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[roseway]

That's an interesting theory, Mick. When I first heard about these service levels (from ADSL24) it was suggested that they directly related to target noise margins, but your theory sounds very plausible. If, as has been suggested, customers will in future be personally able to set the service level on their account, then allowing them to tweak the behaviour of the automatic DLM process, rather than to override it absolutely, would make a lot of sense.

[kitz]

>> 'super stable'

That kinda rings a bell with the 21CN DLM system which is supposed to be slightly different.  I'd need to dig out the stuff I have somewhere, but iirc it was mentioned that these profiles may also apply at some point to WBC.

The long term plan is for users to be able to select and change their own profiles much easier.

[sevenlayermuddle]

I like Mick's theory.

In my case, I had a steady 9dB margin for several years then, owing (stupidly) to an afternoon spent experimenting with different filters and things, I managed too many resyncs and earned my 15dB target margin.  I eventually persuaded demon to get it brought down to 9dB again, but 2 days later it was back at 15dB and it's stayed there ever since (6-7 months ago).

Now, suppose that for some reason that my service option also got changed as a result of that initial sequence of events, either by some misunderstanding by demon, or by some policy within BTw, or whatever, it would explain things nicely...

[jeffbb]

Hi

At long last my SNR target has dropped again  is now 9db.This happened without me rebooting or anything . I did not lose pppoe or any connection WAN or Lan . Its taken about 7 weeks to drop from 12 to 9 . Had quite a bad storm about 10 days ago where errors climbed for a couple hours . Maybe that delayed the change ?? my router has been up continuously for 20 days .

Regards jeff

[kitz]

Thanks for your observations guys.

Interesting another report that there was no resync or loss of PPP session.

[joss]

If I May ask

Is it beneficial to use the DGTeam firm ware to artificially lower your SNRM against the DLM process? I guess what I am asking is, Is it better to leave artificially lowering of the SNRM alone and just hope and pray the DLM will see the line stable and over time lower the SNRM automatically?
Will using the DGTeam firmware to lower SNRM hinder the DLM process.

Sorry if I'm making a hash of this but its the only way I can convay what I wish to ask.

 

[roseway]

If you want DLM to have the best opportunity to reduce your target noise margin automatically, then you should leave it untweaked. If you tweak the target noise margin down, then you are likely to increase the error rate, which can increase the time taken to get the reduction, or stop it happening altogether.

That being said, in my case I did tweak the margin down a bit, but I kept a close watch on the error rate, which was very low. Every case is different of course, so it's hard to make general rules.

[Mick]

I promised to report back and here I am! 

I can confirm that after my ISP asked BT to change my Service Option to 'stable' (from 'super stable') I have for more than a week been holding onto a SNRM of 9dB (previously was nailed at 15dB come what may) and a BRAS of ADSL 6000.   

I have seen some degradation of the sync speed over the week (from 7028 to 6816 kbps) but this could be due to weather conditions and whatever else.  I have recorded a clear increase in average and maximum FECs/CRCs compared to when I was running at 'super stable' setting, but these errors do not seem to have affected my ability to hold onto 9dB!  Furthermore, last Friday we had three power cuts (two of them in really quick succession) and this did not affect DLM's view of my line either.  This was despite the fact that one resync event was below 6815 kbps, which simply resulted in a ADSL 5500 for now.  Under my previous 'super stable' setting any event like this would have forever kicked my SNRM up to 15dB.

I am curious now to see if I may go through an auto-reduction in SRNM after a fortnight (power cuts allowing).  Watch this space!

[roseway]

That's interesting Mick. When your service option was changed from super stable to stable, did your target noise margin immediately reduce to 9 dB?

[Mick]

I didn't want to complicate matters so I omitted this part.  The ISP asked BT to lower the SNRM to 9dB.

The bummer is that BT also changed my FTR to 3200Kb/s (MSR=4000Kb/s), when they previously were FTR=4992Kb/s MSR=6240Kb/s.  I queried this with my ISP.  On the 20CN this would be the result of line (re)training.  However, I am told that on the 21CN, changing the Service Option causes a retrain.  This means that unless the sync speed drops below 3200 kb/s the BT boys are not coming out to check for faults. 

[sevenlayermuddle]

Mick...

