Glossary :: A-C



ACCESS – The general term for the ability of a telecommunications user to make use of a network.

ACCESS LINE – A local loop (q.v.)

ACKNOWLEDGMENT – A signal sent by a receiver to a transmitter to indicate that a message was received correctly.

ACOUSTICAL COUPLER – A type of modem that works by placing the telephone handset into rubber cups to use the acoustical signal from the microphone and receiver.

ACTIVE SENSOR – A measuring instrument in the earth exploration satellite service or in the space research service.

ADAPTIVE DIFFERENTIAL PULSE CODE MODULATION (ADPCM) – A variant of PCM which encodes only the difference between the value of the current sample and the previous one.

ADDRESS – The unique identifier of a terminal on a network.

ADDRESSABILITY – The ability of a network, especially a satellite or cable system, to individually address and thus control (usually to enable decryption) user’s receivers.

ADVANCED DATA COMMUNICATION CONTROL PROCEDURE (ADCCP) – A bit-oriented datalink protocol similar to HDLC.

AERONAUTICAL EARTH STATION – An earth station in the fixed satellite service, or, in some cases, in the aeronautical mobile satellite service, located at a specified point on land to provide a feeder link to a satellite.

AERONAUTICAL MOBILE SATELLITE SERVICE – A mobile satellite service in which mobile earth stations are located on board aircraft.  Emergency position indicating radio beacon stations may also participate in this service.

AERONAUTICAL RADIO INCORPORATED (ARINC) – A not-for-profit corporation owned by the airline, intended to coordinate their use of telecommunications used in the airline industry.

AERONAUTICAL RADIO NAVIGATION SATELLITE SERVICE – A radio navigation satellite service in which earth stations are located on board aircraft.

AIRCRAFT EARTH STATION – A mobile earth station in the aeronautical mobile satellite service located on board an aircraft.

ALLOCATION (OF A FREQUENCY CHANNEL) – Entry in the Table of Frequency Allocations of a given frequency band for the purpose of its use by one or more terrestrial or space radio communications services under specified conditions.  This term also is applied to the frequency band concerned.

ALLOTMENT (OF A RADIO FREQUENCY CHANNEL) – Entry of a designated frequency channel in a plan, adopted by a competent conference, for use by one or more administrations for a terrestrial or space radio communications service in one or more countries or geographical areas and under specified conditions.

ALPHANUMERIC – Any character which is either a letter of the alphabet or a numeral.

ALTERNATE ROUTING – A communication path used if the primary path is not available.

ALTITUDE OF THE APOGEE OR PERIGEE – The altitude of the apogee or perigee above a specified reference point on the earth.

AMERICAN NATIONAL STANDARDS INSTITUTE – An U.S. standards organization and the U.S. representative to the ISO.

AMERICAN STANDARD CODE FOR INFORMATION INTERCHANGE (ASCII) – An American dialect of International Alphabet No. 5 which uses a 7-bit code to represent alphanumeric and control information.

AMPLIFIER – An electronic device that increases the power of a receiver signal.

AMPLITUDE MODULATION (AM) – The technique of varying the strength of a carrier wave in proportion to the strength of a signal.

ANALOG – A signal that can take on any of a continuous range of values between some minimum and some maximum.  Compare digital.

ANALOG CHANNEL – A channel on which the carrier of the information can take on any of a range of values.

ANALOG DATA – Information conveyed by continuously varying some property.

ANALOG TRANSMISSION – Transmission of a carrier wave which varies continuously and can take on any of a range of value of one or more of the properties of the wave.

ANTENNA – A metal structure or wire that picks up or transmits electromagnetic energy through space.

APOGEE – The highest point of geocentric orbit.

ARINC – See Aeronautical Radio Incorporated.

ASSIGNED FREQUENCY BAND – The frequency band within which the emission of a station is authorized; the width of the band equals the necessary bandwidth plus twice the absolute value of the frequency tolerance.  Where space stations are concerned, the assigned frequency band includes twice the maximum Doppler shift that may occur in relation to any point of the earth’s surface.

ASSIGNMENT (OF A RADIO FREQUENCY OR RADIO FREQUENCY CHANNEL) – Authorization given by an administration for a radio station to use a radio frequency channel under specified conditions.

ASYNCHRONOUS – A transmission technique in which each group of bits to be sent is preceded by a “start” character(s) and followed by “stop” characters.

ATTENDED OPERATION – System in which human intervention is required to establish a modem connection and shift over from voice to data mode.

ATTENUATION – The reduction in strength of a signal.

AUDIO FREQUENCY (AF) – The frequencies of sound waves that can be perceived by the human ear, roughly 30 to 20,000 Hz.  Also sometimes called voice frequencies (VF), but actually broader bandwidth.

AUTHORIZED POWER – The maximum signal strength allowed for a particular class of licensed transmitting station.

AUTOMATIC CALL DISTRIBUTION – A system in which incoming calls are distributed among the possible answering stations.

