The PSTN is a public communications system for voice traffic between remote subscribers. The Public Switched Telephone Network (PSTN) is also referred to as Plain Old Telephone Service( POTS), or good old telephone system.
Subscriber connections are established by circuit switching with number dialing. Voice transmission is in analog form in the classic analog telephone network, and in digitized form in the ISDN network.
The telephone network is divided into hierarchies and is organized into a star-shaped line structure. The hierarchy levels are assigned to the exchanges: Local exchange, Long-distance exchange, Node exchange, Main exchange, Central exchange.
The local networks are addressed by their own local network code and provide switching to the terminal. In thelocal networks, user lines are routed via star-shaped cable networks to the local exchange responsible for the access area. Small local networks usually have only one access area, whereas larger ones have several. The local network extends in the access area to the in-house exchange or telecommunications access unit.
In Germany, the long-distance network is controlled by central offices that are interconnected in a mesh. The main offices in the surrounding area are connected to a central office in a star configuration, and the node officesare connected to a main office in a star configuration.