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[Weaver]

I keep seeing my target SNRMs changing in a manner that looks like the action of DLM. I asked AA to turn Openreach DLM off though, and they confirmed that they had done so, which was possible because this is ADSL2, not FTTC/VDSL2.

Is there some reason why the Openreach DLM on/off setting might get reset somehow? Or could the settings get lost during some Openreach maintenance operation?

Twice recently my line 3 upstream target SNRM, which was set by me to 9 dB, has dropped back down to 6 dB in an unexpected retrain / resynch. There have been earlier instances too. Some while back, one line’s downstream target SNRM was set to 6 dB and then later it mysteriously dropped down to 3 dB.

I will of course ask AA about it, but I was just interested to hear others’ experiences.

[aesmith]

Is there some reason why the Openreach DLM on/off setting might get reset somehow? Or could the settings get lost during some Openreach maintenance operation?

Yes. AA disabled DLM for us in 2018 (possibly new for them as the initial comment was "I think I may have turned DLM off completely,").  It got re-enabled a couple of times over the years and AA have disabled it again for us.  Normally that was when an OR guy was fiddling with something, one of them them had a real bee in his bonnet and seemed to almost be offended that our line synched at over 4 meg and he was always knocking it back to a 9dB or 12dB target.

[Weaver]

I’ve met OR men with a similar mindset. The direction of spontaneous change though is such as to make things less reliable but faster.

[XGS_Is_On]

Openreach don't control DLM on ADSL.

[Weaver]

Should I have said BTW to be correct? AA swore to me in the past that they would be happy to ‘disable DLM’ for me (and I am indeed on ADSL2).


Coming back to the current problem. AA pointed out something that I wasn’t aware of. My line 1 does not exhibit this infuriating behaviour, the problem is confined to line 3. The difference about line 1 is that it is set to ‘extra stable’. I think. And line 3 was set to ‘standard stability’.

So when AA pointed this out I hit the line 3 "Change to extra stable" button too. We shall see how this behaves then.

I have no idea what these states do. I have never (knowingly) used them.

I think "Super" is stronger than "Extra". Is that correct? Because the button on the right is currently showing as follows:
        [ Change to standard stability ]  [ Change to super stable ]

The left button is where “[Change to extra stable]” used to be, I presume.

If it is the case that ‘standard stability’ (a phrase that doesn’t mean much) means “reduce the downstream sync rate a bit”, or “increase the downstream target SNRM by some amount”, both of which may be wrong, then that may give better reliability all the time, which is what we want. Maybe it’s to do with line turning rather than DLM. I had always guessed at the latter. I apologise in advance to Kitz, who has probably told me the answers, and since I have never sought to use these tools, I have forgotten the correct definitions of the terms. AA has absolutely nothing worthwhile (that I could find) on their support wiki. Perhaps I should search again.

[kitz]

> Should I have said BTW to be correct?

Yes for ADSLx its BTw... or what ever LLU SP owns your transit backhaul such as TT/Sky

>>  they would be happy to ‘disable DLM’ for me

afaik it is not possible to completely disable the (BTw) DLM*. 
The available ON/OFF (and auto) option relates to Interleave/Error Protection.
Thus why you are still seeing changes for the target SNRM despite you setting it to a higher figure.  BTw clearly state that opting to change the Interleave/Error protection doesnt affect changes being made to the target SNRM.



>>  "Change to extra stable"
>> I have no idea what these states do

There is information about this on the main site. BTw's DLM process.
See DLM Stability Levels.

This is an (ISP) configurable option that determines how harshly and how quickly a line is penalised when errors are seen on the line.
Super Stable is quite harsh, making increasing changes to target SMRM / INP / Interleave error protection/correction rates sooner that the other profiles and it can be difficult for the line rate to 'improve' back to the default profile.  By "improving the line rate", I mean reducing the target SNRm, decreasing the level of interleave/INP to increase the sync speed.
 
Few lines are on the super-stable profile as high target SNRM and Interleaving/INP strip available sync speed and may not be a good thing when you have a long line where due to the overheads could easily halve your available line rate.  iirc BTW used to specifically warn that using high INP values dramatically reduce the line rate. 

-----------------
*There is an ISP custom config, but Im not aware of any ISPs who use this option.

[kitz]

ETA

If I read correctly that its the line on Super Stable that doesnt see the SNRm movements..  then that would make sense.  The line would require a longer period of stability before reducing the SNRm back down when compared to a line on the Standard or Stable profile.

[aesmith]

afaik it is not possible to completely disable the (BTw) DLM*. 
The available ON/OFF (and auto) option relates to Interleave/Error Protection.

I can confirm that AA did something to disable DLM on my line which is 21CN ADSL. Initially he said he was going to "try some profiles" then reported "I have reapplied the profile and I think I may have turned DLM off completely"

The "I think" sounded like it might have been the first time they'd done this.

That was in 2018 and since then the line has remained with a fixed 6dB target irrespective of error rate which is generally around 72 CRC per minute. Occasionally BT re-enables DLM and l need ask AA to disable it again. I think that's maybe happened twice. 

[kitz]

>> "try some profiles"

I wonder...  see the note above linked to the asterisk * in my sentence,  where I said that there is an option for custom profiles in addition to the 3 templates.
 
I am not aware of any ISP that uses a custom profile, but the option is there.  Bearing in mind that the profiles can be set independently of the ISP default for each line, then it could be possible that he dabbled with creating a custom profile for your line.   

This isnt the sort of thing that I'd expect an ISP like Sky or TT to attempt - aside from the fact they use their own DLM on their own MSANs anyhow.
There are a few ISPs like Plusnet or even BT if you can get through to the right person who may switch your profile between Speed/Standard..  but going as far as creating a new custom DLM template (unless used universally) then there's really only AAISP where their CS staff would know enough about the DLM and be confident enough to go the extra mile and try creating a custom profile.
 
I stress that I'm guessing here, but if he managed to create a custom profile that is similar to the BTw "open profile" and then applied it to your line, this would have a similar effect to turning off the DLM.   Actually the more Ive thought about it whilst typing, then the more logical it seems.