 the use of remote concentrators. Analogue Integrated digital networks 29/11 circuits from subscribers' premises are terminated at a concentrator only 60 DMSUs, whereas the analogue network contained over 400 which only requires two PCM systems to connect it to the main GSCs. Each DCCE has junction routes to two DMSUs. Thus, it is exchange. In addition to reducing the per-line cost of call process- still able to connect trunk calls to and from its subscribers in the ing, having larger local exchanges reduces the number of exchanges event of the breakdown of one route or its DSMU. in an area and so reduces the cost of junction plant and tandem The data links used for common-channel signalling between digi- switching. Having fewer local exchanges also reduces the costs of tal exchanges in the PSTN form a separate self-contained network, operation and maintenance. as shown in Figure 29.12. However, this signalling network uses The final stage of this evolution is to extend digital transmission channels in the basic digital transmission bearer network, just as to subscribers' premises. Since a stream of digits is suitable for the does the PSTN. In order to maintain synchronism of the PCM transmission of high-speed data in addition to speech, this enables frames in all the digital transmission links and exchanges, it is an IDN to evolve into an integrated services digital network (ISDN), necessary to distribute synchronising signals from a national refer- whereby many different subscribers' services are provided over the ence clock source. This results in a synchronising network linking same plant. This development is discussed in Chapter 44. it to all the digital exchanges. Finally, centres remote from ex- The IDN of BT (Muir, 1987) is shown in Figure 29.11. Local areas changes have been introduced to collect bulk data (e.g. traffic are served by digital cell centre exchanges (DCCEs). Subscribers statistics, billing, etc.) from exchanges and to provide man-machine obtain access to a DCCE to process their calls via a 