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In the context of the public switched telephone network, routing is the process by which telephone calls are routed around the 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 number unique within that prefix that determines the precise destination.

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 may be 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.

Routing in circuit-switched networks involves creating a path from one customer to another for the duration of each call. Routing decisions are an important part of this process as they determine which channels or circuits are used to connect the customers for the duration of the call. In a PSTN exchange, routing is typically performed using a routing table that contains the pre-defined routes for a connection. In such a system, alternative routes exist, which are specified in the routing tables [1].

In determining routing plans, special attention is paid for example to ensure that two routes do not mutually overflow to each other, otherwise congestion will cause a destination to be completely blocked.

According to Braess' paradox, the addition of a new, shorter, and lower cost route can lead to an increase overall congestion [1, 2]. The network planner must take this into account when designing routing paths.

One approach to routing involves the use of Dynamic Alternative Routing (DAR) [1]. DAR makes use of the distributed nature of a telecommunications network and its inherent randomness to dynamically determine optimal routing paths. This method generates a distributed, random, parallel computing platform that minimises congestion across the network, and is able to adapt to take changing traffic patterns and demands into account [1].

[1] Kelly F.P., Network Routing, Statistical Laboratory, University of Cambridge, 16 Mill Lane, Cambridge CB2 1SB, UK.

[2] Wainwright M., A Small Road Network, Included in: Kennedy I., Teletraffic Lecture Notes, School of Electrical and Information Engineering, University of the Witwatersrand, 2003. EQEQEQE