The USB selective suspend feature allows the hub driver to suspend an individual USB port without affecting the operation of the other USB ports on the hub. Selective suspension of USB devices is especially useful in portable computers, since it helps conserve battery power. Many devices, such as fingerprint readers and other kinds of biometric scanners, only require power intermittently. Suspending such devices, when the device is not in use, reduces overall power consumption. More importantly, any device that is not selectively suspended may prevent the USB host controller from disabling its transfer schedule, which resides in system memory. DMA transfers by the host controller to the scheduler can prevent the system’s processors from entering deeper sleep states, such as hibernate.