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Chronic Cat 6 Concerns

One of the measuring sticks that has historically been used to compare contractor craftsmanship is coming back to haunt us.


April 1, 2004  


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I’m going to go out on a limb and declare that Category 6 is dead. Though shocking, this statement should not be surprising.

Most of you will remember that its development was a long and arduous process. In fact, an addendum was already in the works as the base Cat 6 standard was going to print.

Before going any further I should qualify my first comment. Last year, the commercial building cabling standards body was able to convince IEEE that there is a need for 10Gb/s Ethernet over UTP cable.

Some have been led to believe that you need Cat 6 to support 1000Base-T, yet all the while IEEE clearly identified in the standard that only Category 5e was required.

Others have been sold on the concept of installing Cat 6 cabling because the additional headroom improves network efficiency as fewer re-transmission packets are requested or sent due to greater immunity to noise.

That sounds good, however, if you only test the Permanent Link, the channel specifications are a little loose in that they can hide a questionable link.

Vendor opposition

When the Permanent Link is tested it is done according to the standards and a ‘pass’ or ‘fail’ is issued accordingly. However, if your intention was to get the additional benefits of the improved transmission characteristics (not just PSACR) how do you prove you received your money’s worth?

I have asked several tester man-ufacturers if you can insert the link specifications of the purchased systeminto the tester and establish a higher ‘pass/fail’ requirement and each indicated that could be done.

I did, however, receive tremendous opposition from them. Their concerns were mostly about “which specifications would you use?” To which I replied “well your published ones of course.” “You can’t do that,” they each exclaimed, because lab conditions are different from field conditions.”

Giving them that point I then said “so what numbers can we use in order to ensure our clients, receive all the benefits of your Cat 6 cabling system?” I am still waiting for an answer.

The Cat 6 transmission specifications as we know them today are only capable of supporting 10Gb/s Ethernet 55 m. I should point out that 10Gb/s Ethernet needs all four pairs for transmit and receive.

That in itself is not a show stopper since it’s how we dress the cables in the ceiling space that limit’s the channel’s ability to support 10G Ethernet.

When the cables are neatly bundled together for a distance greater than 10 m the crosstalk from one to cable to another is having an affect at the higher frequencies, hence the 55 m limitation. This interference is called Alien Crosstalk.

10Gb/s Ethernet will use a protocol that extends the usable bandwidth of Cat 6 cabling up to about 500 MHz by canceling out the crosstalk within the cable.

This leaves the crosstalk between cables as the dominant noise source. Even though Alien Crosstalk is relatively low, it cannot be easily cancelled by the electronics and limits the reach to 55 m in the worst case.

Cables with lower Insertion Loss or improved Alien Crosstalk performance can extend this distance further. It is a subject the cabling standards committees are currently reviewing.

It’s kind of funny to learn that one of the measuring sticks we historically used to compare contractor craftsmanship is coming back to haunt us.

As some of you may already be aware, several cabling manufacturers are marketing cables that exceed the current specifications and are robust enough to meet the new ones when published.

The problem is that it’s going to take the standards committee at least a year to complete the addendum and go for industry ballot. Optimistically, the addendum will be published at the end of 2006 or early 2007, which means it will be at least two years before the revised specifications are released.

What amuses me most is that back in the early days of the Category 6 standard development we actually had two grades of cable — 6a and 6b. Even though the Cat 6b specifications were by far greater than those of 6a, it fell by the wayside.

I wouldn’t be surprised if the enhanced Category 6 specifications didn’t end up looking like those of 6b.

If that’s the case you know what the topic of my next View from the Board column will be.

Until then, my advice is to install the best cable you can afford and make sure they aren’t being bundled together.

Mark Maloney, RCDD, is a Senior Consultant with Ehvert Technology Services in Toronto and a member of CNS Magazine’s Editorial Advisory Board.


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