Routing in the PSTN

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

Relationship between exchanges and operators

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.

Call routing

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:

• Either the destination terminal is directly connected to that exchange, in which case the call is placed down that connection and the destination terminal rings.
• Or the call must be placed to one of the neighboring exchanges through a connecting trunk for onward routing.

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.

Some calls however cannot be routed on the basis of prefix alone, for example non-geographic numbers, such as toll-free or freephone calling. In these cases the Intelligent Network is used to route the call instead of using the pre-computed routing tables.

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]

Dynamic Alternative Routing

One approach to routing involves the use of Dynamic Alternative Routing (DAR). 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.

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