What is PSTN? | Public Switched Telephone Network and How It Works


Public Switched Telephone Network (PSTN) has been around for a long time. The main goal is to provide voice services to users regardless of their location.

PSTN is the worldwide collection of interconnected public telephone networks that was designed primarily for analog telephone calls.

PSTN is also called POTS (Plain Old Telephone Service). PSTN is a circuit-switched network. A dedicated circuit is established for the duration of a telephone call.

This contrasts with packet-switching networks, such as the internet, in which messages are divided into packets and each packet is sent individually.

Originally PSTN was only for an analog system, but it is now almost entirely digital. Only the very oldest and most backward parts still use analog.

PSTN uses Signaling System No.7 (SS7) as a signaling protocol. In telecommunication, signaling is the use of signals for controlling communications. In our daily conversation, we are using signaling too, for example, we would give some signals when we want to start a conversation with another person, either by walking closer or by saying something. When we want to end the conversation, we would say “it was a pleasure to talk to you” or “I have to go now”. All these signals or body language is a signaling system for people.

SS7 is used to set up or terminate a telephone call. It also handles caller ID, call waiting, voice message, and any other features except the conversation itself.

PSTN is all about switching. When a call is made, switches at different levels create a fixed-path circuit between two telephones.

When we talk about telephone systems we have to talk about what goes on outside your building, and how telephone calls get to you (or get inside a building).

The first concept you have to understand is PSTN (Public Switched Telephone Network), just like you have a router for the internet, PSTN is basic like the internet for telephones, and this is how telephone calls get routed.

PSTN is always based upon the Circuit Switching telephone network, telephone calls are transmitted as analog signals across a copper wire.

Point to note in PSTN;

  1. Circuit Switching telephone network
  2. Carries analog voice over a telephone lines
  3. Have constant Bandwidth.


There are five classes of telephone switches;

  1. Class 1: Regional Center and International Gateway
  2. Class 2: Sectional Center
  3. Class 3: Primary Center
  4. Class 4: Toll Center
  5. Class 5: Local Exchange.


Customer premise

At the basic level is customer premise, which includes home and business, telephone subscribers. They are PSTN users. Large organizations with 40 or more employees usually go with a PBX (Private Branch Exchange).

PBX is a combination of software and hardware. A PBX allows all phones in the organization to be connected internally and externally with a wide range of services such as caller ID, caller transfer, group call, call forwarding, voice messages, call waiting, etc.

The PBX (Private Branch Exchange) is connected to your local exchange, or central office, a class 5 telephone switch. The distance between telephone user and local exchange is called a “local loop” or a “last mile”.

The local exchange connects with the backbone of PSTN by communicating with a class 4 telephone switch (Toll Center Tandem Office).

Toll Center switches between several local offices, and telephone calls through this center often involved an extra cost. This is why the class 4 switch is called Toll Center.

Class 3: Primary Center

Class 3 telephone switch is Primary Center, which connects class 4 Toll Center downward and connects up with class 2 sectional centers, which provide interstates or interprovincial connections for long-distance calls.

Class 2: Sectional Center, provide interstates or interprovincial connections for long-distance calls. Class 1: Regional Center and International Gateway Manage domestic calls to the other countries. It is where international calls take place

In Summary;

Every area has a central office (offices). Central office means every single house that has a telephone line in it, that telephone line sitting in a house with a jack in a wall that runs all way back to the Central Office. So the wire inside your wall goes out to what is called the demarc point (demarcation) in the back of your house (Home office), then the same wire on the demarc point runs up to the central office (i.e.  Via telephone pole).

Always you have a solid connection from a central office to your house (building).

Also, we can say PSTN is a network of central offices or is the internet for telephone systems for example if want to make calls from Los Vegas to Boston that number will be routed through several central offices. So everybody in PSTN connects to a telephone system through the central office.

NOTE: The telephone company is responsible for the telephone signal from the central office up to the demarc point (outside your house), anything happens inside your building telephone company they are not responsible and your forced to fix it.



  1. PSTN are analog signals
  2. PSTN does not allow simultaneous connections
  3. PSTN is used for small companies.


ISDN is the next revolution step after the PSTN revolution in communication services.

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