e of the line for the make calls within the community than out of it. See also closed duration of the call. user groups. Circular polarisation. Radio transmission mode consisting of a Companding. Refers to a compressing/expanding process for re- circular rotating pattern. ducing the noise transmitted in a signal. Class of service. The differentiation of different service classes Compiler. A computer programme which converts a programme available to a user. written in high level language (source code) into lower level Class X office. US telephone hierarchy, where Class 1 is a re- machine language (object code). See also assembler. gional centre, Class 2 is a sectional centre, Class 3 is a primary Compression. Techniques which reduce the number of bits centre, Class 4 is a toll centre, and Class 5 is an end office. needed to represent the information in a transmitted signal. Clear channel. A transmission channel in which the full channel Concentrator. A device which enables several sources to share the bandwidth is available to the user. same communication medium, where the number of channels Client-server relationship. Usually refers to a service relationship available on the medium is less than the total number of chan- between functional entities, such as managed objects, where the nels required by all the sources. client uses the services provided by the server. Conditioning. Usually refers to the careful balancing of trans- Clock. An electronically generated periodic signal which is used mission line impedances to improve performance, such as fas- to provide a timing reference to the transmitted data or other ter transmission rates and higher quality. circuit functions. Conference call. A telephone call between three or more partici- Closed user group. A sub-group of users sharing common fa- pants. cilities and where communication is restricted to other sub- Connectionless service. A service in which every block of data group members. interchanged between two communicating elements is totally Cluster controlle