Affect of line length – why can I not get the bitrate and/or DSL service that I want?
Since the modulation technique used to carry the high bitrate ADSL signal over your “local loop” telephone line makes heavy use of high frequency signals, the line has to be of a given minimum quality before ADSL can be expected to work properly.
Where the minimum quality required for the maximum bitrate is not available, operation at a lower bitrate may be possible. The bitrate which is possible depends on a number of factors:
4 The length of your telephone line from your house to the nearest telephone exchange. The length must typically be less than 5.5 km. Longer lines usually mean that the following parameters are not met...
4 Signal loss across your telephone line should typically be less than 45 dB.
4 The noise level on your telephone line (poor connections (“fried egg noise”) or crosstalk (humming or overhead conversation or other signals) disturb the ADSL signal. Line noise must typically be less than 55 dB.
4 Line capacitance of your telephone line acts as a filter for the higher frequencies. The value must be less than 180 nF.
4 The cable guage (i.e. the diameter of the copper core) of your telephone line - narrower guage wires cause higher signal attenuation (loss of signal strength) but may have lower capacitance.
A check is usually carried out when you order your DSL line, to check whether your exchange is equipped for ADSL and to test the quality of your individual line – in order to determine what bitrate can be offered and/or guaranteed. In cases where the line fails the test, your order may be rejected. In this case you have two options if you are determined to get service: to request a re-check of your line (if it was borderline), or to indicate that you are willing to accept a lower bitrate and/or forego the bitrate guarantee.