The Weakest Link
Results of a test of 149 new patch cords purchased from 34 distributors, assembly houses and retail and catalogue outlets, were far from encouraging. An astounding 83% of Cat 6 cords did not meet the TIA requirements.
March 1, 2007
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There is an old expression, which says the hardest part of communicating with another person is the final few inches from the listener’s ear to the brain. In some respects, the same is true for today’s network.
The data centre, the cabling and all the other hardware may be state-of -the art. But the last link between the network and the user is the lowly, often forgotten patch cord. And that last link, the patch cord, is very often the weakest link.
At slower network speeds, the patch cord may not have been a major contributor to overall performance.
But with networks running 10GBASE-T, 1000BASE-T and now 10GBASE-T, the quality and performance of every part of the network, including the patch cords, takes on added importance. As with other parts of the network, testing to standards can show which patch cords perform and which will not. And surprisingly, many brand new, just out of the bag patch cords do not (more on that a little later.)
The permanent link and the channel: Before looking at the testing process for patch cords, let’s review how the rest of the cabling is tested. Most new or modified structured cable links undergo a documented certification test of the permanent link.
As the name implies, the permanent link is that portion of the cabling installed on a permanent basis. The cable itself is hidden within walls, under floors and in ceilings, routed in cable trays and conduit. The permanent link is certified from one wall plate jack to the other.
It is possible to certify a link that includes the patch cords on either end. This is called testing the channel, and is performed less frequently than a permanent link test.
A proper channel test requires that the actual patch cords that will be used every day are included during the test.
Two-person test crew
These patch cords must be left in position following the test. It requires channel test adapters on the test equipment to remove measurement effects introduced by the mating of the patch cord to the jack in the channel adapters. It also involves a two-person test crew, one on each end of the link.
There is another way to test the complete channel. A compliant permanent link plus a compliant patch cord will result in a compliant channel. Since the installer most likely certified the permanent link, the tenant or the network manager can add a set of tested patch cords and be assured that the entire link will be compliant to the performance level for which it was tested.
So what is the right way to test patch cords? Do new patch cords need to be tested? What about patch cords that come with test documentation? Or patch cords that are made on site?
There are accepted industry standards that define patch cord tests. TIA/EIA-568-B.2-1 spells out the limits for the two most important test parameters for patch cords, Near End Cross Talk (NEXT) and Return Loss (RL).
An up-to-date certification tool designed to test patch cords can compare test results with acceptable limits, but the operator should know a few things.
Patch cords tend to be very short, typically under three metres. What happens near the ends is more severe, more detectable and more impactful than the phenomena, which occurs in the middle of the cable.
This means the quality of the termination between the cable and the plug is absolutely essential to the patch cord’s performance.
To properly test the patch cord, all measurement interference from the test tool must be eliminated, while every aspect of the patch cord must be included. This requires a dedicated patch cord adapter on the tester that uses an ideal “reference jack” as defined by standards.
This reference jack is the same type used in permanent links, and is performance tested before and after assembly in the adapter.
The goal is to have the highest possible “Mated Connection Performance” between the eight exposed conductors on the patch cord and the matching conductors in the test adapter.
Without this Mated Connection Performance, RL and NEXT measurements will not be accurate.
This also explains why patch cords cannot be tested with channel adapters on the test instrument. Channel adapters are designed to discard the measurement effects of the jack, which is exactly what must be tested when looking at a patch cord.
Channel adapters also test to a standard, which assumes long length and multiple connections. The patch cord standard assumes a very short length and no other connections. This means the pass/fail limits for a patch cord are higher than that for the channel. If a user tests a patch cord using channel adapters, a poor quality patch cord could easily be identified as passing and installed in the system.
Note of caution
As noted above, new installations are typically certified by the installer. The time to test the patch cords is when the network manager starts connecting new devices to the cabling.
A properly conducted patch cord test requires only one operator, and the addition of affordable adapters can turn an existing certification tool into a standards-compliant patch cord tester.
But a note of caution is required here. Today’s certi-fication tools and adapters can test patch cords for Cat 5e and Cat 6. As the standards for Cat 6A are not yet complete, there is not viable way to field test patch cords to Cat 6a.
Cable manufacturers can test their Cat 6A patch cords with their own connectors, but at this point, these tests should be considered proprietary due to lack of a finished standard.
Earlier in this article reference was made to some brand new, right out of the bag patch cords failing to meet performance standards.
The test consisted of 149 new patch cords purchased from 34 distributors, assembly houses, retail outlets and catalogue outlets, all tested with a certification tool using patch cord test adapters.
The results were surprising and not encouraging. Category 5e tests revealed a 69.8% failure rate. Category 6 cord requirements are much more strict, and the data showed that 83% of Cat 6 cords tested did not meet the TIA requirements. These failure rates were roughly equivalent across all purchase channels.
Category 6 failures were predominately NEXT issues; however, many failed both NEXT and RL. No Cat 6 cords failed RL alone. Failed Category 5e cords had smaller failure margins, with NEXT and RL problems more evenly distributed. Many failing cords exhibited damaged or deformed cable, inconsistent assembly techniques, and too tightly coiled packaging.
It was apparent that most cord assemblers do not have the proper manufacturing processes or testing capability to consistently produce compliant Cat 5e or Cat 6 cords.
One Cat 5e assembler had 100% passing samples. They use high quality bulk cable and plugs, combined with good handling, assembly, and packaging techniques.
Another assembler uses similar techniques to produce Category 6 cords. It is possible to produce high volume, fully compliant Category 5e and 6 patch cords if the proper cable, plugs, assembly methods and test gear is used.
Some people prefer to make their own patch cords, and based on the data above, there appears to be some reason to think that is a good idea. However, the data collected from Fluke Networks’ tests shows that self-made patch cords generally have worse performance than manufactured patch cords, and rarely justify the time, materials and level of experience needed.
Patch cords are designed to be made with stranded cable. This version of UTP is far more flexible than the solid cable used in the permanent link. Do not make the mistake of using a length of standard UTP and mating it with patch cord plugs.
The plug and crimp tool was not designed to be used with solid wire, and the resulting connection will likely fail a performance test. The cord itself will also be prone to mechanical failure as the solid wire fatigues from flexing and eventually cracks. If a handmade patch cord
must be used, the safe approach is to test it.
Even though patch cords get used, abused and overlooked, there are several things that can be done to make strengthen this link:
* Make sure that all permanent links have been certified. This is an essential part of every installed or modified link.
* Before attaching equipment to the permanent link, test the patch cords.
* Patch cords should be tested using a certification tool with patch cord adapters. Do not perform a channel test on a patch cord.
* Test new patch cords. Some new cords are susceptible to failure.
* Hand built cords are also prone to failure. If they must be used, make sure they are tested
Hugo Draye is Marketing Manager for Fluke Networks’ Certification Tools. With over 20 years of industry experience, Draye frequently lectures at industry seminars and conferences and his articles appear regularly in the trade press.