Routing in the PSTN

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Routing in the PSTN is how telephone calls are directed through the public switched telephone network. Calls move through many switching systems connected by trunks, and no single path links every operator to every other. Instead, calls travel through several networks to reach their destination.

How numbers are used
- The destination number has an area code and a local number. In the US, the area code is called an NPA and the next part is the NXX (the exchange/prefix).
- Since 2021, many local calls must include the area code due to the national 988 hotline change.

How routing decisions are made
- Each switch uses routing tables to decide the next hop. These tables are created from the network layout, the numbering plan, and traffic data, then downloaded to all switches.
- Most calls within the same network can be routed by area code and prefix. Non-geographic numbers (like toll-free) may use Intelligent Network routing instead of pre-computed tables.

Handling congestion and routing choices
- Routing plans avoid having two routes that overload each other, to prevent a destination from becoming blocked.
- Sometimes adding a new short route can actually make congestion worse (Braess’s paradox).
- Hybrid routing combines fixed (deterministic) routing with flexible (non-deterministic) routing in the same area.
- Some routes require ten-digit dialing to reach the destination.

Trunk capacity and protection
- If many calls take indirect routes, network capacity can drop because those calls use more trunks.
- Trunk reservation reserves part of trunk capacity for directly routed calls to protect performance.

Dynamic routing (DAR)
- Dynamic Alternative Routing is a decentralized method that uses local information, mainly which trunks are available.
- If the direct path is busy, a two-link alternative is tried; if that is also busy, another two-link route may be chosen.
- This approach helps routing adapt to changing traffic patterns.

In short, PSTN routing uses a mix of planned tables and flexible routing to get calls where they need to go efficiently, while managing congestion and changing network conditions.