ADSL broadband (ADSL) is dependent on the electrical characteristics of your telephone line. Or to explain it another way, the operation of ADSL is affected by the amount of 'noise' on your line. This noise is not necessarily audible when you make a telephone call, but it can have a detrimental effect on a digital service such as broadband.

As a rule of thumb, the further you are from your exchange - i.e. the longer the physical length of the cabling between the exchange and your premises - the more 'noisy' your line is likely to be. However, noise can also come from other sources such as interference from electrical equipment within your premises or even from the house or office next door.

Until recently, the ability to order broadband was restricted to premises which were no further than 6km (in terms of cable length) from their exchange. This was judged to be a 'safe' distance where noise levels would be unlikely to affect the provision of broadband service. The only restriction was that lines between about 3.5km and 6km long would not be able to support the higher speed 1Mb and 2Mb services.

But a series of trials has shown that it is possible to provide broadband on lines with much higher levels of noise, albeit subject to certain restrictions.

The main restriction on very long lines is again the speed of product you are able to order. Again, it comes down to the level of noise on your line.

Noise is measured in decibels (dB). As a rule of thumb, lines that have noise of less than 43dB are able to receive all speeds of products (ie 512kbps, 1Mb and 2Mb). When the noise levels go above 43dB you are unlikely to be able to receive the 2Mb service. Once they go above 60dB, there is a possibility that you will not be able to receive broadband at all. (definition from btbroadbandoffice.com/broadband/highest_speed)