Consumer Electronics

SIGNALLING

In telephony context signalling means the passing of information and instructions from one point to another relevant to the setting up or supervision of a telephone call. To initiate a call a telephone subscriber lifts the handset off its rest-goes off hook. This off hook state is a signal to the exchange to be ready to receive the number of the called subscriber. As soon as appropriate receiving equipment has been connected to the line, the exchange signals dial tone back to the calling subscriber who then dials the wanted number. On older exchanges, this information is passed by a rotary dial (Fig. 37.11) by a series of makes and breaks of the subscriber's loop interrupting current flow. On more modern exchanges, voice frequency ...