To recap the last few weeks... One day in early August, following a power cut, my router synced at the default target margin of 15dB but, as often seems to happen after a power cut, the sync speed was much lower than I'd normally get at 15dB. Correspondingly, once the the noise had settled, the actual SNR soon rose to about 18 dB. With such a high margin, my error rates became very low, averaging no more than 20 ES per day.
After a littler over 14 days with these low errors, DLM rewarded me with a 3dB target reduction to 12dB.
That 3dB reduction was on 22 Aug. Since then, with a 12dB margin, my error rates have been not bad, averaging about 150 per day. But that's not been good enough for DLM as I've had no additional reduction in target, despite having let it maintain continuous sync for the last 19 days in case it helped.
So, I've now bravely tweaked my margin again, 'adslctl configure --snr199', which has got me an 18.5dB margin, and obviously a loss in sync speed. But I suspect there will also be a big drop in errors.
It'll be interesting to see, in a little over two weeks time, whether DLM has responded favourably to the lower error rates.
One thing I've already noted is that the earlier reduction has stayed with me. That contrasts with the manual reduction which I persuaded by ISP to get done year ago, and which evaporated 48 hours later. I've seen similar accounts elsewhere of manual overrides quickly reverting. I do wonder whether BTw's DLM interface actually enforces a 48 hour timeout on manual overrides, or at least there may be a tick box for it somewhere on the GUI, that the operators either ignore or don't understand?
edited to get the dates accurate