Packet Switch Stream

In the United Kingdom, Packet Switch Stream (PSS) was an X.25-based packet-switched network, provided by the British Post Office Telecommunications and then British Telecommunications starting in 1980. After a period of pre-operational testing with customers (mainly UK universities and computer manufacturers at this early phase) the service was launched as a commercial service on 20 August 1981. The experimental predecessor network (EPSS) formally closed down on 31 July 1981 after all the existing connections had been moved to PSS.

Companies and individual users could connect into the PSS network using the full X.25 interface, via a dedicated four-wire telephone circuit using a PSS analog modem and later on, when problems of 10-100 ms transmission failures with the PCM Voice based transmission equipment used by the early Kilostream service were resolved, via a Kilostream digital access circuit (actually a baseband modem). In this early 1980s era installation lead times for suitable 4-wire analog lines could be more than 6 months in the UK.

Companies and individual users could also connect into the PSS network using a basic non-error correcting RS232/V.24 asynchronous character based interface via an X.3/X.28/X.29 PAD (Packet Assembler/Disassembler) service oriented to the then prevalent dumb terminal market place. The PAD service could be connected to via a dedicated four-wire telephone circuit using a PSS analog modem and later on via a Kilostream digital access circuit. However most customers, for cost reasons, chose to dial up via an analog modem over the then UK analog telephony network to their nearest public PAD, via published phone numbers, using an ID/password provided as a subscription service.

The current day analogy of ISPs offering broadband always on and dial up services to the internet applies here. Some customers connected to the PSS network via the X.25 service and bought their own PADs. PSS was one of the first telecommunications networks in the UK to be fully liberalised in that customers could connect their own equipment to the network. This was before privatisation and the creation of British Telecommunications plc (BT) in 1984.

...
Wikipedia