ADC KRONE makes Physical Layer Management (PLM)

Karl Tryner
Technical
Services Manager
ADC KRONE
Karl.Tryner
@adckrone.com
Shielded versus Unshielded
The debate re-starts
With the advent of a new level of high
performance network cabling operating at
ultrahigh frequencies and capable of data speeds up
to 10 Gigabit/s, the old debate about whether to use
shielded or unshielded cabling is once again surfacing.
Last time around, with Category 5e and Category 6,
the main concerns were around external interference
from electrical devices and machinery affecting the
performance of the Channel - and emissions from the
data cables interfering with other electronic devices.
Today, with 10 Gigabit/s, the debate centers on Alien
Crosstalk – electromagnetically induced interference
from the data signals in adjacent cables. Karl Tryner,
technical services manager with ADC KRONE unravels
a new twist in the argument.
Twisted pair cabling is used extensively in data and
voice networks to provide the short to medium length
connections (up to 100m) from servers and the like
to switches; and from switches to end users’ PCs,
network printers and other network devices. Twisted
pair cabling has the advantages of relative simplicity
and low-cost Ethernet interfaces in PCs and other
active equipment.
Although structured cabling “categories” are designed
to accommodate the transmission of various so-called
applications like analogue voice, ATM and video, by
far the most common usage is Ethernet - for digital
data, voice over IP and digital video. Over 90% of the
world’s data, in enterprise networks, now uses the
Ethernet standard and indeed, it is now being rapidly
deployed in public networks too.
Twisted pair network cabling consists of four pairs of
wires. Each pair is twisted together to produce what
is called a “balanced pair” with the twist imparting a
degree of immunity from induced noise.
This noise immunity is further improved by the four
pairs being gently twisted together so that conductors
will rarely be parallel to other conductors.
In the case of UTP (unshielded twisted pair) cable
– which currently makes up 95% of the world’s
enterprise data cabling - the four pairs are then
simply enclosed in an insulating sheath usually made
either of PVC or special LSZH (low smoke, zero
halogen) plastic.
An alternative construction is to over-wrap the four
pairs in a metallic foil before the plastic sheath is
applied. This provides an electrical “screen” which,
when earthed, stops external interference from being
introduced into the pairs and also stops the cable from
acting like an aerial and radiating its own signals. These
are referred to by the ISO standards organisation as F/
UTP (overall foil) cables.
In some cases a further level of shielding is achieved
by over-wrapping the four pairs individually with a
metal foil in addition to the overall shield – called
S/FTP cable.
ONE GIGABIT/S
Category 5e and Category 6 are international cabling
standards, with which your cabling installation
should comply if you want to be guaranteed system
operation at up to one Gigabits/s - not both. At the
frequencies involved (100MHz for normal four-pair
Gigabit Ethernet or 250MHz for the rare two-pair
variant) alien crosstalk, the interference between data
cables, is not a major problem. Provided UTP cables are
installed in a loose formation rather than tightly tied
bundles, alien crosstalk is not an issue that would force
design engineers to consider the need for shielded
cables. If electrical noise is a problem, then engineers
would consider the use of a shielded solution or fibre
optics. Other than that, the choice of shielded rather
than unshielded cabling is a matter of user choice and
country-by-country preference. Generally speaking,
for Category 5e/6 shielded cables are thicker than
their UTP counterparts and a little more difficult to
handle and the shielded jack termination process takes
slightly longer than for the unshielded jack. Probably
the main reason, however, that only 5% of the
world’s Category 5e/6 installations currently use
shielded cabling is the simple one of installer
and customer unfamiliarity.
10 GIGABIT/S
With the advent of 10 Gigabit/s Ethernet, and its
need for 500 MHz instead of 100/250 MHz, suddenly
the effects of alien crosstalk between cables became
a major issue. In fact alien crosstalk is one of the
limiting factors in the design and installation of
10 Gigabit/s twisted pair cabling solutions. It is for
this very reason that the shielded versus unshielded
debate has re-opened.
A number of manufacturers and industry experts have
been claiming that the only way to overcome the issue
of alien crosstalk at 10 Gigabit/s is to use a shielded
cabling solution. One cannot argue against the fact
that shielded cabling produces extremely good alien
crosstalk immunity because of its metallic “barrier”.
However, much of this is FUD (fear, uncertainty and
doubt) spread by a number of major manufacturers
who did not have the R&D resources and skills to
advance cable and connector technology to the
level where 10Gigabit/s over UTP with controlled
alien crosstalk was possible. However, UTP solutions
such as our own CopperTen, by using advanced
mechanical design techniques, more than adequately
New
4 ADC KRONE Connecting With Our Customers – Vol.2 No.2 2007