ADSL works but phone dead - mechanism?

[Grunff]

I've been told by several people (including a BT engineer) that ADSL can work on just one wire. Whenever I heard this, I thought "Hmm, really?", and not thought about it any more.

It now appears to have happened to one of our lines. The phone is dead, but the ADSL works great. BT say it's a line break, and will take up to two weeks to repair (yes, it's a business line with TotalCare on it - they did respond in 4 hours, but won't be fixing it for a while ;-).

We have two other lines, but they're both ISDN.

So, what's the mechanism that allows me to get 1M down a single wire?

[Ian Stirling]

Basically, the losses at voice frequencies, and DC may be tens of thousands of times less than at the higher frequencies used for ADSL. Especially, if the line was good beforehand. For example, if really close to the exchange, 20db of losses across the board will make voice unusable, but will only be noticable if you look at the router status page for losses.

[Lurch]

On Sat, 18 Dec 2004 12:37:21 +0000, Grunff strung together this:

Without getting too tecnical, as I can't recall it all off the top of my head, ADSL works on a RF frequency so can 'jump' small breaks in the cable.

[Dave Plowman (News)]

IIRC, it's on an RF carrier, so doesn't need a DC path.

Have you checked for DC on the incoming line? That would show if it's intact to the exchange.

[Dave Liquorice]

I'd be jumping up on down on succesive supervisors to get it fixed PDQ. When I asked one of the local engineers about Total Care he said that they would be down holes in the ground 3AM Christmas morning if they had to do so to repair a Total Care line...

Still think of the compensation for two weeks lack of phone service on a business line... B-)

If it was the ADSL that was dead I could understand the delay but as it's the main voice service that is AWOL that should be sorted really quick.

[Peter Crosland]

If you are on a Total Care line then they should fix it or start taking active steps to do so ASAP. You should escalate the fault higher and higher until they sort it. ADSL needs two wires to work properly.

Peter Crosland

[Lurch]

On Sat, 18 Dec 2004 16:50:40 -0000, "Peter Crosland" strung together this:

But not to work.

[Peter Crosland]

Care to explain that? Unless two wires are connected the link will not function at all.

[Lurch]

On Sat, 18 Dec 2004 19:04:10 -0000, "Peter Crosland" strung together this:

See my other post. ADSL is carried at a radio frequency (RF) so will effectively 'jump' gaps in cables. It does work with one wire only connected without any problems and in some cases can work with none. For an electrical current to flow you are quite correct though.

[Mike]

Only at a pretty poor performance, if you are near the exchange and there is a lot of capacitance to earth at both ends. ADSL depends on balanced lines to reject interfering noise, but the error correcting codes also do the same job so you will get some throughtput anyway.

The ADSL signal works in a series of 4kHz slots from 30kHz up to over 1MHz. Lower slots can encode up to 10 bits whereas higher slots only usually manage 1 bit. But on a perfect system with a short piece of wire ADSL can manage over 8Mbps so getting 1Mbps (if you are actually getting that even) isn't too arduous.

But are you sure the break isn't in your splitter filter in which case the ADSL is getting both A and B lines correctly.

[Peter Crosland]

You are being somewhat disingenuous on this matter. In theory ADSL may work but in practice in such a case the signal to noise ratio is likely to make the link very unreliable with lots of drop outs. The bottom line is that an ADSL line will only function correctly if the telephone circuit is in good order.

[Lurch]

On Sat, 18 Dec 2004 22:22:22 -0000, "Peter Crosland" strung together this:

That's what I said. It does work in practice, sometimes.

[Grunff]

I do a lot of jumping when this happens (2-3 times a year), still doesn't actually achieve anything. Last time I did so much jumoing we had a visit from the BT regional manager. Didn't actually achieve anything.

Lat time we had 2x ISDN (two separate pairs) lines down for 11 days. I think it was about £300 compensation. I can't even begin to calculate the cost to us :-(

In /theory/.

[Grunff]

No, break deff. on the line.

[Grunff]

Just did this now - I don't know what it's supposed to be, but I get ~2V DC. What does that mean?

[Grunff]

As in my reply to Dave L, it just doesn't help. They are totally useless, and don't know the meaning of customer service.

[Andy Hall]

There should be about 50v DC so it means that one or both legs of the line are class F.

[Mike]

It's supposed to be about 42 volts. Does your ADSL work perfectly ?

If so it probably isn't the line at all but a dodgy board or connector at the telephone equipment at their end. The signal comes out of this and then into the ADSL rack then out to you. This really is "spagetti junction" with huge bundles of 50 pairs dangling all over the place and a single damaged or intermittent wire wouldn't surprise me.

You can get VoIP as a backup. This uses the ADSL for voice and whilst you would aquire another phone number it would give you some form of service.

[Grunff]

It's now on/off, but when it's on, I can get about 960Mbit.

Yes, we use voip between our offices, and i have a couple of voip-pots accounts.

[Grunff]

Amazing it still works then!