VoIP Solution Workbook- PSTN (Public Switched Telephone Network)

The public switched telephone network (PSTN) is the network of the world's
public circuit-switched telephone networks, in much the same way that the
Internet is the network of the world's public IP-based packet-switched
networks. Originally a network of fixed-line analog telephone systems, the PSTN
is now almost entirely digital, and now includes mobile as well as fixed
telephones. It is sometimes referred to as the Plain Old Telephone Service
(POTS).

The PSTN is largely governed by technical standards created by the ITU-T, and
uses E.163/E.164 addresses (known more commonly as telephone numbers) for
addressing.

Architecture and context The PSTN was the earliest example of traffic
engineering to deliver Quality of Service (QoS) guarantees. A.K. Erlang
(1878–1929) is credited with establishing the mathematical foundations of
methods required to determine the amount and configuration of equipment and the
number of personnel required to deliver a specific level of service.

In the 1970s the telecommunications industry conceived that digital services
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 (B-ISDN). The B-ISDN vision has been overtaken by the disruptive
technology of the Internet. Only the oldest parts of the telephone network still
use analog technology for anything other than the last mile loop to the end
user, and in recent years digital services have been increasingly rolled out to
end users using services such as DSL, ISDN, FTTP and cable modem systems.

Many observers believe that the long term future of the PSTN is to be just one
application of the Internet - however, the Internet has some way to go before
this transition can be made. The QoS guarantee is one aspect that needs to be
improved in the Voice over IP (VoIP) technology.

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, like
a large private branch exchange (PBX).

Early 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 necesary 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 switchhook, and telephones took advantage
of the exchange principle already employed in telegraph networks. Each telephone
was wired to a local telephone exchange, and the exchanges were wired together
with trunks. 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, culminating in the SS7 network that connected most exchanges by
the end of the 20th century.

Digital Channel Main article: telephone exchange Although the network
was created using analog voice connections through manual switchboards,
automated telephone exchanges replaced most switchboards, and later digital
switch technologies were used. Most switches now use digital circuits between
exchanges, with analog two-wire circuits still used to connect to most
telephones.

The basic digital circuit in the PSTN is a 64-kilobits-per-second channel,
originally designed by Bell Labs, called Digital Signal 0 (DS0). To carry a
typical phone call from a calling party to a called party, the audio sound is
digitized at an 8 kHz sample rate using 8-bit pulse code modulation (PCM). The
call is then transmitted from one end to another via telephone exchanges. The
call is switched using a signaling protocol (SS7) between the telephone
exchanges under an overall routing strategy.

The DS0s are the basic granularity at which switching takes place in a telephone
exchange. DS0s are also known as timeslots because they are multiplexed together
using time-division multiplexing (TDM). Multiple DS0s are multiplexed together
on higher capacity circuits into a DS1 signal, carrying 24 DS0s on a North
American or Japanese T1 line, or 32 DS0s (30 for calls plus two for framing and
signalling) on an E1 line used in most other countries. In modern networks, this
multiplexing is moved as close to the end user as possible, usually into
cabinets at the roadside in residential areas, or into large business premises.

The timeslots 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 of the PSTN use synchronous optical transmission
(SONET and SDH) technology, 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 standardised in ETSI
areas as the V5 interface.

U.S. and Canadian Telephone Switch Hierarchy AT&T PSTN Office
Classification HierarchyIn order to organize automated operator dialing, and
later Direct Distance Dialing (DDD), AT&T divided the various switches in its
network in to a hierarchy containing five levels (or classes). This was a formal
expansion of the network structure that had developed within AT&T Long Lines as
local telephone exchanges had been connected together. As long distance calling
was originally established, it could take up to seven minutes to complete a
connection to another major city, and small points would need to have "call
back" appointments made with long lead times for circuits to be reserved.

While the following discussion refers to AT&T and (principally) to the United
States, it is important to remember that until 1956, AT&T controlled Bell Canada
and thus influenced corporate decisions north of the border. Bell Canada
provided local operations in most of Ontario and Quebec, and both in its
capacity as the largest telecommunications carrier in Canada and because of its
historic operations in the Atlantic and Prairie provinces, dominated decisions
over long distance practices. Canadian authorities agreed that integration of
Canadian long distance services into a trans-national network was valuable to
both countries, so that U.S. and Canadian services were integrated for
networking capabilities at an early stage into what eventually became the
foundation for the North American Numbering Plan Area.

