When the public Prestel service was first launched in March 1979 there was just a single machine in central London to support both editing activity and user access.
In order to support the planned major expansion programme, a new Prestel infrastructure was designed around two distinct types of computer centre: the Update Centre (UDC) where IPs could create, amend and delete their pages of information, and the Information Retrieval Centre (IRC) which supplied mirrored copies of the pages to end users. In practice there only ever was one Update Centre, and this always housed just one update computer, named "Duke", but within six months of public launch there were in addition two dedicated information retrieval computers.
In those early days of the public service all the live Prestel computers were located in St Alphage House, a 1960's office block on Fore Street in the City of London. At the time the National Operations Centre (NOC) was located in the same building on the same floor. The computers and the NOC were later moved to Baynard House, (on Queen Victoria Street, also in the City of London) which acted as a combined UDC and IRC. Both types of machine, together with other development hardware, remained in service there until 1994 when the Prestel service 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 IRC’s were connected to the UDC in a star network configuration, originally via leased line permanent (not packet switched) connections, based on the X25 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 Telenet packet switching network.
The Prestel Mailbox service was originally launched on Enterprise computer to support messaging solely between users on that machine and by 1984 the facility had been rolled out nationwide. This required a further type of Prestel computer dedicated to the exchange of messages. The only example of this type, which became known as Pandora, was co-located with the UDC in Baynard House, London.
Originally Prestel IRC computers were directly dialled by means of an ordinary telephone number (e.g. the Enterprise computer in Croydon was accessed by dialling 01 686 0311. By 1984 the special short dialling codes 618 and 918 were in use in order to give access to the nearest IRC at local telephone call rates, at least across most parts of the UK.
In 1987, the entire local access network was being overhauled and shared with other Dialcom Group companies - users connecting and not automatically logging into Prestel would be greeted with a menu allowing access to Prestel, Telecom Gold, etc.
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