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Author Topic: VDSL Margin up and down  (Read 4087 times)

PhilipD

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VDSL Margin up and down
« on: May 01, 2014, 09:14:54 PM »

Hi

I've had FTTC for a couple of months now with an unlocked HG612 modem and I keep an eye on the line.  I was the 2nd person on the cab and started off with a margin of 10.8, this settled to around 9.5 over the weeks presumably as others have joined, but it didn't fluctuate in the short term and would chart as almost a flat line.

I get max 80/20 sync speeds.

So SNR margin didn't vary much except by a 0.1 or 0.2 when monitored.  My line is all underground from the cab and ADSL was pretty much the same, a stable SNR margin.

Recently I noticed that SNR was going back up to 10.8 then a few seconds later dropping back down again so I set dslstats running and get the constant repeatable fluctuation as attached.  Errors follow the same pattern, with an increase of errors on the lows.  Line is not interleaved at the moment.

While this isn't causing me a problem yet as I still have margin, I'm thinking at some point as more crosstalk takes hold this flapping of SNR could start having a negative effect, it might also cause the DLM to switch to interleaved as errors creep up because bit-swapping can't cope with the swinging SNR and lowering margin due to crosstalk.

No noise on the telephone line.

Just wondered if anyone recognised the pattern?  My first thought was perhaps it was another VDSL connection that was having sync problems, so I was seeing cross-talk as this other line attempted to sync, then it failed, cross-talk goes and SNR pops back up, it then tries to re-sync, cross-talk knocks it down again, however this has been going on for 3 days now constantly all the time.

Probably nothing I can do about it, just a frustration of the technology.  Oh to have a fibre connection and not have to worry about degrading line conditions or noise.

Regards

Phil

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NewtronStar

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Re: VDSL Margin up and down
« Reply #1 on: May 01, 2014, 09:28:53 PM »

I know it sounds like an odd request PhilipD I have incounter this myself last year and it was an excellent Member burakkucat that suggested that I turn off the FTTC Modem after finishing evening Internet browsing and turn it back on again in the morning (6-7 hours) it worked for me.
« Last Edit: May 01, 2014, 09:34:54 PM by NewtronStar »
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burakkucat

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Re: VDSL Margin up and down
« Reply #2 on: May 01, 2014, 11:56:57 PM »

Looking at the SNRM graph, we see that it "yo-yo"s every two minutes. It is if the MSAN/DSLAM's line-card is trying to stabilise the circuit's parameters but has got itself into an oscillatory state (~8.3 mHz (millihertz)  ::)  ).

As N*Star has already explained, the best way to "break that habit" would be to power down the router, power down the modem and disconnect the modem from the line for the minimum of an overnight period. By doing that, it gives the line-card's circuitry time to fully "relax".

When you next want to use the broadband service, just reverse the sequence of the above -- connect the modem to the line, power up the modem and wait for it to synchronise with the MSAN/DSLAM, then power up the router.

Done just the once, it should not induce any adverse effect by the DLM.
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PhilipD

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Re: VDSL Margin up and down
« Reply #3 on: May 02, 2014, 06:47:02 AM »

Hi

Thanks for the information.  I think it is oscillating much more often than two minutes, but the frequency of the chart update is capturing a slightly different pattern as it doesn't sample often enough for this particular issue.  If I open and watch the page refresh on the HG612 stats page which refreshes around once a second, I see this morning SNR 11.1 (highest I've seen it) then the next refresh it drops to around 9, then the next fresh 10.8 or 11.1 and then 9 or 8.8 then back up and so it repeats.  I'd say it is around a 1 second oscillation as it changes with each screen refresh.

I tried the turn-off over night which hasn't changed anything this morning, I didn't disconnect from the telephone line however.

It's strange it goes back up to the 10.8 (or 11+) I saw when I was number two on the cabinet (the engineer told me I was the second on), which had dropped to around 9 or 9.5 after 4 or 5 weeks in a few small increments which just appears to be the usual crosstalk.  There was no oscillation then and it practically flat lined.

It's almost like everyone else on the cab is dropping out for a second leaving me as the only one connected so I get no crosstalk and see the maximum margin my line can have, then they all try and sync again and I get a burst of cross talk.

Like I say at the moment this isn't causing me too much harm, I've enough margin over and above the maximum to ride it out without the modem thinking it needs to drop and re-sync lower, and the errors that are introduced in the downward spike hasn't been enough, yet anyway, to stick me on interleaved.

But as we all know with VDSL (and ADSL before it) these oddities and issues often end up causing a problem sooner or later.

Could it be a faulty modem?  Problem is the only other one I have is an ECI which isn't unlocked.

Regards

Phil
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PhilipD

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Re: VDSL Margin up and down
« Reply #4 on: May 02, 2014, 07:12:03 AM »

Hi

Interesting, happened to glance at the DSLstats chart and it's back to flat lining at around 9.8, which looking at the time it changed coincided with the neighbour leaving for work.  Two events connected? 

I'll keep stats running all day and will see if I can spot a pattern.

Regards

Phil
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NewtronStar

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Re: VDSL Margin up and down
« Reply #5 on: May 02, 2014, 08:46:37 PM »


I'll keep stats running all day and will see if I can spot a pattern.
Phil

I myself don't believe in coincidence when it comes to Broadband issue's ,and one persons Broadband being turned off/on should not cause the SNRM oscillation you have seen recently.

So yes keep the stats running and keep us posted if you see this SNRM oscillating again .
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kitz

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Re: VDSL Margin up and down
« Reply #6 on: May 03, 2014, 11:11:40 PM »

Any further update on a pattern?
It does look like the line card is flapping though.


I could be wrong, but theres not that many people who will turn a modem off these days when they go to work. PC or Laptop yes, but Im not so sure about routers and modems these days.

However it is possible that it could be linked to some sort of EMI from your neighbours property.  I have seen a graph similar to yours in the past, but it was a looooong time ago (8+years?) and I cant recall what it was now - sorry.   Can any of the other 'old regs' recall...  was it the neighbours TV that had to be replaced? We had a bout of REIN type problems all at the same time, all from neighbours properties... including a HiFi that wiped out half the street :(

[Moderator edited to fix a typo.]
« Last Edit: November 29, 2016, 10:14:17 PM by burakkucat »
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PhilipD

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Re: VDSL Margin up and down
« Reply #7 on: May 10, 2014, 09:43:01 PM »

Hi

It stopped oscillating and hasn't started again since as far as I know, one of those odd things.

Regards

Phil
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NewtronStar

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Re: VDSL Margin up and down
« Reply #8 on: May 10, 2014, 10:59:28 PM »

Come Back to reality Kitz  ;D

As with most software and hardware (DLM and MSAN) if it gets into state of no return (LOOP EFFECT) its a we bit like your PC hardware & Windows OS you will need to turn off your PC for the software to re-establish info with the Hardware.
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