Chapter 8: Power Over Ethernet Overview

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

 

Volts managed

 

 

 

per 802.3af

 

by chipset

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Detection

 

Measure whether powered device has the

 

2.7-10.0

 

1.8–10.0

 

 

correct signature resistance of 15–33 kΩ

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Classification

 

Measure which power level class the

 

14.5-20.5

 

12.5–25.0

 

 

resistor indicates

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Startup

 

Where the powered device will startup

 

>42

 

>38

 

 

 

 

 

 

 

 

Normal operation

 

Supply power to device

 

36-57

 

25.0–60.0

 

 

 

 

 

 

 

 

214

GE-DS-82 and 82-PoE Ethernet Managed Switch User Manual

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GE GE-DS-82, 82-POE user manual PoE Provision Process, Stages of powering up a PoE link, Microsemi /PowerDsine Linear Tech