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Although the network was créated using analog voice connections through manual switchboards, automated telephone exchanges replaced most switchboards, and later digital switch technologies were used. Most switches now use digital circuits between exchanges, with analog two-wire circuits still used to connect to most telephones.

The basic digital circuit in the PSTN is a 64-kilobits-per-second channel, originally designed by Bell Labs, called Digital Signal 0 (DS0). 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 (PCM). The call is then transmitted from one end to another via telephone exchanges. The call is switched using a signaling protocol (SS7) between the telephone exchanges under an overall routing strategy.

The DS0s are the basic granularity at which switching takes place in a telephone exchange. DS0s are also known as timeslots because they are multiplexed together using time-division multiplexing (TDM). Multiple DS0s are multiplexed together on higher capacity circuits into a DS1 signal, carrying 24 DS0s on a North American or Japanese T1 line, or 32 DS0s (30 for calls plus two for framing and signalling) on an E1 line used in most other countries. In modérn networks, this multiplexing is moved as close to the end user as possible, usually into cabinets at the roadside in residential aréas, 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 aréas as the V5 interface.