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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.

  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 the terms a bit. Yes, SNR margin is a measure of the difference between signal and background noise. However, high SNR margin means that you WILL get your useful signal through, it means you have more room for variations and less possibility of errors.

SNR = signal / noise , so higher signal, or/and lower noise would increase SNR.
SNR margin = signal - noise (The difference between background noise and useful signal), so again, higher SNR margin also means that you have cleaner/stronger signal.
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?
by John - 2014-11-13 16:29
Some is right, some is wrong.

"SNR" means "Signal to Noise Ratio".
The higher is the better. This is indeed the actual "Volume" of carrier signal. Most Modems do not display this value.

"SNR margin" means "reserves between actual dsl-showtime link speed and actual SNR that is recommended for this link speed in 992.x".

The higher is the better, but explanation in article is wrong. An "SNR margin" of 1 or 2 may synchronize your modem correctly. It shows up the dB of Signal (!)reserves(!), you will need to do successfull handshake from your modem to carrier DSLAM. I also wouldn't recommend this but this may work. You will have no buffer.

Explanation: You'll need 35dB of Signal Noise for 8M sync in one direction, your showtime result is 42db for SNR, then your SNR margin is 7, that is quite good but very, very rare.

This is why asynchronous DSL show higher SNR for Upstream SNR margin, because of your link speed for upstream is mostly way slower. It could be much more, this is what you read.

Annex J becomes quite common in Europe. Modem link speeds are allowed to be handshaked by both sides to the max possible speed. Therefore you'll get way lower SNR margin.

There are also intelligent DSLAMS that do an adaptive rate after handshake, training and showtime mostly in VDSL vectoring zones, but they are pretty rare.
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