Editing Routing in the PSTN (section)

== Call routing ==
{{update|section|date=February 2020}}
Each time a call is placed for routing, the [[telephone number|destination number]] (also known as the called party) 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 [[Telephone numbering plan|prefix]] that may identify a more specific geographic location or a [[telephone exchange]], and the rest of the number.  For example, in the number <span style="color: blue">(301)</span> <span style="color: #0070d0">555</span>-<span style="color: green">1212</span>, 301 is the area code and 555 is the prefix. (The area code is sometimes known as an "NPA," and the area-code and prefix combination is known as an "NPA-NXX.") In the United States, prior to 2021, in some locations, if the call was between two terminals in the same local area (or, two terminals on the same telephone exchange), then the area code could be omitted. With the introduction in October, 2021 of the national suicide hotline number [[988 Suicide & Crisis Lifeline|988]], in most cases, the ability to dial local numbers without the area code was eliminated, and the area code is required for a call to a regular phone number to complete.<ref>{{cite news|url=https://www.jsonline.com/story/news/2021/03/12/area-codes-soon-required-make-phone-calls-wisconsin/6945752002/|title=Wisconsin residents will soon be required to dial the area code in order to make local calls|last=Bentley|first=Drake|date=March 12, 2021|work=[[Milwaukee Journal Sentinel]]|access-date=March 14, 2021}}</ref><ref>{{cite web|url=https://nationalnanpa.com/pdf/PL_556.pdf|date=January 8, 2021|publisher=NANPA|title=Transition to 10-digit dialing (for 988 as 3-digit access to National Suicide Prevention Hotline)|access-date=March 9, 2021}}</ref>

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 [[Network congestion|congestion]].

The routing tables are generated centrally based on the known [[Network topology|topology of the network]], the [[Telephone numbering plan|numbering plan]], and analysis of [[Traffic analysis|traffic data]].  These are then downloaded to each exchange in the [[telephone company|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 telephone number|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 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's paradox]], the addition of a new, shorter, and lower cost route can lead to an increase in overall congestion.<ref name="wainwright">Wainwright M., ''A Small Road Network'', Included in: Kennedy I., Teletraffic Lecture Notes, School of Electrical and Information Engineering, University of the Witwatersrand, 2003.</ref>