Routing in the PSTN is the process of forwarding

telephone calls A telephone is a telecommunications device that permits two or more users to conduct a conversation when they are too far apart to be easily heard directly. A telephone converts sound, typically and most efficiently the human voice, into ...

between the constituent telephone networks that comprise the

public switched telephone network The public switched telephone network (PSTN) provides infrastructure and services for public telecommunication. The PSTN is the aggregate of the world's circuit-switched telephone networks that are operated by national, regional, or local telep ...

(PSTN). Telephone calls are routed across a network of potentially many switching systems, often owned by different telephone carriers. 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 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 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. Adding it to the beginning of one word changes it into another word. For example, when the prefix ''un-'' is added to the word ''happy'', it creates the word ''unhappy''. Particul ...

that may identify a more specific geographic location or a

telephone exchange telephone exchange, telephone switch, or central office is a telecommunications system used in the public switched telephone network (PSTN) or in large enterprises. It interconnects telephone subscriber lines or virtual circuits of digital syste ...

, 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.") Sometimes if the call is between two terminals in the same local area (or, both terminals are on the same

), then the area code may be omitted, but in other places, including the area code is required for the call 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, reac ...

, 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, 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 v ...

is used to route the call instead of using the pre-computed routing tables. In determining routing plans, special attention is paid for example 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 traffic flow through it. The paradox was discovered by the German mathematician Dietrich Braess in 1968. The paradox may have analogies in ...

, 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, r ...

, of switches using a deterministic routing scheme with switches using a non-deterministic routing scheme, such as flood search routing. Routing tables are constructed with no duplicate numbers, so that

direct distance dialing Direct distance dialing (DDD) is a telecommunication service feature in North America by which a caller may, without operator assistance, call any other user outside the local calling area. Direct dialing by subscribers typically requires extra d ...

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 Mathematic ...

subscribers. This may require the use of ten-digit

telephone number A telephone number is a sequence of digits assigned to a landline telephone subscriber station connected to a telephone line or to a wireless electronic telephony device, such as a radio telephone or a mobile telephone, or to other devices f ...

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. An 1989 US patent by

British Telecommunications BT Group plc (trading as BT and 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, broa ...

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 , mottoeng = Literal: From here, light and sacred draughts. Non literal: From this place, we gain enlightenment and precious knowledge. , established = , other_name = The Chancellor, Masters and Schola ...

under Frank Kelly, 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
– Frank Kelly, Statistical Laboratory, University of Cambridge * Gibbens, R. & Kelly, F.P. & Key, P. (1988)
Dynamic alternative routing - modelling and behaviour
, 12th International Teletraffic Congress, 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'' ...