# Software-Defined Heterogeneous Vehicular Networking: The Architectural Design and Open Challenges
## 5 Conclusions and Future Directions  
Conventional vehicular ad hoc networks (VANETs) are capable of facilitating vehicle-to-vehicle (V2V) and vehicle-to-infrastructure (V2I) communications. Nevertheless, such VANETs have several inherent shortcomings, including, but not limited to, lower bandwidths, higher end-to-end delays and unbalanced traffic flows. To overcome such issues, in this article, we propose a highly intelligent, robust and performant next-generation heterogeneous networking architecture for ensuring rapid network innovation to meet the stringent performance requirements of diverse safety-critical vehicular applications. Our proposed SDHVNet architecture offers an abstraction of network entities as SDN switches, thus mitigating the inflexibility in the deployment of heterogeneous radio access technologies, and facilitates an efficient orchestration of the network resources. This, in turn, addresses key issuespertaining to vehicular communication, i.e., guaranteeing ultra-low end-to-end delay for safety-critical vehicular applications, intelligent caching at the network edge, broadcast storm mitigation via efficient slicing of the network, etc.  
A considerable number of architectural design issues still need to be investigated. The sheer number of sensors onboard connected vehicles generates a massive amount of data, whose real-time analysis is indispensable in order to ensure a reliable analysis of the traffic conditions on the road, precise behaviour of vehicles and prediction of traffic vis-a-vis its density and throughput per hour, per lane. Therefore, vehicle-to-cloud communication should augment the conventional V2V and V2I communication for such a highly dynamic and distributed networking environment. Furthermore, the deployment of SDN controllers needs to be handled with caution since passing all intelligence to one centralized controller would not only result in a significant amount of network management overhead, but could also result in a single point of network failure. Therefore, intelligence needs to be passed within the network, and especially at the network edge. However, it is pertinent to mention that placing an SDN controller in every roadside cloudlet would result in frequent handovers, which subsequently would lead to wastage of precious network resources. Hence, appropriate SDN placement schemes need to be investigated. Scalability is also a concern in SDN-based heterogeneous vehicular networks since there are a huge number of vehicles in dense vehicular networking environments, and tracking their run-time positions is not only an arduous task to tackle, but also results in a massive amount of management overhead. Structured vs. non-structured and clustered vs. non-clustered routing protocols should be devised in order to address the scalability issues.  
Conceptualization, A.M., W.E.Z. and Q.Z.S.; investigation, A.M.; methodology, A.M.; supervision, W.E.Z. and Q.Z.S.; validation, A.M., W.E.Z. and Q.Z.S.; writing, original draft, A.M.; writing, review and editing, W.E.Z. and Q.Z.S.  
The corresponding author sincerely acknowledges the generous support of the _Government of the Commonwealth of Australia_ for funding the research at hand via its 'International Research Training Program (Allocation No. 2017560)'.  
The authors declare no conflict of interest.