Data Communications GlossaryData Communications GlossaryIntroduction
This data communications glossary is designed to provide the reader with definitions to the numerous technical terms often found in the data communications and networking environment.
Each entry is alphabetically indexed with several hyperlinks to web resources.It is intended that this reference be useful for data communications students, users, and systems professionals alike As always, the author welcomes any feedback that may improve the content of this online reference.
A
A/B Signaling - A procedure used in T1 transmission facilities . One bit, from each of 24 sub channels in every sixth frame, is used for carrying dial and control information.
Abbreviated Dialing - A feature of some telephone switches which permits users to establish calls by entering fewer digits than the full telephone number.
Access, Access Code - The prefix digits that a telephone user dials to be connected to an outgoing line or trunk.
Access Line - The existing connection between a voice or data customers equipment and a public communications network. That portion of a leased telephone line that permanently connects the user with the Central Office.
Access Method - Two definitions for "access methods" are currently in use:
1) A CPU resident program built to control the flow of data between central storage and the peripheral devices of a host system. The more common access methods of this type would be IBM 's VTAM, TCAM and BTAM.
2) A method for local area network ( LAN) terminals to access the transmission medium. Several types of LAN access methods currently exist: Shared Access, explicit access, contended access and discreet access.
Access Node - Local-Exchange-Carrier owned, Broadband ISDN remote switch which performs grooming, concentration and switching.
Accunet - An AT&T product offering high speed digital service including packet switched, T1 and DDS customer service. Specifically, Accunet offers the services at the following speeds:
1) Accunet T1.5, terrestrial wideband at 1.544 mbps;
2) Accunet Reserved T1.5, satellite -based channels at 1.544 m bps used primarily for video teleconferencing purposes;
3) Accunet Packet Services, packet-switching services;
4) Accunet Dataphone digital service (DDS), private line digital circuits at 2400, 4800, 9600 and 56 kbps.
ACD (Automatic Call Distributor) - A switching system which automatically distributes incoming calls in the sequences they are received to a centralized group of receivers without human interface. If no receivers are available, the calls will be held until one becomes free.
ACK - This is a control character found in bisync protocol. When combined with NAK, the ACK character would indicate that the previously transmit ted data block was correctly received (acknowledge) or incorrectly received (NAK-Negative Acknowledge).
Acoustic Coupler - A kind of modem that uses a standard telephone handset to transmit data over the telephone network. Acoustic couplers are best used in transmitting data at lower speeds (300 Baud) due to the environmental noises present in many office environments, and the acoustic couplers inability to block out the resulting interference.
AC Signaling - A signaling method that relies on alternating current tones or signals to transmit information or control signals.
ACS - An AT&T product (Advanced Communication Service) to provide packet switched Service.
ADCCP (Advanced Data Communications Control Procedures) - USAFS (United States of America Federal Standard) communications protocol, endorsed by the American National Standards Institute.
Address - There are two definitions depending on the usage.
Adjacent - Computer Programs or network devices directly connected by a data link.
ADPCM (Adaptive Differential Pulse Code Modulation) - A CCITT standard defining an encoding technique. ADPCM provides for analog voice transmission.
Aliasing - A condition the occurs when a transmission error fragments a message and the fragments appear as a "good message " at the receiver. Protocols have been developed to prevent aliasing, such as HDLC.
Alphanumeric - A naming convention or character set that could include alphabetic, numeric and punctuation characters.
Alternate Delivery - A process whereby messages to a particular keyboard device/display or output terminal would be re-routed via an alternate path due to out of service or busy conditions. Also known as alternate routing.
American National Standards Institute - (Please refer to ANSI )
Amplitude - A term used to describe the relative height of a sine wave .
Amplitude Modulation - One of three methods to add information to a sine wave carrier signal. By varying the voltage level, the magnitude is modified in accordance with the information to be transmit ted. Contrast with phase modulation and frequency modulation.
Amplifier - A unidirectional, electronic device used to boost (amplify) signals, with the resulting performance measured in decibels.
Ambient noise - Any form of telecommunications interference present in a communications line at all times.
AN - (See Access Node )
Analog, Analog data - Any data in the form of continuously variable physical qualities, which are "analogous" to the data source. Continuously variable as opposed to discretely variable. Contrast with digital data.
Analog loopback - A diagnostic test that forms the loop at the modem 's telco line interface to isolate faults to the analog signal.
ANSI (American National Standards Institute) - A voluntary organization that represents the USA in the ISO, and is responsible for defining ASCII. Members include manufacturers, common carriers, and other standards organizations such as the IEEE.
Answer back - A transmission from a receiving device to the sending device acknowledging a "message received" or "ready to receive" status.
Application Layer - In referring to the OSI model, this is the "top" or seventh layer containing all user or application programs. See OSI layers diagram in Section B.
Application Program - Two definitions exist is computer terminology:
1) (In general) A program written for or by a user that applies to his own work. A program which performs a user function. Contrast with system program.
2) (In data communications) A program that usually resides in DCE used to communicate with DTE that performs a set of specified activities for terminal users.
ARPA - The Advanced Research Projects Agency, credited with designing ARPANET, the first widely used packet-switched network .
ARQ (Automatic Request for Retransmission) - A method of error control where a receiving device notifies a transmit ting device, advising which data blocks were received successfully. Any blocks not successfully received would be re transmit ted by the transmit ting device.
ASCII (American Standard Code for Information Interchange) - Usually pronounced "ask'-ee", this seven-bit-plus parity code set was established by ANSI to provide compatibility between data systems and services.
ASCII terminal - A terminal using ASCII; usually synonymous with asynchronous terminal and with dumb terminal.
ASR (Automatic Send and Receive) - Any one of several devices designed to transmit and receive data unattended, in contrast with RO, "receive only" devices.
Asynchronous, Asynchronous Transmission - Also known a "start/stop", asynchronous refers to having a variable or random time interval between successive characters, operations, or events. Each character, word, or small block is individually synchronized by the use of start and stop bits. Contrast this concept with synchronous.
ATM Forum - An international non-profit organization formed with the objective of accelerating the use of ATM (Asynchronous Transfer Mode) products and services through a rapid convergence of interoperability specifications. In addition, the Forum promotes industry cooperation and awareness. Click here to reach the ATM Forum web site.
AT&T (American Telephone and Telegraph Company) - The USA's major common carrier for long distance telephone lines.
Attenuate, Attenuation - A decrease in power over distance of a signal. Typically, attenuation increases (and the signal level decreases) with both frequency and cable length. Attenuation is measured in terms of levels called decibels. Contrast this concept with gain.
ATTIS (AT&T Information Systems) - A division of AT&T Technologies that supplies and manufactures customer premises equipment (CPE).
Audio frequencies - Frequencies that can be heard by the human ear, usually between 30 Hz and 20,000 Hz.
Auto-answer - A modem that automatically sends an answering tone in response to an incoming call.
Autobaud, Automatic Baud Rate Detection (ABR) - A process that enables a receiving device to accept data from a variety of transmit ting devices, operating at different speeds, stop bits and code levels. ABR usually relies on a sign-on character to provide the information needed by the receiving device to make the necessary transition.
Auto-dial - A modem that automatically originates calls, that is, dials the desired number.
Autodin (Automatic Digital Network) - A digital data communications network that is a part of the U.S. Department of Defense.
Automatic Calling Unit (ACU) - Any one of several type of devices designed to place a telephone call automatically upon receipt of information from a DTE.
Automatic Dialing Unit (ADU) - A device capable of automatically generating dialing digits. Also known as an Auto-Dialer.
Automatic Identification of Outward Dialing (AIOD) - A PBX service feature that identifies the calling extension, permitting cost allocation.
Automatic Route Selection - A PBX feature that permits automatic selection of the lowest cost routing of a call in a network .
Automatic Teller Machines (ATM) - One of several types of remote terminals used by banks to allow customers to perform numerous banking transactions.
Availability - The percentage measurement of computer equipment, system, or network performance that satisfactory data communication service is available. The term satisfactory implies that teleprocessing channels are in working order. The percentage is generally expressed as the ratio of operating time to the sum of operating time plus down time, where MTBF = Mean Time Between Failures and MTTR = Mean Time To Repair.
AWG (American Wire Gauge) -The standard used to define wire size in communications line media.
B
Backup - The hardware and software resources available to recover after a degradation or failure of one or more system components.
Backbone - The major transmission path or facility for a PDN.
Backward Channel - See reverse channel.
Balanced, Balanced Circuit - A network terminated circuit having balanced impedances (between telephone line and network) resulting in a low rate of return loses. Contrast with unbalanced-to-ground.
Band Splitter - A multiplexor (either FDM or TDM) designed to divide a wide bandwidth into several independent, narrower band width channels, for data transmission at a fraction of the total data rate.
Bandwidth - The information-carrying capability of a communications channel or line, expressed in cycles per second ( Hz) between the highest and lowest frequencies of a band.
Baseband, Baseband Transmission - Direct transmission method whereby the transmission medium carries only one signal at a time usually for distances under ten miles.
Baseband Modem - A DCE device also known as a line driver or local dataset.
Base Group - Twelve communication VF paths. A unit of frequency division multiplexing FDM) systems bandwidth allocation.
Baud - A unit of signaling speed or rate, taken from the name of French telegrapher, Emile Baudot. Baud is usually defined as the number of signal level changes per second, regardless of information content of those signals. If each signal event represents only one bit condition, baud is the same as bits per second when each signal event represents other than one bit.
Baudot Code - A five-bit code set designed for asynchronous transmission of data used primarily for teleprinter systems adding one start bit and 1.5 stop bits . Contrast with ASCII and EBCDIC.
BASIC (Beginners All-purpose Symbolic Instruction Code) - A high- level (many "English-like" terms) programming language.
