Routing in the PSTN is the process of forwarding telephone calls between the constituent telephone networks that comprise the

public switched telephone network The public switched telephone network (PSTN) is the aggregate of the world's telephone networks that are operated by national, regional, or local telephony operators. It provides infrastructure and services for public telephony. The PSTN consists o ...

(PSTN). Telephone calls are routed across a network of potentially many switching systems, often owned by different

telephone carrier A telecommunications company is a kind of electronic communications service provider, more precisely a telecommunications service provider (TSP), that provides telecommunications services such as telephony and data communications access. Many t ...

s. Switching systems are connected with trunks. Each switch may have many neighbors. Neighboring switches owned by different operators are connected at interconnect points. The PSTN is a network that uses destination routing to direct calls from origin to the recipient. It is not a full

mesh network A mesh network is a local area network topology in which the infrastructure nodes (i.e. bridges, switches, and other infrastructure devices) connect directly, dynamically and non-hierarchically to as many other nodes as possible and cooperate wit ...

with the

nodes In general, a node is a localized swelling (a "knot") or a point of intersection (a vertex). Node may refer to: In mathematics *Vertex (graph theory), a vertex in a mathematical graph *Vertex (geometry), a point where two or more curves, lines, ...

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

least-cost routing In voice telecommunications, least-cost routing (LCR) is the process of selecting the path of outbound communications traffic based on cost. Within a telecoms carrier, an LCR team might periodically (monthly, weekly or even daily) choose between ro ...

is an important procedure in PSTN routing.

Call routing

Each time a call is placed for routing, the 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

prefix A prefix is an affix which is placed before the stem of a word. Particularly in the study of languages, a prefix is also called a preformative, because it alters the form of the word to which it is affixed. Prefixes, like other affixes, can b ...

that may identify a more specific geographic location or a

telephone exchange A telephone exchange, telephone switch, or central office is a central component of a telecommunications system in the public switched telephone network (PSTN) or in large enterprises. It facilitates the establishment of communication circuits ...

, and the rest of the number. For example, in the number (301) 555-1212, 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 Year 988 ( CMLXXXVIII) was a leap year starting on Sunday of the Julian calendar. Events By place Byzantine Empire * Fall – Emperor Basil II, supported by a contingent of 6,000 Varangians (the future Varangian Guard), organiz ...

, 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. 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 congestion. The routing tables are generated centrally based on the known topology of the network, the

numbering plan A telephone numbering plan is a type of numbering scheme used in telecommunication to assign telephone numbers to subscriber telephones or other telephony endpoints. Telephone numbers are the addresses of participants in a telephone network, reach ...

, and analysis of traffic data. These are then downloaded to each exchange in the 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 A non-geographic number is a type of telephone number that is not linked to any specific locality. Such numbers are an alternative to the traditional 'landline' numbers that are assigned geographically using a system of location-specific area cod ...

such as toll-free or freephone calling. In these cases the

Intelligent Network The Intelligent Network (IN) is the standard network architecture specified in the ITU-T Q.1200 series recommendations. It is intended for fixed as well as mobile telecom networks. It allows operators to differentiate themselves by providing ...

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 Braess's paradox is the observation that adding one or more roads to a road network can slow down overall road traffic, traffic flow through it. The paradox was first discovered by Arthur Cecil Pigou, Arthur Pigou in 1920, and Stigler's law of ep ...

, the addition of a new, shorter, and lower cost route can lead to an increase in overall congestion.Wainwright M., ''A Small Road Network'', Included in: Kennedy I., Teletraffic Lecture Notes, School of Electrical and Information Engineering, University of the Witwatersrand, 2003.

Hybrid routing

Hybrid routing uses numbering plans and routing tables to permit the colocation, in the same

area code A telephone numbering plan is a type of numbering scheme used in telecommunication to assign telephone numbers to subscriber telephones or other telephony endpoints. Telephone numbers are the addresses of participants in a telephone network, rea ...

, of switches using a

deterministic routing In telecommunications, deterministic routing is the advance determination of the routes between given pairs of nodes. Examples: #In a network where routing is controlled by a telephone switch or network switch, switching in which the routes between ...

scheme with switches using a non-deterministic routing scheme, such as

flood search routing In a telephone network, flood search routing is non-deterministic routing in which a dialed number received at a switch is transmitted to all switches, ''i.e.,'' flooded, in the area code directly connected to that switch; if the dialed number is no ...

. Routing tables are constructed with no duplicate numbers, so that

direct distance dialing Direct distance dialing (DDD) is a telecommunications service in North America by which a caller may call any other subscriber outside the local calling area without operator assistance, DDD was introduced in the United States in 1951, on a tri ...

service can be provided to all

network Network, networking and networked may refer to: Science and technology * Network theory, the study of graphs as a representation of relations between discrete objects * Network science, an academic field that studies complex networks Mathematics ...

subscribers. This may require the use of ten-digit

telephone number A telephone number is the address of a Telecommunications, telecommunication endpoint, such as a telephone, in a telephone network, such as the public switched telephone network (PSTN). A telephone number typically consists of a Number, sequ ...

Trunk reservation

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 the capacity of each trunk for directly routed calls.

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 BT Group plc (formerly British Telecom) is a British multinational telecommunications holding company headquartered in London, England. It has operations in around 180 countries and is the largest provider of fixed-line, broadband and mobile ...

describes dynamic alternative routing as:

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.

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 The University of Cambridge is a Public university, public collegiate university, collegiate research university in Cambridge, England. Founded in 1209, the University of Cambridge is the List of oldest universities in continuous operation, wo ...

under

Frank Kelly Francis Kelly (28 December 1938 – 28 February 2016) was an Irish actor, singer and writer, whose career covered television, radio, theatre, music, screenwriting and film. He is best remembered for playing Father Jack Hackett in the Channel 4 ...

, in collaboration with BT Research, and a British patent was granted in 1985. DAR was implemented in British Telecom's national network in 1996.

References

External links

{{FS1037C
Bibliography: Dynamic Alternative Routing
–

, Statistical Laboratory, University of Cambridge * Gibbens, R. & Kelly, F.P. & Key, P. (1988)
Dynamic alternative routing - modelling and behaviour
, 12th

International Teletraffic Congress The International Teletraffic Congress (ITC) is the first international conference in networking science and practice. It was created in 1955 by Arne Jensen to initially cater to the emerging need to understand and model traffic in telephone network ...

, Turin. Via

ResearchGate ResearchGate is a European commercial social networking site for scientists and researchers to share papers, ask and answer questions, and find collaborators. According to a 2014 study by ''Nature'' and a 2016 article in ''Times Higher Education' ...