requencies before switching it and, pressors or echo cancellers. Figure 29.6 shows these between re- after switching, to multiplex it again for transmission over the next gional centres in the North American network. link in the connection. If both the transmission systems and the switching systems employ digital time-division multiplexing, the 29.4.4 Digital transmission need for channelling equipment at the end of each line is eliminated, since channels are both switched and transmitted in PCM frames. If digital transmission is used for telephony (see Chapter 28), there The cost saving is very large. are impairments due to quantisation noise and the noise and jitter on When electromechanical space-division switching is used, ap- transmission links; however, these are normally negligible. The paratus individual to each incoming and outgoing circuit is needed attenuation of each transmission link is zero with zero tolerance. to transmit and receive signalling information associated with it. Thus, the number of links connected in tandem does not affect the This is known as channel-associated signalling. Modern digital overall loss or its variation, as in the case of analogue circuits exchanges use stored-program control (SPC); the information re- considered above. Only the digital error rate accumulates with the quired for controlling connections is handled by their central pro- number of links in tandem. cessors. Thus, for a tandem connection between SPC exchanges, In an integrated digital network, as described in Section 29.5, the signalling information can be exchanged between their central pro- subscribers' lines at the two ends of the connection are the only cessors by means of data links connecting them directly. This is cause of variation of loss between different connections. This elimi- known as common-channel signalling. No signalling apparatus is nates the wide variation of losses encountered in analogue networks required to be associated with individual circuits and this produces and enables every conn