Study On Sefety Data Broadcasting In Vehicular Ad Hoc Networks

VANET, Vehicular Ad-hoc Network, can be used to build efficient Intelli-gent Traffic System based on the developed wireless and network technology,for the purpose of improving the safety and efficiency of road. VANET is one of the most potential subject that is valuable to science and society, attracting the increasing attention from the academia and industry all over the world. Among the numerous applications in VANET, safety services are the most important,which is the original intention that VANET is proposed by researchers. For the life related purpose, safety applications demand highly QoS of data transmis-sion, which needs efficient broadcast protocol satisfying the increasing require-ment. However, speedy movement of vehicles, the highly dynamic topology,and the fading of wireless channel and interference, the transmission of safety data in VANET are facing serious challenges.Based on the existed one-hop and multi-hop broadcast technology, the pa-per studies the broadcast protocols for disseminating safety messages in VANET on three perspectives, that are on the level of physical link, based on the cog-nition of local topology and according to the application services. Based on the analysis of characters of VANET scenarios, broadcast protocols that are sender-oriented and receiver-oriented are proposed, separately. The main re-search contents and contribution of this paper are following:Firstly, a reliable broadcast protocol based on prediction of the minimum expected delay (DPCast) in urban VANET is studied on the level of physicallink. According to the characters of varying traffic density with time and zone and the uneven distribution, a sender-based forwarding scheme for fast dissem-inating messages is proposed in Chapter ?. The global static route attributes and the local dynamic route attributes of city roads are analyzed. The global static route attributes are used to guide the range of forwarder selection, while the local dynamic route attributes are used to model the quality of forwarding of nodes based on the link quality, vehicle movement, channel contention and data interference, which is acting as the metric of selecting the optimal forwarders.In order to alleviate the problem of unstable link, a scheme that receivers are cooperating forwarding packets on the failure of rebroadcasting for specified nodes is proposed, insuring the continual dissemination of safety data. The fi-nal simulation results show that DPCast achieves faster broadcasting with little redundancy as compared to the other represented protocols.Secondly, a fast broadcast protocol based on the sensing of local topology information for normal scenarios (LISCast) is studied based on the cognition of dynamic topology. In order to mitigate the problem of reliability decline in the highly dynamic network for sender-based forwarding scheme, a receiver-based forwarding scheme is proposed in Chapter III. Since waiting-based broadcast protocol may face the problem of forwarding slowly in the sparse network and local broadcast storm in the dense network, a low load scheme is studied to adaptively adjust the parameters for candidates scheduling forwarding priority.Periodical beacons are used to collect local topology information of sender,which will be embedded in the head of safety packet to inform the receivers.On checking up the uniform characteristic information of sender, receivers con-figure the waiting time and probability for forwarding the packet in a distributed way, for the purpose of reduce channel contention and collision. The final simu-lation results show that LISCast improves the broadcast performance in latency and broadcast efficiency as compared to other represented protocols. Besides,LISCast is working in an ad hoc way, which can be expanded to other scenarios.Thirdly, an efficient broadcast protocol based on the safety information quantity of messages in the normal scenario is studied orienting on the appli-cation services. Since most of existing safety protocols schedule the priority of packets for channel access only based on the type of message, ignoring the dy-namic changing emergency of message with time and space, a novel broadcast protocol based on the dynamic emergency degree of messages (EDCast) is pro-posed in Chapter ?. The dynamic information quantity of message, which is measured by the metric of emergency degree, is analyzed in the term of tempo-ral and spatial value. According the distribution and density of vehicles, the one hop coverage gain of message are deduced. Combining the emergency value and one hop coverage, the one hop broadcast utility of message is shown, based on which the priority of channel access of message is designed, for the purpose of maximize the coverage of safety information quantity. In this way, the ex-pectation for vehicles to collect more emergent messages can be satisfied. The final simulation results show that EDCast provides a significant improvement on per-vehicle coverage and delay with little redundancy.