## 3 Camino Architecture for V2X Communication  
The architecture of the CAMINO framework is shown in Figure 2. CAMINO provides flexible, modular support for C-ITS services and embraces hybrid communication by interfacing with a variety of different communication modules. The CAMINO-Core component implements the main functionality that is needed to control the incoming and outcoming data flows between the northbound and southbound interfaces. At the southbound interface, CAMINO-Core interconnects with the different V2X wireless technologies using the different transceiver classes. For example, it can communicate with commercial C-V2X PC5 and ITS-G5 modules via UDP sockets, for which a proprietary header is added to the messages, and with an MQTT broker through TCP over wired or cellular networks. The transceiver classes can interact with the CAMINO-Core services via the Communication (Comm) Controller. The Comm Controller is a component in the CAMINO-Core architecture that is responsible for managing the C-ITS messages that are transceived by the services via the supported communication paths. Messages that are coming from the services will be mapped onto the transceivers based on the configuration settings. The Comm Controller passes the service identifier so that it can be used as part of the Basis Transport Protocol (BTP) header in the ITSG5 and C-V2X PC5 transceiver and wireless modules, or in the topic for the MQTT transceiver. Messages that are received by the transceivers via the southbound interface, will be forwarded and processed by the Comm Controller. Based on the service identifier, derived from the BTP header or MQTT topic, the Comm Controller will pass the message to the correct service. At the northbound interface, CAMINO-Core is integrated with the Distributed Uniform Streaming (DUST) open-source framework [13; 14] that provides interconnection to sensors, actuators, third-party services and HMI by using a publisher/subscriber architecture. More information about the northbound interface is provided in Section 5. The facility layer of the ITS stack, as specified in ETSI, can be managed by the CAMINO-Core. Several C-ITS services are supported, such as Cooperative Awareness (CA), Decentralized Environment Notification (DEN) and Infrastructure to Vehicle Information (IVI). The different services can run in the CAMINO-Core, independently or simultaneously, transmitting standardized messages such as Cooperative Awareness Message (CAM), Decentralized Environmental Notification Message (DENM) and Infrastructure to Vehicle Information Message (IVIM). Additionally, CAMINO allows for the transmission of custom messages and it provides the forwarder service. This service can forward the messages coming to CAMINO-Core via any V2X technology at the southbound interface to the northbound actuators, or can forward incoming message from sensors on the northbound interface to the V2X communication modules on the southbound interface. Figure 2: Overall CAMINO framework architecture. The CAMINO-Core is designed in a way that different V2X communication technology modules can be connected via dedicated interfaces. The interface can differ between the modules that are linked to the unit that runs the CAMINO-Core. The C-ITS services can exchange the same messages over different communication paths in an intelligent and flexible way. As an example, the In-Vehicle Signage service running in the CAMINO-Core can send IVI messages via ITS-G5, C-V2X PC5 and/or 4G depending on the required operation mode. Even more, the same generated message can be easily transmitted over multiple technologies in parallel. At the receiver side, the CAMINO-Core can process messages that are received via multiple communication technologies. Providing support for multiple communication modes, allows flexible testing of C-ITS services over multiple V2X radio technologies.