DC Resistance Unbalance Testing: Easy, Low-Cost Insurance for Your PoE Systems

October 26, 2015

Gigabit Ethernet (1000BASE-T) and Power over Ethernet (PoE) are two widely adopted network technologies. They have proliferated in tandem over the past decade to the point where many enterprises are deploying gigabit Ethernet in the LAN environment and more PoE devices than ever before.

While 10/100BASE-T (i.e., 10 and 100 Mbps) applications required only two cable pairs for transmission, leaving two spare pairs of a four-pair twisted cable available for PoE, gigabit Ethernet requires all four cable pairs for bidirectional data transmission. In this scenario, PoE is delivered over pairs that are simultaneously transmitting data and power.

Understanding PoE and DC Resistance Unbalance

The IEEE standard for PoE was developed to provide low-voltage power to devices over twisted-pair data cabling. Power is provided by a PoE-enabled switch or a mid-span power injector. The power can be used by a wide range of Powered Devices (PD) at the other end, including VoIP phones, wireless access points, sensors, cameras, access control panels and more.

The original IEEE 802.3af standard allows for delivering up to 15.4 W of power over two pairs, while the IEEE 802.3at PoE Plus standard increased the allowed maximum power to 30 W. The latest amendment in development, IEEE 802.3bt™, aims to increase the maximum power to 99.9 W by supplying current on all four pairs.

The IEEE standard specifies two methods to provide power using two pairs of a four-pair data cable—Alternatives A and B. In Alternative B, power is delivered over spare pairs dedicated for power. This is compatible with data transmission methods that only use two pairs, including 10/100BASE-T. In Alternative A, the power is delivered simultaneously with data over the same pairs, which is compatible with both two- and four-pair applications, including 10/100BASE-T and 1000BASE-T.

In Alternative A, current is received and returned using the center tap of a transformer, which splits the current between each conductor of the pair. When the resistance of each wire in the pair is equal, the dc resistance unbalance (the difference in resistance between two conductors) is zero, current is split evenly, and power is available at the Powered Device.

The IEEE standard specifies two methods to provide power using two pairs of a four-pair data cable—Alternatives A and B. In Alternative B, power is delivered over spare pairs dedicated for power

Alternative B takes advantage of the spare pairs in twisted-pair Ethernet cables but is only compatible with 10/100BASE-T applications.

Alternative A is compatible with 10/100/1000BASE-T and transmits power simultaneously with data using the center tap of the transformer.

 

While devices can tolerate some DC resistance unbalance, too much unbalance causes the potential for saturation of the transformer. This can distort the waveform of the Ethernet data signals, causing bit errors, re-transmits and non-functioning data links.

What Causes DC Resistance Unbalance?

DC resistance unbalance can occur in a PoE data link for a variety of reasons. While problems with transformers such as an offset center tap can occur at both PSE and PD, DC resistance unbalance is more often caused by poor workmanship, inconsistent terminations and subpar cable quality.

Consistency in individual conductor terminations is important to prevent DC resistance unbalance. Punching down the individual conductors to the proper IDC tower of a network jack displaces the conductor’s insulation to expose the copper and make the connection. Ensuring proper and consistent seating during this practice isn’t always easy. When two conductors of a pair carrying PoE are terminated inconsistently, DC resistance unbalance can occur.

The overall quality of the cable and connectivity can also impact DC resistance unbalance. Manufacturing quality twisted pair cable requires careful selection of copper conductors and the use of stringent controls to maintain the proper physical geometry of the cable. When a poor quality cable exhibits variations in the diameter of the copper conductors, there is a higher risk for DC resistance unbalance in PoE systems.

Testing for DC Resistance Unbalance

the DSX-5000 measures DC loop resistance as a sum of the resistance of two conductors in a pair while DC resistance unbalance is a measure of the difference in resistance between the two conductors

IEEE Std 802.3™-2012 specifies a maximum DC resistance unbalance of 3% between conductors, meaning that the difference in DC resistance between two conductors is no more than 3% of the total DC loop resistance of a pair. DC resistance unbalance testing verifies that both conductors in a pair have equal resistance and will therefore provide the balanced current needed to effectively support PoE and avoid distortion of the data signals transmitting on the same pair. Unlike other field testers that only test DC loop resistance, the DSX-5000 CableAnalyzer measures both DC loop resistance and DC resistance unbalance.

As shown in Figure 1 below, the DSX-5000 measures DC loop resistance as a sum of the resistance of two conductors in a pair, while DC resistance unbalance is a measure of the difference in resistance between the two conductors.

the DSX-5000 measures DC loop resistance as a sum of the resistance of two conductors in a pair while DC resistance unbalance is a measure of the difference in resistance between the two conductors Figure 1

DC resistance unbalance testing will become an even greater concern as more networks deploy gigabit Ethernet, more PSEs use the Alternative A method of delivering power simultaneously with data, and new powered devices draw more power and current. Don’t let your PoE systems hang in the balance. Requiring DC resistance unbalance testing is an easy, low-cost insurance for today’s PoE systems.

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