User’s Manual: SW-10200
37
Table 7 - Power supplied over data pins
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/
C.2. PoE Provisioning Process
It should be noted that proper preparation is needed prior to initializing a PoE network in order to minimize the risk of
damage to hardware not designed for network-based power provisioning. Despite the fact that adding PoE support to
network devices is a relatively simple operation, read this section carefully and understand this process before attempting to
transfer power through CAT-5 to these devices.
The PSE is a device that manages the power flow over an Ethernet cable. During the detection period, a small voltage
level is induced on the port's output until a PD is detected. 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, during which voltage and power will 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.
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
Table 8 - Stages of PoE
C.2.1. Line Detection
Before power is applied, for safety reasons a valid PD is connected to the PSE's output first. This process is referred to
as "line detection", and involves the PSE seeking a specific 25 KΩ signature resistor. Detection of this signature indicates
that a valid PD is connected, and that provision of power to the device may commence.
The signature resistor is isolated in the PD's PoE front-end, away from the rest of the PD's circuitries till detection is
certified.
