EVEN some professionals in the communications industry seem to have difficulty reconciling the attributes of radio, satellite, metal and optical fibre cables. Not surprising then, there is also a general state of confusion and lack of comprehension in the media. At a superficial level this subject is relatively straightforward, but at the deepest levels of sophistication it still defeats the best brains and machines.

Most people overlook the most important question: what do we want to realise? Should we be wasting satellite and terrestrial radio capacity for fixed broadcast services, or devote this valuable commodity to the real prize, mobility? The explosive growth in mobile phones is but the tip of the iceberg, and is already limited by the finite radio spectrum. But worse is to come, because ultimately, everything will require mobile communication. Planes, trains, trucks, buses, cars, people, computers, cameras, PDAs and much more, will all require wireless communication.

Transmitting information over metal cables, optical fibre or radio involves the movement of energy in time and space in the form of electrons, photons, or electromagnetic waves. Signal dispersion and molecular interaction in the medium sees the energy level diminish with distance from the transmitter irrespective. So, just as a torch gets dimmer when you walk away from it, a radio signal gets weaker with distance.

Next comes the grisly impact of interference. Our species speaks many languages using the same fundamental acoustic mechanisms, and in a noisy street or room we experience hearing and cognitive difficulties. Well, the same is true of signals transmitted over cables, fibre and radio, but the mechanisms are far more complex and numerous. Interference turns out to be the dominant performance limiter for metal cables and radio. Optical fibres are the exception to date, but even this will change as we expand their capacity with more complex signals.

It is now time to debunk a few myths and misunderstandings. Relative information capacities - the ultimate bandwidths - are approximately as follows: optical fibre, 60,000GHz; radio, 300GHz; metal, 3GHz. Today commercial systems are operating with individual signal bandwidths up to: fibre optic, 10GHz; radio, 0.1GHz; metal, 0.5GHz. And what room for expansion? Well, fibre can be expanded almost without limit; radio is significantly limited above 30GHz by atmospheric effects; and metal cables are more or less at the end of the road.

Today optical fibre transports more than 70 per cent of all international telecommunication traffic, and is set to grow. In Britain it is already over 90 per cent. In this context, radio and satellites are, and always will be a drop in the ocean. So those who await the day when metal cables and geostationary or low-Earth orbit satellites will deliver very low-cost wide-band duplex connections to networks will be sadly disappointed. A more likely scenario is fibre all the way, or fibre to a radio station in the street, and satellite focused on high end users and areas where it is difficult to install fibre.

Peter Cochrane holds the Collier Chair for the Public Understanding of Science & Technology at the University of Bristol. His home page is:
http://cochrane.org.uk