Batch Processing - A data processing technique where related transactions are grouped together and transmit ted for processing. Contrast with interactive processing.
BCC (Block Check Character) - A control character added to a block in character oriented protocols (such as Bisync) used for determining if the block was received in error -- such as CRC and LRC.
BCD (Binary Coded Decimal) - A digital system that uses six- bit binary codes to represent decimal digits, providing 64 possible combinations. This was the standard code used on the early IBM 1401 computers.
Bell 103 - An AT&T modem providing asynchronous originate/answer transmission at speeds from 0-300 bps. Often used to describe any Bell 103 or compatible modem.
Bell 113 - An AT&T modem providing asynchronous transmission with either originate or answer capability (but not both) at speeds from 0-300 bps. Often used to describe any Bell 113 or compatible modem.
Bell 201 - An AT&T modem providing synchronous transmission at speeds of 2400 BPS . Often used to describe any Bell 201 or compatible modem.
Bell 202 - An AT&T modem providing asynchronous transmission at 1800 bps and requiring a four-wire circuit for full-duplex operation. Also an AT&T 1200 bps modem providing asynchronous transmission over two-wire, full-duplex, leased line or public telephone networks. Often used to describe any Bell 202 or compatible modem.
Bell 208 - An AT&T modem providing synchronous transmission at 4800 bps. Often used to describe any Bell 208 or compatible modem.
Bell 209 - An AT&T modem providing synchronous transmission over four-wire leased lines at 9600 bps.
Bell 212,212A - An AT&T modem providing asynchronous, full-duplex transmission. Often used to describe any Bell 212 or compatible modem .
Bell 43401 - Bell publication defining the requirements for data transmission over limited distance DC-continuous private metallic circuits supplied by the regional telephone company.
BERT (Bit Error Rate Testing) - A method of data communications line testing, where a bit pattern is compared before and after the transmission to detect errors . BERT tester - used to describe any device designed to perform the above-mentioned function.
Binary - A digital system with two states, 1 and 0. Contrast with octal (8 states), decimal (10 states), and hexadecimal (16 states).
Binary Synchronous Protocol (Bisync, BSC) - An IBM line control procedure which has become an industry standard for data communications. It can express several data codes such as 8- bit EBCDIC, 7-bit ASCII or 6- bit transcode and uses a defined set of control characters and sequences for synchronized transmission of data between stations.
Bipolar Transmission - See polar transmission.
Bit - A contraction of the words: binary digit, representing the smallest unit of information and the basic unit in data communications. A bit can have a zero or a one value (or a mark or space value in data communications.
Bit Oriented - A communications protocol (such as IBM 's SDLC) where control information is encoded in one or more bits. Contrast with byte or character oriented.
Bit Rate - The speed at which binary digits (bits) would be transmit ted over a communications path and usually expressed in "bits per second" ( bps). Bit rate should not be confused with Baud which defines the rate of signal state changes.
Bit Stream - The continuous series of transmit ted bits through a transmission link.
Bit Stripping - When referring to statistical multiplexors, bit stripping involves the removal of the start/stop bits on each async character and transmit ting the data using synchronous techniques.
Bit Stuffing - Also known as zero insertion, bit stuffing is a process used in bit - oriented protocols (such as IBM's SDLC) where a string of "1" bits is broken by an inserted "0" bit to avoid confusing data and SYN characters. Once received, the inserted 0 is removed.
Blank(s) - Used to define a condition of no information in data recording medium and usually indicated by the presence of all zeros or all spaces.
BLERT (Block Error Rate Testing) - A method of data line testing using groups of information transmission blocks to detect errors.
Block - A group or quantity of information composed of any quantity of contiguous bits and/or bytes.
Blocking - A "busy signal" or denial condition occurring in switching systems when circuits or paths become unavailable for call completion. Normally a "busy tone" is transmit ted to the calling party.
Block Multiplexer Channel - An IBM term used to describe a computer peripheral multiplexor channel that interleaves blocks of data. Contrast with byte multiplexor channel and selector channel.
BNC (Bayonet-Neill-Concelman) - A connector used for miniature coaxial cable employing a bayonet locking mechanism. See C connector and N connector. Contrast with TNC.
BOC (Bell Operating Company) - Any one of the 22 local telephone companies spun off from AT&T, effective January 1, 1984, as a result of divestiture and became organized into seven regional Bell holding companies. Subsequent mergers have reduced that number to 4.
Boundary Node - In IBM SNA terminology, a subarea node that can provide certain protocol support for adjacent subarea nodes.
BPS (Bits Per Second) - The rate of information bits transmit ted. The basic unit of data communications rate, very often combined with metric prefixes, such as kbps for thousands of bits per second and mbps for millions of bits per second.
BREAK - A spacing condition that exists longer than 110 milliseconds used as a signal to "break-in" when the opposite party or unit is sending. This is a feature of teletype systems operating in half duplex.
Breakout Box (BOB) - Also known as an EIA monitor, a breakout box serves to monitor the status of signals of the pins of an RS-232C connector or cable and allows signals to be broken, patched, or cross-connected.
Bridge - The interconnecting equipment and techniques used to match circuits to each other ensuring minimum transmission impairment. Bridges function at the data link layer of the OSI model. Contrast with gateway.
Broadband - Two definitions exist in normal usage:
1) As data transmission channel which has a bandwidth greater than voice grade (VG) channels of 3000 Hz and potentially capable of much higher rates. Also known as wideband.
2) In local area networks (LAN), whereby broadband facilities can carry many voices or data channels simultaneously and can also be used for high speed digital data communications at speeds of up to 1.544 mbps, usually via coaxial cable using radio-frequency modems.
Broadcast,Terminal Broadcast - Two definitions apply to broadcast:
1) As a terminal transmission of a message intended for all points on a circuit simultaneously, rather than for a specific station.
2) In LAN technology as a transmission procedure used in a bus topology network sending all messages to all stations despite specific terminal addressing.
BSC (Binary Synchronous) - See Binary Synchronous Protocol .
BTAM (Basic Telecommunications Access Method) - An IBM software routine that functions as the basic access method for 3270 data communications terminals and main storage. BTAM provides the applications program with macro instructions for using the capabilities of the devices supported. BTAM supports start/stop ( asynchronous) as well as synchronous communication.
Buffer - A temporary storage device for blocks of data used to compensate for the difference in either the rate of data flow or the time of occurrence of the events in transmissions between devices.
Buffering - The storing of data in a memory device, allowing the devices to change data rates, perform error checking and error retransmission.
Burst - A group of events occurring simultaneously in time.
Bus, Buss - Two definitions exist in normal usage:
1) A typically electrical data path or channel, with one or more conductors, where all devices receive transmissions simultaneously, such as the input/output bus in a computer.
2) In a LAN environment, such as Ethernet, where all network nodes "listen" to all transmissions. Contrast with ring and star. See also CSMA/CD.
Busy Hour, Prime Time - The period of a day in which the largest number of user transactions occur. Also known as Peak Hour or Peak Period.
Byte - Some set of contiguous bits operated upon as a single unit, usually six or eight bits long. Most character sets use one byte per character, with storage device capacity usually spoken of in K bytes (K = 1024 bytes) and M bytes (M = mega, or million bytes).
Byte Multiplexer Channel - A mainframe input/output channel that provides multiplexing or interleaving of data in bytes. Contrast with Block Multiplexer channel.
C
Cabinet - A physical stand or enclosure designed to rack-mount data equipment and provide easy access to both front and rear panels of the devices contained within. Standard cabinets have 1 3/4" vertical spacing between mounting holes and 19" wide horizontal spacing between mounting rails.
Cable - The combined assembly of one or more conductors within a protective sheath and constructed to permit the use of conductors separately or in groups.
Cable-Based LAN - A local area network (LAN) that uses a coaxial or twisted pair cable as its transmission medium.
Cache Memory - A high-speed computer memory which contains the next most likely instruction or sequence of instructions to be executed upon completion of the present instruction.
Call - A request to connect, or the connection that results from the request, either voice or data. Contrast with minicall and virtual call.
Call Accounting - The recording of data pertaining to start/end times, number of segments, NUI, NTN etc., in packet-switched networks.
Call Detail Recording (CDR) - A PBX feature where each phone call is logged by time and charges.
Called Channel - A channel that can receive but not originate calls. A calling channel can call, but not receive calls, while a called/calling channel can both originate and receive calls. These examples are found in both LAN and packet-switched networks.
Call Forwarding - A telephone service feature that can be programmed to automatically forward calls to another number.
Calling rate - The average number of calls per telephone, determined by dividing the number of busy-hour calls by the number of telephones.
Call setup time - The length of time it takes to establish a switched call between two pieces of DTE.
Call Waiting, Camp-on - Two definitions exist in normal usage:
1) A telephone service allowing a call to a busy telephone to be held while a tone notifies the busy telephone that a call is waiting.
2) In LAN environments; a facility to allow users to wait in a queue if the requested resource is busy. Users are then connected on a first come first serve basis once the resource becomes available.
Capacitance - The ability of an electrical device to hold, store or accommodate electricity.
Card Module, Circuit Card - A printed-circuit board (PC board) designed to plug into a slot in an equipment chassis.
Carrier - A continuous frequency, that can be impressed or modulated with a second information carrying signal.
Carrier Detect - An RS-232 interface modem signal (transmitted on pin 8) that indicates that the local modem is receiving a signal from the remote modem. Also called Data Carrier Detect (DCD) and Received Line Signal Detector (RLSD).
Carrier System - The method of transmitting a number of channels over a single path by modulating each channel on a different carrier frequency at the originating end, then demodulating at the receiving end to return the signals to their original form.
Carrier Wave - The wave upon which a signal is superimposed.
