ection to have a loudness rating within the a further cost saving. It has been estimated that the cost of providing preferred range. inter-exchange signalling by means of common-channel signalling is only about 5% of the cost of channel-associated signalling. A network having compatible digital transmission and switching Integrated digital networks is called an integrated digital network (IDN). The cost savings mentioned above make it economic to use many more direct routes between trunk exchanges and to require fewer tandem routings. Since the analogue networks shown in Figures 29.6 and 29.7 were This enables the hierarchy to use fewer levels. It also makes it designed in the 1960s, there have been great reductions in the economic for an exchange to have routes to two or more switching per-circuit cost of long-distance transmission because of the devel- centres at the next higher level, instead of the single backbone route opment of high-capacity transmission systems using coaxial cables, used in analogue networks. This increases the security of the net- microwave radio and optical fibres. Modern systems using these work. If one of these routes breaks down, traffic can still use the transmission media now employ digital transmission using time-di- other. vision multiplexing (TDM) instead of analogue transmission using Since the introduction of digital exchanges has reduced the per- frequency-division multiplexing (FDM). Pulse-code modulation line cost of switching, quite a high proportion of the cost of an (PCM) is now the preferred transmission technique both for junc- exchange is due to its central processors. This makes it desirable for tion and trunk routes. a digital local exchange to have more subscribers' lines than an During the same period, electronic digital time-division switching systems (Redmill, 1990) have been developed to supersede electro- analogue exchange and SO serve a larger area. Increasing the size of mechanical analogue space-division switching systems. If space-di- exchange areas favours