## 2 Background Preliminaries
### Overview of the Perception Layer Threats and Attacks  
Frequently, CioT systems and MioT environments are subject to similar threats at the perception or sensing layer. However, attacks against the MioT environment sensing layer occur with ferocity, velocity, rapidity, and higher frequency. Hence, we focus our discussion on threats against the CioT perception layer while considering the IioT implementation of a CioT. Further, we consider some studies that have focused on the investigation of threats associated with the various layers of the IioT architecture to provide a quick overview. Some modern attacks on IioT systems mainly target sensors, actuators, communication links, and control systems [19]. Therefore, identifying threats against the perception layer cannot occur in isolation; one must consider the abstraction layers that interface with and control the perception layer. While the study presented in [19] referred to the interface and control abstractions as the data-link layer, the study presented in [20] separated the interface and control abstractions into layers 2 and 3. Layer 2, which interfaces with the perception layer, or layer 1, included the distributed control system (DCS), programming logic control (PLCs), and gateways. Layer 3 included supervisory control and data acquisition (SCADA), data acquisition devices, and the human-machine interface (HMI). Other studies, such as the study presented in [21], described the IioT architecture using three layers: perception (or object), network, and applications. As a result, we consider threats against the perception layer in addition to the network layer. Focusing on the IioT system, a CioT, the study in [20] investigated possible attacks at various levels of the IioT architecture. Attacks at the lower layers, including the perception layer, exploit the vulnerabilities in the communication protocols used by the various IioT/IoT nodes to communicate. At the perception layer, possible attacks include reverse engineering, malware, the injection of crafted packets, eavesdropping, and brute-force searches. Attacks at the layer interfacing with the perception layer include replay, Man-in-the-Middle (MitM), sniffing, wireless device attacks, and brute-force password guessing. The study identified IP spoofing, data sniffing, data manipulation, and malware as possible attacks at the control layer. However, possible attacks against the control layer include attacks against industrial control systems such as SCADA systems. In addition to MitM, these attacks include passive and active eavesdropping, masquerade, virus, trojan horse, worms, denial of service (DoS), fragmentation, Cinderella, and DoorKnob rattling. While the study presented in [20], which limits the DoS attack to the interface layer or layer 2, the study presented in [19] considers DoS attacks to be among the most common threats in the perception and data-link (layers 2 and 3, respectively) layers. Further, the study classified the attacks on the perception and data-link layers based on the wireless communication protocol as jamming DoS attacks, collision DoS attacks, exhaustion DoS attacks, unfairness attacks, and data transit attacks. Collision DoS attacks result from continuous DoS attacks and unfairness attacks result from exhaustion DoS attacks that degrade the system's ability in favor of the attacker. Usually, attacks against the data-link layer aim to speed up or slow down the ICS operations to damage the ICS devices permanently. The study presented in [22] classified the attacks based on an IoT layer architecture consisting of three layers: the perception, network, and application layers. In addition to jamming and reply attacks, attacks against the perception layer include node capture, malicious code injection, and sleep deprivation.