You have likely heard copper network cabling referred to as balanced twisted-pair copper cabling, and maybe you’ve 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?

1. 101 Series: Understanding Copper Cabling’s Balancing Act
Author – Mark Mullins
05-09-2017 1www.flukenetworks.com| 2006-2017 Fluke Corporation

2. 101 Series: Understanding Copper Cabling’s Balancing Act
05-09-2017 2
You have likely heard copper network cabling referred to as balanced twisted-pair copper cabling,
and maybe you’ve 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?
www.flukenetworks.com| 2006-2017 Fluke Corporation
It’s 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.

3. 101 Series: Understanding Copper Cabling’s Balancing Act
05-09-2017 3www.flukenetworks.com| 2006-2017 Fluke Corporation
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 – it’s 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.

4. 05-09-2017 4
The PoE Conundrum
So now that you hopefully understand the difference between common (unbalanced) and
differential (balanced) mode, let’s 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.
www.flukenetworks.com| 2006-2017 Fluke Corporation
101 Series: Understanding Copper Cabling’s Balancing Act

5. 05-09-2017 5
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.
www.flukenetworks.com| 2006-2017 Fluke Corporation
101 Series: Understanding Copper Cabling’s Balancing Act

6. 05-09-2017 6
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.
www.flukenetworks.com| 2006-2017 Fluke Corporation
101 Series: Understanding Copper Cabling’s Balancing Act

7. Thank You!
05-09-2017 7www.flukenetworks.com| 2006-2017 Fluke Corporation