USB devices have two power ratings, low power 100mA or less, and high power, >100mA (500mA).Each port coming out of the root hub of your PC supplies 500mA. When you connect a bus-powered hub, the hub controller requires 100mA and each downstream port is supplied 100mA. Therefore you are only allowed to connect low power devices downstream of a bus-powered hub.To connect high power devices downstream of a hub, the hub must be self powered, not bus powered. This will allow each port of the hub to have 500mA. As well, when you connect a hub or USB composite device downstream of another hub, the first hub must be self-powered.