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 committee 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 for 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 ranges 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 set; allowing each pair set to have fully independent classification and power allocations.
- Power disconnect – A PSE may disconnect power from a PD if no usage has been detected for a period of 320 to 400 ms.
The main reason for a class system is to enable power requirements to be negotiated for better management of the PSE’s power budget. A summary of power classes is illustrated in the nearby chart.
Installation Challenges
One of the potential installation concerns is the potential heat build-up in a cable bundle as higher currents are pushed through the conductors. NFPA 70 (the National Electrical Code) in 2017 added section 725.144, Transmission of Power and Data, that for Class 2 and Class 3 circuits, details the maximum-allowable ampacity of cables based on the size of the individual conductors and the number of cables in a bundle.
Further, Section 840.160 requires compliance with section 725.144 for PoE installations. A new Section 840.170 provides listing requirements for PoE power sources.
Also, higher-current PoE systems may affect the conducting surfaces on RJ-45 connectors via electrical arcing during mating/demating. Arcing may lead to corrosion and pitting damage on the plated contact surface. Leviton has a good white paper on this topic at www.leviton.com/poe (look for the Connectivity for PoE article at the bottom) that recommends using connectors designed to minimize arcing effects, connectors and patch cords with 50 µm gold-plated tines (as specified by ANSI/TIA-1096-A and ANSI/TIA-568-C.2 standards), and metal connector bodies for better heat dissipation.
Additionally, do not forget back-up power for PSE equipment to keep everything running, or consider local device power in the event of PoE failure. UPS systems will need to be rethought with these increasing demands.
Electricians to Take Security Business?
At the recent PSA-TEC, I shared a panel with NSCA Executive Director Chuck Wilson, who pointed out a battle in some states around who installs higher power PoE circuits. Some have begun to think that it is the province of electricians vs. low-voltage contractors. As the power needs of some devices have fallen (LED bulbs) while PoE has gone higher, devices and equipment that were once always electrical installs are now squarely in the sights of low-voltage contractors.
That said, the promise of PoE is undeniable, and it will be most interesting to see new PSE and PD equipment introduced to take advantage of it.
Copies of the IEEE 802.3bt-2018 amendment may be purchased through IEEE/Techstreet at www.techstreet.com/ieee.
Ray Coulombe is Founder and Managing Director of SecuritySpecifiers and the CONSULT Technical Security Symposium. He can be reached at [email protected], through LinkedIn at www.linkedin.com/in/raycoulombe or followed on Twitter, @RayCoulombe.
