2008 Jul 25 at 09:30
Hossein Falaki, graduate student, David R. Cheriton School of Comp. Sci., Univ. Waterloo
The number of smartphones in use is overwhelmingly increasing every year. These devices rely on connectivity to the Internet for the majority of their applications. The ever-increasing number of deployed 802.11 wireless access points and the relatively high cost of other data services make the case for opportunistic communication using free WiFi hot-spots. However, this requires effective management of the WLAN interface, because by design the energy cost of WLAN scanning and interface idle operation is high and energy is a primary resource on mobile devices.
This thesis studies the WLAN interface management problem on mobile devices. First, I consider the hypothetical scenario where future knowledge of wireless connectivity opportunities is available, and present a dynamic programming algorithm that finds the optimal schedule for the interface. In the absence of future knowledge, I propose several heuristic strategies for interface management, and use real-world user traces to evaluate and compare their performance against the optimal algorithm. Trace-based simulations show that simple static scanning with a suitable interval value is very effective for delay-tolerant, background applications. I attribute the good performance of static scanning to the power-law distribution of the length of the WiFi opportunities of mobile users, and provide guidelines for choosing the scanning interval based on the statistical properties of the traces. I improve the performance of static scanning, by 46% on average, using a local cache of previous scan results that takes advantage of the location hints provided by the set of visible GSM cell towers.