n centers would have a Level 1 switching center assigned. One of these four would be the ISC. Each Level 1 center would have one or several Level 2 or secondary centers homing on it. Level 3 or tertiary centers home on a Level 2 switching center. This procedure is illustrated in Figure 8.6. Its hierarchical representation is illustrated in Figure 8.7 setting out the final route. One of the Level 1 switching centers is assigned as the ISC. We define a final route as a route from which no traffic can overflow to an alternative route. It is a route that connects an exchange immediately above or below it in the network hierarchy and there is also a connection of the two exchanges at the top hierarchical level of the network. Final routes are said to make up the "backbone" of a network. Calls that are offered to the backbone but cannot be completed are lost calls. A high-usage (HU) route is defined as any route that is not a final route; it may connect exchanges at a level of the network hierarchy other than the top level, such as between, 1 and 12 in Figure 8.7. It may also be a route between exchanges on different hierarchical levels when the lower-level exchange (higher level number) does home on a higher level. A direct route is a special type of HU route connecting exchanges in the Table 8.1 Traffic Matrix Example - Long-Distance Service (in erlangs) To Exchange From Exchange 7 Homing on meaning subsidiary to in a hierarchical sense. It "reports to." Completed calls are those where a full connectivity is carried out indicated by both calling and called subscriber in the off-hook condition. 8.3 DESIGN OF LONG-DISTANCE NETWORKS Level 1 center Level 3 exchange Level 2 exchange Final route Figure 8.6 A sample network design. Figure 8.7 Hierarchical representation showing final routes. local area. Figure 8.8 shows a hierarchical network with alternative routing. Note that it employs CCITT (ITU-T) nomenclature. Before final dimensioning can be carried out of network switches and trunks, a grade of service criterion mus