# Functional Safety Networks and Protocols in the Industrial Internet of Things Era
## 3 Functional Safety Protocol over Wireless Networks  
The introduction of the IIoT paradigm relies on a pervasive connectivity that can be ensured by the introduction of wireless networks at all the automation levels. Hence, the black channel approach discussed so far represents an ideal approach for porting functional safety protocols over those industrial communication systems not having a specific safety extension. In particular, wireless networks could be adopted, paving the way to the introduction and integration of wireless functional safety networks in IIoT ecosystems. However, this clearly represents a challenging issue, since the unreliability of the communication medium may have a negative impact on the behavior of the safety protocols. Although several analyses and practical experiments have been carried out over the years to address the adoption and the performance of wireless networks in the real-time industrial scenario [4], only some preliminary studies focused on the portability of functional safety protocols on wireless systems. As an example, in [18], the authors refered to WirelessHART and proposed a safety protocol based on an event-triggered technique, called \"Safe-WirelessHART\", whereas in [19], the authors presented an implementation of the Fail Safe over EtherCAT (FSoE) protocol over WiFi. The key aspect, in this context, is represented by the ability of the safety protocols, implemented on wireless networks, to ensure the required performance, particularly in terms of residual error probability and reaction times. Networks of interest are Bluetooth, the IEEE 802.15.4 Wireless PAN, the IEEE 802.11 Wireless LAN and 5G. These are general purpose wireless communication systems that have been (or are going to be, for the case of 5G) considered for industrial applications, possibly in conjunction with purposely developed protocols, and, hence, are good candidates for the black channel approach of functional safety networks. The residual error probability is a performance index typically adopted by functional safety networks, and hence, unfortunately, no data are available yet for wireless systems. However, REP is strictly related to reliability, for which some considerations can be made. Actually, the aforementioned networks, in their legacy versions, are not suitable for industrial communications, mostly due to the non-negligible percentage of lost packets that may affect their behavior. Nonetheless, the introduction of application layer protocols, specifically conceived for the industrial scenario, brought significant improvements. Some examples in this direction are Wireless HART, which relies on IEEE 802.15.4; the Wireless Interface for Sensors and Actuators (WISA), developed on top of Bluetooth; and Wireless networks for Industrial Automation-Factory Automation (WIA-FA), based on IEEE 802.11. These are able to cope with the stringent reliability requirements of the industrial scenario, revealing performance figures close to those of their wired counterparts. Thus, it may be envisaged that wireless functional safety networks based on the black channel approach may be implemented by replacing the wired industrial network stack in Figure 1 with any of the aforementioned wireless ones. The reaction time of a functional safety network is defined as the maximum time in which a safety action, triggered by an event, is performed. A typical case is the sending of the emergency stop command to all the safety nodes of the network, as a consequence of a fault detection. In addition, in this scenario, no specific data are available for wireless systems. However, useful indications can be obtained from the performance of industrial wireless networks. Indeed, the most popular FSN protocols (e.g., Profisafe, FSoE and Ethernet Powerlink Safety) are based on a master-slave relationship, where the safety master regularly polls the slaves, exploiting the services provided by the underlying application layer, at the black channel border.