By the mid-1920s, a revised manual system where "local" toll operators
connected tandem routes (a process formally called Combined Line and Recording)
as needed to complete telephone calls, reduced the process to an average of two
minutes, but still meant that some complex routings might interconnect as many
as sixteen points! As long distance services grew in the Contigous Continental
US (48 states) and Canada, the amount of overhead equipment and people required
to determine and establish Rates and Routes became excessive. As technology
improved, network design included consideration of more automated and defined
procedures. Thus, beginning with a switch installed in Philadelphia PA in 1943,
AT&T began to automate the system, and establish a new switch hierarchy, which
lasted until the breakup of AT&T in the 1980s.

The underlying principle of the five-level hierarchy was to provide economies of
scale by establishing direct connections between centralized call "collection
points" (essentially the Class 4 offices) where economically feasible, and to
provide additional concentration points (Class 1 through 3) to handle overflow
traffic that could not be handled directly, or to handle traffic to locations
which were less likely to be dialed from a given point - usually longer
distances and/or smaller locations in other parts of the North American dialing
plan. The North American plan differed from those of other continents in the
existence of three concentration levels of hierarchy for domestic (here defined
as including all those points "within" the dialing plan) calls, a need not
required where the larger geographic area was broken into several national plan
jurisdictions. However, it is important to note that this was not a strict
hierarchy of absolute levels. If enough call traffic existed between geographic
areas, for example, a Class 4 office could have direct trunk connections not
only to a Class 3 office, but to a Class 2 or Class 1 office, and vice versa.
For example, the Class 2 switch in Toronto (OTORON0101T2) had connections not
only to the Class 1 switch in Montréal (MTRLPQ0201T1), but to the Class 1 switch
in White Plains (WHPLNY0201T1), one of the Class 2 switches in New York City
(NYCMNYAA02T2) and a Class 3 switch in Buffalo (BFLONYFR04T3). Network engineers
re-worked the system as necessary to balance off call completion percentages
with budgetary limitations. In fact, minor changes were made almost every month.

Initially excluded from the development of the North American network were
locations that eventually would become part of the North American Numbering Plan
Area - Alaska, Hawaii, some other United States possessions, various outlying
Northern and rural portions of Canada, and much of the Caribbean. These areas
were handled as International Calls until more advanced computer hardware and
software allowed them to be included in the automated, integrated systems in
later decades. After the spread of stored program control switching, many
services of Class 1 through 3 could be delegated to newer switches in the class
4 and 5 offices, and that portion of the network became obsolete, although it
was partially replaced by the establishment of multiple long distance carrier
networks, connected to the local networks through their points of presence.

Class 1- The class 1 office was the Regional Center (RC). Regional
centers served three purposes in the North American toll network (a) their
connections were the "last resort" for final setup of calls when routes
between centers lower in the hierarchy were not available (b) they were
initially staffed by engineers who had the authority to block portions of the
network within the region in case of emergencies or network congestion -
although these functions were transferred after 1962 to the Network
Control/Operations Center and the distributed Network Management Centers (see
below) (c) they provided collection points (until the development of more
advanced computer hardware and software for toll operators) for circuits that
would be passed along to one of the international overseas gateways (which
operated as special centers outside the formal North American hierarchy). The
regional centers updated each other on the status of every circuit in the
network. These centers would then reroute traffic around the trouble spots and
keep each informed at all times. There were twelve Regional Centers in North
America, ten in the United States, nine of which were operated by AT&T (White
Plains, NY, Wayne, PA, Pittsburgh, PA, Norway, IL [a rural crossroads west of
Chicago at the intersection of US highway 52 and IL highway 71 - an underground
office built with hardened construction to withstand nuclear attack], Rockdale,
GA, St Louis, MO, Dallas, TX, Denver, CO, and Sacramento, CA), one by GTE (San
Bernardino, CA). Two centres in Canada were operated on behalf of the
Trans-Canada Telephone System, one by Bell Canada (Montréal, PQ), and one by
Saskatchewan Telephone, (Regina, SK).

For control and oversight of the entire network hierarchy, AT&T established a
Network Control Center in New York City in 1962, renamed the Network Operations
Center and relocated to Bedminster, NJ in 1977. Engineering supervision was also
centralized in eight regional Network Management Centers. Part of the
realignment and dispersion of functions was to insure maximum network integrity
in the event of a national emergency, a major concern in that era. The basic
structure of this unit, although significantly altered since the AT&T
divestiture in the 1980s, still exists as the Global Operations Center, with
domestic regional centers in Colorado and Georgia.

