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230:, 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
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211:(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
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When congestion causes many calls to use indirect alternative routes that pass through more exchanges than a direct route, the overall capacity of the network is reduced since each of these calls occupies several inter-exchange trunks. Trunk reservation reduces this effect by reserving a fraction of
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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
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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
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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.
272:'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.
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of every operator directly connected to those of every other, which would be impractical and inefficient. Therefore, calls may be routed through intermediate operator networks before they reach their final destination. Efficient
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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.
515:, Francis P. Kelly, Richard J. Gibbens, Peter B. Key, Paul A. Turton, Roger R. Stacey, Martin J. Whitehead, "Routing of network traffic", issued 29 August 1989
234:, 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.
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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.
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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
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474:, Included in: Kennedy I., Teletraffic Lecture Notes, School of Electrical and Information Engineering, University of the Witwatersrand, 2003.
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143:. Each switch may have many neighbors. Neighboring switches owned by different operators are connected at interconnect points.
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Or the call must be placed to one of the neighboring exchanges through a connecting trunk for onward routing.
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Telephone calls are routed across a network of potentially many switching systems, often owned by different
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Please help update this article to reflect recent events or newly available information.
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When a call is received by an exchange, there are two treatments that may be applied:
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Process used to route telephone calls across the public switched telephone network
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Some calls, however, cannot be routed on the basis of prefix alone, for example
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is used to route the call instead of using the pre-computed routing tables.
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scheme with switches using a non-deterministic routing scheme, such as
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Hybrid routing uses numbering plans and routing tables to permit the
321:. Routing tables are constructed with no duplicate numbers, so that
531:
Kelley, Frank (30 September 2018). "Obituary: Richard
Gibbens".
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to direct calls from origin to the recipient. It is not a full
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The routing tables are generated centrally based on the known
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between the constituent telephone networks that comprise the
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that may identify a more specific geographic location or a
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the capacity of each trunk for directly routed calls.
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Dynamic alternative routing - modelling and behaviour
268:. These are then downloaded to each exchange in the
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Gibbens, R. & Kelly, F.P. & Key, P. (1988) "
329:subscribers. This may require the use of ten-digit
49:. Unsourced material may be challenged and removed.
635:, Statistical Laboratory, University of Cambridge
574:Statistical Laboratory, University of Cambridge
363:be the new current nominated alternative route.
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547:"Patent GB8531138: Telecommunications network"
484:Gibbens, Richard; Turner, Stephen (May 1997).
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207:Each time a call is placed for routing, the
163:is an important procedure in PSTN routing.
629:Bibliography: Dynamic Alternative Routing
358:describes dynamic alternative routing as:
109:Learn how and when to remove this message
139:. Switching systems are connected with
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486:"Call routing in telephone networks"
47:adding citations to reliable sources
644:International Teletraffic Congress
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418:Bentley, Drake (March 12, 2021).
130:public switched telephone network
603: This article incorporates
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146:The PSTN is a network that uses
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616:General Services Administration
325:service can be provided to all
34:needs additional citations for
281:toll-free or freephone calling
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124:is the process of forwarding
557:– via Google Patents.
394:General Toll Switching Plan
346:Dynamic alternative routing
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425:Milwaukee Journal Sentinel
356:British Telecommunications
184:This section needs to be
452:. NANPA. January 8, 2021
375:, in collaboration with
323:direct distance dialing
258:topology of the network
611:Federal Standard 1037C
605:public domain material
513:US patent 4862496A
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313:, of switches using a
277:non-geographic numbers
399:Operator Toll Dialing
389:PSTN network topology
315:deterministic routing
283:. In these cases the
58:"Routing in the PSTN"
569:"DAR implementation"
472:A Small Road Network
369:Cambridge University
354:A 1989 US patent by
319:flood search routing
43:improve this article
285:Intelligent Network
148:destination routing
122:Routing in the PSTN
270:telephone operator
264:, and analysis of
217:telephone exchange
209:destination number
161:least-cost routing
137:telephone carriers
337:Trunk reservation
331:telephone numbers
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41:Please help
36:verification
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664:Teletraffic
633:Frank Kelly
377:BT Research
373:Frank Kelly
658:Categories
581:2020-10-19
553:19 October
497:19 October
405:References
307:colocation
251:congestion
69:newspapers
674:Telephony
533:The Caian
431:March 14,
311:area code
154:with the
99:June 2016
642:", 12th
456:March 9,
383:See also
279:such as
132:(PSTN).
669:Routing
327:network
186:updated
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577:. 1996
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555:2020
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