Communications Systems Glossary

C band.  The C band is a band of frequencies used for terrestrial radio relay and satellite transmissions.

Cable.  A cable is one or more electrical conductors in an insulating sheath.

Cable access television (CATV).  Cable access television systems have a common antenna or source from which signals are distributed to a number of users.

Cable modem.  A cable modem is a high-speed modem that transmits and receives data over coaxial cables for TV distribution.

Cambridge ring.  A Cambridge ring is a token-passing LAN developed by the University of Cambridge.

Campus network.  Campus network is another name for a metropolitan area network (MAN).

Capacitance.  Capacitance is the property exhibited by a capacitor, or any wires or devices that are physically arranged to resemble a capacitor.

Capacitor.  A capacitor is obtained by placing two conducting plates near to each other, separated by a non-conducting material called a dielectric.

Carrier.  A carrier is a continuous frequency that is at the desired channel frequency. The carrier is modulated or impressed with a second (information carrying) signal to translate that information from baseband frequencies to occupy the desired channel.

Carrier-sense multiple access (CSMA).  Carrier-sense multiple access (CSMA) is a medium access control technique for multiple-access transmission media. A station wishing to transmit first senses the medium and transmits only if the medium is idle.

Carrier-sense multiple access/collision avoidance (CSMA/CA).  Carrier-sense multiple access/collision avoidance (CSMA/CA) is a refinement of carrier-sense mulitple access (CSMA) in which a station uses a collision avoidance (CA) technique to avoid collisions.

Carrier-sense multiple access/collision detection (CSMA/CD).  Carrier sense multiple access/collision detection (CSMA/CD) is a refinement of carrier-sense multiple access (CSMA) in which a station ceases transmission if it detects a collision using a collision detect (CD) algorithm.

Carrier-to -interference ratio (CIR).  The carrier-to-interference ratio (CIR) is the ratio of the power in the carrier to the power of the interference signal.

Carrier-to-noise Ratio (CNR).  The carrier-to-noise ratio (CNR) is the ratio of the carrier power to the noise power.

Cassegrain.  A Cassegrain antenna has a parabolic reflector that focuses the incoming radiation onto a hyperbolic sub-reflector and thence into the waveguide.

Cell.  In personal communications systems (cellular mobile phone systems) a cell is the geographic area served by a single base station. Cells are arranged so that base-station frequencies can be reused between cells. In asychronous transfer mode (ATM) networks, a cell is a packet of data.

Cell relay.  Cell relay is a fast packet-switching mechanism, which uses fixed-sized packets called cells.

Cellular radio.  Cellular radio is a term for cellular telephony services provided by personal communications systems (PCS).

Centronix.  Centronix is a trade name that has become attached to a 36-pin parallel interface, which is an alternative to serial RS-232 links.

CEPT-1.  A CEPT-1 channel is a 2 Mbps TDM channel.

CEPT-4.  A CEPT-4 channel is a 140 Mbps TDM channel. Also known as E4

Channel.  Strictly, a channel is a single direction path (or simplex path) along which a signal can be sent. However, in common usage the term is synonymous with circuit.

Channel rate.  The channel rate is the data rate on a channel measured in bits per second.

Channel spacing.  The channel spacing is the distance in frequency between channels.

Check bit.  A check bit is a bit attached to a block of data resulting from some form of block coding such as one of the common parity schemes in use.

Cipher text.  The cipher text is the output of an encryption algorithm, which is the encrypted version of a message or data.

Circuit.  A circuit is the medium connecting two communicating devices, or a path between a transmitting and receiving device (there may be one or more intermediary nodes).

Circuit switching.  It is very inefficient for a circuit to be established between any two users who require to converse or transfer information. Circuit switching is a much more efficient use of resources in which each user is connected to a central node or switch that provides the capability of interconnecting any two users for the period of the call.

Closed-circuit Television (CCTV).  Closed circuit TV (CCTV) systems have a common antenna or source from which signals are distributed to users.

Coaxial cable (coax).  A coaxial cable is a transmission medium normally consisting of one central conductor enclosed within a conductive sheath, separated by a dielectric.

Code-division multiple access (CDMA).  Code-division multiple access (CDMA) is a multiple access technique in which each user is given access to the entire channel all the time, with users separated by giving each a unique spreading code.

