Static Vs. Dynamic Power Allocation - Avaya Power Over Ethernet Practical Manual

4 Static vs. Dynamic Power Allocation

Static Power Allocation
This method is used by many vendors because it is cheaper to implement. If a device advertises itself as a
class-1 endpoint but needs only 1-Watt of power, the PSE will allocate 4-Watts of power to the port serving
that device. This is done by logically allocating 4-Watts from the available power pool. Similarly, a class-3
device needing 8-Watts will be allocated 15.4-Watts of power. The problems with allocating the top of the
class range power are:
More power is reserved than is needed. A 1-Watt device will be allocated 4-Watts because the top of the
range for class-1 is 3.84-Watts plus a little more to travel over a possible 100 meters of Cat-5 cable.
Reserved power can deplete the total power pool even though it is not used. The resulting penalty from
logically reserving more power than the PD requires is that available power is "logically" exhausted before
all physical switch ports are used. For example, if a customer is using IP phones and those phones are
PoE class-3, each physical port will be ready to send out 15.4-watts. If each phone only requires 7-watts,
but 15.4-watts are reserved, 8.4-watts per port will logically consume part of the total power pool. Ten of
these physical ports used will strand 84 watts of power from the power pool. 24 ports will strand 201.6
Watts. 96 ports will strand 806.4 Watts. This is a worst case example.
The assumption that each port connected to a Class-3 device should be ready to provide 15.4-Watts at any
time doesn't make sense with IP telephones or any known standards compliant device. Very few if any
devices operationally vary in power needs more than one Watt. An on-hook phone taking 8 watts will
almost never require more than 9-Watts in the off-hook state. Very few if any PDs require more than 11
watts, so the assumption of having to apply 15.4 watts rarely happens.
Static power allocation results in a brute-force application that is wasteful and unintelligent. It is easy to
calculate and deliver power based on the top of the power class, but it results in wasting or "stranding"
power logically from the total power pool. The practical results are:
Incurring low port density. A 48-port switch or switch blade may only provide power to 32 ports
because power was logically exhausted.
Buy larger power supplies. Systems with more than one power supply or chassis based switches
usually have options to buy higher wattage supplies. A larger power supply can mitigate or even
overcome the logical reservation limit, but at a cost of needlessly buying a large power supply you
don't really need. Larger power supplies also cost more to run.
Buy more switches. If larger power supplies don't solve the problem, buying more switches may be
an alternative but can be very expensive. Many fixed switches have an internal power supply that
cannot be upgraded, so buying more may be a solution to low port density.
Manually configure a power ceiling for each port. Today, many vendors have implemented a
feature that allows the administrator to manually set a power limit for each port. While this stops
the wasting of stranded logical power, it is a manual process requiring the switch administrator to
know the power needs of every device. Furthermore, the administrator must manually apply the
power limit to each port and be ready to change that limit as devices move or are changed with
other PDs.
Note: As you will see, Cisco and others use Static power allocation, but Cisco has created commands that
can put a power limit on any number of ports to solve issues of low port density, buying larger power
supplies or buying more switches. Manual configuration and maintenance is still a cost using this method.
MJK
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