Cambridge Digital iTV Trial.

DJ Greaves and M Taunton.

The Cambridge Digital Interactive Television Trial started in September 1994, with ten users connected in the first phase. ATM networking technology is employed throughout the system, with two-way digital data being carried alongside cable TV signals over fibre optic links. In phase two, begun in March 1995, the network and the content server facilities were substantially enhanced, and the number of user sites (homes, schools and businesses) was increased: by January 1996 it approached one hundred.

The partner companies in the consortium which provides and operates the trial infrastructure - the various hardware and software components and the network links - are Online Media, Advanced Telecommunications Modules Ltd (ATML)[1], Cambridge Cable, ICL, and SJ Research. The trial headquarters are located in the offices of Online Media in Cambridge. A second group of companies is responsible for delivering the various services and content accessible via the system. These include the BBC, National Westminster Bank, ITN, Tesco, IPC Magazines, Acorn Computers, and Anglia Television; other organisations involved in the service consortium are BMP DDB Needham, the Post Office, NOP (National Opinion Polls), and the Independent Television Commission.

Facilities presently available to participants in the trial range from home banking via the TV, through software programs for both education and entertainment, general information services, and of course interactive video and audio access on-demand. The last includes both dynamically updated content - news and weather reports, and other topical material such as programme guides, with TV trailers viewable on-demand - and an expanding archive of general interest programmes such as comedies, drama, documentaries and music concerts. Contributors to the audio/visual services include the BBC, ITN and Anglia Television.

Network Topology

The network consists of a number of ATM Switches at Online Media which are connected to server computers and file stores, including file stores from ICL and ATML. 100 and 155 Mbps optical fibres connect Online Media to Cambridge Cable's headend station.

Distribution to the homes reuses the enclosures, power distribtution and holes in the ground used by Cambridge Cable for CATV and POTS distribution, but none of the CATV or POTS equipment. Instead baseband distribution of ATM is made over dedicated 155 Mbps fibres to further ATM switches in the kerbsides and then, from the leaf kerbside boxes there is a star of baseband coax cables with each home at the end of a ray. Homes are served with 2 Mbps duplex ATM.

In each home there is an intelligent set top box with an ARM processor and MPEG decompression chipset. The connection from the street is terminated in the home with a wall socket and the set top box is connected to this with a UTP cable. In the future, other ATM equipment or domestic ATM switches can be plugged into the wall point to expand the home area network.


[1] ATML has now become Virata