Coder-decoder (Codec).  A coder-decoder (codec) transforms analogue data into a digital bit stream (coder), and digital signals into analogue data (decoder).

Collision.  In a carrier-sense multiple access (CSMA) media-access control protocol, a collision occurs when two stations attempt to transmit at the same time.

Collision avoidance (CA).  In collision avoidance, each station listens to ensure that the shared channel is free. If so, a station that desires to transmit waits a fixed time plus a random time before transmitting.

Collision detection (CD).  In a carrier-sense multiple access (CSMA) media-access control protocol, a collision occurs when two stations attempt to transmit at the same time. Since the messages interfere with each other, the collision must be avoided or detected. In collision detection, each station to listen to the network after transmission. If another station has transmitted at the same time, both stations stop transmitting and wait a random period of time before trying to transmit again.

Comite Consultatif International des Radiocommunications (CCIR).  "Comite Consultatif International des Radiocommunications" is the French name for the International Radio Consultative Committee.

Common channel signalling (CCS).  Common channel signalling is an ITU signalling system that had seven standards, although only CCS No 4, No 5, No 6 and No 7 are in common use today.

Common Channel Signalling, Number 7 (CCS#7).  Common Channel Signalling, Number 7 is an ITU common channel signalling standard between all digital exchanges with integrated services.

Companding.  In effect, this compresses the signal range before transmission and expands it at the receiver. This compressing and expanding results in the term companding.

Compression.  Compression is the process by which redundancy is removed from data before transmission or storage.

Conditioning.  Conditioning describes changes made to a signal before transmission to prepare it for the range of distortions that occur in the transmission channel.

Conductance.  The ability of the wire to conduct is called conductance, but this property of the wire is normally considered in terms of how it restricts the flow of electrons, that is its resistance. Resistance is measured in ohms (W).

Conference of European Postal and Telecommunications (CEPT).  The Conference of European Postal and Telecommunications is a 27-country European telecommunications standardisation committee.

Congestion control.  Congestion control is one of a range of techniques implemented within a network to ensure that the network does not become overloaded at any particular point.

Connection-oriented.  A Connection-oriented protocol is one in which a logical connection is established between the transmitting and receiving devices before transmission occurs.

Connectionless.  Connectionless refers to the exchange of data without prior coordination between the transmitting and receiving devices.

Constant Bit Rate (CBR).  Constant bit rate is a service class for asynchronous transfer mode networks (ATM networks) in which the cell rate takes a constant, fixed value specified in the connection contract.

Consultative Committee on International Telegraph and Telephony (CCITT).  The Consultative Committee on International Telegraph and Telephony is an international standards organization.

Contention.  Contention is the result of two or more devices attempting to access the same resource at the same time.

Convolutional code.  A convolutional code extends the concept of a block code to allow memory from block to block. Each encoded symbol is therefore a linear combination of information symbols in the current block and a selected number of preceding blocks.

Copper distributed data interface (CDDI).  Copper distributed data interface (CDDI) is a version of the fibre-distributed data interface (FDDI) LAN standard adapted for twisted-pair copper-wire LANs.

Cordless local area network (Cordless LAN).  Cordless local area network (cordless LAN, or CLAN) is a generic term for wireless local area network (wireless LAN or WLAN).

Crosstalk.  Crosstalk is the undesired effect that a signal transmitted on one circuit or channel of a transmission system creates in another circuit or channel.

Current.  Current is the average flow of charge (electrons) and is measured in amps.

Cyclic code.  Cyclic codes are an important sub-class of linear block codes for error detection, where a new codeword in the code can be formed by shifting the elements along one place and taking one off the end and putting it on to the beginning.

Cyclic redundancy check (CRC).  A cyclic redundancy check (CRC) is a very efficient error detection algorithm that uses a polynomial function to generate the block check characters.

Please contact Argos Press to suggest new words or phrases to be added to our glossary. © Argos Press Pty Ltd, Canberra, 2003-2004. All rights reserved. Please also contact Argos Press for information on licensing our content (including this glossary entry on Communications Systems).

Our other resources include Antennas, Appliances, Broadband Internet, CDMA, Communications Systems, GSM, Modems, Project Management, Radar, Risk Management and Decision Making, Satellite Communications, SMS, Speakers, Systems Engineering, Team Building, Time Management, Transmitters and Receivers, and XML.