Carterphone Decision - The 1968 FCC decision which held that existing telephone company tariffs containing blanket prohibition against the attachment of customer provided terminal equipment to the telecommunications network were unreasonable, discriminatory and unlawful. The FCC declared the telephone companies could set up reasonable standards for interconnection to insure the technical integrity of the network . Following the Carterphone Decision, the telephone companies filed tariffs for protective connecting arrangements to facilitate the interconnection of customer provided terminal equipment (CPE).
CATV (Community Antenna Television) - Also known as "cable television", CATV is one of the most common data communication facilities found on broadband networks based on radio frequency (RF) transmission and generally using 75-ohm coaxial cable.
C Band - The portion of the electromagnetic spectrum used for satellite and microwave transmission with frequencies of approximately 6/4 GHz (giga hertz).
CCITT (The Consultative Committee International Telegraph and Telephone) - The international advisory committee, established under the United Nations that sets international recommendations which oftentimes are accepted as standards. Membership includes PTT 's, scientific and trade associations and private companies. CCITT operates within the International Telecommunications Union (UTI), a United Nations treaty organization in Geneva, to recommend worldwide standards. Note: The CCITT has been known since 1993 as the ITU-T (International Telecommunications Union - Telecommunications Standards Section. Click here to access the ITU web site.
C Connector - A type of connector used for coaxial cables using a bayonet lock. C is named after Carl Concelman. Contrast with TNC and BNC.
CCS - Hundred-call seconds.
CCTV (Closed Circuit Television) - Two definitions for CCTV:
1) A localized video network limited to specific locations in a building or campus.
2) A LAN environment service often found on broadband networks.
CCU (Communications Control Unit) - A communications computer designed to "off-load" the host processor from tasks involving message handling, protocol control, code conversion, error control and application functions. Very often a minicomputer will serve as a CCU.
CD - See Carrier Detect.
Cells - A subdivision of a mobile telephone service area; containing a low powered radio communications system connected to the local telephone service.
Central Office (CO) - The place or building where communications common carriers terminate customer circuits and locate the switching equipment which interconnects those lines. The CO may also be referred to as an exchange, local central office, end office or central exchange .
Central Processing Unit (CPU) - A device designed to execute programmed instructions, perform the logical and arithmetic functions on data and controls input/output functions.
Centralized - Processing with one CPU, which may support remote terminals/job entry stations.
Centrex - A type of PBX service where incoming calls may be dialed or "switched" direct to extensions without operator assistance.
Centronics, Centronics Interface - The 36-pin, byte wide parallel interface designed by the Centronics Co., that set the de facto standard for connecting printers.
CERT (Character Error Rate Testing) - A method of data line testing using test characters to determine error performance.
Chain - A series of linked processing centers where information must pass through on a "store and forward" basis to get to final destination.
Channel - A method for one way transmission, with several definitions of common usage:
1) In and IBM system, a high speed data link that serves to connect the CPU with its peripheral devices. See Block Multiplexor and Byte Multiplexor channels.
2) In State and Federal tariffs, a path for electrical transmission between two or more points without common carrier provided terminal equipment such as a local connection to DTE . Also called a circuit, path or facility.
Channel Interface - See Channel and Interface.
Channel loopback - A diagnostic test that forms the loop at the multiplexer's channel interface . See loopback definition and the related chart in Section B.
Channel, primary - The higher speed of two channels used for transmitting. Also known as Forward or Main channel. Contrast with reverse channel.
Channel, reverse - The slower speed of two channels used for slow- speed data such as error-detection. Contrast with primary channel .
Channel Service Unit (CSU) - A device required by AT&T in order to connect a DTE to a digital transmission line for DDS . The CSU may be integrated into a Data Service Unit ( DSU).
Channel, Voice grade - A channel suitable for the transmission of speech, digital or analog data, or facsimile, typically with a frequency range of about 300 to 3300 Hz.
Character - Any letter, number, punctuation mark or other sign contained in a message, including characters for control functions and for special symbols. See also Control character.
Character Oriented - Used to describe a communications protocol that carries control information encoded in one or more bytes ( octets). Also known as byte oriented. Contrast with Bit oriented.
Character Parity - A method of adding an overhead bit to a character code to provide error-checking capability.
Characteristic Distortion - A distortion in transmission resulting from transients as a result of modulation. The levels of characteristic distortion depend upon the transmission channel and its qualities.
Character Set - A code, such as ASCII or EBCDIC used to represent data in a computer system. A collection of characters including special symbols and functions.
Check Bit - Also known as a parity bit, one non-information bit that is added to a transmitted character to enable a terminal to check each character it processes.
Checksum - The sum of a group of data provided with the group, for checking purposes.
Circuit - In data communications, a circuit is a communications path between two points. In electronic design, a circuit is defined as one or more components that act together to perform one or more functions.
Circuit Switching - A method of communications in which physical circuits are transferred or switched to complete connections for exclusive use until the connection is released. Contrast with packet-switched networks.
Clock, Clocking - A master timing device used to provide the basic sequencing pulses for the operation of any synchronous transmission.
Clear Channel - Any transmission path where the full bandwidth is available to the user.
Closed Architecture - A computer system so designed, that it is only compatible with software and hardware from one particular vendor. Contrast with open architecture.
Cluster - A group of user terminals located near each other and connected to a single controller. Terminals may consist of printers, keyboard devices, microfiche printers, etc.
Cluster Control Unit - A processor designed to control the communications to form a group or cluster of terminal devices. Also known as " terminal control units", they usually provide polling, control, formatting and error handling functions.
Coaxial Cable (Coax) - A two-conductor wire transmission medium known for its wide bandwidth and negligible susceptibility to crosstalk and distortion, since signals are transmitted in a fully enclosed environment.
Coaxial Converter - A device used in IBM systems to convert the 3270 synchronous protocol to asynchronous protocol for asynchronous devices. Also known as a "protocol converter".
Code - A system of unambiguous rules and symbols for use in representing data, such as ASCII and EBCDIC .
CODEC - A device that codes and de codes analog voice signals into and from digital signals. A neologism of "COder" and "DECoder".
Code Level - The number of bits defined in a code set to represent one character.
Collision - Simultaneous use by two stations of a shared transmission medium in a LAN environment. The result of both DTE's attempting to transmit data simultaneously in a half-duplex environment.
Combined Station - Also known as a balanced station, in High-Level Data Link Control (HDLC) protocol, a station capable of assuming the role of either a primary or secondary station.
Command Port - Also known as the master console, used to control and monitor a network or system.
Common Battery - A DC power source in the central office that supplies power to switching equipment and to subscribers.
Common Carrier - Two definitions exist:
1) In general usage, a common carrier is any supplier in an industry that undertakes to "carry" goods, services or people from one point to another for the public in general or for specified classes of the public.
2) In telecommunications, telephone companies "carry" voice and data services over the telecommunications network . Often PTT 's provide these services to the general public. In the United States, a common carrier company that offers communications to the public is subject to state and federal regulation by the FCC, and the PSC / PUC .
Common Carrier Principle - The regulatory concept that limits the number of companies providing needed services in a particular geographic area.
Communications Act of 1934 - The Congressional Act, passed in 1934 setting a goal of universally available, quality telephone service for a reasonable cost. The Act also established the FCC and transferred all foreign and interstate radio and wire transmission to this Commission. It stipulates that prices and regulations for transmission and equipment service be just, reasonable and not unreasonably discriminatory.
Communications Line Control - A computer hardware device, with related software designed to accept data information and perform the necessary control functions for a data communications line.
Communications Satellite - A microwave repeater, orbiting in a typically geosynchronous orbit, 22,300 miles above the earth, designed to relay signals between communications stations.
Communications Monitor - Mainframe software providing a shared interface between applications programs and communications devices, including communications access methods. See IBM 's CICS.
Compandor - A contraction of the words "COMPressor"and" expANDOR", this device serves to reduce the volume range of signals thereby improving the ratio of signal to interference entering the path between the compressor and expandor. It provides the function of a compressor at one point and an expandor at the receiving point.
Composite - The side of a concentrator or multiplexer that includes all of the multiplexed data.
Composite Link - A link, circuit or line that connects a pair of concentrators or multiplexers. Also, any circuit carrying multiplexed data.
Compression - Two forms of compression are typically available:
1) Analog compression, where the bandwidth for analog transmission is reduced. Also known as compaction .
2) Data compression where the number of bits required to transmit data is reduced. This is performed in a variety of ways, often using a unique code to represent groups of repeated bytes or simply using fewer bits to represent the more frequent characters in the data.
Compressor - A device used to perform analog compression . Contrast with compandor.
Computerized Branch Exchange (CBX) - A PBX using a computer with an electronic switching network.
Concentrator - A device which connects a number of circuits which are not all used at once to a smaller group of circuits for cost effective transmission. The data channel is split into two or more channels of average lower speed while the concentrator dynamically allocates space according to the demand in order to allow the greatest possible throughput. Also known as a statistical multiplexer, ATDM or TDM.
Concurrent - The occurrence of two or more events or activities within the same specified interval of time.
Conditioning - Improving the quality of a leased voice-grade (VG) line so that it meets the standards of data transmission, by adding additional equipment to the network .
Connecting Block - A wire or cable terminating block that provides access to a circuit.
Connection - The physical attachment of a path, or the data transmission path itself. Establishing a transmission path.
Connector - An electrical device for making one or more connections such as RJ11C or EIA RS-232 C, typically with male and female components.
Connect Time, Holding Time - The amount of time a switched circuit is in use.
Contended Access - Contrasted with explicit access, contended access is a shared access method found in Local Area Networks (LAN's) which allows first come, first served access by the stations in the network.
Contention - A first come, first served method of line control in which terminals request to transmit. Transmission proceeds if the channel is free, otherwise the terminal must wait.
Control Character - A non-printing character used to initiate, modify, or stop a control function. LF (line feed) is an example of a control character.
