A comprehensive review on vehicular ad-hoc networks routing protocols for urban and highway scenarios, research gaps and future enhancements

Vehicular Ad-hoc Networks (VANETs) have received extensive consideration from the industry and the research community because of their expanding emphasis on constructing Intelligent Transportation Systems (ITS) to enhance road safety. ITS is a collection of technologies and applications that aim to improve transportation safety and mobility while lowering the number of accidents. In VANET, routing protocols play a significant role in enhancing communication safety for the transportation system. The high mobility of nodes in VANET and inconsistent network coverage in different areas make routing a challenging task. As a result, ensuring that the VANET routing protocol has the maximum packet delivery ratio (PDR) and low latency is of utmost necessity. Due to the high dynamicity of the VANET environment, position-based routing protocols are paramount for VANET communication. VANET is subjected to frequent network disconnection due to the varied speeds of moving vehicles. Managing and controlling network connections among V2V and V2I is the most critical issue in VANET communication. Therefore, reliable routing protocols that can adapt to frequent network failures and select alternative paths are still an area to be explored further. Majorly, VANET routing protocols follow the greedy approach; once the local maximum is reached, the packets start dropping, resulting in a lower packet delivery ratio. Therefore, lower PDR is still an issue to be resolved in VANET's routing protocols. This paper investigates recent position-based routing protocols proposed for VANET communication in urban and highway scenarios. It also elaborates on topology-based routing, which was initially used in VANET, and its research gaps, which are the major reason for the advent of position-based routing techniques proposed for VANET communication by various researchers. It provides an in-depth comparison of different routing protocols based on their performance metrics and communication strategies. The paper highlights various application areas of the VANET, research challenges encountered, and possible solutions. Further, a summary and discussion on topology-based and position-based routing protocols mark the strengths, limitations, application areas, and future enhancements in this domain.