# Publish/Subscribe-Middleware-Based Intelligent Transportation Systems: Applications and Challenges
## 4 Application of Publish/Subscribe Middleware in ITS
### Communication Technologies
#### 4.3.1 Peer-to-Peer (P2P) Communication  
P2P has many applications in land transportation. Most protocols standardized under the European standard EN 50159 [91] are P2P protocols that involve two communication partners [83]. The standard specifies the requirements for safety-related communication in railway applications, focusing on the safety measures necessary for communication systems that are essential for the operation and safety of railway systems.  
In road transportation, [PERSON] et al. [15] proposed the ITS Information Platform (IIP), an event-based platform for ITS. The platform comprises two main components: a cloud component that provides canonical publish/subscribe services to traffic management facilities and mobile nodes and a client component that enables pub/sub functionality among mobile nodes. To address situations where cloud communication is infeasible due to latency constraints, the authors introduced a mobile P2P interface within the client component. This interface facilitates direct, infrastructure-independent communication between mobile nodes, such as vehicles, pedestrians, and cyclists, via short-range, high-bandwidth wireless channels. A motivating use case is at unsignalized intersections, where entities must coordinate right-of-way without traffic signals or centralized control. However, a key limitation noted in the paper is that the mobile P2P event broker operates as a best-effort mechanism, lacking guarantees for complete event-subscription matching in highly dynamic environments such as vehicular networks. This constraint poses challenges for safety-critical ITS applications where reliable and timely communication is essential.  
Similarly, [PERSON] et al. [92] proposed an IP Multimedia Subsystem (IMS)-based platform for ITS that supports publish/subscribe communication through standardized IMS services. The system also has IMS Presence Service, which enables vehicles (as presentities)to publish real-time status information, such as location, speed, and operational state, while allowing authorized watchers (e.g., traffic control centers) to subscribe to this data. The architecture integrates supporting components like the Resource List Server (RLS) and XML Document Management Server (XDMS) to manage contact lists and filtering rules, enabling scalable and privacy-aware dissemination of presence information. This publish/subscribe infrastructure supports services such as fleet monitoring, access control to restricted areas, and emergency coordination, with the underlying IMS layer providing robust session control, authentication, and network abstraction across multiple communication technologies. However, the authors acknowledged a potential performance bottleneck: Application Servers (ASs) may become overwhelmed by the volume of notifications with heavy utilization of presence-based services. Although the authors proposed mitigations such as load balancing and multiple IMS domains, the solution's scalability under large-scale real-world deployments remains an open challenge.