remote concen- interfaces for maintenance and making software changes (Waliji, trator (RCU) unless they are situated very close to the DCCE. The 1980). Management centres receive real-time information on traffic concentrator is connected to the main exchange by two low-capac- flows and allow traffic to be re-routed to minimise congestion ity digital transmission systems, usually operating at 2Mbit/s or (Bealby, 1990). These require an administration data network con- 8Mbit/sec. These are diversely routed in order to provide against necting the remote centres to the nodes of the PSTN. It is thus seen failures. Concentrators parented on about 400 DCCEs will replace that a complete IDN contains four separate networks, as shown in over 6000 local exchanges. Figure 29.12. However, all of these use channels in the basic digital The trunk network contains only one level of switching centres. transmission network, making five networks in all. These exchanges are called digital main switching units (DMSUs) It has been shown in section 29.4.1. that an analogue network and are fully interconnected. The digital trunk network contains cannot provide a satisfactory transmission performance between The Trunk Network Security Digital Routes Switching Units Digital Cell Centre Exchanges The Local Remote Network Concentrator Units Figure 29.11 BT (British Telecom) digital network 29/12 Telephone networks should apply to all subscribers. It would avoid abortive calls when subscribers are away and the cost and inconvenience of subscribers having to change their numbers when moving to another town. In order to cater for this, an 'intelligent' network would be required. Every exchange would need to access a complete national data base to determine the location of a called subscriber instead of merely routing the call according to the area code dialled. As shown in SWITCHED DIGITAL NETWORK Figure 29.13, the BT derived services network has been provided SYNCHRONISATION NETWORK with an 'intelligent' network data base (INDB) to provide it