| dc.description.abstract | The prevalence of networked devices capable of wireless connectivity and mobility propels a new data dissemination paradigm -- mobile opportunistic networking (MON). Wherein, mobile nodes discover neighbors in location proximity and exchange messages during their opportunistic contacts in short-time reciprocal communications. Because of frequent link failures and the lack of up-to-date network topology information in MONs, the extit{store-carry-and-forward} data delivery model is employed to relay messages in a delay-tolerant manner. Since inter-node communications occur only during unscheduled meetings between nodes, the performance of data dissemination in MONs is heavily influenced by the dynamic nature of node mobility and contact opportunity in mobile environments.
In MONs, replication-based routing techniques are often used to distribute duplicate messages to increase the chances of delivering messages to a destination, which unfortunately leads to enormous costs of transmission, storage, and energy resources. To reduce the costs associated with replicating messages repeatedly, several essential techniques are required certainly: efficient relay selection, message forwarding, transmission scheduling, and buffer management. The study in this dissertation proposes one joint buffer management and message scheduling scheme and two novel routing schemes for data dissemination in MONs: (1) Quota-Based Routing and Buffer Management with Heuristic Strategies in Opportunistic Ad Hoc Networks; (2) Exploiting Mobile Contact Patterns for Message Forwarding in Mobile Opportunistic Networks; (3) Exploiting Group Mobility for Message Dissemination in Mobile Opportunistic Networks.
The study in this dissertation first discusses the understanding of contact patterns and contact-driven knowledge regarding the novel schemes of efficient relay node selection and message forwarding inside node communities. In Chapter
ef{chap2}, we show the findings that contact frequency is heterogeneous between relay nodes, i.e., some nodes may never establish contacts with the others, whereas some nodes may contact more than one node. Thus, nodes have contact periodicity or repeating patterns, and nodal contacts in communities are transient in nature.
Although previous studies proposed various buffer management and scheduling schemes, their efforts were mainly based on several premises, e.g., the availability of global network knowledge, unlimited bandwidth capacity, and homogeneous contact patterns. Prior research has paid less attention to the theme of joint buffer management and relay selection. In Chapter
ef{chap3}, we propose a novel scheme, named Quota-Based Routing Scheme with Finite Buffer Management (QRBP). This scheme selects only a portion of nodes that have higher mobility, and then delegates those nodes to carry a certain quota of message replicas and disseminate those messages to nodes with lower mobility. Nodes can manipulate message scheduling and dropping to improve the successful delivery rate according to heuristic strategies based on several measures of the quota value, remaining time-to-live (TTL), and contact rate with the destination. Simulation results manifest that the QRBP scheme obtains better message delivery performance than Epidemic, SprayAndWait, and Temporal Closeness and Centrality-Based (TCCB) routing and DropOldest (DO), DropNewest (DN), DropRandom (DR), and Space-Time-Drop (ST-Drop) buffer management schemes.
In Chapter
ef{chap3}, we consider the practical situation that some nodes move quickly over a larger area, which leads to different movement patterns in a network map. Then, our study notices that replicating only a quota of any particular message can still sustain the efficiency of message delivery provided with only a sufficient number of high-mobility nodes. Our study in Chapter
ef{chap4} further releases this restricted consideration and appeals to a general case, which is to replicate messages multiple times to increase the chances of reaching a destination. In particular, this general study should examine the intrinsic properties of contact patterns and identify two contact types, i.e., regular and sporadic contacts in MONs. Contacts occurring periodically are defined as $regular$, and occasional contacts are termed as $sporadic$. Chapter
ef{chap4} presents an efficient message forwarding scheme, named Regular and Sporadic Contact-Based Routing (RSCR), which is based on contact patterns among mobile nodes. First, we derive the regular and sporadic contact patterns based on the mean of inter-contact durations with respect to any node pair in MONs. Then, we jointly use the derived contact patterns together with a remaining TTL value to accurately determine whether or not to hand over messages when two nodes encounter during movement. Simulation results show that the proposed scheme attains the same delivery rate at a lower cost as compared with Epidemic and PRoPHETv2 schemes.
Following the efforts of efficient buffer management, scheduling, and relay selection schemes in Chapters
ef{chap3} and
ef{chap4}, we aim to investigate contact patterns inside groups/communities in Chapter
ef{chap5}. Conventional schemes often analyzed direct contacts between nodes, observed contact formation for an extended period, and evaluated a static graph for community formation in MONs. However, such formation methods can result in communities with weak contact relationships and incorrectly appointing random nodes to a community. Instead, our study examines essential factors such as contact consistency in a certain period, contact frequency, and contact duration to form $k$ groups of which nodes render strong contact patterns. Further, we specify a new metric, temporal-tie strength, by utilizing contact duration time, contact frequency, and aggregate contact properties with respect to intra- and inter-group contact patterns during a specific time window. We propose a novel routing scheme, named Temporal-Tie-Strength Group-Based Routing (TS-GBR), which is able to improve the cost-effective performance of message forwarding in MONs. Our simulation with several real-life data sets demonstrates the efficiency of the proposed scheme as compared with Epidemic, TCCB, Transient Community-based (TC), and Dynamic Transient Social Community (DTSC) routing schemes.
Therefore, the contribution of this dissertation can promote the efficiency of message delivery and relay selection schemes in MONs. We believe that these efforts to MONs can be integrated to the current and new emerging wireless communication systems such as Internet of People (IoP), Device-to-Device communication (D2D), Social Internet of Things (SIoT), unmanned aerial vehicles (UAVs), Edge computing, and Vehicle-to-Everything (V2X) in the coming future.
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