Cat 6 has a need for more controlled terminations, which didn't exist with those
friendly, rosy Cat 5 and Cat 5e installation standards of
days past. Often, varying degrees of technique would
lead to varying degrees of performance, but sufficient
headroom in the standards ensured an adequate buffer
for a bit of ham-fistedness. But now with Cat 6 standards, it's critical
that the cabling system of any high-speed network be capable of minimising
or eliminating inconsistent installation methods, which cause problematic (and thus,
expensive) performance variations. So, you're not allowed to screw up. Not even a bit.
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Each little imperfection along the way won’t necessarily cause a connection to fail a test on its own. But each one eats into the limited Cat 6 headroom by a few points of a dB, and often more. It only takes an accumulation of these little things and the link becomes about as much use as an ashtray on a motorbike.
Cable Behaviour
At DC and low frequencies, the current flows through a wire in the usual way. As the frequency gets higher (like the 33 MHz of 100BaseT) the wire starts to radiate or act like an arial and the current only flows in the skin of the wire. Some of the energy is actually travelling as radio waves along the length of the wire and we start to say hello to Mr Crosstalk and his rather unpleasant family..
At 75 Mhz, which is the frequency used for Gigabit Ethernet on UTP cable, more of the energy is on the outside and the potential for unwanted crosstalk increases.
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Once we get to the 250 Mhz limit for Cat 6, the wire is going full-on ballistic and acts just like a wave-guide. Most of the electrical energy travels from one end to the other outside the conductor as electromagnetic energy. Also quite a lot of it travels within the plastic of the insulation, so the sheathings and ties are no longer just mechanical devices for protecting and holding the wires together, they are now an integral part of the dielectric of the cable. The sheath becomes a significant factor in the cable’s impedance make-up. Blimey!
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At these frequencies, any kinking, squashing, stretching or using it to strangle clients has a proportional effect on impedance and a consequent knock-on effect on transmission performance. It shags your cable, big-time.
Termination Problems
Cut back the sheathing a bit too far on your cat 5e? Bollocks, who cares. Just shove it in there and watch it work anyway. Yeah, I can make network cable, just watch me. However, at Cat 6 frequencies, we said the wave energy is actually flowing through the sheath. The sheath is a dielectric and cutting it away changes the impedance of the pairs, causing echoes (return loss) and upsetting the geometry affecting the crosstalk performance. Each manufacturer has different recommendations, but a good yardstick is not more than 3mm from the jack.
Ah yes, it's all about wire routing these days Egor, not just 'pair untwist' you uncouth bastard. Tiny details like the routing of each wire at the back of the jack are critical. And so, and this is gonna kill you, is the order in which the crimping punch-downs are done. A wire can easily be damaged during termination and/or affect another wire. Man, and if it wasn't hard enough already..
Another issue is too much untwist - just like Cat 5. However, I've found some references which suggest that with Cat 6, up to 2dB of NEXT (whatever that is) can be saved by maintaining the twist up to the point of termination. If the lay of the wires doesn’t suit this, an extra half twist should be added.
Hmmm..it's probably not a bad idea to have a play with this Cat6 stuff first, before you go on site and make yourself look like a prat in front of the client.
Horizontal Cabling
Ah, there's lots of potential for screw-ups in this area! The internal geometry of Cat 6 cables is so sensitive that you are talking about several dB’s worth of loss and probably looking at total replacement of any cable that suffers even light damage. A typical installation example is kinked cables. I know. Happens all the time. The installer sheepishly straightens out the livid buckling in the cable afterwards, hoping nobody noticed, but it’s too late! The plastic cross-shaped separator inside the cable has deformed and will never return to the right shape, no matter how much prodding and caressing, so you've gotta replace the whole frickin' thing.
Naturally with this new and advanced technology, things can and do get even worse.
