for the time the line is used. This type of connection service is gaining importance because both frame relay and ATM use it. See also packet switching, virtual circuit.

permissions See access permissions.

Personal Digital Assistant (PDA) A variety of hand-held computer that provides functions such as personal organization features like a calendar, note taking, database manipulation, access to a calculator, and communications. For communication, a PDA uses cellular or wireless technology that is often built into the system, but that can be supplemented or enhanced by means of a PC Card.

petabyte See byte.

phase change rewritable (PCR) A type of rewritable optical technology in which the optical devices come from one manufacturer (Matsushita/Panasonic) and the media comes from two (Panasonic and Plasmon).

physical layer The first (bottommost) layer of the OSI reference model. This layer addresses the transmission of the unstructured raw bit stream over a physical medium (the networking cable). The physical layer relates the electrical/optical, mechanical, and functional interfaces to the cable and also carries the signals that transmit data generated by all of the higher OSI layers. See also Open Systems Interconnection (OSI) reference model.

piercing tap A connector for coaxial cable that pierces through the insulating layer and makes direct contact with the conducting core.

ping (Packet Internet Groper) See Packet Internet Groper (ping).

plenum The space in many buildings between the false ceiling and the floor above, used to circulate warm and cold air throughout the building. The space is often used for cable runs. Local fire codes specify the types of wiring that can be routed through this area.

Plug and Play (PnP) Refers to the ability of a computer system to automatically configure a device added to it. Plug and play capability exists in Macintoshes based on the NuBus and, since Windows 95, on PC-compatible computers. Also a set of specifications developed by Intel and Microsoft that allows a PC to configure itself automatically to work with peripherals such as monitors, modems, and printers.

point-to-point configuration Dedicated circuits that are also known as private or leased lines. They are the most popular WAN communications circuits in use today. The carrier guarantees full-duplex bandwidth by setting up a permanent link from each end point, using bridges and routers to connect LANs through the circuits. See also Point-to-Point Protocol, Point-to-Point Tunneling Protocol, and duplex transmission.

Point-to-Point Protocol (PPP) A data link protocol for transmitting TCP/IP packets over dial-up telephone connections, such as between a computer and the Internet. PPP was developed by the Internet Engineering Task Force in 1991.

Point-to-Point Tunneling Protocol (PPTP) PPTP is an extension of the Point-to-Point Protocol which is used for communications on the Internet. It 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. See also Virtual Private Networks.

polymorphic virus A variant of file infector virus that is named for the fact that it changes its appearance each time it is replicated. This makes it difficult to detect, since no two versions of the virus are exactly the same. See also file infector.

presentation layer The sixth layer of the OSI reference model. This layer determines the form used to exchange data between networked computers. At the sending computer, this layer translates data from a format sent down from the application layer into a commonly recognized, intermediary format. At the receiving end, this layer translates the intermediary format into a format useful to that computer's application layer. The presentation layer manages network security issues by providing services such as data encryption, provides rules for data transfer, and performs data compression to reduce the number of bits that need to be transmitted. See also Open Systems Interconnection (OSI) reference model.

Primary Domain Controller (PDC) The server that maintains the master copy of the domain's user accounts database and that validates logon requests. Every network domain is required to have one, and only one, PDC. See also domain, domain controller.

print queue A buffer in which a print job is held until the printer is ready to print it.

Project 802 A subgroup of the IEEE, originally formed in 1980, that defined network standards for the physical components of a network, the network interface card, and the cabling, which are accounted for in the physical and data-link layers of the OSI reference model. See also Appendix B.

protocol The system of rules and procedures that govern communication between two or more devices. Many varieties of protocols exist, and not all are compatible, but as long as two devices are using the same protocol, they can exchange data. Protocols exist within protocols as well, governing different aspects of communication. Some protocols, such as the RS-232 standard, affect hardware connections.

