 is definitely richer than any analog signalling system but not as rich as commonly used trunk signalling systems, which we will look at next. Trunk Signalling Interexchange signalling in circuit-switched networks is referred to as trunk signalling. Two types exist: Channel Associated Signalling (CAS) and Common Channel Signalling (CCS). CAS systems preceded CCS systems, which offer better capacity, reliability, and flexibility, and at present are pro- gressively being replaced by them. Local Figure 1-14 Exchange Terminal exchange DSS-1 message flow. TRUNK SIGNALLING (1) SET UP Call attempt is received. TRUNK SIGNALLING (2) ALERT The phone begins ringing. TRUNK SIGNALLING (3) CONNECT The user picks up. TRUNK SIGNALLING (4) CONNECT ACK Conversation TRUNK SIGNALLING (5) DISCONNECT Remote user hangs up. TRUNK SIGNALLING (6) RELEASE TRUNK SIGNALLING (7) REL. COMPLETE Chapter 1 Channel Associated Signalling In CAS systems, the signalling and the voice associated with a certain call are transferred along the same path. In- band signalling, which was described previously, is an example of CAS. CAS can be used in both analog and digital systems. Digital systems that use CAS implement a specific channel for line signalling. Line signalling controls the establishment and release of voice channels (see Figure 1-15). The line signalling associated with all voice channels in a trunk is trans- ported in a single channel. European systems, for instance, reserve time slot number 16 in each PCM link. Thus, the sixteenth time slot carries line sig- nalling associated with channels 1 through 15 and 17 through 31 (time slot 0 is used for frame synchronization). Some examples of line signalling are: idle line, seizure of line, answer, and charging pulses. On the other hand, register signalling, which handles addressing infor- mation, is transported through the voice path. Some examples of register signals are: callee's number, callee's status, and caller's number. Since reg- ister signals are sent only during call set-up, when there 