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
The more commonly used SNR margin, as described below is sometimes abbreviated as simply SNR as well.
SNR margin (dB, a.k.a. noise margin) is the ratio by which the signal exceeds the minimum acceptable amount (minimum SNR) to sustain a certain speed. It is normally measured in decibels. SNR margin is often confused and used interchangeably with SNR. Many DSL modems and wireless devices (notably dd-wrt open source router firmware) use SNR margin, only denoted as "SNR". SNR margin is simply calculating the difference between signal (RSSI) and noise to get the SNR margin as a positive number expressed in db.
SNR margin = signal(dBm) - noise(dBm)
For example, if singal (RSSI) = -55db, and noise = -80db, then:
(-55db signal) - (-80db noise) = 25 SNR margin
Higher SNR/SNR margin numbers repesent 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.
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 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. 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 (stronger signal over background noise), and above 6dB is acceptable.
Note: For DSL, the further you are from the exchange, the lower your SNR and the higher your attenuation will be.