[Gllug] ADSL on BT phone wires

[Ryland, Peter]

On Wed, 2006-08-09 at 09:03 +0100, Chris Bell wrote:
> Hello,
>    The standard BT phone line carries DC volts, and the A and B legs of the
> pair should be connected the correct way round at the BT termination.
>    I am very suspicious about further information given to me, so can anyone
> please confirm or rubbish the following:
> 
>    I have been told that ADSL is phase sensitive, and should also arrive at
> the customer's termination phase correct relative to the normal phone, and
> that occasionally the ADSL is not connected correctly at the exchange,
> resulting in the wrong phasing. This is sometimes sorted at the local
> termination rather than correcting the exchange fault.
> 
>    I suspect that the fault seen (not on my line) was really just a poor
> insulation displacement connection, and any re-make would have sorted the
> problem, rather than a phase reversal.

Well, for starters, "phase" is the wrong word.  The word is "polarity".
Phase is specifically to do with AC (and "not in phase" or "out of
phase" means that the voltage and current are not in sync due to load
with an imaginary* component) which is only present when the phone is
ringing**.

The thing is that *any* (non-trivial) DC equipment is not just sensitive
to polarity, it generally will not work at all if plugged in the wrong
way around.  And whilst adsl modems generally don't care about the DC
power from the line, they will still be sensitive to it.

Pete

* As in complex numbers type imaginary, not as in make believe. :-)
** In Australia, a 50V AC signal is provided on the line to signal an
incoming ring; I'd guess that it works much the same way here.


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