Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. The public switched telephone network (PSTN) also referred to as the Plain Old Telephone Service (POTS) is the network of the world's public circuit-switched telephone networks. Originally a network of fixed-line analog telephone systems, the PSTN is now almost entirely digital in its core and includes mobile as well as fixed telephones. The technical operation of the PSTN utilises standards created by the ITU-T. These standards allow different networks in different countries to interconnect seamlessly. There is also a single global address space for telephone numbers based on the E.163 and E.164 standards. The combination of the interconnected networks and the single numbering plan make it possible for any phone in the world to dial any other phone.

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