# Publish/Subscribe-Middleware-Based Intelligent Transportation Systems: Applications and Challenges
## 4 Application of Publish/Subscribe Middleware in ITS
### Autonomous Vehicles  
Autonomous vehicles are changing the landscape of both public and private land transportation. However, they face challenges in handling complex sensor data fusion, managing interconnected functions in intelligent vehicles, ensuring scalability and interoperability, achieving deterministic high-speed communication, and coping with dynamic communication patterns, all of which require advanced technologies and adaptive software platforms for reliable and real-time data exchange. As a solution to these challenges, [PERSON] et al. [74] proposed a system that integrates Open Platform Communications Unified Architecture (OPC UA) [75], Time-Sensitive Networking (TSN) [76] with Automotive Open System Architecture (AUTOSAR) Adaptive [77; 78] to provide deterministic high-speed communication. AUTOSAR Adaptive Platform is a service-oriented communication platform for standardized data exchange. OPC UA is a well-established communication standard [79] for modern industrial automation that supports publish/subscribe architecture, enhancing scalability. Publish/subscribe is designed to work with TSN, a collection of standards that adds real-time aspects to standard Ethernet [80] networks. TSN allows services with different requirements to share a single physical network. Publish/subscribe middleware also allows communication between different types of autonomous vehicles. In [81], the authors explored new messaging mechanisms for two-way communication between autonomous vehicles, focusing on scenarios where a rover (using Real-time Operating System 2 (ROS2)) and a drone (using PX4) either work independently or coordinate tasks. The PX4-FastRTPS Bridge connects the PX4 and the ROS2, enabling them to exchange messages between their components through the publish/subscribe model. The authors found that the microRTPS bridge facilitates communication and control between the rover and drone, effectively enabling seamless interaction and bidirectional communication. However, the system's reliance on protocol bridges introduces additional latency and potential points of failure, particularly under network congestion or limited bandwidth [82]. [PERSON] [PERSON] et al. [83] investigated how safety-critical railway applications can achieve reliable and secure communication within on-premise SIL4 cloud environments. An on-premise SIL4 cloud refers to a private cloud environment that is located and managed by an organization, specifically designed to host safety-critical applications and meets the highest safety integrity level (SIL4) as defined by CENELEC (European Committee for Electrotechnical Standardization) functional safety standards [84; 85]. The study emphasizes the importance of adhering to the EN 50159 safety communication standard [86]. It also evaluates the middleware Rail Safe Transport Application (RaSTA), DDS, and OPC UA protocols for compatibility with safety-critical railway requirements. The authors discovered a system architecture where DDS and OPC UA operate in a safe partition alongside safety components, separated from the non-safety-critical components partition. This design ensures safe communication over unreliable networks (black channels) by implementing a safety layer that handles errors such as delays, sequence faults, and unauthorized access. Their evaluation confirms that DDS and OPC UA Safety fulfill key safety requirements, support publish/subscribe models, and meet criteria such as authenticity, integrity, and timeliness, making them viable for scalable and secure railway communication systems. However, they found that RaSTA has limited security features. They also found that it is limited to point-to-point on-premise SIL4 cloud applications. [PERSON] et al. [12; 87] developed a DDS-middleware-based safety system for underground railway environments. It uses DDS middleware (OpenSplice version 5.5) to ensure the sharing of crucial information throughout the network, even in the event of a disconnect in communication.