Avoiding the rush hours

WiFi continues to be a prime source of energy consumption in mobile devices. This paper observes that, despite a rich body of research in WiFi energy management, there is room for improvement. Our key finding is that WiFi energy optimizations have conventionally been designed with a single AP in mind. However, network contention among different APs can dramatically increase a client's energy consumption. Each client may have to keep awake for long durations before its own AP gets a chance to send packets to it. As the AP density increases in the vicinity, the waiting time inflates, resulting in a proportional decrease in battery life.We design SleepWell, a system that achieves energy efficiency by evading network contention. The APs regulate the sleeping window of their clients in a way that different APs are active/inactive during non-overlapping time windows. The solution is analogous to the common wisdom of going late to office and coming back late, thereby avoiding the rush hours. We implement SleepWell on a testbed of 8 Laptops and 9 Android phones, and evaluate it over a wide variety of scenarios and traffic patterns (YouTube, Pandora, FTP, Internet radio, and mixed). Results show a median gain of up to 2x when WiFi links are strong; when links are weak and the network density is high, the gains can be even more. We believe SleepWell is a desirable upgrade to WiFi systems, especially in light of increasing WiFi density.