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Patch Cords Meet Their Match

A recent study found that there are vast differences in the ways cords are made, and the testing and quality control they receive.


May 1, 2003  


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Patch cords in structured cabling systems are often overlooked, as they are considered non-differentiated commodities. Nothing could be further from the truth. Cabling is normally installed long before furniture or active equipment, and over 95 per cent of all new installations are tested to the permanent link model, which excludes the patch cords at both ends.

The idea is that the link is tested and certified for the promised level of performance (normally Category 5e/Class D or Category 6/Class E), and then patch cords are added later when the network is installed.

This model works well if the performance of the patch cords meets the performance of the installed link – which often is not the case.

BUYER BEWARE

Unfortunately, the majority of the patch cords on the market today that are labeled as Category 6 fail to meet the minimum requirements of ANSI/TIA/EIA-568-B.2-1 and ISO/IEC 11801:2002.

The reason this problem is not widely reported is because the test outlined in the standards requires the use of a laboratory grade network analyzer that has been qualified to meet the TIA and IEC requirements.

Since most patch cord assembly houses could not afford the equipment or personnel to test cords (and neither could their customers), many unknowingly bad cords are sold. Yet the cords still carry printing on their jackets claiming they meet international standards.

What that writing actually means is the stranded cable material might have been statistically sampled. What then happens is the cable is cut into segments and plugs are added to the ends.

The real ‘magic’ for patch cord performance is in the plug termination method and quality.

In a study of patch cord manufacturing facilities, it was learned there are vast differences in the ways cords are made, and the testing and quality control they receive:

Some facilities use high quality components, work from a well-considered design, really know what they’re doing, take great care to provide high levels of consistency, and then do 100 per cent transmission testing on each cord;

Many facilities do spot transmission testing on random samples, and 100 per cent continuity (wiremap) testing on all cords;

Some do transmission testing, but in an incorrect manner. For example, they run a “channel” test on the patch cord, or test the cord as part of a channel;

Some cord material is low impedance (90-95 ohms), contributing to a return loss ‘bounce’ when connected to a 100 ohm system;

Most facilities assume good transmission performance “by design,” and do 100 per cent continuity-only testing, and

Some facilities do only random continuity testing, in effect using customers as their quality control department.

Yet with all this wide variation in quality, to the layman most cords will look the same. They will virtually all have some kind of “certified by a familiar logo” designation.

As we have shown, what that usually means is the cord material from which the cords where made was certified, but not the cord itself.

THE WEAKEST LINK

If you consider the entire structured cabling channel, from the PC to the switch, the weakest link is the modular plug. This is the point that has the potential for the lowest performance.

Why? Pairs get untwisted and jammed into a small space, they are crossed over each other and split, and then they are put in parallel with flat plates. Often, mechanical crimps are used to hold the cable in the plug.

These crimps can crush and deform the conductors, creating impedance changes that contribute to return loss. Cords take a lot of abuse; they are pulled around desks and run over by chair wheels, stretched tight around fixtures and flattened by heavy furniture.

When you consider that the goal is to try to continue the same matched electrical performance of the horizontal cable, it’s a marvel that manufacturers of patch cords can mimic the transmission of the cable so well through two modular plugs and a length of stranded cable.

And just where are these patch cords located? They are the closest parts of the structured cabling system to the active components. They are placed where the outbound signal strengths are highest, and inbound signals are weakest.

A small impedance anomaly that causes a three or four per cent reflection does a lot more damage to the integrity of the signal transmission when it is located at a few feet from the end (in patch cords) versus somewhere in the middle of a link. This is also true for NEXT anomalies.

Network owners and installers need to consider channel performance, not permanent link performance, when they are specifying structured cabling requirements.

The cable plant is likely to have a much longer life cycle than the active equipment, so planning should anticipate all future needs for bandwidth and capacity.

Marginal cords might be suitable today for 10/100BASE-T Ethernet, but not for Gigabit Ethernet or future applications. Advanced applications tend to use multiple pair transmission schemes and bidirectional communication on the same pair(s), which makes the performance of the patch cord vital to the quality or error rate of the application.

Besides, if you’re paying for Category 6/Class E, shouldn’t you be sure you’re getting it?

Category 6 installations have some special requirements. The performance of Category 6 is much higher than Category 5 or 5e, especially for NEXT and return loss.

For optimum performance plugs and jacks must be centered and well matched. As a result of the many studies to define component specifications, the variability between plug and jack is now much better understood, and incompatibility issues are diminishing.

However, it is still vital that for Category 6 systems you follow the recommendation of the supplier and use only approved patch cords. Otherwise, there is a real risk you will have a ‘good cord’ that is not well matched to your system and suffer degraded channel performance.

An independent study was recently completed on patch cord quality. Random samples of Category 5e and 6 patch cords were purchased from normal distribution channels, and then tested for performance. The results may surprise you. Only 30 per cent of the Category 5e and 17 per cent of the Category 6 patch cords passed TIA requirements.

What can end users and installers do? How can you tell if you have a good cord? They all appear similar, and all have official-looking certification stamps along the sides.

Clearly, a wiremap test is insufficient. You can probably assume that your cords at least have proper continuity, but after that you’re on your own.

Testing cords in the channel is much better but not sufficient either. Why? Permanent links with sufficient headroom can use marginal patch cords and still pass channel requirements, but if the same patch cord is added to a marginal permanent link, the channel would fail.

Then there is the issue of repeatability, and how well the cords stand up to being flexed or coiled or run over by chair wheels. For years we made field tester cords from patch cords sent to us from suppliers.

We found performance varied widely, and we were forced to do 100 per cent incoming inspection to ensure the performance matched our standards (which were admittedly tougher than necessary for normal office use). Would these cords all pass wiremap? Sure. Would they work fine for a 10/100 application? Most of them would. What about their performance after a great deal of flexing, coiling, and uncoiling on Gigabit Ethernet? Many would not make the grade.

NEW TEST TOOLS AND METHODS

Installers and end-users really only have a couple of options. First you can follow the recommendations of your supplier, and only choose to buy approved cords that are designed to go with the installation specified.

In most cases this is the simplest way to avoid potential performance degradation, especially on Category 6 installations.

Another alternative is to test the cords yourself. Field testers are now available that have optional patch cord adapter fixtures that are designed with special hardware an
d software to exactly meet TIA patch cord test requirements.

In fact, these products are already in use at many patch cord manufacturing facilities worldwide. This provides a means to check legacy cords, as well as verify incoming product to meet requirements consistency from cord to cord.

The news of new patch cord testing tools and methodologies is good for the industry as a whole.

Network owners and installers finally have absolute assurance that the patch cords they are using are standards compliant and will work with their new category 6 cabling system.

Similarly, connector manufacturers, cable manufacturers, and the makers of quality patch cords now have the means to validate that their product is better than cheap alternatives.

Brad Masterson is Canadian Product Manager for Fluke Networks. Involved in the field of networking and network testing since 1995, he is a Certified Engineering Technologist registered with OACETT, and a member of BiCSI and Cabling System’s editorial advisory board.