Routing in the PSTN
Encyclopedia
Routing in the PSTN is the process used to route telephone calls
across the public switched telephone network
. This process is the same whether the call is made between two phones in the same locality, or across two different continents.
. Each exchange has many "neighbours", some of which are also owned by the same telephone operator, and some of which are owned by different operators. When neighbouring exchanges are owned by different operators, they are known as interconnect points.
This means that there is really only one virtual network in the world that enables any phone to call any other phone. This virtual network comprises many interconnected operators, each with their own exchange network
. Every operator can then route calls directly to their own customers, or pass them on to another operator if the call is not for one of their customers.
The PSTN is not a fully meshed network
with every operator connected to every other - that would be both impractical and inefficient. Therefore calls may be routed through intermediate operator networks before they reach their final destination
. One of the major problems in PSTN routing is determining how to route this call in the most cost effective and timely manner.
(also known as the called party) is entered by the calling party into their terminal. The destination number generally has two parts, a prefix
which generally identifies the geographical location of the destination telephone, and a number unique within that prefix that determines the specific destination terminal. Sometimes if the call is between two terminals in the same local area (that is, both terminals are on the same telephone exchange
), then the prefix may be omitted.
When a call is received by an exchange, there are two treatments that may be applied:
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
, 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 can be routed based only on their prefix using these routing tables.
Some calls however cannot be routed on the basis of prefix alone, for example non-geographical numbers, such as 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 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' paradox
, the addition of a new, shorter, and lower cost route can lead to an increase overall congestion. The network planner must take this into account when designing routing paths.
Telephone
The telephone , colloquially referred to as a phone, is a telecommunications device that transmits and receives sounds, usually the human voice. Telephones are a point-to-point communication system whose most basic function is to allow two people separated by large distances to talk to each other...
across the public switched telephone network
Public switched telephone network
The public switched telephone network is the network of the world's public circuit-switched telephone networks. It consists of telephone lines, fiber optic cables, microwave transmission links, cellular networks, communications satellites, and undersea telephone cables, all inter-connected by...
. This process is the same whether the call is made between two phones in the same locality, or across two different continents.
Relationship between exchanges and operators
Telephone calls must be routed across a network of multiple exchanges, potentially owned by different telephone carriers. The exchanges are all connected together using trunksTrunking
In modern communications, trunking is a concept by which a communications system can provide network access to many clients by sharing a set of lines or frequencies instead of providing them individually. This is analogous to the structure of a tree with one trunk and many branches. Examples of...
. Each exchange has many "neighbours", some of which are also owned by the same telephone operator, and some of which are owned by different operators. When neighbouring exchanges are owned by different operators, they are known as interconnect points.
This means that there is really only one virtual network in the world that enables any phone to call any other phone. This virtual network comprises many interconnected operators, each with their own exchange network
Core network
A core network, or network core, is the central part of a telecommunication network that provides various services to customers who are connected by the access network. One of the main functions is to route telephone calls across the PSTN....
. Every operator can then route calls directly to their own customers, or pass them on to another operator if the call is not for one of their customers.
The PSTN is not a fully meshed network
Mesh networking
Mesh networking is a type of networking where each node must not only capture and disseminate its own data, but also serve as a relay for other nodes, that is, it must collaborate to propagate the data in the network....
with every operator connected to every other - that would be both impractical and inefficient. Therefore calls may be routed through intermediate operator networks before they reach their final destination
Destination routing
Destination routing is a term used in telecommunications to define a sequential pathway that messages must pass through to reach a target destination.-Overview:...
. One of the major problems in PSTN routing is determining how to route this call in the most cost effective and timely manner.
Call routing
Each time a call is placed for routing, the destination numberTelephone number
A telephone number or phone number is a sequence of digits used to call from one telephone line to another in a public switched telephone network. When telephone numbers were invented, they were short — as few as one, two or three digits — and were given orally to a switchboard operator...
(also known as the called party) is entered by the calling party into their terminal. The destination number generally has two parts, a prefix
Telephone numbering plan
A telephone numbering plan is a type of numbering scheme used in telecommunications to allocate telephone numbers to subscribers and to route telephone calls in a telephone network. A closed numbering plan, such as found in North America, imposes a fixed total length to numbers...
which generally identifies the geographical location of the destination telephone, and a number unique within that prefix that determines the specific destination terminal. Sometimes if the call is between two terminals in the same local area (that is, both terminals are on the same telephone exchange
Telephone exchange
In the field of telecommunications, a telephone exchange or telephone switch is a system of electronic components that connects telephone calls...
), then the prefix may be omitted.
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
Network congestion
In data networking and queueing theory, network congestion occurs when a link or node is carrying so much data that its quality of service deteriorates. Typical effects include queueing delay, packet loss or the blocking of new connections...
.
The routing tables are generated centrally based on the known topology of the network
Network topology
Network topology is the layout pattern of interconnections of the various elements of a computer or biological network....
, the numbering plan
Telephone numbering plan
A telephone numbering plan is a type of numbering scheme used in telecommunications to allocate telephone numbers to subscribers and to route telephone calls in a telephone network. A closed numbering plan, such as found in North America, imposes a fixed total length to numbers...
, and analysis of traffic data
Traffic analysis
Traffic analysis is the process of intercepting and examining messages in order to deduce information from patterns in communication. It can be performed even when the messages are encrypted and cannot be decrypted. In general, the greater the number of messages observed, or even intercepted and...
. These are then downloaded to each exchange in the telephone operator
Telephone operator
A telephone operator is either* a person who provides assistance to a telephone caller, usually in the placing of operator assisted telephone calls such as calls from a pay phone, collect calls , calls which are billed to a credit card, station-to-station and person-to-person calls, and certain...
s network. Because of the hierarchical nature of the numbering plan, and its geographical basis, most calls can be routed based only on their prefix using these routing tables.
Some calls however cannot be routed on the basis of prefix alone, for example non-geographical numbers, such as toll-free or freephone calling
Toll-free telephone number
A toll-free, Freecall, Freephone, 800, 0800 or 1-800 number is a special telephone number which is free to the calling party, and instead the telephone carrier charges the called party the cost of the call...
. In these cases the Intelligent Network
Intelligent network
The Intelligent Network , is the standard network architecture specified in the . It is intended for fixed as well as mobile telecom networks...
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' paradox
Braess' paradox
Braess's paradox, credited to the mathematician Dietrich Braess, states that adding extra capacity to a network when the moving entities selfishly choose their route, can in some cases reduce overall performance...
, the addition of a new, shorter, and lower cost route can lead to an increase overall congestion. The network planner must take this into account when designing routing paths.