| Summary: | 博士 === 國立成功大學 === 資訊工程學系 === 102 === Vehicular Ad-Hoc Network (VANET) communication has recently become an increasingly popular research topic in the area of wireless networking, as well as in the automotive industry. The goal of VANET research is to develop a vehicular communication system to enable the quick and cost-efficient distribution of data for the benefit of passengers’ safety and comfort. One of the most important parameters in simulating vehicular networks is the node mobility. It is important to use a realistic mobility model so that results from the simulation correctly reflect the real-world performance of a VANET. The previous version of the MObility model generator for VEhicular networks (MOVE) did not support interactive simulations, nor allow realistic vehicular network simulations that considered a radio model, and users could also not rapidly generate a mobility trace with specific probability distributions for the destination selection of each car (meaning that each car needs to be added individually). We thus extend MOVE by adding interactive simulations via the Traffic Control Interface (TraCI) in order to carry out rerouting to reduce travel time, provide realistic radio models, and enable destination selection under various probability distributions. We observe the effects of the various details of the resulting mobility models on the performance of VANET, and show that selecting an appropriate level of detail in the mobility model is important with regard to the performance of the VANET simulation.
The results of this work show that node mobility in a vehicular network is strongly affected by the drivers’ behavior, which can change road traffic at different levels. For example, drivers’ preferences with regard to path and destination selection can affect the overall network topology. Therefore, in order to understand the effect of network topology on VANET performance, we examine the effects of preferred routes using four examples: turning decisions at an intersection, choice between fastest and shortest paths, decision to re-route when encountering traffic jam, and destination selection. We also discuss how different destination selection models affect two practical intelligent transport system (ITS) application scenarios: traffic monitoring and event broadcasting. We observe that the network performance is generally better when one model’s drivers’ choose routes using the shortest path model, as compared to using the fastest path model. We show that a destination selection model can have different effects for different ITS applications. Furthermore, we observe that simulation results are not significantly affected by different node density settings when cars pick their destination following a Pareto distribution. Finally, we show that more nodes might concentrate at the center of the map when the uniform distribution is chosen to model the destination selection.
Finally, given that some cars might not have the networking capability to join the vehicular network or access the Internet, we propose a collaborative bandwidth sharing protocol (CBSP) built on top of MultiPath TCP (MPTCP), as this allows multihoming to enable users to buy bandwidth on demand from neighbors (called helpers). Although MPTCP provides required multihoming functionality, the current implementation of packet scheduling in MPTCP leads to the following two problems when the packet scheduler considers both round trip time (RTT) and available congestion window (cwnd) to dispatch packets (called RTT scheduling in this thesis). (1) Out-of-order packets: packets might be sent to slower links due to limited cwnd, thus increasing the number of out-of-order packets. (2) Biased-feeding: the fastest link might always be selected to send packets whenever it has available cwnd, while other links are only selected when the fastest link has no available cwnd. CBSP uses virtual interfaces to bind the subflows of MPTCP to avoid modifying the implementation of MPTCP, and to flexibly manage the interfaces of smartphones for each helper. We employ the Scheduled Window-based Transmission Control (SWTC) scheduling algorithm to improve the performance of packet scheduling in MPTCP. The results show that SWTC can more efficiently reduce the number of out-of-order packets than RTT scheduling under various network heterogeneities, and also avoid biased-feeding.
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