DC Resistance Unbalance Testing for PoE Desktop Computing | Fluke Networks
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The Next Big Step for PoE

Mark Mullins

The much-anticipated higher levels of PoE have been quite the buzz in the industry. Now that the 802.3bt standard for PoE that includes Type 3 and Type 4 PoE is expected to be ratified early next year, we will start to see more devices forego their AC power connection for PoE.

And there’s one device in particular that has long been the most important item in every office and on every desk – the computer. That’s right, PoE computing is the next big step for PoE. And we’re not talking about thin clients or tablets – we’re talking about actual desktop computers around which offices have long designed their AC power infrastructure with receptacles at every desk and every cubicle.

DC Resistance Unbalance in PoE Desktop Computing

What Are the Power Levels?

In the upcoming 802.3bt standard, Type 3 PoE will deliver a maximum of 60W of DC power, with 51W available for the device, and Type 4 will deliver a maximum of 90W, with 71W available for the device. While the current Type 2 PoE (IEEE 802.3at) that delivers a maximum of 30W, with 25.5W available for the device, gave rise to some PoE computing capabilities, these have been limited to very small devices with restricted processing power and limited applications.

The higher power offered by Type 3 and Type 4 PoE, combined with the fact that computer processors have become more powerful while using far less energy, means that that we now have actual desktop computers able to be powered via PoE. And the advancement of LED screen technology means that these computers can feature large (20-inch or greater) high-definition screens. They may be touch screen or use common peripherals such as keyboards and mice, and they are powerful enough to run the latest version of Windows® and the full Microsoft® Office suite in conjunction with high-definition video. And PoE computers are far more energy efficient, consuming about half the power of an equivalent desktop computer.

Already there are desktop computers available that run on 60W PoE via Cisco’s UPOE, and when Type 3 and Type 4 are ratified next year, we will see even faster and more powerful PoE desktop computers hitting the market. In fact, the 90W of PoE that will be delivered by Type 4 will likely give rise to multi-screen PoE computing systems and larger screens that could reach upwards of 46 inches.

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Over All Four Pairs

Type 3 and Type 4 PoE used for desktop computers deliver power over all four pairs simultaneously with data. This is achieved by applying common-mode voltage that evenly splits the current between each conductor in the pairs. For the power to be evenly split on the pair, the DC resistance of each conductor must be equal, or balanced. Any difference in resistance between two conductors is referred to as DC resistance unbalance, and it’s not a good thing.

While PoE devices can tolerate some DC resistance unbalance, too much unbalance causes saturation of the transformer, which can cause Ethernet data signals to become distorted. This is pretty big deal for desktop computers. And in four-pair Type 3 and Type 4 PoE systems, it’s not just the DC resistance unbalance on each pair that matters – excessive DC resistance unbalance between multiple pairs can cause PoE to stop functioning.

While too much DC resistance unbalance can be avoided by choosing quality cable from reputable manufacturers and by ensuring quality workmanship with consistent terminations, it’s a good idea to test for it as it could have a huge impact on the performance of PoE desktop computing. Industry standards specify a maximum DC resistance unbalance of 3% between conductors in a pair, and the upcoming IEEE 802.3bt standard also now requires the DC resistance unbalance between any two pairs to be no more than 7% of the total parallel resistance of the two pairs.

With the advent of PoE desktop computing comes huge benefits – safer low-voltage connections, cost and energy savings, faster deployments, increased flexibility and the ability for network UPS systems to keep computers running in the event of a power outage. And the good news is that you can make sure these PoE desktop computers will function by using the DSX CableAnalyzer™ series of copper cable certification testers that have the ability to quickly test DC resistance unbalance within a pair and DC resistance unbalance between pairs.