A | B | C | D | E | F, G | H, I | J, K, L | M | N, O, P, Q | R | S | T, U, V, W, X, Y, Z


Acoustic Coupler - A device that converts digital signals into audio signals, enabling data to be transmitted over the telephone lines via a conventional telephone.

Active/Passive Device - A device capable of supplying the current for the loop (active) and a device that must draw its current from connected equipment (passive).

Address - A unique sequence of letters or numbers for the location of data or the identity of an intelligent device.

ANSI (American National Standards Institute) - The principal standards development body supported by over 1000 trade organizations, professional societies and companies. U.S.A.'s member body to ISO (International Standards Organization.

ASCII (American Standard Code for Information Interchange) - A seven-bit-plus parity code established by ANSI to achieve compatibility between data services.

Asynchronous Transmission - Transmission is controlled by start and stop bits at the beginning and end of each character.


Bandwidth - The range of frequencies available for signalling; the difference expressed in Hertz between the highest and lowest frequencies of a band.

Baud - Unit of signalling speed. The speed in baud is the number of discrete conditions or events per second.

Bell - AT&T standards for devices that transmit over telephone lines; e.g. modems.

Bell 212 - An AT&T modem- providing, full duplex, asynchronous, 1,200 bps data transmission for use on the telephone network.

Bisynchronous Transmission (BSC) - A byte or character- oriented IBM communications protocol that has become the industry standard. It uses a defined set of control characters for synchronized transmission of binary coded data between stations in a data communications system.

Bit (Binary Digit) - The smallest unit of information in a binary system; a 1 or 0 condition.

Block Check Character - Used to check transmission accuracy, a character transmitted by the sender after each message block and compared with a block check character computed by the receiver.

BPS (Bits Per Second) - Unit of data transmission rate.

Break- Out Box (BOB) - A testing device that permits the user to test line conditions.

Buffer - A temporary-storage device used to compensate for a difference in data rate and data flow between two devices (typically a computer and a printer); also called a spooler.

Bus - A data path shared by many devices (e.g. multipoint line) with one or more conductors for transmitting signals, data or power.

Byte - A binary element string functioning as a unit. Eight-bit bytes are most common. Also called a "character".


Carrier Detect - Same as Received Line Signal Detector. AN RS-232 modem signal that indicates to an attached terminal that the modem is receiving a signal from a remote modem.

CCITT (Consultative Committee International Telegraph and Telephone) - An international association that sets worldwide communications standards (e.g. V.21, V.22, V.22bis).

Character - Letter, numerical, punctuation or any other symbol contained in a message.

Clamping Voltages - The "sustained" voltage held by a clamp circuit at some desired level.

Clear To Send (CTS) - Modem interface signal that indicates to the DTE device to begin transmission.

Clock - Timing signals used in synchronous transmission. More generally: the source(s) of timing signals sequencing electronic events.

Communication Protocol - The rules governing the exchange of information between devices on a data link.

Composite Line - The line or circuit connecting a pair of multiplexors or concentrators.

Concentrator (Statistical Multiplexor) - A device used to divide a data channel into two or more channels of lower average speed, dynamically allocating channel space according to demand in order to maximize throughput.

Conditioning - The addition of equipment to a leased voice-grade channel, enabling the channel to meet specifications for data transmission.

Contention - The facility provided by the dial network or a data PABX which allows multiple terminals to compete on a first come, first-served basis for a number of computer ports.

CPU (Central Processing Unit) - Portion of a computer that directs the sequences of operations and initiates the proper commands to the computer for execution.

CRC (Cycling Redundancy Check) - An error detection scheme in which the block check character is the remainder after dividing all the serialized bits in a transmission block by a predetermined binary number.

Crossed Pinning - Configuration that allows two DTE devices or two DCE devices to communicate.

Crossover - Conductor which runs through the cable and connects to a different pin number at each end.

Crosstalk - The unwanted transmission of a signal on a channel that interfaces with another adjacent channel.

CRT (Cathode Ray Tube) - A television-like picture tube used in a terminal.

CTS (Clear to Send) - An RS-232 modem interface control signal which indicates that the attached DTE device is ready for transmission.


DCE (Data Communications Equipment) - Devices that provide the functions required to establish, maintain and terminate a data transmission connection; e.g. a modem.

DDS (Dataphone Digital Service) - An AT&T communications service in which data is transmitted in digital rather than analog form.

Demodulation - The process of retrieving digital (computer) data from a modulated analog (telephone) signal.

Dial Network - A network that is shared among many users, any one of whom can establish communication between desired points, when required, by use of a dial or push button telephone.

