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Power Over Ethernet Interoperability Guide by Sanjaya Maniktala

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2-Event Classification
As mentioned previously, 1-event classification is a one-way street: The PD is simply telling the PSE (on being queried) what class it belongs to and therefore what power it demands. It does not turn around and seek to identify the power delivery capabilities of the PSE. When the AT standard came about, with an entirely new class, one that was not a subset of existing classes, that situation needed to change. For one, there were already switches/hubs out there that did not support medium-power (30 W). So a new medium-power PD may find itself connected to an older switch/hub—one that does not support its power requirement. In that case, the PD would need to “know” what type of PSE it was connected to, so it could, for example, ...

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