http://www.kitz.co.uk/adsl/linestats.htm :
An explanation of SNR and SNR Margin *
The SNR Margin is not the same as the Signal to Noise Ratio (SNR), although it is related.
The SNR is the signal-to-noise ratio. It is the ratio between the strength of the signal and the background noise on the line and is universally expressed as a logarithmic ratio (decibels or dB).
For a given line the SNR remains essentially fixed whatever the speed you are running.
ADSL routers and modems actually report the SNR Margin, and the term used to describe this varies but is generally “SNR Margin”, “Noise Margin”, “Margin”, “Receive Margin”.
The SNR Margin is the difference between the actual SNR and the SNR required to run at a given speed.
For example, if you need 20dB of SNR to run at 512Kbps, and the actual line SNR is 45dB, then the SNR Margin is 25dB.
The main reason why SNR margins fall as the line speed increases is that the required SNR increases.
For example, if increasing the speed from 512Kbps to 2MBps raises the required SNR from 20db to 30dB then the margin left will fall from 25dB to 15dB (as 45dB-30dB = 15dB.)
The one refinement is that if it is necessary for a faster speed to use some higher frequencies then these will have inherently lower SNRs and that will also contribute to the reduced SNR margin somewhat.
The SNR Margin is there to absorb fluctuations in interference on the line, and therefore to keep the service reliable. The figure of 6dB is normally accepted as enough headroom to make the link reliable, but some lines need more - 10dB is often accepted as a safer target.
Note that the target figures in the list are a reasonable summary, but it is the "SNR Margin" that is being talked about, not the "SNR". Despite this, it also does not invalidate the point that the most important factor on the reliability of a line at a given speed is the SNR Margin.
Also, as neither the SNR or SNR Margin can be established until a line is enabled (or validated with special test gear), then the estimated attenuation has to be used to come up with a viable speed. Once a line is enabled and working, it is possible - at least in theory - to gather information about the actual SNR and SNR Margin and use that to work out the actual speed the line will be able to sustain. This is essentially what will happen with the MaxDSL launch towards the end of the year.
* The above explanation has been slightly adapted from an original post made by Steve J (nopr0700) on adsl guide forums. - Due to the fact that forum posts are often archived, I have reproduced his information on this page. Thanks Steve.
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