Probing queries in wireless sensor networks

For queries in wireless sensor networks, empty sets may be returned as query results which could confuse users a lot and users obtain no useful information about the monitored objects from the empty sets. To solve the problem, this paper proposes methods to provide users with approximate answer sets in the case where no sensing data satisfies the query conditions. The approximate answer sets can be used not only to answer the query approximately but also to guide users to modify their queries for further probing the monitored objects. The distance between sensing data and a query and the dominating relationship between sensing data are first defined. Then, three algorithms for processing probing queries are proposed, which compute the best approximate answer sets that consist of the sensing data with the smallest distance from given queries. All the algorithms utilize the dominating relationship to reduce the amount of data transmitted in sensor networks by filtering out the unnecessary data. Experimental results on real and synthetic data sets show that the proposed algorithms have high performance and energy efficiency.