Not having been around these forums for a while,
I was fascinated to find this thread today. It's very frustrating that the workings of DLM are well documented in respect of sync, SNRM, IP Profile etc, and seem to more or less work, most of the time; yet the way that DLM handles the Target Margin seems shrouded in mystery, myth and magic.
I wonder if any of the techies here can make anything of my recent experience -
I'd been coasting along with my 52·5dB line syncing at around 5200, 6dB Target, until the cable just down the road was JCB'd by a careless contractor on 20 May. After the repair, I was down to 250kbps and a 12dB Target, and as expected the sync increased after 3 days, to 3776, with a low error count. As long as YouTube and BBC iPlayer worked, I was quite happy, and interested, to see how long it took for the Target to reduce.
Until then I'd always left the router on 24/7, so when nothing had happened by mid-August, I tried re-booting every day around mid-morning for a few weeks - still no change. At the beginning of October, I thought I'd better throw in the towel and ask Plusnet to request a reduction by BTw, but before I got round to it, I lost ISP connection around lunchtime on 6 Oct, due to a MUX failure in the local exchange. Sync was OK, but was at 576/288 with a margin of 31dB - just as though I was suddenly on a 0·5Mb fixed rate line!
The MUX was fixed by late afternoon, and I was back to 3776 at 12dB, but within a matter of minutes, the router re-synced of its own accord at around 4350 at 9dB. The following day around lunchtime, again without prompting, the router re-synced at 5120 at 6dB!
It would seem, then, that the DSLAM was stuck for nearly 6 months, and was somehow unstuck by a MUX failure.