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Routing in the PSTN is the process by which telephone calls are routed around the public switched telephone network. Telephone exchanges are connected together with trunks. Each call that is to be routed contains a destination number that has two parts, a prefix which generally identifies the geographical location of the destination telephone, and a unique number that determines the precise equipment.
The exchange uses pre-computed routing tables, which are generated by batch processing at central locations based on the known topology of the network, the numbering plan, and analysis of traffic data. These are then downloaded to telephone exchanges at intervals. There are several alternative routes to any given destination, and the exchange can select dynamically between these in the event of link failure or congestion.
Because of the hierarchical nature of the numbering plan, and its geographical basis, most calls can be routed based only on their prefix. One exception to this are intelligent network services with non-geographical numbers, such as toll-free or freephone calling.
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