On Tue, 5 Feb 2008 17:00:37 -0800, "EpsilonRho" wrote:

:Problem solved. The AT&T technician changed the pair arriving to my house
:with some unused ones (this is the second time) and now the impedance of TIP
:& RING is virtually the same and not changing. No interruption has occurred
:in 24 hours! The conclusion of the story is that the impedance of the line
:should be the same for the positive and the negative, if it is not the case
:the common mode rejection is greatly degraded making the line susceptible to
:external disturbances.
:Thank you again to you all,
:Gene
:
:

Glad to know the problem is solved. It seems that the tech/liney just took pot
luck on both occasions and grabbed whatever free pair was available in the
cable. The first time he struck out and the second time he got lucky. I'll bet
he didn't log that first pair he tried (or your original pair either) as
'faulty', so the next customer who experiences similar symptoms will have an
even greater chance of getting a dud pair. Bloody, lineys, - duh.. what's a line
test?

As far as telephone lines are concerned you don't say that "each leg of the line
is the same impedance". It is simply a balanced twisted pair which means that
providing that no leakage, insulation or earth faults exist on either leg then
the DC resistance of each leg will be exactly equal. The pair should then
perform in accordance with appropriate standards, barring other problems of
course, (such as a double jumper connecting your pair to another pair which is
unterminated) It might still be balanced in a DC sense but that stub pair
creates all sorts of reflections. That is why it is important to do a proper
test on each pair that is to be used for DSL.