The PSTN network architecture had to evolve over the years to support increasing numbers of subscribers, calls, connections to other countries, direct dialing and so on. The model developed by the United States and Canada was adopted by other nations, with adaptations for local markets.
The original concept was that the "telephone exchanges are arranged into hierarchies, so that if a call cannot be handled in a local cluster, it is passed to one higher up for onward routing. This reduced the number of connecting trunks required between operators over long distances and also kept local traffic separate.
However, in modern networks the cost of transmission and equipment is lower and, although hierarchies still exist, they are much flatter, with perhaps only two layers.
Most automated "telephone exchanges use digital switching rather than mechanical or analog switching. The "trunks connecting the exchanges are also digital, called circuits or channels. However analog "two-wire circuits are still used to connect the "last mile from the exchange to the telephone in the home (also called the "local loop). To carry a typical phone call from a "calling party to a "called party, the analog audio signal is digitized at an 8 kHz "sample rate with 8-bit resolution using a special type of nonlinear "pulse code modulation known as "G.711. The call is then transmitted from one end to another via telephone exchanges. The call is switched using a call set up protocol (usually "ISUP) between the telephone exchanges under an overall "routing strategy.
The call is carried over the PSTN using a 64 "kbit/s channel, originally designed by "Bell Labs. The name given to this channel is "Digital Signal 0 (DS0). The DS0 circuit is the basic granularity of "circuit switching in a telephone exchange. A DS0 is also known as a "timeslot because DS0s are aggregated in "time-division multiplexing (TDM) equipment to form higher capacity communication links.
A "Digital Signal 1 (DS1) circuit carries 24 DS0s on a North American or Japanese "T-carrier (T1) line, or 32 DS0s (30 for calls plus two for framing and signaling) on an "E-carrier (E1) line used in most other countries. In modern networks, the multiplexing function is moved as close to the end user as possible, usually into cabinets at the roadside in residential areas, or into large business premises.
These aggregated circuits 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 use "synchronous optical transmission, for example, "SONET and "Synchronous Digital Hierarchy (SDH) technologies, 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 standardized in "ETSI areas as the "V5 interface.
Voice quality over PSTN networks was used as the benchmark for the development of the "Telecommunications Industry Association's TIA-TSB-116 standard on voice-quality recommendations for IP telephony, to determine acceptable levels of audio delay and echo.