What is PSTN?

Public Switched Telephone Network (PSTN), is the worldwide circuit-switched telephone network, interconnected by switching centers, It comprises public and private networks of telephone lines, fiber optic cables, microwave transmission links, cellular networks, communications satellites, and undersea telephone cables, thus allowing any telephone in the world to communicate with any other. Originally the PSTN was a network of fixed-line analog telephone systems. Now it is almost entirely digital in its core and includes mobile as well as fixed telephones.