other direct routes between centres at different levels which violate this pattern. In 1984, the CCITT made recommendations for an international If it is assumed that a country is already divided into local- numbering plan to cover integrated services digital networks exchange areas and the locations of their exchanges have already (ISDNs) (CCITT, 1989b). This extends the existing numbering been decided, a routing plan should be developed to determine: plans for PSTNs. Thus, a subscriber's ISDN access will normally be indistinguishable from PSTN access and will be provided by the 1. Which exchanges should be interconnected by direct junctions, same local exchange. and which connections made indirectly via tandem switching The maximum length of international numbers is extended to 15 centres. digits. The area code becomes a network destination code (NDC). 2. The number and location of tandem switching centres. This arises from the present situation whereby some 'area' codes 3. The number of levels of tandem switching to be used in the have been used for access to mobile networks and special services network. 29/16 Telephone networks 4. Whether automatic alternative routing is to be used and, if so, having routes to more than one centre at the next-higher level, under what conditions. makes possible complex alternative routing schemes. Thus, if a call encounters congestion at a higher level in the hierarchy, it is possible The routing plan must be consistent with the plans for numbering, to 'crank back' to an exchange at a lower level and choose a charging, transmission and signalling. different outgoing route there. Large groups of circuits are more efficient than small groups Automatic alternative routing will re-route calls away from a because of their higher occupancy (i.e. traffic per circuit), as ex- high-usage route whenever it is unable to carry traffic. This can plained earlier. If there is a large amount of traffic between two happen if there is a breakdown instead of high traffic. This is exchanges, it i