# Coordination of Congestion and Awareness Control in Vehicular Networks
## 6 Conclusions  
This paper has presented and evaluated COMPASS, a novel methodology for the cross-layer coordination of protocols in vehicular networks that act or depend on the same communication parameters. The proposal has been designed to solve possible negative interactions or conflicts that may arise between such protocols when operating independently of each other. The proposed coordination methodology provides significant benefits over alternative methodologies such as integrating the conflicting or interacting protocols. In particular, the coordination methodology is independent of the protocols to be coordinated, their specific implementations, and the communication parameters over which these protocols actuate. In addition, the coordination methodology does not require changing the original protocols, which facilities the evolution of vehicular networks and standards, and the coexistence with legacy solutions. The proposal has a low computational complexity which facilitates its real-world implementation and possible impact on standardization. COMPASS can be implemented as part of the Management layers specified for ITS communications architectures in ETSI or ISO. The proposed methodology has been initially designed to demonstrate the feasibility to effectively coordinate congestion and awareness control protocols. However, this study has also demonstrated the capacity of the proposed methodology to coordinate additional protocols that operate over the same parameters. Such capacity has been demonstrated considering the coordination of vehicular congestion, awareness and topology control protocols under diverse context conditions.  
The conclusions of this study identify possible future research directions. For example, COMPASS could be further fine-tuned and optimized under more dynamic scenarios and real road topologies. COMPASS could also be designed so its operation is adapted based on the context conditions of each vehicle. Such adaptation could be designed to approximate the optimal solution so that all application requirements are satisfied and the maximum channel capacity is not reached. Other alternative approaches to select the communication parameters within the ICS could also be investigated. COMPASS could also be proposed for congestion control in C-V2X or LTE-V. In fact, C-V2X Mode 4 as specified under 3 GPP Release 14 identifies the need for congestion control but does not specify a particular congestion control protocol. ETSI and 3 GPP are currently analyzing whether the current DCC solution could be valid for C-V2X Mode 4. This study has also shown than under certain conditions the IEEE 802.11p channel capacity might not be sufficient to simultaneously satisfy the application requirements of all vehicles. Novel mechanisms to increase the channel capacity could be considered and be further investigated, including the use of higher-order modulation and codingschemes, compression techniques, the combination of network coding and multi-hop beaconing, or the joint optimization of data rate, transmission power and packet transmission frequency, among others. In addition, the definition of priority levels among protocols or vehicles could be investigated to tackle situations where the channel capacity is insufficient.  
Conceptualization, J.G.; methodology, J.G. and M.S.; software, M.S.; validation, M.S.; formal analysis, M.S.; investigation, J.G. and M.S.; writing-original draft preparation, M.S.; writing-review and editing, J.G.; supervision, J.G.; project administration, J.G.; funding acquisition, J.G.  
This work was supported in part by the Spanish Ministry of Economy and Competitiveness and FEDER funds under the projects TEC2014-57146-R and TEC2017-88612-R.  
The authors declare no conflict of interest.