Abstract- rate of collision. However, the design of

Abstract- Road safety and traffic management applications require stable communication with minimum disturbances. Due to high mobility of nodes in VANET, exchanging information with another node and Road Side Units requires reliable channel conditions. Important applications such as safety and traffic management needs channel with low rate of collision. However, the design of an efficient Medium Access Control (MAC) is a challenging task due to dynamic topology change. Anyway, many TDMA based MAC protocols are designed for VANETs to minimize collisions and increase reliability. However, its performance significantly decreases when one or more number of nodes join the network. However, the process does not work well while the number of requests reaches certain threshold value.  It leads to increase in packet loss and entry-delay. Therefore, we present Cluster based contention avoidance (CBCA) that divides nodes into number of groups according to direction, distance and velocity and provides opportunity to send data based on traffic situations. Here, bandwidth request adaptively controlled by RSUs.Key Words- VANETs, MAC layer, Contention Avoidance, Bandwidth.1. Introduction: Mobile Ad hoc Networks (MANETs) is the area that has recently received considerable attention. One promising application of mobile ad hoc networks is the development of Vehicular Ad hoc Networks (VANETs). MANETs are self-forming network, which can function without the need of any centralized control. Each node in an ad hoc network acts as both a data terminal and a router. The nodes in the network then use the wireless medium to communicate with other nodes in their radio range. The benefit of using ad hoc networks is it is possible to deploy these networks in areas where it is not feasible to install the needed infrastructure. VANETs provides wireless communication among vehicles and vehicle to roadside equipment. The performance of communication depends on how better the routing takes place in the network. Routing of data depends on the routing protocols being used in the network. Ad Hoc networks are collection of self-governing mobile nodes. VANETs is the emerging area of MANETs in which vehicles act as the mobile nodes within the network. VANETs is the wireless network in which communication takes place through wireless links mounted on each node. Most of these applications depend on services to disseminate warning messages, which are alert messages sent by a vehicle to warn other vehicles of any potential danger. In the coming future, vehicles will not only distribute information about themselves and their environment using warning messages, but they will also be able to communicate with other vehicles and the infrastructure via multi-hop wireless communications. VANETs have many possible applications, ranging from inter-vehicular communication and file sharing to obtain real-time traffic information (such as jams and blocked streets).  Fig. 1.1: Communication between two vehiclesVehicular ad hoc networks are expected to implement wireless technologies such as dedicated short-range communications (DSRC) which is a type of Wi-Fi. Other candidate wireless technologies are cellular, satellite, and Wi-MAX. Vehicular ad hoc networks can be viewed as component of the intelligent transportation systems (ITS). As promoted in ITS, vehicles communicate with each other via inter-vehicle communication (IVC) as well as with roadside base stations via roadside-to-vehicle communication (RVC). Various forms of wireless communications technologies have been proposed for intelligent transport systems.Mainly two types of communication happen in VANETs.1. Vehicle to vehicle (V2V)2. Vehicle to infrastructure (V2I)Every vehicle in VANETs is equipped with GPS, On board unit, which contains processing elements. These communication elements provide number of services to their users. There are many routing issues in VANETs like link failure, packet loss, delaying and computational cost.In general, every node acts as a router in sending data packets from source to destination there are a many problems inspired as to work on the routing protocols. Due to advancement in transport technology VANETS have attracted great interest from both industry and research side. The main aim is to build intelligent transport system to enhance driving safety, entertainment services and traffic optimization. VANET provides Vehicle-Vehicle (V2V) Communication and Vehicle-Infrastructure (V2I) Communication. VANET is defined as a group of vehicles dynamically establishing a network without any existing infrastructure. It consists of set of vehicles fixed with communication device called On-Board Units and Road-Side Units, which is used to communicate with vehicles. Due to this dynamic nature, there will be frequent disconnection of links between vehicles, which leads to delay, and effects network performance. Another dimension of VANET uses Dedicated Short Range Communication (DSRC) with 5.9 GHZ bandwidth particularly for V2V and V2I. Existing System: In Medium Access Control (MAC) Protocol, nodes share common channel for sending and receiving the data. However, inefficient usage of channel may lead to packet collisions, waste of bandwidth and delay. Sharing channel efficiently and fairly between nodes is a challenging issue. In general, MAC protocols are broadly classified into two categories: Contention-based and Contention-free 5. In Contention-based approach, every node will ask for bandwidth when it needs to transmit e.g. CSMA. In Contention-free approach, the access to channel is pre-allocated like TDMA and FDMA where the medium is reserved 6. Therefore, designing an efficient MAC protocol is required that satisfies Quality of Service (QoS) requirements. An interesting area of research in the field of VANETs is TDMA based MAC protocols where time slots are divided and distributed to all vehicles. Only one vehicle must access channel at each time slot. All the vehicles without any code sequence but different timings use the same frequency. Proposed Work:In Proposed, Vehicles are grouped into Clusters. Each cluster is divided into two regions internally. The two regions are named as Region A and Region B. Every cluster has a cluster head. Cluster head is elected such that it must be in the starting of Region A so that it maintains the cluster for a long period compared to other vehicles. When a vehicle requests for bandwidth cluster head takes responsibility of requesting RSU and allocating bandwidths accordingly. Vehicles in Region A needs more bandwidth compared to vehicles in Region B. This is controlled by cluster heads using Queues A and B. Based on the requests of cluster heads RSU allocaates bandwidth to the vehicles. For dynamic networks, allocation of bandwidth is critical because vehicles are moving in different directions with different speeds. RSU must monitor status of each cluster in terms of bandwidth usage because cluster may request more bandwidth based on current situation but the same situation will nor remain same. So due to this bandwidth will be wasted. Based on packet flow bandwidth should be allocated but not on request. When a cluster heaad vehicle is about to leave the respective cluster, the next incoming vehicle into the cluster is elected as cluster head.Conclusion:To fully utilize the scarce bandwidth in a wireless network and to improve the contention-resolution process of a CSMA network Cluster based Contention avoidance is proposed. The analytical and the simulation results reveal that the CBCA scheme outperforms the TDMA schemes in terms of network throughput, transmission delay, and drop probability. Due to its simplicity and efficiency, the CBCA scheme can easily be implemented in the physical layer of any wireless CSMA network. Future research directions include applying the CBCA scheme to the wireless mesh mode and implementing it in the medium access control layer.References:1 F. Yu and S. 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