What's the use for an SN margin of 25dB?

Any advice on following would be much appreciated.

I have a TG585 v7.

After several years of rock solid syncing at 8Mb (all figures downstream) I was upgraded (Demon) to ADSL2+ in August. After a couple of days it settled down to sync at 10Mb with attenuation 36dB and SN margin varying from 8-10dB. This went on without interruption for several weeks.

However in the last few weeks it has started retraining every few hours (say, between 2 and 10 hours), to between 2 and 5Mb with a SN margin of between 15 and 25 dB. It never goes back up without my intervention, but when I reboot it always comes back up to 10Mb.

Can anyone suggest, in a general sense, what might cause this behaviour?

1. it surely cannot be noise on the line because, all the SN margins are so healthy?
2. will it never retrain itself back up, even when the SN margin is 25dB?
3. I can't see a way of finding out from the TG585 what caused the retrain. Can it be set to a verbose form of logging?

DAVID COLEMAN

1. Demon/C&W may have put you onto a 10Meg profile, thus while you have spare margin it goes no faster. and/or they've set a 10dB target margin, and as noise wanders around you may see 25dB during the quiet periods

2. Modems never retrain back to a higher speed, without someone doing something. There is a standard to support this but its very flaky and only one UK provider uses it at the moment.

3. Retrains are either noise, or a reboot usually. Recording the noise would require a large amount of data collection, some modems support DMT software which can let you track this on a PC to some extent.

Many thanks. I'll return to the hunt for noise and take up the profile issue with Demon.