Cable television companies which installed massive surplus information-carrying capacity when they dug up London's roads have been rewarded for their foresight by a Pounds 6 million higher education contract.

Test transmissions began this week on a 155 megabit per second (Mbps) optical fibre ring with 34 Mbps branches, which could link 100 academic sites in the London area by the year 2000.

The United Kingdom Education and Research Networking Association awarded the contract for the London-wide network to ICn, a consortium of London's six major cable television franchisees. Of the other four bidders, only BT could offer the geographical coverage required. ICn won on price.

The Pounds 4 million cost of connecting the first 24 sites (see map) comprises approximately Pounds 1.5 million capital cost plus five years rental at about Pounds 500,000 a year. "We have signed a contract that allows expansion up to about 100 sites," said Bob Day, UKERNA's development director. "The Pounds 6 million would be a breakpoint at about 50 sites."

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Institutions are expected to save Pounds 500,000 a year when they relinquish their low-speed connections to the national academic network JANET and re-route their JANET and Internet traffic through the London ring. While laying cables to carry television programmes between neighbouring franchise areas, the broadcasters installed 48-fibre cables where a single fibre might have sufficed. They set up ICn to sell their network's spare capacity to London businesses, which use it for private phone networks, videoconferencing and for continuously "mirroring" data between computer sites as a precaution against disasters.

London's universities are likely to use their broadband network for video lectures, telesurgery, and remote access to supercomputers. Multi-site institutions could use the network to cut the cost of their internal telephone and security systems.

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UKERNA has its own fibres, not shared with any other organisation, and can easily upgrade the network's capacity in stages to 2.4 Gbps as demand increases and more sites are connected.

Academic "metropolitan area networks" exist or are under development in Manchester, Bristol and South Wales. Scotland has four such projects. But London's will be much the biggest because of the concentration of higher education institutions in the London area.