# A Controller Switching Mechanism for Resilient Wireless Sensor-Actuator Networks
## 2 Related Work
They employed TDMA for the MAC layer and Bluetooth 5.0 for the PHY layer for communication between robot and remote controller. Communication latency was approximately 2 ms. In [26], the authors proposed a fast feedback control over wireless multihop networks at update intervals of 20-50 ms. They used LWB as a network protocol to determine its advantages over TDMA of WirelessHART and IEEE 802.15.4e. They designed a dual-processor platform that includes an application processor and a communication processor to achieve the predictable and efficient execution of all application tasks and message transfers. Application tasks exclusively execute on an application processor, while the wireless protocol executes on a dedicated communication processor. In addition, studies have been conducted to solve problems caused by limited network bandwidth and network energy consumption problems in WSANs. The authors of [27] introduced a holistic control architecture that integrated an LWB and control strategies for rate adaptation and self-triggered control \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Features** & **LoRaWAN [13; 18]** & **BLE [15; 19]** & **IEEE 802.15.4 [17; 20]** \\ \hline Frequency & 169, 433, & 2.4 GHz & 780, 868, 950 MHz, \\ Band & 868, 915 MHz & 2.4 GHz & 2.4 GHz \\ Bandwidth & 0.5–125 KHz & 80 MHz & 2 MHz \\ Data Rate & 290 bps–100 kbps & 125 kbps–2 Mbps & 100–250 kbps \\ Medium & & & CSMA/CA, TDMA, DSME, \\ Access & & & TSCH, LLDN \\ Communication & & & \\ Range & 2–15 km & 10 m & 100 m \\ Topology & Star & Mesh & Mesh \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Comparison of three wireless networking for industrial CPSs. strategy. Similarly, the authors in [28] combined resource reallocation and resource savings using self-triggered control. The control system informs the communication system at run time about its resource requirements, and the communication system leverages this information to reallocate resources. WSANs should be fault-tolerant to ensure the stability of the physical system. Fault tolerance refers to the ability of an architecture to tolerate and overcome system faults by implementing corrective actions. Various mechanisms were developed to overcome the malfunction or failure of nodes due to power depletion and environmental impacts [29]. To resolve sensor node failure in WSANs, the authors in [30] proposed an algorithm that assigns a hardware condition to each node by cellular learning automata. They introduced a routing algorithm on the basis of the status of the nodes to reduce the energy consumption of the networks. On the other hand, a failure avoidance technique of the actuator was proposed to solve the actuator energy-constrained problem in WSANs [31]. They adopted a mobile charger to prevent the energy depletion of actuators. Unlike sensor and actuator nodes in WSANs, controllers do not have energy problems but may fail due to environmental impacts. However, research on the failure of the controller in WSANs is insufficient. Therefore, new techniques are required to ensure the stability of physical systems during controller failure. Fault tolerance is typically achieved by introducing redundancy in the software and/or hardware. Controller switching is one of the methods of ensuring fault tolerance with a backup controller as a redundancy. In [32], the authors presented a system-level Simplex architecture to preserve safety in the presence of logical faults and applied it to an inverted pendulum. When the inverted pendulum state passed the edge of the recoverable region, the safety controller took over and prevents system collapse. In [33], the authors use supervised learning to distinguish between disturbances and malicious behavior. Once the support vector machine detected that an attack occurred, the control was switched from the primary programmable logic controllers (PLC) to a secondary PLC that executed the same controller in parallel. However, none of the works considered network protocols and controller failures.