The early success of wireless sensor networks has led to a new generation of increasingly sophisticated sensor network applications, such as HP's CeNSE. These applications demand high network throughput that easily exceeds the capability of low-power 802.15.4 radios that are most commonly used in today's sensor nodes. To address this issue, this paper investigates an energy-efficient approach to supplementing an 802.15.4 based sensor network with high bandwidth, high power, longer range radios such as 802.11. Exploiting a key observation that the high bandwidth radio achieves low energy consumption per transmitted bit of data due to its inherent transmission efficiency, we propose a hybrid network architecture that utilizes an optimal density of dual-radio (802.15.4 and 802.11) nodes to augment a sensor network having only 802.15.4 radios. We present a cross-layer mathematical model to calculate this optimal density, which strikes a balance between the low energy per bit of the high-bandwidth radio and the low sleep power of 802.15.4 radio. Experimental results obtained using a wireless testbed reveal that our architecture improves the average energy per bit, the time elapsed before half of the nodes drain their battery, and the end-to-end delay by 62%, 106%, and 73% respectively, compared to a network that uses only 802.15.4 radios.
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