The Research On Safety-applications-oriented Message Dissemination Mechanism In VANETs

The rapid development of transportation industry also brings problems like the traffic accident,the traffic jam and the air pollution etc.In the vehicular ad hoc network(VANET),V2V-based broadcast can fast disseminate safety messages between vehicles within the whole network and hence expand drivers' perception vision,which will reduce the accident probability and ensure the transportation reliability.Periodic safety message(PSM)and emergency warning message(EWM)constitute the two types of safety message in VANET.PSM is propagated periodically within one-hop vehicles via the method of single-hop broadcast.Existing fixed-period single-hop broadcast protocols will result in a larger network overhead when the traffic density is high;while most adaptive period single-hop broadcast protocols also present some limitations such as incomprehensive influence factors,complex models and so on.Therefore,this paper proposed a fuzzy logic based adaptive single-hop broadcast protocol(FL-ASB).This protocol takes into consideration four factors including the velocity,the one-hop neighbor node number,the received signal strength and the visibility,and establishes an adaptive broadcast period adjustment model based on fuzzy logic,realizing the adaptive adjustment of PSM broadcast period and the balance between instantaneity and network overhead.EWM owns a higher priority in VANET,and therefore needs to be transmitted to further vehicles with a higher delivery ratio in shorter time.Hence,EWM is usually transmitted with the method of multi-hop broadcast.The next-hop forwarder selection mechanism and the collision avoidance mechanism are key factors influencing the instantaneity and reliability of multi-hop broadcast protocol.Some existing multi-hop broadcast protocols are unable to adjust dynamically the partition of communication range and the contention window size according to the change of traffic flow,which results in a large latency and a poor extensibility.In order to tackle the above problems,this paper proposed a traffic flow theory based dynamic multi-hop broadcast protocol(TF-DMB).Based on the traffic flow theory,this protocol simulates the distribution principle of vehicles under different traffic density,and realizes the following two dynamic characteristics.One dynamic characteristic is the dynamic partition of communication range,which will help locate candidate forwarder vehicles at a time,and therefore reduce the one-hop latency.Another is the dynamic adjustment of contention window size according to the traffic density,which will help reduce the the collision probability.This paper conducts the performance verification through the joint simulation of VanetMobisim and NS2.Firstly,a two-way four-lane highway of design speed 120km/h scenario is simulated via VanetMobisim and then the trace file is inported into NS2,where the performance verification of FL-ASB and TF-DMB is conducted respectively in the field of latency,network overhead,delivery ratio etc.Simulation results prove that FL-ASB and TF-DMB possess better performance than other similar protocols.