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. Neighboring switches owned by different operators are connected at interconnect points.
The PSTN is not a full mesh network with the nodes of every operator directly connected to those of every other, which would be impractical and inefficient. Therefore, calls may be routed through intermediate operator networks before they reach their final destination. Efficient least-cost routing is an important procedure in PSTN routing.

Call routing

Each time a call is placed for routing, the destination number is entered by the calling party into their terminal. The destination number generally has two parts, an area code which generally identifies the geographical location of the destination telephone, and a telephone number unique within that area code that determines the specific destination terminal. The telephone number may be subdivided into a prefix that may identify a more specific geographic location or a telephone exchange, and the rest of the number. For example, in the number 555-1212, 301 is the area code and 555 is the prefix. Sometimes if the call is between two terminals in the same local area, then the area code may be omitted, but in other places, including the area code is required for the call to complete.
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 operator's network. Because of the hierarchical nature of the numbering plan, and its geographical basis, most calls between telephone numbers on the same network can be routed based on their area code and 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.

Dynamic Alternative Routing

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

Hybrid routing

Hybrid routing uses numbering plans and routing tables to permit the colocation, in the same area code, of switches using a deterministic routing scheme with switches using a non-deterministic routing scheme, such as flood search routing. Routing tables are constructed with no duplicate numbers, so that direct distance dialing service can be provided to all network subscribers. This may require the use of 10-digit telephone numbers.