## 1 Introduction  
The term operational technology (OT) refers to technologies and systems that are used to monitor, control, and automate different physical processes in industrial systems, including but not limited to manufacturing, energy, and transport sectors. OT can be defined as \"hardware and software that detects or causes a change through the direct monitoring and/or control of industrial equipment, assets, processes, and events\" [1]. Typically, OT systems include programmable logic controllers (PLCs), supervisory control and data acquisition (SCADA) systems [2], and distributed control systems (DCSs) [3]. Compared to Information Technology (IT), which mainly deals with data processing and distribution, OT's main function is managing the physical processes [4]. OT is often mixed or used interchangeably with another close term: Industrial Control Systems (ICSs) [5]. Still, although rather close, these concepts have some differences. While OT is a wider term that covers all hardware and software systems used to control and monitor physical processes, ICS is a specific category within OT and refers to control systems and devices used to operate industrial processes; in other words, ICSs are a subset of OT. OT integrates hardware and software for general process operational management, while ICSs concentrate on control and automation systems designed for industrial use only. It is also possible to face the closely related term of Cyber-Physical Systems (CPSs). The field of CPS is extensive, covering conventional ICSs, SCADA, and smart systems based on the Internet of Things (IoT). In scholarly discussions, the term CPS is frequently used, whereas in the industrial sector, the term Industrial Internet of Things (IIoT) is more commonly adopted for industrial control systems that include manufacturing, process control, and associated fields [6]. Utilizing OT enables organizations to significantly improve the efficiency, quality, and reliability of processes being managed via OT. Integration of OT and IT has led to the emergence of Industry 4.0, which is known to optimize the computerization of Industry 3.0, and it is formed by combining IT and OT for monitoring and control processes, maintenance and predicting failures, and many other functions [7]. However, the use of OT, which historically was used as isolated environments, in combination with general-purpose IT and OT, has raised questions related to their security and safety risks, especially taking into account that OT is widely used to control critical infrastructure. As the primary objective of cybersecurity in OT is to ensure the safe operation of physical assets at all times, there is a clear need to combine safety and security concerns with control systems to handle cyber-attacks on safety-critical systems that have the potential to cause real harm in the physical world [8]. As OT networks merged with IT networks, the differing priorities have created ongoing challenges that remain unresolved. IT networks strongly focus on authentication and authorization, which align with the confidentiality and integrity components of the CIA triad. On the contrary, OT networks were historically centered around availability, making it common to presume authentication and authorization through physical access to a secure and isolated OT network. However, this traditional belief in a completely trusted and isolated setting is no longer valid [9]. OT cybersecurity has gained increasing attention since 2010 [8]. Attempts are being made to incorporate the Intrusion Detection System/Intrusion Prevention System (IDS/IPS) functionalities of IT networks into OT networks. However, the lack of standardized protocols and interfaces presents major challenges for detecting threats [6]. The authors of [8] also stress attention on the following features of OT: the use of purpose-built technologies and protocols, long life-cycle compared to IT systems, difficulties in OT upgrades that may require plant shutdowns, and issues related to reliability and intellectual property.