| Summary: | 博士 === 國立中央大學 === 通訊工程學系 === 106 === The mobile opportunistic network (MON) is an emerging thread of mobile and wireless networks. In MONs, message distribution takes advantage of node mobility and opportunistic encounters with other nodes over infrastructure-less networks. Network topologies in MONs are fragmented dynamically, and persistent end-to-end paths between any pairs of source and destination nodes cannot be guaranteed. Hence, many studies resort to store-carry-and-forward transfer model originating from the delay/disruption tolerant networks (DTNs) architecture to deliver messages in MONs. In this environment, message delivery often applies the message replication methodology to contend with network dynamics, thus increasing the opportunity that destinations can receive message replicas of an original message from a source node in a network. However, repeatedly replicating messages among mobile nodes consumes significant resources like local buffer space on nodes and limited bandwidth during inter-node communications. To reduce resource cost accompanied with repeatedly replicating messages in a MON system, (1) relay selection, (2) transmission scheduling, and (3) buffer management are three crucial research dimensions that must be resolved certainly. To this purpose, the study in this dissertation addresses chiefly on buffer management, message scheduling, and routing aspects, and results in several contributions for message delivery in MONs: technically, (1) Enhanced Buffer Management for Message Multicasting, (2) Probabilistic Routing with Contact Periodicity and Regularity, and (3) On Exploiting Temporal Periodicity for Message Delivery.
Although previous studies proposed various buffer management and scheduling policies, their efforts mainly contributed to a simple scenario of message unicasting from a source to a singular destination in a network. When message multicasting towards multiple destinations is considered, previous solutions performed inefficiently since the essence of their efforts were not optimized for message multicasting with different performance measures. Therefore, the first part in this dissertation proposes an efficient buffer management and scheduling scheme, called E-GBSD, on the base of a generic replication-based routing model for message multicasting in MONs. The proposed design elegantly extends an optimal knowledge-based scheduling and drop policy, and derives a new utility function to prioritize messages in a buffer for maximizing the successful delivery rate in a network. Simulation results manifest that E-GBSD outperforms not only the original policy but also several buffer management policies under message multicasting in MONs.
Probabilistic routing with message replication is recognized as an important methodology that can attain high performance of successful message delivery. Prior probabilistic routing studies mainly depended on delivery predictability with contact frequencies among nodes. However, our study finds that the accuracy of delivery predictability is sensitive and unreliable during time periods of low contact frequency. Conventional probabilistic routing schemes with flat queuing policies, like First-in and First-out, may result in a looping problem that will cause nonnegligible transmission overhead. Hence, the second part in this dissertation presents the probabilistic routing based on contact periodicity and regularity (PRCPR) to maintain the performance of probabilistic routing in MONs. This scheme considers the residual duration of a current period instead of the original delivery predictability when nodes undergo some periods of low contact frequency. This scheme can thus enable nodes to accurately determine whether or not to hand over messages when two nodes encounter during movement. Besides, this study also proposes a lightweight buffer management policy to cope with the message looping problem. Simulation results manifest that the proposed scheme obtains better delivery performance that could be enhanced at most 10% with moderate transmission overhead than conventional probabilistic routing scheme when message traffic is congested.
Recently, some studies argued that human mobility involves some periodicity as a result of various realistic mobility traces and analyses. Whereas routing in MONs is sensitive to node mobility and network dynamics, periodicity has become an important factor for routing design in MONs. Hence, to exploit the properties of periodicity and contact relationship among nodes, the third part in this dissertation proposes an efficient message forwarding scheme, named Temporal Periodicity for Message Delivery (TPMD), which is based on the contact periodicity and contact similarity. This study effort identifies the characteristics of contact periodicity among nodes in a temporal scale, and formulates a period quantification procedure. Referring to temporal periodicity in MONs, we conduct extensive simulation to examine TPMD and show its efficiency on improving successful delivery rate and lowering message overhead as compared with the famous PRoPHETv2 scheme.
Therefore, the contribution of this dissertation can promote the efficiency of message delivery and management in MONs. We believe that these efforts to MONs can be integrated to the emerging network technologies such as Device-to-Device Communications, Vehicle-to-Everything Networking, Edge Computing, and Information-Centric Networking in the coming future.
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