s ex- changes with call-routing and charging information, together with SIGNALLING NETWORK management information for both the network operator and the TRANSMISSION NETWORK service providers (Webster, 1989) ADMINISTRATION NETWORK 29.7 Numbering and charging INTEGRATED DIGITAL NETWORK 29.7.1 National schemes Figure 29.12 Component networks of IDN In order to establish a connection through a network, it is necessary for a caller to inform the switching centre of the required address of the subscriber being called, i.e. the called subscriber's directory every possible combination of subscribers. However, digital trans- number. This determines both the route for the call and the charging mission links have zero loss; thus, this will apply to any connection rate. Thus, a numbering plan is required to allocate a unique number of links set up in an integrated digital network. The only variation to each subscriber connected to the network. At first, each numbe- in overall attenuation between different connections is that due to ring scheme applied only to a single exchange and exchanges were differences in the lengths of the subscribers' lines. This represents identified by the names of their towns. Later, linked number an enormous improvement and can ensure that all calls through the schemes (Flood, 1975) were applied to multi-exchange areas. network have a loudness rating within the preferred range. In a linked numbering scheme, the 'local' numbering scheme covers a number of exchanges in the area, SO that a call from any exchange in the area uses the same number to reach a particular Network services subscriber. Thus, the first part of a subscriber's directory number is an exchange code and the remainder is the subscriber's number on The chief service provided by the PSTN is to make connections for the exchange. For example, a 5-digit linked numbering scheme has communication between its subscribers. A call is made by the a theoretical capacity for ten 4-digit exchanges or 100 3-digit calling subscriber dialling the d