I. Introduction
The rapid improvement in physiological sensors and low power integrated circuits, together with the increasing use of wireless networks has enabled the development of a new generation of wireless sensor networks known as Wireless Body Area Networks (WBANs). A WBAN usually consists of a limited number of heterogeneous sensors that are attached to clothing or on the body, and a fusion center, typically a personal device, e.g. cellphone or PDA. Such networks enable a wide range of innovative applications in health, sports, lifestyle and emergency situations. However, practical realization of WBANs faces a number of unique challenges [3]. In particular, energy efficiency, which is the focus of our work, constitutes a significant factor in long-term deployment of WBANs.