Download

1.3 DIGIT ANALYSIS AND ROUTING 13Figure 1.2-1. Called numbers. SN, subscriber number; NN, national number.Figure 1.2-2. Called number formats on international calls. NN, national number; IN,international number.NPA. In Fig. 1.2-1, exchange C is in the NPA of D, and exchanges A and B arenot. Exchange A has received the NN of S 2 . If A routes the call on a trunk of TG 1or TG 2 , it deletes the area code from the received number and sends the SN of S 2 .However, if A routes the call on TG 3 , it has to send the NN of S 2 . Exchanges BandC always send the SN.A similar digit deletion occurs on international calls. Figure 1.2-2 shows a callfrom subscriber S in country X to a subscriber in country Y. An ISC sends a nationalor international number, depending on whether it routes the call on a direct trunk onan ISC in the destination country, or on a trunk to an ISC in an intermediate country.1.3 DIGIT ANALYSIS AND ROUTING1.3.1 Destinations and Digit AnalysisConnections for interexchange calls are set up along paths that have been predeterminedby the network operator. A route is a path to a particular destination. Anexchange determines the call destination by analyzing the called number and thenselects an outgoing trunk in a route to the destination.We need to distinguish two destination types. The final destination (FDEST) of acall is the local exchange that serves the called party. An intermediate destination