Public switched telephone network
Public Switched Telephone Network is usually abbreviated to PSTN
The public switched telephone network is a world wide communication system consisting of copper telephone lines, fibre-optic cables, microwave towers, satellite systems and undersea cables.
A telephone call is routed to its destination by one or more switching centres.
It includes mobile telephones as well - they too are routed through to a PSTN switching center.
It was once a purely analogue system but it is now almost all digital with fibre-optic cables carrying the heaviest traffic such as between continents.
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Click on this link: PSTN