Tracking down a external fault

We are currently connected with O2 adsl2+ but similar symptoms have also occured with Plusnet adsl in the past.

We're about 4km from the exchange so appreciate we're not going to get an amazing connection but when it works the connection will reliably run at about 4..5Mbps which it has done for a number of years.

However January last year we started getting disconnections every so often and the line speed fell away dramatically to about 1Mbps and it stayed like that for a few months. This was all with Plusnet.

I then moved to O2 and and the speeds were similarly poor but after a little while they came back and I was getting about 5.5mbps.

This lasted for a few months until about October last year when speeds dropped again to about 1mbps and running anything less than a 9db line profile would result in constant disconnections.

O2 have been very good investigating things and BT have been out 4 or 5 times but the net result is they've struggled to find the root cause of the problem.

Now part way through the day without anything changing at home the connection was playing up and after restarting the router the connection is back up to about 5.5mbps and working beautifully on a 6db noise margin profile and the line will even run a bit quicker with about a 4db noise margin (I have a modified DG834GT to play with the line profile).

I'd like to think this speed will stay permanently but going on past performance I'm not that hopeful.

I have an old wired landline phone that when the speeds are slow I can plug in via the microfilter and hear noise when the modem is connected. This noise is not there now
so I'm guessing this interference probably has something to do with the problem and that it is caused externally.

Can anyone think of a way to track down the cause or is it just one of those things I'll have to live with?

The stats from the router are currently

ADSL Link Downstream Upstream
Connection Speed 5376 kbps 1165 kbps
Line Attenuation 47.5 db 23.6 db
Noise Margin 7.7 db 9.8 db

When the connection plays up the attenuation is the same but the noise margin starts similar to the above for a few seconds before jumping up to about 17db on the downstream.

Upstream is not affected.

Any thoughts gratefully received.

the noise margin jumps UP ? is that after a resync to a lower speed ?

Well I thought it was too good to last and the connection has just dropped and won't reconnect unless I reset the router to the standard 9db noise margin profile to get a stable connection and the speed has just dropped to 3mbps whereas earlier on that noise profile was good for a stable 5mbps connection.

And yes the SNR does increase when the speed drops and I'm aware this is odd and it's one of the things that confuses me.

The current stats are

ADSL Link Downstream Upstream
Connection Speed 2919 kbps 1148 kbps
Line Attenuation 47.0 db 23.6 db
Noise Margin 17.1 db 9.6 db

When it initially connects it briefly shows an expected db of say 8db and then it jumps up high to around 17db.

I'd guess that your connection is experiencing a long burst of high noise (maybe a faulty streetlight starting up or something similar). If the noise lasts long enough to cover the period where your router re-establishes the connection it will do so at a slower rate. When the noise stops you are left with such a large margin it will probably not lose the connection the next time the noise kicks off.

The immediate answer is to reboot your router and you'll go back to the default margin and decent sync. Now you are on LLU rather than a BTw based product you won't even suffer from a lowered IPprofile until the system catches up with your faster sync.

The problem is that often the noise stays for a long time, in the last case about 3 months and no amount of rebooting the router made any real difference.

I've just rebooted the router this morning and got

ADSL Link Downstream Upstream
Connection Speed 3882 kbps 1165 kbps
Line Attenuation 47.5 db 23.7 db
Noise Margin 14.3 db 9.6 db

You're also right about the lack of DLM, it's one of the reasons I moved to an LLU provider as often the drop outs where not for very long but long enough to cause the connection to get slower and slower over time.

Edited by StuB (Wed 02-Feb-11 09:56:43)

BT REIN engineers often use a portable AM radio, off-tune on Medium or Long Wave, to track down the source of interference. See if you do pick up any noise on an AM radio band; if so you can wander about the local area to find where the noise is strongest. (Radios are directional so you would need to turn it about.)

I've recently used this technique to help track down a faulty power brick on a neighbour's PC.

I presume you've read up about ring wire isolation/removal and locating the router close to the master socket. That level of noise pickup suggests that it would be very apparent using a MW radio as outlined above. It also suggests that you haven't successfully isolated the ring wire.

Is there any audible noise on the line (you can either use the quiet line test or simply dial a 0 for a quick check).

Have you tested it with all your electrical kit turned off apart from the router and maybe a pc but preferably a laptop running on battery?

I get the same results with only the router plugged into the test socket of the master socket with all other extensions disconnected and nothing else switched on in the house.

Also the ring wire is not connected.

On the quiet line test yesterday using an old wired phone I could hear a small amount of crackling but very little.

When the router speeds are very low I can hear the router making quiet screeching noises on the phone (but not if I use our dect cordless ones) that obviously you shouldn't hear. Turn the modem off and it stops and then back on and once it starts to try and sync the noises appear again.

The noises are the kind of thing I'd expect if the microfilter was playing up but I've tried this with three routers and six microfilters and the problem is the same so I'm pretty sure the problem is not related to an internal issue.

I'll try the idea above with the am radio to see if I can find an interference source.

I actually have vague recollections that I used to be able to hear the radio when using the phone when we first moved in about 7 years ago and BT came and fitted a filter where the cable enters the building before the master socket and it stopped the radio on the phone line.

Edited by StuB (Wed 02-Feb-11 22:10:08)

it sounds like your line is hr dis, you need another sfi visit.

say to the engineer that there is noise present on the line when your modem is syncing up/ in sync. ask him if he wouldnt mind to try his latop and voyager or next gen tester along with a phone at the dp and cab.

he should be able to isolate what section the noise is in by a process of elimination, if he is still using a laptop and voyager this can be a total pain in the ass but it is the only way to find it. until of course it goes totally dis.

if you are fed overhead and the cable has not yet been changed that should be the first thing that is tried.

Thanks for the response.

The next time the connection drops significantly I'll call o2 and see if I can get them to send an engineer again and pass on your comments.

Also while I can guess the meaning of 'hr dis' and 'sfi visit' due to the context they're used in what do they actually stand for?

We are also fed by an overhead cable and the house is old and I would imagine the cables going to the house are also old too.