Design Considerations for Sensor Networks with Gateways

In a large class of sensor network deployments, a small subset of the sensors covering the sensor field is equipped with special communications capability to communicate with operators outside the sensor field. These sensors play the role of gateways for off-field communication in the sense that all communications toor out of the field is through these nodes, and the other non-gateway nodes are only capable of sensor-to-sensor communication. This design achieves a lower cost by concentrating expensive communication devices in a small subset of nodes. An important problem in designing such gateway-based sensor networks is determining the number of gateway nodes needed, their location in the sensor field, and the automation of the sensor-to-gateway association for off-field communication. These design considerations are addressed in this paper. In determining the number of gateways the tradeoff is between performance and cost. As the number of gateways increases, less traffic load is placed on each gateway and its surrounding nodes, resulting in longer network lifetime and larger off-field aggregate transmission capacity. However, with a larger number of gateways the network may be too costly to deploy as gateway nodes are more expensive than non-gateway sensor nodes. We develop and analyze models that allow us to determine the optimal number of gateways and their location in the sensor field. We also provide initial results with respect to determining the needed number of fusion nodes. While the presence of multiple gateways offers a higher degree of off-field communication reliability, a sensor will need to select one of the gateways at a time for off-field communication. In this paper, we also propose a dynamic sensor-to-gateway association protocol. Based on current energy levels, the distributed protocol dynamically assigns sensors to gateways in such a way that the overall transmission load is balanced among the different gateway regions over the lifetime of the sensor field.