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In telecommunication, a common control is an automatic telephone exchange arrangement in which the control equipment necessary for the establishment of connections is shared by being associated with a given call only during the period required to accomplish the control function for the given call. The first examples deployed on a major scale were the Director telephone system in London and the panel switch in the Bell System. Direct control telephone exchanges became rare in the 1960s, leaving only common control ones.
Systems which have control subsystem as an integral part of the switching network itself were known as direct control switching systems. Systems in which the control subsytem is outside the switching network are known as Common control systems. Strowger exchanges are usually direct control systems, whereas crossbar, electronic exchanges including all stored program control systems are common control systems. Common control is also known as indirect control or register control.
During the 1970s, common control exchanges became stored program control exchanges, using in the 1980s common-channel signaling in which the channels that are used for signaling are not used for message traffic.
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