First to be aware of: Analog system

In the beginning there was a scientist, named Alexander Graham Bell who invented a machine called Telephone. If anyone speaks on telephone, phone set converts the sound into electrical signal and then transmit it through Copper wire.

This electric transmission of voice is a system where air pressure waveform of the sound is corresponded through electric voltage level representation. In this system the voltage or the current is said to be an analogy of the sound produced by human, thus it is called analog system in telecommunication.

If you make international call then this electrical signal goes through fiber optics, communication satellites or undersea telephone cables. The receiving phone set at the other end will finally get that electric signal and reconvert it into sound.

This whole network of voice communication is called Public Switched Telephone Network (PSTN), which is sometimes referred to as plain old telephone service (POTS).

In PSTN, connection is maintained between two telephone sets for the duration of call, i.e. a circuit is maintained here. Thus, PTSN use circuit-switched telephone network system for connecting calls where switch means a system that connects telephone calls.

Someone pick up the receiver, dial a number and the other party answer the call then they basically open the circuit. Anyone during the call time hang up means the circuit is closed.

In early telecommunication, every phone call needed its own dedicated copper wire connecting the two phones used .The process was costly because of say for 5 minutes call, two callers may have to keep busy a 5 hundred kilometer long stretching copper wire and thus they paid a lot for the call.

Telephone conversion over PSTN are more efficient and cheap today because for the intervention of digital technology.