Public Switched Telephone Network or PSTN

Each telephone is plugged into a telephone jack.Each jack is connected to a drop wire by inside wiring and then to a cable with other wires. The cable is then connected to a telephone exchange or wire centre.

            When one wants to contact another person  through the phone, who is also connected to the same exchange in a similar manner, an electronic equipment in the exchange connects both  the telephones of the callers together. This is termed as  a local call.

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             The exchange is also connected to far off exchanges through similar cables. When a person calls a person connected with a far off exchange , it is a trunk call. Most of the exchanges in the world are now connected to each other, forming a Public Switched Telephone Network or PSTN.