LinkPower
LPS1000 Adjustable Voltage PoE Switch
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Powered Device (PD) is a device that receives power from PSE. More and more networks
today have attachments such as IP phones, wireless LAN access points, and IP cameras all
designed as PDs. Due to the different power requirements needed by various PDs,
IEEE802.3af defines an option to classify PDs into classes according to their power
consumptions. Following the process of classification, a PD informs the PSE of its power range
so the PSE can apply more efficient power. Table 1 details the class and the corresponding
power levels delivered from PSE and received by PD.
Class
Usage
Maximum Power level
0
Default
0.44 to 12.94
1
Optional
0.44 to 3.84
2
Optional
3.84 to 6.49
3
Optional
6.49 to 12.95
4
Reserved PSE classify as Class 0
Table 1. PD Power Classification
There are two different modes included in the 802.3af standard. Mode A is to apply the power
over the same data pairs (pins 1,2,3 and 6) as the data that is being transported. This is true of
both 10/100 and 1000BASE-T networks. Mode A is sometimes referred to as phantom power.
Mode B allows the power to be carried over the unused pairs (pins 4, 5, 7 and 8). However, a
pre-standard PSE or injector using power over the unused pairs may not operate with an
802.3af PD due to various power negotiations occurring between the PSE and PD, such as
detection and classification.
Figure 3. Comparison of Midspan PSE and Endspan PSE
