The IEEE 802.3af standard specified recognizes procedures for PD device; with those behaviors and through the electrical signals returned from PD, the PSE will known the attached device support power over Ethernet technology and the inquired power. The procedures show on the right site.

There are 2 procedures is very important for the PSE, one is 25K ohm detection resistor should applied in 802.3af PD and PD device should claim its own power consumption by classification resistor in honesty.

Some of device manufacture claimed their product support Power over Ethernet, but those PD does not equipped detection resistor and will cause PSE can not recognize and forwarding power to PD.

After the 25K ohm resistor detected by PSE, PSE will perform classification and powering the PD. In the IEEE 802.3af standard, it specified the classification ID with different power consumption level as following table.

In the PD power classification table, the class ID 4 is reserved for the future standard – IEEE 802.3at High power standard. Although the IEEE 802.3at forum still not release it, but have decided use the ID 4 as high power PD classification ID. The Korenix JetNet 4706 and JetNet 3706 PoE switch obey the IEEE 802.3at pre-standard and feeding high power to PD when the classification ID is 4.