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Top1. Introduction
Increasing vehicle traffic congestion, road accidents and user demands for internet connection for various applications have led to the evolution of intelligent transportation systems (ITSs). To make these applications possible Vehicular Adhoc Networks (VANETs) has grown out (Zeadally, Hunt, Chen, Irwin, & Hassan, (2010). The Dedicated Short Range Communication (DSRC) is a specific designed protocol based on which VANETs provide a wireless communication in vehicular networks (Kenny, 2011.). DSRC technology in 5.9 GHz band supports to various safety and non-safety applications in VANETs. To support communication in VANETs, the Institute of Electrical and Electronics Engineers (IEEE) created a new standard termed as Wireless Access in Vehicular Environments (WAVE) based on DSRC. This new standard WAVE is an improved standard of 802.11 which includes 802.11p protocol and IEEE 1609 standard (Han, Dianati, Tafazolli, Kernchen, & Shen, 2012) and also provides better data rate and improves coverage range in vehicular networks scenarios. VANETs is a special class of Mobile Adhoc Networks (MANETs) with unique characteristics and it is different from MANETs due to the high mobility of vehicles, variation of density in VANETs and no issue of power consumption in vehicles.
Along with the services like Avoiding collisions, Real Time traffic management to deal with safety issues, VANETs also provide support to non-safety situations such as weather information, internet access etc. to different users. There are three types of communications in VANETs: Vehicle to Infrastructure (V2I) communication, Vehicle to Vehicle (V2V) communication and Hybrid Vehicle (HV) Communication (Kumar, & Dahiya, 2016). For communication to occur in vehicular networks, each vehicle must be equipped with some sort of radio interface or sensors or On Board Units (OBUs). This installed device further helps in exchanging the information with the Road Side Units (RSUs) or other vehicles. In HV communication both V2I and V2V communications take place.
High mobility of vehicles in VANETs leads to generate a highly dynamic network topology and a short V2V & V2I communication time. To address and overcome the issue of frequent partitions due to high mobility in the network, Vehicular Delay-Tolerant Networks adopt the all the related characteristics of Delay-Tolerant Networks (DTNs) (Pererira, Casaca, Rodrigues, Vasco, Soares, Triay, & Cristina, 2012). In VDTNs, along the roads, a number of apart Road Side Units (RSUs) are deployed and these RSUs are linked to vehicles via internet. Since, it is hard to deploy sufficient Road Side Units along the road due to deployment cost of RSUs. Therefore, there is a distance defined as uncovered area, which is the distance between two neighboring RSUs in which there is no coverage of any RSUs. The time during which a vehicle remains in that uncoverage area is defined as outage time. In the case when the communication link between Source vehicle (SV) and Destination Vehicle (DV) is inaccessible in an uncovered area, then Relay Vehicle (RV) plays an important role to disseminate messages from SV to DV. Various Relay Vehicle selection schemes and algorithms are proposed for data dissemination in an uncoverage area and to reduce outage time in VANETs. Furthermore, if number of SVs and DVs are more than one, then clustering plays an important role in maintaining connectivity among vehicles in VDTNs. Clustering is applied in VANETs to divide the larger network in to smaller groups of mobile vehicles and improve routing, information dissemination and data gathering (Cooper, Franklin, Ros, Safaei, & Abolhasan, 2017). To select Cluster Head (CH), various clustering techniques or algorithms are proposed in VANETs. In cluster based VANETs, firstly, a cluster of vehicles is formed and then Cluster Head (CH) is selected through beaconing process. Due to lesser effect of mobility, low congestion of vehicles, high reliability, high stability and cooperative communication make, clustering is important in VANETs. In this paper, we propose a Clustering based Optimal Relay Vehicle (CORV) selection scheme for VANETs in which first of all a cluster is formed and then CHs are selected based on their position and stability. RSU determines one cluster and its CH depending upon its coverage range and remaining connection time ‘T’. Based on requested data or missing data, RSU determine Destination Vehicle and then CHs forward the requested data to DVs.