Digital Data - Information transmitted in a coded form (from a computer represented by discrete signal elements.

Dip Switches - Switches for opening and closing leads between two devices.

Downloading - The process of sending configuration parameters, operating software or related data from a central source to remote stations.

DSR (Data Set Ready) - An RS-232 modem interface control signal which indicates that the modem is ready for transmission.

DSU (Digital Service Unit) - The interface between a user's data DTE device and a digital data service.

DTE (Data Terminal Equipment) - Devices acting as data source, data sink, or both (e.g. a terminal).

DTMF (Data Tone Multiple Frequency) - The audio signalling frequency on touchtone, push button telephones.

DTR (Data Terminal Ready) - An RS-232 modem interface control signal which indicates to the modem that the terminals are ready for transmission.

Dumb Terminal - Both hard-copy and VDT type ASCII asynchrounous terminals that do not use a data transmission protocol and usually send data one character at a time.


EBCDIC (Extended Binary Coded Decimal Interchange Code) - An eight-bit code used primarily on IBM equipment. The code allows for 256 characters to the sending station for verification of data integrity.

EIA (Electronic Industries Association) - A standards organization in the USA specializing in the electrical and functional characteristics of interface equipment.

EMI/RFI (Electromagnetic Interface/Radio Frequency Interference) Filtering - Protection from "background noise" that could alter or destroy data transmission.

Emulation - The imitation of a computer system, performed by a combination of hardware and software, that allows programs to run between incompatible systems.

Eprom - Read-only, non-volatile, semiconductor memory that is erasable via ultra violet light and reprogrammable.

Even Parity - A "dumb" terminal data verification method in which each character must have an even number of "on" bits.

F, G

FCC - Federal Communications Commission.

Firmware - A computer program or software stored permanently in Prom or Rom.

Flow Control - The procedure for regulating the flow of data between two devices; prevents the loss of data once a device's buffer has reached its capacity.

Frequency-Division Multiplexor (FDM) - A device that divides the available transmission frequency range into narrower banks, each of which is for a separate channel.

Frequency Shift Keying (FSK) - A frequency modulation technique in which one frequency represents a mark and a second, a space.

Full Duplex (FDX) - Transmission in either direction, at the same time.

H, I

Half Duplex (HDX) - Transmission in either direction, but not at the same time.

Handshaking - Exchange of predetermined signals between two devices establishing a connection.

Hertz (Hz) - A measure of frequency or bandwidth. The same as cycles per second.

Hexadecimal Number System - The number system with the base of sixteen. In hexadecimal, the first ten digits are 0-9 and the last six digits are the letters A-F.

IEEE (Institute of Electrical and Electronic Engineers) - An international professional society that issues its own standards and is a member of ANSI and ISO.

Interface - Signal characteristics and meanings of interchanged signals.

J, K, L

Joule - A unit of energy expended when a force of 1 newton (unit of measurement) moves the point of application 1 meter in the direction of the force. (1 watt = 1 joule/sec.)

Jumper - A wire which connects a number of pins on one end of a cable only, such a looping back Request to Send (RTS) from Clear to Send (CTS).

LDM (Limited Distance Modem) - A signal converter which contains and boosts a digital signal so it may be transmitted much further than a standard RS- 232 signal.

LED (Light Emitting Diode) - A semiconductor light source that emits visible light or invisible infrared radiation.

Leased Line - A telephone reserved for the exclusive use of leasing customers. Also called a Private Line.

Line Driver - A signal converter that conditions a digital signal to ensure reliable transmission over an extended distance.

Line TurnaroundM - The reversing of transmission direction from sender to receiver or vice versa when using a half-duplex circuit.

Local Area Network (LAN) - A data communications system confined to a limited geographic area with moderate to high data rates (100 Kbps to 50Mbps). The area served may consist of a single building, a cluster of buildings or a campus-type arrangement. The network uses some type of switching technology, and does not use common carrier circuits - although it may have gateways or bridges to other public or private networks.

Loopback - Type of diagnostic test in which the transmitted signal is returned to the sending device after passing through all, or a portion of, a data communications link or network.


Mainframe - A large-scale computer system that can house comprehensive software and several peripherals.

Menu - The list of available software functions for selection by the operator, displayed on the computer screen.

Modem (Modulator-Demodulator) - A device used to convert serial digital data from a transmitting terminal to a signal for transmission over a telephone channel, or to reconvert the transmitted signal to serial digital data for acceptance by a receiving terminal.

Modem Eliminator - A device used to connect a local terminal and a computer port in lieu of the pair of modems that they would ordinarily connect to.

