The public switched telephone network (PSTN) is the concatenation of the world's public circuit-switched telephone networks, in much the same way that the Internet is the concatenation of the world's public IP-based packet-switched networks. Originally a network of fixed-line analog telephone systems, the PSTN is now almost entirely digital, and now includes mobile as well as fixed telephones.
The basic digital circuit in the PSTN is a 64-kilobit-per-second channel, originally designed by Bell Labs, called a "DS0" or Digital Signal 0. 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.
The DS0's are the basic granularity at which switching takes place in a telephone exchange. DS0's are also known as timeslots because they are multiplexed together in a time-division fashion. Multiple DS0's are multiplexed together on higher capacity circuits, such that 24 DS0's make a DS1 signal, which when carried on copper is the well-known, T-carrier system, T1 (the European equivalent is an E1, containing 32 64 kbit/s channels). In modern networks, this multiplexing is moved as close to the end user as possible, usually into cabinets at the roadside in residential areas, or into large business premises.
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.
Within the access network, there are a number of reference points defined. Most of these are of interest mainly to ISDN but one - the V reference point - is of more general interest. This is the reference point between a primary multiplexer and an exchange. The protocols at this reference point were standardised in ETSI areas as the V5 interface.
Only the very oldest and most backward parts of the telephone network still use analog technology for anything other than the last mile loop to the end user, and in recent years digital services have been increasing rolled out to end users using services such as DSL and ISDN.
In the 1970s the telecommunications industry conceived that digital services would follow much the same pattern as voice services, and conceived a grandiose vision of end-to-end circuit switched services, known as the Broadband Integrated Services Digital Network (B-ISDN). The B-ISDN vision has been overtaken by the disruptive technology of the Internet.
Many observers believe that the long term future of the PSTN is to be just one application of the Internet - however, the Internet has some way to go before this transition can be made: see the article on Voice over IP for more on this subject.
Note: there are also a number of large private telephone networks which are not linked to the PSTN, usually for military purposes. There are also private networks run by large companies which are linked to the PSTN only through limited gateways, like a large PABX system.