## 1 Introduction
This is especially true in the case of low-power communications and in the presence of node mobility. With these characteristics, the quality of service (QoS) of the network cannot be always guaranteed. A natural result is that control applications will suffer from time-varying delay and packet loss, both of which could significantly degrade the control performance, or even cause system instability. Therefore, WSANs must be well designed when deployed to control applications. The design of WSANs featuring node mobility is investigated in this paper for control applications. The overall goal is to enhance the reliability of WSANs so that the required performance of control applications is guaranteed in dynamic, lossy environments. In particular, our focus is on dealing with unpredictable packet loss caused by unreliable link quality in mobile WSANs, without considering the effects of time-varying delay. The reasons behind this choice of focus can be briefly explained as follows. Firstly, since a packet loss can be equivalently regarded as a delay with a magnitude of infinity, from the viewpoint of control, packet loss is a factor that often has more significant impact on the resulting control performance than delay. Secondly, several methods have been presented in the literature to cope with time-varying delay in control loops closed over wireless sensor networks [17; 18; 19; 20; 21], while the packet loss problem that arises in WSANs is yet to be investigated. An application-level design methodology will be developed for WSANs based on an experimental study of the link quality properties and a compensation method for packet loss. The main contributions of this paper include:  
* The link quality of WSANs is characterized in terms of packet loss rate through experiments on a real deployment. The experimental results provide important insight into how the WSAN should be designed from the application point of view. Moreover, they are also of great value to the design and evaluation of sensor network protocols and algorithms. * A simple yet efficient method is developed to deal with unpredictable packet loss at actuator nodes. It can significantly improve the QoS of WSANs under unreliable channel conditions, and facilitates the implementation of (mobile) control applications over WSANs. * The proposed approach is evaluated and verified using trace-based simulations that extract data from the real experiments. In this way, real characteristics of the wireless links are taken into account in performance evaluation. Promising results are presented and analyzed. The proposed design methodology is a _generic_ solution in the sense that: 1) It does not require any modification to low layers such as physical layer, MAC layer, and transport layer within the network protocol stack. Only the application layer is involved. Therefore, it is independent of the underlying communication protocols, for example, MAC and routing protocols, utilized in the WSAN. 2) It does not require any knowledge about the models of the physical systems to be controlled or the design of the control algorithms. It is suitable for a wide range of control applications. 3) The proposed algorithm is computationally cheap yielding only a small runtime overhead. This meets well the general WSAN design requirements stemming from the constraints on data processing capacity and energy consumption in actuator nodes, thus making the proposed approach applicable to various WSA platforms subject to resource constraints. This paper is organized as follows. Section 2 briefly reviews related work with respect to WSAN, control over WSNs, link quality analysis, and packet loss handling. Section 3 discusses the architecture and design challenges of WSANs from an application perspective. In Section 4, the properties of link quality are captured in terms of packet loss rate through experiments on a real WSAN. Aiming to improve the reliability and QoS of WSANs, Section 5 proposes a method to handle packet loss.