DSL Mystery (to me at least)

I'll have one last go at getting an understanding of this and a possible cause for what I'm seeing.

For many years I had a good, solid sync at 8128 with 9dB attenuation. According to the stats I have this was with an SNR target of 15dB too!

Now, I am lucky if I sync at > 7000. Previously I had zero errors also, but now if I ring my landline (without ANYTHING else connected and using (various) filters in the test socket) I will see numerous (CRC) errors appear. I also get errors if I connect a phone and dial out but much fewer. I think it is these CRC errors which is keeping my target SNR high?

Today I had the pair changed in the first leg and this has made no difference - I think the next leg is direct to me.

So why does a ringing current cause these errors? Tried it with 3 different routers/modems, all exactly the same result. The SNR target was changed once for me down to 12dB and I sync'd at 8128 again but exactly 48 hours later it reverted.

TIA.
Chris.

a bit of damaged cable or a bad joint in the line somewhere might cause noise in response to the passing of audio frequencies or dc current around the voice loop.

Thanks Phil, makes sense - it's not likely to be a line card/config issue then I presume.

So I'm stuffed really I guess. I've already used my favours up with BT and the ISP won't touch it because it's still way above the fault margin - short of ordering a new line.

it's not likely to be a line card/config issue then I presume.

wouldn't think so, although there is an ADSL / voice splitter at the exchange end that could go wrong in the same way a microfilter can.

Interesting... and I doubt very much I'd get anything done about it even if I could demonstrate the issue as I still sync at 7000...

Is there any upper threshold to SNR target margin? Is it likely that my speed will continue drop as more errors are produced when I receive phone calls etc. or will it reach a level and stay there? I'm guessing the latter...

Is there any upper threshold to SNR target margin

15 dB, I believe.

Hi
Have you tried the quiet line test ? If that fails then BT have no option but to fix the line . Do not mention adsl.
Regards Jeff

Thanks Jeff - yes I did and it is quite clear sadly. It really is quite a short line and they've already replaced the first leg. I think the problem is at the switch - really wish I knew a local friendly BT engineer :0(

I've created a perl script now which is recording all stats I can get at from the router (seeing as SNMP is not complete for this device). I noticed:

After exactly 24 hours I accrued one CRC error down, within 15 minutes of that the SNR margin went from 15 to 14. So, in 24 hours just one RX error, no TX errors. I then dialled my number (with no phones connected) ... bam! 24 TX errors 28 RX errors.... SNR margin back to 15 ... I really don't think that's how it should be...

Edited by deleted (Sun 30-Aug-09 19:56:43)

You mean you get this even if no phones are connected? Are you sure that using the test socket disables all the connections in the house, e.g. not an old ringer device lurking somewhere

100% Andrew... nothing but a pair 'o wires, a microfilter and my router....