A fault-tolerant model of wireless sensor-actor network

In a wireless sensor and actor network (WSAN), a group of sensors, actors, and actuation devices are geographically distributed and linked by wireless networks. Sensors gather information for an event occurring in the physical world and send them to actors. Actors can perform appropriate actions on actuation devices by making a decision on receipt of sensed information from sensors. Sensors are low cost, low powered devices with limited energy, computation, and wireless communication capabilities. Sensors may not only stop by fault but also suffer from arbitrary faults. Furthermore, wireless communication is less reliable due to noise and shortage of power of sensors. Reliable realtime communication among sensors, actors, and actuation devices is required in WSAN applications. In order to realize the reliability and realtimeness, we newly propose a multiactor/multi-sensor (MAMS) model where each sensor sends sensed information to multiple actors and each actor receives sensed information from multiple sensors in an event area. Actors are required to causally/totally order events from multiple sensors and actions on actuation devices. In addition, multiple actors may perform actions on receipt of sensed information. Multiple redundant executions of an action on each device have to be prevented and conflicting actions on each device from multiple actors have to be serialized. In this paper we discuss how to realize reliable, ordered delivery of sensed information to actors from sensors on the basis of global time and how to reliably and non-redundantly perform actions with realtime constraints.