# Software-Defined Heterogeneous Vehicular Networking: The Architectural Design and Open Challenges
## 2 Background and Motivation  
Vehicular networking is one of the key technologies that caters to the realization of a variety of the aforementioned vehicular safety applications, i.e., forward collision warnings, emergency vehicle assistance, vulnerable pedestrian collision mitigation, blind intersection warnings and hazardous location alerts. These applications thus allow for a collection and dissemination of useful contextual information between the vehicles (vehicle-to-vehicle (V2V) communication), among the vehicles and infrastructure (vehicle-to-infrastructure (V2I) communication), among the vehicles and supporting network (vehicle-to-network (V2N) communication) and between the vehicles and vulnerable road pedestrians (vehicle-to-pedestrian (V2P) communication), thereby strengthening the basis for the promising paradigm of vehicle-to-everything (V2X) communication, as depicted in Figure 1. A secure and low-latency communication between the vehicles and among the vehicles and the supporting infrastructure and network is quite critical to the successful implementation of such applications. V2X communication makes vehicles an integral part of the Internet of Things (IoT) landscape [5]. Accordingly, the emerging yet promising paradigm of the Internet of Vehicles (IoV) has also recently started taking its place in the research literature [6; 7]. Over the past several years, the IEEE 802.11p/DSRC has been considered as the _de facto standard_ for the implementation of numerous vehicular networking applications and services. IEEE 802.11p/DSRC is considered as a short-range wireless technology that originally evolved from the WiFi standard and primarily operates in a 5.9-GHz ITS bandwidth [8]. While DSRC provides a fast two-millisecond over-the-air latency, its standard performance degrades to a significant extent in urban scenarios with abundant high-rise buildings and intersections, leading to considerable blockage in the line-of-sight communication. Other limiting factors include fading, the high mobility of vehicles, and uncoordinated medium access mechanisms [9]. On the contrary, the Third-Generation Partnership Project (3 GPP) has recently promulgated the notion of C-V2X, i.e., cellular vehicle-to-everything communication,  
Figure 1: Towards seamless, ubiquitous vehicle-to-everything (V2X) communication. a technological paradigm using existing and developing cellular standards for a diverse array of vehicular connectivity applications and use-cases [10]. C-V2X is currently being developed as part of the 3 GPP objectives to accelerate the development of cellular systems from 4G to 5G by incorporating enhancements to LTE Broadcast and LTE Direct. LTE Broadcast would facilitate both V2I and V2N communication by leveraging traditional cellular infrastructure, wherein messages can be broadcast from V2X servers to numerous vehicles concurrently, while the individual vehicles can unicast the messages back to the server [11]. Enabling V2I and V2N communications is enormously advantageous for several vehicular applications, i.e., receiving alert messages from the traffic management authorities warning of traffic accidents and conditions several miles ahead up the road or communicating with a smart parking facility to locate and reserve the available parking space automatically. LTE Direct would enable robust V2V communication with a low latency of about one millisecond, at distances of up to hundreds of meters and, more notably, both in-coverage and out-of-coverage of the traditional cellular infrastructure [12; 13]. However, the aforementioned technologies are not yet capable of supporting a gigabit per second data rate for sharing of onboard raw sensor data (i.e., from visual cameras, radars and LiDARs) between the vehicles and with the infrastructure [3].