A USB device can be either powered by the upstream Host or Hub port, or have its own power supply. If the device is powered by the upstream port, it is considered Bus-powered and can draw no more than 100mA before it is enumerated. After enumeration, it can be configured to draw up to 500mA. The USB specification includes mechanisms for sensing over-current conditions and shutting down a device in an over-current state. If a device has its own power supply, which is common not only for devices that require more than 500mA but also for battery-powered devices, then it is considered self-powered.