The graph on this slide shows the PD detection, classification, and power ON sequence. The 2-event hardware classification used in PoE+ essentially repeats the 802.3af voltage probe twice. Each voltage probe of the PD results in a pulse of current which corresponds to a particular power level. Initially, the PSE asserts a voltage pulse in the range of 15.5V - 20.5V on the data or spare lines. The PD responds with a current of up to 40mA which conveys one of four power classes to the PSE. Class 4 current indicates the PD is able to accept up to 25.5W. The second pulse is a signal to the PD that the connected PSE is indeed a high-power PSE able to source the higher power levels associated with the 802.3at standard. The PD responds with a class 4 current level, confirming that it is a high power PD requiring the maximum power level.