Poe and poe+ applications, Power device (pd) discovery, Power classes – Allied Telesis AlliedWare Plus Operating System Version 5.4.4C (x310-26FT,x310-26FP,x310-50FT,x310-50FP) User Manual
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Power over Ethernet Introduction
Software Reference for x310 Series Switches
C613-50046-01 REV A
AlliedWare Plus
TM
Operating System - Version 5.4.4C
22.5
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Power priority of the PD being supplied power
■
Backup power conservation to extend UPS battery life
The IEEE 802.3at standard provides a capability for power re-negotiation with LLDP-MED.
PoE and PoE+ Applications
Products designed to the IEEE 802.3af (PoE) standard and IEEE 802.3at (PoE+) standard
provide the benefits of lower installation costs, installation flexibility, and remote power
monitoring and device management. Products supporting IEEE 802.3at can use higher
power levels, along with dynamic power management when using LLDP-MED to
exchange configuration data.
Power Device (PD) discovery
The first step for PSE equipment is to determine whether a device plugged into a port is a
valid Powered Device (PD). If it is, it will require power as well as network communication
through the attached LAN cable.
The IEEE 802.3af-2003 and IEEE 802.3at-2009 standards for device detection involves
applying a DC voltage between the transmit and receive wire pairs, and measuring the
received current.
A PSE will expect to see approximately 25K Ohm resistance and 150nF capacitance
between the transmit and receive wire pairs for the device to be considered a valid PD. A
range around these values is specified in the IEEE 802.3af and IEEE 802.3at Power Ethernet
standards.
The PSE will check for the presence of PDs on connected ports at regular intervals, so that
power can be removed when a PD is no longer connected. Legacy (pre-IEEE 802.3af Power
Ethernet standard) PDs are also detected by the PSE by default. See
.
Power classes
Once a PD is discovered, PSE initiates a PD classification test by applying a DC voltage to
the port. If the PD supports optional power classification it will apply a load to the line to
indicate to the PSE the classification the device requires.
Since PDs may require differing power ranges, the IEEE 802.3af and IEEE 802.3at Power
Ethernet standards classifies PDs according to their power consumption. By providing the
PSE with its power range, the PD allows the PSE to supply power with greater efficiency.
The power classes as outlined by IEEE 802.3af and IEEE 802.3at are as follows showing the
different PD classes and the PSE power output for each corresponding PD power range:
PD Class
Power Available at PD
Power Supplied from PSE
0
0.44 W to 12.95 W
15.4 W
1
0.44 W to 3.84 W
4.0 W
2
3.84 W to 6.49 W
7.0 W
3
6.49 W to 12.95 W
15.4 W
4
12.95 W to 25.5 W
30 W