What is considered good DSL Noise margin / SNR ?
Noise in communications is a combination of unwanted interfering signal sources, such as crosstalk, radio frequency interference, distortion, etc.
Noise margin is the ratio by which the signal exceeds the minimum acceptable amount. It is normally measured in decibels.
Signal-to-noise ratio (SNR), often referred to as noise margin is defined as the power ratio between a signal (meaningful information) and the background noise (unwanted signal):
SNR = signal / noise
Higher numbers repesent cleaner signals, with less noise. In some instances interleaving can help raise the noise margin to an acceptable level.
6dB or below noise margin is bad, it will experience no synch or intermittent synch problems
7dB-10dB is fair but does not leave much room for variances in conditions
11dB-20dB is good with little or no synch problems (if no large variation)
20dB-28dB is excellent
29dB or above is outstanding
Note that there may be short term bursts of noise that may drop the margin, but due to the sampling time of the management utility in your modem, will not necessarily show up in its interface.
Some DSL routers, in addition to the actual SNR, may define signal-to-noise margin (SNR margin) as the difference between the actual SNR and the SNR required to sync at a specific speed. For example:
actual SNR = 44dB
SNR to sync at 8Mbit/s = 35dB
SNR margin = 44-35= 9dB
As with actual SNR, the higher that SNR margin number, the better and above 6dB is acceptable.