Around the world, there are more than 200 million individuals accessing the Internet from home. For the vast majority of this number, Internet connection speeds are defined by the performance of a voiceband modem operating over a pair of twisted copper wires—in theory, a maximum of 56 Kbps; in practice, almost always, significantly less. Modest e-mails are fine, simple web pages usually acceptable, but once you start downloading anything into the low MB range, then response times extend into tens of minutes. A superhighway may span the continents, but for the typical home-based Internet user the crucial last-mile connection is the communications equivalent of a dirt tract.

One need not be an entrepreneurial genius to recognize the market potential in improving home data rates; subject, of course, to the small matter of prices. Dedicated microwave of cable connections offers multi-MB performance but at a cost orders of magnitude beyond what would be even remotely acceptable to domestic consumers.

If a new, dedicated connection is ruled out on cost grounds, the next obvious step is to try and harness some pre-existing network. Possible methods are, the co-axial cable used to distribute TV programmes (not quite the attractive proposition as it might first appear), and finally, our old friend—the twisted-pair telephone network. For the present, it is the last candidate that is the clear favourite.

The bandwidth of the standard twisted-pair telephone line, around 3KHz, has been carefully chosen to match the requirements of conventional telephony. It is wide enough to give a reasonably clear voice signal, at the same time filtering out any high-frequency components that might result in cross-talk as radiated signals ‘leak’ into adjacent twisted-pair connections. Advances in microelectronics and signal processing mean that it is now possible to operate such lines at significantly higher bandwidths, relying on complex intelligent modems to counteract the effects of cross-talk. ISDN and the various flavours of DSL both depend on this approach.

In the case of ISDN (the basic data rate being 64 Kbps), it is not a huge advance on the voice-band modem, but for DSL, research indicates that rates of up to 8 Mbps should be possible. However, even DSL has its limitations. Unresolved cross-talk and attenuation problems will prevent some telephone lines being used. Also, the bulk of DSL installations, especially to domestic users, are likely to be in the form of Asymmetric Digital Subscriber Line (ADSL), where to reduce the so-called near-end cross-talk problem the upstream bit rate (from the home to the local exchange) is significantly lower than the downstream rate.

Bandwidth Allocation

If the prospect of linking your home PC to 25 Mbps of wireless bandwidth is beginning to look like an attractive prospect, then the best advice is probably not to get too excited. A wireless system can only be deployed if the required bandwidth has been made available.