|Reprinted with permission from the e-book, Network Cabling Help
| Below are of some of the terms used in high performance cable testing, and a description of what they mean.|
The first thing to understand about testing data cables is the ACR. This stands for attenuation to crosstalk ratio. The pink area in the graph is the attenuation. Attenuation is the reduction in signal strength over the length of the cable and frequency range. The blue area is the crosstalk. Crosstalk is the external noise that is introduced into the cable. So, if the two areas meet, the data signal will be lost because the crosstalk noise will be at the same level as the attenuated signal.
ACR is the most important result when testing a link because it represents the overall performance of the cable. The question becomes "What causes the signal to attenuate and where does the crosstalk come from?"
DC loop resistance
The name 'crosstalk' comes from the telecommunications industry. You may have heard a faint conversation in the background while on the phone yourself. This is caused by the electromagnetic effect between adjacent telephone wires. In the transmission of data, cross talk is at its highest level in the RJ45 connection as it enters the cable, or at the 'Near End'. The term 'Near End' is slightly confusing because data can travel in both directions, and the NEXT test is carried out in both directions automatically by the tester, so the NEXT result is relative to the end of the cable that it was carried out on.
The twists in a cable help to cancel out the effects of NEXT and the more twists there are, the better the cancellation, however, the twists also increase attenuation, so there is a trade off between NEXT cancellation and attenuation. The twist rates in data cables are optimised for the best overall performance. The twist rates are also varied for each pair within the cable to help combat crosstalk.
This stands for Power Sum Near End Cross Talk and is actually just a calculation. When a tester carries out the NEXT test, it measures the cross- talk on each pair as affected by each of the other three pairs individually, PSNEXT is simply the addition of the three NEXT results for each pair. So this is the combined effect that a pair would be subject to when used in a network that supports a four pair transmissions method, e.g.. Gigabit Ethernet.
|FEXT, ELFEXT and PSELFEXT
Basically, Far End Cross Talk (FEXT) is like NEXT, but it is measured at the far end (well that seems logical!). On its own FEXT doesn't mean too much, because the length of the cable determines how much the signal is attenuated before it can affect the pairs at the far end. To compensate for this, and to provide a more meaningful result, the attenuation is subtracted from the FEXT test and the result is then called Equal Level Far End Cross Talk (ELFEXT).
And of course, no test parameter these days would be complete without adding the results together for each pair and calling it a Power Sum measurement, so now we have Power Sum Equal Level Far End Cross Talk or PSELFEXT for short.
This is the propagation delay, or the time it takes for the signal to travel from one end of the cable to the other. It is not very important on it own because its value is directly proportional to the length of the cable. What is important is the relationship between the delays on each of the four pairs. This brings us nicely on to ...
Now this is important. Delay skew is the difference between the fastest and slowest pairs. Some networks use a four-pair transmission method. This means that the signal is split into four, sent down the four pairs in the cable and re-combined at the far end. It is essential that the signals reach the far end at near enough the same time, otherwise the signal will not be re-combined correctly.
Reprinted with permission from Network Cabling Help
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This was first published in February 2005
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