On 2/1/08 9:07 PM, in article
, "EpsilonRho"
wrote:

I am having sporadic problems with DSL. I have established that the source
of problem is the phone line, because I have connected two different laptops
directly to the AT&T outside box in order to bypass the house wiring
completely and the problem persists with both PCs.

What kind of problems? Do you hear it on the line?




I called the AT&T customer service and the technician confirmed that the
line was unbalanced (squirrels chewing on the wiring). To solve the problem,
he just swapped the old pair with an unused one. The situation has now
improved somewhat but still the reliability is not 100%.



To satisfy my curiosity, I loaded the phone line (again bypassing the house
wiring) with one 2Kohm resistor. The measured loop current is ~15mA (well
below the range of the max. REN load).

REN is not a factor in loop current considerations, only the phone (430 Ohms
DC) and the loop resistance to the serving switch or Mux line card are.

I was expecting to find a symmetrical
voltage change at the TIP (RED) and the RING (GREEN). To my surprise the
TIP voltage dropped from 0V to –19V and the RING voltage went up from –51.5V
to –49.30V. Obviously the line is totally unbalanced.

Without knowledge of the serving arrangement and signals that may be
present, neither the readings nor the conclusion are not of any value.




Is my assumption correct that the line should be at least DC balanced when
referred to ground?

No.

It should be balanced for transmission, but not necessarily for DC.

Loop current and transmission are both with respect to the Ring and Tip, NOT
to ground. I don't have your loop specs, but it may be you are being served
by a Pair-gain Mux, in which case the 2000 Ohms you tested with is much too
high.






TIA,

Gene



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