The definitive test involves measurements of line capacitance in 'nanofarads'. Capacitance acts as a filter, allowing certain frequencies to pass and blocking others. In the case of telephone wires, which were not designed for ADSL use, a high capacitance level can prevent the frequencies used by ADSL from being transmitted properly. Capacitance is affected by cable length, insulation, moisture etc. Line 'noise', which is measure in dB on a logarithmic scale and as a signal/noise ratio, is unrelated to capacitance, which is the important function being tested. A capacitance up to about 47 nanofarads is usually accepted as being adequate.
If the line is noisy or it fails the BT line test for ADSL, ask BT, phone 151, to 'lower the gain' on the phone line, if it is in the high position.

ADSL technology can provide maximum downstream (Internet to customer) speeds of up to 8 megabits per second (Mbps) at a distance of about 6,000 feet (1,820 meters), and upstream speeds of up to 640 kilobits per second (Kbps).

You might wonder, if distance is a limitation for DSL, why it's not also a limitation for voice telephone calls. The answer lies in small amplifiers called loading coils that the telephone company uses to boost voice signals. Unfortunately, these loading coils are incompatible with ADSL signals, so a voice coil in the loop between your telephone and the telephone company's central office will disqualify you from receiving ADSL.

Other factors that might disqualify you from receiving ADSL include:

• Bridge taps - These are extensions, between you and the central office, that extend service to other customers. While you wouldn't notice these bridge taps in normal phone service, they may take the total length of the circuit beyond the distance limits of the service provider.
• Fibre-optic cables - ADSL signals can't pass through the conversion from analogue to digital and back to analogue that occurs if a portion of your telephone circuit comes through fibre-optic cables.
• Distance - Even if you know where your central office is (don't be surprised if you don't -- the telephone companies don't advertise their locations), looking at a map is no indication of the distance a signal must travel between your house and the office.