Class 2- The class 2 office was the Sectional Center (SC). The sectional
center typically connected major toll centers within one or two states or
provinces, or a significant portion of a large state or province, to provide
interstate or interprovincial connections for long-distance calls. At various
times, there were between 50 and 75 active class two offices in the network.

Class 3- The class 3 office was the Primary Center (PC). Calls being made
beyond the limits of a small geographical area where circuits connected directly
between class 4 toll offices would be passed from the toll center to the primary
center. These locations use high usage trunks to complete connection between
toll centers. The primary center never served dial tone to the user. The number
of primary centers in the network fluctuated from time to time, ranging between
150 and 230.

Class 4- Main article: Class 4 telephone switch The class 4 office is
the Toll Center (TC), Toll Point (TP), or Intermediate Point (IP). A call going
between two end offices not directly connected together, or whose direct trunks
are busy, is routed thru the toll center. The toll center is also used to
connect to the long-distance network for calls where added costs are incurred,
such as operator handled services. This toll center may also be called the
tandem office because calls have to pass through this location to get to another
part of the network. Toll centers might have been operated either as interstate
facilities, under the operation of AT&T Long Lines (GTE in a few cases), or by
local telephone companies, handling long distance traffic to points within a
particular operating company territory. Class 4 offices continue to exist,
although with considerable changes, as they handle local exchange company
interconnections, locally charged or long distance rated, or provide facilities
for connection to long distance company points of presence.

Class 5- Main article: Class 5 telephone switches The class 5 office is
the local exchange or end office. It delivers dial tone to the customer. The end
office, also called a branch exchange, is the closest connection to the end
customer. Over 19,000 end offices in the United States alone provide basic dial
tone services.

In modern times only the terms Class 4 and Class 5 are much used, as any tandem
office is referred to as a Class 4. This change was prompted in great part by
changes in the power of switches and the relative cost of transmission, both of
which tended to flatten the switch hierarchy. The breakup of the Bell System,
and the need for each of the surviving regional operating companies to handle
long distance interconnections, also promoted the inclusion of inter-regional
and international processing through larger Class 4 offices.

International Overseas Call Centers The special requirements of placing
calls to locations outside main Canadian/United States points meant that these
calls were handled by special operators in locations where connections could be
monitored to other countries. The technology to automate these connections began
to develop in the 1960s (see Bell Laboratories Record 42:7, July-August 1964),
and as the decade of the 1970s progressed, North American customers who were
served by electronic offices began to be able to directly dial to an increasing
number of international points (service between ESS offices in New York and
London began on March 1, 1970). However, since points could not be connected
until equipment in both countries was converted to electronic switching,
implementation to many locations took some time, and while the majority of calls
began to be connected via automated systems by the 1990s - after the termination
of the five-level hierarchy - the majority of countries were still connected via
manual intervention until the beginning of the 21st century

U.K. Telephone Switch Hierarchy The forerunner of British Telecom, the
General Post Office, also organized its intercity trunk network along similar
hierarchical lines to that of North America. However, because of the
significantly smaller geographic area involved, fewer levels of connection were
required, and no formal numbering of class offices was made.

There were a few special exceptions to the following description, notably those
involving Northern Ireland, some of the Channel Dependencies, and the few
locations in England which were served by non-GPO companies, such as Hull and
Portsmouth.

In the early days of manual exchanges, outlying areas (eventually called
dependent exchanges) were connected through progressively larger locations
(eventually called group switching centres) into one of the main cities -
Birmingham, Edinburgh, Glasgow, Liverpool, London, and Manchester. As automation
began to be established in the network, this was refined into a system of
approximately fifty tandem locations for Group Switching Centres, with an
additional layer of perhaps a dozen Wide Area Tandems to provide for busy
periods, emergency routing, etc. There were also some additional Local Tandems
to handle traffic in the London Metropolitan Area without involving the GSCs,
although this was a later development, as it required common control signalling
for identification.