Modulation - Modifying some characteristics of a wave form.

Multidrop Line - A single communications circuit that interconnects many stations, each of which contains terminal devices.

Multiplexor - A device used for division of a transmission facility into two or more subchannels, either by splitting the frequency band into narrower bands (frequency division) or by allotting a common channel to several different transmitting devices one at a time (time division).

Multipoint Line - A single communications line or circuit interconnecting several stations; usually requires a polling mechanism to address each connected terminal.

N, O, P, Q

Node - A point of interconnection to a network. Normally, a point at which a number of terminals are located.

Null Modem - A device that connects two DTE devices directly by emulating the physical connections of a DCE device.

Packet - A group of bits transmitted as a whole on a packet- switched network.

Parallel Transmission - Transmission mode that sends a number of bits simultaneous over separate lines. Usually unidirectional.

Parity Bit - A bit that is set at "0" or "1" in a character to ensure that the total number of 1 bits in the data field is even or odd.

Parity Check - The addition of noninformation bits that make up a transmission block to ensure tat the total number of 1's is always even or odd.

Point-to-Point (Link) - A connection between two, and only two, pieces of equipment. Polling - A means of controlling devices on a multipoint line.

Private Line - A telephone line tat does not go through the central office and is reserved for exclusive use of a single customer.

Protocol - A formal set of conventions governing the formatting and timing of message exchange between two communicating systems.

Pulse Dialing - Older form of phone dialing, utilizing breaks in DC current to indicate the number being dialed.


RAM (Random Access Memory) - Semiconductor read-write volatile memory. Data stored is lost if power is turned off.

Request-to-Send (RTS) - An RS-232 modem interface signal which indicates that the DTE has data to transmit.

ROM (Read-Only Memory) - Nonvolatile semiconductor memory manufactured with predefined data content, permanently stored.

RS-232 - Interface between data terminal equipment and data communication equipment employing serial binary data interchange.

RS-423 - Electrical characteristics of unbalanced voltage digital interface circuits.

RS-449 - General-purpose 37-pin and 9-pin interface for data terminal equipment and data circuit-terminating equipment employing serial binary data interchange.

RTS (Request-to-Send) - Physical modem interface control signal from DTE, requesting clearance to transmit.


SDLC (Synchronous Data Link Control) - IBM standard protocol superseding BSE.

Serial Transmission - The most common transmission mode in which information bits are sent sequentially on a single data channel.

Shielding - Protective covering that eliminates electromagnetic and radio frequency interference.

Short Haul Modem - A signal converter which Conditions a digital signal to ensure reliable transmission over DC continuous private line metallic circuits without interfering with adjacent pairs in the same cable.

Simplex Transmission - Data transmission in one direction only.

Space - Absence of signal. In telegraph communications, a space represents the open condition or no current flowing. A space impulse is equivalent to a binary 0.

Start Bit - In asynchrounous transmission, the first bit or element in each character, normally a space, which prepares the receiving equipment for the reception and registration of the character.

Statistical Multiplexor - A device that allows a single channel to carry information from multiple devices simultaneously.

Stop Bit - In asynchronous transmission, the last bit, used to indicate the end of a character which serves to return the line to this idle state.

Straight-Through Pinning - RS-232 and RS-422 configuration that matches DTE to DCE, pin for pin.

Synchronous Modem - Modem that carries timing information with data.

Synchronous Terminal - A data terminal that operates at a fixed rate with transmitter and receiver in synchronization.

Synchronous Transmission - Transmission in which data bits are sent at a fixed rate, with the transmitter and receiver synchronized. Synchronized transmission eliminates the need for start and stop bits.

Time-Division Multiplexor (TDM) - A device that accepts multiple channels on a single transmission line by connecting terminals, one at time, at regular intervals-interleaving bits or characters from each terminal.

T, U, V, W, X, Y, Z

Time-Sharing - A method that allows several interactive terminals to use on computer. Transient - An abrupt change in voltage, of short duration.

Unbalanced Line - A transmission line in which the magnitudes of the voltages, on the two conductors are not equal in respect to ground.

V.35 - CCITT standard governing data transmission at 56 Kbps using 60-108 KHz group band circuits.

Voice-Grade Line - A channel that is capable of carrying voice frequency signals.

Volatile Memory - A storage medium that loses all data when power is removed.

Wideband - A communications channel that has greater bandwidth than voice grade lines.

X-ON/X-OFF (Transmitter On/Transmitter Off) - Control characters used for flow control, instructing a terminal to start transmission (X-ON) and end transmission (X-OFF).

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