On Jul 12, 15:52, McFadden, Mike wrote:
> Sridhar wrote
> >How do you find out the exact number of feet you're out from the CO?
> I think you call up the telco and ask them the question and then they run
> some test and report back to you.
It's a time-domain reflectometer test. Basically you send a narrow pulse
down the cable and look for a spike coming back. You can do it with a
pulse generator and an oscilloscope. Trigger the scope from the pulse
generator, and display the voltage on the cable. Any discontinuity in the
cable will show up as a bump in the trace (a discontinuity might be an
imperfect splice, or a sharp bend in a coaxial or twisted pair cable which
crushes the cable and distorts the geometry), and unless the end is
perfectly terminated, it will cause a reflection which will show up as a
spike. Then if you know the cable characteristics, specifically the
velocity factor, measuring the time between the pulse and the bumps or
spikes allows you to work out how far down the cable they are.
The same technique is used for optical fibres. A couple of weeks ago, we
had a bunch of about 30 spliced (previously they were connected via two
sets of patch panels). The guy who did the splices produced a very fancy
optical TDR, which showed the graphs of the returns as well as the times
and distances. He was even able to point out where there were patch panels
on the far end and estimate how long the attached patch leads were.
--
Pete Peter Turnbull
Network Manager
University of York
Received on Thu Jul 12 2001 - 19:15:31 BST