in a system in which a host device is in communication with a peripheral device through communications device connected to a usb port , the usb host device polls the communications device to constantly regardless of the power state of the usb communications device . while in the low power mode , the usb communications device generates nak packets , indicating that the device has no data to send . the nak packets are generated in hardware , thereby allowing the usb host continues operating while unaware of the state of the peripheral device . external events can be used to trigger the usb communications device to exit the low power state without communicating with the usb host , and without the usb host altering its behavior .