Intermittent Fault causes ATM CRC errors

Hello all,

This is my first post here at thinkbroadband so please be gentle with me

OK so this is a bit of a stumper and because it's intermittent ZEN are reluctant to pass it on to BT just yet!

I'm using a Draytek Vigor 2820Vn on an ADSL 2+ line (G.992.5)
Sync Speed 19.7 Mb DOWN 12.3Mb UP
SNR Margin 3
Loop attenuation 22

Speed Test gives 17MB down and 0.9 Mb Up

From time to time I get no, or very slow throughput trying to load web pages. During this event the router is ramping up hundreds of ATM cell CRC errors every few seconds, even if I am doing nothing on the Internet. A BT quiet line test shows the line has no noise.

During the event the SNR Margin drops to 1
After the event the SNR Margin is back at 3 and no CRC errors and a speed test gives 17Mb up and 0.9 Down again.

The router does not need to be rebooted, but doing so makes no difference.

The effect is almost like a denial of service attack flooding the router with ATM CRC errors.

I'm guessing that it is some kind of noise or crosstalk but have no idea how to get this escalated. Since the problem is intermittent the chances of an Openreach engineer being on site during an event is slim.

Has anyone heard of, or experienced this kind of problem. Any advice or diagnostic help would be appreciated.

Thanks

John.

A new microwave cooker perhaps?

RobertoS

Wish it was that simple

I believe OpenReach have specialists for cases of intermittent noise (which is probably what this is). Trouble is that these days you could well be charged a lot of money if they don't find anything which is why Zen are reluctant to advise this. Unless you can track down the noise (new business using industrial equipment perhaps) then you may find it very difficult to progress this any further.

Ultimate solution would be full fibre - but unless you are lucky enough to live in an FTTPoD area with a good chunk of spare change then this is not going to help.

Thanks ian72

Can't even get FTTC They Fibred the whole estate EXEPT our cabinet (Which is only 100 yards away from another cabinet which has been fibred)

I hate problems like this because they are so difficult to resolve.

I've worked in IT for years and have often had to try and work on intermittent issues - they are as painful for the fixer as for the fixee. People always assumed I thought they were lying if they couldn't reproduce the problem - trouble is without seeing it you often can't get the info you need to get anywhere near fixing it.

I know how you feel and wish I could offer some nugget that would give light at the end of the tunnel, but I'm afraid I can't do anything so helpful as that.

Almost certainly an "external event" generating noise. IT may not be in your house, but could be next door or anywhere along the line.

Is there a pattern to the bursts? Are they regular, every XXX minutes? Or do they tend to occur at specific times? How long are they? A few seconds or a couple of minutes?

Get RouterStatsLite and get it running over several days monitoring Sync and SNR to see if there are patterns to the problem and if it is literally switch on and off, or ramps up slowly.

Hello MHC

and thanks for your help

There is no strict pattern to the events, however since this started, every Saturday and Sunday morning between 10:00am and 1:00pm I will experience an event which lasts around 1 to 1.5 hours after this the line stabilises with a couple of CRC errors every few minutes. maybe later on in the day there will be shorter bursts of errors. Late on Sunday evening all the CRC errors stop, and the line is rock solid.

During the week I might randomly get shorter bursts (today for instance nothing at all until 4:30pm'ish then around 5000 CRC errors in 20 minutes, then the line has stabilised again.

It's really weird and very frustrating.

I'll download RouterStats - Lite and see if I can gather more information, Thanks for suggesting this.

John

RSL will run continually - set it for 15 second samples and to auto save the graphs. At least then you have some records. Locating the problem is another issue entirely!

Roberto's suggestion about a microwave, may sound silly but it could well be one of those close to the line, or an arc welding set, or ...

Then the age-old questions re Sky boxes, DECT phones, their respective power supplies, newly failed filters, and so on. I just went for the first thing I thought of that might have been recently replaced.

Edited by RobertoS (Mon 04-Aug-14 18:55:56)

If it were me, the first thing I;d be doing is asking Zen to get then target SNR set back to 6db, making the service a tad more stable may well mean these 'blips' aren't an issue.

Thanks RobertoS,

All the usual suspects were worked through and eliminated before posting here. Replaced filters, moved router to test socket etc. (No Sky box here!).

I will swap the Draytek router for an old Netgear when the problem reappears just to eliminate the Draytek - but I'm not convinced the Draytek router is the cause. It does look like noise from somewhere.

Also the telegraph pole is right outside my front door so the cable run is only about 8 metres.

The cable run goes right back to the exchange. At any point in that run it could pick up noise.

It is more likely the underground run from pole to cabinet to exchange could be picking up noise.

I had a case about 4 to 5 years back - friend of a friend. His ADSL would basically go down some evenings, but only when he was out. His neighbours suffered too although there were only a few with ADSL from a sparsely populated cabinet. It turned out to be him!! He ran a mobile burger van from a lay-by around 1 mile from his house and the cabinet was along side. He would put his generator on the verge - sometimes near the cabinet and sometimes not - when close up the interference was enough to affect the ADSL lines. At a guess the generator was sitting above te cable feeding him and his few neighbours down the lane.

ROFL
Nice one .

Thanks ian72

And from the pole it goes underground and along to the cabinet where it emerges briefly before its onward subterranean journey to the exchange (just over a mile away)

The most likely places where noise would get injected are

1) Cable from house to pole (acting like an aerial)
2) Junction box at top of pole
3) D side in the cabinet
4) Exchange JB

But wherever it's happening it's a pain

In reply to a post by Zarjaz:

If it were me, the first thing I;d be doing is asking Zen to get then target SNR set back to 6db, making the service a tad more stable may well mean these 'blips' aren't an issue.

Hello Zarjaz,

I spoke with Zen support today and they have set the noise margin back to 6db at the cost of about 1Mb sync speed (now 18.1 down and 12.1 up).

So far no ATM CRC errors - time will tell

Well I hope it continues. Speed is good, but worth nothing without stability.

Keep us posted.

OK so Zen have altered the noise margin and moved it back to 6db.

The only effect this has had is to reduce the routers sync speed from 19.7Mb to 18.0Mb.

The problem still exists as described.

Can anyone say why the router would receive these ATM cell CRC errors even when there is ZERO IP traffic to and from the internet?

Don't forget we are talking about hundreds of errors every few seconds (150,000 per hour)

There is a constant traffic of packets at the sync/connection speed between you and the exchange even when no data is passing. Think of empty envelopes.

When you use the connection, data goes into those packets.

Basically the CRC error rate is unlikely to change whether the connection is in use or not.

Hello again everyone,

thought I would come back and update you all on the outcome of this issue.

A very competent OR engineer came out and spent around 3 hours on site. As predicted there wasn't a problem when he arrived , but fortunately it did show up during his visit.

ZEN Internet support were predicting that the problem was being caused by REIN but this turned out to be not the case.

The OR engineer conducted extensive tests and concluded that the issue was being caused by cross talk.

The cabinet I am connected to fortunately has two cables serving it "E" side, and after finding me a pair in the other cable all is now well.

Conclusion.

Persistence pays off!


NOTE to moderators - is there any way to mark a thread as solved?