FYI, I think There is another rumoured relationship between FTR and target margin, and I'd be interested to know if you feel that part of your recent experiences can confirm or deny it...

...according to one of the BT patent papers, DLM won't increase the margin if it would reduce your speed to less than FTR.  Further, it assumes a speed drop of 800kbps for each increment, so it won't increment unless you're already syncing at 800 above FTR.

Even if it's true, I don't think it affects your current circumstances as your line as DLM was able to apply a 15dB margin even at your earlier fault rate.   So the worst scenario that could happen is your margin could increase to 15 again, just as it did before.

[Mick]

...according to one of the BT patent papers, DLM won't increase the margin if it would reduce your speed to less than FTR. 

Well, in my case it increased from 9 to 15dB and this reduced my sync speed to just below my FTR.  My ISP said that if we were to call out BT to investigate luck would have it that their modem would sync just above my FTR and they would register a no fault found.

Further, it assumes a speed drop of 800kbps for each increment, so it won't increment unless you're already syncing at 800 above FTR.

I wish it were like this, I would have stayed at a 9 or even 12dB SNRM, but as I said it increased straight up to 15dB.  This happened more than once on the occasions that I asked my ISP to reduce the SNRM to 9dB.

[sevenlayermuddle]

Thanks Mick.

It's interesting, if disappointing, to know that patent doesn't exactly descibe DLM. 

However, the bit about going straight from 9dB to 15dB is consistent with the BT patent,  whereby a line that is 'very poor' will move two steps towards 'stability'.  It also reflects my own experience shortly after getting an ISP-requested reduction.  If the definition of 'very poor' varies with service option, it's all consitent with your theory.

Speculating further...  whatever 'flow chart' the off-shore ISP call centres use for dealing with customers requesting anything that involves BT, the first step could easily be to 'move to a more stable service option', before even bothering to talk to BT.  But that would be entirely the wrong thing to do if the request is for a target SNMR reduction....

[Mick]

It's interesting, if disappointing, to know that patent doesn't exactly descibe DLM. 

I think that my experience alone is not enough to deduce a pattern on this matter.  It may well be that the patent works in this way, but it is allowed to go only just below the FTR value (by how much?)  My sync speed was below FTR but only just (like 50-80 kbps with a Netgear router from what I recall).  As the long days arrived in Spring I was soon able to sync at higher than FTR speeds using a different router.

Speculating further...  whatever 'flow chart' the off-shore ISP call centres use for dealing with customers requesting anything that involves BT, the first step could easily be to 'move to a more stable service option', before even bothering to talk to BT.

Aha!  But the ISPs have to talk to BT to effect a change in the Service Option setting.  As I understand it this is all performed at the exchange equipment.  Have a look my attached ticket log to see if it makes sense - BT mention that a lower sync speed will not go below the FTR! 

[attachment deleted by admin]

[sevenlayermuddle]

I think that my experience alone is not enough to deduce a pattern on this matter.  It may well be that the patent works in this way, but it is allowed to go only just below the FTR value (by how much?)  My sync speed was below FTR but only just (like 50-80 kbps with a Netgear router from what I recall).  As the long days arrived in Spring I was soon able to sync at higher than FTR speeds using a different router.

The way the patent describes it, the decision to raise the margin is taken based on a prediction of your resultant speed, rather than an actual measurement of the result.  So, in the extreme, if your speed was FTR+801 then DLM could raise the margin.  But of course the 800 speed variation is just an estimate, and it's quite possible it could actually drop by 802 or more.  I don't see any mention in the patent of anything that would rectify such cases.

Aha!  But the ISPs have to talk to BT to effect a change in the Service Option setting.  As I understand it this is all performed at the exchange equipment.  Have a look my attached ticket log to see if it makes sense - BT mention that a lower sync speed will not go below the FTR! 

I can't say I've followed or understood everything that was said in your ticket log, but I do see the bit about 'line has been remotely stabilised' but that the resultant lower line rate '...wont breach FTR'.  That does seem consistent with the idea that the DLM algorithm is designed so as to not reduce speed to less than FTR.

I must say, your ticket makes fascinating reading even though I can only guess at what most it means.  And I'm impressed that your ISP has (a) had the willingness and determination to see it through, and (b) been willing to be so open with you about it all.