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Chapter 8: Power Over Ethernet Overview
196 GE-DS-242-PoE Managed Ethernet Switch User Manual
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