# SDN-Based Secure Common Emergency Service for Railway and Road Co-Existence Scenarios
## 1 Introduction  
Today, the cohabitation of rail and road environments is unavoidable in many countries, due to the existence of level crossings with varying levels of safety (presence or absence of barriers, for example) or the deployment of trains in an urban environment, cohabiting with cars, as can be seen today in China [1]. These points of intersection are proving to be excitentogenic, in particular, due to human error, especially for car drivers, major emergency braking constraints in the rail environment (several hundred meters for high-speed lines), and lack of synchronization between railway and car environments [2]. As a result, securing the coexistence of these two worlds has become a priority for many regions and public and private rail companies. To achieve this goal, the first idea implemented was to define new services within each domain. For example, in the railway domain, the European Railway Traffic Management System (ERTMS) [3], based on the Global System for Mobile Communications--Railway (GSM-R) [4], supports the enhanced Railway Emergency Call (eREC) [5]. The eREC is based on the Voice Group Call Service (VGCS) with additional preemption features. Whena user initiates an eREC call, the request arrives at the serving GSM-R network as shown in Figure 1, which creates a group call to all the entities in the same area, as well as to the area of the dispatcher/controller. The call is created even if no resources are initially available for it, as the network disconnects any other, lower-priority, call to free resources and prioritizes the eREC communication. Similarly, in road communications, the European Telecommunications Standards Institute (ETSI) has defined two main message types: the Cooperative Awareness Message (CAM) and the Decentralized Environmental Notification Message (DENM) [6]. CAM messages provide periodic updates about the vehicle's status, while DENM messages transmit specific, often emergency-related, information. These messages can indicate obstacles, lane changes, or sudden slowdowns, and are disseminated quickly in the local area via vehicle-to-vehicle (V2V) communications and more broadly through roadside infrastructure. Therefore, it can be inferred that both sectors have mechanisms to manage emergencies but no unified system exists across them. However, a growing idea today is to design joint services, interconnecting road and rail services. Indeed, having a common service to distribute the emergency alert could offer several benefits. It could improve communication between different emergency services and it could also ensure that both the domains have the same information at the same time. This will help to build coordinated and efficient responses to emergency situations and to establish a common understanding of an emergency situation that can improve decision making and response time [8]. Along with these, sharing civil and network/telecommunication infrastructures for railway and road coexistence scenarios restricts duplication and minimizes direct investment towards marginal areas. In this way, it reduces the capital expenditure (CapEx) and operating expense (OpEx) [9; 10]. However, effectively addressing and managing emergency or critical situations within the operational contexts of railways and roads remains an ongoing and open area of inquiry [11]. Software-defined networking (SDN) is one of the technologies that could guarantee fast, efficient distribution of information across shared or independent communication infrastructures, particularly for critical or emergency situations. SDN can oversee and manage the entire network as a single entity and eliminates the need for individual device configuration. It allows for precise routing and prioritization of traffic based on specific needs. It also offers advanced data traffic differentiation and centralized network control. SDN controllers facilitate centralized information collection and processing, unlike traditional networks that require comprehensive information exchange for security.