Some phone calls trigger noise spike and anomalous low sync

Hi everyone,

I've been wrestling with this one for years, on and off. It's a problem with my parents' ADSL and since I'm staying with them at the moment I thought I'd take another swing at it. I'd register, but I'm currently afflicted, and it would take me a while!

I'll start with a description: Once in a while, when a phone call is being made/received, there is audible crackling on the line and of course the noise margin (normally 11-13 dB) nosedives until sync is lost. When the modem (a Belkin N1 - would not recommend to a friend) resyncs with the exchange, a really, really low speed is negotiated (sometimes as low as 135Kbps). After this sync, my router will show a massive noise margin, perhaps 35dB. Interestingly, I have never seen this problem cause the upstream bandwidth to drop, is this fixed at the exchange, even on Max, or is it just capped to 448Kbps? Can it go lower?

As soon as I notice this, I reset the modem and it then resyncs at a normal speed and noise margin (typically 2500-3000 Kbps / 11-13dB. Obviously, I am then trapped on a ridiculously low IP profile, and it can take days for the exchange to adjust it upwards again, even if there's a huge gap between profile and current sync speed.

Details -

ISP - Nildram aka TalkTalk Business aka Opal (formerly with Zen, until they temporarily priced themselves out of the market, same problem existed)
OS - Various Macs running Leopard or Snow Leopard
Wholesale number checker - Expects 2Mbps on fixed (sounds about right to me knowing what the line is actually capable of 99.9% of the time), or 250Kbps - 1Mbps on Max, but obviously that's based on the problems caused by this anomaly.
Attenuation - very stable at 54dB down / 31dB up


Things I've done to try to resolve this -

-Swapped out all microfilters, more than once, actually.

-Eliminated the possibility of faulty connected equipment - This problem has persisted through about 4 different modems, some wired, some wireless, and 3 different DECT phone systems. All of the equipment has changed over the years, the fault has not.

-Isolated the master socket from the extensions as much as possible - The system is a bizarre combination of star-wired and daisy-chained and the master socket is pre-NTE5 so there's no test socket. There were originally two sockets wired directly to the DP, via a junction box, with a further 4 (four!) extensions daisy-chained from the capacitor-less non-master. Since only the two terminations connected directly to the DP were being used I have eliminated the four chained extensions from the circuit by removing the relevant wires from the crimpers at either end of the extension cables (ie, at the extension socket itself, and at the secondary termination box).

-Isolated master by disconnecting the 2nd termination - Okay, I opened up the box and disconnected everything in order to see if the problem would continue. It did. I don't know if I'm technically allowed to do this, with the demarcation point being rather fuzzy on a star-wired system, but, frankly, all of the Openreach equipment in this house is decades obsolete anyway, so would they really care?

Any thoughts/advice? At this stage I will try anything.

Thanks,
Ben.

the lowest downstream sync on MaxDSL is 160k and usually it takes a router or DSLAM fault or a bad line fault to drop to that from a few meg normal sync.

If you're down to a single socket on the end of an incoming pair then you have the best wiring.

As well as the filters your end there's the equivalent at the exchange, some of those will be getting quite old now so failure isn't out of the question. If it's the line card a lift and shift to another one might help. If it's the line then I guess it needs to be caught in faulty mode and have an engineer track down the fault.

It sounds like a similar line fault to the one I had. That's the one that 5 OperReach visits failed to fix and saw me give up and move to cable.

If you hear noise try the BT line tester (it will almost certainly say there is nothing wrong) then report it as intermittent noise on the line. You may get lucky. If not you can try your luck with the ISP but they are often very reluctant to call out OpenReach.

I had the same fault and 20 engineers and 4 lift and shifts later all is well again

Thanks for the advice everyone, I've long since suspected that the problem might be at the exchange and it's nice to hear that I may actually be correct.

One last question: If I have the chance to have the connection changed from Max to 2Mbps fixed, will this stop the profile slipping low? At this stage, I have absolutely no problem with the sync dropping during approx 1/10 phone calls, as long as the resync is at a decent speed, and I know that stability at that kind of speed is not a problem the rest of the time. I don't know a lot about how ADSL works: would this involve actual engineer work at the frames, or is it all done via software?

Thanks again,
Ben.

If I have the chance to have the connection changed from Max to 2Mbps fixed, will this stop the profile slipping low?

it'll stay low at 2000 and with the reduced upstream speed. It'll go off rather than syncing below 2M.