Can Copper problems upstream of cab cause VDSL problems?

We have had problems with our overhead since we moved here 7 years ago - it finally broke just after we had BT Infinity installed last summer and the overhead length to the roof line was replaced, and jointed to an old length down the wall which was left untouched (difficult to replace you see as it disappears behind a porch).

All was great, until a few months ago when we started to have problems again with intermittent noisy phone calls, but also VDSL drop outs. Then just over a week ago the noise got chronic and our profile got borked down from 65Mbps to 42Mbps.

So reported the fault online and the line checker actually detected something and offered an appointment which is in for next Wednesday. All set I thought - we'll get the whole cable replaced this time as it's bound to be the wall cable....

However this week BT appear to have replaced the trunk copper cable up the hill from the cab towards the exchange (vans with cable drums, jumper wire on pavement, disturbed pavement lids etc.), and despite several Quiet Line tests I can no longer detect any noise problem on voise calls.

My profile is still stuck at 52Mbps though

Now, the question is, could a problem with the trunk cable on the exchange side of the Fibre Cab actually have caused my VDSL disconnects and profile disruption? I had thought that this must have pointed towards my house connection rather than a problem with the copper on the other side of the cab....

I also need to know whether I should maintain the appointment on Wednesday, or am I risking a NFF charge if in fact they have already fixed the problem by virtue of the new trunk cable over the past few days?

Finally, can I persuade them to reset my profile if the trunk cable was the reason for it getting borked in the first place?

Many thanks for any advice!

Edited by Colin_London (Sat 18-May-13 18:44:05)

Now, the question is, could a problem with the trunk cable on the exchange side of the Fibre Cab actually have caused my VDSL disconnects and profile disruption?

No.

Post deleted by RobertoS

In reply to a post by Zarjaz:

No.

I know I'm asking to be shot down doing this, but here goes...

I know there's a low-pass filter of some kind on the exchange-side of the tie cables within the FTTC cab, and this is what ensures that only the voice signal passes from the upstream cable into the FTTC cab electronics.

Therefore I understand that there's no way a proper signal can get through the block, and interfere with the VDSL2 signal.

But...

Is there *no* type of fault on the upstream line that could interfere with the downstream line?

I would have thought that some kind of intermittent contact between the A and B lines could have an effect, perhaps caused by water damage.

Surely the loop start / ground start signalling gets passed through the cabinet, otherwise how does the switch know when you pick the phone up?

The damage to the trunk cable was almost certainly due to damage / water ingress from gas main works that took place last year - I remember seeing the exposed BT cable with some nasty bashes in it at the time (probably from jack hammers). If the insulation between legs on a pair has been damaged then surely that it going to affect the performance of the whole line even on the other side of the cabinet, and thus its capability to transmit DSL signals?

In reply to a post by Colin_London:

Surely the loop start / ground start signalling gets passed through the cabinet, otherwise how does the switch know when you pick the phone up?

When you go off-hook, the resistance drops from something like 10megohms to 1000 ohms or less. The exchange detects this by the current flowing in the line.

The filter doesn't prevent this from happening; it only adds components that makes sure any frequency above normal speech gets blocked out.

In reply to a post by Colin_London:

even on the other side of the cabinet, and thus its capability to transmit DSL signals?

That's the question.

I'm good at knowing what the software should be doing, and have a grasp on what goes on in the copper when it is working. But I'm far from skilled in knowing what happens in the copper in fault situations, or what all the fault situations even are. Zarjaz is the man for that part...

I would have thought that water ingress could indeed cause voltage problems, resistance problems, and shorting problems... and that all of those could get transmitted onto the D-side. But I could be wrong.

If the VDSL Cab had something like a PABX line card in it, and effectively 'bridged' two separate phone lines on either side of the cabinet then I would agree that there could be no interaction between a fault on the E side of the cab and the D side.

But I don't think they are isolated from each other by the cab in that way, and you still have a transmission line at VF and below whose DC performance can be affected by a fault at any point?

Zarjaz is the man for that part...

I admire your confidence. !

A damaged cable will give you audible noise, from the resulting earth and battery contacts, low insulation, HR's etc. If this damage is on the E-side, then it cannot be affecting the transmission of the VDSL signal down the D-side.

I wonder if the OP has unlocked his modem to look at attainable rates ? The audible noise could well be from a mullered E-side, but his drop in sync rate is going to be something D-side related, cross talk or some such.

Given the general size of components used in such filters (and the space available in cabinets) , I suspect that they are all High (ADSL/VDSL) - Pass/Reject designs, rather than Low (Audio)- Pass/Reject designs.

For Audio Filters, it is close to Henries and micro-Farads that are required, implying very significant physical sizes.

For VHF/UHF Filters, the values are in the micro-Henry and pica-Farad range, resulting in very small physical sizes, as presently demonstrated in the domestic ADSL/Phone Splitters.

Hence in the Splitters, it is VHF/UHF Reject filters in the Phone leg, whilst the ADSL leg is "straight-through", ie no filtering.

It may be possible to demonstrate the likelihood of problems of the nature described, by shorting the appropriate pins on the Phone side of such Splitters.

Picure of the filters in a cabinet - post #72
ECI Cabinet filters