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The PSTN
The Public Switched Telephone Network (PSTN) is the collection of all the electronics and their interconnections that allow us to make a telephone call.
In the local scene there is a local telephone company that allows you to call other phones in your metropolitan area. This telephone company is called a Carrier. Since they connect you locally, they are called a Local Exchange Carrier (LEC- pronounced: lek) or a Competitive Local Exchange Carrier (CLEC – pronounced: see-lek). When you want to connect to another city, state or country, usually, regional carriers or long distance carriers are playing a part.
In your local area, there are some impressively massive electronics systems that connect all the phones together and also connect to the regional and long distance carriers. These are called Central Office Switches, sometimes referred-to as Central Offices or just Switches.
Regional Carriers and Long Distance Carriers also use these Switches.
In addition to these impressive electronics systems there are wires, fiber optics, RF (microwave) and free space optical links that interconnect the Switches. The microwave energy that is radiated into space from the RF interconnects of the PSTN, is quite a bit higher power than the energy used in the Relay Stations (Towers) of the cellular telephone network.
The world is crisscrossed with these interconnects, The majority of the traffic that is carried by the PSTN, is hosted on fiber optic cable.
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