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1.3 DIGIT ANALYSIS AND ROUTING 15Figure 1.3-1. Routes for calls from A to D (intra-LATA calls).As perceived by an exchange, a route to a destination is an outgoing trunk group(TG). In Fig. 1.3-1(b), the route set at exchange A for destination D consists of trunkgroups TG 1 ,TG 3 , and TG 4 .Each exchange has a list of routes that can be used for a destination. The lists fordestination D at exchanges A, X, and Y are:ExchangeRoutes for Destination DA TG 3 ,TG 4 ,TG 1X TG 2 ,TG 5Y TG 6Alternate routing is the procedure by which an exchange selects an outgoingtrunk for a call when there are several routes to a destination. In this procedure,the order in which the routes are listed specifies the sequence in which an exchangechecks the outgoing trunk groups for available trunks. In this example, exchange Afirst tries to find an available trunk in its first-choice route TG 3 . If a trunk is available,A seizes the trunk. If not, it attempts to find an available trunk in its secondchoiceroute TG 4 , and so on. If none of these routes has an available trunk, exchangeA aborts the setup of the call.In alternate routing, the TGs to a destination are ordered such that the first-choiceroute is the most direct one (passing through the smallest number of intermediateexchanges), the second-choice route is the most direct one among the remainingroutes, and so on. In Fig. 1.3-1(b), the arrows indicate the selection sequences atexchanges A and X.