Value on 21CN

[#1]

What are you expecting from DLM, when you are forcing the SNR margin via DMT?

[#2]

Ben,
From previous experience on 20CN forcing the noise margin has no effect on DLM it still seems to eventually drop the default noise margin assuming that the uncorrected errors don't get too high - this is why I prefer not to push the margin below 9dB as a 3dB drop from a 6dB level would be a disaster.
As far as I can tell the system seems to look at the actual noise margin, error rate and connectivity to decide what to do.
Whether DLM on 21CN is different I don't know but give it a few weeks and I may find out.
At present I am more interested in monitoring the stability and the error rates at this speed.
Previously on 20CN the highest stable speed was 3776kbps as above that the error rate could leap to levels that caused a resync and even below that it could have severe transient problems.
The thing which you need to understand is that the switch to 21CN which means a change to the connection in the exchange has fixed a major problem which multiple BT engineers failed to rectify or indeed diagnose.
Once I have a clear idea as to the long term line stability I may ask for a retrain unless the ability to alter the default noise margin by yourselves goes live.

[#3]

Change of plan.
Although the line remained stable the error rates are too high for comfort (I put it down to the weather) so I have dropped back to the default noise margin of 12.1dB which gives a sync speed of 3264kbps and will monitor it for 24 hours.

[#4]

It's interesting that you can trick / force DLM by forcing the target SNR with DMT. I would have thought that DLM would look at it's intended SNR margin to compare against stability, rather than the actual margin. Keep us informed, we're all learning

[#5]

In my experience you can get away with forcing the margin with DMT as long as the line remains stable, but if the line drops just once then the margin instantly reverts to the previously set DLM value. In other words, the actual DLM margin value isn't being changed on a permanent basis. This value is set by line conditions.
@Jim...  has your line dropped of it's own accord? If so, what happened to the margin when it resynced?

[#6]

In my experience it holds the change even after an involuntary resync but that may depend on the router.
With the Netgear it holds it until the router reboots but with my Speedtouch it would hold it even after a router reboot by using the saveall telnet command
Note that the change is the offset from the default value not an absolute value.
Since going to 21CN I haven't had a loss of sync due to line drops or instability despite having an enormous number of errors yesterday evening which made me revert to the default margin.
I had obviously pushed it too high in adverse weather conditions

[#7]

Having found that the error rate with a 4000kbps sync (3,500kbps IP profile) can get far too high I have now made a minor tweak to the default noise margin dropping it down by 1dB from 12.1dB to11.1dB (that is what the router reports) which gives a sync speed of 3616kbps and an eventual IP profile of 3000kbps.
This looks to be the most the line will support in the long term so I will now stop fiddling, unless something horrible happens, and just monitor the results

[#8]

Success

[#9]

and after a jump in errors this afternoon the system bounced my default noise margin up to 15dB - it appears that 21CN is much more aggressive than 20CN
Forced it back down to 9dB and will see what happens

[#10]

It appears that the drop for less than 20 minutes triggered a drop in profile as it is now down to 2,500
Interesting the IP profile hadn't dropped at 18 hours after the low sync but has now - seems a very long delay