Other standards govern data transmission, including the parameters and handshaking signals such as XON/OFF used in asynchronous (typically, modem) communications, as well as such data-coding methods as bit- and byte-oriented protocols. Still other protocols, such as the widely used XMODEM, govern file transfer, and others, such as CSMA/CD, define the methods by which messages are passed around the stations on a LAN. Protocols represent attempts to ease the complex process of enabling computers of different makes and models to communicate. Additional examples of protocols include the OSI reference model, IBM's SNA, and the Internet suite, including TCP/IP. See also systems network architecture (SNA), Transport Control Protocol/Internet Protocol (TCP/IP).

protocol analyzers See network analyzers.

protocol driver The driver responsible for offering four or five basic services to other layers in the network, while "hiding" the details of how the services are actually implemented. Services performed include session management, datagram service, data segmentation and sequencing, acknowledgment, and possibly routing across a WAN.

protocol stack A layered set of protocols that work together to provide a set of network functions.

Proxy server A firewall component that manages Internet traffic to and from a local area network (LAN). The proxy server decides whether it is safe to let a particular message or file pass through to the organization's network, providing access control to the network, and filters and discards requests as specified by the owner, including requests for unauthorized access to proprietary data. See also firewall.

public data network (PDN) A commercial packet-switching or circuit-switching WAN service provided by local and long-distance telephone carriers.

punchdown block A wiring terminal, or series of terminals, into which cable can be plugged or "punched down." It is designed for environments that require a centralized location for all cabling for ease of making changes;. wiring running to the jacks can be more easily organized and maintained.

punchdown tool A specialized tool used to "punch down" cable wires into a wiring terminal. Using this tool ensures a solid connection.

PVC See permanent virtual circuit (PVC).

PVC (polyvinyl chloride) The material most commonly used for insulating and jacketing cable.

Q

quad shielding Cable that contains two layers of foil insulation and two layers of braided metal shielding.

R

RAID See redundant array of independent disks (RAID).

random access memory (RAM) Semiconductor-based memory that can be read and written to by the microprocessor or other hardware devices. The storage locations can be accessed in any order. Note that the various types of ROM memory are also capable of random access. However, the term RAM is generally understood to refer to volatile memory, which can be written as well as read. See also read-only memory (ROM).

read-only memory (ROM) Semiconductor-based memory that contains instructions or data which can be read but not modified. See also random access memory (RAM).

redirector Networking software that accepts I/O requests for remote files, named pipes, or mail slots and sends (redirects) the requests to a network service on another computer.

Reduced Instruction Set Computer (RISC) A type of microprocessor design that focuses on rapid and efficient processing of a relatively small set of instructions. RISC design is based on the premise that most of the instructions that a computer decodes and executes are simple. As a result, RISC architecture limits the number of instructions that are built into the microprocessor but optimizes each so it can be carried out very rapidly, usually within a single clock cycle. RISC chips execute simple instructions faster than microprocessors designed to handle a much wider array of instructions. They are, however, slower than general-purpose CISC (complex instruction set computing) chips when executing complex instructions, which must be broken down into many machine instructions before they can be carried out by RISC microprocessors.

redundancy system A fault-tolerant system that protects data by duplicating it in different physical sources. Data redundancy allows access to data even if part of the data system fails. See also fault tolerance.

redundant array of independent disks (RAID) A standardization of fault-tolerant options in five levels. The levels offer various combinations of performance, reliability, and cost. Formerly known as redundant array of inexpensive disks (RAID).

redundant array of inexpensive disks (RAID) See redundant array of independent disks (RAID).

remote-boot PROM (programmable read-only memory) A special chip in the network interface card that contains the hardwired code that starts the computer and connects the user to the network, used in computers for which there are no hard disk or floppy drives. See also diskless computers.

remote user User who dials in to the server over modems and telephone lines from a remote location.

repeater A device that regenerates signals so that they can be transmitted on additional cable segments to extend the cable length or to accommodate additional computers on the segment. Repeaters operate at the physical layer of the OSI reference model and connect like networks, such as an Ethernet LAN to an Ethernet LAN. They do not translate or filter data. For a repeater to work, both segments that the repeater joins must have the same media-access scheme, protocol, and transmission technique.

requester (LAN requester) Software that resides in a computer and forwards requests for network services from the computer's application programs to the appropriate server. See also redirector.

resources Any part of a computer system. Users on a network can share computer resources, such as hard disks, printers, modems, CD-ROM drives, and even the processor.

rewritable optical An optical disk that can be written to more than once.

