Delay Tolerant Networks (DTNs) provide networking in infrastructure-less environments, e.g., deep space, rural areas, disaster areas, underwater fields, etc. In DTNs, the current TCP/IP model cannot work well due to lack of continuous end-to-end connectivity. A store-carry-forward message delivery scheme and custody transfer mechanism is used in DTNs to confirm the reliable transfer of bundles with custody transfer request among nodes, by delegating the responsibility of custody-bundle transfer through intermediate nodes in a hop-by-hop manner. Note that a bundle is the protocol data unit in DTNs. The intermediate nodes keeping custody bundles are called custodians. Each custodian must reserve a sufficient amount of storage and energy for receiving and holding the custody bundles until their successful delivery or delivery expiration. Due to a shortage of storage capacity, custodians sometimes face storage congestion, where they have to refuse to receive any custody bundle from other nodes. In addition, each battery-powered node has to be awake while holding the bundles. Since each custodian also generates its own custody bundles, it is naturally selfish in behaviour and rejects requests of custody transfer from other nodes to save its storage as well as its energy. Intuitively, this problem is aggravated in long-term isolated networks. In such a situation, some movable vehicles referred to as message ferries can solve the storage congestion problem by actively visiting the network and gather bundles from custodians. Note that message ferries are equipped with storage enough to carry collected bundles to the destination, i.e., a base station referred to as a sink node, and it can also supply energy to the custodians if required. When there are multiple isolated networks referred to as clusters, message ferries have to periodically visit those clusters and collects bundles from custodians there. This network architecture is suitable for wide-area sensing. Note that each node in a cluster can directly/indirectly communicate with other cluster members through multi-hop communication but cannot communicate with nodes in other clusters due to long distances among them. A message ferry helps the inter-cluster communication by acting as a mediator between each cluster and the outer world via the sink node which serves as a connector to the Internet or to other sink nodes. In such ferry-assisted multi-cluster scenario, inter-cluster communication and intra-cluster communication should be carefully considered to minimize the total mean delivery delay of bundles. This thesis mainly focuses on these inter-cluster and intra-cluster communications by combining three research studies. The intercluster communication has been studied by two studies: 1) Grouping clusters, and 2) Optimal visiting order of isolated clusters. And, the intra-cluster communication has been studied by self-organized data aggregation technique among selfish nodes in an isolated cluster.

K. Habibul Kabir, Studies on Ferry-Assisted Multi-Cluster Delay Tolerant Networks, Ph.D. Dissertation, Osaka University, Ph.D. Dissertation, March 2011.