Page 16 of 130
1.3 PSTN (Public Switched Telephone Network)
PSTN is known as the “world’s most complex machine”. It is the world’s public telephone network
and is consist of copper wires, fiber optic cables, microwave transmission links, communications
satellite, cellular networks and telephone sets.
This is all inter-connected by switching centers that is located within your city allowing any telephone
to dial anywhere in the world.
The PSTN is almost entirely digital in its core but the telephone line going into individual homes are
mostly analog using copper wires.
Since the invention of the telephone, the PSTN has grown proportionately with the increased
demands to communicate. Switching services beyond metropolitan areas were soon developed,
increasing the size and complexity of the central office.
Before we discuss what PSTN is, first you need to understand what makes PSTN function, POTS!
Plain Old Telephone Service – Offered by your Telecommunications provider as a phone line.
Many POTS network connected together makes up the PSTN.
We will discuss all the components one by one which comprises a part of PSTN with POTS network in it. Also
we will discuss what types of wires and specific tools are used in different departments which enable us to
say HELLO and stay connected with our family and friends.
Central Office
As we discussed in the previous unit, the device in the CO that receives the signal from the telephone is called
the Switch (also known as Line Equipment, Office Equipment).
It is responsible for –
Supplying power to the telephone circuit.
Responding to signals sent from the telephone
Sending the ring signal to your telephone when a call is made to you
Communicating information, such as caller ID, to your telephone
Getting your call on the path from your telephone, to the telephone of the person you are
calling
Maintaining and delivering the features of your phone service (i.e. name/number display)
Figure 7: Office Equipment
