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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.

  User Reviews/Comments:
by Goulburn - 2013-02-26 00:14
This is incorrect. Consider SIGNAL MARGIN like two folks talking in a crowded room. If they can speeak softly to each other the Noise Margin is low (6db) if they have to speak loudly or shout the noise margin is 10db or 20db.

Its a measure of how much louder you have to speak, or how much gain in db needs to be applied to the signal for it to be understood at the receiving end. British Telecomm (BT) tries to achieve the best speed at 6db. Basically the lower this figure the faster the download speed will be in Mbps.

It is true some adsl copper wire lines are noisy and so gain has to be applied to the signal, say 10db or 20db or higher, when that happens the download speed deceases.
by Philip - 2013-03-05 17:27
I believe you're confusing noise with SNR (Signal-to-noise ratio, or margin).

SNR = signal / noise , so higher signal, or/and lower noise would increase SNR.
by anonymous - 2013-07-19 21:02

SNR at Wikipedia -->

TP-LINK ASDL Stat Explanation -->
by anonymous - 2013-08-22 16:56
Noise margin has nothing to do with dB. Noise margin is how much noise you can have (1V, 2V, 2.5V, etc) before a "1" is no longer discernible as a '1" and a "0" is no longer discernible as a "0."

Not sure why dB are mentioned at all. That's not the proper unit or method for determining noise margin, you're talking about something unrelated to noise margin- though I don't know what the proper way to refer to it is.
by anonymous - 2013-09-05 15:42
Err. EXACTLY the other way around. High noise margin means quiet room and you can whisper to one another. Low noise margin means noisy room and your whispering would be drowned out. You'd keep losing track of the conversation and having to repeat yourself to get the message accross; analogous to resending packets and losing sync.
by Philip - 2013-09-29 10:28
"High noise margin means quiet room "... That's exactly what it's been stated in the FAQ.
by anonymous - 2014-07-26 10:46
Do those apply for ADSL2+ as well?
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