Subscriber Trunk Dialing The dialing codes used by trunk operators to
connect calls were originally assigned and established to ensure speed with
pulse dialing equipment. With the advent of subscriber dialed calls, numbering
patterns were reassigned to provide for mnemonic methods of improving customer
performance. STD codes all began with 0. The largest cities, which had seven
digit local numbers, were allocated special codes - London, 01; Birmingham, 021;
etc. Smaller towns were typically allocated a code based on the first letters of
their name, translated into digits on the telephone dial. For example, OXford
translated into 09 on the British phone dial, so the original STD code for
Oxford was 0096. However, because of subscriber dialing errors, there was an
early decision to eliminate codes which began with "00" and Oxford soon became
0865, the 86 standing for UNiversity.

Some of the smallest towns connected to the trunk network only through nearby
switches. In those cases, STD codes were composed of combination of the code for
the nearby switch, plus some additional digits that were unused in that nearby
switch, but which served two purposes (1) to identify the end location, and
allow the nearby switch to complete the call (2) to "pad out" the overall
length of the dialing string, since a small town might only have a three-digit
telephone number, and allow the network to move to a more-standard number
length.

As step offices became rarer, Subscriber Trunk Dialing Codes no longer followed
the original rules, and were significantly revised in the mid-1990s, with
further changes as wider use of mobile phones and non-BT competition came into
the UK market. There are now some 70000 local exchange codes in use in the UK.
The largest trunk carrier, British Telecom, connects the local network through
some 60 transit (tandem) switches.

French Telephone Switch Hierarchy The early history of the
telecommunications switching network in France is, unusual for this country, one
of decentralized development. Early telephone exchanges were installed by local
communities, often by private companies, and only later taken over by the French
government. As a result, by 1930, France was served by almost 25,000 local
exchanges, but almost half of these had less than five subscribers.
Additionally, telephones were not considered important for residential customers
(nor for small businesses), so France had a low penetration rate of telephone
subscribers.

Under these conditions, early network development revolved around two major
distinctions, "Paris" and "not Paris." Within metropolitan Paris, automated
step switches appeared, with a level of tandem switching, before World War II.
In the rest of the country, automation was confined to major cities, with a high
level of manual intervention.

World War II heavily damaged the French telephone system, so that by the end of
the War, only about 140 automatic exchanges (mostly in Paris and its banlieues)
and 228 manual exchanges were fully operational. Repair of much of the network
had been deferred during the war due to lack of parts, as well as co-opting of
technical personnel for German military needs.

Recovery was rapid after the war, and the extensive damage in some ways helped
the modernization of the system as new technology was introduced. The DGT
(Direction Générale des Télécommunications) introduced automated operator
dialing of long distance connections, generally using the INSEE codes as "area
codes" for the various departments - with special handling for Paris. These
codes subsequently became public as customer dialing of long distance calls
began to be introduced in the late 1960s and early 1970s.

The network had a minimal hierarchy, with most connections routed into a central
tandem in each department, and from there to Paris. As greater installation of
private telephones, for both small business and private residences, increased in
the 1970s, direct connections among the tandems in adjacent regions were
installed, and a three-level tier of switches, local, tandem, and regional
interconnection was implemented, with final routing through Paris. Also, during
the 1970s and 1980s, the smaller rural switches were replaced and combined with
nearby automated offices, and a closed numbering scheme was adopted for dialing
consistency.

In common with most countries, the development of technology allowed for
different networking, and the maintenance of a formal hierarchy disappeared into
a distributed network. By the mid-1990s, a revised structure had appeared,
reflected by the replacement of the old departmental area codes by the
assignment of regional codes and a major renumbering scheme for strategic
planning, privatization, and deregulation under the auspices of ART, the
Autorité de régulation des télécommunications (Regulatory Authority for
Telecommunications - since 2005, ARCEP, as responsibility for postal services
was added). After 1996, the country prepared for complete deregulation of the
telephone network. Thus, the local exchanges (zones à autonomie d'acheminement)
are connected somewhat differently by various carriers. However, the largest of
these, based upon the (partially) privatised former government network, is a
two-level long distance hierarchy, based on 80 CTS (centre de transit
secondaire) and 8 CTP (centre de transit primaire) locations. In addition, there
are 12 CTI (centre de transit internationaux) for connections to areas which are
not integrated into the French telephone network [note that some overseas
locations are considered "domestic" for telecommunications purposes].

Retrieved from
"http://en.wikipedia.org/wiki/Public_switched_telephone_network"

____________________________________________________________________

Copyright © voipsolutionjournal.com - 2007 All Rights Reserved - VOIP Solution Journal
VoIP Solution Journal
Selecting a VoIP Solution Phone System for Home and Business.