Re: Again on ADSL line

On Sat, 1 Mar 2008 14:31:16 -0800, "BobW" <nimby_NEEDSPAM@xxxxxxxxxxxxxx> wrote:


:TDRs have their limitations on long loops. The high frequency loss of the
:loop is large enough such that you can't see a "sharp" reflection. A long
:pulse TDR will get there, and back, but the "hump" that returns if very
:tough to pick out-of-the-noise.
:
:Also, If the other end of the loop happens to be terminated with 100ohms at
:high frequencies (as the case if that line is served with DSL) and 600ohms
:at lower frequencies (depending on the POTS line card when it's "on hook"),
:then you won't get a farthest-end reflection, anyway. So, you won't be able
:to deduce its length with a TDR. Either way, you need to do something at the
:other end of the loop.
:
:The phone companies spend big bucks on test equipment. I used to work for a
:company that made some of this stuff. We made large amounts of money selling
:to the telcos -- at least until the market got saturated with our equipment.
:If they (telcos) could have figured out a way to do things like loop length
:determination WITHOUT the expensive central office equipment or some
:craftsperson sitting at the MDF then they would have done it.
:
:BobW
:
:

The telco's do have the equipment to determine distance to fault testing by
remote control. As an example, several months back, on a Friday evening, a
reticulation contractor cut through the 50 pair local street distributor cable
about 100M away from my house. Being an ex telco tech myself I managed
(eventually) to talk to the rostered testing officer who was located in Tasmania
about 4,000km away. He tested my line and came back with the result that the
cable was broken 961M from the exchange. In this case, the distance to fault
would have been determined by a TDR test because the broken cable produces a
discontinuity reflection.

Even the local fault techs can do line tests from their laptops. These access
the cellphone network and the telco's remote test facility. However, such tests
cannot determine line loop resistance unless the fault tech disconnects the
customer wiring and places a loop on the line.

There are devices which can be fitted to the line at the customer premises
http://www.heuer.com.au/tmsa41/r41_data***.pdf which will allow remote
disconnnection and line loop resistance testing to be conducted without human
intervention.

.

Quantcast