[Gllug] ADSL on BT phone wires

[Chris Bell]

On Wed 09 Aug, Ryland, Peter wrote:
> 

> 
> 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**.

   Sorry, yes.

> 
> 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
> 

   The ADSL must be tapped off using at least one capacitor, rather than
just a transformer which would act as a DC short circuit, and I assumed that
non-electrolytic capacitors would be physically larger but more reliable. It
then depends on whether the signal polarity really matters, and that would
require deeper knowledge of the system.
   The problem occurred on an existing phone line that was upgraded for
testing connections to AOL, not the main BT ADSL, and is connected via an
extension. It worked at one end of the extension, not the other end. I was
not involved, but am suspicious about the explanation given.

-- 
Chris Bell

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