RG-58 A /U Stranded-core coaxial cable. The version of this cable used by the United States military is known as RG-58 C/U.

RG-58 /U Solid-core coaxial cable.

rights Authorization with which a user is entitled perform certain actions on a computer network. Rights apply to the system as a whole, whereas permissions apply to specific objects. For example, a user might have the right to back up an entire computer system including the files which the user does not have permission to access. See also access permissions.

ring topology A topology in which computers are placed on a circle of cable. There are no terminated ends. The data travels around the loop in one direction and passes through each computer. Each computer acts as a repeater to boost the signal and send it on. Because the signal passes through each computer, the failure of one computer can bring the entire network down. The ring may incorporate features that disconnect failed computers so that the network can continue to function despite the failure. See also token passing, Token Ring network.

RIP See Routing Information Protocol (RIP).

RISC See Reduced Instruction Set Computer (RISC).

RJ-11 A four-wire modular connector used to join a telephone line to a wall plate or a communications peripheral such as a modem.

RJ-45 An eight-wire modular connector used to join a telephone line to a wall plate or some other device. It is similar to an RJ-11 telephone connector but has twice the number of conductors.

ROM See read-only memory (ROM).

routable protocols The protocols that support multipath LAN-to-LAN communications. See also protocol.

router A device used to connect networks of different types, such as those using different architectures and protocols. Routers work at the network layer of the OSI reference model. This means they can switch and route packets across multiple networks which they do by exchanging protocol-specific information between separate networks. Routers determine the best path for sending data and filter broadcast traffic to the local segment.

Routing Information Protocol (RIP) A protocol that uses distance-vector algorithms to determine routes. With RIP, routers transfer information among other routers to update their internal routing tables and use that information to determine the best routes based on hop counts between routers. TCP/IP and IPX support RIP.

RS-232 standard An industry standard for serial communication connections. Adopted by the Electrical Industries Association (EIA), this recommended standard defines the specific lines and signal characteristics used by serial communications controllers to standardize the transmission of serial data between devices.

S

SAP See service access point (SAP).

SAP (Service Advertising Protocol) See Service Advertising Protocol (SAP)

SCSI See Small Computer System Interface (SCSI).

SDLC See Synchronous Data Link Control (SDLC).

sector A portion of the data-storage area on a disk. A disk is divided into sides (top and bottom), tracks (rings on each surface), and sectors (sections of each ring). Sectors are the smallest physical storage units on a disk and are of fixed size—typically capable of holding 512 bytes of information apiece.

sector sparing A fault-tolerant system also called hot fixing. It automatically adds sector-recovery capabilities to the file system during operation. If bad sectors are found during disk I/O, the fault-tolerant driver will attempt to move the data to a good sector and map out the bad sector. If the mapping is successful, the file system is not alerted. It is possible for SCSI devices to perform sector sparing, but AT devices (ESDI and IDE) cannot.

security Making computers and data stored on them safe from harm or unauthorized access.

segment The length of cable on a network between two terminators. A segment can also refer to messages that have been broken up into smaller units by the protocol driver.

Sequenced Packet Exchange (SPX) Part of Novell's IPX/SPX protocol suite for sequenced data. See also Internetwork Packet Exchange/Sequenced Packet Exchange (IPX/SPX).

Serial Line Internet Protocol (SLIP) Defined in RFC 1055, SLIP is normally used on Ethernet, over a serial line; for example, an RS-232 serial port connected to a modem.

serial transmission One-way data transfer. The data travels on a network cable with one bit following another.

server A computer that provides shared resources to network users. See also client.

server-based network A network in which resource security and most other network functions are provided by dedicated servers. Server-based networks have become the standard model for networks serving more than 10 users. See also peer-to-peer network.

server message block (SMB) The protocol developed by Microsoft, Intel, and IBM that defines a series of commands used to pass information between network computers. The redirector packages SMB requests into a network control block (NCB) structure that can be sent over the network to a remote device. The network provider listens for SMB messages destined for it and removes the data portion of the SMB request so that it can be processed by a local device.

service access point (SAP) The interface between each of the seven layers in the OSI protocol stack that has connection points, similar to addresses, used for communication between layers. Any protocol layer can have multiple SAPs active at one time.

