exchange in each NPA, and assume all spare NPA capacity is used, there would be 792 NPAs in the United States, each with a toll exchange. Allow for a three- or four-level hierarchy and the importance of fan-out becomes evident. Figure 8.4 shows one-quarter of a three-level hierarchy network, where the top level is mesh connected with four transit exchanges. The fan-out concept assumes a pure hierarchy without high-usage routes. HU routes tend to defeat the fan-out concept and are really mandatory to reduce the number of links in tandem to a minimum. 8.3.7 Network Design Procedures A national territory consists of a large group of contiguous local areas, each with a toll/toll- connecting exchange. There will also be at least one international switching center (ISC). In larger, more populous countries there may be two or more such ISCs. Some may call these switching centers gateways. They need not necessarily be near a coastline. Chicago is an example in North America. So we now have established three bases to work from: 1. There are existing local areas, each with a long-distance exchange. The term telephone density should not mislead the reader. Realize that some "telephone lines" terminate in a modem in a computer or server, in a facsimile machine, and SO on. 8.3 DESIGN OF LONG-DISTANCE NETWORKS To other level 1 switches Level 1 Level 2 Level 2 Level 2 Level 2 Level 2 Level 2 00000000 00000000 Level 3 switches Figure 8.4 A three-level hierarchy with initial fan-out of six and subsequent fan-out of eight. The highest level consists of four transit exchanges, but only one is shown. 2. There is one or more ISCs placed at the top of the network hierarchy. 3. There will be no more than four links in tandem on any connection to reach an ISC. As mentioned previously, Point 1 may be redefined as a long-distance network consisting of a grouping of local areas probably coinciding with a numbering (plan) area. This is illustrated in a very simplified manner in Figure 8.5, where T, in CCITT terminology, is a higher-level ce