xchange is not the local exchange of subscriber B, it transmits signaling information to the next exchange that connects the circuit further. 2.6.2 Signaling The control unit of the local exchange receives the subscriber signaling, such as dialed digits, from the subscriber line and makes consequent actions according to its program. Usually the call is routed via many exchanges and the signaling information needs to be transmitted from one exchange to another. This can be done via channel associated signaling (CAS) or common channel signaling (CCS) methods (Figure 2.9). 2.6.2.1 CAS When a call is connected from a local exchange to the next exchange, a speech channel is reserved between exchanges for this call. At the same time another channel is reserved only for signaling purposes and each speech path has its own dedicated signaling channel while the call is connected. This channel can be, for example, a signaling channel in time slot 16 of the pri- mary PCM frame as explained later in Chapter 4. The main phases of signal- ing between exchanges are shown in Figure 2.10. First the speech channel and the related signaling channel are seized from exchange A to exchange B. The Telecommunications Network: An Overview Subscriber A Subscriber B Exchange Exchange Seizure B-exchange seizes channel Seizure acknowledgment and device for signaling Address information B-number received, ringing signal to B B-answer B-subscriber lifts Speech channel is the handset connected through Conversation Clear-back B-subscriber hangs up A hangs up or time limit is exceeded Clear-forward The call is disconnected Figure 2.10 CAS between exchanges. Then the telephone number of subscriber B is transmitted to exchange B, which activates the ringing signal. When subscriber B answers, the speech connection is switched on and the conversation may start. If subscriber B hangs up first, a clear-back (CBK) signal is transmitted from exchange B to A. Exchange A responds with a clear-forward (CLF) sig- nal when subscriber A hangs up or when the tim