Networks Europe March/April 2019 - Page 33

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