A Fault-Tolerant Model of Wireless Sensor-Actor Network Keiji Ozaki † , Kenichi Watanabe † , Satoshi Itaya † , Naohiro Hayashibara † , Tomoya Enokido †† , and Makoto Takizawa † Tokyo Denki University † , Japan E-mail {kei, nabe, itaya, haya, taki}@takilab.k.dendai.ac.jp Rissho University †† , Japan E-mail eno@ris.ac.jp Abstract 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 re- ceipt of sensed information from sensors. Sensors are low cost, low powered devices with limited energy, computa- tion, 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 de- vices is required in WSAN applications. In order to realize the reliability and realtimeness, we newly propose a multi- actor/multi-sensor (MAMS) model where each sensor sends sensed information to multiple actors and each actor re- ceives 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 se- rialized. In this paper, we discuss how to realize reliable, ordered delivery of sensed information to actors from sen- sors on the basis of global time and how to reliably and non-redundantly perform actions with realtime constraints. Keywords: Wireless Sensor-Actor Networks, Fault- tolerance, Redundant action execution, Global-time based synchronization 1. Introduction A wireless sensor and actor network (WSAN) is a collec- tion of sensors, actors, and actuation devices linked by wire- less medium to perform distributed sensoring and acting tasks [1, 2, 16]. Sensors gather information about physical world. Actors are capable of making a decision on appro- priate actions for information gathered by sensors and per- form the actions. WSAN is used in microclimate control, home automation, environmental monitoring, target track- ing [1, 2]. There are many discussions on how to reliably and efficiently broadcast messages among sensors and ac- tors [5, 16]. WSAN is one of the most significant technolo- gies to realize ubiquitous societies [22]. Sensors are low-cost, low-power devices which are equipped with limited energy, computation, and wireless communication capabilities. Sensors may stop, even mal- function [13] due to the out-of-charge and fluctuation of observed phenomena in the physical world. In addition, the wireless communication between sensors and actors is less reliable, i.e. messages may be lost due to noise. We discuss how to make WSAN tolerant of faults of sensors and wire- less communication links. If some event occurs in an event area of the physical world, sensors gather physical phenom- ena of the event and send sensed information to actors. In our multi-actor/multi-sensor (MAMS) model, each sensor sends sensed information to multiple actors and an actor re- ceives sensed informations for a same event from multiple sensors in order to be tolerant of faults of sensors and wire- less networks. Even if some sensors are faulty and messages are lost in the wireless link, each actor can receive proper sensed information from the other proper sensors. If sensors are arbitrarily faulty [13], an actor takes a kind of majority- based decision on sensed information from multiple sen- sors. If multiple actors receive the same sensed informa- tion, the same action may be performed multiple times by the actors even if the action should be performed only once for the sensed information. On receipt of sensed informa- Proceedings of the Ninth IEEE International Symposium on Object and Component-Oriented Real-Time Distributed Computing 0-7695-2561-X/06 $20.00 © 2006 IEEE