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 lines, fiber optic cables, microwave
transmission links, cellular networks, communications satellites, and
undersea telephone cables, all interconnected by switching centers,
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
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.
4.1 Network topology
4.2 Digital channels
4.3 Impact on IP standards
5 See also
7 External links
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 dialing 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
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
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, ISDN, FTTx, 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 (PBX).
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
Bell Telephone Company in the United
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
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.[clarification needed]
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
Cellular Network routing
Main article: PSTN network topology
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
The original 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.
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 exchanges 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
circuits 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.
Digital Signal 1 (DS1) circuit carries 24 DS0s on a North American
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 technology.
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 (POTS)
Via Net Loss
^ Kushnick, Bruce (7 January 2013). "What Are the Public Switched
Telephone Networks, 'PSTN' and Why You Should Care?". Huffington Post
Blog. Retrieved 11 April 2014.
^ "TIA TSB-116". Global.ihs.com. Retrieved 2011-11-20.
Cable protection system
Mobile phone signal
Cable protection system
Prepay mobile phone
The Telephone Cases
Timeline of communication technology
Undersea telegraph line
Edwin Howard Armstrong
John Logie Baird
Alexander Graham Bell
Jagadish Chandra Bose
Lee de Forest
Erna Schneider Hoover
Charles K. Kao
Alexander Stepanovich Popov
Johann Philipp Reis
Vladimir K. Zworykin
Free-space optical communication
Network switching (circuit
Public Switched Telephone
World Wide Web