Database applications in wireless sensor networks very often demand data collection from sensor nodes of specific target regions. Design and development of spatial query expressions and energy-efficient query processing strategy are important issues for sensor network database systems. The existing sensor network database systems lack the needed sophistication for the space calculation of the target sensor nodes; hence, unnecessary query/data transmissions are required between the sensor nodes and the server. This paper describes our spatial operations and energy-efficient query processing methods that are designed and implemented in our sensor network database system called SNQL + . With a set of spatial operators based on geometric parameters, such as Envelope, NearBy, Distance, Direction, and set theoretic operators, SNQL + allows sensor network applications to easily specify the target space of interest. Our energy-efficient query processing strategy implements an in-network query management based on the lowest common ancestor (LCA) algorithm, so that the query processing cost for calculating the target spaces is greatly reduced by avoiding the need of heavy query/data transmissions between the base-station and target nodes. Performance evaluation shows that our proposed design and implementation of spatial query expressions and processing strategy achieve improved energy efficiency for database operations in the wireless sensor network.
Due to the random movement of sensor nodes in uncontrolled mobile sensor networks, the distribution of sensor nodes may not be uniform over the area of network coverage. Querying a target space in which mobile sensors are unavailable at the time of the data request will lead to no return of the result. In this paper, this problem is referred to as sensor void problem, which leads to difficulties in data management and application design activities. We address the sensor void problem and offer solutions with a location-aware time-constrained query processing strategy. The key idea of the proposed strategy is to allow mobile sensor nodes to keep track of time validity and target location matching checks for the execution of the queries received from the base station. Programmers can specify a certain degree of time sensitivity in their queries according to their applications' semantics. We implemented the proposed mobile query processing scheme into our previously developed sensor database system. A performance evaluation shows how the proposed query processing strategy effectively handles the sensor void problem with various parameters of an uncontrolled mobile sensor network.
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