# A Controller Switching Mechanism for Resilient Wireless Sensor-Actuator Networks
## 2 Related Work  
Wireless networking is a key technology to realize industrial CPSs. Various organizations and alliances have developed many different protocols that operate in various frequency bands. These protocols can be divided into various communication features, such as communication range and frequency band. Table 1 shows the features of three widely used wireless network protocols for industrial cyber-physical systems. Long-range wide area network (LoRaWAN) is a wide-range network protocol designed to support the transmission of 2-15 km with low-power consumption. LoRaWAN uses unlicensed sub-gigahertz radio frequency bands such as 169, 433, 868 (Europe), and 915 MHz (North America) [13]. LoRaWAN has a star topology, which means that each node needs to directly communicate with a gateway. Therefore, citywide environmental sensors, and streetlamp control and monitoring are the main applications rather than device-to-device communication [14]. Bluetooth low-energy (BLE) and low-rate wireless personal area networks (LR-WPAN) are short-range network protocols [15]. BLE is the RF-based technology that utilizes received-signal-strength indicators. Industry use cases of BLE are mostly real-time location systems and indoor positioning systems. IEEE 802.15.4 LR-WPAN was developed to obtain information regarding production environments and to monitor physical systems in the 2.4 GHz unlicensed band [16]. The superframe applied in IEEE 802.15.4 is divided into active and inactive sections. The inactive section is divided into a content access period (CAP) and a content free period (CFP); for CAP, a carrier sense multiple access/collision avoid (CSMA) is used, and for CFP, a time division multiple access (TDMA) is used. The problem of IEEE 802.15.4 is that CSMA/CA cannot provide a bounded delay and high packet-delivery ratio to reach the final destination when a large number of nodes exist. In addition, the maximal number of TDMA slots that can be increased is limited to 7. To solve this problem, IEEE 802.15.4e extends the previous 802.15.4 standard by introducing MAC behavior modes. IEEE 802.15.4e provides MAC modes of distributed synchronous multichannel extension (DSME), time-slotted channel hopping (TSCH), and low-latency deterministic networks (LLDN) [17]. DSME MAC increases the slots of TDMA in a superframe and implements multichannel frequency hopping. TSCH MAC improves transmission rate by dividing time into slots and changing channels for every slot. LLDN MAC was designed for very-low-latency single-hop and single-channel networks. The recent introduction of actuator nodes enabled the realization of WSANs [21]. Recent efforts aimed at improving the reliability of networks on the basis of their real-time states. The most common WSAN protocol is WirelessHART, which is based on the physical layer of IEEE 802.15.4. The TDMA protocol of WirelessHART and IEEE 802.15.4e provides predictable communication latency. However, network-induced time delays and data losses are inevitable in networked control systems. The disadvantage of TDMA-based communications is that retransmissions frequently occur owing to the low packet transmission rates; retransmissions also increase the overall time delay. Because one TDMA slot must be allocated for one-hop transmissions, end-to-end delays are exacerbated for retransmissions of multihop transmission. They also require a considerable amount of time to recover in the event of network failures. Therefore, TDMA protocols are not suitable for fast controller switching. Much research on close feedback loops over WSANs considers the stability of the physical system. To ensure the stability of the physical system, studies on WSANs aim to reduce time delays, and improve reliability and real-time performance [22; 23; 24]. More recently, designs of WSANs have been moving to support fast update intervals to keep up with the physical dynamics over multihop communication. In [25], the authors demonstrated closed-loop control over wireless single-hop with inverted pendulum systems.