Query Forwarding in Wireless Sensor Networks*

The main function of a wireless sensor network is to sense the outside environment and report the data to a processing center. Therefore, both sensing and communication roles should be considered for a sensor network. Except for the mobility aspect, sensor networks inherit almost all properties of ad hoc networks. Therefore, sensor nodes are supposed to report data to a common sink instead of simply communicating with each other. During the data aggregation process, the sink node generates query information and sends the query to the network through broadcasting. All the sensor nodes will receive it and report the data back to the sink if the sink is interested in the data. Because the number of sensor nodes is very large, flooding is not the best way to do broadcast, since flooding incurs redundant broadcast and consumes substantial amount of energy on useless forwarding. Our main goal is to reduce the number of the redundant rebroadcasts. In the literature, there are some other proposed solutions for energy efficient broadcasting schemes in wireless sensor networks ([1]-[6]). Our scheme can be simply implemented without complicate information exchanges among sensor nodes. We define hot spots in the network area, and make each node in a hot spot able to directly communicate with any node in a neighboring hot spot. So we build a connected backbone if all the hot spots are occupied with at least one node. We provide two methods for hot spot placement: grid placement and hexagon placement. We compare the difference between these two placements. The set of hot spots constitutes a backbone in a sensor network. The size and the placement of the hot spots affect the connectivity of the sensor network. We can limit the forwarding of the same query inside the same hot spot by using probabilistic method or electing a cluster head inside a single hot spot. The larger the number of hot spots is, higher is the connectivity. However, larger number of hot spots also leads to more redundant forwarding. The number of hot spots is depended on the size of the hot spot and the placement of the hot spots. We plan to look for better approaches for mobile wireless sensor networks in the near future by applying topology control.