  There is a growing interest in design and implementation of cyber-physical
control systems over wireless sensor and actuator networks (WSANs). Thanks to
the use of wireless communications and distributed architectures, these systems
encompass many advantages as compared to traditional networked control systems
using hard wirelines. While WSANs are enabling a new generation of control
systems, they also introduce considerable challenges for quality-of-service
(QoS) provisioning. In this chapter we examine some of the major QoS challenges
raised by WSANs, including resource constraints, platform heterogeneity,
dynamic network topology, and mixed traffic. These challenges make it difficult
to fulfill the requirements of cyber-physical control in terms of reliability
and real-time. The focus of this chapter is on addressing the problem of
network reliability. Specifically, we analyze the behavior of wireless channels
via simulations based on a realistic link-layer model. Packet loss rate (PLR)
is taken as a major metric for the analysis. The results confirm the
unreliability of wireless communications and the uncertainty of packet loss
over WSANs. To tackle packet loss, we present a simple solution that can take
advantage of existing prediction algorithms. Simulations are conducted to
evaluate the performance of several classical prediction algorithms used for
packet loss compensation. The results give some insights into how to deal with
packet loss in cyber-physical control systems over unreliable WSANs.
