# Multi-Layer Problems and Solutions in VANETs: A Review
## 3 Discussion  
A lot of research had been done to in order to support a scalable VANET. Even with all the research there is still a gap between the currently used standards in industry and the research community. With the emergence of the 4G and 5G technology, the future VANETs will most likely converge with these technologies. * Software Defined Networks (SDN) and VANETs: The field of SDN has seen a tremendous interest by researchers in the recent past. The concept of SDN which has mainly been designed for wired networks and data centers can also be applied for the wireless networks including the VANETs. [PERSON] [60] has performed a study of using the SDN for the VANETs. It has presented network architecture, benefits of using SDN in vehicular communication, and comparison of SDN based routing with the conventional routing algorithms used in VANETs. By segregating the data and control planes of the vehicular network, it is possible to improve the overall performance of the network. The proposed architecture consists of the SDN Controller, SDN wireless node, and the SDN based RSU. The controller acts as an intelligence center and controls network behavior. The wireless node (a vehicle) and the RSU belong to the data plane. The paper also highlights the different control modes. The benefits of a software defined vehicular network will be the ease of path, power, and channel selection. It will also support a wide array of services. The Simulation from [61] also shows the routing protocol in SDN being superior to the traditional routing in VANETs (AODV, DSDV, OLSR etc). More research is needed to demonstrate the efficiency of using the SDN based VANETs that can support a scalable network and has infrastructure support. And on top of that the security aspects should also be kept in mind. * Cognitive Radios and VANETs: There are several open issues regarding the use of CR in VANETs. These issues include the coordination between the licensed and unlicensed network users, duration of spectrum opportunities, random movement of vehicles, multiple points of observation, interference, delay, and security [62]. The ever challenging nature of VANETs imposes many new challenges to the deployment of the CR. The CR should have the capabilities of self-optimization and adaptability to overcome these challenges. Moreover, the cost of deploying a CR in VANETs should be kept in mind. * Hybrid Protocols; Hybrid MAC and Routing Protocols have recently seen some interest. The main idea is to extract the advantages of one protocol in a certain situation and use another protocol for another condition. This concept can be applied to VANETs in terms of the traffic loads and topology of the network. For example, using a contention based MAC under low traffic conditions and a scheduling based MAC under heavy traffic conditions [63]. Similarly, the Hybrid Routing Protocols [64; 65] are being researched in order to optimize the performance of the network and improve the throughput, end to end delay, bandwidth utilization, packet delivery ratio, and support scalability. Hybrid Congestion Control Algorithms discussed before are also being researched in order to combine the benefits of both Proactive and Reactive approaches. The complexity and additional overheads of these protocols needs further research. * Internet of Vehicles (IoV) and Vehicular Clouds (VCs); The fairly new concept of Internet of Vehicles (IoV) which has emerged from the Internet of Things (IoT) has captured the interest of several researchers working in the area of vehicular networks. With high speed mobile internet, there is a big room for the emerging technologies like the IoV. The traditional VANETs lack the capacity to handle the vast information available regarding other vehicles in the network. The IoV will allow the vehicles to be permanently connected to the internet to provide different services such as traffic management, road safety, and infotainment [66]. It will enable the exchange of information between vehicles, pedestrians, sensors, road side units etc.