[BUGA] ADSL line resilience

[Greg 'groggy' Lehey]

As many of you know, a couple of weeks ago I finally got ADSL
installed.  It worked well, modulo a few interesting stories getting
routing to work (see my diary,
http://www.lemis.com/grog/diary-jun2005.html, for further details).

Earlier this week Telstra had their almost-annual line failure and
inability to honour the Customer Service Guarantee.  See
http://www.lemis.com/grog/Telstra/ for details on that.  They're
getting better, though: this time it took them only 2½ working days to
fix the problem (the CSG allows 2).  In some ways, that's progress.

Obviously, the thing that was different this time was that I had the
ADSL line.  The router gives line quality statistics.  That's probably
what found the problem in the first place, long before the phones
failed.  I've written some scripts to maintain line quality
information (see http://wwww.lemis.com/grog/dslstats.html).  The
interesting thing was that the phones failed, but the ADSL line
didn't, It was only down for a total of 75 minutes while the linesmen
physically replace the cable.  During the phone outage, the S/N margin
actually improved.

Why is that?  All phones failed, including the one on the same line as
the ADSL.  Why did the ADSL line not fail?  My current guess is that
they disconnected the DC voltage at the exchange, but that the ADSL
line doesn't require DC voltage.  Can anybody clarify?

Greg
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