Research On Traffic Flow Density Based Broadcasting Technologies In Vanets

VANETs (Vehicle Ad-hoc NETworks) are a special kind of Mobile Ad Hoc NETworks (MANET), which consist of lots of highly mobile vehicles moving on the roads. The networks have the characters such as self-organized, centerless, multi-hoped and high mobility. In VANETs, many applications mainly focus on disseminating of emergency safety messages and traffic condition information to all vehicles in a certain geographical area. Consequently, broadcasting is very important for VANETs. However, in the real traffic environment, the variable traffic flow makes the density changes rapidly, and decreases the performance of VANET broadcasting. For catering the requirements of broadcasting in VANETs with different densities, a series of multi-hop broadcast protocols which are designed based on the variable traffic flow density are proposed for VANETs.In the traffic jams, the node density of VANETs is saturated. The broadcast storm can make the networks congested, and decrease the broadcast efficiency of VANETs. Accordingly, a probability based broadcasting protocol NPPB is proposed for VNAETs, which is designed for the saturated density in traffic jams. In this protocol, an adjustable power parameter is designed to decide the retransmission probability to reduce collisions and competitions. Analyses and simulations show that NPPB can mitigate the influence of broadcast storm, the reliability is ensured, and the broadcast efficiency is enhanced.Under the free flow traffic conditions, the density of VANET is variable. It is difficult for the protocols that are based on fixed parameters to broadcast efficiently. The efficiencies are heavily affected by the variety of node density. Correspondingly, a node coverage information based broadcasting protocol is proposed for VANET. Additionally, the reliability of the protocol is enhanced by selecting redundant retransmission nodes for tolerant the fault of key nodes and links. The distributed on demand mechanism is employed to get nodes'connectivity information, by which the relaying node is selected freely from density's influence. And the broadcast storm is mitigated efficiently. Analyses and simulations show that the protocol has achieved good performances such as short latency, low control overhead and low load. The reliability is enhanced by paying suitable additional communications.When the vehicles move by group, it is suitable to construct a layered network to broadcast information in VANET. A cluster based broadcast protocol VWCP is proposed for VANETs. The combined weighted methodology is adopted to select the cluster heads. The traffic rules and road traffic characteristics are taken into account to make the architecture more stable and reduce the additional communication for cluster building and maintaining. A simulation experiments is designed to validate the performances of the protocol, in which a practical mobility model is designed according to the relationship between space headway and velocity of vehicles. Result show that the cluster architecture if simple and stable, the control overhead is low and the load is reduced efficiently, at the same time the reliability is guaranteed. This protocol can meet the efficient broadcasting requirements of VANET in the wavy vehicle distribution traffic.Most traditional wireless broadcast protocols are designed under the assumption of the connectivity of networks. However, in the road traffic environment, the density of the traffic flow is so low as to cause the network disconnection and the failure of broadcast service. This dissertation presents a delay tolerant broadcast protocol named DTBP for VANETs. A combined weighted algorithm is proposed to select suitable nodes for relaying when the link is broken. By designing the retransmission moments selection strategy, the rebroadcasting moments are set appropriately when the contacts occur. Simulation results show that this broadcast protocol achieves high broadcast coverage ratio in sparse VANETs. And efficient broadcasting service is provided in the intermittently connected VANETs.