A Robust And Energy-Efficient Gradient-Based Routing Protocol For WSNs

Wireless Sensor Network (WSN) is a new emerging technology that is of increasing interest in many military, civil and scientific applications. WSN is a self-configuring network which consists of a large number of low-cost tiny sensor nodes that are equipped with capabilities of sensing, computing, and wireless communication. These smart sensor nodes can collectively sense, process, and disseminate a wide range of complex environmental data to a central monitoring unit.Limitation on available resources is a major challenge in wireless sensor networks. Due to high rates of unexpected node/link failures, robust data delivery through multiple hops also becomes a critical issue. Traditional routing protocols waste energy and bandwidth for they keep maintaining routing tables via a periodic update process. Although the proposal of reactive on-demand philosophy where routes are established only when required is a major step in wireless (sensor) networks, it still requires nodes to keep information about routes. However, constantly changing topologies resulted from nodes' movements, malfunction and dynamic channel conditions make the routing state upkeep difficult. In this paper, a state-free gradient-based forwarding (SGF) protocol is proposed to address these challenges.Nodes running SGF do not maintain states of neighbors or network topology and thus can scale to very large networks. Without using routing tables, SGF builds a gradient called cost field that provides each node the direction to forward data. To conserve energy, the gradient is established based on the minimum energy consumption of transmitting a packet from the node to the sink so that data can be transmitted along the energy-efficient paths. In the data transmission stage, the forwarder of a routing node is selected among multiple candidate nodes through a distributed contention process. The opportunistic forwarding implicitly maintains all possible routes and hence manifests robust performance. To alleviate collisions introduced by contention-based forwarding, a logarithmic function is introduced to effectively spread out nodes' deferral times. Moreover, the maintenance of gradient is purely driven by data transmissions and hence incurs little overhead. Simulation results show that SGF achieves significant energy saving and outperforms several existing data forwarding protocols in terms of packet delivery ratio and end-to-end delay.