## 2 Related Works
### Routing  
Infrastructure-aided hybrid routing uses Road Side Unit (RSU) to help the vehicular nodes to select the idle channel and relay node [42]. The channel is selected using the Belief Propagation Algorithm (BPA), which aggregates the belief from the vehicles and RSU to make the final belief selection. The relay node is selected using the computation of message delivery time; it selects the neighbor node that has minimum message delivery time. This paper proposes a hybrid communication scheme in which two nodes communicate only when they have a consensus about the common idle channel. The major objective of this paper is to combine cognitive capabilities with routing techniques to overcome network connection issues and spectrum scarcity. If there is no common idle channel between RSU and the vehicle, then the vehicle needs to store and carry the packet until there is a common idle channel exist between them. This intern increases the transmission delay. Routing is performed in VANET by using Ant Colony Optimization (ACO) algorithm [43]. Based on the ACO algorithm, a traffic-aware position-based routing protocol is developed. This protocol is an enhanced version of the geographical source routing protocol. The ant-based algorithm is used to find the route that has optimum network connectivity. Vehicles are calculated as a weight for every street segment proportional to the network connectivity of that segment. To find the optimum route between the source and destination, the source vehicle determines the path on the street map with the minimum total weight for the complete route. In general, ACO has a higher convergence time which increases transmission delay.  
During route selection, the trustworthiness of the SUs plays a pivotal role [44]. Incompletely predictable vehicular ad-hoc networks where vehicles move in a certain range or just in a particular tendency. A trustworthiness evaluation-based routing protocol is introduced to find the perfect route between the source and destination. In this protocol, the trustworthiness of each individual is calculated using the cloud depending on the attribute parameters uploaded by the corresponding vehicle. According to the results of the cloud, vehicles select appropriate forward nodes and complete the routing. Though consideration of trust value increases security level, it is insufficient to achieve better transmission efficiency. In VANET, routing is performed using spatial distribution-based connectivity aware routing protocol [45]. This protocol divides the street into multiple segments and aggregates the vehicle number and position distribution to access the connectivity quality of the segments. The proposed protocol evaluates the connectivity of the whole streets, in which shadowing of the overpasses is also taken into account if the street is multilevel. In this process, routing is performed using two modes, namely intersection mode and street mode. In intersection mode, when a vehicle at the current intersection receives a packet to be forwarded, it chooses a direction to forward the packet to a neighbor intersection. In street mode, the greedy-based approach is used to select the optimal relay inside Current Street. The involvement of the greedy approach increases the route selection time, which is inefficient.