Control Program - A program containing many software routines that would normally have to be put into each individual application program, routines such as console interruptions, error handling and interruptions from a communications terminal.
Control Signal - Any signal that is used for synchronization, remote testing or status. An example would be a control character such as CD ( carrier detect) which is an RS-232 control signal announcing the presence of a carrier.
Control Unit - A hardware device designed to coordinate the sequence of input/output operations of a peripheral device and the CPU, interpreting code d instructions and initiating commands to execute instructions.
Conversational Mode - The online interaction between the user and the computer, with a dialog taking place.
Core - Two definitions exist for this term:
1 - In general, the main storage portion of a mainframe computer system, also referred to as CPU storage. Archaic term.
2 - In data communications, the portion of a fiber optic waveguide through which light is transmit ted, usually 8 to 12 microns in diameter ( single-mode fiber) and from 50 to 200 microns (multimode fiber).
CPE (Customer Provided Equipment) - Any form of telephone equipment including telephone instruments, answering machines, modems, etc. provided solely by the customer but attached to the telephone company lines.
CPI (Computer PABX Interface) - A LAN environment term used to describe a Digital Equipment Corp. technology using T1 transmission involving 56 k bps channels. This is a voice/ data standard representing a move toward an open architecture network.
CPS (Characters Per Second) - A data flow measuring unit where circuits carry bits forming a data character.
CPU -See Central Processing Unit.
CR (Carriage Return) - A control character in either the EBCDIC or ASCII character sets used to position the cursor at the left margin on a CRT terminal or position the printing mechanism at the left margin on a printer.
CRC (Cyclic Redundancy Check) - An error checking control method in which the block check character (BCC) is the remainder after dividing all the serialized bits in a transmitted block by a prespecified binary number.
Crossbar Switch - A switch having numerous vertical and horizontal paths and electromagnetically operated by a mechanical method of interconnecting any one of the vertical paths with any one of the horizontal paths. (Obsolete)
Cross Connect - A hardware device that is used to inter connect multiplexers with line terminating equipment and other multiplexers.
Crosstalk - Any undesirable transfer of energy form one circuit, called the disturbing circuit, to another, called the disturbed circuit.
CRT (Cathode Ray Tube) - A television type screen in some computer terminals used for the visual display of input/output information. CRT is commonly used as a synonym for CRT terminal.
CSMA (Carrier Sense Multiple Access) - A contention-based LAN access method where terminal stations listen prior to transmitting, send a packet of data, then free the line for other stations to transmit . While stations do not transmit until the line is clear, transmission collisions still occur.
CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) - A CSMA protocol using a slotted TDM to minimize a collision reoccurrence. For optimum results, CSMA/CA will work best if the time slot is short as compared to the length of the packet.
CSMA/CD (Carrier Sense Multiple Access with Collision Detection) - A LAN access method where all stations attached to the network listen for transmissions before attempting to transmit. If two or more devices begin transmit ting at the same time, each stops for a random period of time before attempting to re transmit. See also IEEE 802.3 and Ethernet.
CSU (Channel Service Unit) -A digital DCE unit located on the user site to provide a DDS channel for use with the logic and timing recovery circuitry. See DSU.
CTS (Clear To Send) - An RS-232 modem interface control line indicating the attached terminal device ( DTE) may transmit. The CTS signal is sent in reply to the DTE's RTS on pin 5.
CUG (Closed User Group) - A predefined set of terminal users that accept connection requests ( calls) from only user in their group. Often this applies to the sending of messages as well.
Current Loop - A method of connecting and transmit ting signals to Teletype data terminals, where a mark is represented by the presence of current and a space is represented by the absence of current (USA). In other countries, a mark is represented by current in one direction, while a space is represented by current in the other.
Cursor - A rectangular block of light or underline, occasionally blinking, that serves as a character position marker to locate positions on a screen and moved manually by the keyboard.
CPE (Customer Provided Equipment) - Telecommunications or data processing equipment that is provided by the customer and connected to the telephone company facilities.
CVSD (Continuous Variable Slope Delta modulation) - A method of digit ally encoding speech using a one- bit sample to encode the difference between two successive signal levels, usually 32,000 times a second.
CXR Carrier - A data communications signal, similar to CD ( carrier detect) that is used to indicate the intention to transmit data.
Cycle - One iteration or a loop through a set of logical steps.
Cyclic Redundancy Check - See CRC.
D
DAA (Data Access Agreement) - Any DCE approved by a common carrier that allows privately owned terminals to be connected to the common carrier 's network. Modems manufactured today for the public network have built-in DAA's.
DACS (Digital Access and Cross-connect System) - AT&T central office switching equipment which allows T1 carrier or any of the 64 k bps sub channels to be switched or cross connected to another T1 carrier.
Data - The representation of facts, instructions or concepts in a structured manner suitable for communication.
Data Acquisition - A method of recording and measuring data from physical devices.
Database - An organized collection of information.
Data Channel - The data transmission path between two or more stations.
Data Circuit - A telecommunications medium for the transmission of information in analog or digital form.
Data Collection - A procedure where data arriving from several sources, is combined at one location in a file or queue, prior to processing.
Data Communication - The processes, facilities and equipment used to transport encode d information from one point to another.
Data Dictionary - A listing of all the data names and elements in a system.
Data Encryption Standard (DES) - A cryptographic algorithm endorsed by the National Bureau of Standards (NBS) to encrypt data using a 56 bit key.
Data Entry - The inputting of data into a computer system for processing.
Datagram - A capability in a packet-switched network where a complete message may be contained in the data field of a packet, not usually implemented on today's packet data networks (PDN). See minicall .
Data Integrity - The performance of a data communications system, ideally indicating an absence of undetected errors.
Data Link - The physical connection that includes all necessary equipment for two devices to communicate.
Data Link Control -The management of transmitted data over communications circuits using appropriate hardware and related software.
Data Link Layer - The second layer in the OSI model that establishes, maintains and released data link connections between the network layer and physical layer. While the data link layer is not responsible for error correction, it is responsible for error detection, transmission and reception of datagrams, packet reception and local addressing.
Data Mode - The status of a DSU or modem transmitter where the Request To Send and Data Set Ready circuits are prepared to send data.
Data Network - A telecommunications system consisting of a number of terminals able to access each other via communication lines and switching methods.
Data-Over-Voice (DOV) - A technique used in FDM allowing the combination of voice and data on the same line. DOV usually employs twisted pair cables assigning some of the unused bandwidth for data transmission.
Data PBX - A digital transmission circuit switch allowing users to select from a number of circuit paths. Contrast with PBX .
Dataphone - An AT&T trademark identifying the communications equipment furnished by AT&T for data communication service.
Dataphone Digital Service (DDS) -An AT&T private line service for transmit ting data over a digital system. The digital technique allows for more efficient use of the transmission facilities, since no modems are required, resulting in lower error rates and costs than with analog systems. AT&T filed for DDS with the FCC in 1974.
Data Service Unit (DSU) - A device used in conjunction with a digital network, replacing the modem in the sense that the DSU provides remote and local testing, loop equalization and the logic and timing needed to provide a standard EIA or CCITT interface. DSU's usually have an integrated Channel Service Unit (CSU).
Data Set - An AT&T trademark synonymous for modem. See modem.
Dataspeed - An AT&T marketing term used to describe a variety of data communications devices.
Data Stream - The transmission of characters and data bits through a channel.
Data Switch - A device used to connect data processing equipment to network lines, offering flexibility in line /device selection.
Data Terminal Equipment (DTE) - A term used to describe numerous data processing equipment such as computers, terminals, controllers and printers.
Data Transfer Rate, Data Rate - The measure of the speed of data transmission, usually expressed in bits per second. Synonymous with speed, the data rate is often incorrectly expressed in baud .
dB (deciBel) - A unit of measuring the relative strength of a signal parameter such as power, voltage, etc., equal to 1/10 Bel. The number of deciBels is 10X the logarithm of the ratio of the measured quantity of the reference level.
D bit (Delivery Confirmation Bit) - A bit used in CCITT X.25 packet-switched networks to request end-to-end acknowledgement.
dBm - The absolute measure of signal power where 0 dBm is equal to one milliwatt at 1000 Hz terminated by 600 ohms impedance. A level of power measurement in the telephone industry based on 600 ohms impedance and 100 Hz frequency.
DCD (Data Carrier Detect) - See CD.
DCE (Data Communications Equipment) -The device installed on premises to provide the functions needed to establish, maintain and terminate a connection as well as the signal conversion required for communications between the DTE and the telephone line or data circuit . Typically, DCE is a modem.
D-Conditioning - A common carrier service designed to control the harmonic distortion and improve the signal-to-noise ratio. D-conditioning is currently being offered in 9600 bps service with complex modems on voice grade private lines.
DDCMP (Digital Data Communications Message Protocol) - A DEC data communications line protocol.
DDD (Direct Distance Dial) - The North American telephone dial system enabling users to call subscribers outside of their local area without operator assistance. In the United Kingdom and other countries, the service is known as STD (Subscriber Trunk Dialing).
DDS (Dataphone Digital Service) - A digital service offered on private lines and eliminating modems. DDS is offered inter-LATA by AT&T (as an ACCUNET offering) and intra-LATA by BOC's.
DEC (Digital Equipment Corporation) - A major manufacturer of minicomputers and related hardware and software.
Decimal - A digital system that has ten possible states, 0 through 9.
DECnet - A network approach developed by DEC that permits inter connection of DEC computers using DDCMP.
Decentralized - A processing method where intelligence is located at several remote locations of the same processing system.
DeciBel - See dB.
Dedicated Line - Any telecommunications line that is reserved for the exclusive use of a customer and is not dialed. A dedicated line may be either point-to-point or multi point. Also known as a leased line or a private line.
