ADSL - The technology behind the speed

In Britain today BT will announce its plans to deliver nationwide the high speed connectivity technology ADSL. Rupert Goodwins explains the technology behind the revolution

For anyone who can remember the early days of online, ADSL (Asymmetric Digital Subscriber Line) seems little short of a miracle. In the early 80s, the fastest modems available were 1200bps -- almost two thousand times slower than BT's top ADSL speed, and eight thousand times slower than the best the technology can manage. Over fifteen years, modems slowly -- and expensively -- got faster, until they reached 56k.

ADSL goes so much faster than that because it doesn't treat the telephone line like a telephone line. Ordinary modems make noises in the same frequency range as humans use to talk: ADSL uses radio frequencies instead. In fact, an ADSL link has over 250 separate radio channels all transmitting simultaneously and each carrying as much information as an old modem. One of the main reasons ADSL is happening now is that chips have got so good, 250 modems will fit onto a single piece of silicon together with the circuitry for the equivalent of 250 radio transmitters and receivers, all tuned to their own frequency.

Because these frequencies are much higher than humans can hear, the old voice channel is left untouched -- thus giving ADSL the unique feature that frees the line to make a telephone call at the same time you're using it for data. And because there are so many channels, if there's interference on one or two then the ADSL modem can just slow the data on the affected channels and carry on pretty much as before. It's like widening the M25 from six lanes to six hundred.

However, unlike a modem you can't just buy an ADSL box, plug it into your phone socket and connect to an ISP. The ADSL signals won't go through the telephone exchange, so the phone company must install equipment (called a DSLAM) at their end of your line to connect to its own network. And the phone company also has to fit the ADSL modem at your end, to make sure the signals don't interfere with the rest of your phones, answering machines or whatever else you have connected. Once you've got everything fitted, though, it's just like being connected to a local area network that happens to be the Internet -- ADSL doesn't go through the telephone network itself, it can easily be left connected twenty four hours a day, seven days a week. ADSL doesn't, in general, have any time-based charges. You pay for the connection, and that's that.

Of course, there have been worries. Will ADSL signals radiate away from the telephone cabling and interfere with radios? Will it block other services, such as ISDN, running on wiring alongside the ADSL line? Can it reliably work with the sort of telephone lines we find in the UK? Tests have shown there to be a few problems, but nothing insurmountable. Because radio frequency signals are less robust than voice frequencies, the telephone line must be under around 15,000 feet from the exchange for 2 megabits/second ADSL -- but 80% or more of the UK's phone lines are fine.

Go to the ADSL News Special with today's news, technical information and insights from the UK's leading comms. journalist, Rupert Goodwins.