CCITT term for international network with wideband. switching centres. Broadcast. The transmission of information to several users sim- Centrex. A system in which the PABX function is provided as a ultaneously, such as in television or radio broadcasts. virtual group within a public telephone exchange. Buffering. The process in which data is temporarily stored in Channel. Referes to the path for transmission of information. It memory for transmission at a later date. This technique allows may be physical or logical. communicating devices with different operating speeds to in- Channel associated signalling. System in which signalling is terface to each other. physically and permanently associated with a speech channel Burst error. When several symbols in succession, within a trans- even when no signalling information is being transmitted. mitted data, are corrupted due to causes such as interference. Channel bank. Refers to equipment which multiplexes low speed Burst mode. A method of modulation in which a four wire circuit data into high speed data channels. It can also perform other is established over two wires by alternately sending bursts of function such as signalling. The equipment is usually sited in a signals first in one direction and then in the other. telephone central office. Glossary of terms 62/21 Characteristic impedance. Usually refers to the impedance which Coherent demodulation. A demodulation process using a refer- a transmission line would present to a signal entering it, if the ence carrier signal which is synchronised in phase and fre- line were infinitely long. quency with the carrier used in the modulation process. Charge retention. The ability of a battery or capacitor to hold its Collision. Refers to the interference between packets of data which charge under specified conditions. are transmitted by two stations simultaneously on the same Chatter. The variation in contact resistance between relay or medium. switch contacts, when they first close. It is measured after the Comfort noise. B