Routing in the PSTN

Routing in the PSTN is the process used to route telephone calls across the public switched telephone network.

Telephone calls are routed across a network of potentially many switching systems, often owned by different telephone carriers. Switching systems are connected with trunks. Each switch may have many neighbors. When neighboring switches are owned by different operators, they are known as interconnect points.

The PSTN is not a fully meshed network with every operator connected to every other, which would be both impractical and inefficient. Therefore, calls may be routed through intermediate operator networks before they reach their final destination. An importance procedure in PSTN routing is the determination of the most efficient and least costly route.

Each time a call is placed for routing, the destination number (also known as the called party) is entered by the calling party into their terminal. The destination number generally 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 specific destination terminal. Sometimes if the call is between two terminals in the same local area (that is, both terminals are on the same telephone exchange), then the prefix may be omitted.

When a call is received by an exchange, there are two treatments that may be applied:

Each exchange in the chain uses pre-computed routing tables to determine which connected exchange the onward call should be routed to. 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.

The routing tables are generated centrally based on the known topology of the network, the numbering plan, and analysis of traffic data. These are then downloaded to each exchange in the telephone operators network. Because of the hierarchical nature of the numbering plan, and its geographical basis, most calls can be routed based only on their prefix using these routing tables.

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