Issue link: https://www.e-digitaleditions.com/i/1414667
40 I ICT TODAY Other features implemented in IEEE802.3bt that are important for smart building and PoE lighting applications include: Maintain Power Signature (MPS) Deploying large numbers of PDs leads to considerable standby power. For example, in PoE LED lighting appli- cations, it is critical to reduce the standby power due to the large number of ports in a typical application. Also, while the light is off, the port should stay on and consume as little power as possible. Auto Class Classification is the process where the PSE detects the PD's power requirements by using the physical layer. Type 3 and Type 4 PSEs may choose to implement an extension of the physical layer classification known as auto class. The purpose of auto class is to let the PSE determine the actual maximum power drawn by the connected PD. The PSEs implementing auto class will measure the power consumption of the connected PD throughout a defined period of time; during this time, the PD will consume the maximum power it will ever require. The PSEs can set the maximum power output based on the power drawn during auto class plus margin. Extended Power IEEE802.3bt defines the minimum power that the PSE should source and maximum power that the PD should expect to receive. There is some amount of power that is "saved" for dissipation on the cables in the worst-case scenario of 100-meter (≈328.084 ft) cables. The previous standard, IEEE802.3at, considered the worst case. Fol- lowing that approach means that for a PSE sourcing 90 W output, the PD should expect that only 71 W and 19 W are "saved" for dissipation on the cables in case the PD is deployed 100 meters (≈328.084 ft) from the PSE. Perpetual PoE is an enhancement feature that helps enable PDs (light fixtures) connected on certain ports to continue to receive power during a soft reload of the switch (PSE). However, is that always the case? What if the PSE and PD are in close proximity? If the cable losses are much lower, can less power be saved? The idea of the extended power feature is to allow the PD (or PSE) to use the maximum available power based on the cable's true total resistance. Once the PD measures cable resistance, it can calculate the power that will be lost on the cable and claim the worst-case reserve to be used by the PD. At worst, the power reserve will be the same as in the current standard, but it also can be much lower. Backward Compatibility Although new features were added, higher power is supported, and some algorithms were changed to ensure interoperability. The idea is that the system will work with legacy Type 1 and Type 2 devices. This should work automatically, as long as the PSE is capable in terms of power of supporting the PD and both are standard compliant. Should the PD require higher power (IEEE 802.3bt PD) and the PSE cannot support it (IEEE 802.3af PSE), the PD will either remain off or will turn on and consume only the power available from the PSE. There are other important PoE features not defined in IEEE803.3bt but supported by PSE vendors that are particularly important for POE lighting and smart building applications. Fast and perpetual PoE, for instance, are enhancing the availability and uptime of port power after a power failure. Perpetual PoE: Perpetual PoE is an enhancement feature that helps enable PDs (light fixtures) connected on certain ports to continue to receive power during a soft reload of the switch (PSE). This feature enables devices to be powered during a software reboot. It is a critical feature for PoE connected lighting.