Equal Level Far End Crosstalk (ELFEXT)
ELFEXT is a calculated result, rather than a measurement. It
is derived by subtracting the attenuation of the disturbing
pair from the Far End Crosstalk (FEXT) this pair induces in
an adjacent pair. This normalizes the results for length. Consider
the FEXT and attenuation measured on two links constructed of
the same materials with the same workmanship, but different
lengths.
50 m link example:
FEXT = 45 dB and Attenuation = 11 dB
ELFEXT = 45 - 11 = 34 dB
Another way to understand ELFEXT is to think of far-end Attenuation
Crosstalk Ratio (ACR) as the same thing.
Results Interpretation
Compare the results of measurements made from both ends of
the link to the appropriate ISO or TIA limits. There are 12
ELFEXT measurements made from each end, for a total of 24.
This is because the attenuation can vary slightly depending
upon which pair is energized. So as an example, the field
tester will energize Pair 1 and listen on Pair 2 at the far
end. Then it will energize Pair 2 and listen on Pair 1 at
the far end.
ELFEXT that is too high is indicative of either excessive
attenuation, higher than expected FEXT, or both.
Troubleshooting Recommendations
The same factors that contribute to NEXT problems contribute
to FEXT problems. Troubleshooting for ELFEXT means troubleshooting
NEXT and attenuation problems, just as you would for ACR problems.
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