Allied Telesis AT-S62 - Manual (Page 347)

AT-S62 Management Software Menus Interface User’s Guide
Section II: Advanced Operations 347
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:
ˆ 0 - 0.44 W to 12.95 W
ˆ 1 - 0.44 W to 3.84 W
ˆ 2 - 3.84 W to 6.49 W
ˆ 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 352.
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.