Research On Hierarchical Multi-path Routing Mechanism For Urban Vehicular Ad Hoc Networks

Urban Vehicular Ad hoc Networks(VANETs)take the vehicles running on the road as basic nodes,construct the network in form of self-organization,and achieve communication between nodes by multi-hop relay.It can provide the functions and applications such as driving safety,information sharing and interactive entertainment for drivers and passengers,and support to the deployment of vehicle automatic driving,intelligent transportation management and smart city construction by its communication service.Furthermore,due to the advantages of a large number of nodes,flexible form of organization,no communication cost,etc.,VANETs can provide a sharing and free communication resource platform for large urban areas,which are different but can be interconnected and complementary to cellular networks.As a particular type of Mobile Ad hoc Networks(MANETs),VANETs have some unique features such as the large number and wide distribution of nodes,high mobility and limited trajectory by urban streets of vehicles,unstable network connections and frequent changes of topology,which result in the inadaptability of traditional routing protocols and algorithms to VANETs environment.As far,related applications based on multi-hop and unicast communication,such as resource sharing,information collection and user interaction,are are still developing slowly.Therefore,to design a targeted and efficient routing mechanism is the key of utilizing effectively the networks.In this paper,we propose a hierarchical,source-routing and multi-path VANETs routing mechanism and design multiple quality evaluation parameters for network delivery path,which can provide the targeted routing services for the applications with different Quality of Service(Qo S)requirements by adjusting weights.The main contents and innovations of this study are as follows:(1)A hierarchical network structure based on the actual environment of urban road network is proposed.In the structure,all vehicles are assigned to different regions and different intersections to be manage hierarchically.In the region layer,the urban road network is divided into several regions on the basis of the potential network communication needs.Each region is a logical node and the range can be adjusted dynamically according to the traffic volume within it.In the intersection layer,the logical nodes are set up taking urban intersections as center.As the underlying work unit of control plane,the intersection node is in charge of managing the identity registering and location updating for the vehicles in adjacent streets,completing the topology calculation and quality evaluation for its connected street networks,and then providing the services of node aggregation and link aggregation for the upper layer(i.e.,region layer).The hierarchical network structure can effiectively solve the problems of flat management caused by the large number and wide distribution of vehicle nodes.(2)Based on the concept of separation of data plane and control plane in software defined network(SDN),the idea of separating the control plane into information collection plane and routing decision plane is proposed.Information collection plane is mainly responsible for vehicle node identity registration,topology information collection and further process,logical topology aggregation of upper layer,etc.Especially,it can provide the service of information processing similar to the middleware for the raw data of different types and from different sources,transform them into available topology information of network state,and then generate and maintain a hierarchical global network topology in real time for later routing process.According to current network state,the logical nodes in each layer of routing decision plane are in charge of location service for destination node,intra-domain and inter-domain path query,multi-path evaluation and routing decision-making.The targeted design effiectively expands the sources and channels of information,enhances the integrity of topology generated and increases the accuracy of routing decision.(3)According to the characteristics that vehicles can only move along exiting urban streets and their trajectories can be predicted,through the study of related traffic flow models,a novel prediction model for vehicle positions based on the free flow velocity and the classification of queue and individual vehicles is proposed.The model does not emphasize the prediction accuracy of each vehicle's position,instead,focuses on the relative distance of successive vehicles in the one-dimensional network consisting of street vehicles,then to calculate the connectivity probability and evaluate the delivery delay for the street network between adjacent intersections.The simulation results show that the evaluations of the connectivity probability and delivery delay of the street network calculated by the proposed prediction model are more accurate than other existing prediction methods.(4)According to communication requirement of different functions and applications,combined with the features of VANETs connection,multiple path quality evaluation parameters including connectivity probability,delivery delay and connection strength are defined.In the routing process,the quality value of the path can be calculated by adjusting the weights of these parameters to meet different Qo S requirements.In addition,considering the distance between paths,the proposed routing mechanism can calculated multiple alternative paths for the unstable multi-hop connection in VANETs.Once the current delivery path is broken,the source node can quickly switch to the next path to continue the data transmission,which can reduce the delivery delay and improve the routing efficiency.The simulation results show that,compared with other traditional VANETs routing protocols and methods,the proposed routing mechanism in this paper significantly improves the performance in terms of packet delivery ratio and delivery delay.