Service Advertising Protocol (SAP) Allows service-providing nodes (including file, printer, gateway, and application servers) to advertise their services and addresses.

session A connection or link between stations on the network.

session layer The fifth layer of the OSI reference model. This layer allows two applications on different computers to establish, use, and end a connection called a session. This layer performs name recognition and functions, such as security, needed to allow two applications to communicate over the network. The session layer provides synchronization between user tasks. This layer also implements dialog control between communicating processes, regulating which side transmits, when, for how long, and so on. See also Open Systems Interconnection (OSI) reference model.

session management Establishing, maintaining, and terminating connections between stations on the network.

sharing Means by which files are publicly posted on a network for access by anyone on the network.

shell A piece of software, usually a separate program, that provides direct communication between the user and the operating system. This usually, but not always, takes the form of a command-line interface. Examples of shells are Macintosh Finder and the MS-DOS command interface program COMMAND.COM.

shielded twisted-pair (STP) cable An insulated cable with wires that are twisted around each other with a minimum number of twists per foot. The twists reduce signal interference between the wires, and the more twists per foot, the greater the reduction in interference (crosstalk).

shielding The woven or stranded metal mesh that surrounds some types of cabling. Shielding protects transmitted data by absorbing stray electronic signals, sometimes called noise (random electrical signals that can degrade or distort communications), so that they do not get onto the cable and distort the data.

short A disruption in an electrical circuit that occurs when any two conducting wires or a conducting wire and ground come in contact with each other.

signal bounce The process by which, on a bus network, the signal is broadcast to the entire network. The signal travels from one end of the cable to the other. If the signal were allowed to continue uninterrupted it would keep bouncing back and forth along the cable and prevent other computers from sending signals. To stop the signal from bouncing, a component called a terminator is placed at each end of the cable to absorb free signals. Absorbing the signal clears the cable so that other computers can send data. See also terminator.

Simple Mail Transfer Protocol (SMTP) A TCP/IP protocol for transferring e-mail. See also application protocols, Transport Control Protocol/Internet Protocol (TCP/IP)

Simple Network Management Protocol (SNMP) A TCP/IP protocol for monitoring networks. SNMP uses a request and response process. In SNMP, short utility programs, called agents, monitor the network traffic and behavior in key network components in order to gather statistical data which they put into a management information base (MIB). To collect the information into a usable form, a special management console program regularly polls the agents and downloads the information in their MIBs. If any of the data falls either above or below parameters set by the manager, the management console program can present signals on the monitor locating the trouble and notify designated support staff by automatically dialing a pager number.

simplex transmission One-way transmission of data.

Simultaneous peripheral operation on line (spool) Facilitates the process of moving a print job from the network into a printer.

SLIP (Serial Line Internet Protocol) See Serial Line Internet Protocol (SLIP).

Small Computer System Interface (SCSI) A standard, high-speed parallel interface defined by the ANSI. A SCSI interface is used for connecting microcomputers to peripheral devices, such as hard disks and printers, and to other computers and LANs. SCSI is pronounced "scuzzy."

SMB See server message block (SMB).

SMDS See Switched Multimegabit Data Services.

SMP See symmetric multiprocessing (SMP).

SMTP (Simple Mail Transfer Protocol) See (Simple Mail Transfer Protocol) SMTP.

SNA See Systems Network Architecture (SNA).

SNMP See Simple Network Management Protocol (SNMP).

software Computer programs or sets of instructions that allow the hardware to work. Software can be grouped into four categories: system software, such as operating systems, that control the workings of the computer; Application software, such as word-processing programs, spreadsheets, and databases, which perform the tasks for which people use computers; network software, which enables groups of computers to communicate; and language software, which provides programmers with the tools they need to write programs.

SONET See Synchronous Optical Network (SONET).

spanning tree algorithm (STA) Implemented to eliminate redundant routes and avoid situations in which multiple LANs are joined by more than one path by the IEEE 802.1 Network Management Committee. Under STA, bridges exchange certain control information in an attempt to find redundant routes. The bridges determine which would be the most efficient route, then use that one and disable the others. Any of the disabled routes can be reactivated if the primary route becomes unavailable.

spread-spectrum radio technology A technology that provides for a truly wireless network. Spread-spectrum radio broadcasts signals over a range of frequencies, avoiding the communication problems of narrow-band radio transmission.

