A cable modem executes periodic ranging with a cable modem terminal system and records a time point at which the periodic ranging is completed. The cable modem obtains an un-ranging time interval during which the cable modem terminal system permits the cable modem not to execute periodic ranging with the cable modem terminal system, and sets a sleep time interval during which the cable modem does not execute periodic ranging with the cable modem terminal system according to the un-ranging time interval. Then, the cable modem shuts off a power amplifier in the cable modem in the sleep time interval if the cable modem does not transmit packets during the preset time interval before the recorded time point.