nal circuits with terminating unit Local exchange National ext. circuits with two-wire Four-wire switching exchange Local exchange switching International switching center Subscriber's line Two-wire switching (ISC) with virtual with terminating unit international connecting PC Primary center Figure 8.3 An international connection to illustrate the maximum number of links in tandem for such a connection. (From Figure 6/G.101 of Ref. 2.) 2 It should be noted that there are connectivities with "no links in trandem." This is an own-exchange connec- tivity, where the calling and called subscriber terminate their subscriber loops in the same exchange. 8.3 DESIGN OF LONG-DISTANCE NETWORKS The PSTN network designer should comply with this CCITT criterion, in that for a national connection, there should be no more than four links in tandem. The reason CCITT/ITU-T set this limit was to ensure transmission QoS. As we add links in tandem, transmission quality deteriorates. Delay increases and we include here processing delay because of the processing involved with a call passing through each switch. End-to-end bit error rate deteriorates and jitter and wander accumulate. Transcontinental calls in North America generally need no more than three links in tandem, except during periods of heavy congestion when a fourth link may be required for an alternate route. 8.3.4 Numbering Plan Areas The geographical territory covered by the long-distance network will be broken up into numbering plan areas (NPAs). In North America, each NPA is assigned a three-digit area code. In other parts of the world, two- and even one-digit area codes are used. NPA size and shape are driven more by numbering capacity and future numbering requirements. Numbering plan administrators are encouraged to design an NPA such that it coincides with political and/or administrative boundaries. For example, in the United States, an NPA should not cross a state boundary; in Canada, it should not cross a provincial boundary. NPAs are also important for establishing