CABLING
insertion loss is directly related to the length of a link, one
cause could be excessive length (failing length testing is a
pretty good indicator). Insertion loss failures can also be
caused by non-twisted or poor quality patch cables, high
impedance connections or use of the wrong category of
cable for the application being tested. And once again,
you’ve got to make sure the correct autotest was selected
for the cabling under test.
Let’s talk about crosstalk
If your cable link fails or nearly fails near-end crosstalk
testing (i.e. NEXT and PSNEXT), in addition to poor quality
components, you might be looking at an installation
issue. Excessive compression caused by inappropriate
bundling practices, such as the use of plastic cable ties,
can cause near-end crosstalk, as can too much untwisting
at connection points, split pairs or the inappropriate use
of couplers. But beware – using the wrong link adapter on
your testing or testing near an excessive noise source could
also be the problem.
If you fail far-end crosstalk (i.e. ACR-F and PS ACR-F), the
general rule of thumb is to troubleshoot near-end crosstalk
problems first. Correcting those will almost always correct
far-end crosstalk problems. And when it comes to failing
alien crosstalk testing (i.e. ANEXT, PSANEXT, AACRF and
PSAACRF), this too could potentially be fixed by addressing
near-end crosstalk issues first. But it could also be a result
of unbalanced cable, meaning the design of the cable is
such that conductors aren’t properly balanced to be able
to cancel out injected noise. Failing transverse conversion
loss (TCL) or transverse conversion transfer loss (TCTL)
mode conversion parameters is an indicator of poor
balance and often corresponds to failed alien crosstalk
testing.
Pièce de Résistance
Last but not least, if your cable fails DC Resistance, you
may have excessive cable length or poor connections due
to oxidised contacts or marginally attached conductors. It’s
also possible that you’re using too small of a cable gauge,
a poor-quality cable with variations in its geometry or a
cable type you really shouldn’t be using, such as copper
coated aluminium (CCA), copper coated steel and other
non-standard conductors. The resistance on some of these
cables can be 55% greater than a quality solid copper cable
of the same diameter, and this can cause greater heating
of the cable in PoE applications. And besides, non-standard
cables like CCA cannot legally be installed to meet National
Electric Code fire safety ratings, resulting in code violations
and increased risk of fire. n
33