| As mobile Ad hoc networks have the great flexibility and scalability without depending on any pre-existing infrastructure, they have been widely used in different kinds of application environments such as industry, family, office and military now. However, comparing to normal mobile networks, the mobile Ad hoc networks have poorer stability and higher probability of failures (including soft failures, hard failures and network failures). Hence, it is especially necessary to research the fault tolerance and reliability assurance techniques for the mobile Ad hoc networks.Firstly, the thesis analyzes the characteristics of the mobile Ad hoc networks in details, including lack of center nodes, self-organization, multi-hop routing, poor security, limited power source, narrow wireless bandwidths, dynamic topological structures and so on. The challenges that the mobile Ad hoc network checkpointing are facing are also pointed out. In addition, the related works about checkpointing protocols in the distributed environment, the normal mobile networks and the mobile Ad hoc networks are introduced and analyzed in details.Considering the characteristics of cluster-based Ad hoc networks, this thesis proposes a hybrid checkpointing strategy which combines the techniques of synchronous checkpointing and asynchronous checkpointing, namely the mobile terminals in the same clusters use the synchronous checkpointing strategy, and the terminals in different clusters use the asynchronous strategy. The hybrid strategy cannot only avoid cascading rollback among the processes in the same clusters, but also avoid too much message transmission among the processes in different clusters. It can also reduce the communication delay. In order to ensure the consistency of global states, the correctness criterion of hybrid checkpointing, including checkpoint taking, rollback and unable to be eliminated, are proposed. The checkpoint and rollback recovery algorithms are also presented. Moreover, the processing mechanism of handoff and the maintenance strategy of cluster relations based on process state transition are put forward, to ensure the correctness of checkpoints when the locations of the mobile terminals change. Experiments show that the proposed hybrid checkpointing strategy is a preferable trade-off method which synthetically takes all kinds of resource constraints of Ad hoc networks into account, and has the advantages such as less of dependence to cluster heads and shorter recovery time compared to the pure synchronous and pure asynchronous checkpoint methods. Aiming at the features of multicast communication, this thesis first analyzes the problems of checkpointing in this kind of communication environment, and proposes a communication induced checkpointing model. In order to save energy, based on the state transition graph of the mobile terminals, an adjustment strategy of checkpoint time intervals is put forward, which takes some factors like the failure probability and sleeping probability into account. Base on the model, an improved communication induced checkpointing strategy is proposed, including the rules of checkpint taking and elimination, and using the message receiving and sending lists to organize and manage the multicast messages, which can save storage space effectively. Moreover, in order to reduce needless rollback, the multicast center nodes are responsible for recording the rollback dependence relations among processes, and they can get the set of rollback processes from the dependence relations when some processes rolls back. Because the proposed strategy can avoid bringing extra communication overhead to the receivers of the multicast messages, it can be applied to multicast communication environment well. By comparing with the pure synchronous, pure asynchronous and basic communication induced checkpointing strategies, the experimental results verify the proposed improved communication induced checkpointing strategy has good performance on storage overhead, recovery time and so on. |
PDF Full Text Request