Research On Vehicular Ad Hoc Network MAC Mechanism For Traffic Safety Messages

Vehicle Ad Hoc Networks(VANET) has caused intensive concern fromgovernments, academic community and industrial community around the world. Trafficsafety messages transmission is a kind of important application in VANET. Thetransmission delay of traffic safety messages should be less than100ms, and the packetreception rate of traffic safety messages should be more than90%for many trafficsafety applications. Traffic safety messages transmission is the fundamental guaranteefor many traffic safety applications as emergency brake warning informationtransmission, Lane-changing information transmission. Nevertheless, traditional trafficsafety message broadcast mechanism cannot meet the requirement of traffic safetymessage transmission. Therefore, it is very necessary to meet the requirement of trafficsafey message transmission as much as possible for the traditional traffic safetymessage broadcast mechanism.In this dissertation, according to the reliable and rapid transmission requirement oftraffic safety messages, traffic safety messages broadcast mechanism in VANET areresearched and analyzed. Through analyses, we can study the problem of thesemechanism. A efficient safety messages mechanism is proposed. Main innovativecontributions of the thesis are enumerated as follows:Fristly, VANET traffic safety messages broadcast mechanism are researched andanalyzed. We analysis the design ideology, the design scheme and protocol’s superiorand defect. IEEE802.11broadcast protocol is mainly analyzed. The performance of themechanism in VANET is analyzed. The main problem of IEEE802.11protocol fortraffic safety messages are hidden and exposure station problem, Ack frame problemand contention window problem.Secondly, a new mechanism is proposed named Vehicle Density AdaptiveBroadcast(VDAB) on the problem of IEEE802.11. VDAB mechanism collect andestimate the vehicle density initiatively. Then according the vehicle density, the channelcontention window of the802.11MAC layer adjuste dynamically. Therefore, trafficsafety messages requirement could be satisfiy.Thirdly, performance analysis of VDAB is proposed. The simulation results show that the frequency increases as the density of vehicles, the transmission distance andvehicle status information, the traffic safety information successfully reception rate isdeclining, while the transmission delay is increasing. When the vehicle densityincreases, packet reception rate of VDAB increased approximately10%and the averagedelay decreased by about15%than the standard802.11MAC. When the transmissiondistance increases, packet reception rate of VDAB increased about5%; With theincrease in the frequency of the vehicle status information, packet reception rate ofVDAB increased by approximately9%and the average delay is reduced by about23%.Fourthly, based on the NS-2simulation platform, the propose protocol simulationmodule is developed. The simulation results show that the improved protocol couldimprove the traffic safety information packet reception rate and reduce the transmissiondelay.