# Software-Defined Heterogeneous Vehicular Networking: The Architectural Design and Open Challenges
## 3 The State-of-the-Art in Intelligent Transportation Systems: An Overview
It is also highly pertinent to mention that the recent research focus has shifted from the conventional cryptography-based security solutions (i.e., based on the certificates and public key infrastructures) to a number of trust management schemes since: (a) vehicles in a vehicular network are highly dynamic in nature and are randomly dispersed throughout the network; (b) the presence of a seamless networking infrastructure cannot be guaranteed at all times; and (c) a cryptography-based solution could be easily compromised due to insider attacks, which are not only one of the most common security attacks, but are also extremely difficult to detect and handle [27]. Furthermore, research studies evaluating the technical feasibilities and performance analyses of wireless networking technologies supporting diverse vehicular applications have been conducted. A study evaluating the performance of heterogeneous vehicular networks (i.e., comprised of DSRC, LTE and WiFi) for both V2V and V2I communication has been delineated in [28]. An application layer handoff scheme has also been envisaged that not only guarantees optimal utilization of availablewireless technologies, but further ensures minimizing of corresponding backhaul communication requirements. In [29], a signalling game mechanism has been proposed for warranting an _always best connected_ service for vehicles traversing a geographical region equipped with heterogeneous networks. A heterogeneous network with aims to satisfy both safety and non-safety communication requirements of autonomous driving has been delineated in [30]. The study presented an enhanced protocol stack and also conceived the communication messages indispensable for supporting autonomous driving vehicles. Furthermore, a multi-tier heterogeneous adaptive vehicular networking architecture so as to ensure reliability and low latency for safety-critical message dissemination in a vehicular networking environment has been presented in [31]. The said architecture integrates LTE and DSRC technologies for balancing the network traffic via offloading the packet forwarding from the cellular networks. The architecture encompasses both high-tier nodes (i.e., public authority-operated vehicles such as buses, taxis or any other recognized authority's operated vehicles) and low-tier nodes (i.e., private vehicles). The high-tier nodes broadcast beacons with relevant information via the DSRC, whereas low-tier nodes receiving the beacons are registered with the high-tier nodes and communicate with the infrastructure via DSRC, and not LTE. Hence, all V2V communication takes place via the registered high-tier nodes, which primarily act as message relays. In [32], the authors presented a heterogeneous vehicular networking framework in order to meet the communication requirements of numerous ITS applications and services, along with a comparison of different radio access networks' candidate technologies. The authors opined that in contrast to DSRC, LTE is suitable for V2I communication, whereas DSRC is much more practical than LTE D2D for V2V communication. Off late, the emerging yet promising paradigm of SDN has been exploited for vehicular networks. In [33], a brief survey of existing and future challenges of SDN-based vehicular networks has been highlighted. In [34], an SDN-based vehicular communication architecture has been envisaged so as to provide a far more agile configuration capability and to enable rapid network innovation on-demand. Use-cases relevant to _adaptive protocol deployment_ and _multiple tenants' isolation_ have been highlighted to discuss the advantages of the said architecture in detail. In [35], a scalable and responsive SDN-enabled vehicular networking architecture, facilitated with mobile edge computing, has been suggested to minimize the data transmission time and for improving the quality-of-experience (QoE) of the vehicular users for a diverse range of latency-sensitive applications.