AUTOMATIC REQUEST FOR REPTITION (ARQ) – An error correction system in which the receiver asks the sender to repeat information received incorrectly.

AZIMUTH – The angle between an antenna beam and the meridian plane, measured along a horizontal plane.



B Channel – a 64 kbqs channel, part of the ISDN system, usable for voice or data.

BANDWIDTH – The qamut of frequencies used.

BASE STATION – A stationary station in the land mobile service which can communicate with other mobile stations.

BASEBAND – The range of frequencies inherent in a source of information.

BASEBAND SIGNALING – Transmission of information at its original range of frequencies.

BAUD – A measure of the signaling speed of a circuit, the baud rate is the number of discrete signal conditions per second.

BAUDOT CODE – A 5-bit code for expressing alphanumeric and control information, used in telegraphy.

BEL – A unit of strength of a signal, equal to 10 decibels (q.v.)

BELL OPERATING COMPANY – Telephone companies were part of AT&T before the breakup. They are owned by the seven regional holding companies.

BINARY CODED DECIMAL – An encoding scheme in which each decimal digit is encoded as a 4-bit binary number.

BINARY SYNCHRONOUS – A character oriented synchronous datalink protocol that allows frames of data of arbitrary length. ASCII or EBCDIC is used for encoding the characters.

BIT – Binary digit, the smallest piece of information possible, usually expressed as either a “1” of a “0.”

BIT ERROR RATE – The fraction of bits transmitted which are received in error. Sometimes expressed as the number of bits received correctly per bit received in error.

BIT RATE – The rate at which bits are transmitted, expressed as bits per second.

BIT STUFFING – A technique of interesting a 0-bit in a transmitted datastream used in bit-oriented protocols to ensure that six 1s never appear consecutively.

BIT-ORIENTED PROTOCOL – A datalink protocol in which the bit, rather that the character, is the fundamental unit.

BLOCK CHECK CHARACTER – A set of bit sent at the end of a binary synchronous frame used to check the frame for accuracy.

BLOCK UP CONVERTER (BUC) – translate L-band input frequency to either C-, X- or Ku-band.

BRITISH TELECOM – The national telecommunications organization in the U.K. British Telecom International (BTI) is responsible for communication outside the U.K.

BROADBAND – Referring to a bandwidth greater than the baseband bandwidth, or greater than a voice frequency bandwidth.

BROADCAST SATELLITE SERVICE (BSS) – The satellite service designed to bring primarily video entertainment from satellites directly to consumers via high power satellites and small user antennas. More commonly referred to as DBS, Direct Broadcast Service.

BROADCASTING SERVICE – A radio communications service in which the transmissions are intended for direct reception by the general public. This service may include sound and television transmission.

BROADCASTING STATION – A station in the broadcasting service.

BUFFER – A computer storage device used to receive, hold, and then release a data stream to allow other devices of different transmission speed to communicate.

BUNDLING – The practice of offering several services as a package.

BYPASS – The use of transmission facilities other than those of the local telephone company network.

BYTE – Eight bits.



C-BAND – Frequencies in the 4 to 6 GHZ range used both by terrestrial microwave links and for satellite links.

CABLE – One or more conductors contained within a protective insulating cover.

CABLE TELEVISION – Any system that receives transmission from program sources and distributes them to users (usually homes) via coaxial cable, usually for a fee.

CARRIER – A continuous (usually high frequency) electromagnetic wave which can be modulated by a signal to carry information.

CARRIER POWER (OF A RADIO TRANSMITTER) – The average power supplied to the antenna transmission line by a transmitter during one unmodulated radio frequency cycle.

CARRIER SYSTEM – A technique of frequency multiplexing in which individual signals are carried on separate carrier waves of different frequencies.

CARRIER-TO-NOISE RATIO (C/N) – The ratio of the strength of the carrier signal to that of the noise in a channel, usually expressed in decibels.

CARTERPHONE DECISION – The 1986 court decision that allowed non-Bell equipment to be connected to the telephone network.

CASSEGRAIN FEED – An optical system used in telescopes and satellite antennas in which the feed is placed behind the dish, and the radio waves are brought to a focus there after being reflected first from the main dish and then from a subreflector placed near the dish’s prime focus.

CEEFAX – A tradename I the U.K. for a videotex service.

CELLULAR MOBILE RADIO – A land mobile telephone service provided by a grid of lowpower computerconnected and controlled cells, rather than by one highpower station. The user may travel from cell to cell, and the system maintains track and switches the links accordingly.

CENTRAL OFFICE – The office in a telecommunications network which is directly connected to the end user.

CENTROINCS INTERFACE – A popular parallel datalink protocol often sued to connect to a computer to printer.

CENTUM CALL SECONDS – A measure of traffic on a network, equal to 100 seconds of usage.

CHANNEL – An electrical path between two devices. Also called link, channel, path, line.

