Public Switched Telephone Network (PSTN) has been around for a long time. Main goal is to provide voice services between users regardless of their location.
When we talk about telephone systems we have to talk about what goes on outside your building, how telephone call get to you (or get inside a building).
The first concept you have to understand is PSTN (Public Switched Telephone Network), just like you have router for internet, PSTN is basic like internet for telephones, and this is how telephone calls get routed.
What is PSTN?
PSTN sometimes refers as POST (Plain Old Telephone Services) is the interconnected telephone systems over which telephone calls are made via a copper wire.
PSTN always based upon the Circuit Switching telephone network, telephone calls are transmitted as analog signals across a copper wire.
Point to note in PSTN;
How PSTN work?
Every area has a central office (offices). Central office means every single house that have a telephone line in it, that telephone line siting in a house with a jack in a wall that actually runs all way back to the Central Office. So the wire inside your wall goes out to what called demarc point (dermarcation) in back of your house (Home office), then the same wire on demarc point runs up to the central office (i.e. Via telephone pole).
Always you have a solid connection from a central office to your house (building).
Also we can say PSTN is network of central offices, or is the internet for telephone systems for example if want to make call from Los Vegas to Boston that number will be routed through a number of central offices. So everybody in PSTN connects to a telephone system through central office.
NOTE: The telephone company is responsible for telephone signal all the way from central office up to the demarc point (outside your house), anything happen inside your building telephone company they are not responsible and your forced to fix it.
ISDN is next revolution step after PSTN revolution in communication services.