3 IEEE 802.3af – how it works
The phases between connecting a PD, power flow to the PD and power termination are:
PD (Powered Device) Detection
PD Classification or actual power determination
Power management, termination and PD re-discovery
PD Detection:
The goal of this phase is to detect a valid PD. When a PD is connected to a PSE (Power
Sourcing Equipment), a small DC current is sent from the PSE. A PSE is typically a switch (End-Span) or a
power injector (Mid Span). If the resistive load of a PD is sensed at 25K
Ω
(+/- 1.25K
Ω
), the device may
meet the 802.3af requirements for power. Not only does the resistive load need to meet the 25K
Ω
level,
but the PD must also meet the resistance range during a short voltage ramp-up period. The idea is to only
power devices that are valid PDs and to protect non-PDs that could be damaged if they received power.
In a short amount of time, voltage is increased from 2.7 Volts to 10.1 Volts in 1.0 Volt increments. The
resistance must remain between the range of 23.75K
Ω
and 26.25K
Ω
or power is removed almost
immediately. This is generally known as the 802.3af resistive signature – adhering to both an initial
resistive range and that same range during a voltage ramp-up period. Any device that does not present this
resistive signature, will not receive power from the PSE. Additionally, any “signature” in the zones beyond
23.75 – 26.25 K
Ω
presents an invalid or non-compliant signature and power will not be applied. See
appendix B for specifics on invalid and non-compliant signatures. Capacitance and inductance values are
also measured when validating a PD to the IEEE 802.3af standard
.
Table 1 – Parameters for a valid PD power signature
Valid PD Detection
Ramp-up Conditions
Resistive Signature Range
Initial Resistive load for a potential PD
23.75K
Ω
– 26.25K
Ω
Required resistance during voltage ramp-up
2.7V – 10.1V
23.75K
Ω
– 26.25K
Ω
Input capacitance
2.7V – 10.1V
0.05µF – 0.12µF
Input Inductance
2.7V – 10.1V
≤
100µH
Note: Power in the 802.3af standard is expressed in watts assuming 48 volts to the PD. By design, a PD
must accept a voltage variance. The standard mandates that “any PD will withstand 0 to 57 volts
indefinitely without damage”.
The IEEE 802.3af standard allows power on either the spare pairs or the signaling pairs. Ethernet uses
signaling wire pairs 1,2 & 3,6. Power over these used pairs is sometimes called phantom power or inline
power. Most End-Span PSEs (switches) use the inline power scheme, but they can also use the spare
pairs 4,5 & 7,8 as an option.
Mid-Span PSEs use only the spare pairs 4,5 & 7,8 to transmit power. All PDs are required to accept power
on either spare or signaling pairs. All Avaya products comply with this requirement and can be powered
three ways; from the signal pair, the spare pair or one pair (7,8) as a further option to support external
power supplies like a “brick” type transformer.
PD Classification:
After detection, the goal of this next phase is to supply appropriate power. This can be
done based on the IEEE 802.3af power class of the PD, or more intelligent algorithms can sense how much
power is actually needed.
MJK
Copyright ©
2006 Avaya Inc. All Rights Reserved.
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