References:
IEEE Std 802.3af-2003 (Amendment to IEEE Std 802.3-2002, including IEEE Std 802.3ae-2002), 2003
Page(s):0_1-121
White Paper on Power over Ethernet (IEEE802.3af)
http://www.poweroverethernet.com/articles.php?article_id=52
Microsemi /PowerDsine
http://www.microsemi.com/PowerDsine/
Linear Tech
http://www.linear.com/
The PoE Provision Process
While adding PoE support to networked devices is relatively painless, it should be realized that power
cannot simply be transferred over existing CAT-5 cables. Without proper preparation, doing so may
result in damage to devices that are not designed to support provision of power over their network
interfaces.
The PSE is the manager of the PoE process. In the beginning, only small voltage level is induced on the
port's output, till a valid PD is detected during the Detection period. The PSE may choose to perform
classification, to estimate the amount of power to be consumed by this PD. After a time-controlled
start-up, the PSE begins supplying the 48 VDC level to the PD, till it is physically or electrically
disconnected. Upon disconnection, voltage and power shut down.
Since the PSE is responsible for the PoE process timing, it is the one generating the probing signals prior
to operating the PD and monitoring the various scenarios that may occur during operation.
All probing is done using voltage induction and current measurement in return.
Stages of powering up a PoE link
Stage Action
Volts
specified
per 802.3af
Volts
managed
by chipset
Detection
Measure whether powered device has the
correct signature resistance of 15–33 kΩ
2.7-10.0 1.8–10.0
Classification
Measure which power level class the resistor
indicates
14.5-20.5 12.5–25.0
Startup Where the powered device will startup >42 >38
Normal
operation
Supply power to device 36-57 25.0–60.0
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