## 1 Introduction  
The use of wireless communication systems in automation and real-time control applications is increasing at a steady rate. Wireless systems have the distinct advantage of reducing equipment installation complexity through the lack of a need for wiring and harnessing, enabling easier trouble-shooting and system re-configuration; this reduces the long-term maintenance requirements associated with wired systems [1; 2; 3; 4]. In addition, wireless sensor/actuator networks (WSANs) can potentially provide the device interconnectivity needed for a range of industrial control and monitoring functions across a wide range of operating environments [1; 2; 3; 4]. However, wireless systems are generally perceived in a negative sense for real-time and safety-related applications such as the sensor/actuator networks needed for industrial process control systems [5; 6; 7]. The use of wireless technologies in these applications poses several severe problems, which include out-of-order packet transmissions, high levels of packet jitter and high probabilities of packet losses; these problems are especially problematic in systems with strict timing constraints [4; 5; 6; 7]. Although much progress has been made in recent years, to date most industrial applications of wireless technology have mainly been restricted to soft real-time process monitoring and data acquisition applications, in which interruptions to the wireless service do not lead to unacceptable loss of control or damage to equipment [4; 7]. When control loops have been closed by wireless equipment in industrial situations, developers have often been forced to take drastic actions; for example the enforcement of \"blackout zones\" around the feedback control loops in question [7]. Indeed, as discussed by [4] and [7], the use of wireless technology in feedback control applications presents arguably the largest ongoing challenge in the domain, and is the focus of the current paper. The particular structure of WSAN architecture under consideration is as depicted in Figure 1, which is common in many industrial situations [1; 2; 4; 7; 8; 9]. In such a system, a distributed \"sensor\" node samples and processes the controlled variable, and communicates this information to a \"central controller\" node for control signal calculation. The controller communicates the control signal to a distributed \"actuator\" node for final processing and application of the controls to the plant manipulated variable. Although not explicitly indicated in the figure below, we assume that a wired network (typically Ethernet based) may also be present between the wireless interface and the central controller node. In addition, although again not explicitly indicated, we assume that the paths between sensor-controller and controller-actuator nodes may consist of multiple hops which are possibly routed through more than one bridged LAN. This would be a typical arrangement for a Distributed Control System (DCS) for an industrial plant. No specific assumptions are made regarding the operation principle of the underlying wired and wireless networks or as to whether the underlying protocol(s) are of the decentralized (Multi-Master) or centralized (Master-Slave) type, but it is well suited to either (i) a flexible TDMA scheme with polled Media Access Control (MAC) and support for retransmission in case of detected errors (e.g., see [8; 9; 10; 11; 12; 13]) which many industrial networks are based upon or (ii) a non-specialized architecture such as WiFi, along with a protocol such as sNTP (simple Network Time Protocol) for node clock synchronization. In this paper, the concern is with ameliorating the effects of sensor and controller packet losses in such a sampled-data Distributed Control System (DCS). Such packet losses present as temporary interruptions to the inter-node information flows in Figure 1. The techniques described in this paper provide extensions to earlier techniques developed by the authors [14], which overcome some of the deficiencies identified in the previous work.