# Publish/Subscribe-Middleware-Based Intelligent Transportation Systems: Applications and Challenges
## 4 Application of Publish/Subscribe Middleware in ITS
### Infrastructure
#### 4.1.2 Vehicular Ad Hoc Networks (VANETs) Infrastructure  
VANETs are composed of mobile nodes (vehicles) equipped with on-board units (OBUs) and often fixed nodes in the form of Roadside Units (RSUs) [53; 54]. Figure 9 shows a typical VANET system. They facilitate both Vehicle-to-Vehicle (V2V) and Vehicle-to-Infrastructure (V2I) communications, commonly using protocols such as IEEE 802.11p to enhance traffic safety and efficiency [53]. The network topology is highly dynamic, with vehicles forming a mesh topology that continuously reconfigures itself as they move without the help of a central coordinator. Also, the nodes relay messages hop-by-hop to enable multihop reach beyond direct wireless range [54]. RSUs help bridge vehicles to broader networks, traffic management centers, or the Internet [54]. VANETs are integral to ITS, supporting applications like passenger safety, traffic management, and road condition monitoring. They also leverage IoT technologies to improve transportation efficiency, safety, and comfort, ultimately playing a key role in developing autonomous vehicles [53]. Researchers stress the importance of VANET and middleware in improving ITS performance. [PERSON] et al. [55] explored reliable communications within VANETs, a key component of ITS. Its strengths lie in improving real-time safety alerts, traffic management efficiency, flexible communication modes, and adaptability to various traffic conditions. They high light challenges in VANET communication, emphasizing high mobility, unstable network conditions, and heterogeneous communication technologies that affect reliability and performance. To address these issues, the study suggests multipronged communication models that integrate Vehicle-to-Vehicle (V2V), Vehicle-to-Infrastructure (V2I), and Vehicle-to-Broadband Cloud (V2B) communication, ensuring seamless data exchange between vehicles, RSUs, and cloud-based services. The authors argue that middleware solutions are critical in managing heterogeneous networks, facilitating adaptive switching between Wi-Fi, cellular, and dedicated vehicular networks (802.11p and WAVE) to maintain stable and low-latency communication. However, it faces challenges such as the complexity, cost of implementation, and potential security and privacy issues. [PERSON] et al. [56] proposed V-ICE, a publish/subscribe-based communication model for timely black ice warnings in ITS. It leverages Information-Centric Networking (ICN) and a hierarchical namespace architecture. It also uses Roadside Units (RSUs) as intermediaries to ensure vehicles subscribe to relevant road segments and receive updates when black ice is detected. The study introduces V-ICE-R (an extended version of V-ICE) that integrates the Content-Oriented Publish/Subscribe System (COPSS) [57] middleware to enhance reliability and timeliness, ensuring notifications are received even when vehicles temporarily lose network connectivity. Simulation results demonstrate that V-ICE-R significantly outperforms server-based and broadcast-based approaches, reducing the percentage of cars encountering black ice without prior notification to 0.32% in a 30-min event scenario and 1.45% in a high-frequency 6-min scenario, compared to 19.30% and 66.04% for server-based solutions, respectively. V-ICE-R also minimizes network congestion, generating 145,758 messages in the 30-min scenario. However, the server-based polling produces 222,241 messages, and the broadcast-based flooding results in 5.99 million messages, demonstrating the efficiency of event-driven messaging in reducing redundant data transmission. The system also employs message filtering mechanisms at RSUs to suppress duplicate reports from multiple vehicles, optimizing network traffic and system performance. Through SUMO-based simulations in Luxembourg, the study validates that publish/subscribe middleware provides a scalable, low-latency, and reliable solution  
Figure 9: A typical VANET.