CHANNEL GROUP – A block of 12 telephones channels, transmitted together.

CHARACTER – A of the letters, numbers, and other symbols used as part of a written message.

CHARACTER STUFFING – In the binary synchronous protocol, a technique for following any character to be a part of the data stream.

CHARACTERISTICS FREQUENCY – A frequency that can be easily identified and measured in a given emission. A carrier frequency may, for example be designated as the characteristic frequency.

CURCUIT – see Channel.

CIRCUIT SWITCHING – A network technique in which a physical connection is made between the sender and receiver for the duration of the transmission.

CIRCULAR POLARIZATION – A mode of transmission in which signals travel in a rotating corkscrew-like pattern as they are downlinked to earth. By utilizing both right-hand and left-hand circular polarization, the satellite’s transmission capacity is doubled.

CLASS OF EMMISION – The set of characteristics of an emission, designated by standard symbols, e.g., type of modulation of the main carrier, modulating signal, type of information to be transmitted, and also if appropriate, any additional signal characteristics.

CLOSED CIRCUIT TELEVISSION – A television signal carried, usually via microwave or coaxial cable, between two more locations, but broadcast for general reception.

CLUSTUER CONTROLLER – A device used to control a number of terminals.

C/N – Carrier-toNoise ratio; the ratio of the power of a satellite signal to the received noise expressed in decibels.

COSTAL EARTH STATION (CES) – One of the land-based nodes in the maritime satellite services which is connected to the public telephone network.

CODEC – Coder/Decoder, a device which converts an analog signal into and/or from a digital signal.

CODE DIVISION MULTIPLE ACCESS – See spread spectrum multiple access.

COLLOCATION – The placing of several satellites near each other in orbit. Co-location allows a single fixed receiving antenna to receive signals from all the satellites without moving from one satellite to another (tracking).

COMMON CARRIER – A regulated telecommunications company that will carry messages systems within a telephone network use to exchange control information.

COMMON CHANNEL INTEROFFICE SIGNALING – The network that switching system within a telephone network use to exchange control center.

COMMON CONTROL SWITCHING ARRANGEMENT – A service which provides users with intercity private switched lines.

COMMUNICATIONS SATELLITE – An artificial satellite, usually placed in geostationary orbit, used to relay radio transmission.

COMMUNITY ANTENNA TELEVISION (CATV) – An earlier name for what are now termed cable TV companies.

COMMUNITY RECEPTION (IN THE BROADCAST SATELLITE SERVICE) – The reception of emissions from a space station in the braodcastingsatellite service equipment, which in some cases may be complex and have antennas larger than those used for individual reception. Intended for use by a group of the general public at one location or through a distribution system covering a limited area.

CAMPANDOR – Compressor/Expander, a device to improve the signal-to-noise ratio of a transmitted signal by compressing the dynamic range at the sender, and expanding it to its original value at the receiver.

COMPRESSOR – A device to compress the dynamic range of a transmitted signal.

COMPUTER INQUIRY – Two watershed FCC actions to establish the difference between and degree of regulation of various telecommunications, telescoping, and data processing services, and determine what organizations could provide various kinds of services.

COMSAT – The Communications Satellite Corporation, the U.S. signatory to the Intelsat and Inmarsat organizations.

CONFORMAL ARRAY – An antenna that “conforms” to the surface of the structure carrying it. A conformal array may be built into the skin of an aircraft. Conformal arrays are lighter and smaller than conventional flat arrays.

CONSENT DECREE – An agreement between the U.S. government and the Bell system that, along with the Modified Final Judgement (q.v.), mandated the breakup of the Bell system.

CONSULTATIVE COMMITTEE ON INTERNATIONAL RADIO (CCIR) – A working body of the ITU which provides recommendations for telecommunications standards and practices in the fields of radio communication.

CONSULTATIVE COMMITTEE ON INTERNATIONAL TELEGRAPHY AND TELEPHONY (CCITT) – A working body of the ITU which provides recommendations for telecommunications standards and practice in the fields of telegraphy and telephony, including data transmission.

CONTENTION – A method by which terminal on a line request permission to transmit.

CONTROL CHARACTER – A character that is interpreted as a signal to control the transmission, rather than as an item of the message being sent.

CONUS – The contiguous United States; i.e., all the states (including the District of Columbia) except Alaska and Hawaii.

COORDINATION AREA – The area associated with an earth station outside of which a terrestrial station sharing the same frequency band neither causes nor is subject to interfering emissions greater than a permissible level.

CROSS-TALK – The unwanted leakage of signal between supposedly independent channels.

CROSS-SUBSIDIZATION – The practice of using revenues from (usually unregulated) services to pay for other (usually regulated) services.

CUSTOMER PREMISES EQUIPMENT (CPE) – Telecommunications equipment located at the user’s site rather than in a central office.

CYCLICAL REDUNDANCY CHECK – A mathematical algorithm to produce an error-checking code within a data stream.


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