was sold by BT to a private company. Each IRC normally housed two information retrieval computers, although in some IRCs in London just a single machine was present. IRCs were generally located within major telephone exchanges, rather than in BT Data Processing Centres, in order to give room for the extensive communications requirements. Exchange buildings were ideally suited to housing the large numbers of rack mounted 1200/75 baud modems and associated cabling as well as the racks of 16-port Multi-Channel Asynchronous Communications Control Units (MCACCUs) or multiplexors from GEC which gave the modems logical access into the computers. In the new infrastructure, IRCs were connected to the UDC in a star network configuration, originally via leased line permanent (not packet switched) connections, based on the X.25 protocol, operating at 2.4 kilobits per second (kbit/s). By mid 1981, these private circuit links had been replaced with dedicated 4-wire X25 circuits over the new public Packet Switch Stream (PSS) network operating at 4.8 kbit/s. By June 1980, there were four singleton retrieval computers in London, plus six other machines installed in pairs at IRC sites in Birmingham, Edinburgh and Manchester. Fully equipped IRC machines had a design capacity of 200 user ports each but these first ten machines were initially only capable of supporting approximately 1,000 users between them, expandable later to 2,000 users. By September 1980, there were five IRC machines in London plus pairs of machines at Birmingham, Nottingham, Edinburgh, Glasgow, Manchester, Liverpool and Belfast offering a total of 914 user ports. Further IRC's were planned at Luton, Reading, Sevenoaks, Brighton, Leeds, Newcastle, Cardiff, Bristol, Bournemouth, Chelmsford and Norwich by the end of 1980. In some of these locations where there was insufficient Prestel traffic to warrant siting an IRC computer, the plans were to site multiplex equipment in a suitable exchange building from where connections were made over X25 to the nearest proper IRC. As at the end of 1980, there was actually a total of 1500 live computer ports available and by July 1981, the number of IRC computers has been expended to 18, increasing the coverage of the telephone subscriber population from 30% to 62%. In 1982, using the multiplexor technique described above, a virtual IRC was created in Boston, Massachusetts giving access to a machine in the UK known as Hogarth in order to provide Prestel services to subscribers from across the United States via the