Delay Distortion - Sometimes referred to as "envelope delay", this form of distortion results from different propagation speeds of signals at different frequencies. Within any form of transmission medium, certain frequencies will travel slower than others. The delay distortion is measured in microseconds of delay relative to the delay at 1700 Hz, and can potentially wreck havoc on data transmission.
Delay Equalizer - A device designed to perform corrective action over delay distortion by making the delay substantially constant over a desired frequency range.
Delivery Time - A measurement of time beginning with the start of transmission at the transmit ting terminal to the reception at the receiving terminal.
Demodulation - The opposite of modulation, the process of retrieving data from a carrier signal.
Demodulator - The internal portion of a modem designed to convert the received analog line signals back to digital form.
Demultiplexing - The opposite of multiplexing where a composite signal is broken into is component channels.
Destination - A field in a data message that contains the address of the terminal to which a data message is directed.
Diagnostics, Diagnostic Programs - Programs or procedures used to check equipment, communications links or networks and pinpoint faulty components. Diagnostics may be used by the computer engineer or may be called in by the supervisory programs automatically.
Dial Line - Also known as dial-up line or dial-in line, any communications line that must be dialed. Contrast with Dedicated line .
Dial Network - A term synonymous with public telephone network . See also dial line.
Dibit - A grouping of two bits so that in combination there are four possible values: 00, 01, 10, 11. Each possible value of a dibit is encode d as a unique carrier phase shift (called four- phase modulation).
Differential Phase Shift Keying (DPSK) -The modulation technique used in the Bell 201 modem. Also, see dibit .
Digit, Digital - A discretely variable signal as compared with analog, which is continuously variable. Data signals are coded in discrete and separate pulses.
Digital Data - Any information represented by digital code .
Digital Error - Digital transmission where a 0 signal is mistakenly understood to be a 1 signal.
Digital Loopback - A diagnostic technique for testing the digital circuitry of a communications device, usually a modem . The loop is formed at the modem 's DTE interface, either initiated locally or remotely via communications circuits.
Digital Service Unit (DSU) - See Data Service Unit.
Digital Switching - A process whereby a connection is established and maintained through program control. Digital information is routed between input and output.
Digital Transmission - The direct transmission of discrete pulses as opposed to analog transmission.
DIP (Dual In-line Pins or Dual In-line Package) - an electronic component package or IC (integrated circuit chip) characterized by two rows of connecting pins.
DIP Switch - A switch possessing the physical characteristics of dual in-line packages (DIP).
Direct Current (DC) - Electrical current that travels in only one direction in a circuit, from + to -.
Direct Current Loop (Current Loop) - A method of interfacing teleprinters whereby a digital signal is transmit ted, usually at 20 milliamperes DC .
Direct Distance Dialing - See DDD.
Direct Inward Dialing (DID) - A PBX feature where an external caller is able to call an extension without an operators assistance.
Direct Outward Dialing (DOD) - A PBX feature where an internal caller at an extension is able to call an external number without an operators assistance.
Disconnect Signal - The signal that is transmit ted from one end of a subscriber line or trunk to indicate at the other end that the established connection should be disconnected.
Discrete Access - An access method found in LAN environments using a star topology, wherein each station has its own separate connection to maximize the LAN's switching capabilities.
Distortion - The unwanted changes in signal or signal shape from its true form occurring during transmission between two points, e.g. amplitude distortion, delay distortion.
Distributed Architecture - A LAN using a shared communications medium.
Distribution Frame - A structure designed for terminating wires of a telephone central office, PBX, or private exchange and for allowing easy change of connections via cross-connecting wires.
Distributed Processing - A method of data processing where intelligence is located at several remote sites of the same processing system and connected via a data communications network.
Divestiture - The breakup of AT&T by the federal court based on an antitrust agreement reached between AT&T and the U.S. Department of Justice, effective January 1, 1984.
DMA (Direct Memory Access) - The accessing of computer memory from input/output and peripheral controllers without going through the arithmetic processing unit.
DLC (Data Link Control) - A telecommunications protocol designed to control, set up, perform error checking and terminate the transfer of information between two stations on a data link . Compare with HDLC and SDLC .
DMI (Digital Multiplexed Interface) - An AT&T voice/ data PABX standard designed for using T1 transmission in LAN environments. DMI uses 64 k bps channels which represents moving toward an open architecture environment using ISDN. Compare with CPI.
DNIC (Data Network Identification Code) - A 4-digit PDN code used for identification in a packet-switched network.
DOD (Department of Defense) - One portion of the USA executive government responsible for military concerns, including data communications and also responsible for some LAN related protocols and standards. TCP/IP and some FIPS are under the jurisdiction of the DOD.
Domain - A term used often in IBM SNA to describe a host-based systems services control point (SSCP) and the physical units (PU's), logical units (LU's). links and related resources.
DOS (Disk Operating System) - An operating system designed for microcomputers to instruct a disk-based system to schedule or supervise work, manage resources, and operate and control attached peripherals.
Down-Link - A geosynchronous/geostationary satellite and the signal used to transmit information to an earth station. Contrast with Up-link.
Down Loading, Downline Loading - The process of sending software or related data from a central source to an individual station or personal computer.
Downtime - The period of time when all or part of a computer system or data network is not available to the user community because of a failure. Time required for preventive maintenance is not typically included. See also availability.
DPSK - See differential phase key shifting.
DQM (Data Quality Monitor) - A device used to measure the data bias level above or below a threshold.
Driver - A software module (short for "line driver") under the control of the processor used to control an input/output port to an external device.
Drop Cable, Drop - In LAN environments, the cable that serves to connect a branch to the main, or bus cable, and attaches DTE.
Drop, Subscriber's; Customer's Drop - The line from a telephone cable to a subscriber's building.
DSR (Data Set Ready) - An RS-232 modem interface control signal which indicates to the attached terminal the modem is connected, powered-on, and ready.
DS-0 (Digital Signal level 0) - A standard digital signal or channel operating at 64 kbps.
DS-1 (Digital Signal level 1) - A standard digital signal or channel operating at 1.544 mbps carried on a T1 facility.
DS-1C (Digital Signal level 1C) - A digital signal operating at 3.152 mbps.
DS-2 (Digital Signal level 2) - A digital signal operating at 6.312 mbps carried on a T2 facility.
DSU - See data service unit.
DSX-1 (Digital Signal Cross-Connect level 1) - The set of parameters used where DS-1 digital signals (T-1) are cross-connected.
DTE - See: Data Terminal Equipment.
DTR - An EIA RS-232 interface control singal sent from the Data Terminal on pin 20,indicating to the modem that the terminal is ready for transmission
DTMF (Dual Tone Multi-Frequency) - A term used to describe the audio signaling frequencies where each key generates two audio tones. Contrast with pulse dialing.
Dumb Terminal - Any terminal unable to perform error detection or use a communications protocol, such as a Teletype or Teletype compatible terminal. Usually an asynchronous, ASCII device operating at 9600 bps or higher. Contrast with intelligent terminal.
Duplex - Any communications facility providing transmission in both directions simultaneously, often referred to as "full duplex". Contrast with "half-duplex".
Duplex Transmission - The simultaneous, independent, 2-way transmission of data in both directions. Contrast with "half- duplex transmission".
Duplexing - A process using redundancy, whereby duplicate processors, files or circuitry would be available to enable the system to carry on its work. Providing an alternate system in this way is also called "Tandem".
DUV (Data Under Voice) - The normally unused spectrum at the lower end of a radio channel used to provide a T1 (1.544 m bps) channel for digital services.
E
Earth Station - The transmitter and related antenna located on earth for communication with a satellite.
EBCDIC (Extended Binary Coded Decimal Interchange Code) - An 8- bit character code, standard for many IBM systems offering 256 possible combinations of characters.
Echo - The reflection or return of transmitted data.
Echo Canceler - A device to suppress echo's (similar to an echo suppressor) without speech clipping and able to operate during two-way transmissions.
Echo Check - A method of checking for data transmission errors by returning the received data to the sending end for comparison with the original data.
Echo Distortion - An impairment in telephone lines caused by electrical reflections located at distant points where line impedances are dissimilar.
Echoplex - One method of checking data integrity by returning characters to the sending station for verification. This process requires simulated full duplex operation.
Echo Suppressor - A device used by telephone companies to block the receive side of the line during the time that the transmit side is in use.
ECMA (European Computer Manufacturers Association) - A trade organization and member of the ISO issuing data communications standards. Its membership includes western European computer manufacturers and suppliers.
EEPROM (Electrically Erasable Programmable Read-Only Memory) - A PROM that can be cleared or erased using electrical signals rather than the ultraviolet light required for EPROM.
EFS (Echo Free Seconds) - The measurement, in seconds, of the percentage of time data is transmit ted error free.
EIA (Electronic Industries Association) - A trade association recommending data communication standards, with RS-232 the best known. EIA is comprised of American electronics manufacturing corporations and also contributes to ANSI. See ANSI.
EIA Interface - Data transmission signal characteristics designed with universal standardization for data communication including duration, current and voltage for hardware devices. See also EIA.
Electronic Mail - The delivery of mail or messages, either all or in part via a public or private data communications system.
Electronic Switching System (ESS) - A computerized, digital telephone switching system, manufactured by AT&T, utilizing a stored program to control the switching function. With ESS, custom calling features such as Call Waiting, Call Forwarding and Three-Way calling are available to the subscriber.
EM (End of Medium) - Also known as End of File, the EM character is sometimes used to indicate the physical end of data recorded on a medium .
EMI (Electromagnetic Interference) - A level of undesirable radiation or interference, oftentimes reduced through the use of shielded cables. The FCC has defined acceptance levels for EMI.
Empty Slot Ring - A LAN environment practice whereby an empty packet would circulate through each station in a LAN ring. A single bit within the header of the packet indicates if messages are present and, if so destination and source addresses are also contained within.
