Security Business

MAY 2019

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32 Security Business / / May 2019 Tech Trends BY RAY COULOMBE Power Play The new IEEE 802.3bt-2018 amendment aims to solve the problem of higher power and more efficient Power over Ethernet delivery On Sept. 27, 2018, IEEE formally approved IEEE 802.3bt-2018 and published it as an amendment to the 802.3 standard in January. Titled Physical Layer and Management Parameters for Power over Ethernet over four pairs, this amendment “represents a substantial change to the capabilities of Ethernet with standardized power,” according to IEEE’s abstract. “The power classification information exchanged during negotiation is extended to allow meaningful power management capability…These enhancements solve the problem of higher power and more efficient standardized Power over Ethernet (PoE) delivery systems.” For me, the surprise was that the IEEE did not stop at the 60 W High PoE (PoE++) level but went for the gold by taking it 50-percent higher. This may help to explain the length of time to approve the amendment. The enabler for this was bringing all four pairs of a data cable into play. Understanding PoE Classifications How much power can PoE deliver? Depending on what you read, you may see 90W or 100W. Not finding a definitive answer through ordinary searches (Wikipedia was about the best), I purchased the standard and found that there is a real risk of oversimplifying. First, some terminology and definitions. PSE stands for Power Sourcing Equipment which, as the name implies, is the device (switch or mid-span) sourcing the power over Ethernet cabling (or to a conversion device) to the equipment using the power, known as a PD, which stands for Powered Device. What is available at the PD is less than the level of power sourced due to losses in the cabling. Both PSEs and PDs are characterized by four “types” and eight “classes.” Type 1 corresponds to the original PoE – IEEE 802.3af – providing power up to 12.95W at the PD. It encompasses four power classes (0-3); and PSE voltage ranges from 44.0 to 57.0V. Type 2, known as PoE+, was first implemented in 802.3at – adding a power Class 4 up to 25.5W at the PD; and PSE voltage from 50.0 to 57.0V. Type 3, referred to as PoE++ or High PoE, provides up to 51W at the PD. This is now defined in the new amendment – it covers power classes 5 and 6, and PSE voltage remains from 50.0 to 57.0V. Type 4 provides up to 71W at the PD and encompasses power classes 7 and 8. Note that the PSE has to provide a minimum of 90W at its physical interface and no more than 99.9W; thus, this is where the 90W/100W applies, but the average power at the PD over a sliding one-second interval cannot exceed the PD levels mentioned above. PSE voltage remains from 50.0 to 57.0V, which calculates to up to 1.7 amp total current supplied from the PSA over four pairs. Several other significant changes were implemented, including: • Four-pair operation – Type 3 operation allows voltage supply over data pairs, spare pairs, or all four pairs. Type 4 operation is four-pair. • Multiple signatures – A single-signature PD shares the same detection signature and classification signature between both pair sets. A detection signature is based on the PSE sending initial voltage to the PD’s 25 K ohm resistor to determine a valid PoE device and begin a classification process. Classification signatures are based on two subsequent “probing events” to establish a class for the device’s power needs. A dual-signature PD has an independent signature on each pair

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