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Power Over Ethernet Interoperability Guide by Sanjaya Maniktala

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Class Levels in Layer-1 Classification
Now we come to the actual classes in physical-layer classification. When the AF standard was being created, there were only a handful of power classes under consideration, based on existing types of devices in the market. These power levels were called Class 0, Class 1, Class 2, and Class 3. Class 4 was kept as a placeholder for the future but was largely undefined at the time.
1.  Class 0 is in effect a “classless default,” and it means that the PD (max) power rating is anywhere between 12.95 W (corresponding to 15.4 W measured at the PSE end as per Fig. 4.2), down to almost zero. Almost zero, in this case, is just enough power drawn by the PD so as to prevent the PSE from disconnecting it thinking there ...

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