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. These RF microwave systems are referred-to as point-to-point links. The microwave energy that is radiated into space from the RF interconnects (point-to-point links) of the PSTN, is quite a bit higher power than the energy used in the Relay Stations (Towers) of the cellular telephone network. The microwave radiation on these PSTN point-to-point links is typically confined to narrow radiation beam widths. This tends to keep the radiation away from the ground so that radiation danger or radiation health is not typically a concern. Therefore from a health perspective, the point-to-point links and thier attendant accessory systems do not usually pose a danger to people. Probably the greatest threat from such links is in a city where these links are on top of buildings and other nearby buildings are close to the radiation path.
The world is crisscrossed with these interconnects, The majority of the traffic that is carried by the PSTN, is hosted on fiber optic cable.