## 2 Related Work  
While significant effort has been made in research and development of WSNs in recent years and tremendous advancements have been achieved with respect to deployment, localization, MAC protocols, power control, topology control, routing, distributed signal processing, and security [22], WSANs are a relatively new research area with limited progress. [PERSON] and [PERSON] [1] described research challenges for coordination and communication problems in WSANs. [PERSON] and [PERSON] [2] discussed the opportunities and challenges for service-oriented sensor/actuator networks. [PERSON] _et al._[23] studied the route design problem for mobile actuators and developed a practical algorithm to reduce the waiting time of sensors. [PERSON] _et al._[3] presented a sensor-actuator coordination model based on an event-driven partitioning paradigm. [PERSON] _et al._[24] deployed a large heterogeneous WSAN on a working farm to explore sensor network applications that can help manage large-scale farming systems. A power-aware many-to-many routing protocol can be found in [9]. Despite their contributions in WSAN, none of the networks designed in these works are particularly for real-time control applications. Sensor networks have started to attract the attention of control engineers. [PERSON] _et al._[10] developed distributed ad-hoc network algorithms to facilitate executing control procedures in a distributed manner. [PERSON] [11] prototyped a light monitoring and control application as a case study of WSANs. [PERSON] _et al._[25] illustrate the main challenges in developing real-time control systems for pursuit-evasion games using a large-scale sensor network. A mixed model for design, analysis, and synthesis of control algorithms within sensor networks has been presented in [26]. [PERSON] _et al._[27] dealt with some of the design issues of a highly modular and scalable implementation of a WSAN for factory automation applications. Considering networked control systems (NCSs) over WSNs, [PERSON] _et al._[17] developed a gain scheduler to cope with time-varying delay induced by dynamic changes in the number of hops in multi-hop communications. [PERSON] _et al._[18] also considered the effect of time-varying delay caused by multi-hop communication, and proposed a predictive control scheme with a delay estimator. Various design challenges associated with control over wireless networks have been addressed in these papers, but the impact of packet loss as a result of unreliable communications in WSANs, particularly those with mobile nodes, on the performance of the control applications remains an open issue, and needs to be investigated systematically. Experiments have been conducted for analysis of the link quality in sensor networks, e.g., [28-31]. The authors reported, respectively, their measurements of the packet delivery performance of sensor networks of different sizes in different environments. Some important characteristics with respect to, e.g., packet loss rate in WSANs, have been captured in these papers; but none of the analysis hasintended for real-time control applications. For example, the relationship between the resulting control performance and the link quality has not yet been characterised. Also, no methods have been developed in these papers to address the observed unreliable packet delivery. In the control community, effort has been made for packet loss compensation. A recent survey on this topic can be found in [32]. Despite their differences, most of existing packet loss compensation methods have the common features that: 1) they depend heavily on the knowledge about the accurate models of the physical systems to be controlled, and, possibly, the controller design; and 2) the relevant algorithms are computationally intensive. Due to these reasons, they are impractical for real systems lacking well-established mathematical models. In particular, they are not the desirable solutions for resource-constrained WSANs because of too large computational overheads.