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In this paper we study two-layered heterogeneous sensor networks where two types of nodes are deployed: basic sensor nodes and cluster head nodes. The basic sensor nodes are simple and have limited power supplies, while the cluster head nodes are much more powerful and have much more power supplies, which organize sensors around them into clusters. Such two-layered heterogeneous sensor networks have better scalability and lower overall cost than homogeneous sensor networks. We propose to use polling to collect data from sensors to the cluster head since polling can prolong network life by avoiding collisions and reducing idle listening time of sensors. We will focus on finding energy efficient and collision-free polling schedules in a multi-hop cluster. To reduce energy consumption in idle listening, a schedule is optimal if it uses minimum time. We show that the problem of finding an optimal schedule is NP-hard, and then give a fast on-line algorithm to solve it approximately. We also consider dividing a cluster into sectors and using multiple non-overlapping frequency channels to further reduce the idle listening time of sensors. We conducted simulations on the NS-2 simulator, and the results show that our polling scheme can reduce the active time of sensors by a significant amount while sustaining 100% throughput.
Sensor networks, heterogeneous networks, clusters, polling, multi-hop polling, scheduling.

M. Ma, Z. Zhang and Y. Yang, "Energy-Efficient Multihop Polling in Clusters of Two-Layered Heterogeneous Sensor Networks," in IEEE Transactions on Computers, vol. 57, no. , pp. 231-245, 2007.
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