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Operating Rules
External Switch Power
Switch Priority Class
Port priority classification can be used by the switch to allocate power to
ports. It is a prioritization scheme by which the user can assign a low (default),
high, or critical priority to any given port. This assignment is done through
the command line interface (see the software manual that came with your
switch) of the switch and alters the hardware port-number priority for power
allocation.
■ Low - Default. This priority class receives power only if all other priority
classes are receiving power. If there is enough power to provision PDs on
only some of the ports with a low priority, then power is allocated to the
ports in ascending order, beginning with the lowest-numbered port in the
class until all available power is in use.
■ High - This priority class receives power only if all PDs on ports assigned
with a critical priority are receiving full power. If there is not enough
power to provision PDs on ports assigned with a “High” priority, then no
power goes to the low priority ports. If there is enough power to provision
PDs on only some of the “High” priority ports, then power is allocated to
the “High” priority ports in ascending order, beginning with lowest-
numbered high priority port, until all available power is in use.
■ Critical - This priority class always receives power. If there is not enough
power to provision PDs on all of the ports configured for this class, then
no power goes to “High or Low” priority ports. If there is enough power
to provision PDs on only some of the “Critical” ports, then power is
allocated to the “Critical” ports in ascending order, beginning with the
lowest-numbered port in the class.
Line Loss
A certain amount of power is consumed from the switch to the powered device
(typically a 16% loss), which can be influenced by cable length, quality, and
other factors. The IEEE 802.3af specification has addressed loss of power by
providing more power than a powered device requires. As well, depending
upon the classification (Class 0-3) of the device, the switch will provide more
or less power to address the specific power needs of that end device.
PD Power Classification
A PD is classified based on the maximum power it draws across all input
voltages and operational modes. The most common class is 0, in which the
switch will allow a maximum draw of 15.4 watts per port. As an example, 15.4
watts - Power Loss (16%) = 12.95 watts. See table 2-2.
Operating Rules
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