Allied Telesis AT-S62 - Manual (Page 322)

Chapter 20: Power Over Ethernet
Section II: Advanced Operations 322
Power allocation is dynamic. Ports supplying power to powered devices
may cease power transmission if the switch’s power budget has reached
maximum usage and new powered devices, connected to ports with a
higher priority, become active.
PoE Device
Classes
The IEEE 802.3af standard specifies four levels of classes for powered
devices. The classes are defined by power usage. The classes are:
❑ 0 - 0.44 W to 12.95 W
❑ 1 - 0.44 W to 3.84 W
❑ 2 - 3.84 W to 6.49 W
❑ 3 - 6.49 W to 12.95 W
(The standard actually specifies five levels; the fifth is reserved for future
use.)
You cannot adjust this on a powered device. It is set by the
manufacturer. This is mentioned here because you can view the class of
a powered device through the switch’s management software. To view
this information, refer to Displaying PoE Status and Settings on page
327.
You might notice that according to the IEEE standard the maximum
amount of power a powered device should consume is 12.95 W. So why
does the switch offer up to 15.4 W per port? It has to do with line loss.
Some power is lost on the twisted pair cable as it travels from the switch
to the device. For those devices needing 12.95 W, the extra watts act as
compensation for the possible loss.