BT Infinity to Plusnet Fibre - 12mb Speed Drop

[#1]

Sorry - been hit by the lurgy over the weekend. Will answer soon...

[#2]

Quote from: N3ViT

I am trying to get the jist of your post. Is it what, that DLM has kicked in because of migration and thus slapped my speed down or that this would have happened anyway? I can believe the former but I can't believe the latter that it was just ultimately bad timing after consistently good speeds/pings before hand. It's just too coincidental. Its like claiming a sore head would have happened anyway, regardless of me banging my head. I can appreciate it if the migration has triggered DLM by default and that has seen some unneeded reason to rectify my line.

There is nothing about the migration that would have triggered DLM. There are only two things that matter to DLM: the number of resyncs, and the number of errors on the line.
In general, the number of resyncs is visible to you, but the number of errors is pretty much invisible - unless you are streaming a lot of video, and happen to see/hear the glitches. For most data use, the TCP protocol that underpins any download will automatically cope with an errors on the line. The number of errors that are sufficient to trigger DLM will have a barely noticeable effect on a TCP-based download
Therefore having "consistently good speeds/pings beforehand" is pretty much the experience of *every* user prior to DLM intervention. Small drops in speed, yes, but probably well within the definition of "consistently good". Even when I knew I was incurring 4% packet loss (which is a long, long, way above the DLM intervention threshold), I too could describe my line as "consistently good speeds/pings". DLM can be very trigger-happy, and will certainly trigger at error levels below anything you can see for yourself.
Right now, you feel it is too coincidental. But I can pretty much guarantee that, with a cab standing for over 2 years, you have been gradually succumbing to an increasing error rate.

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it's been a while since I was last with a LLU provider and I don't know what shenannigans goes on with non-BT ISPs these days so I took them at their word.

Note that "non-BT ISP" is not the equivalent of "LLU provider". Plusnet are not an LLU provider.

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Quote from: WWWombat The final speed test from Plusnet matches up with the BTW test, which itself shows an IP profile of 61.4Mbps; that maps to a sync speed of perhaps 63.4Mbps. That would make for a 8-9Mbps drop; that is well within the kind of drop that is seen when DLM intervention occurs. Again, it can happen at any time, randomly. And this again feels like someone telling me I would have had a sore head regardless of banging my head.

I guess the problem is that, in analogy terms, you have indeed been banging your head, but without noticing. DLM is a bit like your subconscious fitting a crash helmet, before your head gets sore.

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Quote from: WWWombat It currently looks like you've experienced a gradual increase in noise - which is visible in reduced speeds when a new sync occurs - ending, finally, with a sudden intervention of DLM. And these syncs happen randomly? So why did this one happen to be the catalyst?

Separate the thinking into two separate things here.
1) The times when small drops in speed are noticed - those are not "random" syncs; those would be caused by you triggering it, or a power cut etc.
What is happening is that noise increases happen randomly, but your line (that starts with a 6dB SNR margin taking account of the initial noise) sees a reduction in that margin - to perhaps 5 or 4 dB. It survives this with no change in speed, no resyncs.
However, when you do a resync yourself, the new speed will take account of the small increase in noise, as it resets the margin back to 6dB.
But ... if there isn't a resync, then there wouldn't be a speed drop.
2) The times when a large drop in speed is noticed, with an increase in latency - those are random.
DLM intervention isn't, of course, random - there will have been a trigger. But it appears to be random to you.
Generally the resync will be triggered in the small hours - between midnight and 8am.
Why did this one happen to be the catalyst? "This one" in what sense?
DLM does count the number of resyncs, so that can indeed (sometimes) be the catalyst. However, for most people, most of the time, the resync is a part of the consequence, not the catalyst. The resync is just the way that DLM gets the modems to turn on the FEC/interleaving protection that it feels is needed, after it has set a new line profile.
The actual catalyst would be the error rate. Something totally invisible.

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Yes, from the info they gave me back, it does look like DLM has now taken affect: Quote 0.128M-67M Downstream, Interleaving High - 0.128M-20M Upstream, Error Protection Off

That downstream setting is quite strange: normally the lower figure of the speed range would be around half of the upper figure.

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But my download nor ping diminished before, so what happens? Does it built up over time and just let a massive burst out? Or is it a case that a lot of other people in our town joined up FTTC at the time I migrated?

