Public switched telephone network

Public switched telephone network

Overview
The public switched telephone network (PSTN) is the network of the world's public circuit-switched
Circuit switching
Circuit switching is a methodology of implementing a telecommunications network in which two network nodes establish a dedicated communications channel through the network before the nodes may communicate. The circuit guarantees the full bandwidth of the channel and remains connected for the...

 telephone network
Telephone network
A telephone network is a telecommunications network used for telephone calls between two or more parties.There are a number of different types of telephone network:...

s. It consists of telephone line
Telephone line
A telephone line or telephone circuit is a single-user circuit on a telephone communication system...

s, fiber optic cables, microwave transmission
Microwave transmission
Microwave transmission refers to the technology of transmitting information or power by the use of radio waves whose wavelengths are conveniently measured in small numbers of centimeters; these are called microwaves. This part of the radio spectrum ranges across frequencies of roughly...

 links, cellular network
Cellular network
A cellular network is a radio network distributed over land areas called cells, each served by at least one fixed-location transceiver known as a cell site or base station. When joined together these cells provide radio coverage over a wide geographic area...

s, communications satellite
Communications satellite
A communications satellite is an artificial satellite stationed in space for the purpose of telecommunications...

s, and undersea telephone cables, all inter-connected by switching centers, thus allowing any telephone in the world to communicate with any other. Originally a network of fixed-line
Outside plant
In telecommunication, the term outside plant has the following meanings:*In civilian telecommunications, outside plant refers to all of the physical cabling and supporting infrastructure , and any associated hardware located between a demarcation point in a switching facility and a demarcation...

 analog
Analog signal
An analog or analogue signal is any continuous signal for which the time varying feature of the signal is a representation of some other time varying quantity, i.e., analogous to another time varying signal. It differs from a digital signal in terms of small fluctuations in the signal which are...

 telephone systems, the PSTN is now almost entirely digital
Digital
A digital system is a data technology that uses discrete values. By contrast, non-digital systems use a continuous range of values to represent information...

 in its core and includes mobile
Mobile phone
A mobile phone is a device which can make and receive telephone calls over a radio link whilst moving around a wide geographic area. It does so by connecting to a cellular network provided by a mobile network operator...

 as well as fixed telephones.

The technical operation of the PSTN utilizes standards created by the ITU-T
ITU-T
The ITU Telecommunication Standardization Sector is one of the three sectors of the International Telecommunication Union ; it coordinates standards for telecommunications....

.
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Encyclopedia
The public switched telephone network (PSTN) is the network of the world's public circuit-switched
Circuit switching
Circuit switching is a methodology of implementing a telecommunications network in which two network nodes establish a dedicated communications channel through the network before the nodes may communicate. The circuit guarantees the full bandwidth of the channel and remains connected for the...

 telephone network
Telephone network
A telephone network is a telecommunications network used for telephone calls between two or more parties.There are a number of different types of telephone network:...

s. It consists of telephone line
Telephone line
A telephone line or telephone circuit is a single-user circuit on a telephone communication system...

s, fiber optic cables, microwave transmission
Microwave transmission
Microwave transmission refers to the technology of transmitting information or power by the use of radio waves whose wavelengths are conveniently measured in small numbers of centimeters; these are called microwaves. This part of the radio spectrum ranges across frequencies of roughly...

 links, cellular network
Cellular network
A cellular network is a radio network distributed over land areas called cells, each served by at least one fixed-location transceiver known as a cell site or base station. When joined together these cells provide radio coverage over a wide geographic area...

s, communications satellite
Communications satellite
A communications satellite is an artificial satellite stationed in space for the purpose of telecommunications...

s, and undersea telephone cables, all inter-connected by switching centers, thus allowing any telephone in the world to communicate with any other. Originally a network of fixed-line
Outside plant
In telecommunication, the term outside plant has the following meanings:*In civilian telecommunications, outside plant refers to all of the physical cabling and supporting infrastructure , and any associated hardware located between a demarcation point in a switching facility and a demarcation...

 analog
Analog signal
An analog or analogue signal is any continuous signal for which the time varying feature of the signal is a representation of some other time varying quantity, i.e., analogous to another time varying signal. It differs from a digital signal in terms of small fluctuations in the signal which are...

 telephone systems, the PSTN is now almost entirely digital
Digital
A digital system is a data technology that uses discrete values. By contrast, non-digital systems use a continuous range of values to represent information...

 in its core and includes mobile
Mobile phone
A mobile phone is a device which can make and receive telephone calls over a radio link whilst moving around a wide geographic area. It does so by connecting to a cellular network provided by a mobile network operator...

 as well as fixed telephones.

The technical operation of the PSTN utilizes standards created by the ITU-T
ITU-T
The ITU Telecommunication Standardization Sector is one of the three sectors of the International Telecommunication Union ; it coordinates standards for telecommunications....

