 a rates and tariffs scheme. 3 We know a priori that each NPA will have at least one long-distance exchange. It may be assigned more. This long-distance exchange may or may not colocate with the POP (point of presence). 4 We now have made the first steps in determining exchange location. In other countries this exchange may be known as a toll-connecting exchange. 8.3.5 Exchange Location We have shown that the design of the long-distance network is closely related to the layout of numbering plan areas or simply numbering areas. These exchanges are ordinarily placed near a large city. The number of long-distance exchanges in a numbering area is dependent on exchange size and certain aspects of survivability. This is the idea of "not having all one's eggs in one basket." There may be other reasons to have a second or even a third exchange in a numbering area (NPA in the United States). Not only does it improve survivability aspects of the network, but it also may lead the designer to place a second exchange near another distant large city. Depending on long-distance calling rates and holding times, and if we assume 0.004 erlangs per line during the busy hour, a 4000-line long-distance exchange could serve some 900,000 subscribers. The exchange capacity should be dimensioned to the forecast long-distance traffic load 10 years after installation. If the system goes through a 15% expansion in long-distance traffic volume per year, it will grow to over four times its present size in 10 years. Exchange location in the long-distance network is not very sensitive to traffic. 8.3.6 Hierarchy Hierarchy is another essential aspect in long-distance (toll) network design. One impor- tant criterion is establishing the number of hierarchical levels in a national network. This deals with how much a telephone company charges for a telephone call. 4POP, remember, is where the local exchange carrier interfaces with long-distance carriers. This whole concept of the POP is peculiar to the United States and occurred when the Bell System