Off the dirt-track, into the fast lane

Cable TV modems may be able to move data 1,000 times faster than normal modems, but there are some problems to overcome first ...

Next time a cable company rep knocks on your door, hesitate before you hurl abuse at him. He could be offering the only decent method of accessing the Internet.

Whether he is or not depends on whether the cable companies manage to introduce what should be their next big thing - the cable modem. In theory, this will allow you to plug your computer into a cable TV line and download data at up to 30 megabits per second, more than 1,000 times the speed of today's top-of-the-range modems. This really is the information superhighway, rather than the information super dirt-track with which most of us are familiar.

A cable modem works by sending data over a spare or cleared channel on a cable TV system. A special card is fitted to the PC, which is then plugged into the cable television system. Then, hey presto, you're on the outside lane of the highway.

Or that is the theory. But trials, mainly in the United States, are coming up with a legion of problems.

First, what sounds like the simplest part of the equation, putting the special card into the PC, is turning out to be a nightmare: it may take engineers several hours to configure them.

Second, the cable network itself is causing difficulties. Traditionally, cable systems have a centre, known as the head end, from where the cable company feeds programmes into the system. In the good old days, all these cables connected to the head end via coaxial cable (like the thick line that connects your TV to its aerial). But these systems were only designed to carry TV pictures, and only in one direction.

What the trials are finding is that some networks are better than others at transmitting data. Those that have been upgraded with fibre-optic connections to neighbourhood boxes which connect to around 500 to 1,000 homes fare the best. Old, entirely coaxial, analogue systems struggle to use this technology. Most networks in the UK are modern, but rely on co-ax - so there are likely to be problems here, too.

Then there is the question of the return path - from the modem user back to the head end. Some systems, notably SURFboard from General Instruments, take the simple step of ignoring the cable system for this, relying instead on the ordinary telephone line as the return path. So while the Net can talk to you at 27 megabits a second, you talk to it at a mere 28.8 kilobits a second. This is fine if you want to send a simple instruction down the line, and to get high speed access to the World Wide Web in return. But how will such a system cope when, for example, businesses want to send video clips to and from each other?

The performance of a modem on a cable TV system depends on the way the system has been built and how many people are on-line. If the modem can be installed into a cable system so that the system can incorporate lots of modems, each servicing only a limited number of homes, it will work much better. But if each modem has to serve a large number of houses, it will struggle. The more people using a modem, the slower it will work, although the exact effect of extra load is complex. This could cause serious problems if everyone wanted to watch video at the same time.

More fundamentally, connection to a cable modem does not mean you would automatically get high-speed access to the World Wide Web. For while the cable company can supply a 10mbps connection to your home, once you are out on the Net you are at the mercy of the same slow connections that dog other users.

For the cable companies, cable modems represent a major learning curve. They have had little or no experience of data, and there are horror stories of companies connecting up homes with fast cable modems but installing a hopelessly inadequate link to the Net at the head end. But cable modems do represent a great opportunity for the cable companies. First, the bigger operators such as TCI, the world's largest cable company, are setting up their own Net access companies. This is causing concern to companies such as AOL and Compuserve, which are now busy striking alliances with cable companies.

Second, if someone knocked on your door and offered you Net access at up to 1,000 times the speed of your current set-up for not much more than you were already paying, you might be interested. The fact that you also had to have cable TV would probably not put you off.

Cable modems are just starting to take off. International standards are likely to be set this year which will allow for an explosion of cable modem development.

Already there are major trials in the US, and there are stirrings in the UK. TeleWest, the cable company, is carrying out pilots in academic institutions in Scotland and Essex, and plans domestic trials this summer. Other cable groups are also contemplating experiments. If these are successful, we could suddenly find cable modems hitting the headlines. Maybe the information super highway is not a joke after all.