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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

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 (dB, a.k.a. noise margin) is the difference between the actual SNR and minimal SNR required to sync at a specific speed. It is normally measured in decibels. 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 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.

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).

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

  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.
by anonymous - 2014-12-04 12:30
Noise margin in a digital circuit is different than the noise margin on an analogue line.

ADSL modulation takes the digital 1s and 0s in your computer and converts them into analogue signal suitable for sending down a phone line. That's why you measure it in dB - it's an analogue signal when it's on the line.
by anonymous - 2015-01-07 08:22
In response to the idiotic notion that decibels are nothing to do with Noise Margin in DSL communication circuits:- In communications system engineering, noise margin is the ratio by which the signal exceeds the minimum acceptable amount. It is normally measured in decibels. Since we are discussing DSL circuits, decibels most definitely IS the correct unit of measure for Noise Margin. Noise Margin (in circuits) is the amount of noise that a circuit can withstand. Noise Margins are generally defined so that positive values ensure proper operation, and negative margins result in compromised operation, or perhaps outright failure.
by Bb engineeer - 2015-01-22 18:38
Most of you have it half right. Snr is the ratio of useful signal compared to background noise. Snr margin is the extra signal strength needed to overcome the noise (the noisy room is a good example, the lower the Snr, the quieter you can talk). Noisy lines need a bigger margin.

Broadband is a trade-off between speed and stability. A higher Snr margin means the dslam (actually msan these days) has to do more work so, although the line is more stable, it is slower. Generally a margin increase of 1dB will drop sync speed by about 3mbps. (I could have got those numbers backwards).

The dlm at the exchange adults bigger margins when multiple losses of sync are detected, giving a slower, more stable connection.

Before people wade in to argue, I'm a broadband support agent with one of the UK's biggest Bb providers, and deal with this daily.
by Bb engineeer - 2015-01-22 18:47
But to answer the question, UK lines aim for a margin of 3dB, most are stable at 6db, anything over 12dB is worrying and will be very slow.

3dB margin will usually give about 20mbps sync.
by anonymous - 2015-02-21 11:58
Thanks for actually answering the question!
by anonymous - 2015-03-11 07:06
Could you please advice how to manage the noise margin, like if the noise margin is very high (31db) how can we minimize and what is the possible impact.

Thank you
by anonymous - 2015-04-13 02:58
A few notes:

1) dB is not a unit, it's simply a way to look at things in a logarithmic scale. Instead of saying 100, 1000, 1000000, I could say 20 dB, 30 dB, 60 db. (dB is simply 10 times log_{10} = the number of zeros after the one). The common use of dB when discussing noise is due to the high amount of variability in signal and noise amplitudes, which would otherwise necessitate use of cumbersome numbers.

2) There's no need to ever want to reduce your signal to noise margin. It could be high because noise is low, which is a good thing. The British engineers looking to minimize SNR are relying on persistently high noise to look at the signal-to-noise ratio as a measure of the signal's amplitude, which is a really bad practice because some lines have less noise, and because they could simply look at the amplitude of the signal directly instead.
by anonymous - 2015-12-01 18:14
I have a SNR Margin Down=3.5dB, and have had disconnections since the loss of the phone line for two minuets. My ISP provider is BTand since this happened the hub has disconnected the internet for about 1 minuet, it does this about every 8 hours or so on and have been advised that it can take up to 5 days to stabilize.
Can you please advise thanks.
by Derek Noffke - 2016-01-25 06:52
I found the "opposite" opinions on whether the SNR margin is better high or low confusing.

I did some reading in Wikipedia and I came to the conclusion that the reason for the opposite opinions is likely due to an assumption of what the SNR margin is representing.

By my reading the SNR margin is negotiated between the subscriber ADSL device and the provider and represents the percentage of the theoretical data throughput that will be used.

If the line is perfect then 100% capacity may be used (small SNR margin)

If the line is noisy then for the same sync speed much less data is transmitted (larger SNR margin)
by Juan - 2016-01-29 06:57
Sorry, guys, but I am of the opinion that a low SNR is bad, and a high one is good. Perhaps you are mistaking SNR for attenuation? With attenuation, higher dB's is good and low dBs is bad.

My ISP's system backs this up. See my connection details from their dashboard below, and the definitions in brackets:
ADSL Router Results (Telkom Exchange to Your Location)

General Status:
There no defects on the line.

Maximum Sync Rate
(Maximum speed at which the DSLAM and your modem will communicate.)

Current Downstream Sync
(The current download speed established between the DSLAM and your modem.)

SNR (Signal-to-Noise Ratio)
(Signal-to-Noise Ratio on the line. A value below 6dB can cause problems.)

(The amount of resistance on the line. A value below 55dB is good.)

DSLAM Results (Your Location to Telkom Exchange)

General Status
There no defects on the line.

Maximum Sync Rate
Maximum speed at which the DSLAM and your modem will communicate.)

