The International network of its primary centres directly connected and fewer levels in its hierarchy than a country which is large and sparsely populated. The introduction of international subscriber dialling (ISD) necessi- These differences are illustrated in Figures 29.6 and 29.7 which tated the introduction of a world-wide automatic-switching net- show, respectively, the analogue networks of North America work. In 1964 the CCITT recommended a plan for a hierarchy of (Clarke, 1952) and of BT (British Telecom) (Barron, 1959). The automatic international exchanges. This plan is shown in Figure former has four levels of trunk exchanges (toll offices classes 1 to 29.8. 4), whereas the latter had only three. These were called group The international network has three levels of switching centre switching centres (GSCs), district switching centres (DSCs) and (centre du transit) designated CT1 to CT3. The international gate- main switching centres (MSCs). The DSCs and MSCs were collec- way exchanges in each country, which connect national to interna- tively called trunk transit exchanges and the network interconnect- tional circuits, are centres CT3, whereas centres CT1 and CT2 ing them was the trunk transit network. The large number of direct interconnect only international circuits. A complete international circuits between GSCs handled most of the trunk traffic. Only a connection consists of three parts: small proportion of trunk traffic reached the transit network. How- 3.5dB 3.5dB Cosited 4.5dB 4.5dB 4-Wire switched Trunk junctions exchanges Local junction 4.5dB 2-Wire switched 4.5dB exchanges Junction tandem exchange Figure 29.7 BT (British Telecom) former analogue network 29/8 Telephone networks a national network and its constituent local networks. The specified 0.5dB grade of service determines the number of circuits required on each route within it, i.e. the dimensioning of the network. 0.5dB 0.5dB 0.5dB 29.4 Transmission 29.4.1 General 0.5dB The overall (i.e. mouth to ear) performance of a telephone connec-