. These standards allow different networks in different countries to interconnect
Interconnection
In telecommunications, interconnection is the physical linking of a carrier's network with equipment or facilities not belonging to that network...

 seamlessly. There is also a single global address space
Address space
In computing, an address space defines a range of discrete addresses, each of which may correspond to a network host, peripheral device, disk sector, a memory cell or other logical or physical entity.- Overview :...

 for telephone numbers based on the E.163 and E.164
E.164
E.164 is an ITU-T recommendation that defines the international public telecommunication numbering plan used in the PSTN and some other data networks. It also defines the format of telephone numbers. E.164 numbers can have a maximum of fifteen digits and are usually written with a + prefix...

 standards. The combination of the interconnected networks and the single numbering plan make it possible for any phone in the world to dial any other phone.

History


The first telephones had no network but were in private use, wired together in pairs. Users who wanted to talk to different people had as many telephones as necessary for the purpose. A user who wished to speak whistled into the transmitter until the other party heard.

Soon, however, a bell was added for signalling, and then a switch hook, and telephones took advantage of the exchange principle already employed in telegraph networks. Each telephone was wired to a local 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...

, and the exchanges were wired together with trunks
Trunking
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...

. Networks were connected together in a hierarchical manner until they spanned cities, countries, continents and oceans. This was the beginning of the PSTN, though the term was unknown for many decades.

Automation introduced pulse dialing between the phone and the exchange, and then among exchanges, followed by more sophisticated address signaling including multi-frequency
Multi-frequency
In telephony, multi-frequency signaling is an outdated, in-band signaling technique. Numbers were represented in a two-out-of-five code for transmission from a multi-frequency sender, to be received by a multi-frequency receiver in a distant telephone exchange...

, culminating in the SS7 network that connected most exchanges by the end of the 20th century.

The growth of the PSTN meant that traffic engineering techniques needed to be deployed to deliver quality of service
Quality of service
The quality of service refers to several related aspects of telephony and computer networks that allow the transport of traffic with special requirements...

 (QoS) guarantees for the users. The work of A.K. Erlang
Agner Krarup Erlang
Agner Krarup Erlang was a Danish mathematician, statistician and engineer, who invented the fields of traffic engineering and queueing theory....

 established the mathematical foundations of methods required to determine the capacity requirements and configuration of equipment and the number of personnel required to deliver a specific level of service.

In the 1970s the telecommunications industry began implementing packet switched network
Packet switched network
A packet-switched network is a digital communications network that groups all transmitted data, irrespective of content, type, or structure into suitably sized blocks, called packets...

 data services using the X.25
X.25
X.25 is an ITU-T standard protocol suite for packet switched wide area network communication. An X.25 WAN consists of packet-switching exchange nodes as the networking hardware, and leased lines, Plain old telephone service connections or ISDN connections as physical links...

 protocol transported over much of the end-to-end equipment as was already in use in the PSTN.

In the 1980s the industry began planning for digital services assuming they would follow much the same pattern as voice services, and conceived a vision of end-to-end circuit switched services, known as the Broadband Integrated Services Digital Network
Broadband Integrated Services Digital Network
In the 1980s the telecommunications industry expected that digital services would follow much the same pattern as voice services did on the public switched telephone network, and conceived a grandiose end-to-end circuit switched services, known as Broadband Integrated Services Digital Network...

 (B-ISDN). The B-ISDN vision has been overtaken by the disruptive technology
Disruptive technology
A disruptive technology or disruptive innovation is an innovation that helps create a new market and value network, and eventually goes on to disrupt an existing market and value network , displacing an earlier technology there...

 of the Internet.

Today, only the oldest parts of the telephone network still use analog technology for anything other than the last mile
Last mile
The "last mile" or "last kilometer" is the final leg of delivering connectivity from a communications provider to a customer. The phrase is therefore often used by the telecommunications and cable television industries. The actual distance of this leg may be considerably more than a mile,...

 loop
Local loop
In telephony, the local loop is the physical link or circuit that connects from the demarcation point of the customer premises to the edge of the carrier or telecommunications service provider's network...

 to the end user, and in recent years digital services have been increasingly rolled out to end users using services such as DSL
Digital Subscriber Line
Digital subscriber line is a family of technologies that provides digital data transmission over the wires of a local telephone network. DSL originally stood for digital subscriber loop. In telecommunications marketing, the term DSL is widely understood to mean Asymmetric Digital Subscriber Line ,...

, ISDN, FTTx and cable modem
Cable modem
A cable modem is a type of network bridge and modem that provides bi-directional data communication via radio frequency channels on a HFC and RFoG infrastructure. Cable modems are primarily used to deliver broadband Internet access in the form of cable Internet, taking advantage of the high...

 systems.

