Here's a set of metrics for evaluating your ADSL line quality. I found this quoted on several DSL forums:
20bB. and below = Outstanding
20dB-30dB. = Excellent
30dB-40dB. = Very Good
40dB-50dB. = Good
50dB-60dB. = Poor and may experience connectivity issues
60dB. and above = Bad and will experience connectivity issues
For S/N Margin Readings:
6bB. or below = Bad and will experience no line synchronization or intermittent synchronization problems.
7dB-10dB. = Fair but does not leave much room for variances in conditions.
11dB-20dB. = Good with little or no synchronization problems
20dB-28dB. = Excellent
29dB. or above = Outstanding
Decibel dB Rule of Thumb
Every 3dB represents a doubling or halving of power. i.e. 9dB is twice the signal or power compared to 6dB. Decibel usually represents a margin, i.e. the difference between a background signal and the subject signal. For more information, click here.