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The key to successful PoE implementation
Power over Ethernet (PoE) technology has revolutionized the way network devices are powered and connected. It enables power and data to be transmitted over a single Ethernet cable, reducing the need for separate power and data outlets near the devices. This technology has made it possible for devices such as cameras, phones, and wireless access points to be powered remotely from the floor distributor.
One of the so far mostly overlooked challenges of PoE technology is the issue of DC resistance unbalance. In a PoE system, 50V DC is transmitted as phantom power over the two wires of selected pairs in an Ethernet cable. I.e. the DC power supply voltage will be fed into the center tap of the signal transformers and the current is traveling equally on the two wires of the pair. If the two DC currents in the pair are equal, the magnetic flux created in the respective coils of the signal transformers cancel each other out, resulting in no net magnetic flux in the balun.
However, if there is an unbalance in the resistance between the wires in a pair, the currents on the two wires will not split equal. Are the two DC currents not the same, the magnetic flux generated by each current in its part of the coil will not cancel each other out. One flux will dominate and may saturate the ferrites of the transformer. This leads to increased attenuation loss and the system won’t be able to transmit data signals anymore, resulting in interruptions in the data communication.
Consequently, when using remote power supply (PoE), reducing resistance unbalance is an absolute precondition. It is therefore vital to control the resistance unbalance throughout the complete channel – including any patch cords.
Different resistance measurements in a wire pair
DC Resistance Unbalance expresses, how much the DC resistances of the two conductors of a pair differ from each other. This is not the same as the “Resistance” or “DC Loop Resistance”
commonly seen in usual field measurements. The figure explains the difference between the two measurements.
Unbalance resistance in a cable pair, may be caused by substandard cable and connector manufacturing, poor installation practices or unreliable termination technology that degrades over time.
Cable and connector manufacturing mistakes that would result in resistance unbalance, are typically resulting in fails in other parameters as well. They are most likely recognized in normal quality assurance measurements and should therefore not reach the market in the first place.
Poor installation practice unfortunately could occur any time. Properly trained installers (QPP partners) however know how to handle the products properly. Visual inspections mostly allow to evaluate, whether an installation has been done properly. Generally, if increased DCRU is recognized it is good practice to re-terminate the affected connectivity to eliminate the possibility of termination mistakes.
The last reason, an unreliable termination, unfortunately is very hard to find. If an unreliable wire termination ages, it is very likely that the uncoordinated increase in resistance will cause DCRU. Very often, resistance unbalance develops over time and is the first indication of something wrong. Transmission parameters often remain within acceptable limits, while resistance unbalance already reaches prohibiting levels. With unreliable termination technology resistance unbalance can occur unpredictable and suddenly in a previously properly working link. To prevent time consuming and annoying search for the cause of transmission failures, the use of cabling components with reliable and stable long term termination quality is the best way to prevent this kind of problems.