Emulation - Designing a device or program to perform or imitate something else, i.e.; IBM 37XX emulation or IBM 3270 emulation.
Encoding, Decoding - Formatting data into a pattern suitable for data communication.
Encryption - A method of data protection whereby a bit stream would be changed to include additional bits, or appear as a random sequence of bits to an unauthorized observer.
End Office - The first central office that a subscribers telephone line is connected to over the access line. Also, the end switching office for a dialed connection.
ENQ (Enquiry) - A control character used in the ASCII code set to request identification status.
EPROM (Erasable Programmable Read Only Memory) - A form of computer memory that is non-volatile but may also be erased via the use of ultraviolet light for reuse. See EEPROM and PROM.
Equalizer - A capacitor or coil like device used by modems to compensate for distortions caused by telephone line conditions.
Equalizer, Adaptive - An equalizer able to change dynamically to compensate for distortion caused by telephone line conditions.
Erlang - A data communications measurement standard for rating traffic and usage.
Error - A term used to describe a deviation from the expected, especially if data integrity is jeopardized.
Error Correction - A method to insure data integrity in received data, performed by retransmission requests to the sending station (source), or by manipulating the received data. See ARQ and FEC.
Error Rate - The measure of data integrity given as the blocks, bits or characters incorrectly received, versus the number transmit ted. Error rate is sometimes seen as a rate of one error every one million bits.
ESS - See electronic switching system.
Essential Facilities - A term used in packet switched environments to define the standard facilities found on all networks . Compare with additional facilities.
Ethernet - The de facto standard LAN of the Xerox corporation and later sponsored also by DEC and Intel Corp. Characterized by 10 m bps baseband transmission using CSMA/CD, Ethernet uses coaxial cable and is similar to the standard LAN recommended by IEEE 802.3.
ETX (End of Text) - A control character preceding a BCC, indicating a message conclusion.
Exchange - One or more central offices and equipment belonging to the telephone company designed to administer communication service to a particular area.
Exchange, Private Automatic (PAX) - A privately operated dial telephone exchange designed to prohibit calls to or from the public telephone network.
Exchange, Private Automatic Branch (PABX) - A user owned (private) automatic telephone exchange that may be a data PABX, voice PABX or voice/data PABX.
Expandor - A transducer-like device with the capability to expand the input voltages for a given range of amplitude.
Explicit Access - Contrasted with contended access, explicit access is a method of shared access found in LAN environments, allowing stations to make use of the network individually for a certain time period. Each station receives a turn, but must also wait for its turn.
Extended Addressing - A facility found in bit-oriented protocols allowing larger addresses to be used. IBM SNA adds two high- order bits to the basic address.
Extended Area Service (EAS) - A common carrier telephone service providing calls to a designated area extending beyond the local exchange .
External Modem - A modem designed to be physically located externally from the terminal device instead of internally as in an integrated modem.
F
Facility - Two definitions exist in networking technology:
1) The computer system capabilities due either to software or hardware.
2) The data communications lines and equipment required to build a circuits and transmission networks.
Fading - Interference from radio transmission signals or microwave communication causing a received signal to deflect from the target.
Far-End Crosstalk - Crosstalk interference occurring in the same direction as the signal. Contrast with near-end crosstalk.
Fast Select - A packet-switched transmission method whereby the user is able to transmit small amounts of data (approx. 128 characters) with the call request packet, instead of transmit ting the data information in packets following the call request packet. This method allows a user to have small amounts of information arrive at the destination quickly.
FAX (Facsimile Terminal) - An image transmission system designed to reproduce the communicated image (such as documents or photographs) on a paper forms.
FCC (Federal Communications Commission) - A board of seven presidential appointees empowered to regulate all USA interstate communications systems as well as all overseas communications originating or terminating in the USA. The FCC was created by the Communications Act of 1934.
FCS (Frame Check Sequence) - A method for error detection in bit-oriented protocols, normally consisting of a 16-bit fields.
FDM (Frequency Division Multiplexing) - A method of multiplexing where a data lines bandwidth is divided into channels and assigning a specific range of frequencies to each channel.
FDX (Full Duplex) - Transmitting data in both directions simultaneously. FDX can occur on either two or four wire circuits.
FE (Format Effecters) - The control characters used to control information displayed on a monitor or printer.
FEC (Forward Error Correction) - The inclusion of additional data contained in a transmit ted block, to be used by the receiver in case of errors.
FEP (Front-End Processor) - A data communications device designed to offload the host processor from the task of message routing between application programs and user terminals, error correction and other communications processing functions.
FEX, FX (Foreign Exchange Service) - A service designed to connect the subscriber's telephone to a remote exchange, providing what appears to be local telephone service. May be a virtual service.
FF (Form Feed) - A control character found in both ASCII and EBCDIC code sets requesting a printer to advance to the top of the next page or form.
Fiber Loss - The weakening of light signal strength in fiber optic transmission.
Fiber Optics, Fiber Optic Cable - A transmission medium using plastic or glass fibers to carry light rays containing information. See optical fibers.
Field - A reserved area of a display monitor for a specific type of information. Also, a component of a data base record.
Figures Shift (FIGS) - A control character used in the Baudot Code to enable the printing of symbols and numbers by actually allowing a physical shift of the carriage. See also Letters Shift.
File - A collection of related data records directed toward some purpose and sequenced in a particular manner.
File Server, File Server protocol - A LAN station or protocol designed to allow application programs to share and store data files.
Filter - The electronic device or devices used to attenuate undesirable signals within frequencies in the transmission circuits and pass through, unchanged, desirable signals within frequencies.
FIPS (Federal Information Processing Standard) - A US government approved standard for computer processing and data communications.
Firmware - The software designed within hardware devices, usually permanently stored in a ROM or PROM chip. Firmware may also be temporarily stored within an EEPROM or EPROM chip, as well.
Fixed Loss Loop (FLL) - An FCC classification requirement limiting the output of a modem to 4 db if the modem is programmable. See permissive device (modem).
Flag - A field used in bit-oriented protocols where a character or bit field would be used to separate the data on either side of the flag.
Flat Rate Service - A telephone service available in certain geographic areas, entitling the subscriber to an unlimited number of calls within a predefined area for a fixed rate.
Flow Control - A method of preventing the loss of data whereby the transfer of messages or characters to a receiving device would be controlled via the use of a control character (s) such as X-ON (transmit on) or X-OFF (transmit off), allowing the receiving devices buffer to drain before accepting more data.
FM (Frequency Modulation) - One of three different methods of transmit ting digital information on an analog line. FM changes the carrier frequency to different values. Also see AM and PM.
Foreign Attachment - Any equipment not owned or provided by the telephone company, but attached to the telephone companies lines. See also CPE.
Foreign Exchange Service - See FEX.
Format - A method for structuring transmitted data whereby the positioning of information and related control data would insure identification at the receiving device.
FORTRAN - A high-level programming language (named for its purpose as a FORmula TRANslator), commonly used to create mathematical and scientific programs.
Forward Error Correction - See FEC.
Four-Wire Circuit, Four-Wire Line - A data communications circuit or line using 2-pairs of conductors, one pair for the transmitting channel and the other pair for the receiving channel.
Four-Wire Equivalent Circuit, Four Wire Equivalent Line - Accomplishing the same method of transmission as described above but using only one pair of conductors. Transmitting and receiving channels are "isolated" on the circuit by using different carrier frequencies for the two channels.
Fox Message - A test pattern or message often repeated continuously during diagnostic testing for problem isolation. Example: "THE QUICK BROWN FOX JUMPED OVER THE LAZY DOGS 1234567890".
Frame - Several definitions appear in common data usage:
1 - The overhead control characters used to surround the data in an information or text frame.
2 - On a T-1 circuit, a frame refers to the 24 bytes plus one framing bit (a total of 193 bits).
3 - The sequence of bits and bytes in a transmitted block.
Frame Check Sequence - See FCS and CRC.
Framing - A control method used with TDM digital channels using control bits to identify channels such as the formatted version of T1.
Frequency - The number of times that a sine wave would repeat, or cycle in one second.
Frequency Range - The lowest to the highest frequency that could be transmit ted over a band.
FSK (Frequency Shift Keying) - A method of frequency modulation (FM) where one frequency would represent a mark (one) and another frequency would represent a space (zero).
Full Duplex - See FDX.
Foreign Exchange - See FEX.
G
Gain - A deciBel (dB) measurement of amplitude. The gain is amplified whenever the signal passes through a repeater, antenna or amplifier.
Gain Hits - Any form of undesirable signal surge resulting in the possibility of corrupted data. AT&T standards suggest a maximum permissible threshold of less than eight gain hits within a fifteen minute period.
Garbage - A slang term often used to describe corrupted data .
Gateway - A network station designed to interconnect two otherwise incompatible devices or networks. Occasionally, a gateway may perform protocol conversion and packet assembly/disassembly ( PAD) functions. Gateways operate at the fourth through seventh layers of the OSI model. Contrast with bridge .
Gaussian Noise - A line noise whose amplitude is characterized by the Gaussian distribution such as white noise, ambient noise or hiss.
General Switched Telephone Network (GSTN) - Same as public telephone network (PTN).
Geosynchronous, Geostationary - The path of an orbiting communications satellite at the correct speed and distance over the earth so as to appear stationary as the earth rotates.
GFI (Group Format Identifier) - The first 4 bits in a packet header ( X.25 packet-switched networks) containing the D bit, Q bit and modulus value.
gHz (Gigahertz) - A measurement of frequency equal to 10 to 9th power.
Grade of Service - A percentage measurement of incomplete, delayed or blocked calls.
Ground - The electrical common conductor.
Ground Start - A method of signaling designed to detect that a circuit is grounded at the far end.
