The IEEE 802.3af standard defines, but does not mandate, the use of power classes. This means a PD
may advertise one, and only one power class. This is important for non-Avaya PSEs that logically allocate
power from the total power pool and is discussed later in this paper. The following chart describes power
(in watts) required by PDs and provided by PSEs.
Table 2 – IEEE 802.3af power definitions for each class of PSE and PD devices
Class
Usage
Minimum Power from PSE
Power Range at PD
0
Default
15.4w
0.44w – 12.95w
1
Optional
4.0w
0.44w – 3.84w
2
Optional
7.0w
3.84w – 6.49w
3
Optional
15.4w
6.49w – 12.95w
4
Reserved
Treat as Class 0
Future Use
The preceding chart not only describes the power ranges for endpoints, but also the power minimums for
PSEs. The difference between the maximum PD power and the maximum PSE delivered power is needed
to “push” the power across a potential 100 meters (328 feet) of copper twisted pair between the PSE and
the PD. There is a natural attenuation of power from the PSE to the PD as the power encounters the
impedance of the copper wire. Therefore, more power is needed from the PSE so the full PoE class
maximum wattage is delivered to the PD.
IEEE 802.3af PD power class is derived from a classification signature measured in milli-Amps. The chart
below describes the values that a PD presents to a current under a constant voltage. The values returned
to the PSE then classify that PD into an IEEE power class.
Table 3 – Values used to assign PDs into IEEE 802.3af classes
Classification Signature
Conditions
Values
Current for Class 0
14.5V – 20.5V
0 – 4 mA
Current for Class 1
14.5V – 20.5V
9 – 12 mA
Current for Class 2
14.5V – 20.5V
17 – 20 mA
Current for Class 3
14.5V – 20.5V
26 – 30 mA
Current for Class 4
14.5V – 20.5V
36 – 44 mA
PD Management, Termination and Rediscovery:
This final stage is steady state power management
including continual power parameter sensing and detecting PD absence or violation of required parameters.
Steady state powering involves a continual sensing of valid parameters. An example is a sudden short
circuit of the wire pairs carrying power. This condition is detected and power is almost instantly halted by
the PSE. If a PD is drawing power and is removed from the PSE port, power is again halted because
another device could use that same PSE port and be damaged by erroneously receiving power. Lastly, if
resistance, capacitance or inductance lies in the values listed in Appendix B, power is removed because the
signature is now invalid or non-compliant. This is the end of the 802.3af material. The remaining sections
are vendor specifics on implementing IEEE 802.3af PoE.
MJK
Copyright ©
2006 Avaya Inc. All Rights Reserved.
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