In its fibre optic experiment due to start next year (CI No 1,159) British Telecommunications Plc will test two optical fibre systems that it says will provide a blueprint for its 21st century communications network. The UKP5m experiment will involve optical fibre links transmitting stereo television, high fidelity stereo radio, telephone calls and other services. About 500 business and residential customers in Bishops Stortford, Hertfordshire will take part in the trial that will help British Telecom assess two types of fibre system, Telephony Over Passive Networks, TOPN, and Broadband Integrated Distributed Star, BIDS. Passive Networks, the cheaper system, uses a series of inert fibre splitters, or couplers, to siphon off services to each customer on the network; initially these will be confined to telephone speech and low-speed data, but Telecom says the system will be upgraded to a Broadband Passive Optical Network, BPON, carrying television, hi-fi radio and video library. Three variations of TPON will be tested: a single line passive network providing phone services on an all-optical network with fibre right up to customer’s premises, serving up to 128 customers on a time-division multiplexed basis; small business passive network for customers needing two to 30 exchange lines; and street pass ive networks, similar to the single line version, but with the optical system terminating at street-level, with the final connection to houses being made over a conventional pair of copper wires similar to the FibretotheCurb system being tried out by Nynex Corp in New York (CI No 1,123). Broadband Integrated Distributed Star, a more ad-vanced system, uses electronic switches to route customer services that have been selected. The BIDS project will start in March 1990 with TPON following in September 1990 and being upgraded to BPON in March 1991; British Telecom says the Department of Trade & Industry has agreed to relax the restrictions that prevent it carrying television services over its main network.