ADSL is really very clever. It uses the spare capacity in normal telephone lines to transmit data between you and us and back again, whilst the line is used normally for voice calls. The technical wizardry behind ADSL has been available for a few years. We’ve been using it for most of this time and the performance this technology gives is excellent. Now with BT’s co-operation we deliver this service thoughout the country. There are two aspects of ADSL you might wish to understand – How the network works and what happens on your telephone line.
(Figure 1) shows your premises (office or home), the BT network, our premises and the Internet. We are part of the Internet so any traffic through us feed onto or from the rest of the Internet – right across the globe. The ADSL service moves data between you and us, over BT’s network. Once you’ve signed up, we enable your account here, the service is installed, and then you log on. From that point the connection between you and us is permanently established. Data flows between us a bit like data flows round any computer network. That is what we mean by "Always On" it is there all the time unless you switch off. If you switch your PC off or there is a power failure or suchlike, you simply get everything working normally and log back on, to resume normal service. With this service you get access to e-mail, all the web sites across the world, and all the usual facilities on the Internet.
(Figure 1)
The obvious question is "How does ADSL work over telephone lines, and what happens to normal voice or fax telephone calls?" Telephone lines operate a bit like a TV cable coming from your aerial, except that information moves in both directions rather than just one way for TV. The TV cable carries signals for all the TV channels which you can watch, of course. You select just one at a time to watch, but the TV still receives all the programmes down the aerial. To provide ADSL service, your telephone line operates similarly to the TV cable. Two of the channels are for normal calls: one for each direction to and from you; and two channels are used for ADSL internet traffic channel, up and down.
(Figure 2)
Figure 2 shows how the telephone line operates between your business or home, and the local telephone exchange. From there, the ADSL Internet traffic is taken back to us (over special phone lines) and normal telephone calls go out over the telephone network as before. There is no interference between ADSL and normal voice calls, or vice versa. You can use your telephone line for making and receiving telephone calls just as now.