SPX (Sequenced Packet Exchange) See Sequenced Packet Exchange (SPX)

SQL See structured query language (SQL).

STA See spanning tree algorithm (STA).

stand-alone computer A computer that is not connected to any other computers and is not part of a network.

stand-alone environment A work environment in which each user has a personal computer, but works independently, unable to share files and other important information that would be readily available through server access in a networking environment.

standard Ethernet A network topology that transmits at 10 Mbps over a baseband wire and can carry a signal 500 meters (five 100-meter segments). See also thicknet, 10Base5.

star topology A topology in which each computer is connected by cable segments to a centralized component called a hub. Signals transmitted by a computer on the star pass through the hub to all computers on the network. This topology originated in the early days of computing with terminals connected to a centralized mainframe. The star topology offers centralized resources and management. However, because each computer is connected to a central point, much cable is required in a large installation, and if the central point fails, the entire network goes down. See also hub.

stealth virus A variant of file infector virus. This virus is so named because it attempts to hide from detection. When an antivirus program attempts to find it, the stealth virus tries to intercept the probe and return false information indicating that it does not exist.

STP See shielded twisted-pair (STP).

stripe set A form of fault tolerance that combines multiple areas of unformatted free space into one large logical drive, distributing data storage across all drives simultaneously. In Windows NT, a stripe set requires at least two physical drives and can use up to 32 physical drives. Stripe sets can combine areas on different types of drives, such as Small Computer System Interface (SCSI), Enhanced Small Device Interface (ESDI), and Integrated Device Electronics (IDE) drives.

structured query language (SQL) A database sublanguage used to query, update, and manage relational databases. Although not a programming language in the same sense as C or Pascal, SQL can be used either in formulating interactive queries or embedded in an application as instructions for handling data. The SQL standard also contains components for defining, altering, controlling, and securing data.

SVC See switched virtual circuits (SVC).

Switched Multimegabit Data Services (SMDS) A high-speed, switched-packet service that can provide speeds of up to 34 Mbps.

switched virtual circuit (SVC) A logical connection between end computers that uses a specific route across the network. Network resources are dedicated to the circuit, and the route is maintained until the connection is terminated. These are also known as point-to-multipoint connections. See also virtual circuit.

switching See packet switching.

symmetric multiprocessing (SMP) SMP systems, such as Windows NT Server, use any available processor on an as-needed basis. With this approach, the system load and application needs can be distributed evenly across all available processors.

synchronous Synchronous communication relies on a timing scheme coordinated between two devices to separate groups of bits and transmit them in blocks called frames. Special characters are used to begin the synchronization and check its accuracy periodically. Because the bits are sent and received in a timed, controlled (synchronized) fashion, start and stop bits are not required. Transmission stops at the end of one transmission and starts again with a new one. It is a start/stop approach, and more efficient than asynchronous transmission. If an error occurs, the synchronous error detection and correction scheme implements a retransmission. However, because more sophisticated technology and equipment is required to transmit synchronously, it is more expensive than asynchronous transmission.

Synchronous Data link Control (SDLC) The data-link (data transmission) protocol most widely used in networks conforming to IBM's SNA. SDLC is a communications guideline that defines the format in which information is transmitted. As its name implies, SDLC applies to synchronous transmissions. SDLC is also a bit-oriented protocol and organizes information in structured units called frames.

Synchronous Optical Network (SONET) A fiber-optic technology that can transmit data at more than one gigabit per second. Networks based on this technology are capable of delivering voice, data, and video. SONET is a standard for optical transport formulated by the Exchange Carriers Standards Association (ECSA) for the ANSI.

Systems Network Architecture (SNA) A widely used communications framework developed by IBM to define network functions and establish standards for enabling its different models of computers to exchange and process data. SNA is a design philosophy that separates network communication into five layers. Each layers, like those in the similar ISO/OSI reference model, represents a graduated level of function moving upward from physical connections to applications software.

T

T1 line A high-speed communications line that can handle digital communication and Internet access at a rate of 1.544 Mbps (megabits per second).

