mple of guasi-associated signalling between A and B and between B and A C.C.S. may be associated or quasi-associated signalling. In asso- The later requirement for ISDN indicated the need for a more ciated (Figure 43.6) the c.c.s. link is between the same locations as advanced c.c.s. system than No.6. This resulted in the CCITT No.7 the traffic circuits served. c.c.s. system (see Section 43.3.1.) In Figure 43.7, associated signalling applies AC and CB. The signalling for traffic circuits AB is routed ACB (and BCA) in the 43.3 Digital network signalling quasi-associated mode, C being a signal transfer point. Should associated c.c.s. be equipped AB, on failure of any one of the Digital transmission and s.p.c. digital switching systems in tele- associated signalling links AB, AC or BC, backup quasi-associated phone networks form Integrated Digital Networks (IDN). The logi- signalling ACB, ABC or BAC may apply to maintain service; C, cal evolution of IDN is to Integrated Services Digital Networks or A acting as signal transfer points. (ISDN). This is achieved by extending the IDN to the user by providing Integrated Digital Access (IDA). Present ISDN is narrow- band (N-ISDN), based on the 64kbit/s channel capacity of the 43.2.1 CCITT No.6 C.C.S. system digital networks. Figure 43.8 identifies the signalling requirements for N-ISDN. This was the pioneer c.c.s. system (Welch, 1979; CCITT, 1988; Dahlbom, 1972). Its main features are: 43.3.1 CCITT No.7 inter-exchange C.C.S. system The pioneer CCITT No.6 c.c.s. system was produced primarily for 1. Signalling bit rates 2.4kbit/s (analogue link), 4kbit/s and the analogue environment, the digital versions being produced by 56kbit/s (digital link). Other rates as required. subsequent study, but retained the basic features of the analogue 2. Error control; error detection by redundant coding and error system (Welch, 1979). In the result, No.6 has limitations for ISDN. correction by retransmission. The No.7 system is optimised for the ISDN. The CCITT No.7 Signal units (SUs) 28 b