With both the 10GBASE-T application standard for 10Gb/s Ethernet over copper and the category 6A cabling standard now ratified, there has been much flurry over UTP versus F/UTP and S/FTP cabling systems.

by Carrie Higbie

While a consideration, channel costs are only a part of the equation in today's 10Gb/s world. In order to properly evaluate the benefits of each 10Gb/s copper cabling option, it is important to understand all the facts. This paper will provide current facts regarding 10Gb/s channels, pathways, labor and performance.

The systems supported in the 10GBASE-T 802.3an standard are:
Limited distance category 6 (designed for existing installations)
Category 6A (shielded and unshielded)
Category 7 (fully shielded only)
Understanding the feasibility of different types of 10Gb/s capable copper systems requires a brief explanation of alien crosstalk. Alien crosstalk (AXT) is cable to cable noise that is seen at higher frequencies and is more problematic in unshielded (UTP) systems. The noise coupling occurs on like pairs (for instance blue-white to blue-white) as they will have roughly the same twist lay. This is not a concern with shielded systems as the shield prevents alien crosstalk from affecting adjacent cables.

Existing category 6 systems are the most costly for 10Gb/s due to the amount of mitigation practices necessary to support 10Gb/s, which translates into labor dollars. Mitigation techniques for category 6 include unbundling all cables, changing to shielded patchcords, reterminating to 6A hardware, and energizing every other port only, to name a few. Testing these systems requires a significant amount of time because after applying one mitigation technique, you may need to try another and retest, then another and retest, etc. For this reason, it is recommended that new installations utilize a category 6A system or better for 10Gb/s transmissions.

In order to mitigate the effects of alien crosstalk at 10Gb/s, a new cabling category/class was introduced. Designated as Augmented category 6 or category 6A by the TIA and,as class EA within ISO/IEC, the standard applies to screened F/UTP systems as well as UTP. Category 7/Class F was accepted without modification and in fact for years was the only standardized 100 meter 10 Gigabit capable copper solution.

Among the most immediately noticeable differences between category 6 and 10Gb/s capable category 6A is that the maximum allowable diameter of the category 6A cable has been increased to 0.354" (9.1mm). This increase allows for the separation between the disturbing cable and the cable that is disturbed in UTP configurations. Because F/UTP cable eliminates AXT through the use of an overall foil screen, this additional separation is not required and has typically allowed category 6A F/UTP cable to be smaller than its UTP counterparts. While there are some newer smaller diameter 10Gb/s capable UTP cables on the market, it is still important that they pass the AXT testing parameters in the 6-around-1 configuration. End users should be aware of waiving AXT field testing for UTP systems. This is being done in some cases because AXT field testing can add up to 3.5-4 hours for each 24-port patch panel, but in these cases, the manufacturer and the end-user are taking a risk that they wouldn't take if installing a shielded system.

In order for UTP cables to remain viable as a 10Gb/s option, other factors must be considered. One is pathway fill. Should the time come where the pathway fill is greater than 40%, the cables on the bottom may become crushed and the resulting deformation of the cable will undo the separation benefits. This will certainly affect AXT and other performance parameters. Noise will be introduced back into the system and this is one of the hardest problems to troubleshoot as it is sporadic and intermittent.

Noise, in the form of AXT, occurs not only between cables but in closely placed connectors. In order to mitigate AXT at the patch panel, various methods are used such as increased spacing and varying punch down practices, the latter of which may increase installation time and certainly increases the potential for errors. In some installations, one end of the cable is wired differently than the other, further complicating installation. The same AXT protection offered by shielded cable is extended to shielded connectors, eliminating the need for varied termination practices.

Those who argue against shielded systems state the additional termination time and cost of grounding/bonding as an additional cost. The fact is that you must have a grounding and bonding (earthing) system for every network, even if it is 100% fiber. The only incremental, albeit small, is the cost to install one piece of #6 AWG wire per 24 port patch panel that attaches to the equipment rack which should already be attached to the existing grounding system. While a screened or shielded connector may take slightly longer to terminate than a UTP connector, this is not enough to impact the savings and performance benefits realized.