T1 service The standard digital line service. It provides transmission rates of 1.544 Mbps and can carry both voice and data.

tap A connection to a network. This usually refers specifically to a connection to a cable.

TCP See Transmission Control Protocol (TCP).

TCP/IP See Transport Control Protocol/Internet Protocol (TCP/IP).

TDI (transport driver interface) See transport driver interface (TDI).

TDR See time-domain reflectometer (TDR).

Technet (Microsoft Technical Information Network) See Microsoft Technical Information Network (TechNet).

Telnet The command and program used to log in from one Internet site to another. The Telnet command and program brings the user to the login prompt of another host.

terabyte See byte.

terminator A resistor used at each end of an Ethernet cable to ensure that signals do not reflect back and cause errors. It is usually attached to an electrical ground at one end. See also signal bounce.

terminator resistance The level of resistance in a terminator, measured in ohms. It must match the network architecture specification. For example, Ethernet using RG-58 A/U thinnet cable requires a 50-ohm resistor in the terminator. Terminating resistance that does not match the specifications may cause the network to fail. See also ohm.

thicknet (standard Ethernet) A relatively rigid coaxial cable about 0.5-inch in diameter. Typically, thicknet is used as a backbone to connect several smaller thinnet-based networks because of its ability to support data transfer over longer distances. Thicknet can carry a signal for 500 meters (about 1640 feet) before needing a repeater.

thinnet (thin-wire Ethernet) A flexible coaxial cable about 0.25-inch thick. It is used for relatively short-distance communication and is fairly flexible to facilitate routing between computers. Thinnet coaxial cable can carry a signal up to approximately 185 meters (or about 607 feet) before needing a repeater.

throughput A measure of the data transfer rate through a component, connection, or system. In networking, throughput is a good indicator of the system's total performance because it defines how well the components work together to transfer data from one computer to another. In this case, the throughput would indicate how many bytes or packets the network could process per second.

time-domain reflectometer (TDR) A troubleshooting tool that sends sonar-like pulses along a cable looking for any kind of a break, short, or imperfection that might affect performance. If the pulse finds a problem, the TDR analyzes it and displays the result. A good TDR can locate a break to within a few feet of the actual separation in the cable.

token A predetermined formation of bits that permits a network device to communicate with the cable. A computer cannot transmit unless it has possession of the token. Only one token at a time can be active on the network, and the token can travel in only one direction around the ring. See also token passing, Token Ring network.

token passing A media access control method in a Token Ring network in which a data frame, called a token, is passed from one station to the next around the ring. See also token, Token Ring network.

Token Ring network A network in which computers are situated on a continuous network loop through which a token is passed from one computer to the next. Computers are centrally connected to a hub called a Multistation Success Unit (MAU) and are wired in a star configuration. Computers use a token to transmit data and must wait for a free token in order to transfer data. See also token, token passing.

TokenTalk An expansion card that allows a Macintosh II to connect to an 802.5 Token Ring network.

tone generator and tone locator Standard wiring tools used for troubleshooting. The tone generator is used to apply an alternating or continuous tone signal to a cable or conductor and is attached to one end of the cable. A matching tone locator is used to detect the correct cable at the other end of the run. These tools are also referred to as a "fox and hound."

tone locator See tone generator and tone locator.

topology The arrangement or layout of computers, cables, and other components on a network. Topology is the standard term that most network professionals use when referring to the network's basic design.

tracert A Trace Route command-line utility that shows every router interface through which a TCP/IP packet passes on its way to a destination.

trailer One of the three sections of a packet component. The exact content of the trailer varies depending on the protocol, but it usually includes an error-checking component (CRC).

transceiver A device that connects a computer to the network. The term is derived from transmitter/receiver; thus, a transceiver is a device that receives and transmits signals. It switches the parallel data stream used on the computer's bus into a serial data stream used in the cables connecting the computers.

Transmission Control Protocol (TCP) The TCP/IP protocol for sequenced data. See also Transport Control Protocol/Internet Protocol (TCP/IP).

