VDSL Performance & Tuning

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#693216 28-Sep-2012 23:28

Cool, yeah that looks better - powered off the fritzbox for a while and now it's looking decent; I'll see how it goes.

Now I'm wondering if I should have powered off when putting the target SNRM down to 13... not gonna touch it though it barely gained me more than 1mbps so I'll just stick with 14.

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stevehodge

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#693231 29-Sep-2012 01:00

SamF:
I'm only going on the Chorus docs which state that DLM makes its determination based on mean time between errors. This has a direct mathematical correlation to CRCPM.

Can you post exactly what it says?

SamF:
As for weekly changes, DLM is able to make changes weekly, but it doesn't necessarily mean it will make a change every week. It is also able to make changes much more quickly if it deems the line to be too unstable.

So that doc says that DLM can only make 1 change per week unless it wants to make more? I'm losing faith in that doc...

SamF:
I also think, based on a number of people's experiences, that a few weeks ago, DLM wasn't making changes for anyone which could explain your month on DLM-1 without any changes.

Unless it wasn't making any changes for basically the whole month I was on DLM-1, it still treated us very differently for what appear to be similar error rates. That inconsistency has me thinking that there is a lot we don't know about DLM. Also note that I did have a couple of non-DLM resyncs in that month and yet did change from DLM-1.

Anyway I thought I'd try to confirm if there was a period when DLM wasn't making changes based on what has been reported in this thread. On August 16th you posted that you thought DLM might not be working. On the 25th and 26th you said you thought that DLM hadn't been working for a couple of weeks, but was again. I don't think there is a lot of evidence that DLM wasn't making changes in mid August - people reported changes on 10th and 13th. There is a period of about 3 weeks in early September, but with so few people here, it's not very definitive. Here's the complete list:

Jul 23 - stevehodge went from DLM-2 to DLM-1 (I was in Oz when this happened, so I'm not sure if it was DLM initiated or not, but it wasn't at 4:10am so I suspect not)
Jul 29 - SKYMAX reported going from DLM-6 (possibly DLM-4) to DLM-2 (seemed to be user initiated resync)
Jul 30 - SamF reported going to from DLM-4 to DLM-2 (seemed to be user initiated resync)
Jul 31 - SKYMAX had a DLM resync
Aug 2 - SKYMAX had a resync to DLM1
Aug 10 - sidefx had a DLM resync from DLM-6 to DLM-3
Aug 13 - _Allan had a DLM resync from DLM-3 to DLM-6
Aug 21 - SamF had a resync
Aug 22 - jjnz1 had a DLM resync from DLM-1 to DLM-2
Aug 24 - SamF had a resync
Aug 25 - Napster reported being back on DLM-1
Aug 27 - Instinct reported being back on DLM-2 after 4 days on DLM-1
Aug 27 - stevehodge had a DLM resync from DLM-1 to DLM-2
Sep 1 - SamF and sidefx started experimenting with higher SNRM targets
Sep 18 - sidefx went from DLM-3 to DLM-6
Sep 20 - sidefx went from DLM-6 to DLM-2
Sep 22 - sidefx has a DLM resync but stayed on DLM-2
Sep 24 - sidefx went from DLM-2 to DLM-1
Sep 24 - helis went from DLM-6 to DLM-2 (2 days after first connection)
Sep 25 - SamF had a DLM resync but stayed on DLM-2
Sep 26 - helis had a DLM resync but stayed on DLM-2
Sep 27 - SamF had a DLM resync from DLM-2 to DLM-1
Sep 28 - SamF had a DLM resync from DLM-1 to DLM-2
Sep 28 - stevehodge had a DLM resync from DLM-2 to DLM-1

The pattern of changes seems consistently to be DLM-3 -> DLM-6 -> DLM-2 -> DLM-1. A lot of people have had DLM resyncs with no change while on DLM-2. Someone mentioned that they thought that the Fritz doesn't always report DLM-4 properly - I have a theory that perhaps some of those "still on DLM-2" resyncs are actually moving people from DLM-4 to DLM-2, i.e. perhaps the pattern is 3->6->4->2->1. That would make sense as it matches the list Napster provided.

DLM seems to have 2 modes. Sometimes it makes a lot of changes in a short period (1-4 days, usually 2 days between changes), as if it is searching for the right profile. Sometimes it seems to go almost exactly a month between changes: me on DLM 1 from Jul 23 to Aug 27, and on DLM-2 from Aug 27 to Sep 27; SamF on DLM-2 from Aug 24 to Sep 25; sidefx on DLM-3 from Aug 10 to Sep 18. There is no evidence of changes on a weekly schedule. My theory is that if you have been on a particular profile for more than 4 or 5 days then DLM will not change you until at least a month after your previous change.

I don't think there is much evidence that adjusting the SNRM target will trigger DLM changes. Both SamF and sidefx didn't see any DLM changes for at least a couple of weeks after starting to make changes. However when DLM did makes changes for them (about a month after it's previous change) it did seem to go back to rapid changes for them. We don't have enough data to tell if this was coincidence or not.

stevehodge

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#693232 29-Sep-2012 01:07

helis: I thought this too but I have downloaded 10gig in one hit at speeds of 2-3MB/s and had no CRC errors during the 40mins or what ever it was and now I've come back to see I've had 4 CRC errors in the last 15mins. I doubt I've had 50meg of data transfer over the link in the last 15mins.

