Editing Routing in the PSTN (section)

==Dynamic alternative routing==
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.

A 1989 US patent by [[British Telecommunications]] describes dynamic alternative routing as:<ref>{{US patent reference|number=4862496A|issue-date=29 August 1989|inventor=Francis P. Kelly, Richard J. Gibbens, Peter B. Key, Paul A. Turton, Roger R. Stacey, Martin J. Whitehead|title=Routing of network traffic}}</ref><blockquote class="templatequote">A call between two nodes interconnected by a direct link is first offered to the direct route, and if that is blocked it is offered to a currently nominated two-link alternative route between the two nodes. If that route is busy, the call is lost, and a randomly chosen two-link route is assigned to be the new current nominated alternative route.</blockquote>Thus the chosen route varies to take changing traffic patterns and demands into account. The technique was the subject of Richard Gibbens' doctoral thesis at [[Cambridge University]] under [[Frank Kelly (mathematician)|Frank Kelly]],<ref>{{Cite journal|last=Kelley|first=Frank|date=30 September 2018|title=Obituary: Richard Gibbens|journal=The Caian|volume=2017-18|pages=117}}</ref> in collaboration with [[BT Research]], and a British patent was granted in 1985.<ref>{{Cite web|date=December 1985|title=Patent GB8531138: Telecommunications network|url=https://patents.google.com/patent/GB8531138D0/en?oq=GB8531138|access-date=19 October 2020|via=Google Patents}}</ref> DAR was implemented in British Telecom's national network in 1996.<ref>{{Cite web|date=1996|title=DAR implementation|url=http://www.statslab.cam.ac.uk/~frank/DAR/bt.html|access-date=2020-10-19|website=[[Statistical Laboratory, University of Cambridge]]}}</ref>