Critical Nodes Evaluation Of Opportunistic Networks Based On Topological Condensation Graph

Opportunistic network is a type of mobile Ad Hoc networks that establishes communication through the movement of network nodes without the need of perfect communication domain between the source and the target node.The data transmission of the Opportunistic network is realized by the Story-Carry-Forward routing mechanism.In the process of multi-hop forwarding of information,the network is faced with problems such as network segmentation and connectivity degradation caused by the failure of key nodes,which will affect data transmission.Therefore,evaluating critical nodes in the Opportunistic network is very important for optimizing and maintaining the network and enhancing network's connectivity.The thesis examines the evaluation methods for critical nodes in the opportunistic networks.By analyzing the research status of critical nodes evaluation methods and the communication characteristics of the opportunity network,the paper proposes a critical nodes evaluation method based on topological condensation graph.According to the time-varying characteristics of the opportunistic network,the topology condensation graph is used to obtain the topology information of the dynamic change of the opportunistic network.The main idea is to transform the topology of the dynamic change of the opportunistic network into a series of network snapshots in a time window using the time discretization method,and define the weights of the inter-nodes according to the contact information between the nodes in opportunistic network.Combined with the sparseness of the opportunistic network and the intermittent connection between nodes,the second-order node degree is defined to reflect the local importance of the nodes in the opportunistic network.The connection strength is defined to reflect the relationship importance of the nodes in the opportunistic network.The key domain importance degree is defined to reflect the global importance of the nodes in the opportunistic network.The second-order node degree,the connection strength,and the key domain importance are used to evaluate the key nodes of the opportunity network.On the basis of analyzing the correlation between the three evaluation indicators,the Euclidean distance is used to combine the three evaluation indicators to quantify the importance of the nodes.Multiple time windows are set according to the sparsity degree of the connection between nodes inthe topological condensation graph.The optimal time window is determined through experiments.The thesis carries out simulation experiments on the Opportunistic Network Environment(ONE)and processes experimental data with MATLAB.Three typical experimental scenarios are designed from the point that the failure of critical nodes will lead to different degrees of segmentation of the network,the experimental results verify the effectiveness of the proposed method.The method has higher evaluation accuracy compared with the betweenness centrality method and the weighted k-shell decomposition method.Experiments were carried out for 10 minutes,20 minutes and30 minutes respectively,the results show that the method has higher evaluation accuracy when the time window is taking for 20 minutes.