Research On Medium Access Control Protocol Of Vehicular Ad Hoc Network

As we all known, the living standard is improving constantly. Besides enjoying convenience brought by vehicle, people want it to become their own mobile information center. In other word, people hope that, information acquisition, telecommuting and entertainment application should be available in vehicle. Vehicular Ad Hoc Network emerges at this proper moment. Not only can VANET supply real time information such as driving assistance and emergency alert, but also onboard entertainment, navigation and Internet access. Without doubts, VANET will assure more safer, efficient and comfortable driving experience.Based on the research of VANET multi-media traffic (including data traffic) Quality of Services (QoS), intensive research and analysis has been made about the Qos performance of multi-media traffic in vehicular environment, on condition of using standard IEEE 802.11 medium access control protocol. Main contributions are as follow:1.Analyze standard IEEE 802.11e EDCA mechanism deeply by observing the performance of real time multi-media in vehicular environment. Our research reveal that although EDCA outperform DCF by differentiating traffic priorities, it has no mean of decreasing the delay/delay jitter of multi-media flows in heavy load and high relative mobility scenario. So EDCA can not satisfy the Qos required by multi-media traffic.2.Aiming at the problem that network is incapacitated for the Qos performance of multi-media traffic in heavy load and relative high vehicular velocity condition, we start with the MAC channel contention mechanism and raise ACW-EDCA(Adaptive Contention Window EDCA,ACW-EDCA) scheme. ACW-EDCA adjusts value of initial and current contention window dynamically to decrease the medium access time consumption/delay jitter. Simulation result show that ACW-EDCA lower average delay by 30% and delay jitter by 25% both in static and slow mobility scenario, comparing to standard EDCA mechanism. But the delay/delay jitter performance of audio and video in high relative mobility scenario are still not as good as expected.3.In view of the video & audio Qos problems remain exist in ACW-EDCA method under the relative high vehicular velocity condition, this paper improve ACW-EDCA with Wireless Frame Burst mechanism(WFB) and propose a scheme named WFB+ACW-EDCA. Based on the frame burst theory used in gigabit Ethernet, this mechanism fully exerts the short frame feature of real time multi-media traffic and decreases average delay/delay jitter. Compared with standard EDCA, WFB-EDCA gains 40% improvement in average delay/delay jitter and enhances multi-media audio traffic Qos in VANET.4.Regarding to ACW-EDCA enhances audio and video traffic Qos on the cost of reducing data traffic throughput, this paper presents a scheme designated as Active Notify EDCA on the basis of ACW-EDCA. That is AN+ACW-EDCA. In accordance with the transmitting ability of MAC layer, AN-EDCA regulates data amount from TCP layer dynamically. Then the average throughput is elevated. The simulation demonstrates that, both in relative static and high mobility scenario, the average throughput of AN-EDCA exceeds standard EDCA by 30%.5.Make through research and systematic analysis to the standard EDCA module of NS2. Develop the ACW-EDCA, WFB-EDCA, AN-EDCA modules which are mentioned in this paper.