Another advantage of using a meter — certify the initial installation. Keep records of the test results. Ideally, the cables can be certified before the walls close and again at final fitout. Now is the time to deal with site damage and the numbers will help settle disputes. Later, if problems develop, you can easily measure a change.

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I’m trying, not very successfully I’m afraid, to come up with a quick explanation of why one must be gentle with category cables, why a simple continuity check is not the whole story about cable health, and why a high tech meter is the better tool.

Consider this: If one was attempting to distribute off the air analog TV, cable TV, or satellite signals, one would pick a 50, 75, or 300 Ohm scheme. Everyone knows (or should know) that there is a family of fittings with each of these schemes and that there are consequences for mixing fittings and cables. When there is an impedance mismatch, some energy is absorbed at the transition and a reflection is generated. In long runs (of analog TV) one can actually perceive “ghost” images. In shorter runs there is some smearing of the image that is hard to characterize. The physical aspects of the cable and fittings are extremely important. A crush point or too sharp bend changes the impedance, causing some signal loss and reflections at the transition.

For the sake of discussion let’s send a single pulse through our transmission line (cable). We have a signal starting at point A, meeting impedance issues at B and C, terminating at D. When the pulse hits B most of the signal will continue to C, but a portion will reflect back to A. If things are not quite right at A, a portion of BA will be reflected and head back to B. Since everything travels at the same speed per foot, the reflections arrive little later and combine with the actual signal. The same process is working between B and C, C and D. In a sense we launch a single pulse from A toward D, but a smeared blizzard arrives at D.

This is over simplistic, but I hope that you get a sense of what is happening.

In our Gigabit network, too tight bends, crushes, and splices result in impedance changes and a smearing of the nice digital signals we are pushing. A little smearing is tolerated, but if the smear is still cooking at the expected time of the next pulse arrival, we are in trouble.

The sophisticated testers allow us to put some numbers on the maximum successful transmission rate. Lower data rates are less bothered by cable issues.

Water in a cable that was designed to be filled with air, will cause a dramatic impedance discontinuity. But, the wire map is fine.

By the way the TDR testers measure the reflection. Assuming that the cable is uniform to the problem area, it is easy enough to calculate the distance to the fault. It is very handy to know where a shark bit the undersea cable.

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Edit: By the way when we are laying out transmitters, we fuss with the SWR (Standing Wave Ratio). This is another facet of the same coin. For transmitting antennas we would like maximum energy to be launched into the air, not reflected back to be absorbed by the cable and transmitter. Multi-megawatt pulses meeting a soggy, wet section of cable can result in some interesting surprises (as the water absorbs a little of that energy). In receiver systems an unfortunate SWR will result in a bad signal to noise ratio.

Wikipedia TDR article.