Ground Station - Also known as an earth station, a ground station is designed to send (transmit) and receive signals to and from a communications satellite.
Group Addressing - Any address that is shared by two or more devices or stations.
Group Channel - An organization on telephone carrier systems, whereby a full group is a channel equivalent to 12 voice grade channels at 4000 Hz for a total of 48 kHz . A half-group has the equivalent bandwidth of six voice grade channels (24 kHz). By not subdividing into voice grade facilities, group channels can be used for high-speed data communication . See also wideband.
Guard Band, Guard Frequency - Two definitions are commonly found in data communications:
1 - The unused frequencies between sub channels in FDM systems used to separate channels, thereby preventing crosstalk.
2 - A single carrier tone used to indicate that a communications line is prepared to transmit data.
H
Half-Duplex Transmission (HDX) - The common use definition is a circuit designed for transmission in either direction but not both directions simultaneously. Contrast with FDX, full-duplex transmission.
Hamming Code - A method of forward error correction (FEC) named for its inventor and designed to detect and correct a single bit received in error.
Handset - The part of a telephone that contains both receiver and mouthpiece.
Handshaking, Handshake - Communications line interplay used to establish a data path via the exchange of predetermined signals, usually performed by communications protocol or modems.
Hardcopy - The printed output of a computer, in readable form.
Harmonic - The frequencies that combine as multiples of some basic or fundamental frequency.
Harmonic Distortion - An impairment of a transmission line caused by erroneous frequency generations along the line.
Hardware - The physical equipment that comprise a computer system, including mechanical, or electromechanical devices, as opposed to the computer program. Contrast with software .
Hardware Interface - Physical hardware used in the inter connection of computer/data terminal devices and modems.
Hardwired - The permanent connection of data communications links, lines or cables and related devices.
Hardwired FEP - A non-programmable front-end processor ( FEP).
HASP (Houston Automatic Spooling Priority) - Also known as Job Entry Subsystem (JES), HASP is a control program adopted by IBM for the transmission of jobs to computers and the control of devices and data lines.
HDLC (High-level Data Link Control) - A bit protocol developed by the ISO to be the international standard communications protocol, similar to IBM 's SDLC.
Head-End Unit - A LAN environment hardware device found on a broadband network that uses separate frequencies for multiple services.
Header - The control block or blocks of data added prior to the actual message, either a packet or a transmission block .
Hertz (Hz) - The same as cycles per second, used as a measurement of bandwidth or frequency.
Hexadecimal - A base 16 numbering system representing the states as 0 through 9 followed by A through F. Any eight-bit byte can be represented by two hexadecimal digits.
Hierarchical Network Structure - A network plan whereby all functions are categorized into specific areas or layers, each having a specific role.
Hierarchical Switching - Used to describe a switching methodology used in LAN 's where the switching is done in stages. In a star topology, this is called star switching.
High Frequency (HF) - Any frequency level that falls in between 3 and 30 mHz.
High Pass - A predetermined filter frequency level, above which all frequencies may pass. Contrast with low pass .
High Performance Option (HPO) - An alternate means of conditioning a communication circuit, similar to D1 conditioning.
Hit, Line Hits - A slang term or expression to describe line noise or other forms of interference causing data communications line failure or corrupted data.
Holding Time - The time it takes to establish a telephone connection. Factors include telephone equipment, busy periods, etc.
Horizontal Redundancy Checking (HRC) - A method for data error checking whereby redundant information would be included in the data to be checked. Contrast with LRC ( Longitudinal Redundancy Checking).
Host, Host Computer - The central or controlling computer in a data communications network, usually providing database access, programming languages, etc.
Hub - A DDS office designed to multiplex T1 data streams from a number of local offices into signals suitable for transmission.
Hybrid - Any combination of two or more technologies. For example, DDCMP is a hybrid character/bit data communications protocol.
Hyper Group - See also jumbo group.
Hz - See Hertz.
I
IBM (International Business Machines Corp.) - One of the leading computer hardware and software manufacturing companies in the world, having many de facto standards such as the IBM PC and 3270 terminals.
IC (Integrated Circuit) - Also known as a microchip, and as a semiconductor device, an IC performs numerous functions in data processing such as processing, data storage and program storage. See also PROM, EEPROM, EPROM, ROM and RAM.
IDF (Intermediate Distribution Frame) - A connecting frame or such device used to connect data communications equipment by the use of connecting blocks.
Idle Character - Similar to SYN character and NUL character.
IEC (Interexchange Carrier) - An FCC licensed common carrier permitted to carry subscribers transmissions inter- LATA, or if approved by a PUC or PSC, intrastate.
IEEE (Institute of Electrical and Electronic Engineers) - An international institute that issues its own standards and is a member of ANSI and ISO, perhaps best known for its development of IEEE Project 802. Click here to visit the IEEE web site.
IEEE Project 802 - The IEEE development team credited with the creation of the 802 series of local area network (LAN) standards.
IEEE 802.2 - A LAN standard for the data link layer used with other Project 802 standards such as IEEE 802.3, IEEE 802.4, IEEE 802.5. See also: ISO.
IEEE 802.3 - A LAN standard for the physical layer using the CSMA/CD access method, similar to Ethernet, within a bus topology. See also: ISO.
IEEE 802.4 - A LAN standard for the physical layer, using a token- passing access method within a bus topology and functioning similarly to MAP. See also: ISO.
IEEE 802.5 - A LAN standard for the physical layer, using the token ring or token passing access method on a ring topology.
IEEE 488 - An IEEE standard parallel interface, oftentimes used to connect data communications equipment using connecting blocks.
Impedance - The signal effects due to a varying current, resistance capacitance and inductance. Line impedance (ac resistance), if increased via inductance will allow a greater amount of power to be transmit ted with less current, but at a higher voltage.
Impulse Noise - A communications line interference caused by electrical action, lightning, on/off movement of switching equipment, etc., and appearing as high amplitude and short duration.
Impulse Hits - Also known commonly as "spikes", impulse hits are errors of impulse noise adversely affecting data communication. AT&T recommends no greater than 15 impulse hits per 15 minute period.
IMS/VS (Information Management System/Virtual Storage) - An IBM software product designed for both batch processing and data communications-based transaction processing.
Induction, Inductance - A method of loading a circuit or line to help reduce attenuation at voice frequencies (VF).
Information - Raw data that has been organized into a meaningful context for a computer user.
Infrared - A method of data transmission using infrared light to transmit data on fiber optic medium or open-air transmission over short distances.
Information Bit - A data bit, having value as a component of a message or text, as opposed to an overhead bit used for addressing or error control.
Input/Output (I/O) - Moving data between a peripheral device and the CPU, or simply a term used to describe a type of peripheral device.
Insertion Loss -The loss of signal power due to the connection of communications units possessing different impedance values.
Integrated Circuits - A complex electronic circuit providing all the capability of circuits containing resistors, diodes, capacitors, etc., and remaining functionally equivalent without these components.
Integrated Services Digital Network (ISDN) - A CCITT standard for digital communication systems designed to transmit voice, data, video and other digital communication having major effects on the design of multiplexers and other communication equipment.
Intel - A microchip manufacturer and one of the sponsors along with Xerox Corp., of Ethernet.
Intelligent, Intelligence - A term used to describe a microprocessor controlled device able to perform sophisticated tasks relying on software, and therefore programmable.
Intelligent Port Selector - See data PBX.
Intelligent TDM - See concentrator.
Intelligent Terminal - A microprocessor controlled terminal able to perform sophisticated tasks relying on software, and therefore programmable.
Interchange Circuit - A circuit with an associated interface connector having designated connector pins for data, timing or control functions.
Interconnect Company - A company that provides communications terminal equipment to be connected to telephone lines.
Interconnect Industry - The industry involved in the design, sales and service of equipment designed to connect to telephone lines.
Interface - Two related definitions exist in common computer usage:
1) A common boundary or connection defined by identical signal and physical inter connection characteristics and their meanings.
2) The necessary equipment needed to provide a shared boundary connection.
Interface Processor - A specialized communications device used for the purpose of interfacing computers and terminals to a network.
Interference - The undesirable noise or disturbance that occurs on a communications channel, caused by natural or man-made signals.
Intermodulation Distortion - A form of distortion created by two analog frequencies whose amplitude causes a third erroneous frequency, which corrupts the data signal representation.
International Access Code - The prefix digits required for placing an overseas telephone call.
International Record Carrier (IRC) - A common carrier providing international telephone channels for voice or data.
International Telecommunications Union (ITU) - The United Nations agency organized to establish standardized telecommunications procedures, regarding frequency allocation and radio regulations world-wide.
Internetwork Router - A Local Area Network (LAN) device used for communications between sub networks, transmit ting only the messages for the correct sub network. Inter network routers function within the network layer of the OSI model.
Interoffice Trunk - The telephone channel between two local central offices.
Interrupt - A device generated signal notifying the CPU that it needs its assistance. Such a signal may arise from either software or hardware and is usually task oriented.
Interrupting Equipment -The devices used to break the ringing generator's output into ringing and silent periods and creating the busy and ringback tone pulses, etc.
Intertoll Trunk - The telephone channel between toll offices in a different telephone exchange.
Intra-office Trunk - The trunk or path connection within the same central office.
ISDN - See Integrated Services Digital Network.
ISO (International Standards Organization) - The International, voluntary standards organization, closely aligned with the CCITT perhaps best noted for its OSI model and OSI communications protocol. Membership includes other international organizations issuing standards, with ANSI being the American representative. Click here to access the ISO web site.
Isochronous - A form of data transmission in which the transmitter would use a synchronous clock to send messages, while the receiving device does not, and detects messages by framing with start/stop bits, similar to asynchronous transmission.
ISU (Integrated Service Unit) - One device designed to combine the functions of both a DSU and a CSU.