There are a number of large private telephone networks which are not linked to the PSTN, usually for military purposes. There are also private networks run by large companies which are linked to the PSTN only through limited gateways
Gateway (telecommunications)
In telecommunications, the term gateway has the following meaning:*In a communications network, a network node equipped for interfacing with another network that uses different protocols....

, like a large private branch exchange (PBX).

PSTN operators


The task of building the networks and selling services to customers fell to the network operators
Telephone company
A telephone company is a service provider of telecommunications services such as telephony and data communications access. Many were at one time nationalized or state-regulated monopolies...

. The first company to be incorporated to provide PSTN services was the Bell Telephone Company
Bell Telephone Company
The Bell Telephone Company, a common law joint stock company, was organized in Boston, Massachusetts on July 9, 1877 by Alexander Graham Bell's father-in-law Gardiner Greene Hubbard, who also helped organize a sister company — the New England Telephone and Telegraph Company...

 in the United States.

In some countries however, the job of providing telephone networks fell to government as the investment required was very large and the provision of telephone service was increasingly becoming an essential public utility
Public utility
A public utility is an organization that maintains the infrastructure for a public service . Public utilities are subject to forms of public control and regulation ranging from local community-based groups to state-wide government monopolies...

. For example, the General Post Office
General Post Office
General Post Office is the name of the British postal system from 1660 until 1969.General Post Office may also refer to:* General Post Office, Perth* General Post Office, Sydney* General Post Office, Melbourne* General Post Office, Brisbane...

 in the United Kingdom brought together a number of private companies to form a single nationalised company
State ownership
State ownership, also called public ownership, government ownership or state property, are property interests that are vested in the state, rather than an individual or communities....

.

In recent decades however, these state monopolies were broken up or sold off through privatization
Privatization
Privatization is the incidence or process of transferring ownership of a business, enterprise, agency or public service from the public sector to the private sector or to private non-profit organizations...

.

Regulation of the PSTN


In most countries, the central government has a regulator dedicated to monitoring the provision of PSTN services in that country. Their tasks may be for example to ensure that end customers are not over-charged for services where monopolies may exist. They may also regulate the prices charged between the operators to carry each others traffic
Interconnection
In telecommunications, interconnection is the physical linking of a carrier's network with equipment or facilities not belonging to that network...

.

Network topology



The PSTN network architecture had to evolve over the years to support increasing numbers of subscribers, calls, connections to other countries, direct dialling and so on. The model developed by the US and Canada was adopted by other nations, with adaptations for local markets.

The original concept was that the 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...

s are arranged into hierarchies, so that if a call cannot be handled in a local cluster, it is passed to one higher up for onward routing. This reduced the number of connecting trunks required between operators over long distances and also kept local traffic separate.

However, in modern networks the cost of transmission and equipment is lower and, although hierarchies still exist, they are much flatter, with perhaps only two layers.

Digital channels



As described above, most automated 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...

s now use digital switching rather than mechanical or analog switching. The trunks connecting the exchanges are also digital, called circuits or channels. However analog two-wire circuit
Two-wire circuit
In telecommunication, a two-wire circuit is characterized by supporting transmission in two directions simultaneously, as opposed to four-wire circuits, which have separate pairs for transmit and receive. In either case they are twisted pairs. Telephone lines are almost all two wire, while trunks...

s are still used to connect the last mile
Last mile
The "last mile" or "last kilometer" is the final leg of delivering connectivity from a communications provider to a customer. The phrase is therefore often used by the telecommunications and cable television industries. The actual distance of this leg may be considerably more than a mile,...

 from the exchange to the telephone in the home (also called the local loop
Local loop
In telephony, the local loop is the physical link or circuit that connects from the demarcation point of the customer premises to the edge of the carrier or telecommunications service provider's network...

). To carry a typical phone call from a calling party
Calling party
The calling party is a person who initiates a telephone call over the public switched telephone network, usually by dialing a telephone number....

 to a called party
Called party
The called party is a person who answers a telephone call. The person who initiates a telephone call is the calling party....

, the analog audio signal is digitized at an 8 kHz sample rate with 8-bit resolution using a special type of nonlinear pulse code modulation known as G.711
G.711
G.711 is an ITU-T standard for audio companding. It is primarily used in telephony. The standard was released for usage in 1972. Its formal name is Pulse code modulation of voice frequencies. It is required standard in many technologies, for example in H.320 and H.323 specifications. It can also...

. The call is then transmitted from one end to another via telephone exchanges. The call is switched using a call set up protocol (usually ISUP) between the telephone exchanges under an overall routing strategy
Routing in the PSTN
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....

.

The call is carried over the PSTN using a 64 kbit/s channel, originally designed by Bell Labs
Bell Labs
Bell Laboratories is the research and development subsidiary of the French-owned Alcatel-Lucent and previously of the American Telephone & Telegraph Company , half-owned through its Western Electric manufacturing subsidiary.Bell Laboratories operates its...

