What is considered good DSL Noise margin / SNR ?
Noise (dBm) in communications is a combination of unwanted interfering signal sources, such as crosstalk, radio frequency interference, distortion, etc.
Signal-to-noise ratio (SNR) is defined as the power ratio between a signal (meaningful information) and the background noise (unwanted signal):
SNR = signal / noise
6dB or below SNR is bad, you will experience no sync, or intermittent sync problems
7dB-10dB is fair but does not leave much room for variation in conditions
11dB-20dB is good with little or no sync problems (if no large variation)
20dB-28dB is excellent
29dB or above is outstanding
The more commonly used SNR margin, as described below is sometimes abbreviated as simply SNR as well.
SNR margin (a.k.a. noise margin) is the difference between the actual SNR and minimal SNR required to sync at a specific speed. It can be simplified to: the difference between actual signal and signal required to sync. It is normally measured in decibels. It is in essence a buffer that allows for fluctuations in SNR without dropping the connection. SNR margin is often confused and used interchangeably with SNR. Some NAT wireless routers, for example (notably dd-wrt open source router firmware) use SNR margin, only denoted as "SNR".
For example, to calculate SNR margin:
If actual measured SNR = 45db
SNR to sync at 8Mbit/s = 35db
SNR margin = 45-35 = 10db
Higher SNR/SNR margin numbers indicate cleaner/stronger signals, with less background noise. The higher the SNR margin the more stable the connection. In some instances interleaving can help raise the noise margin to an acceptable level.
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 display both the actual SNR, and the signal-to-noise margin (SNR margin) as a separate value, which (again) is the difference between the actual SNR and the SNR required to sync at a specific speed. As with actual SNR, the higher that SNR margin number, the better (stronger signal over background noise). SNR margin is the buffer between actual current SNR and the SNR required to sync.
For DSL, the further you are from the exchange, the lower your SNR and the higher your attenuation will be.
At peak times, the noise may increase as your provider's DSLAM becomes congested.
Fluorescent lights and other sources of EMI close to the modem can affect the SNR as well.
See also: SG DSL Speed Calculator