# Multi-Layer Problems and Solutions in VANETs: A Review
## 1 Introduction  
Roadside accidents have increased significantly due to high traffic on highways and urban areas. In order to provide a reliable driving experience and avoiding accidents, vehicular communication was introduced to provide safety and to increase the travel experience of the user by providing on demand infotainment services. For this purpose, Wi-Fi based standard like DSRC was introduced in order to fulfill the requirements of the users for both safety and non-safety applications. While LTE based vehicular networks are being researched for the future in order to overcome the shortcomings of the DSRC.  
The Dedicated Short Range Communication (DSRC) technology is being used for the safety and non-safety applications in vehicular communication. The allocated bandwidth of 75 MHz in 5.9 GHz spectrum band is being utilized for this purpose [1]. The IEEE Wireless Access in Vehicular Environment (WAVE) 1609 is being used as a standard for communication between vehicles. The WAVE communication model is similar to the Open System Interconnect (OSI) model with some modifications on different layer and some extensions to the management layer. Research in the past has focused on many areas from the Physical, MAC, Network, Application to the Security and Management Layers. Each Layer has its own challenges that add up to the overall performance of the DSRC. For a large number of vehicles, the congestion can severely degrade network performance which affects mostly the safety applications of the vehicles. Supporting the reliability and scalability for V2V safety communications has been the main objective of many researches in the recent past by improving the Quality of Service (QoS) parameters of the network and resolving the scalability issue.  
The DSRC architecture shown in Figure 1 includes the vehicles equipped with On Board Units (OBUs). The OBUs interact with the Road Side Units (RSUs) for the exchange of messages and applications. The RSUs are in turn connected to the external network which includes the internet cloud. The V2I communication takes place between the RSUs and OBUs for providing the internet services, weather conditions, and other infotainment applications. The V2V communication between the vehicles allows the exchange of safety messages which can avoid accidents.  
Several factors influence this aforementioned communication. In this paper, we explain these factors that influence the performance of VANETs by explaining the work done on each layer, challenges at every layer, and the future of VANETs. The Physical layer has been studied in terms of channel selection and estimation schemes, diversity schemes, propagation models, and cognitive radios. Whereas the MAC layer has been thoroughly researched for Multi-Channels, contention window size adjustment, hidden/exposed nodes, contention and scheduling based algorithms, and congestion control algorithms. The Routing layer has been studied in terms of different routing protocols and the advantages of cross layer routing protocols. Finally, future research directions and ongoing research in the fields of VANETs are discussed.  
The rest of the paper is structured as follow: Section 2 discusses the Multi-Layer Challenges in VANETs, along with the proposed solutions. Section 3 focuses on the current trends in research and the future work, and finally Section 4 concludes the paper.