Public switched telephone network
The public switched telephone network (PSTN) is one of the world's public circuit-switched telephone networks. Originally PSTN is a network of fixed-line analogue telephone systems. Today, the PSTN is totally digital in its core and also includes mobile along with fixed (plain old telephone service, POTS) telephones.
Early History
The first telephones were in private use, had no network and were wired together in pairs. If a user wanted to talk to different people, he had as many telephones as necessary for that purpose. A user who wished to speak whistled into the transmitter till the other party heard him. Soon, a bell was added for giving signal, and then a switch hook was also added, and telephones took advantage of the exchange principle that was already implemented in telegraph networks. Each telephone was connected through wires to a local telephone exchange, and the exchanges were connected together with trunks. Networks were connected together in a hierarchical manner till they spanned to cities, countries, continents and oceans. This was the beginning of the PSTN, the term PSTN was not known for many decades.
Pulse dialling between the phone and the exchange was introduced through Automation, and then among exchanges, followed by more sophisticated address signalling including the multi-frequency.
Digital Channel
Although this network was made by using analogue voice connections through manual switchboards, then automated telephone exchanges replaced most switchboards, and later the digital switch technologies were used. Now most switches uses digital circuits between exchanges, with analogue two-wire circuits still used to connect to most telephones.
The basic digital circuit used in the PSTN is a 64 Kbit/s channel, that was originally designed by Bell Labs, designated Digital Signal 0 (DS0). To carry a typical phone call from a calling party to a called party, the analogue audio signal is then digitized at an 8 kHz sample rate by using 8-bit pulse code modulation (PCM). The call is then transmitted from one point to another via telephone exchanges. The call is switched using a signalling protocol (Signalling System 7) between the telephone exchanges by an overall routing technique.

A DS0 circuit is the basic system of circuit switching in the telephone exchange. DS0s are aggregated in time-division multiplexing (TDM) equipment to form higher capacity communication links therefore a DS0 is also known as a timeslot. In modern networks, the multiplexing function is now moved close to the end user as much as possible, usually into cabinets at roadside in residential areas, or into large business premises etc.
The timeslots are conveyed from the initial multiplexer to the exchange over a set of equipment which is collectively known as access network. The access network and inter-exchange transport of the PSTN uses the synchronous optical transmission.
Within the access network, there are a number of defined reference points. Most of these are of interest mainly to ISDN but one i.e., the V reference point is of more general interest. This is the reference point between a primary multiplexer and an exchange.