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Computer Networks/Networks

Notes 4.3 Public Switched Telephone Network

Generally, a telephone system found in home and offices is a part of the Public Switched Telephone
Network (PSTN). It is accessed by telephones, private branch exchange trunks and data
arrangements. Hence, PSTN may be referred as a public communication system that provides
local, extended local and long distance telephone service to the subscribers. The PSTN uses a
circuit-switched telephone almost similar to the IP-based packet-switched network for Internet.
The PSTN is made available to the public through a group of common communications carriers
who have agreed to exchange calls and connections on behalf of their subscribers as per the
international law and the laws of the country of operation.

Notes Originally, PSTN was based on analog circuits but it is now entirely digital in
nature and includes fixed line and mobile telephone services.

The PSTN follows technical standards developed by the ITU-T. Its telephone numbers are based
on E.163/E.164 addresses. The PSTN has several telephone exchanges networked together to
form a nationwide and worldwide) telephone communications system. All telephones in the
PSTN are networked in such a way that any phone can make a call to any other phone within the
country or outside the country as per international laws prescribed by ITU.

Figure 4.4: PSTN Structure

ISDN

NID CO CO CO CO

Local loop, analog
voice, copper CO FEX FEX
twisted pair CO
Trunk lines
Digital carrier
(Bundles of Copper pair) FEX FEX IEX

ISDN - Integrated Services Digital
Network
NID - Network Interface Design
CO - Central Office
FEX - Foreign Exchange
IEX - International Exchange

A phone or telephone set acts as an interface device to provide the interface to the PSTN. It
enables a subscriber to dial a phone number to make a call. The simple layout of the PSTN as
shown in the Figure 4.4 is consisted of the following components:
(a) Telephone Set: The telephone set provides the dialing function and converts the voice to
electrical signal so as to traverse through copper wires and other associated circuits to

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