# Deep Neural Networks for Spatial-Temporal Cyber-Physical Systems: A Survey
## 5 Challenges and Future Research Directions
For example, network protocols (UDP and TCP by determining the reliability and latency of data transmission), data serialization protocols (JSON, protocol buffers by affecting the data size, encoding/decoding overhead), compression protocols (by scaling/shrinking the data size during transmission to improve the network performance), real-time communication protocols (MQTT, DDS by providing low-latency, publish-subscribe messaging for timely data delivery), and synchronization protocols (PTP, NTP by ensuring time synchronization in distributed systems, which aides coordinated processing. Cross-platform sensor-actuator communication remains a challenging issue, and it is important to design a comprehensive quality of service framework and satisfy the performance requirements of CPS. In addition, sensor failure is another remaining challenge because most CPS heavily rely on sensing data for the sake of control and motoring purposes. The entire CPS will not function well if there are failures of some sensors within the ecosystem. Thus, the deployment model shall be thoroughly studied to guarantee the robustness of sensor deployment (e.g., coverage, connectivity). Therefore, it is critical to design a holistic solution to ensure the overall performance of CPS by considering all components and their integration as one complex system. The realization of the performance satisfaction of CPS systems depends on the performance with respect to computing, control, and communication. Thus, it is critical to design the modeling and optimization techniques to integrate all components (sensing, networking, computing, and data). Some existing research efforts have been conducted to address the integration of some components (sensing, control, networking, and data). Nonetheless, to enhance the performance of CPS, how all its components interact and interplay jointly, leading to a unified design and optimization strategy, is worth investigation. * **Security:** CPS has unique system requirements and security challenges. Specifically, the confidentiality, integrity, and availability (CIA) security paradigm has been widely used to design security standards for information technology-driven systems. For example, availability is a crucial property regarding security and an essential requirement of a CPS. Different threats (DoS attacks, malware propagation, etc.) could affect the availability of CPS. Under this situation, computing and networking components in the CPS shall employ effective mitigation measures so that malicious computing requests and traffic can be detected in time and the impact of such attacks can be effectively mitigated. For CPS integrity, an ML model that depends on real-time data inputs is critical for the realization of highly dependable and trustworthy CPS (transportation infrastructure, manufacturing infrastructure, etc.). Data fidelity is crucial for the CPS, as it is the information that can accurately simulate and direct the physical system in response to environmental changes accurately and quickly. In CPS, an adversary could compromise the integrity of sensing data by intercepting the communication channel using either a man-in-the-middle (MITM) attack or the commands transmitted by the programmable logic controllers. Thus, security measures (device authentication, etc.) shall be in place to prevent unauthorized users from changing data. Although solutions based on cryptography have been promoted in the context of CPS, such as those that use TLS, HMACs, or other authentication and integrity guarantees. Historically, such countermeasures have not been widely used due to hardware restrictions and the relative computational cost of deploying protocols and mechanisms. Table 7, summarizes the challenges of utilizing the DNN in CPS spatial-temporal data. Based on the requirements of performance and security in CPS, we consider the following fundamental research challenges that are required for further research concerning the performance of DNNs and the performance requirement of CPS.