# Software-Defined Heterogeneous Vehicular Networking: The Architectural Design and Open Challenges
## 1 Introduction  
Over the past few decades, the promising notion of vehicular ad hoc networks (VANETs) has been thoroughly studied and well-researched by researchers in both academia and industry [1]. However, the emerging and promising paradigms of cloud computing, fog and/or edge computing, software-defined networks (SDN) and network functions virtualization have not only completely revolutionized the wireless networking industry, but have further spurred considerable innovative advancements for the transportation sector. This is coupled with other recent significant technological advances pertinent to the evolution of connected and autonomous vehicles and pervasive usage of numerous state-of-the-art sensory devices installed onboard vehicles that facilitate in a diverse range of cooperative vehicular safety applications, i.e., forward collision warnings, emergency vehicle assistance, (vulnerable) pedestrian collision mitigation, blind intersection warnings and hazardous location alerts, amongst many others. These safety applications are not only critical in nature, but further require a low-latency infrastructure with a maximum tolerable delay ranging between 10 ms and 50 ms [2]. Furthermore, modern-day connected vehicles are equipped with on average 100 sensorsonboard, and this number is anticipated to increase up to 200 towards the end of the year 2020 [3]. These sensors not only generate the bulk amount of data, but also play an indispensable role in creating and sharing of ambient intelligence for vehicular cooperative communication. Furthermore, as per an estimate of Intel [4], an averagely-driven connected vehicle (i.e., a personal vehicle used for day-to-day routine purposes and not for any commercial operations) in the near future would generate around 4000 MB (40 TB) of data for every eight hours of its driving. This is in addition to the vehicular user's data consumption, which on average stands at 650 MB per day and is expected to reach 1.5 GB per day by 2020. The questions, therefore, arise as: (a) how to tackle such a flood of data so that the meaningful information could be accumulated, processed and utilized for the above-referred vehicular safety applications; (b) which particular radio access technologies would be able to facilitate the transmission of such sort of a meaningful information with higher data rates and lower end-to-end delay; and finally, (c) where this all processing (i.e., compute and storage) needs to be tackled; as sending these data back to the remote back-end servers would not only require excessive bandwidth, but may also result in excessive load on the backhaul, thereby increasing the network management overhead and compromising the service-level objectives of diverse vehicular safety applications. The emerging and promising paradigm of software-defined networking (SDN) indicate a possible solution to these vehicular networking challenges. SDN has been conceived and subsequently deployed for wired networks. However, as of late, there is a rapid shift of interest towards deploying SDN for both the wireless and ad hoc domains. This has, in turn, stimulated the interest of the academic community to look into the possibility of designing SDN-based vehicular networks that would not only enable secure and high bandwidth communication services, but may also provide low latency for the safety-critical vehicular applications. SDN de-couples the control plane from the data plane, and the overall management and orchestration of network resources is carried out via a logically-centralized programmable controller. This, therefore, facilitates enabling a vendor-independent control of the entire network for both network carriers and enterprises, in turn considerably simplifying the network design and operations and laying out the foundations for a highly flexible and programmable networking infrastructure. Hence, given a programmable SDN controller, it is easier to configure disparate network devices and to deploy a wide array of new applications instantly.