Alien Crosstalk and how to stop it

John Siemon, CTO and VP Operations of cabling specialist Siemon explains shielded cabling and converged cabling

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Alien Crosstalk and how to stop it
By  Georgina Enzer Published  June 25, 2012

John Siemon, CTO and VP Operations of cabling specialist Siemon explains shielded cabling and converged cabling.

Network and cabling solutions provider Siemon is conducting a global tour to educate the marketplace on the benefits of shielded cabling over legacy UTP,  or unshielded twisted pair cabling.

Siemon is also discussing the adoption of 10GBASE-T networks, which is being driven by equipment from Cisco, Intel and HP.

10GBASE-T, or IEEE 802.3an-2006, is a standard released in 2006 to provide 10 Gbit/s connections over unshielded or shielded twisted pair cables, over distances up to 100 metres.

10GBASE-T has latency in the range 2 to 4 microseconds compared to 1 to 12 microseconds on 1000BASE-T. As of 2010 10GBASE-T silicon was available from several manufacturers with claimed power consumption of 3-4 W per port at structure widths of 40nm and with 28nm in development, power will continue to decline.

“10GBASE-T is here. We have been talking about it for a long time, it is poised to take off this year and is fuelled by numerous announcements at the recent Interop conference in Las Vegas. We feel very strongly that even though there is some recognition of Cat6 in operations in 10GBASE-T we think that attempting to run 10G over Cat6 is a mistake. We very strongly believe that shielded cabling is the best solution for any 10GBASE-T network and we are emphasising that point in our seminars.” says John Siemon, CTO and VP Operations for Siemon.

Shielded Cabling
The primary reason shielded cabling is best for 10GBASE-T is that, although the chipset for 10GBASE-T is very advanced in terms of its ability  to cancel out and accommodate internal noise from the twisted pair cable, one major limitation is its tolerance to external noise, the uncorrelated noise from other cables, known as alien crosstalk.

“In addition to providing superior alien crosstalk performance, the shield provides general immunity to external noise for 10GBASE-T networks,” states Siemon.

Siemon says that 10GBASE-T data centres will further the broad-based market adoption and acceptance of shielded twisted pair solutions.

“As far as we are concerned, the cabling of choice for 10GBASE-T should be shielded twisted pair because our models show that a typical 100m, 4-connector Cat6A shielded channel provides 80% higher channel capacity than a comparable Cat6A UTP channel up to 500MHz.Considering the relatively small cost differential between Cat6A UTP and shielded systems, the channel capacity provided by the latter make it the clear winner,” adds Siemon.

One of the limiting factors of 10GBASE-T is alien cross talk, but there are alien crosstalk mitigation methods which provide spatial separation between the cables and create a distancing effect between each of the cables.

“The crosstalk between pairs within a cable is controlled by varying the twist rates, which has enabled twisted-pair to perform well up to the maximum frequencies specified for 6A. Alien crosstalk is dominant between like-coloured pairs when cables with the same basic construction are grouped into a bundle. When bundled, like-coloured pairs, which have the same twist rate couple strongly when they are not separated by a shield. Although Cat6A UTP cables contain design elements that control this type of coupling, they are not as effective as a shield in reducing alien crosstalk,” according to Siemon.

Currently in data centres density is a major consideration. Because space often comes at such a premium, the tendency is to push pathway fill to the limit. In circumstances when pathways are filled to capacity, the cable is subject to significant strain, which effects alien crosstalk performance far more for UTP than it does for shielded cabling.

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