## 2 Related Work  
Over the last two decades, several radio access technologies have been proposed in order to cover different aspects of vehicular communications and support the demanding requirements that are posed by V2X applications. In this context, predominantly two radio technologies can be identified for V2X communications: the short-range wireless communication based on the IEEE 802.11p [15; 16] and the cellular standards for V2X [17; 18] have been developed by the 3 GPP.  
Within the IEEE 802.11p-based communications, two standards have been developed: ETSI C-ITS in Europe [19; 20] and DSRC in North America [21; 22]. These radio technologies facilitate the formation of wireless ad-hoc networks whenever vehicles or RSUs are within the coverage range of each other, thus enabling direct communications among vehicles and with the roadside infrastructure.  
In the context of ETSI C-ITS, there exist two main open-source software implementations of the ETSI C-ITS protocol stack, namely, OpenC2X [23] and Vanetza [24]. These implementations include the core features and protocols of ETSI C-ITS standard and they are described as follows. The networking and transport layers rely on IP and TCP/UDP for the transmissions of non-safety messages, whereas the GeoNetworking protocol and Basic Transport Protocol (BTP) are used for the transmissions of safety-related messages. The GeoNetworking [25] is an ad-hoc routing protocol designed for multi-hop communications using geographical addressing. The BTP is a connectionless and unreliable packet transport protocol similar to UDP. The facilities layer defines two fundamental message types for the exchange of information between ITS stations, i.e., Cooperative Awareness Message (CAM) [26] and Distributed Environmental Notification Message (DENM) [27]. The CAM is a periodic message that provides status information to neighbouring vehicles and RSUs. Its rate can be varied in-between 1-10 Hz, depending on vehicle dynamics and the congestion of the wireless channel. The DENM is an event-triggered message controlled by the ITS application. Furthermore, both OpenC2X and Vanetza can be configured to work with a commercially available MK5 OBU from Cohda Wireless [28]. The interoperability tests using OpenC2X and Vanetza with MK5 OBU have been successfully validated through the exchange of CAM and DENM messages [29; 30].  
The C-V2X has been standardised by the 3 GPP in Release 14 [7; 31]. The lower layers of C-V2X are specified by the 3 GPP for radio access, whereas the upper layers (i.e., applications, facilities, networking, transport and security) are reused from the ETSI C-ITS and DSRC standards. This, in turn, allows one-to-one mapping of the existing applications that were already developed using ETSI C-ITS or DSRC and ensures interoperability with the emerging C-V2X ITS applications.  
In this work, we have developed an experimental OBU based on OpenC2X. This choice is motivated by the flexibility provided by OpenC2X's modularised architecture, the existing implementation of a Local Dynamic Map (LDM) for data persistence, and the interface to reading a vehicle's speed through an On-Board Diagnostics (OBD) adapter. We have extended OpenC2X incorporating several new key software modules that require fulfiling the requirements of the ACCA service described in the next section.