Yeah, but CRC errors don't happen at a consistent rate. They are generally caused by outside interference. So your 10GB download may have been during a period when there was little interference, while now your 4 CRC errors in 15 minutes indicates higher interference. Perhaps if you downloaded 10GB in that 15 minutes you might have seen 10 errors instead of 4.

At the moment I'm seeing about 50 CRC errors in most hours (with a couple of spikes and one period from 9pm to midnight with only 25 CRC errors per hour). I'll try downloading something for the next hour and see what happens.

helis

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#693290 29-Sep-2012 08:40

I was changed to DLM1 this morning but results are not looking good. I'm not sure if I should leave it be or increase the SNR and reboot.

55ms ping to Sydney is nice.

sidefx

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#693291 29-Sep-2012 08:40

@stevehodge: The throughput when downloading 10 gig vs nothing is so many magnitudes greater that even if you've got periods of little interference you'd still sort of expect far far far greater number of CRC errors per minute while downloading large things.

You're right that we just don't have enough sample points to draw many conclusions on DLM though. The theory about fritzbox not reporting DLM4 properly and the order actually being DLM3->6->4->2->1 seems logical though!

It would be great if everyone on VDSL on the forums could post any and all DLM resyncs + stats! :)

@helis: Nice! yeah tough call on changing SNRM or leaving it. Changing it *seems* to have worked well for me, but of course YMMV.

What server are you testing to in Sydney? Some of them give me around 30ms since moving to DLM1.

"I was born not knowing and have had only a little time to change that here and there." | Octopus Energy | Sharesies
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helis

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#693300 29-Sep-2012 08:54

Internode. I get 15ms ping to Christchurch. I'm in Dunedin so that's why mine might be a little higher.

I changed SNR to 13. resynced and CRCPM is dropping nicely. It needs to settle down at this point so will update in a couple of hours.

Also I'm only synced at 40.4Mbps now. Not too much of a drop.

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SamF

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#693328 29-Sep-2012 10:02

@helis. From evidence I've seen, unless you get your CRCPM down to at least under 0.4 and possibly as low as 0.2 (unconfirmed) in order to stay on DLM-1.

With respect to the order of DLM profile changes, there is no 'preset' order, but according to the chorus docs, DLM can only change one level at a time depending on the line conditions at that point.

Also, CRC errors don't necessarily have to scale with data volumes. If errors are introduced by external sources (interference), you may only see errors when the interference is active.

helis

96 posts

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#693338 29-Sep-2012 10:46

haha back on DLM2. I have put my SNR back to 12. Im happy on DLM2. 16ms off pings isn't enough for me to stress over it. My ping times as it stands are already less than Slingshot ADSL so that'll do me.

fellaintga

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#693372 29-Sep-2012 12:21

I am happy on DLM-1 which took about 6 weeks to get onto. Do you want to know the secret to getting on this profile and staying there. Leave your connection alone. I am sure if you have a half decent connection you will get there. I have also seen a connection switch to DLM-1 only a week after it was installed so just leave it alone and let it sort it self out.

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blair003

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#693375 29-Sep-2012 12:35

fellaintga: I am happy on DLM-1 which took about 6 weeks to get onto. Do you want to know the secret to getting on this profile and staying there. Leave your connection alone. I am sure if you have a half decent connection you will get there. I have also seen a connection switch to DLM-1 only a week after it was installed so just leave it alone and let it sort it self out.

+1

(although I guess it's pretty easy to +1 this when I'm also only DLM-1 after just leaving it do its thing)

SamF

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#693377 29-Sep-2012 12:43

You can potentially get to DLM-1 by leaving your connection alone if your line is good enough (as mentioned), however, if your line isn't brilliant you will have to tweak SNRM etc to get to DLM-1 and stay there.

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helis

96 posts

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#693379 29-Sep-2012 12:48

fellaintga: I am happy on DLM-1 which took about 6 weeks to get onto. Do you want to know the secret to getting on this profile and staying there. Leave your connection alone. I am sure if you have a half decent connection you will get there. I have also seen a connection switch to DLM-1 only a week after it was installed so just leave it alone and let it sort it self out.

Took me 7.5 days from when my VDSL went active to get DLM-1 but too many errors to maintain the profile. But as I said the only benefit is 15ms off pings. Not the end of the world. On DLM-2 I have a nice sync rate and low crc errors which is fine by me.

blair003

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#693380 29-Sep-2012 12:48

@SamF But then you are risking connection quality/stability for the sake of slightly lower latency, so its a bit of a toss up in my view.

sidefx

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#693383 29-Sep-2012 13:19

blair003: @SamF But then you are risking connection quality/stability for the sake of slightly lower latency, so its a bit of a toss up in my view.

Nah you're just losing a bit of throughput for the sake of better latency I believe. Connection quality\stability should be better with higher target SNRM. I'd say on my line it was worth it - looking at my error rate when I dropped the SNRM back down there's no way it would leave me on DLM1 without the changed SNRM.

"I was born not knowing and have had only a little time to change that here and there." | Octopus Energy | Sharesies
- Richard Feynman

stevehodge

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#693393 29-Sep-2012 15:21

sidefx: @stevehodge: The throughput when downloading 10 gig vs nothing is so many magnitudes greater that even if you've got periods of little interference you'd still sort of expect far far far greater number of CRC errors per minute while downloading large things.

I did some testing last night. I transferred about 17GB in 3 hours, which is only about 20% of the available bandwidth but should have been enough to see some effect. There was no clear increase in CRC errors.

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