ted in bit quences of zeros by a special pattern (for details on zero code positions 2 to 8 of time slot 0 of every other frame. Bit position 1 suppression schemes, refer to Section 21.7). carries the International bit, while frames not containing the frame The assumption on the DS1 stream was that at least one eighth, alignment signal, i.e. the odd frames, are used to carry National and i.e. 12.5%, of the incoming bits were 1s. If that ratio was not International signalling bits and alarm indication for loss of frame maintained, synchronisation would be lost and an error would alignment. The loss of frame alignment alarm is announced by result. When the DS1 contains voice channels, work around tech- alarm bit A, which is the third bit of the odd. To avoid imitation of niques like changing the eight 0 to a 1 do not create major problems, the frame alignment signal, bit 2 of the odd frames is set to 1. since the distortion in the voice message is minimal and the human Time slot 16 in each CEPT 30 frame is used to transmit such ear would not notice it. In contrast, changing a customer data bit is signalling data as on-hook and off-hook conditions, dialling digits not acceptable, as that may imply a substantial difference in the and call progress. Since a common channel is dedicated for the received data (e.g. 0001 and 1001 are quite different numbers). signalling data of all voice circuits, this method of signalling is Therefore, one bit out of eight in the customer channel is reserved referred to as common channel signalling. The signalling consists 21/10 Time division multiplexing 16 frames = 1 multiframe 32 8-bit time slots per frame Frame Multiframe alignment alignment 11011 Odd frames 00YXX Frame 0 I1ANNNNN Even frames Frames 1-15 = International bit = Multiframe Alignment Signal N = National bit = Extra bit A = Alarm Indication Signal (AIS) = Loss of multiframe alignment alarm bit FAS Frame Alignment Signal =4 signalling bits concerning channel i and channel i Figure 21.7 CEPT PCM-30 frame and multiframe comp