I notice BT have recently published another DLM patent, this time for a system that includes different stability options. It may or may not describe BT's own DLM in current deployment (or for 21cn), but I'm betting that it does.
see:
http://www.freepatentsonline.com/EP2169980A1.htmlClick on 'download pdf' rather than trying to read the html version which doesn't render properly.
Like all patents, it looks like it's been written by an engineer and then encrypted into gobbledegook by their legal beagles, so reading it is hard going.
But it's probably worth the struggle as it's loaded with intriguing details. For example, it explicitly defines, (table 2) various thresholds for resync intervals, and time between errors, that are needed to get you penalised by a higher target, or rewarded by a reduced target.
I've yet to absorb it, and I may regret drawing any early conclusions, but to whet your appetites...
...It looks you need less than one error in 24 hours (86400 seconds) to eventually qualify for a reduction on the 'stable' profile, whereas one error in 2.4 hours is what's needed on the 'aggressive' profile. I'm not yet sure what it means by 'errors'.
...Ten retrains in an hour always gets you naughty points, regardless of the profile. This specifically includes power-off resets.
- 7LM
PS: It's also largely about area-wide events, such as thunderstorms. It seems that if a high percentage of lines in a given neighbourhood suddenly starting retraining frequently, as a thunderstorm is likely to affect many lines, then the retrains aren't counted for profiling. So a thunderstorm should no longer get you sent to 15dB jail, no matter how many times you own line retrains.