# Software-Defined Heterogeneous Vehicular Networking: The Architectural Design and Open Challenges
## 3 The State-of-the-Art in Intelligent Transportation Systems: An Overview  
A brief glimpse of the research literature reveals that a number of research studies have surveyed the potential challenges and limitations for devising an efficient ITS. In [19], the authors presented a comprehensive overview of the LTE-based V2X standardization activities in terms of their scope, probable use-cases, and associated service requirements. Challenges of dense vehicular environments and higher mobilities along with numerous technical design considerations have also been addressed. A survey of heterogeneous vehicular networks outlining research issues, challenges and solutions pertinent to heterogeneity at both the medium access control and network layers has been presented in [20]. In [21], the authors outlined a systematic investigation of existing vehicular communication systems in terms of (their) potential benefits, limitations, diverse vehicular applications and system requirements and proposed a layered-5G vehicular networking architecture comprised of a generic cloud layer, a core network cloud layer, a radio access network layer encompassing diverse radio access networks and vehicles and roadside units' space. Furthermore, a study of automotive sensing technologies employed for active safety measures has been surveyed in [3] and opened 5G mmWave communication as the only viable option for high-bandwidth connected vehicles. In [22], a brief survey of both academic and industrial advances for realizing the notion of IoV has been presented along with a debate on the potential challenges and research issues in the implementation of the V2X connectivity. Furthermore, a survey deliberating on the state-of-the-art vehicular localization techniques, their performance and applicability to autonomous vehicles has been presented in [23], wherein the authors primarily focused on sensor-based technologies (GPS, inertial motion units, cameras, radars, LiDARs and ultrasonic sensors) to determine the position of vehicles on a specified coordinate system and employed cooperative techniques (i.e., V2V and V2I communication via several wireless communication technologies) in order to enhance the locational accuracy and reliability. In [24], the authors investigated the relationship between big data and IoV within a vehicular context and primarily focused on how the IoV facilitates the big data acquisition, ensures a seamless, ubiquitous big data transmission and enhances the storage and computational abilities for the same. It further deliberated on a big data-enabled IoV and evaluated how big data mining could bring considerable advantages to the IoV development in certain aspects, including, but not limited to, network characterization, protocol design and performance evaluation. Security is also one of the indispensable components in designing a highly efficacious and cooperative ITS and therefore demands careful consideration. A self-contained and systematic survey encompassing security, trust and privacy-related challenges pertaining to VANETs has been presented in [25], wherein the authors outlined several anonymous authentication mechanisms, location privacy protection schemes, trust management models along with their efficacy and various types of network simulators, mobility simulators and integrated simulation platforms. In [26], the authors presented a comprehensive survey of the recent state-of-the-art VANET security architectures, frameworks, security standards and protocols, classification of several critical vehicular security attacks and their probable solutions and challenges that act as the bottlenecks in the evolution of secure ITS architectures along with future research directions.