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Author Topic: DLM has put me on a 3 dB (ish) target SNRM  (Read 2463 times)

roseway

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DLM has put me on a 3 dB (ish) target SNRM
« on: June 03, 2017, 11:40:05 AM »

Over the last few days my target SNRM has fallen from 6 dB to 3 dB (ish) in three stages, and my downstream connection speed is up to 78630 kbps, from a previous low of ~69000. It happened in three stages (all dB values approximate):

29 May 17:00 6 dB --> 5 dB
31 May 16:00 5 dB --> 4 dB
3 June 11:10  4 dB --> 3 dB

If you look at my stats on MDWS, don't be misled by the fact that the SNRM was ~4 dB before 29 May. This was the aftermath of a power cut some weeks earlier, when my modem reconnected at a higher than normal speed, before several crosstalk disturbers came online and dropped the SNRM to ~ 4 dB.
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  Eric

WWWombat

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Re: DLM has put me on a 3 dB (ish) target SNRM
« Reply #1 on: June 03, 2017, 12:18:35 PM »

Looking back 90 days, to see the previous "normal" speed, it looks like you have jumped in steps of

69.4Mb -> 71.4Mb -> 74.8Mb -> 78.6Mb

Not quite as much as my "rule of thumb" would suggest, but the reductions seems to be mainly in the first leg, and circumstances could easily have changed in those ~90 days.

Your line looks susceptible to accidentally "running low" (low SNRM, I mean) - a natural side-effect of crosstalk taking effect sometime after the initial sync. When targeting 6dB, there's some buffer to cope. When targeting 3dB, there isn't a lot of headroom, so lots of errors could happen "innocently". I wonder how DLM will cope with such scenarios: will it figure that one simple resync will solve the problem, or will it drop a line into an unnecessary banding or interleaving configuration?
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roseway

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Re: DLM has put me on a 3 dB (ish) target SNRM
« Reply #2 on: June 03, 2017, 01:12:47 PM »

The "running low" situation happens here every time there's a power cut. We don't get them very often, but I tend to cherish them for the beneficial effect on my connection speed afterwards. As you say, it will be interesting to see what happens next time.
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  Eric

kitz

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Re: DLM has put me on a 3 dB (ish) target SNRM
« Reply #3 on: June 03, 2017, 10:00:55 PM »

Interesting.

Quote
I wonder how DLM will cope with such scenarios: will it figure that one simple resync will solve the problem, or will it drop a line into an unnecessary banding or interleaving configuration?

Although forcing a resync would be the ideal situation,  I doubt DLM is clever enough to detect that the low SNRM could be an effect of resyncing before crosstalkers. 

If DLM decides that some action is necessary, then the logical process would be to reverse the target SNRm steps.

Actually whilst typing this, I just recalled that I think that may have been what it did on Williams line.  TBH I didn't pay much attention as it was whilst I couldnt type much at the time,  nor was he uploading to MDWS for me to be able to independently verify what he said he was seeing. 
He reckoned he had been capped, but TBH I doubted that because the sync speeds werent any of the figures I would expect to see if the line had been capped.

----   

ETA -Just checked the emails - No the sync speeds don't look like capped, more like increases in target SNRm


« Last Edit: June 03, 2017, 10:03:13 PM by kitz »
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banger

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Re: DLM has put me on a 3 dB (ish) target SNRM
« Reply #4 on: June 03, 2017, 10:57:13 PM »

@Kitz I am currently bouncing between 3 and 4dB. Been going on for weeks now DLM keeps trying 3dB but ES shoot up into 1000s then DLM puts me back to 4dB with hardly any ES. Happened just yesterday DLM applied 3dB but after about half a day reverted to 4dB.
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Tim
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Chrysalis

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Re: DLM has put me on a 3 dB (ish) target SNRM
« Reply #5 on: June 03, 2017, 11:05:33 PM »

thats a horrible situation looping between 3 and 4db so the green range is too big
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banger

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Re: DLM has put me on a 3 dB (ish) target SNRM
« Reply #6 on: June 03, 2017, 11:35:51 PM »

Apparently so Chrys. as soon as my sync gets to 74 I get loads of ES but when it drops down to 70 I hardly get any ES so must be on the cusp of some noise somewhere.
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Tim
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roseway

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Re: DLM has put me on a 3 dB (ish) target SNRM
« Reply #7 on: June 04, 2017, 07:28:55 AM »

That's a bad bit of design in DLM then. It would seem that your best option would be to cap your connection at 70 Mbps, if your modem supports it.
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  Eric

ejs

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Re: DLM has put me on a 3 dB (ish) target SNRM
« Reply #8 on: June 04, 2017, 07:35:16 AM »

thats a horrible situation looping between 3 and 4db

What actual problems is it causing? You could just ignore the stats and leave it alone.
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tubaman

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Re: DLM has put me on a 3 dB (ish) target SNRM
« Reply #9 on: June 04, 2017, 01:36:22 PM »

Over the last few days my target SNRM has fallen from 6 dB to 3 dB (ish) in three stages....

