[kwikbreaks]

The DLM question is easy to answer. DLM runs 24x7 on BTw systems from day 1 onwards. The only significance of the 10 days for ADSL is that the lowest sync rate during the first 10 days is recorded and no fault is accepted if the sync rate remains within 70%(iirc) of the recorded minimum. I don't know what it is with FTTC but quite probably the same.

The way it works is simple too. There is no "line training" - how do you train a length of twisted pair cable? Nor any "test to see what speeds can be achieved" on BTw systems although I believe Sky may start artificially low then go higher. All that happens on BTw is that there is a default target noise margin of 6dB set and the router/DSLAM negotiate the highest sync rate with that margin at the time. Increasing noise may cause the margin to fall. If it falls far enough sync is lost and then re-established still with that noise margin so as the noise was higher than means a lower sync. This is an ongoing process. If the software determines there have been excessive resyncs the target margin gets increased in 3dB steps up to (iirc) 15dB in an attempt to achieve stability. A long period of stability (typically a couple of weeks) is rewarded with a reduced noise margin - only down to 6dB although some ISPs will manually set it at 3dB. Really bad lines can be set at a fixed rate rather than rate adaptive.

A BRAS profile (maximum datarate) is set based on sync. The old system was an immediate reduction for lower sync and delayed increase - sometimes as long as 5 days. The newer system makes closer to instant changes but as it was developed after the days when I took much of an interest in this stuff I don't know the details. The equipment datarate gets passed to ISPs and they sometimes have delays updating theirs so you can get "stuck profiles" with lower data rates than the sync could support.

The problems I saw in my days on cable were what I suspect were cowboy installers juggling customer connections to cabinet taps leading to short outages and twice in my case out of spec power levels that had to be fixed. The biggest problem though was congestion which kept raising its ugly head and I think is down to the low capacity local pipes cable has coupled with VM's determination to keep "unlimited" in their advertising.

I never really had many problems in my ADSL days apart from a duff line which never got fixed when I moved house and that drove me to cable. The sort of problems I saw reported on boards were frequently down to poor internal house phone wiring. Other issues after ADSL had been around a few years were cheapskate ISPs cramming far more customers on the expensive BT kit than it could sensibly support leading to huge peak time slowdowns. Early LLU operators for the most part were first rate but some of the later arrivals were poor too.


I haven't been on FTTC long enough to see any problems at all so far.