Twisted Pair Overview
Twisted pair cabling is the dominant choice for today's LANs.
At the Category 5e performance level or above, there are an
overwhelming number of options. Let's first identify, compare
and contrast these
cabling alternatives.
At the most basic level, installers use continuity checkers
to verify end-to-end connections. These testers provide a
full wire map, measure length with a TDR, and provide additional
useful functions. This kind of tool is great for testing voice
cabling, making quick checks on data cabling, and for the
growing residential LAN wiring market.
Today's cable standards for this kind of cabling requires
several measurements to be made in order to certify that the
cabling meets stated performance requirements. Some tests
are performed worldwide, while others are specific to the
U.S. or Europe. Each of these standards has unique pass/fail
limits, which vary depending upon the category and link definition.
All standards require that installed links pass three tests.
The first is called wire map. Wiremap verifies end-to-end
pin-to-pin connectivity, and checks for split pairs. Any miswires,
breaks, opens, shorts, crossovers or splits should be detected.
Another key measurement used to qualify LAN cabling is attenuation.
Every electromagnetic signal loses strength as it propagates
away from its source, and LAN signals are no exception. Attenuation
increases with temperature and frequency. Higher frequency
signals are attenuated much more than low frequency signals.
This is one reason why a cable may have correct pin-to-pin
continuity, pass low speed traffic like 10BASE-T perfectly,
yet not be able to handle 100BASE-T. With Category 5 copper
cabling, attenuation is remarkably consistent from manufacturer
to manufacturer. Cables that have attenuation performance
much better than standard Category 5 numbers usually have
increased copper diameter or slightly higher impedance.
The most important test in qualifying the performance of
network cabling is Near End Crosstalk (NEXT). Crosstalk occurs
when signals from one pair of wires radiate and are picked
up by an adjacent pair of wires. Crosstalk increases with
frequency, so that just as for attenuation, a category 3 cable
may be fine for 10BASE-T, but can't handle 100BASE-T.
Keeping the pairs tightly twisted and well-balanced is the
best way to minimize crosstalk. This tight twisting causes
opposing electromagnetic fields to more effectively cancel
each other, thus reducing emissions from the pair. Category
5e cable is much more tightly and consistently twisted than
category 3 cable, and uses better insulation materials, which
further reduce crosstalk and attenuation. EIA/TIA 568B requires
that all UTP terminations be properly twisted to within 0.5
inches of all connections.
Standards also require that length be measured. Length measurement
may seem straightforward but it can actually be tricky. A
basic link or permanent link should not exceed 90 meters and
a channel should not exceed 100 meters. The accuracy of a
length measurement is affected by several factors, including
nominal velocity of propagation (NVP) of the cable; twist
length vs. physical sheath length, and impulse dispersion
over long lengths.
When you are measuring length with a field test tool, you
are usually measuring time delay, and converting it to a length
estimate based upon an assumption of signal speed.
Nominal Velocity of Propagation (NVP) refers to how quickly
signals travel in a cable. It is expressed as a percentage
of the speed of light. Incorrectly set NVP is a very common
error. If your NVP is set for 75% and the actual cable's NVP
is 65%, that's a 10% error from the start. Furthermore, NVP
is unique to each pair and also varies with frequency. For
category 3 cables and hybrid category 5 cables, NVP can vary
by up to 12% between pairs.
In addition, the copper conductors in UTP are twisted, so
the actual length of the wire is longer than the length of
the cable jacket. On a 1000-foot spool, you could easily have
1020 feet of copper.
It's for all of these reasons that you should only consider
length results from hand held testers to be good approximations
and not precise values.
Attenuation to Crosstalk Ratio (ACR) is a measurement that
determines the effective signal-to-noise ratio of a cabling
link. ACR is simply the difference between the NEXT and the
attenuation. It is a measure of the strength of the signal
that survives attenuation from the far end relative to crosstalk
noise. For example, imagine an instructor standing in front
of a room giving a lecture. The goal of the instructor is
to be heard by the students. The volume of the instructor's
voice is a key factor in determining this, but it isn't as
important as the difference between the instructor's voice
and the background noise. The instructor could be speaking
in a very quiet library, so that even a whisper could be heard.
But imagine that same instructor, speaking at the same volume,
at a noisy football game. The instructor would have to raise
his voice, so that the difference between his voice (the desired
signal) and the cheering crowd (the background noise) is enough
for him to be heard. That's ACR.
Emerging technologies require these new measurements. Return
loss is the ratio, expressed in decibels, of the fractional
amount of signal reflection caused by an impedance mismatch.
Return loss is increasingly important when trying to get premium
performance from UTP. Manufacturers of very high quality UTP
have taken special care to ensure impedance is very uniform
throughout the link, and also that all components are very
well matched. So while return loss wasn't a big issue when
Category 5 cabling first appeared, it is an important differentiator
for Cat 5E and Cat 6 cabling.
Power-sum NEXT (PSNEXT) is actually a calculation, not a
measurement. PSNEXT is derived from an algebraic summation
of the individual NEXT effects on each pair by the other three
pairs. PSNEXT and FEXT (discussed below) are important measurements
for qualifying cabling intended to support 4 pair transmission
schemes such as Gigabit Ethernet.
Far End Crosstalk (FEXT) is similar to NEXT, except the signal
is sent from the local end and crosstalk measured at the far
end.
FEXT by itself is not a useful measurement. This is because
FEXT is highly influenced by the length of the cable, since
the signal strength inducing the crosstalk is affected by
how much it has been attenuated from its source. For this
reason, Equal Level FEXT or ELFEXT is measured instead. ELFEXT
simply subtracts attenuation from the result, so that the
result is normalized for attenuation (length) effects. Then,
just to make things interesting, we also have power-sum ELFEXT,
or PSELFEXT.
Category 6 testing requires three things you don't have in
your current Level II tester:
1. Support to accurately
make all the new measurements
2. Dynamic range to measure
FEXT and return loss at Category 6
levels
correctly
3. 250 MHz of bandwidth
For this reason many installers are in the process of replacing
their field testers with new products designed to fully support
Category 5/5e/6 requirements.
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