Installation pitfalls in Cat 6 cabling
the advent of Category 6 requirements in cabling, several issues have came to
light from a manufacturing and installation perspective. Dileep Kumar
highlights some of these issues and the precautions to be taken during installation
Due to Category 6 cabling, no longer does the installer
have sufficient buffer to allow for expedient installation. A more stringent
requirement for link and channel testing calls for more stringent installation
practices. This, coupled with the need to revisit terminations when failures
occur, has made for tedious and frustrating installations, with lost time and
money for all involved. The need for more controlled terminations hasnt
existed for Cat 5 or Cat 5e installation standards. Often work practice variation
would amount to some degree of variation within the tested result, but sufficient
headroom ensured adequate buffer for such anomalies. But now with Category 6
standards, it is paramount that the structured cabling system of any high-speed
network must be capable of minimising or eliminating inconsistent installation
methods that cause costly performance variations.
Its also worth pointing out that many of the
items explained below wont necessarily cause a link to fail a test on
its own. But each one eats into the limited Cat 6 headroom by a few points of
a dB, often more. It only takes an accumulation of these little things and the
Before getting in to the installation details, let
us understand how the cable behaves at higher frequencies and the wave guide
effect of wires.
At DC and low frequencies, the current in a wire flows
through the wire as you would expect. As the frequency gets higher, like the
33 MHz of 100BaseT the wire starts to radiate or act like an aerial, 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 become susceptible to alien
At 75 Mhz, the frequency used for Gigabit Ethernet
on UTP cabling, more of the energy is on the outside and the potential for alien
But once we get to the 250 Mhz limit for Cat 6 the
wire is acting just like a wave-guide. Most of the electrical energy travels
from one end to the other outside the conductor as electromagnetic energy. Quite
a lot of it travels within the plastic of the insulation, so these 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 cables impedance make-up.
At these frequencies, any kinking, crushing or elongation
of the cable has a proportional effect on impedance and a consequent knock-on
effect on transmission performance.
Sheathing cut back too far on Cat 5ewho cares?:
But at Cat 6 frequencies, 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 upsets the geometry affecting the crosstalk
performance. Each manufacturer has different recommendations, but insist on
not more than 3mm from the jack.
Wire routing, not just pair untwist: Tiny details like
the routing of each wire at the back of the jack are critical. So, believe it
or not, is the order in which the punch-downs are done. A wire can easily be
damaged during termination and/or influence another. The best order depends
on which end of the cable youve got hold of, and the direction the cable
is leaving the jack.
Too much untwist: You know this one. But with Cat 6
theres quite a bit of field experience now, which indicates that up to
2dB of NEXT can be saved by maintaining the twist up to the point of termination.
If the lay of the wires doesnt suit this, an extra half twist should be
added. Why not make up and test some channels in the workshop with any new jack/cable
combination before you go anywhere near siteit could save you a lot of
Theres loads here that can and does go wrong.
The internal geometry of Category 6 cables is so sensitive that you are talking
about several dBs worth of damageand probably looking at total replacement
of any cable that suffers this fate. Weve seen all these sorts of things
that frequently happen on sitelike kinked cables (the installers
straightened it out, but its too late, the plastic separator inside has
deformed and will never return to the right shape).
And in fact there are many other little things that
affect the dBslike avoiding the prolific use of nylon cable ties. Nylon
cable ties should only be used as a cable retainer at the patch panel and outlet
jack in accordance with the manufacturers instructions. Never use the
nylon type on Cat 6 horizontal runsthey pinch and cause local impedance
to increase. Only use hook and eye (e.g. Velcro) types but use them
sparingly. If you are running cables in tray or basket, tie only where absolutely
necessary. Dont run the cables in nice neat bundlesthe more the
cables are parallel the worse will be the alien crosstalk effects. Were
not talking rats nest, but when it comes to Cat 6, neat is
bad! Also, if you need to use cable ties dont put them at regular spacingsthe
impedance effects will cause standing waves, which can lead to transmission
Cable depth on trays or when laid directly onto concrete
floors has issues too. The weight of the cable above causes cable ties, grit,
and irregularities in the pathway surface to dig into the lower cables. The
cross sectional shape and volume effects the rate of temperature build up from
the transmission signal energy. This is insignificant for a shallow total depth
of cables so it will not be a problem, but deep pile is out of fashion. Both
of these issues effect the impedance characteristics of the cabling.
The other things weve found to have significant
effects on site are nicks, cuts and grazes in the sheath. On one job, several
cables in one run all failed. Checking with a TDR (time domain reflectometer)
all showed a major reflection at the same distance. The problem was that another
trade had dropped a piece of metal studding onto a cable bundle and then someone
had trodden on it. The studding was now hidden amongst the nicked. Most of those
damaged cables needed replacement.
We strongly recommend that you lay-in Cat 6 cable in
preference to pulling it in. If youre forced to pull-in, be very careful
around corners. Its a pain at the time, but its even worse if you
have to go back and replace the cables to get the installation through test.
In fact 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 benchand you can terminate them in good clean, light and controlled
Obviously, all the points we made about termination
problems apply herethe other point is how to best dress the cables into
the cabinet/frame. Use as many ties (Velcro type) as you need to install 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 dont tighten them.
Finally, weve found quite a few failures on links
that worked fine before the cable was shoved back into the wall box or floor-box.
Think whats going to happen to the cables as you push back. If they are
going to kink, crush or over-bend youre going to have problems.The author
is Technical & Product Manager - PremisNET, Krone Communications.
- If youve never done Cat 6 before you can avoid high failure
rates by seeking the manufacturers advice.
- You not only need generic Cat 6 training, you need manufacturer-specific
training on the termination techniques for the particular cable/connector
- Be incredibly kind to the cable.
- Above all, read the instructions. It could save you thousands of
rupees on a job.
He can be reached at firstname.lastname@example.org