Some of it seems counter-intuitive at first. For example, when you're trying to do
something normal and seemingly harmless, like using cable ties. Nylon cable
ties should only be used as a cable retainer at the patch panel and outlet
jack, in accordance with the manufacturer’s instructions. You must never
use the nylon type on Cat 6 horizontal runs, as they pinch and cause
local impedance to increase. Yes, they do. Recommendations are to
only use ‘hook and eye’ types like Velcro, but use even them sparingly.
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If you are running cables in a tray, tie only where it's absolutely necessary. And don’t run those cables in nice neat bundles either, because the more the cables are parallel, the worse will be the alien crosstalk effects! Yes folks, neat is bad, which will come as a relief to some of the people I know. Also, if you need to use cable ties, don’t put them at regular spacings - the impedance effects will cause 'standing waves', which can lead to transmission problems.
There's more. Cable depth on trays or when laid directly onto concrete floors has issues too. The weight of the cable above causes cable ties, little pieces of crap, and irregularities in the pathway surface, to dig into the lower cables. The cross sectional shape and volume affects the rate of temperature build up from the transmission signal energy. This is negligible for a shallow depth of cables and shouldn't be a problem, but 'deep pile' is totally out of the question. Both of these issues effect the impedance characteristics of the cabling.
Other things which can have significant effects on Cat 6 performance are nicks, cuts and grazes in the outer sheathing. In one example from the internet, several cables in one run all failed. Checking them with a TDR (time domain reflectometer) showed a major reflection at the same distance. Which is bad. Turned out that some other shitforbrains tradesman had dropped a piece of metal studding onto a cable bundle and then someone else had trodden on it, just for good measure. The studding was now hidden amongst the nicks in the cabling. Most of the damaged cables needed replacement.
When installing the stuff (which you won't of course be doing, unless you have the right ticket), lay-in the Cat 6 cable in preference to pulling it in. If you’re forced to pull-in, be very careful around corners. It’s a pain in the arse at the time, but it’s even worse if you have to go back and replace the cables when none of them work afterwards.
Pre-fabrication or partial pre-fabrication comes into its own with Cat 6, since you can draw the cables off the drums straight onto the bench and you can terminate them in good, clean, light, spider-free, sandwich-munching conditions, away from neanderthal tradesmen.
The Patch Panel
All the previous points made about termination problems apply here too. One main point is how to best dress the cables into the cabinet/frame. You can use as many Velcro type ties as you need, to install the vertical runs. Then, when all the cables are in place, remove any that are not absolutely necessary to support the Cat 6 cables. And on the horizontal runs, keep them to a bare minimum and don’t tighten them. Ever. Not even a little bit.
The Outlets
To finish on a lighter note after all this doom and gloom, a Cat 6 modular plug can be plugged into Cat 5/e RJ45 Jacks. The physical dimensions of Cat 6 RJ45 plugs and jacks are identical to Cat 5/e plugs and jacks. Indeed, they are backward compatible with Cat 3, Cat 5, and Cat 5e plugs and jacks. Splendid.
A final word of caution. There are examples of failures on outlets which worked fine after being wired up, before some gorilla shoved the Cat 6 cable back into the wall box. Think what’s going to happen to the cables as you push them back. If they are going to kink, crush or over bend, then you’re going to have problems, aren't you Egor.
So in summary, if you do decide to use Cat 6, treat the stuff like it's made of exploding glass. Cat 5e is said to support Gigabit Ethernet, so that's where I'm going. Watch this space for more on this intriguing new technology..
- DK/AM
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I have just re-wired all my LAN connections in Office with CAT6, replacing CAT5E. Now, When I connect a PC with Windows XP, it works fine. But if I connect Windows 7, it doesn't connect. Physical Connection is present. Hence, I tried to reduce speed to Half Duplex 10Mb and now it works. But performance is poor. The main reason why i bought CAT6 is because of performance. Now i am stuck with performance crappier than CAT5E.
Anyone can help??
Try Mike's suggestion here:
http://social.answers.microsoft.com/Forums/en-US/w7network/thread/051271a6-838b-44cf-8e35-dca9e18ec444