Thepublic switched telephone network (PSTN) is the aggregate of the world's telephone networks that are operated by national, regional, or localtelephony operators. It providesinfrastructure and services for publictelephony. The PSTN consists oftelephone lines,fiber-optic cables,microwave transmission links,cellular networks,communications satellites, andundersea telephone cables interconnected byswitching centers, such ascentral offices, networktandems, and international gateways, which allow telephone users to communicate with each other.
Originally a network offixed-lineanalog telephone systems, the PSTN is now predominantly digital in itscore network and includesterrestrial cellular,satellite, andlandline systems. These interconnected networks enable global communication, allowing calls to be made to and from nearly any telephone worldwide.[1] Many of these networks are progressively transitioning toInternet Protocol to carry their telephony traffic.
The technical operation of the PSTN adheres to the standards internationally promulgated by theITU-T. These standards have their origins in the development of local telephone networks, primarily in theBell System in the United States and in the networks of European ITU members. TheE.164 standard provides a single globaladdress space in the form oftelephone numbers. The combination of the interconnected networks and a globaltelephone numbering plan allows telephones around the world to connect with each other.[2]
Commercialization of the telephone began shortly after its invention, with instruments operated in pairs for private use between two locations. Users who wanted to communicate with persons at multiple locations had as many telephones as necessary for the purpose. Alerting another user of the desire to establish atelephone call was accomplished by whistling loudly into the transmitter until the other party heard the alert. Bells were soon added to stations forsignaling.
Later telephone systems took advantage of the exchange principle already employed in telegraph networks. Each telephone was wired to atelephone exchange established for a town or area. For communication outside this exchange area,trunks were installed between exchanges. Networks were designed in a hierarchical manner until they spanned cities, states, and international distances.
Automation introducedpulse dialing between the telephone and the exchange so that each subscriber could directly dial another subscriber connected to the same exchange, butlong-distance calling across multiple exchanges required manual switching by operators. Later, more sophisticated address signaling, includingmulti-frequency signaling methods, enabled direct-dialed long-distance calls by subscribers, culminating in theSignalling System 7 (SS7) network that controlled calls between most exchanges by the end of the 20th century.
The growth of the PSTN was enabled byteletraffic engineering techniques to deliverquality of service (QoS) in the network. The work ofA. K. Erlang 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 implementingpacket-switched network data services using theX.25 protocol transported over much of the end-to-end equipment as was already in use in the PSTN. These became known aspublic data networks, or public switched data networks.
In the 1980s, the industry began planning for digital services assuming they would follow much the same pattern as voice services and conceived end-to-end circuit-switched services, known as theBroadband Integrated Services Digital Network (B-ISDN). The B-ISDN vision was overtaken by thedisruptive technology of theInternet.
At the turn of the 21st century, the oldest parts of the telephone network still used analog baseband technology to deliver audio-frequency connectivity over thelast mile to the end-user. However, digital technologies such asDSL,ISDN,FTTx, andcable modems were progressively deployed in this portion of the network, primarily to provide high-speed Internet access.
As of 2023[update], operators worldwide are in the process of retiring support for both last-mile analog telephony and ISDN, and transitioning voice service toVoice over IP via Internet access delivered either viaDSL,cable modems orfiber-to-the-premises, eliminating the expense and complexity of running two separate technology infrastructures for PSTN and Internet access.
Several large private telephone networks are not linked to the PSTN, usually for military purposes. There are also private networks run by large companies that are linked to the PSTN only through limitedgateways, such as a largeprivate branch exchange (PBX).
The task of building the networks and selling services to customers fell to thenetwork operators. The first company to be incorporated to provide PSTN services was theBell Telephone 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 essentialpublic utility. For example, theGeneral Post Office in the United Kingdom brought together a number of private companies to form a singlenationalized company. In more recent decades, these state monopolies were broken up or sold off throughprivatization.[3][4][5]
The architecture of the PSTN evolved over time to support an increasing number of subscribers, call volume, destinations, features, and technologies. The principles developed in North America and in Europe were adopted by other nations, with adaptations for local markets.
A key concept was that the telephone exchanges 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. Modern technologies have brought simplifications
Most automated telephone exchanges use digital switching rather than mechanical or analog switching. Thetrunks connecting the exchanges are also digital, called circuits or channels. However analogtwo-wire circuits are still used to connect thelast mile from the exchange to the telephone in the home (also called thelocal loop). To carry a typical phone call from acalling party to acalled party, the analog audio signal is digitized at an 8 kHzsample rate with 8-bit resolution using a special type of nonlinearpulse-code modulation known asG.711. The call is then transmitted from one end to another via telephone exchanges. The call is switched using a call set up protocol (usuallyISUP) between the telephone exchanges under an overallrouting strategy.
The call is carried over the PSTN using a 64kbit/s channel, originally designed byBell Labs. The name given to this channel isDigital Signal 0 (DS0). The DS0 circuit is the basic granularity ofcircuit switching in a telephone exchange. A DS0 is also known as atimeslot because DS0s are aggregated intime-division multiplexing (TDM) equipment to form higher capacity communication links.
ADigital Signal 1 (DS1) circuit carries 24 DS0s on a North American or JapaneseT-carrier (T1) line, or 32 DS0s (30 for calls plus two for framing and signaling) on anE-carrier (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 theaccess network. The access network and inter-exchange transport usesynchronous optical transmission, for example,SONET andSynchronous Digital Hierarchy (SDH) technologies, although some parts still use the olderPDH technology.
The access network defines a number of reference points. Most of these are of interest mainly to ISDN but one, theV 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 inETSI areas as theV5 interface.
Voice quality in PSTN networks was used as a benchmark for the development of theTelecommunications Industry Association's TIA-TSB-116 standard on voice-quality recommendations for IP telephony, to determine acceptable levels of audio latency and echo.[6]
In most countries, the government has aregulatory agency dedicated to provisioning of PSTN services. The agency regulate technical standards, legal requirements, and set service tasks may be for example to ensure that end customers are not over-charged for services where monopolies may exist. These regulatory agencies may also regulate the prices charged between the operators tocarry each other's traffic.
In theUnited Kingdom, the copper POTS and ISDN-based PSTN is being retired in favour ofSIP telephony, with an initial target of completion in December 2025, but delayed to January 2027.[7]
Several other European countries, including Estonia, Germany, Iceland, the Netherlands, Spain and Portugal, have also retired, or are planning to retire, their traditional networks.[8][9][10]
Countries on other continents are also performing similar transitions.[9]
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