PSTN, the Public Switched Telephone Network, is a circuit-switched network that is used primarily for voice communications worldwide, with more than 800 million subscribers. Originally a network of fixed-line analog telephone systems, the PSTN is now almost entirely digital and also includes mobile as well as fixed telephones.

The basic digital circuit in the PSTN is a 64-kilobit-per-second channel, known as "DS0" or Digital Signal 0. DS0's are also known as timeslots because they are multiplexed together in a time-division fashion.To carry a typical phone call from a calling party to a called party, the audio sound is digitized at an 8 kHz sample rate using 8-bit pulse code modulation.

Multiple DS0's can be multiplexed together on higher capacity circuits, such that 24 DS0's make a DS1 signa or T1 (the European equivalent is an E1, containing 32 64 kbit/s channels).

The timeslots are conveyed from the initial multiplexer to the exchange over a set of equipment collectively known as the access network. The access network and inter-exchange transport of the PSTN use synchronous optical transmission (SONET and SDH) technology, although some parts still use the older PDH technology.

For more than a hundred years, the PSTN was the only bearer network available for telephony. Today, the mobile telephone over wireless access network, which is carried through the PSTN trunking network, is becoming increasingly popular. Other bearer networks for voice transmission include integrated service digital network (ISDN), Digital Subscriber Line (DSL), Asynchronous Transfer Mode (ATM), frame relay and the Internet VOIP.

Today typical PSTN: Public Switched Telephone Network for voice and data communications with both wireless and wire line access

PSTN: Public Switched Telephone Network

Related Terms: Circuit Switching, VOIP

Reference Links:
http://www.ericsson.com/support/telecom/part-b/: Public Switched Telephone Network