. The name given to this channel is Digital Signal 0
Digital Signal 0
Digital Signal 0 is a basic digital signaling rate of 64 kbit/s, corresponding to the capacity of one voice-frequency-equivalent channel...

 (DS0). The DS0 circuit is the basic granularity of circuit switching
Circuit switching
Circuit switching is a methodology of implementing a telecommunications network in which two network nodes establish a dedicated communications channel through the network before the nodes may communicate. The circuit guarantees the full bandwidth of the channel and remains connected for the...

 in a telephone exchange. A DS0 is also known as a timeslot because DS0s are aggregated in time-division multiplexing
Time-division multiplexing
Time-division multiplexing is a type of digital multiplexing in which two or more bit streams or signals are transferred apparently simultaneously as sub-channels in one communication channel, but are physically taking turns on the channel. The time domain is divided into several recurrent...

 (TDM) equipment to form higher capacity communication links.

A Digital Signal 1
Digital Signal 1
Digital signal 1 is a T-carrier signaling scheme devised by Bell Labs. DS1 is a widely used standard in telecommunications in North America and Japan to transmit voice and data between devices. E1 is used in place of T1 outside North America, Japan, and South Korea...

 (DS1) circuit carries 24 DS0s on a North American or Japanese T-carrier
T-carrier
In telecommunications, T-carrier, sometimes abbreviated as T-CXR, is the generic designator for any of several digitally multiplexed telecommunications carrier systems originally developed by Bell Labs and used in North America, Japan, and South Korea....

 (T1) line, or 32 DS0s (30 for calls plus two for framing and signaling) on an E-carrier
E-carrier
In digital telecommunications, where a single physical wire pair can be used to carry many simultaneous voice conversations by time-division multiplexing, worldwide standards have been created and deployed...

 (E1) line used in most other countries. In modern networks, the multiplexing function is moved as close to the end user as possible, usually into cabinets at the roadside in residential areas, or into large business premises.

These aggregated circuits are conveyed from the initial multiplexer to the exchange over a set of equipment collectively known as the access network
Access network
An access network is that part of a telecommunications network which connects subscribers to their immediate service provider. It is contrasted with the core network, which connects local providers to each other...

. The access network and inter-exchange transport use synchronous
Synchronization
Synchronization is timekeeping which requires the coordination of events to operate a system in unison. The familiar conductor of an orchestra serves to keep the orchestra in time....

 optical transmission, for example, SONET
Sonet
Sonet may refer to:* Sonet Records, European record label* Synchronous optical networking * Saab Sonett...

 and Synchronous Digital Hierarchy (SDH) technologies, although some parts still use the older PDH
Plesiochronous Digital Hierarchy
The Plesiochronous Digital Hierarchy is a technology used in telecommunications networks to transport large quantities of data over digital transport equipment such as fibre optic and microwave radio systems...

 technology.

Within the access network, there are a number of reference point
Reference Point
Reference Point is the first album by Acoustic Alchemy released in 1990 for jazz label GRP and their fourth album overall.Containing some of the band's more popular tracks, such as the title track "Reference Point", "Same Road, Same Reason" and "Cuban Heels", the nine-track album is also the only...

s defined. Most of these are of interest mainly to ISDN but one – the V reference point – is of more general interest. This is the reference point between a primary multiplexer and an exchange. The protocols at this reference point were standardized in ETSI areas as the V5 interface
V5 interface
V5 is a family of telephone network protocols defined by ETSI which allow communications between the telephone exchange, also known in the specifications as the local exchange , and the local loop...

.

Impact on IP standards


Voice quality over PSTN networks was used as the benchmark for the development of the Telecommunications Industry Association
Telecommunications Industry Association
The Telecommunications Industry Association is accredited by the American National Standards Institute to develop voluntary, consensus-based industry standards for a wide variety of ICT products, and currently represents nearly 400 companies...

's TIA-TSB-116 standard on voice-quality recommendations for IP telephony, to determine acceptable levels of audio delay and echo.

See also

  • Via Net Loss
    Via Net Loss
    Via Net Loss is a network architecture of telephone systems using circuit switching technologies deployed in the 1950s with Direct Distance Dialing and used until the late 1980s. The purpose of the VNL plan and a five-level long distance switching hierarchy was to minimize the number of trunk...

  • Plain Old Telephone Service (POTS)
    Plain old telephone service
    Plain old telephone service is the voice-grade telephone service that remains the basic form of residential and small business service connection to the telephone network in many parts of the world....

  • Managed Facility Voice Network
    Managed Facility Voice Network
    A managed facilities-based voice network, or MFVN, is a physical network owned and operated by a voice service provider that delivers traditional telephone service via a loop start analog telephone interface. MFVNs are interconnected with the public switched telephone network and provide dialtone...