Avaya Power Over Ethernet Practical Manual page 8

Dynamic Power Allocation
This method is more expensive to implement in a switch or Mid-Span, but is less expensive to operate and
allows full port density. Dynamic power allocation senses how much power is required and delivers that
exact amount of power to the port leading to the PD. Dynamic, means that as the power requirements
change, the power delivered to that port also changes accordingly. Since dynamic power allocation doesn't
depend on the advertised PD class, the algorithm isn't confined to delivering 15.4-Watts for every port.
Therefore, the size of the power supply can be smaller than a static power allocation design.
Unlike the preceding section on static power allocation, if a device advertises itself as a class-1 endpoint but
needs only 1-Watt of power, the PSE will allocate 1-Watt of power to the port serving that device. There is
no logical reservation from the available power pool. Similarly, a class-3 device needing 8-Watts will be
allocated 8-Watts of power. There are no known problems with dynamically allocating the exact amount of
power needed. In fact the advantages over the static allocation scheme are:
No more power is reserved than is needed. A 1-Watt device will be allocated 1-Watt.
The total power pool is not affected by reservations. There is no wasted power because there is no concept
of reserving power from a pool. There is no logical pool to consider. Each device is given the amount of
power it requires.
The assumption that each port connected to a Class-3 device should be ready to provide 15.4-Watts at any
time still doesn't make sense with IP telephones or any other standards compliant device. Again, very few if
any devices operationally vary in power needs more than one Watt. A phone taking 8 watts will almost
never require more than 9-Watts. If a PD required 12.95 watts, 15.4-Watts would be supplied by the
802.3af specification. If, theoretically, all ports required 15.4-Watts, you could run out of available power
due to a smaller power supply. This is not a known event in industry to date because very few if any Class-
3 PDs require a full 12.95-Watts.
Dynamic power allocation results in an intelligent application that is applied to any vendor's PD. The
practical results of this method are:
Full port density. A 48-port switch or Mid-Span will provide power to all 48 ports.
No need to buy larger power supplies. Operational costs are lower.
No need to buy more switches to compensate for a logical reservation of power. The equipment
footprint and operational/maintenance requirements are kept to a minimum.
There is no manual configuration needed by a network administrator. Errors are avoided and time
is saved because there is no manual effort required to administer power for PDs. Changes can be
administered automatically or with a minimum of administrator effort.
MJK
Copyright ©
2006 Avaya Inc. All Rights Reserved. 8