Transport Control Protocol/Internet Protocol (TCP/IP) An industry standard suite of protocols providing communications in a heterogeneous environment. In addition, TCP/IP provides a routable, enterprise networking protocol and access to the Internet and its resources. It is a transport layer protocol that actually consists of several other protocols in a stack that operates at the session layer. Most networks support TCP/IP as a protocol.

transport driver interface (TDI) An interface that works between the file system drives and the transport protocols, allowing any protocol written to TDI to communicate with the file system drivers.

transport layer The fourth layer of the OSI reference model. It ensures that messages are delivered error free, in sequence, and without losses or duplications. This layer repackages messages for efficient transmission over the network. At the receiving end, the transport layer unpacks the messages, reassembles the original messages, and sends an acknowledgment of receipt. See also Open Systems Interconnection (OSI) reference model.

transport protocols Protocols that provide for communication sessions between computers and ensure that data is able to move reliably between computers.

"Trojan horse" virus A type of virus that appears to be a legitimate program that might be found on any system. The Trojan horse virus can destroy files and cause physical damage to disks.

trunk A single cable, also called a backbone, or segment.

trust relationship Trust relationships are links between domains that enable pass-through authentication, in which a user has only one user account in one domain, yet can access the entire network. User accounts and global groups defined in a trusted domain can be given rights and resource permissions in a trusting domain even though those accounts do not exist in the trusting domain's database. A trusting domain honors the logon authentication of a trusted domain.

T connector A T-shaped coaxial connector that connects two thinnet Ethernet cables while supplying an additional connector for a network interface card.

twisted-pair cable A cable that consists of two insulated strands of copper wire twisted together. A number of twisted-wire pairs are often grouped together and enclosed in a protective sheath to form a cable. Twisted-pair cable can be shielded or unshielded. Unshielded twisted-pair cable is commonly used for telephone systems. See also shielded twisted-pair (STP), unshielded twisted-pair (UTP).

U

UART See universal asynchronous receiver transmitter (UART).

UDP (User Datagram Protocol) See (User Datagram Protocol (UDP)

Uniform Resource Locator (URL) Provides the hypertext links between documents on the World Wide Web (WWW). Every resource on the Internet has its own location identifier, or URL, that specifies the server to access as well as the access method and the location. URLs can use various protocols including FTP and HTTP.

uninterruptible power supply (UPS) A device connected between a computer, or another piece of electronic equipment, and a power source, such as an electrical outlet. The UPS ensures that the electrical flow to the computer is not interrupted because of a blackout and, in most cases, protects the computer against potentially damaging events such as power surges and brownouts. Different UPS models offer different levels of protection. All UPS units are equipped with a battery and loss-of-power sensor. If the sensor detects a loss of power, it immediately switches over to the battery so that the user has time to save his or her work and shut off the computer. Most higher-end models have features such as power filtering, sophisticated surge protection, and a serial port so that an operating system capable of communicating with a UPS (such as Windows NT) can work with the UPS to facilitate automatic system shutdown.

universal asynchronous receiver transmitter (UART) A module, usually composed of a single integrated circuit, that contains both the receiving and transmitting circuits required for asynchronous serial communication. Two computers, each equipped with a UART, can communicate over a simple wire connection. The operation of the sending and receiving units are not synchronized by a common clock signal, so the data stream itself must contain information about when packets of information (usually bytes) begin and end. This information about the beginning and ending of a packet is provided by the start and stop bits in the data stream. A UART is the most common type of circuit used in personal-computer modems.

Universal Serial Bus (USB) A serial bus with a data transfer rate of 12 megabits per second (Mbps) for connecting peripherals to a microcomputer. USB can connect up to 127 peripheral devices to the system through a single, general-purpose port. This is accomplished by daisy chaining peripherals together. USB is designed to support the ability to automatically add and configure new devices and the ability to add such devices without having to shut down and restart the system.

unshielded twisted-pair (UTP) cable A cable with wires that are twisted around each other with a minimum number of twists per foot. The twists reduce signal interference between the wires. The more twists per foot, the greater the reduction in interference (crosstalk). This cable is similar to shielded-twisted pair (STP), but lacks the insulation or shielding found in STP cable.

UPS See uninterruptible power supply (UPS).

URL See Uniform Resource Locator (URL).

USB See Universal Serial Bus.

user account Consists of all of the information that defines a user on a network. This includes the user name and password required for the user to log on, the groups in which the user account has membership, and the rights and permissions the user has for using the system and accessing its resources.

