Routing in the PSTN
Routing in the PSTN is the process used to route telephone calls across the public switched telephone network. This process is the same whether the call is made between two phones in the same locality, or across two different continents.
Relationship between exchanges and operators 
Telephone calls must be routed across a network of multiple exchanges, potentially owned by different telephone carriers. The exchanges are all connected using trunks. Each exchange has many "neighbours", some of which are also owned by the same telephone operator, and some of which are owned by different operators. When neighbouring exchanges are owned by different operators, they are known as interconnect points.
This means that there is really only one virtual network in the world that enables any phone to call any other phone. This virtual network comprises many interconnected operators, each with their own exchange network. Every operator can then route calls directly to their own customers, or pass them on to another operator if the call is not for one of their customers.
The PSTN is not a fully meshed network with every operator connected to every other - that would be both impractical and inefficient. Therefore calls may be routed through intermediate operator networks before they reach their final destination. One of the major problems in PSTN routing is determining how to route this call in the most cost effective and timely manner.
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. The network planner must take this into account when designing routing paths.
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.
- Wainwright M., A Small Road Network, Included in: Kennedy I., Teletraffic Lecture Notes, School of Electrical and Information Engineering, University of the Witwatersrand, 2003.