Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!cs.utexas.edu!rice!uw-beaver!ubc-cs!eric!joplin!dleblanc
From: dleblanc@joplin.mpr.ca (David LeBlanc)
Newsgroups: sci.electronics
Subject: Re: Electrical pulse on telephone lines?
Keywords: ring
Message-ID: <1967@eric.mpr.ca>
Date: 15 Dec 89 01:03:13 GMT
References: <4208@crdgw1.crd.ge.com> <4910@blake.acs.washington.edu> <42002@improper.coherent.com>
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Reply-To: dleblanc@joplin.UUCP (David LeBlanc)
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Organization: Microtel Pacific Research Ltd., Burnaby, B.C., Canada
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In article <42002@improper.coherent.com> dplatt@coherent.com (Dave Platt) writes:
>This was discussed in comp.dcom.telecom some time ago.  As I recall,
>the explanation is that the telco sends a measured voltage pulse down
>the phone lines periodically.  By measuring the current flow and
>certain other things, they can check the quality of the phone line
>and detect some physical problems as they develop (e.g. insulation
>failure, etc.).  The phone-switch in your central office does this
>automatically, at a time when the line is idle (usually late at night),
>and enters a trouble-report in the database if anything unusual
>is detected.

Yup, its technically called "Time Domain Reflectometry".  Any
transmission line has two important properties : a finite propagation
velocity and a characteristic impedance.  When energy impinges upon an
interface between two transmission lines of differing impedance a
portion of the energy is reflected and a portion is transmitted (the
exact proportions determined by the impedance mismatch - a dead short or
open results in 100% reflection - a perfect match results in 100%
transmission).

The telephone cable is a transmission line.  By launching a pulse which
propagates down the line at a known velocity and by measuring the
reflections sent back and the time at which they arrive back at the
central office (eg. microseconds later) locations and severities of
faults can be determined amazingly accurately.  There are optical TDR
systems for fiber which can resolve position down to millimeters and
losses (reflections) down to < 1dB !!

Have fun with your chirps.

Dave LeBlanc
dleblanc@joplin.mpr.ca