User Datagram Protocol (UDP) A connectionless protocol, responsible for end-to-end data transmission.

user groups Groups of users who meet online or in person to discuss installation, administration, and other network challenges for the purpose of sharing and drawing on each other's expertise in developing ideas and solutions.

UTP See unshielded twisted-pair (UTP) cable.

V

vampire tap (piercing tap transceiver) An Ethernet transceiver housed in a clamp-like device with sharp metal prongs that "bite" through thicknet cable insulation and make contact with the copper core. The transceiver's DIX (DB15) connector provides an attachment for an AUI cable that runs from the transceiver to either the computer or a hub or repeater. Along thick coaxial cable, which includes bands spaced 2.5 meters (8 feet) apart, a vampire tap is inserted into each of these bands; an AUI, DIX, or DB15 connector then attaches a cable from the tap to the computer or other device to be added to the Ethernet network.

virtual circuit A series of logical connections between a sending computer and receiving computer. The connection is made after both computers exchange information and agree on communication parameters that establish and maintain the connection, including maximum message size and path. Virtual circuits incorporate communication parameters such as acknowledgments, flow control, and error control to ensure reliability. They can be either temporary, lasting only as long as the conversation, or permanent, lasting as long as the users keep the communications channel open.

Virtual Private Network (VPN) A set of computers on a public network such as the Internet that communicate among themselves using encryption technology. In this way their messages are as safe from being intercepted and understood by unauthorized users. VPNs operate as if the computers were connected by private lines.

virus Bits of computer programming, or code, that hide in computer programs or on the boot sector of storage devices such as hard-disk drives and floppy-disk drives. The primary purpose of a virus is to reproduce itself as often as possible; a secondary purpose is to disrupt the operation of the computer or the program.

voltmeter See digital voltmeter (DVM).

volume set A collection of hard disk partitions that are treated as a single partition, thus increasing the disk space available in a single drive letter. Volume sets are created by combining between 2 and 32 areas of unformatted free space on one or more physical drives. These spaces form one large logical volume set which is treated like a single partition.

W

wide area network (WAN) A computer network that uses long-range telecommunication links to connect networked computers across long distances.

winipcfg A diagnostic command specific to Microsoft Windows 95 and 98. Although this graphical user interface (GUI) utility duplicates the functionality of ipconfig, its GUI makes it easier to use. See also ipconfig.

wireless bridge A component that offers an easy way to link buildings without using cable.

wireless concentrator A component that acts as a transceiver to send and receive signals while communicating with network interface cards.

wireless network An emerging networking option that consists of wireless components that communicate with a network that uses cables in a mixed-component network called a hybrid.

workgroup A collection of computers that are grouped for sharing resources such as data and peripherals over a LAN. Each workgroup is identified by a unique name. See also domain, peer-to-peer network.

WORM (Write-Once Read-Many) See Write-Once Read-Many (WORM).

World Wide Web (the Web, or WWW) The Internet multimedia service that contains a vast storehouse of hypertext documents written in HTML. See also Hypertext Markup Language (HTML).

Write-Once Read-Many (WORM) Any type of storage medium to which data can be written only once, but can be read any number of times. Typically, this is an optical disk whose surface is permanently etched using a laser, in order to record information.

X

X.25 A recommendation published by the CCITT that defines the connection between a terminal and a packet-switching network. A packet-switching network routes packets whose contents and format are controlled standards such as those defined in the X.25 recommendation. X.25 incorporates three definitions: the electrical connection between the terminal and the network, the transmission or link-access protocol, and the implementation of virtual circuits between network users. Taken together, these definitions specify a synchronous, full-duplex terminal-to-network connection. Packets transmitted in such a network can contain either data or control commands. Packet format, error control, and other features are equivalent to portions of the HDLC protocol defined by the ISO. X.25 standards are related to the lowest three levels of the OSI reference model.

X.400 A CCITT protocol for international e-mail transmissions.

X.500 A CCITT protocol for file and directory maintenance across several systems.

XNS (Xerox Network System) Protocol developed by Xerox for their Ethernet LANs.

Z

Zones Logical groupings of users and resources in an AppleTalk network.