Link Limitation

From a network design perspective a connectivity consists of one or more links in tandem.2 We define a link as the transmission facilities connecting two adjacent switches. CCITT in Rec. E.171 (Ref. 1) states that for an international connection there shall be no more than 12 links in tandem. This is apportioned as follows:

• 4 links in the calling party's country;

• 4 links in the called party's country; and

• 4 international links.

This concept is illustrated in Figure 8.3.

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.)

2It should be noted that there are connectivities with "no links in trandem." This is an own-exchange connectivity, where the calling and called subscriber terminate their subscriber loops in the same exchange.

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.

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