ITDM (Intelligent Time Division Multiplexer) - A device, also known as a statistical multiplexer designed to assign time slots on demand, rather than on a fixed sub channel basis.
ITI (Interactive Terminal Interface) - A term used in packet- switched networks used to define a PAD supporting network access by asynchronous terminals.
IVDT (Integrated Voice and Data Terminal) - A dual function device incorporating both a terminal keyboard/display and voice telephone.
J
Jabber, Jabbering - The flow of continuously sent garbage from a failed terminal, resulting in a lockup of a LAN for other users.
Jack - Either a three-conductor or a "mini-plug", RJ11 type receptacle designed to accept the corresponding plug for an electrical connection.
Jamming - The disturbance or interference of open-air radio transmission to deliberately prevent communication.
JES (Job Entry Subsystem) - The control program and procedure for directing host processing of a job or series of jobs and related tasks in an IBM host environment.
Jitter - One form of line distortion caused when a transmit ted signal deviates from its reference timing position, causing errors, especially in high speed transmission.
Job - A set of data, including programs, files and instructions to a computer known as Job Control Language (JCL, in the IBM realm).
Jumbo Group - The highest frequency division multiplexing (FDM) carrier system multiplexing level that contains 3600 voice frequency (VF) or telephone channels in six master groups and also known as hyper group.
Jumper, Jumper wire - The wire/wires used to cross-connect circuits for testing or diagnostic purposes.
K
Ka Band - The frequencies used for satellite communications, approximately in the 30/15 gHz range.
kbps (Kilo Bits Per Second) - Thousands of bits per second (BPS) equal to 10 to the third power. See also BPS.
Keying - A method of encoding data by modulating the carrier either by phase or frequency.
kHz (kilo Hertz) - Abbreviation for 1000 Hertz (cycles per second). See also Hertz.
KSR (Keyboard Send and Receive) - A teleprinter transmitter and receiver that can only transmit from the keyboard. Contrast with RO and ASR devices.
Ku Band - The frequencies used for satellite communications, approximately in the 14/12 gHz range.
L
LAD (Local Area Data) Channel - See Bell 43401.
LADT (Local Area Data Transport) - A common-carrier offered communication service that transmits both voice and 4800 bps data simultaneously over the same telephone line.
LAN (Local Area Network) - The data communications facilities used to provide communications within a limited geographical area, normally up to 6 miles, using medium to high data rates between 9600 bps to 100 mbps. LAN 's may have bridges or gateways to other networks but are usually confined to a building or cluster or buildings, frequently referred to as a "campus".
LAP (Line Access Procedure) - A term used in packet-switched networks to define the data-link layer level protocol specified in the CCITT X.25 interface standard, superseded by LAPB.
LAPB (Line Access Procedure, Balanced) - A term used in packet- switched networks to define a link initialization procedure designed to establish and maintain communications between the DTE and DCE. All PDN's currently support LAPB, which involves the T1 timer and N2 count parameters.
LASER (Light Amplification by Simulated Emission of Radiation) - A fiber optic method of data transmission using very high frequency beams of light with great information carrying capacity.
LATA (Local Access and Transport Area) - A divestiture related subdivision that resulted in approximately 184 local telephone serving areas in the United States. The areas of LATA 's are designated primarily by demographics, and are distinguished separately from long-distance service.
Latency - The waiting time, or delay between a stations request to a transmission channel, and completion.
Layer - A term used to define one level of a hierarchy of functions, as specified by the OSI reference model. Divisions of other protocols, such as IBM 's SNA, sometimes will correspond to one or more OSI layers.
L Band - The microwave transmission frequencies approximately in the 1 gHz range.
Leased Line, Private Line, Dedicated Line - A point-to-point or multipoint communications line for voice/data leased from a common-carrier, usually on a monthly basis.
LED (Light Emitting Diode) - An electrical component, offering greater reliability than an ordinary bulb, used to indicate status information. LED's are also used as a fiber optic transmission source.
Letters Shift - A control character used in the Baudot Code to enable the printing of alphabetic characters. See also figures shift.
Light Wave - Fiber optic transmission using very high frequency light beams.
Line - A multipoint or point-to-point communications medium .
Line Discipline - An archaic term for line protocol.
Line Driver, Local Dataset - A DCE device that functions as a signal converter, "driving" the digital signal directly across a communications path. Oftentimes, line drivers are used to ensure reliable transmission beyond the 50-foot RS-232 limit. Also known as short-haul modems.
Line Hit - An occurrence of noise and other forms of electrical interference on a transmission circuit causing data to become corrupted.
Line Printer - An output device designed to print an entire line of data as a unit. Contrast with character printer.
Line Protocol - A communications program designed to perform data transmission functions over a network.
Line Speed -The maximum, reliable, rate of transmission, dependent upon the capability of a modem, expressed in bps.
Line Switching - Switching, whereby a circuit is established between sending and receiving lines. Contrast with message switching.
Line Turnaround -The reversal of direction in a half-duplex communications circuit (HDX).
Link, Data Link - A point-to-point circuit, functioning as one component of a network.
Link Layer - See data link layer.
Loading - The practice of adding loading coils to add inductance to a communications circuit to reduce amplitude distortion.
Loading Coil - Also known as an induction coil, a loading coil is normally used to compensate for wire capacitance and serves to boost the level on voice-grade frequencies for circuits exceeding approximately 18,000 feet.
Local Analog Loopback - Performing an analog loopback at the line side of the local modem, in order to test the local modems analog output circuitry. See analog loopback.
Local Attachment - Connecting a control unit or peripheral device directly to an IBM host channel.
Local Central Office, Local Exchange - See End Office.
Local Channel Loopback - Performing a channel loopback at the input or channel side to the local concentrator/multiplexer for diagnostic testing. See channel loopback.
Local Composite Loopback - Performing a composite loopback at the output or composite end of the local concentrator/multiplexer for diagnostic testing. See composite loopback.
Local Area Dataset - A device designed to condition an RS-232 signal over a dc continuous metallic circuit without interfering with the adjacent pairs in the same telephone cable. Local datasets normally conform with Bell 43401 standards. See also line driver.
Local Digital Loopback - Performing a digital loopback at the DTE side of the local modem. See digital loopback.
Logical Channel, Logical Connection - See virtual circuit.
Logical Channel Number - A number assigned in packet-switched networks whenever a virtual call is placed. Up to 4095 separate logical channels may exist on a single link.
Logical Group, Logical Group Number - The process of dividing logical channels in packet-switched networks into one of 16 logical groups.
Longitudinal Currents - Current flowing in the same direction in the two conductors of a cable pair or pair of wires.
Loop, Local Loop - The closed loop circuit formed by the users device/devices and the cable pair and other conductors that connect it to the central office switching equipment.
Loopback Test, Loopback - One of several diagnostic tests used to evaluate and compare the signal transmit ted with the returned signal for use in determining the operational status of the equipment and related transmission paths.
Loop Start - A method to indicate an off-hook condition between an analog telephone set and a switch. By picking up the receiver, the loop will close and will allow dc current to flow.
Loss - A reduction in a signals level as expressed in deciBels. See also attenuation.
Low Frequency (LF) - Frequencies occurring approximately in the 30 to 300 kHz range.
Low Pass - A filtering device designed to allow frequencies below a predetermined level to pass. Contrast with high pass .
LRC (Longitudinal Redundancy Check) - A method for error detection, often called horizontal parity checking, whereby the BCC consists of bits based on either odd or even parity for all the characters in the transmission block.
LTRS (Letters Shift) - A code character used to command a Teletype - like device to perform a physical shift for alphabetic characters . See also figures shift.
M
MAC (Media Access Control) - An access control protocol defined under IEEE 802 which includes variations for the token ring, token bus and CSMA/CD.
Magnetic Medium - Any medium designed for data storage using magnetic pulses to record information, such as magnetic tape, diskettes or disks.
Mainframe - A large-scale computer, capable of processing large amounts of data with very fast processing, including control units and peripheral devices supplied by one vendor, examples of which would include IBM, Unisys, Control Data, and others. Often, mainframe systems will have a closed architecture.
Main Network Address - A term used in IBM 's SNA to define the logical unit (LU), network address, within VTAM .
Manchester Code, Manchester Encoding - A binary signaling mechanism in which each bit period is divided into two complementary halves, combining data and clock pulses.
MAP (Manufacturing Automation Protocol) - A General Motors Corp. token-passing bus designed for factory environments that appears very similar to IEEE 802.4.
Marginal Relay - A relay designed to operate only on a specified current flow which is greater than the current normally flowing in the circuit.
Mark - Three definitions exist in normal usage:
1) In telegraph communications, a mark indicates the closed, current -flow condition.
2) When used in data communication, a mark indicates a no-traffic state for asynchronous transmission, a binary 1.
3) A mark may also indicate the idle condition, contrast with space. See mark-hold.
Mark-Hold - Transmitting a steady mark to indicate the normal no- traffic line condition.
Mark-to-Space Transmission - Switching from a marking impulse to a spacing impulse.
MASER (Microwave Amplification by Simulated Emission of Radiation) - A device designed to generate a microwave signal with low-noise properties.
Master Clock - The timing signal or signal mechanism used by all stations in a network for synchronization.
Master Group (MG) - A term used in Frequency Division Multiplexing ( FDM) whereby an assembly of 10 supergroups (600 voice frequency channel s) would occupy adjacent bands in the transmission spectrum to provide simultaneous modulation and demodulation.
Master Station - Three definitions exist normal usage:
1) A monitor station, used in LAN token-passing ring environments allowing recovery from error conditions, such a busy, duplicate or lost tokens.
2) The main unit that controls and polls the nodes in a multipoint circuit.
3) The unit that controls the slave station in a point-to-point circuit.
Matrix - Two common definitions exist in normal usage:
1) A rectangular array