As well as transmitting information generated by the users of the telephone network, it is also necessary to transmit signalling information. Signalling refers to the protocols that must be established between exchanges so that the users who are communicating with each other can exchange information. There are signals that indicate when a subscriber has picked up the telephone, when they can start to dial a number, when another subscriber calls, signals that let the communication link be maintained, etc.

In the E1 PCM system signalling information can be transmitted by two different methods: the Common Channel Signalling (CCS) method and the Channel Associated Signalling (CAS) method. In both cases the time slot TS16 of the basic 2 Mbit/s frame is used to transmit the signalling information.

For CCS signalling, messages of several bytes are transmitted through the 64 kbit/s channel provided by the TS16 of the frame, with these messages providing the signalling for all the channels in the frame. Each message contains the information that determines the channel that is signalling. The signalling circuits access the 64 kbit/s channel of the TS16, and are also common to all the channels signalled. There are different CCS systems that constitute complex protocols. In the following section and by way of example, Channel Associated Signalling will be looked at in detail. Channel Associated Signalling is defined in the ITU-T recommendation G.704, which defines the structure of the E1 frame.