101 Series: Understanding Copper Cabling’s Balancing Act

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101 Series Understanding Copper Cablings
Balancing Act Author Mark Mullins
www.flukenetworks.com 2006-2017 Fluke
Corporation
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101 Series Understanding Copper Cablings
Balancing Act
You have likely heard copper network cabling
referred to as balanced twisted-pair copper
cabling, and maybe youve heard of the testing
parameter referred to as DC resistance unbalance.
But what does all this balance-related
terminology really mean, and why should you care?
Its All About Balance Electrical signals travel
in one of two modes common mode or differential
mode. Differential mode is considered balanced
where the signal travels over two conductors in
reference to each other, essentially travelling
in opposite forces. In contrast, common mode, or
unbalanced, signals travel simultaneously and are
referenced to ground.
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101 Series Understanding Copper Cablings
Balancing Act
Because balanced cables carry signals that
reference each other, they typically feature a
symmetrical design, like the twisted pairs you
see in a network cable. Unbalanced cables include
those where the conductors share a common axis
like coax cable. Note its important to
understand that DC power and electromagnetic
noise from external sources is injected into the
cable in common mode. When common mode noise is
coupled onto two conductors in a perfectly
balanced twisted-pair, the balanced design
cancels out the signal to achieve noise immunity.
However, balanced twisted-pair cables are never
perfectly balanced, which is why we have
performance parameters for balance as specified
in industry standards.
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101 Series Understanding Copper Cablings
Balancing Act
The PoE Conundrum So now that you hopefully
understand the difference between common
(unbalanced) and differential (balanced) mode,
lets look at how this relates to transmitting DC
power over twisted-pair cabling in Power over
Ethernet (PoE) applications. When power is
delivered simultaneously with data in 4-pair
applications, the power is transmitted as a
common-mode voltage that is equally split between
each conductor of the pair. Here is where balance
plays a role again. If the DC resistance of
each conductor in the pair is perfectly equal,
there is no difference in resistance. This is
what allows the common-mode voltage of the power
to be evenly split. Too much difference in
resistance can distort the data signals, causing
errors, retransmits and even non-functioning data
links. Again, achieving perfect balance is
impossible, and that is why we also have
performance parameters for this type of balance
as well.
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101 Series Understanding Copper Cablings
Balancing Act
Disrupting the Balance So what causes all this
imbalance that can adversely impact data signals?
Unbalance can be caused by poor workmanship,
inconsistent terminations and subpar cable
quality. Practices such as ensuring minimum
bend radius and maintaining pair twist as close
to the point of termination as possible are key
to maintaining balance. And when it comes to
preventing DC resistance unbalance in PoE
applications, consistency in individual conductor
terminations is critical. The overall quality
and maintaining proper physical geometry of the
cable is also critical to maintaining balance.
When a poor-quality cable exhibits variations in
the diameter, concentricity (roundness), contour
and smoothness of the copper conductors, there is
a higher risk for unbalance.
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101 Series Understanding Copper Cablings
Balancing Act
Testing for It When it comes to cable
certification for maintaining proper balance to
minimize noise interference, mode conversion is
the concern. This phenomenon happens when a cable
is not balanced and noise injected into the cable
in common mode causes the voltages to be unequal
and degrades network performance. The two
standards-based mode conversion parameters that
indicate balance are Transverse Conversion Loss
(TCL)  and Equal Level Transverse Conversion
Transfer Loss (ELTCTL), which are measured by
injecting a differential mode signal and then
measuring the common mode signal within a pair.
For both parameters, the smaller the common mode
signal, the better the balance. When it comes
testing differences in resistance, DC resistance
unbalance is the concern and industry standards
specify a maximum DC resistance unbalance between
conductors. This verifies that both conductors in
a pair have equal enough resistance to enable the
common-mode current needed to effectively support
PoE and avoid distortion of the data signals
transmitting on the same pair. Thankfully, DSX
CableAnalyzer has ability to measure both
differential and common mode signals, means that
it is capable of verifying both types of balance
through mode conversion testing and dc resistance
unbalance testing. And as data speeds advance,
and more PoE devices deliver power simultaneously
with data, verifying balance is becoming more
important than ever.
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