## 3 CPS Architecture, Layers and Components
### Physical Layer  
The physical layer includes the objects of the physical system that are utilized in the real world. In this layer, the CPS system involves sensors and actuators in which information is transferred through sensors to the cyber world, and the actuator operates according to commands. Physical processes are performed by sensing and actuating in a continuous-time domain. Sensor-based devices such as radio frequency identification tags (RFID) and global positioning system (GPS) collect data in real-time to track objects of the physical systems. Data examples include heat, location, electric consumption, sound, and light signals [35]. Sensors collect data according to its type from the local or wide range and transfer it to the network layer which is later sent to the application layer for further analysis. Efficient security protocols are required according to the capability of devices for reliable communication between these layers. In a complex system, the physical system has constraints due to the power of an external battery. To solve this problem, the physical layer connects to the computation system using a network layer.  
\begin{table}
\begin{tabular}{l l l l} \hline \hline
**Ref.** & **Methods** & **Dataset** & **Findings** \\ \hline
[13] & Memetic & NSL-KDD \& KDD99 & PSO with higher accuracy \\ \hline
[14] & SC4 ID algorithm & UNM \& ADFA-LD & A new, more accurate approach for handling abnormal system calls. \\ \hline
[15] & SVM-ANN & NSL-KDD & High performance by a hybrid model \\ \hline
[16] & PCA-LDA-SVM & KDD-CUP 99 & Dimensionality reduction \\ \hline
[22] & Deep learning & KDD-CUP 99 \& NSL-KDD & Deep learning model with reliable outcomes. \\ \hline
[23] & Sparse autoencoder & CICIDS 2017 & Uses trigonometric simplexes \\ \hline
[24] & SVM & NSL-KDD & The logarithmic marginal density ratio \\ \hline
[25] & MSML & KDD-CUP 99 & Multi-level intrusion detection \\ \hline
[27] & Q learning & CRAWDAD & Federated jamming \\ \hline
[28] & DT, KNN, NB \& DNN & KDD-CUP 99, open-stack cloud & Socket programming and OpenStack firewall \\ \hline
[29] & Fuzzy logic & DDoS attack (T-shark) & Dynamic DDoS attack detection \\ \hline
[30] & SVM-KNN-PSO & KDD 99 & High precision ensemble model utilising a weighted method. \\ \hline
[31] & MDRA & KDD-CUP 99 & Real-time attack detection \\ \hline
[32] & MINDFUL & KDD-CUP 99, UNSW-NB 15, CICIDS & Multi-channel for deep feature learning \\ \hline
[33] & Deep hierarchical & NSL-KDD \& UNSW-NB15 & Data balancing using SMOTE \\ \hline
[34] & DT-RFE & KDD-CUP 99 \& NSL-KDD & Stacked approach \\ \hline \hline \end{tabular}
\end{table}
Table 1: Comparative analysis of the existing approaches.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Layer** & **Function** & **Attack** & **Target Area** & **Safety Measure** \\ \hline \hline \multirow{4}{*}{**Safety Measure**} & & Code Injection & & \\  & & Botnets & Security & Firewall \\  & & Malware & Privacy & Strong Authentication \\  & & Trojans & Authentication & Strong Authorization \\  & & Worms & Safety & Trust Management \\  & & Buffer Overflow & & \\ \hline \hline \end{tabular}
\end{table}
Table 2: Layered architecture with complete functionality and attack details.  
In order to build a re-configurable radio-based environment of the propagation channel and increase the received signal power, re-configurable intelligent surfaces (RIS) have been studied as a possible approach. A study focused on joint beamforming design and optimization for hybrid satellite-terrestrial relay networks with RIS support, when links from the satellite and base station (BS) to numerous users are blocked [36]. A satellite system should be secure and energy efficient in designing a communication system. A design is proposed by the researcher to improve security and decrease power consumption in [37].