In the case of channel associated signalling (CAS), each 64 kbit/s telephone channel is assigned 2 kbit/s for signalling. This signalling is formed by a word of 4 bits (generically known as a, b, c and d) that is situated in the TS16 of all the frames sent. Each TS16 therefore carries the signalling for two telephone channels.

Given that there are only 4 signalling bits available for each channel, to transmit all the signalling words from the 30 PCM channels that make up a 2 Mbit/s frame (120 bits) it is necessary to wait until the TS16 of 15 consecutive frames have been received. The grouping of frames defines a CAS signalling multiframe, which consists of a set of the TS16 of 16 consecutive E1 frames.