Mine has done exactly the same thing and it has gained me 5 Mbps (40 - 45) on the DS side . The US has stayed at 6dB and the speed is pretty much unchanged.
My error stats haven't changed much so I'm keeping my fingers crossed that it'll stay that way.
Clearly Openreach are rolling this out widely right now.
 :)
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WWWombat

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Re: DLM has put me on a 3 dB (ish) target SNRM
« Reply #10 on: June 04, 2017, 03:35:33 PM »

Interesting.

Although forcing a resync would be the ideal situation,  I doubt DLM is clever enough to detect that the low SNRM could be an effect of resyncing before crosstalkers. 

It is indeed interesting. It wouldn't take much for DLM to identify that a simple resync is enough - it just needs to check whether the recent actual SNRM is significantly below the defined target SNRM (be it 6, 5, 4 or 3dB). But ... I don't think DLM has historically collected the SNRM data to be able to make that decision from.

If DLM decides that some action is necessary, then the logical process would be to reverse the target SNRm steps.

Actually whilst typing this, I just recalled that I think that may have been what it did on Williams line.

As banger has subsequently replied, his line does indeed seem to reverse that final step - and does so quite nicely. I have been relieved to see that it seems to take this reversal step within its stride.

I imagine, therefore, that DLM has to now try to keep a green/amber/red status for each of the possible target SNRM values, so it can identify what to step back to, and for human-based (engineering centre of excellence?) intervention.

William's line was the first (visible) line that got the XdB treatment during the trial, and followed a pattern of 6-5-4-3-6. At the time, I wondered whether the step back to 6dB was a poor "undo" function, or if it was a deliberate pre-stage to see what the line was capable of at each target, rather than proper live functionality.
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WWWombat

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Re: DLM has put me on a 3 dB (ish) target SNRM
« Reply #11 on: June 04, 2017, 03:45:10 PM »

thats a horrible situation looping between 3 and 4db so the green range is too big

It might indicate a problem with hysteresis. In this case, that 1dB step is enough to get the statistics to jump from zero ES to over 1,000. What else could DLM do? Beyond learning over a longer period that 3dB seems to be never viable, and to try it less often?

@Banger's line looks good enough to hold the 70Mbps speed, but the SNRM graph is not perfect. Whatever target is set in place, the ongoing operation seems to show that SNRM can suddenly drop by up to 1dB (ie this isn't a "normal" diurnal variation).

If that imperfection ever gets ironed out, then who knows? Perhaps the line could work at 3dB ... and you'd expect DLM to try it out at some point.

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banger

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Re: DLM has put me on a 3 dB (ish) target SNRM
« Reply #12 on: June 04, 2017, 06:05:07 PM »

I am happy with 70, but will just let DLM do its stuff and was wondering if it will settle on 4dB or keep trying 3dB but it looks like 3dB is on the cusp of some noise. It doesn't seem to last long on 3dB as ES goes up to 1000s and virtually zero on 4dB. Just wondering if it will settle after so many tries?
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Tim
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Chrysalis

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Re: DLM has put me on a 3 dB (ish) target SNRM
« Reply #13 on: June 04, 2017, 08:15:50 PM »

What actual problems is it causing? You could just ignore the stats and leave it alone.

The problem is network outages for the resync.

Thats why amber is a very large range to stop DLM moving between 2 profiles.
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Chrysalis

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Re: DLM has put me on a 3 dB (ish) target SNRM
« Reply #14 on: June 04, 2017, 08:17:30 PM »

It might indicate a problem with hysteresis. In this case, that 1dB step is enough to get the statistics to jump from zero ES to over 1,000. What else could DLM do? Beyond learning over a longer period that 3dB seems to be never viable, and to try it less often?

@Banger's line looks good enough to hold the 70Mbps speed, but the SNRM graph is not perfect. Whatever target is set in place, the ongoing operation seems to show that SNRM can suddenly drop by up to 1dB (ie this isn't a "normal" diurnal variation).

If that imperfection ever gets ironed out, then who knows? Perhaps the line could work at 3dB ... and you'd expect DLM to try it out at some point.



It could learn so e.g. 3 failed attempts blacklists the line from 3db until a DLM reset. Stability is king over performance.

Notice how I didnt throw a fit when my line got banded at 74mbit, as I know it failed at 80mbit, but 74mbit is stable.  I am happy to be banded as it ensures my line doesnt sync at an unstable rate.
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