Sleep modes of wireless network cards are used to switch these cards into low-power state when idle, but large time-out periods and frequent wake-ups can reduce the utility of this approach. Modern processors offer the ability to switch CPU voltages or clock frequencies and therefore reduce CPU energy consumption, however, that can reduce the sleep durations of a network device, adversely affecting the achievable energy savings. This paper describes an approach in which multiple resource managers cooperate to reduce a mobile device's energy consumption. This system-level approach is based on the integrated management of a real-time CPU scheduler, the frequency scaling capabilities of a modern processor, a QoS packet scheduler, and the low-power sleep mode of a wireless network card.