In the above diagram, the PSE (Power Sourcing Equipment) is the PoE device and the PD, powered device (mode B), is the application. For a compliant power supply, on the left hand side, this diagram shows a non-PSE switcher hub, which simply provides data over the lines. The PSE is the actual power supply. Data is passed through the power supply but the unit also provides power over the unused pairs to the end station or the PD.