ADSL for your medium-term needs Given that the world is already wired for standard analogue telephones and that it would be difficult to completely rewire it, the search is on for technologies which allow us to squeeze much more out of standard twisted-copper telephone lines. ADSL or Asynchronous Digital Subscriber Line aims to be that standard. ADSL transmits up to 6 Mbps to a subscriber (a high-speed down channel useful for Internet surfing), and as much as 640 kbps more in both directions (for video conferencing or sending data out from the home/office). Such rates expand existing access capacity by a factor of 50 or more without new cabling. Finally a standard telephone line is also included. All of these channels are separated from each other. ADSL is promoted by a Forum setup in December 1994. It seeks standards that are compatible world-wide. Already many European and American telecoms companies are involved and testing ADSL systems. US and European standards based on ADSL are being ratified. ADSL uses advanced digital signal processing and creative algorithms to squeeze so much data through twisted-pair telephone lines. In addition, many advances have been required in transformers, analogue filters, and A/D converters - the hardware of the switchboards and routing equipment. ADSL is good for up to distances of 2.7 to 5.5 km, depending on wire quality and the number of switching stations. This is enough to connect 95% of phone users to the local telephone exchange and it's optic-fibre network backbone. For mobile PC users the advantages of ADSL are no more that for any other fixed line system. However, given the ubiquity of standard telephone interfaces and the cheapness of modems, even in PC Card formats, ADSL means that mobile PC users could plug in anywhere. Today's computers can cope with ADSL throughput rates. As telecoms costs are reduced, remote networking, e-mailing, and Internet access become cheap and easy everywhere. VDSL for longer term capacity With little experience of digital data transfer, we cannot image the amount of data a fully digital world would cause. Yet experts realise that even standards like ADSL will not be enough once all video is digital and its use is widespread. VDSL is planned to carry on where ADSL stops. By shortening the planned distances, VDSL squeezes even higher transfer rates from old-fashioned copper wires. VDSL is still in the formation stage. The maximum downstream rate under consideration is between 51 and 55 Mbps over lines up to 1000 ft (300 meters) in length. Downstream speeds as low as 13 Mbps over lengths beyond 4000 ft (1500 meters) are also in the picture. Upstream rates in early models will be asymmetric, just like ADSL, at speeds from 1.6 to 2.3 Mbps. Both data channels will be separated in frequency from bands used for POTS and ISDN, enabling service providers to overlay VDSL on existing services. VDSL is a recognition of the financial realities of telecommunications. Fibre-optic cable to every home may be an ideal but it is horrendously expensive. Given that telecoms is becoming increasingly competitive and that prices (and therefore margins) are falling, even new cables in each road may be too expensive. By limiting new cabling to the main routes VDSL will allow telecoms companies to offer high-capacity bandwidth at relatively low cost to cover digital transmission needs for several decades to come. For the mobile PC user the major benefits are again the ubiquity and ease of connection. Both ADSL and VDSL are "behind the scenes" technologies. Users don't "see" any of it. All they need are computers powerful enough to cope with the data rates VDSL plans to offer!