The public switched telephone network (PSTN) is the aggregate of the world's circuit-switched telephone
networks that are operated by national, regional, or local telephony
operators, providing infrastructure
and services for public telecommunication
. The PSTN consists of telephone line
s, fiber optic cable
s, microwave transmission
links, cellular networks
, communications satellite
s, and undersea telephone cables
, all interconnected by switching center
s, thus allowing most telephones to communicate with each other. Originally a network of fixed-line analog
telephone systems, the PSTN is now almost entirely digital in its core network
and includes mobile
and other networks, as well as fixed telephones.
The technical operation of the PSTN adheres to the standards created by the ITU-T
. These standards allow different networks in different countries to interconnect seamlessly. The E.163
standards provide a single global address space
for telephone numbers. The combination of the interconnected networks and the single numbering plan
allow telephones around the world to dial each other.
Commercialization of the telephone began in 1876, 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 a telephone call
was accomplished by whistling loudly into the transmitter until the other party heard the alert. Bells were soon added to stations for signaling
, so an attendant no longer needed to wait for the whistle.
Later telephones took advantage of the exchange principle already employed in telegraph networks. Each telephone was wired to a telephone exchange
established for a town or area. For communications outside this exchange area, trunks
were installed between exchanges. Networks were designed in a hierarchical manner until they spanned cities, countries, continents and oceans.
Automation introduced pulse dial
ing between the telephone and the exchange, so that each subscriber could directly dial another subscriber connected to the same exchange, but long-distance calling
across multiple exchanges required manual switching by operators. Later, more sophisticated address signaling, including multi-frequency
signaling methods, enabled direct-dialed long-distance calls by subscribers, culminating in the Signalling System 7
(SS7) network that controlled calls between most exchanges by the end of the 20th century.
The growth of the PSTN meant that teletraffic engineering
techniques needed to be deployed to deliver quality of service
(QoS) guarantees for the users. The work of A. 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 implementing packet-switched network
data services using the X.25
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 end-to-end circuit-switched services, known as the Broadband Integrated Services Digital Network
(B-ISDN). The B-ISDN vision was overtaken by the disruptive technology
of the Internet.
At the turn of the 21st century, the oldest parts of the telephone network still use analog technology for the last mile loop
to the end user. However, digital technologies such as DSL
, and cable modems
have become more common in this portion of the network.
Several large private telephone networks 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
, such as a large private branch exchange
The task of building the networks and selling services to customers fell to the network operators
. The first company to be incorporated to provide PSTN services was the Bell 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 essential public utility
. For example, the General Post Office
in the United Kingdom brought together a number of private companies to form a single nationalized company
. In more recent decades, these state monopolies were broken up or sold off through privatization
In most countries, the central 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. These regulatory agencies may also regulate the prices charged between the operators to carry each other's traffic
The PSTN network architecture had to evolve over the years to support increasing numbers of subscribers, calls, connections to other countries, direct dialing and so on. The model developed by the United States and Canada was adopted by other nations, with adaptations for local markets.
The original concept was that the telephone exchange
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.
Most automated telephone exchange
s 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
s are still used to connect the last mile
from the exchange to the telephone in the home (also called the local loop
). To carry a typical phone call from a calling party
to a called 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
. 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
The call is carried over the PSTN using a 64 kbit/s
channel, originally designed by Bell Labs
. The name given to this channel is Digital Signal 0
(DS0). The DS0 circuit is the basic granularity of circuit switching
in a telephone exchange. A DS0 is also known as a timeslot
because DS0s are aggregated in time-division multiplexing
(TDM) equipment to form higher capacity communication links.
A Digital Signal 1
(DS1) circuit carries 24 DS0s on a North American or Japanese T-carrier
(T1) line, or 32 DS0s (30 for calls plus two for framing and signaling) on an E-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 the access network
. The access network and inter-exchange transport use synchronous
optical transmission, for example, SONET
and Synchronous Digital Hierarchy
(SDH) technologies, although some parts still use the older PDH
Within the access network, there are a number of reference points 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
Impact on IP standards
Voice quality over PSTN networks was used as the benchmark for the development of the Telecommunications Industry Association
's TIA-TSB-116 standard on voice-quality recommendations for IP telephony, to determine acceptable levels of audio delay and echo.
* Managed facilities-based voice network
* Plain old telephone service
* Via Net Loss
Switched telephone networking