What happens over time is that each new subscriber adds to the noise level that your line experiences. Each cancellation will take noise away. The consequences are hard to describe; I'm sure I explained it in one of the earlier posts, but here is the summary:
1) The amount of crosstalk noise depends on where their copper pair sits in the line bundle relative to your, and for how long (distance) they share proximity; this means that the timing of any additional noise is random, and the amount of the additional noise is random.
2) The first consequence of additional noise is that your modem detects the noise while measuring the SNR level; it measures a lower SNR level. Most of the time, your line will survive this change, so your speed/ping continues without change. If you could see the statistics at this time, however, you'd see that you were running at a slightly reduced margin.
3) After a while, these noise increases (and SNR reductions) accumulate. If they get so far along, your modem might start to fail to detect the DSLAM's signal, known as an LOS (loss of signal) which would trigger a resync. This new resync would show slightly lower speeds, and no change in ping.
4) As an alternative to (3), you perform a resync yourself. Perhaps it is a power cut, or someone pulls the wrong lead out somewhere. Again, the accumulated noise increases mean that the new sync speed will be slightly lower, with no change in ping.
5) The second consequence of additional noise is that such noise interferes with reception of data from the DSLAM - perhaps causing a bit error in the bitstream received in your modem every now and again.
6) Your modem detects these bitstream errors, and keeps a count of them; it reports these counts to the DSLAM every 15 minutes. The errors can occur even when you aren't performing a download; if the fault happens in a packet involved in a download, then TCP in your computer will adjust to cope. When streaming, you may notice glitches. For the most part, you will see no change to speeds or pings.
7) As time gos by, and noise accumulates, the error rate increases. Most of the time, you cope with the increased error rate without notice.
8.) Eventually (and without any apparent notice), the increased error rate goes over the threshold that triggers DLM. Perhaps there was another subscriber added - and this effect is likely to go on forever. Or perhaps a burst of noise was caused by something else - a drill, a motor, a fan, a fluorescent light - and is really a one-off. Either way, you probably still won't have seen any change to your speeds or pings - but DLM certainly noticed.
9) Overnight, DLM tallies up the errors - and sees that your line has gone over the threshold for a 24-hour period. It sets a new line profile for your line, sometime between midnight and 8am, which triggers a resync. During the resync, the modems use the new line profile to turn on FEC and interleaving, which markedly reduces speed and increases pings.
10) At this point, you finally notice.
If you look through that list, you don't get to see any visible impacts except
a) slightly slower speeds after a manual resync (though sometimes it could be an automatic resync, or just some overnight maintenance)
b) marked reduction in speeds & increase in latency when DLM intervenes.
There is no real warning shot. The point is that you almost never see any of the causes or the consequences until DLM has intervened. Especially with the locked-down Openreach modem; I've yet to figure out why Openreach chose that, and (personally) I hate not knowing.

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Obvious questions:

I didn't notice anything from your answers; we couldn't see any dramatic shift on day 3, that might have indicated a DLM reset followed by immediate re-intervention. Likewise, noticing the speed drop on day 10-ish probably shows that things "just happened" but without much conviction one way or the other.

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As above, it appears DLM is acting strong on my line: 0.128M-67M Downstream, Interleaving High - 0.128M-20M Upstream, Error Protection Off

Indeed. The "high" interleaving setting suggests that DLM has intervened more than once, and that the errors continued through an initial "low" setting. On a "high" setting, the ping times are more likely to be 18-20ms higher than without.
But it begins to suggest to me that there is more noise here than just that explained by additional crosstalk. It is something of a cliche, but christmas lights have been known to have an impact on ADSL - I'm sure that FTTC connections aren't immune to them.

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There's been no modem change, I didn't take their router either. I've had an ASUS RT-n66u for the past 16 months too. As I said, all has been dandy on the line up until migration.

That's another variable removed.

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I did setup a ping graph around the time I opened the support ticket (it has calmed down now, but there was a lot of spiking max ping beforehand): http://www.thinkbroadband.com/ping/share/ec1aee0cb831806d71a892fa9340c0c1.html

I can't see a graph on that page. Do you have another example?

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Some of the internal stats, let me know if others are more useful:

I don't think they tell me much; others may have a better idea there.

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My main question is, will DLM tone itself down over a reasonable time? Or is it set once its went Rambo on my line? I think I can make do with the speed decrease until my contract is up if nothing can be done to fix it.

DLM is certainly capable of reducing intervention, and removing it too - but it isn't very fast. The fact that you've gone to a high level of interleaving is perhaps a sign that things aren't "simple" though.
The problem here will be knowing whether your line is working in a way that gives DLM a chance to want to remove itself - so it is almost certainly going to help if you can get access to better statistics from the modem, and then track them properly (did I give a pointer to the Kitz forum for this?). Doing this helps give you a chance to identify if the issue is at certain times of day, or if there is some other correlating feature.