It 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.
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.
According to Braess's paradox, the addition of a new, shorter, and lower cost route can lead to an increase in overall congestion.
Routing tables are constructed with no duplicate numbers, so that direct distance dialing service can be provided to all network subscribers.
[4] Dynamic alternative routing (DAR) is a method of decentralized dynamic routing of telephone calls that uses only a limited amount of local information, which consists of trunk reservation thresholds, or simply knowing the collection of outgoing trunks from the exchange.
The technique was the subject of Richard Gibbens' doctoral thesis at Cambridge University under Frank Kelly,[6] in collaboration with BT Research, and a British patent was granted in 1985.