Current Upstream Sync
(The current download speed established between the DSLAM and your modem.)

SNR (Signal-to-Noise Ratio)
(Signal-to-Noise Ratio on the line. A value below 6dB can cause problems.)

(The amount of resistance on the line. A value below 55dB is good.)
by Luis - 2016-03-02 10:27
I'm sorry guys but I consider that in essence this article is correct, so most comments are unnecessary, redundant or superfluous. Some are correct and clarify the subject though.
by Roy - 2016-05-01 09:25
A high SNR margin is both good and bad.

It's good in the sense that the line will be stable and won't drop out very often due to interference.

However higher connection speeds need higher SNRs, so having more margin means that the connection speed is slower than it might otherwise be. To illustrate this, making some numbers up based on the original post:
SNR to sync at 9Mbit/s = 38dB
SNR to sync at 8Mbit/s = 35dB
SNR to sync at 7Mbit/s = 32dB

So if the actual line SNR was 45dB:
SNR margin at 9Mbit/s = 7dB
SNR margin at 8Mbit/s = 10dB
SNR margin at 7Mbit/s = 13dB

So in this made-up example, a 7Mbps connection would have an SNR margin of 13dB which would be a stable connection, whereas a 9Mbps connection would have an SNR of 7dB - faster but less stable.

As has been said elsewhere, a target of 6dB SNR margin is the usual starting point. At the moment I've got a downstream SNR margin of 6.2dB and an upstream SNR margin of 12.1dB. The modem reports a max upstream data rate of 4Mbps but the data rate is capped at 2Mbps hence the high SNR margin. So those numbers tell me the upstream connection is running more slowly than it could do, but it's pretty bombproof.
by MiniB - 2016-05-02 04:18
what about this measurements:

Loop attenuation [dB] 11.0 dB 34.3 dB
Signal attenuation [dB] 11.0 dB 34.0 dB
SNR 9.6 dB 7.7 dB

Maximum attainable rate 20100 kbps 758 kbps
Current rate 12284 kbps 769 kbps
Output power 18.5 dBm 12.3 dBm

Line operational mode ADSL2+ over POTS(G.992.5 Annex A) non-overlapped
Measure mode DELT

I'm connected in a ARAM equipment, I don't know estimated lenght of circuit. Can I go to upper speed, like 14 or 16Mbps or I will reach bad line parameters?
by David - 2016-05-17 14:23
So here is what I got. I work for a ISP.
This is my line going to my house. FYI I'm about 2 miles out from the CO.
I have my line set for Interleaved as I have and get a lot of errors on the line. I have tried Fast which is a option for the system to ignore errors on the line but it's less stable.

So you can go slow and steady with Interleaved on or Fast and reckless with Fast.

My line is set up on G.dmt.
ADSL2+ will do more than 8000kbps is all.

My line 2 miles away from the CO

Service State IS
Line Status Up
Rate Mode Adapt At Startup
DMT Standard 992.5A (G.DMT over POTS)
CPE Vendor BDCM (ver A2pB033g)
Up Time 135405 sec
Downstream Upstream
Line Rate 2859 kbps 187 kbps
Margin 6.3 dB 22.5 dB
Power 18.7 dBm 12.5 dBm
Attenuation 44.0 dB 22.8 dB
Max Attain Rate 4016 kbps 648 kbps
Actual Delay 0.0 ms 0.0 ms
Actual INP 0.00 sym 0.00 sym

This is a line that is blocks away from the CO
As you can see it's set up on ADSL2+ since the customer wanted more than the basic pkg.

Service State IS
Line Status Up
Rate Mode Adapt At Startup
DMT Standard 992.5A (ADSL2+ over POTS)
CPE Vendor BDCM (ver A2pB033g)
Up Time 68513 sec
Downstream Upstream
Line Rate 11999 kbps 1084 kbps
Margin 22.8 dB 7.5 dB
Power 18.6 dBm 12.5 dBm
Attenuation 13.0 dB 6.3 dB
Max Attain Rate 25268 kbps 1208 kbps
Actual Delay 8.0 ms 4.0 ms
Actual INP 1.79 sym 0.64 sym

The closer you are to your CO the better or at least in this Town.
CO = Telecom Central Office.

Most of the time we will try a new loop to see if running a new wire from the poll to the house make anything better. Also changing out your phone jack and IW in the house will help.
Setting up a NID repair for your ISP is also a idea if your line is bad or if you have more than the normal amount of drops.
FYI even when a new loop is placed it does not mean things will be great. Best of luck if you would like them to change out a Main Line from the CO to your house as that will take block and blocks out of service. No CO will change out a main line as it will drop phone and internet and 911 service's to many. Like a power outage.

Hope this helps a little.
by anonymous - 2016-09-04 23:00
OP is correct. I have found digital signal needs at least 9dB SNR for reliability. Any less and you get problems.
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