High Quality Content by WIKIPEDIA articles! 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 Telecom 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.