CABLING
By Mark Mullins, Global
Communications Manager and
Manager of Americas Field
Marketing, Fluke Networks
www.flukenetworks.com
When it comes to cabling what looks neat and tidy
may not actually be the best for data
transmission and performance
While neatly combed bundles of cables might look really
nice in exposed overhead cable trays and in racks and
cabinets, this aesthetically pleasing deployment isn’t
always a thing of beauty when it comes to performance.
Sometimes, this can be hard to explain to your customers
who seem to care more about how the cable looks versus
how well their data transmits.
Some things are better kept natural
Whenever Category 6A UTP cables capable of supporting
10 Gig applications are combed or tightly bundled, you
risk increased alien crosstalk (AXT) that can wreak havoc
on high-speed transmission. Why? Alien crosstalk is caused
by interference on a cable pair from a pair in an adjacent
cable, and it became an issue with Category 6A cabling
characterised up to 500 MHz, simply due to the higher
level of frequency and noise. So, it’s easy to understand
that anytime cables are evenly combed or tightly bundled
together so that they lay adjacent to one another, alien
crosstalk will increase; which is precisely why worst-case alien
crosstalk testing involves six around one.
By increasing the level of noise within the channel, alien
crosstalk decreases signal-to-noise ratio and degrades
the bandwidth capabilities of the cable, potentially
preventing the cable from supporting 10 Gig transmission.
This is precisely why standards bodies such as the
Telecommunications Industry Association (TIA), as well as
most cable manufacturers, recommend letting cables have
a ‘natural lay’ in pathways and conduit. But even with this
recommendation, there are still some customers that require
cables to be combined in neat bundles. There are even tools
available for quickly and efficiently straightening, combing
and organising cables into multiple bundles with ease.
Tightly fastening of cable bundles every eight to 12
inches via nylon tie-wraps is also not recommended. Nylon
tie-wraps can place too much pressure, that actually
changes the physical geometry of the cable, deforming
or modifying the twisted pairs and further degrading
performance. If fastening bundles of cables, for example,
within cable managers at the rack, loose Velcro straps are
the recommended method. And besides, Velcro straps can
be more easily removed and replaced during moves, adds
and changes.
From a cable testing standpoint, combed and tightly
bundled cables might exhibit Power Sum Alien Near-End
Crosstalk (PSANEXT) and Power sum attenuation to alien
crosstalk ratio far-end (PSASCRF) levels that either fail
standards performance specifications or leave you with very
little headroom, while too-tight nylon tie-wraps can exhibit
a wide range of performance issues – and there is nothing
beautiful about that at all.
21
It’s not just aliens you should worry about
While increased alien crosstalk caused by tightly bundled
cables is a concern for Category 6A cables in 10 Gig
applications, this isn’t the only issue of concern. If you’re
installing cables that will deliver higher power – Power over
Ethernet (PoE) like 60W Type 3 PoE or higher – bundling
cables can also cause heat rise, and the larger the bundle
the more the cables will heat up. And it’s even more of a
concern with lower category cables like Category 5e and 6.
While other factors can also increase the temperature
within a cable like the overall cable construction and
surrounding ambient temperature, cables in the middle of
the bundle transmitting 60W PoE or higher can’t adequately
dissipate heat. And when a cable’s temperature rises,
so does insertion loss – the loss of signal power that can
prevent proper data transmission.
Since insertion loss is directly related to the length of
a link, which is why there are specific distance limitations
called out in industry standards, TIA standards recommend
reducing the link length (otherwise known as derating)
when installing cables at elevated temperatures. But even
if the ambient temperature isn’t high, cables in the middle
of large bundles carrying higher levels of PoE may heat up
to the point of degrading performance. So once again, it’s
recommended to loosely group cables rather than bundling.
So, what is the lesson here? While tightly bundled cables
might be a thing of beauty to some, beauty is only in the
eye of the beholder. The real beauty is when your cables
perform in a way that results in reliable data transmission.
Now you just need to educate and convince your customers
of the same. n
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