r. A device which handles remote communica- self contained with control information. No dedicated circuit is tions processing for several terminals or workstations. set up between the two elements. See also connection oriented C-message weighting. The characteristics of a filter which ap- service. proximates the acoustic frequency response of a circuit existing Connection oriented service. A service in which a dedicated cir- between two telephones. cuit is set up between the communicating elements and is Co-channel interference. Interference between two subscribers, maintained for as long as the service is being used. The circuit using the same channel but in different cells of a cellular mobile may be real or virtual. See also connectionless service. radio system. Connect time. The time for which a line is being used. Codec. Refers to a coder/decoder, which is an analogue to digital Contact bounce. The bounce of metal contacts, such as on a relay, converter circuit interfacing between an analogue and digital when they first close, causing the electrical circuit to be made signal. It is usually an integrated circuit. and broken at high frequency. Code division multiple access. An access method for several de- Contact resistance. The electrical resistance between metal con- vices sharing a common transmission channel, which often uses tacts, such as of a switch, relay or connector. spread spectrum techniques for modulation. Contention. The situation where multiple sources compete for use Coding gain. Parameter used to evaluate the effectiveness of an of the same resource. For example contention arises when error correcting code. several sources compete for use of a common local area line in Coherent. A beam of light in which the individual waves have the the Ethernet system. same phase relationship. 62/22 Glossary Control character. Refers to the extra characters added to a trans- Decibel. Usually written as dB. It is a unit of measure for the mitted message, which are not part of the main message but strength (or