Research On Multihop Broadcast Protocol Under Different Connected Networks In Vehicular Ad Hoc Network

A vehicle ad hoc network(VANET)is one of the important components of an intelligent transportation system and plays a key role in ensuring traffic safety and improving traffic efficiency.In a VANET,vehicles warn surrounding vehicles in advance through multi-hop broadcasting to reduce traffic accidents.The main objectives of multi-hop broadcasting are to reduce transmission delay,improve transmission efficiency,and guarantee the success transmission to the target area.Existing multi-hop broadcast protocols are inadequate in adapting to different application scenarios and communication networks.On the one hand,those protocols treat emergency messages as the same type,so they cannot meet the different requirements of different types of emergency messages in terms of delay and transmission efficiency,and are applicable to single type scenario;on the other hand,they divide communication networks into connected networks and disconnected networks,but simplifies the process of switching between the two kinds of networks,so the selected relay cannot achieve the transmission target of different network connectivity states,which easily leads to message loss.Based on the emergency message types and network connectivity states,this paper proposes two multi-hop broadcast protocols,the main contents of which are as follows:1)A hybrid relay selection scheme(HRSS)is proposed based on the diversity requirements of emergencies.Based on the three main characteristics of emergency messages,i.e.,urgency,variability,and effectiveness,this paper defines a kind of control message to reflect the requirement of different emergency messages.According to the control message,HRSS can adaptively switch between two improved relay selection strategies,i.e.,HRSS-S(HRSS-Sender)and HRSS-R(HRSS-Receiver).HRSS-S takes advantage of the centralized conflict-free and considers link quality and distance to select relays,which is dedicated to transmitting delay-sensitive messages to reduce latency.HRSS-R takes advantage of real-time of vehicle position,which is dedicated to transmitting delay-insensitive messages to improve transmission efficiency.Simulation results show that HRSS outperforms existing schemes in simultaneously meeting the requirements of delay and transmission speed for different emergency messages.2)An adaptive relay schemes based on network connectivity state(ARSNCS)is proposed.Based on four network connectivity states,i.e.,connected state,connected-todisconnected state,disconnected state,and disconnected-to-connected state,four different relaying strategies are proposed in this paper.For the connected state,a minimum waiting interval is set for neighboring candidate relay vehicles to avoid broadcast storms.For the connected-to-disconnected and disconnected-to-connected states,a higher relay priority is given to the vehicle traveling in the same direction as the message transmission direction to improve the probability of message transmission to the target area.For the disconnected state,a responsibility transfer mechanism is proposed,in which the vehicle traveling in the same direction as the message propagation direction assumes the relaying responsibility to maximize the transmission efficiency.ARSNCS can adaptively select the corresponding relaying strategy to reach the transmission goals for different network connectivity states.Simulation results show that ARSNCS outperforms existing schemes in terms of broadcast success rate and time delay.