Noise margin and one line much slower than the other

I have two BT lines here installed. One is connected to BT Total Broadband (Option 3 Unlimited) and the second line is connected with IDNET. The IDNET line is much faster for downloads than the BT Total Broadband line. 15MB vs. less than 9MB with BT. The only big difference in the line stats between the two lines is the reported Noise Margin Downstream.

Here are the stats for both lines

BT Total Broadband Line
Downstream: 8876 Kbps
Upstream 1191 Kbps

Line attenuation (Down/Up) 31.0 dB / 15.7 dB
Noise margin (Down/Up) 12.2 dB / 6.2 dB
Latency type: Interleaved

IDNET Line
Downstream: 15226 Kbps
Upstream 1295 Kbps

Line attenuation (Down/Up) 33.0 dB / 17.3 dB
Noise margin (Down/Up) 6.2 dB / 6.9 dB
Latency type: Unknown (it's not reported by the router)

Some notes:

1. I've recently disconnected Sky HD Box (Saturday) and DECT phones (today) from the BT line to see if that might help. I'm not sure if the numbers would improve immediately or if I would have to wait a longer time? That's the only difference between the two lines in terms of connectivity.

2. Both routers (Home Hub 2.0 and a Netgear DGN3300) connect to the same 2 port BT wall socket. I've tested the same Netgear router on the BT Total Broadband line over a period of months but it doesn't make any difference to the speeds.

3. I've tried switching the filters and the cable that connects from BT wall socket between the routers to see if that might be causing a problem, but it makes no difference.

As far as I know, the BT line should be performing better than IDNET because the attenuation numbers are lower? I'm guessing the noise margin must be what's dragging the connection speeds back so dramatically. It's the only thing i can see that could be difference. But I'm not sure what I can do to improve it.

Anyone have an idea what might be going wrong here?

Thanks in advance!

The target SNR noise margin may have been raised from 6dB to 12dB on your BT line. Does it say 12dB shortly after a router restart?

Yep. And it was 11.3 dB earlier this morning before I restarted a couple of times testing things. But now it's always coming back at 12.x dB. Connection has been rock stable. Both lines were connected for 260+ hours until last week when I manually restarted them.

I've also had both these lines for more than a year. The performance difference has always been the same.

Edited by deleted (Wed 18-Jan-12 13:44:03)

The normal target SNR margin is 6dB. It is raised in 3dB steps if the line is unstable (too much noise). If the noise stops it is meant to drop again in 3dB steps back to 6dB. Each drop is said to require 2 weeks of stability. Sometimes the drops fail to happen.

You could ask your ISP to reset your target SNR margin to 6dB. Getting them to understand the request may take perseverance though and their may be a special form of words they need, e.g. can my line be retrained. Make sure the request is for the phone number associated with the BT as ISP line.

It is probably worth getting the error stats (CRCs, HECs, FECs) from the line first. The command to do that are modem specific, or try http://kitz.co.uk/adsl/frogstats.php

Is the bell wire disconnected? See http://www.dslzoneuk.net/socket.php?type=html

Yeah the connection has never really gone beyond the 8mb-9mb level in 15 months. Whether disconnecting Sky and eliminating DECT will help I don't know. I know both of those used to cause problems in the past. But it's been a long time since I've really paid attention to any of this stuff so technology and interference prevention may have improved?

Ideally I can get this BT line performing as well as the secondary one. I'd like to cancel the second line, but the way it's looking I might have to put BT Unlimited on the IDNET one, cancel the existing BT line/connection, etc. Huge hassle.

It looks from the BT support forums that they are able to do the reset. But I'll need to wait 3 days of continuous uptime before they will even consider helping.

Thanks for your time and help with this. It's much appreciated. We'll see what happens.

Are the lines routed differently much inside the house? If not I'd expect noise to affect them both in a similar way.

Good luck getting the reset and I hope it gives a decent sync increase.

Just to add to John2007's post...

It looks like you are on a decent ADSL2+ 21CN connection - the default target SNR margin is now 3db if everything is stable. This would improve your sync even more.

There's a good chance your SKY box / DECT phone have/are causing problems so you need to be happy that's all sorted.

Your provider (love your username!) should have much more control over your line these days so should be able to help you

Good question John. You've reminded me of something important I think! There is one big difference between the two lines now I remember. There's a second phone connection port by the Sky satellite box port connection in another room. The BT engineer asked which number I wanted that second BT line port connected to when he came for the installation. He said he could wire that secondary port to either from the dual BT socket port in the main entrance hall. And of course, I asked him to hook that port up to the BT line that is now suffering from the slower speeds now.

I guess this must be wired as some kind of internal extension. I'm not sure exactly. You can't see the wires the port is in the wall. But I bet it has the potential to wreck the line quality of my main line.

Could that be triggering the problems we've been discussing? And if so, I guess I'm going to have to pay for a BT engineer callout to disconnect it or just ditch the line and migrate everything to the second line. The latter is almost certainly going to be far cheaper.

And the Sky HD box was previously connected to that second single BT port in the other room until Saturday.

Is the sky box connected via a microfilter? That is all the sky box needs