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By leveraging device-to-device communication, opportunistic networks promise to complement infrastructure-based networks, by enabling communication in remote areas or during disaster and emergency situations, as well as by giving rise to novel applications, such as location-based sharing. Yet, to become feasible in practice and accepted by users, it is crucial that opportunistic communication be energy-efficient. Through extensive and detailed measurements and analysis, we show in this paper, that all of today's device-to-device communication technologies suffer from two grave energy consumption problems: very expensive neighbor discovery and unfair connection maintenance. We consider the two most well-known technologies -- Wi-Fi Direct and Bluetooth, and a third solution based on the WLAN access point mode -- WLAN-Opp. We carefully design a measurement setup which allows us to isolate the energy consumption of individual operations (e.g. CPU sleeping/waking up, scanning/listening for neighbors etc) for thesetechnologies and compare the technologies based on these measurements. Our analysis reveals that neighbor discovery can quickly drain a device's battery, depending on the required scanning frequency. In addition, once a connection is established, the "host" of the connection consumes two to five times the energy needed by a "client". To solve this unfairness problem, we propose a strategy that periodically alternates the hosting role among the peers. Further, we minimize the cost of the role switching operation by using the distribution of the residual connection time of two peers to calculate an adaptive switching period. We examine the trade-off between fairness and switching cost on real-world connection traces and show that our scheme largely outperforms static role switching. Finally, we demonstrate that our fair role switching scheme is also effective when run on real devices.

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