The growth of the PSTN was enabled by teletraffic engineering techniques to deliver quality of service (QoS) in the network.
In the 1970s, the telecommunications industry began implementing packet-switched network data services using the X.25 protocol transported over much of the end-to-end equipment as was already in use in the PSTN.
At the turn of the 21st century, the oldest parts of the telephone network still used analog baseband technology to deliver audio-frequency connectivity over the last mile to the end-user.
However, digital technologies such as DSL, ISDN, FTTx, and cable modems were progressively deployed in this portion of the network, primarily to provide high-speed Internet access.
As of 2023[update], operators worldwide are in the process of retiring support for both last-mile analog telephony and ISDN, and transitioning voice service to Voice over IP via Internet access delivered either via DSL, cable modems or fiber-to-the-premises, eliminating the expense and complexity of running two separate technology infrastructures for PSTN and Internet access.
[3][4][5] The architecture of the PSTN evolved over time to support an increasing number of subscribers, call volume, destinations, features, and technologies.
A key 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 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).
A DS0 is also known as a timeslot because DS0s are aggregated in time-division multiplexing (TDM) equipment to form higher capacity communication links.
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.
Voice quality in PSTN networks was used as a 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 latency and echo.
In the United Kingdom, the copper POTS and ISDN-based PSTN is being retired in favour of SIP telephony, with an original completion date of December 2025, although this has now been put back to January 2027.