 is used for in-slot signalling, the speech analogue signalling. A much more efficient way of transferring bits being reduced to 7. This only gives two signal conditions (0000 information between s.p.c. exchanges is to provide a bi-directional and 1111 ---) in each direction, a very limited repertoire. high speed data link between the two processors over which they The later D2 version adopts a 12-frame multiframe. Bit 8 of the transfer signals in digital form by coded bit fields. A group of traffic time slots in every sixth frame (called signalling frames) is used for circuits (many hundreds) would thus share a common channel signalling, giving two in-slot signalling bits per time slot. This slows signalling (c.c.s.) link. All C.C.S. signalling is link-by-link. the signalling relative to D1, reduces the speech bits in the signalling C.c.s. gives rise to the following requirements which do not arise frame time slots to 7, but increases the signal repertoire. with signalling on traffic circuit systems: 1. High order error rate performance. 43.1.5.3 Interregister signalling 2. Signalling backup to maintain service on failure or unaccept- able performance. Applied with common control switching, the signalling terminals 3. Assurance of traffic circuit continuity as, unlike on-traffic-cir- being associated with the registers (CCITT, 1988a; Miller, 1971). cuit signalling, c.c.s. does not establish traffic path integrity. The system deals primarily with address signalling and is separate 4. Circuit label included in the signal messages to give traffic from line signalling systems (Figure 43.2). circuit identity. Traffic circuits Switching Switching C.C.S. link Signalling Signalling Figure 43.6 Example of associated C.C.S. between A and B 43/8 Signalling systems Traffic circuits Switching Switching Traffic circuits Traffic circuits Signalling Signalling Switching C.C.S. link C.C.S. link Associated signalling between Signalling AC and between BC. Acts as a signal transfer point for A - B and B A signalling Figure 43.7 Exa