runk network Regional tandem exchanges (secondary trunk switching centres) Local tandem exchanges (primary trunk Local switching centres) network Local exchanges Subscriber lines Figure 29.5 National telecommunication network each local exchange has junctions to a tandem exchange, SO this rest of the world. In CCITT terminology, these are called CT3 provides a convenient interface with the trunk network. In CCITT exchanges (centres du transits). terminology, it is called the primary trunk switching centre. In the A national network can be seen to consist of a hierarchy of UK it was called a group switching centre (GSC) and in North interconnected networks, as follows: America a class 4 office. A GSC in the UK usually acted as a trunk exchange, as a tandem exchange and as a local exchange serving A subscriber's private network. For example, this may consist nearby subscribers. However, in a large city, these functions may be of extension lines connected to a private automatic branch performed by separate exchanges. exchange (PABX) for telephony or a local-area network (LAN) Just as it is usually uneconomic for all local exchanges to be for data. directly interconnected in a multi-exchange area, it may be unecon- The subscribers' distribution network, which is sometimes omic for all primary trunk switching centres to be directly intercon- called the access network or local-loop plant. This connects nected. Thus, a primary centre may have direct trunk routes to those subscribers' telephones or PABXs to a local exchange. nearest to it, but traffic to other primary centres is routed via tandem The junction network, which interconnects a group of local switching centres, known as secondary trunk centres, each serving exchanges and connects them to the primary trunk centre. a region of the country. Again, these may not be fully interconnected 4. The trunk network, which interconnects primary centres and traffic between secondary centres is routed via tertiary centres. throughout the country. It can be seen that a comple