Performance Analysis Of MAC Layer In Vehicular Ad-hoc Networks

With the continuous development of social economy and the steady growth of traffic,Vehicular Ad hoc Networks which are used to improve the utilization rate of the road, thetraffic safety and the comfort level of the road use, has been the development direction offuture transportation. But in the VANETs the wireless channel have the characteristics oftime-varying, path fading, interchannel interference, noise interference etc. Besides, thenodes in the networks are always high-speed moving, and the network topology have thecharacteristics of dynamic change.So the original ad hoc network is difficult to satisfy therequirement of people for VANETs. For the characteristics of VANETs, how to improvethe VANETs technology in the aspect of channel capacity, access delay, antijamming andsecurity is very necessary.First of all, In order to improve the performance of802.11protocol model used inVANETs，we established a new802.11Ext protocol model that Communication module isdivision more reasonable and the simulation process is more accurate.802.11Ext is a802.11protocol improved for VANETs features. Besides, to analyze the problems that isthe unfair of the channel access and the low utilization ratio of network channel causedby the network topology changing frequently, we try to optimize the network model andsolve the problems through the adoption of suitable for802.11Ext DDCF algorithm. Thealgorithm firstly according to the change of network load records the channel competitiontime and free time, and then make used of these values to adjust load factor todynamically adjust the smallest window initialization values.Secondly, in view of the characteristics of the VANETs, we respectively set up802.11p,802.11a and802.11b protocol model, validate the efficiency of802.11p. thensetting the802.11p network parameters by used of NS2simulation software, we analyseand evaluate the effects of the network parameters for the IEEE802.11p protocol model,and.Finally, we present a capacity algorithm of the network system based on the QoS. The algorithm requires each node know clearly that the time of transmission datashappened, so we approximate the DCF model regarded as the P-persistent on CSMAaccess mechanism. At the same time, in order to enhance multi-channel transmissionperformance of service channels, we use the reserve access mechanism for servicechannels. We mathematically analyze the throughput performance of reservationmechanism and random selection. From the result of the analysis, we compare these twomechanism and come to the conclusion that reserved channel access mechanism insystem throughput and network capacity is more superior to select random channelaccess mechanism.