Sudden reduction in SNR

Hi

I've had a good connection on my ADSL2+ line with Be for over a year, syncing at around 14000 down with a 6.5db SNR and very stable.

This weekend something has happened, to me it seems like some sort of REIN or perhaps it is simply crosstalk with a newly enabled ADSL line. I noticed the SNR margin had dropped to 3db, and upstream had also lost 1 db. The line remained up all night at these reduced margins and the error rate didn't increase.

During yesterday while watching the SNR values in DMT I saw the line recover 3 times, where it immediately went back up to 6.5db, where it stayed for a minute or so before jumping back down.

What is interesting is that the SNR across virtually all tones has dropped, see the chart below where I have superimposed the SNR chart of how the tones normally plot and how it is now.

My question is, does this look like cross talk (which I probably can't do anything about) or is it some electrical noise causing the problem?

To troubleshoot and rule out issues at my end, I had exactly the same results and SNR plot using a completely different router and nothing was in common as I used a different RJ11 cable, different filter and power brick at the test socket which is in a different room in the house. A quiet line test is the same as it is always, quiet.

I don't think the problem is carried in the mains as the same result is seen when running from a UPS isolated from the mains.

http://img520.imageshack.us/img520/1973/snrprofile.png

Regards

Phil

I see exactly the same thing (although on a much slower sub-5Mbps line).
For years the noise levels have been relatively stable (just the usual 2dB day to night fluctuations).
Then suddenly last Christmas everything went nuts and I see the same odd gaps and an overall drop across all tones.
It will do this in phases; sometimes it will be stable at a ludicrously high speed (for me, anyway); other times it will suffer badly across all tones (which on my long line, generally results in a re-sync and a speed drop of up to 1Mbps).
Then it will be fine for days/weeks before doing it all again for a couple of hours (before returning to normal).
I've been assuming it's a new, but rarely used, appliance the neighbours got last Christmas (they're away at the moment so my line has been stable for nearly four days, at a much too high 4689kbps, with almost zero variation in SNR/margin).

The only thing I can suggest is using DMT (or maybe Routerstats if you want to track over a longer period) to keep an eye on your margin and error counts (because this can indicate noise issues better than SNR changes).
See if the problem continues and see if there's a pattern (e.g. everytime Eastenders is on, etc.).

My money is on either a new plasma telly (or something else which generates a whole load of noise), someone has just had ADSL connected (or ADSL1 upgraded to ADSL2+) on an adjoining cable (or property).
Then again, it could just be thunderstorms.

Ade

ADSL2+ with BE
DL Sync Blimey....4707kbps now!
UL Sync 1088kbps
DG834GT with DGTeam firmware

Hi

Thanks for the reply.

After some investigation I'm inclined to think it is interference coming from our neighbours. We are partially connected one side, and using a radio tuned to pretty much any empty frequency on the LW or MW scale when placed near the party wall gives a very audible buzzing noise, it is at it's highest volume when placed against the wall downstairs which would be their kitchen the other side.

The noise is a similar buzzing I get when the radio is put next to anything in our own house that has a switch mode power supply, but it dies down very quickly just moving a metre or so away. The noise from next door is going through a solid brick wall and is picked up by the radio when pointed in the same direction but from the opposite side of the house, all be it quieter, it's definitely the same buzz and nothing I've tested of my own is audible more than a metre or so away line of sight!

Unfortunately we've had words with the same neighbour about audible noise, so I don't really want to approach them until I know for sure so will keep the radio on quietly when on the computer and when/if the buzzing stops I'll check the noise profile, if it goes back to normal while the noise has stopped at least I know it is that noise causing the problem and can safely assume the neighbours are the cause and can speak to them. I'm pretty sure they are on cable, which is a shame as if on ADSL they might be having problems as well and be glad to know. As they have a phone in their kitchen and our lines probably share a cable I can easily imagine the noise could also be picked up by their telephone line and passed to ours.

I've had a Loss of Service last night as well with a resync at a slower speed which is unusual for my line so it is definitely having an effect whatever it is.

Also the noise has changed slightly, as it has moved along a little bit and is affecting less tones at the start, perhaps the equipment is going more and more faulty and will shortly break completely, fingers crossed.

Regards

Phil

I have had the exact same problem, I actually had the problem ongoing for well over a year, perhaps even a year and a half or more but can no longer clearly remember.

I had posted dmt graphs on these forums when I used to go on about it a lot, and those graphs clearly showed a similiar thing where my snr margin would 'suddenly' drop and the affect would be across the entire frequency range. Noone could suggest what would cause interference across the entire range, I concluded it was either crosstalk or a fault at the dslam with the signal power outputs.

This problem went on for a long time as I said, BT tried a pair swap first, when they did the pair swap I still had the problem but it the extenct was reduced. On the old pair it was extreme it was able to knock out my line on interleaving at a 15db noise margin. (snrm reduced 18-20db on the worst tones). On the new pair it became manageable as the affect was low enough my router could stay synched through it however I would get packet loss until it recovered. Eventually when I was on aaisp they forced BT into moving me to a new dslam (not new port actual new dslam), so I got moved from an old alcatel dlsam onto a newer TI dslam and this made a very significant change, again the affect was still there but it was now affecting only a smaller range and lower affect, the improvements the TI dslam had such as proper bitswapping enabled my line to stay up and have no packet loss when it occured.

Now the patterns for me were every weekday between 7.30 and 8am it would start and then it would go on and off all day up to around 6pm. Sometimes it would stay on all day. Weekends and holidays like christmas it stopped.

When I moved to LLU ukonline service the problem just vanished, it was gone completely. My line was normal for a while until last week as I think its now came back but not too bad, my line (SRA enabled) loses about half a meg synch about 7.50am for about 10 minutes every morning now and then recovers. It then loses the same amount of synch in short bursts during the morning, its usually ok in afternoons and again isnt occuring at weekends.

