ackground noise inserted by the receiver to com- bounce time and before the contact resistance settles to a steady pensate for the fact that no sound is transmitted during speech state value. silence periods in certain transmission systems. Checksum. The process of summing a group of data items in order Common air interface. Standard for CT2 to ensure all handsets to perform a check, usually on the integrity of the data. can operate with all telepoint base stations. (Telepoint allows Choke packet. Packet used in data flow control, where the station use of CT2 telephones outside homes and offices.) detecting the congestion sends a control packet to the sending Common carrier. Term usually applied to a public or private tele- station in order to reduce its data generation rate. communication operator, or PTO, who can provide services to Churning. A generic term refering to frequent changes. For private individuals and businesses. example a customer may churn (change) his key system for a Common channel signalling. Signalling system where one sig- larger PABX. Also refers to the frequent movement of sub- nalling channel is associated with many speech channels and scribers, which means that many more telephone installations provides a direct data link between control processors within are required than there are new customers. exchanges. Ciphertext. A message after it has been encrypted from plaintext Communications network. Asystem which carries services by the in such a way that the recipient can recover the original (plain- exchange of information, such as voice and data, between text) message. co-operating physical or logical elements. Circuit. In telecommunications it usually refers to a link connect- Community of interest. A community or group of subscribers who ing two or more nodes. generally share the same interests, and are often geographically Circuit switching. The process used to provide a line between two close to each other. This means that they are more likely to users, in which they have exclusive us