## 1 Introduction  
Wireless sensor networks (WSNs) [1,2] have been studied for about 10 years. Today this field is widely supported by an increasing number of dedicated journals such as ACM Trans. on Sensor Networks and Int. J. of Distributed Sensor Networks, conferences such as SENSYS (ACM Conf. on Embedded Networked Sensor Systems), IPSN (ACM/IEEE Int. Conf. on Information Processing in Sensor Networks), and DCOSS (IEEE Int. Conf. on Distributed Computing in Sensor Systems), and commercial companies such as Crossbow, Ember, Sentilla, Dust Networks, Microsoft, Intel, and Sun Microsystems, to mention just a few. Numerous special issues of renowned journals on sensor networks have been published and special sessions of leading conferences organized, with more expected to appear in the future. Since its inception, WSN has grown into a hot research area at a tremendous pace. A large number of institutions and researchers around the world have set their feet in this field, and launched various research and development projects. Significant advances have been achieved in almost all aspects, including architecture, hardware, software, system design, supporting tools, standards, applications, etc [3]. WSNs are designed to gather information about the state of physical world and transmit sensed data to interested users, typically used in applications like habitat monitoring, military surveillance, agriculture and environmental sensing, and health monitoring. In most cases, they are unable to effect on the physical environment. In many applications, however, only observing the state of the physical system is not sufficient; it is also expected to respond to the sensed events/data by performing corresponding actions upon the system. For instance, in a fire handling system, the actuators need to turn on the water sprinklers upon receipt of a report of fire. This need for actuation heralds the emergence of wireless sensor/actuator networks (WSANs) [4-7], a substantial extension of sensor networks that feature coexistence of sensors and actuators. WSANs enable the application systems to sense, interact, and change the physical world, e.g., to monitor and manipulate the temperature and lighting in a smart office or the speed and direction of a mobile robot. It is envisioned that WSANs will be one of the most critical technologies for building the network infrastructure of future cyber-physical systems [8]. They will revolutionize the way we interact with the physical world. As shown in Figure 1, a WSAN is a networked system of geographically distributed sensor and actuator nodes that are interconnected via wireless links. Both sensor and actuator nodes are normally equipped with certain data processing and wireless communication capabilities, as well as power supply. In most situations, sensor nodes are stationary whereas actuator nodes, e.g., mobile robots and  
Figure 1: A wireless sensor and actuator network. unmanned aerial vehicles, are mobile. Sensors gather information about the state of physical world and transmit the collected data to actuators through single-hop or multi-hop communications. Upon receiving the required information, the actuators make the decision about how to react to this information and perform corresponding actions to change the behavior of the physical environment. The base station is principally responsible for monitoring and managing the overall network through communications with sensors and actuators. It is not until very recently that the area of WSAN has begun to emerge, regardless of one-decade research activities in WSN. Relatively very little work has been conducted on WSAN. In particular, quality of service (QoS) management in WSANs is an area yet to be explored [4]. WSANs are application-oriented, especially when used for cyber-physical computing. Therefore, QoS has to be supported by WSANs in order to achieve end users' satisfaction with the services that the system provides.