 equivalent lines of System 12 awards have been accumulated from centralised management of the operation and maintenance features 22 countries. of the exchange. It also permits the provision of supplementary System 12's distributed architecture allows it to cover a very subscriber services, both exchange and network based, and includ- broad spectrum of exchange applications with respect to both size ing the possibility of digital access to voice and data services. This and network hierarchical level. Its expandable digital switching 32/8 Telephone exchanges network and distributed control provide economical solutions for 1. Rural areas: remote subscriber units and small exchange con- exchanges ranging from a few hundred of lines up to over 100000 figurations: up to 488 lines. lines. For transit exchanges up to 60000 trunks may be handled. Urban and metropolitan areas: medium to large scale local With respect to network levels, System 12 may be applied as local, exchanges: 512 to over 100000 lines. tandem, national and international transit exchange. Also combined 3. Toll exchanges: up to 60000 trunk terminations. local transit exchanges are possible. 4. Transit applications: tandem, toll, international, and inter-con- The list of individual exchange sites clearly shows the variety of tinental exchanges. applications: from small and large locals via numerous transits and 5. Network Service Centres (NSCs). combined exchanges up to several international transit exchanges including the highest possible level. All exchanges throughout the range provide a full set of modern 32.5.3 Developments and future services and features to subscribers and network operators. System 12 independent exchanges are designed to cover both The digital switching System 12 has the flexibility that is necessary individual and multi-exchange network applications. They provide to allow economic extension in terms of the number of lines and a full range of maintenance and administrative features including new services. It also offers high faul