Before understanding Public Switched Telephone Network (PSTN), We must know about Circuit Switching Technique.

Circuit Switching Technique: Communication through circuit switching means that there is a communication path between the two stations. And this communication path is linked through a “sequence of links” between nodes of the network.

Circuit Establishment: for establishing an end-to-end connection before data transferring, Some segments of a circuit can be a link that is dedicated, and some other segments can be shared.

Data transferring: data transferring happens from the source to the destination. Depending on the network’s nature, This data can be analog data or digital data. The connection in general is a full-duplex.

Circuit disconnection: it Terminates the connection at the last of transferring of data, after completion of data transfer the connection is terminated. So this disconnection is made by any user, disconnection means intermediate links are removed from the sender side to receiver side.

Advantages of Circuit Switching:

  1. Long continuous transmission can be possible because a continuous transmission route gets established.
  2. The dedicated path makes sure that there is a steady and constant data rate of communication.
  3. Intermediate delays are not there, when there is circuit establishment. So it is suitable for real time communication which consists of both voice and transmission.

Disadvantages of Circuit Switching:

  1. Circuit Switching establishes a connection between end sides. And this connection is not useful for sending any other data.
  2. Requirement of bandwidth is very high even when there is low data volume.
  3. To establish the connection, time requirement is very high.
  4. When the resources are allocated to one connection, they cant be used by another connections anymore.

Diagrammatic Representation:

Circuit establishment between two telephones connected by circuit switched connection. And these blue boxes are switching offices and they have connections with other switching offices. These black lines are connecting the switching offices and they will form a permanent link between the offices. When we request for a connection, these white dotted lines denotes the links that are established between the switching offices, so that a permanent circuit is established between the communicating areas. The links will stay as long as the communication continues to be.

Public Switched Telephone Network (PSTN) depends on circuit switching. For connecting one phone to another phone, this phone call is connected through many switches that is operating on a local area, regional level and international level. So when we make a call, a particular dedicated circuit gets activated and when the call ends, it gets deactivated.

The main use of PSTN is to transform human voice in a recognizable form.

Public Switched Telephone Network (PSTN) is the worldwide collection of public telephone network that are interconnected which was designed first for analog telephone calls. Public Switched Telephone Network (PSTN) is also referred to as the landlines/ Plain Old Telephone Service(POTS).It is the “traditional circuit-switched telephone network”. Since the late 1800s this, system has been in general use.

By using underground copper wires, this old telephone network has provided businesses and houses with a reliable thing to communicate with anyone around the world for generations and generations. The operators used to sit at a switched board, literally connecting one piece of copper wire to another so that the call would travel all over the country

The current Public Switched Telephone Networkconsists of copper telephone lines, fiber optic cables and communication satellites, microwave transmission links and under-sea telephone lines. It can be linked to the cellular networks.

The PSTN operation follows the ITU-T standards.

Public Switched Telephone Network phones are used as standard form of communication. But there was a steady decrease over the last decade. There are currently “972 million PSTN telephone subscriptions” in use worldwide, so far that’s the lowest tally in this century. The present telephone networks uses the DMS-100 digital switches.

EVOLUTION OF Public Switched Telephone Network

In 1875 Alexander Bell formed the American Bell Telephone Company.

In 1876,Alexander Bell made the improvement in telegraphy and made the first voice transmission through wire. Then Bell Telephone Company opened its first Switching office in New Haven in 1878.

How do Public Switched Telephone Network Phone Lines work?

Let us consider a situation when you dial a number from your own phone. An let us consider few steps to understand how POTS works:

Step 1: Your telephone set converts sound waves into electric signals. These signals are then transmitted into a terminal using a cable.

Step 2: This terminal will collect the electric signals and transmit these signals to the central office(CO).

Step 3: The central office, then routes the calls as electric signals through fiber optic cable, The fiber optic cable carries and sends these signals as light pulses to their final destination.

Step 4:The call is connected to an office( this is a regional hub that is useful for sending these calls to central offices that are distant) or these signals are sent to a central office which are useful for local calls.

Step 5: When the call which u made reaches the right office, this sign is then converted as a electrical signal and is then connects to a terminal.

Step 6: This terminal connects this call to the particular telephone number. While getting the call, the set of telephone  will convert the electrical signals to sound waves back.

It takes less time for your call to reach the desired location. Fiber optic cables and a global network of switching centers are useful for this process.

Circuit route for an average distance call:

The telephone system consists of three major components:

  1. Local loops: these are the loops that are analog that are twisted pairs going into houses and businesses.
  2. Trunks: these are digital fiber optics that connects the switching offices.
  3. Switching offices: these are from where the calls are moved from one trunk to another trunk.

The caller and callee while they in call, the medium consists of End office, Toll office Intermediate switching office which has very high bandwidth intertoll trunks on both sides.

The Local loops consists of 3 types:

  1. Modems
  2. ADSL
  3. Wireless Local loops


A modem is a “modulator-demodulator” is a hardware device that converts the data into a suitable format so that it can be transmitted from one computer to another computer using telephone wires. This modulates one or more carrier signals to encode the digital data for sending and it also demodulates the signals which decode the sent information.

The main usage is it transmits the signal easily and decodes reliably to produce the digital data that is origibak.

ADSL (Asymmetrical Digital Subscriber Lines): It is a type of Digital subscriber Line(DSL) technology, a data communications technology which gives faster data transmission through copper cable than conventional model which provides.

The bit rate and bandwidth of ADSL are asymmetric, Providers sell ADSL for consumers as a service for  accessing internet and for downloading information from the browser, but it is not serving content that can be accessed by others.

ADSL equipment Configuration: In this configuration a telephone company should install a NID ( network Interface Device) in the costumer’s premises. Telephone company’s end is marked by small plastic box and costumer’s property start is also marked by this “small plastic box”.

Close to NID there is a splitter and analog filter which will  separate the 0 to 4000Hz band used by the POTS from the data , The POTS signal is routed into the telephone which is already or a fax machine.

ADSL is only installed from the telephone exchange for smaller distances, that is less than 4 kms.

Wireless Local Loops:  They uses wireless links but not copper wires to connect subscribers to the local central office.

WLL Architecture consists of PSTN, Switch function, and Local exchange office that contains WANU which means Wireless Access Network Unit, trans-receiver WLL controller, and AM HLR(Access Manager Home Location Register) HLR stores the details of all local WANU’s whereas WLL controller controls the wireless local loop components Authentication Manager is responsible for Authentication. WANU’s function includes Authentication, operation and maintenance and also routing and trans-receiving voice and data and WASU(Wireless Access Subscriber Units) it is present at the house of subscriber, it connects the subscriber to WANU and the power supply and consists of voice, Fax, Modem, and Data can be given.

I hope you have read and understood about the Public Switched Telephone Network.