In general, the Bus driver has responsibility for tracking the device power states of all devices on its bus, and transitioning the Bus itself to only those power states that are consistent with those of its devices. This means that the Bus state can be no lower than the highest state of one of its devices. However, enabled wake-up events can affect this as well. For example if a particular device is in the D2 state and set to wake up the system, and the bus can only forward wake-up requests while in the D1 state, then the Bus must remain in the D1 state even if all devices are in a lower state. Note that the explicit command for entering the D3 state may be the removal of power.
Device is on and running.
It is receiving full power from the system, and is delivering full functionality to the user. Buses in D1 cannot do anything to the bus which would force devices on that bus to lose context. Attains greater power savings than D1. Device is off and not running. Device context is lost.