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[Rhubarb]

Hi.

I'm seeing a drop from 6db to 4db in downstream SNR when my phone is off-hook. There is a similar but much smaller effect on the upstream reading ~0.25db. What puzzles me a little is that the drop appears the same across D1, 2 & 3 and yet there is negligible impact on U0, 1 & 2. (I suspect that this might be because the upstream bins are less lightly 'loaded'.) I'm on 17a profile with speeds of: Upstream rate = 12728 Kbps, Downstream rate = 50826 Kbps.

I'd expect to see some impact on the line when the phone is off hook but this seems a bit high to me.

My socket is a Mk2 which was an early VDSL engineer install.

Can anyone give me an idea of what sort of drop in SNR I should expect to see?

Cheers - Andy

Edited to change Mk3 to Mk2 type filter.

[burakkucat]

Can anyone give me an idea of what sort of drop in SNR I should expect to see?

With a good, known to be reliable, telephone a small drop in the SNRM is to be expected. However anything of the order of 0.5 dB or greater should be regarded with suspicion.

The usual suspect is a joint which is developing HR or semi-conductive tendencies.

Is the SNRM drop a repeatable event? Have you tried using the ring-back option of 17070, to put a high-ish AC voltage across the pair? And does the AC burst then result in a less noticeable SNRM drop, which increases with the passage of time?

[Rhubarb]

Hi. Thanks for the reply. 

Yep, the effect is repeatable and it happens with both phones in the house, a DECT phone and a conventional wired handset plugged into the master socket.

I've tried the dial tone suggestion by ringing in and letting it ring before answering, strangely enough the downstream SNR rises about 1 db while ringing in but then drops about 2 db again once the phone is answered. (I'd hoped to post a trace from DSLStats but have given up for now!) Ah! ...

On the first drop I picked the phone up straight away, on the second I let the phone ring first.

Cheers - Andy

[burakkucat]

I am fairly convinced that we are looking at the results of a corroded/corroding joint, showing semi-conductive tendencies, somewhere in your circuit.

The problem, at the moment, is deciding where to look . . .

[Rhubarb]

The problem, at the moment, is deciding where to look . . .

Indeed.  The cable from the A & B in the house to the pole is one continuous piece (changed about 12 years ago) so there's nothing I can look at. I've got a new Mk3 filter on the way 'just in case' so I'll have a look in the back of the box when I change that later in the week.

If I come up with anything else or find a solution I'll post back.

Thanks again for your help - Andy