Due to the fact it got fixed when I moved to ukonline I really did suspect something in the exchange side, since when I moved off BT equipment it stopped, now it is came back I am less convinced although still strongly suspect that, this new problem could of course be something different and unrelated.

I think crosstalk is a strong possibility in your case and could have been in my case also. I guess what will be interesting is if that buzzing noise stops when it recovers.

Hi

Thanks for your reply.

It's annoying for me as the buzzing through the radio never stops and it is definately from next door, or it could be next door to them I suppose! Neither does my line show any periods of recovery now, but that doesn't really prove much until I can see the line recover along with the buzzing stopping or it stops and the line remains the same.

It has also affected upstream where typically I'd get 1390 to 1400, it has now dropped down to 1345 or less, and I'm not sure if that is typical with crosstalk or not to have an affect on both upstream and downstream so suddenly. Would crosstalk knock of 3 to 4 db of SNR margin, I don't know enough about it really? It's odd as the downstream SNR reduction kicks in at tone 55 upto around tone 260, and this drifts around a little bit as well.

I had wondered if at the exchange they've had airconditioning issues during the hot weather and perhaps have brought in some portable units that are electrically noisy so only a temporary problem, maybe that is just wishful thinking.

The reduction in 3-4db happened when my ADSL sync had been up for 20+ days solid and hardly fluctuating from around 6db noise margin, which is how it has been for over a year. It remained up at that lower margin for around another 12 hours before resyncing at a lower speed. It was during that first period by chance I was monitoring and saw the margin recover 3 times for a few minutes before dropping back but at that early stage I hadn't done anything with the radio to check for noise.

The loss of services now are annoying although yet to happen at a time I'm using computer, but I fear things will be worse when heading into winter and there is lots of other random noise about adding to whatever is constantly affecting the line now. It's going to be case of upping the SNR margin and getting a sync speed of around 10 Meg or less I expect which is a shame. The line is already interleaved so I don't have that as an option.

I can't see the point in complaining or raising a fault with the ISP as acording to the BT database for ADSL2+ we should only get 7 Megs, and only 5 Megs on ADSL1, so upping the margin lots we would still be greater than 7 Megs and it would be "that is all you can expect from your line" type response, even though for well over a year we've had >14 Meg and completely stable.

I'll update here if I get any joy resolving this.

What I would strongly recommend for us techies is to get a DMT chart screen shot of your line when it is working normally, and get an MW radio tuned to 612KHz and walk around your house to checking the noise profile, that way if things go wrong with stability and line speed you stand a much better chance of narrowing down the issue.

Edit: Well just as I was about to post this the radio in the other room against the party wall fell totally silent! The bad news the line SNR profile has remained exactly the same, with the same reduction across the tones and the plot from DMT didn't change in response to the noise stopping.

At least I know it isn't the neighbours, although not entirely happy they are bombarding our house with electrical noise, however it is having no effect on our broadband.

Regards

Phil

yeah I think you a victim of crosstalk, which only a pair swap has a chance of fixing, or the other line turning their adsl off.

Can crosstalk have this kind of impact, most defenitly yes. It is why when possible and if enough pairs engineers try to have lines that dont have adsl inactive sandwiched in between lines that do.

To get an idea of the impact of crosstalk, especially in the very early days of adsl2+, people using dmt would see a very significant jump in signal strength at the point where adsl1 signal ends, clearly their adsl1 part of the signal was impacted with crosstalk.

Hi

Yes I'm inclined to agree it is crosstalk as it affects most of the ADSL1 frequency space, lets hope they don't upgrade to ADSL2+.

The other persons wire must be running along side mine all the way from the exchange. I doubt I can get a pair swap because of it and if I did it could be worse.

This ADSL is all bit fragile isn't it

Regards

Phil

I have something similar to you.

In my case my attenuation has gone from my normal 27db to 30db causing my SNR to go down.
I used to connect at 16Mb , stable no problems, came back from holidays my connection as only connecting at 14.1Mb.
I tried checking the master socket the same numbers, the master socket was installed in 2001 by BT, so never had to change it.

I did have some issues previously where the voice line was affected and BT corrected it, BE has asked me to do tests and they all show the same thing, is there any ways i can request a new line to my flat.

I was thinking of asking for a new line and then drop the original line, but someone has mentioned this may actual be detrimental to what i want to achieve, as it might degrade the line for both.

Sorry to get in your thread, didn't want to hijack it, found your issue very similar!

[IMG]http://www.speedtest.net/result/317666832.png[/IMG]

Hi

Do you have any tone plots from DMT you can compare before and after?

In my case, just last week, my original speeds have returned and the crosstalk has gone. Of course the person causing the cross-talk may be on holiday and has turned off their router, so it might drop again when they get back.

I don't think adding a new line then dropping the old one is a guarantee of anything, and if anything speed decreases for crosstalk are only going to become worse as more ADSL2+ lines are enabled now BT have updated most exchanges. This means crosstalk not just on the lower frequencies but on the higher ones as well.

I'd give it a couple of weeks, you never know it may be temporary.

Regards

Phil

.........as more ADSL2+ lines are enabled now BT have updated most exchanges

I presume you really mean most URBAN exchanges !!!

There are a large number of people on rural/semi rural exchanges who did not get broadband at all until fairly recently - August 2004 in my case - and there are STILL some who cannot get it at all.

My exchange's:

21CN PSTN switchover target: Q2 2011

21CN WBC status: Not available

So is this real world or a BT/Ofcom fantasy???
....BT have updated most exchanges ???

A pessimist is simply a realistic optimist

DrayTek Vigor 2710n

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