Alien crosstalk still a threat to data centers

As cabling becomes more densely packed, the threat of electromagnetic disruption caused by alien crosstalk is getting worse, despite new cabling solutions designed to keep it out of the data center.

With the release of augmented Category 6 (Cat 6a), and Category 7 (Cat 7) cabling, some IT pros might be lulled into believing that alien crosstalk (AXT) is about to become extinct. But according to industry experts, the reality is that many shops haven't upgraded to the new cabling, and for them it's worse than ever.

For server farmers, alien crosstalk is the ghost in the machine, the one with the comical name that carries a threat nobody thinks is funny. Left unchecked, alien crosstalk can cripple your network, leaving you grappling with downtime headaches and the looming misery of a massive cable retrofitting.

AXT, in plain English, is electromagnetic noise that can occur in a cable that runs alongside one or more other signal-carrying cables. It has been dubbed "alien" because this form of crosstalk -- a disturbance caused by the electric or magnetic fields of one telecommunication signal affecting a signal in an adjacent circuit -- occurs between different cables in a group or bundle, rather than between individual wires or circuits within a single cable.

Because it resembles noise rather than signals, alien crosstalk degrades the performance of a communications system by reducing the signal-to-noise ratio (S/N), the measure of a signal's strength relative to the background noise. Alien crosstalk is a much bigger headache than traditional crosstalk because, unlike the simple crosstalk caused by single interfering signal, it cannot be eliminated by phase cancellation, a method for stabilizing signals.

Recent strides in Cat 6a and Cat 7 cabling, which provide shields around pairs of cables, have given data centers a solution that severely reduces, and in many cases completely eliminates, AXT. But for server farms that still rely on old-school, lower-grade cabling software such as Cat 5 that don't provide adequate protection from alien crosstalk, the issue is real and exacerbated because more than ever, data centers are trying to cram more stuff into a footprint it outgrew years ago.

Cabling expert Carrie Higbie, global network applications market manager for Watertown, Conn.-based Siemon Co., agreed.

"It can shut [your data center] down," Higbie said. "If you get too much crosstalk, [it can either] negotiate to a lower speed or you get so many bit errors that it is effectively not working."

According to Tom Condon, senior consultant for Chicago-based, System Development Integration, the most common symptom of AXT is a degradation of capacity in the network system. Condon said as packets -- the units of data that are routed between an origin and a destination -- go through your cables, they create interference from other sources, or crosstalk. Should a packet misfire, you have to resend, and you may ultimately end up with a bunch of junk traffic on that cable, while the actual throughput is very small.

But Condon said alien crosstalk is a problem that can be prevented.

"[Alien] crosstalk is a performance issue. As long as you have the right kind of cables and they are shielded, it won't be an issue -- when you lay the cable, as long as they're put in places so they're not next to high voltage electrical lines or unshielded radio transmitters -- it should be fine," Condon said.

Another reason so many data centers are grappling with AXT is carelessness. Alien crosstalk can be rather easily minimized, or even eliminated, by avoiding bundling cables together and by not running them parallel within close range. And even if you are forced to run cables parallel to each other, cables can be surrounded by an electromagnetic shield to prevent electronic fields from entering or leaving the cable, isolating them from each other.

Tony Lock, chief analyst for U.K.-based Bloor Research, said while Cat 5 cabling is still a prevalent solution in many data centers, alien crosstalk isn't a problem unless IT folks push the boundaries of what their cabling and their network can do. But if AXT does infiltrate their system, the ever-increasing demands placed on data centers can make it more dangerous than ever.

"If it's bad enough, there is a noticeable degradation of service," Lock said. "These days there are more and more real-time systems, and interlinking between different platforms. … You want to ensure the service works as quickly as possible, and if that's an issue and you don't want degradation."

According to Higbie, the best way to ensure a proper end result is to be cognizant of AXT's dangers from the get-go, and avoid overrunning your data center with cabling, because it creates a noise field with the potential to put your server farm on ice.

"The lousier your cable, and the lousier your install is, the worse it is," Higbie said. "The more tightly you have them bundled, the worse the problem is going to get."

Let us know what you think about the story; e-mail: Luke Meredith, News Writer

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