In reply to a post by MrSaffron:

2 days, 10 days or 100 days the DLM is always active, how fast it gets down to a relatively unchanging speed depends on not just the line but the random noise environment it is subject to

Thanks - I get it. I probably expressed what the engineer told me incorrectly. DLM is constant, but how long would you expect the line profile speed to settle to a (more or less) constant level after a profile reset?

I understood the engineer to mean that DLM would start with the max I pay for and drift down until the error rate became acceptable, and that it would do this over a few cycles, over (presumably) a few days in order to fully sample line conditions.

Assuming there is no change to the physical nature of the line between the cabinet and my house, this seems (to me) a reasonable understanding of DLM, but I'm very happy to stand corrected!

This morning I get approximately the same stats from my router as the 'After' data I posted yesterday. Happy days! For the time being....

Pete

Correct the profile starts at an open setting, and while its usual to drift down in various steps as the error rate is reduced it might take 2 days or it might take 30.

10 days is probably a good average, but what you have at 10 days can still change and that is the key point.

Also it is not always down, if the first few days are bad but following weeks are perfect the DLM can recover on its own.

Yes, I get it, thanks. Given I was stable for a year or two on ~33Mbps download and then, recently it fell to ~26Mbps this must have been DLM adjusting downwards, as you said.

Hopefully the recabling the OpenReach engineer implemented between pole and house will have corrected any physical issue that would have caused DLM to dial me down.

All the best,

Pete

Your before and after stats are quite telling.
Look at the very high noise margins.

The line is capable of around 55Mb, you pay for 40Mb, but it looks like DLM previously capped/banded your line at 27.4Mb.

DLM caps lines for multiple resyncs in a short time period.
Don't reboot the router multiple times. Turn it off completely if you are having any work done that involves power being turned on and off.
DLM sees repeated disconnections as instability wether they are done manually or not.

The DLM reset has removed the cap and got you back to 40Mb.
As long as the line doesn't resync multiple times the sync speed shouldn't reduce again.

The engineer discussing 10 days training is a reference to the ADSL DLM.
The FTTC DLM starts its thing immediately after a line goes live (or right after your DLM reset).

not read everything but yeah high noise margin indicating possible REIN/SHINE noise or a flappy line (on/off a lot)

also, do yourself a massive favour and get rid of the [censored] Hub One - it is a BT HH 5A with the poor Lantiq chipset - mine and its replacement they sent out kept slowing my throughput (not sync) after 3-4 days... soon as I put a BT HH 5B in which has a broadcom chipset (which are well known for high speeds and solid stability) my speed increased slightly and never had a problem..
a little known fact is that BT replaced the HH 5A (Lantiq) as quick as they could with the HH5B (Broadcom) because it was causing lots of problems...

since then I have put in a Zyxell VMG8924-B10A (Broadcom - unlocked) which allows me to pull advanced stats from it using DSL-stats program.

used it as proof when plusnet speed weent from 80Mb to 55Mb and Openreach changed the MGALS mid live connection..."well the MGALS has dropped by 'X'Mb lets drop the mininum speed by the same 'X'Mb too" I complained face to face with an Openreach boss about that, they cant do that mid a live conection otherwise you could have me paying for 80Mb and let it whittle down to 5Mb and keep me paying the premium.

one of the issues is (and no offence to the enginners who DO know a lot) - but their remit is "in and out as fast as possible...not there to fix speed , only to get it working and stable" - every single engineer says that, when i start showing them print offs of graphs of SNRM, Line Attenuation, QLN, - BEFORE and AFTER they shrug and say "you know more than me mate"... says a lot about them, the UK biggest Internet Provider???

it should also be stated that MAX ATTAINABLE RATE - is always 'inflated' by Interleaving, so if your not on fastpath (like me) so error protection is on... my sync is now 64Mb, it says my max attainable is now 77Mb but really its 70Mb, if errorprotection is turned off I get 69Mb and it reports max as 70Mb - thats about 10% less, for me but could change depending on the speed itself

Just a quick follow-up from OP. The recable seems to have fixed it. Here are the stats from my router from the date of the engineer's visit:

Date Down Up
22/01/2020 39950 8904
23/01/2020 39950 8897
24/01/2020 38171 8886
28/01/2020 33478 8852
31/01/2020 38171 8803
03/02/2010 33478 8894
05/02/2020 33478 8834
07/02/2020 34956 8849

Thanks to everyone who helped me here!

Pete