te national network contains a hier- archy of exchanges, as shown in Figure 29.5. At the apex of the There are variations in practice and in terminology throughout the hierarchy there will be one or more international gateway exchange- world. In North America, junctions are called trunks and trunk S, which provide the interface between the national network and the circuits are called toll circuits. In CCITT terminology, networks (2) 29/6 Telephone networks Class 1 Offices Trunk circuit 0.5 to 1.5dB Class 2 Offices Trunk circuit 0.5to 1.5dB Class 3 Offices Trunk circuit 0.5to 1.5dB Class 4 Offices VNL +2 Junction circuit Junction circuit 2 to 4dB Ave 4, max 6dB Class 5 Offices Junction circuit Ave 2, Max 3dB Final route High usage Junction circuit Junction circuit route Ave = Average TC = Toll Centre X = Subscriber PC = Primary Centre EO = End office (local office) SC = Sectional Centre T = Tandem office RC = Regional Centre VNL = Via Net Loss ES = Echo Suppressor Figure 29.6 North American analogue network and (3) together are called the local network; however, in the UK A subscribers' private network may use a private automatic ex- this term is usually reserved for (2) alone. change (PAX) which, unlike a PABX, does not connect its lines to In some countries, where subscribers' distribution areas are large, the PSTN. On the other hand, a private network may extend over it is economic to locate the switching equipment directly associated many different locations using circuits in networks (2), (3) and (4) with a group of subscribers at a location close to the subscribers but rented from the PTO. Multinational companies may have private remote from the exchange. This equipment is called a remote circuits extending through the international network to link their concentrator (Flood, 1975). It concentrates traffic from a number of offices in different countries. subscribers' circuits onto a smaller number of circuits leading to the In a national network, as shown in Figure 29.5, the minimum exchange. A concentrator m