SNR Margin and Speed

[#1]

I have been following another thread and was struck by this post by jameseh

Quote

Your sync speeds DROPPED because your noise margin ROSE. A rise in noise margin is consistent with a voice fault occurring on your line, such as a level of interference.  If the interference is fixed, then I would expect your noise margin to DROP and your sync speeds to RISE.

I had assumed that the SNR Margin reported by the router was the difference between the actual SNR value and the SNR value to run at a specific speed as described here http://www.kitz.co.uk/adsl/linestats.htm#SNR
Perhaps the more technically erudite can explain to me how an increase in noise can increase the SNR margin at router resynch (this is nothing to do with the default margin set by the exchange on a standard line)

[#2]

I think the magic missing word was "target" in this case.
There was noise, the DSLAM decided to raise the TARGET noise margin, resulting in slower speed on a resync with a higher real noise margin resulting from the raised target.
That said, if there's ongoing noise there will be a loss of speed as less useable signal gets through, irrespective of the above.

[#3]

The post was from the 21CN thread http://community.plus.net/forum/index.php/topic,74251.0.html and the noise margins being discussed are over 20dB so shouldn't be anything to do with the default margin unless the 21CN system has increased the default noise margin above the usual 15dB

[#4]

In that case it's probably the second situation I described.
Normally if there's a lot of noise the signal cannot be maintained and a resync occurs. If the noise is still there during resync it will be slower as less signal can get through. The noise margin at resync may well be close to the target, but if the noise then disappears the margin will be much higher.