This thesis summarizes research achievements of novel routing protocols suggested for vehiclead-hoc networks. Currently, ubiquitous networks that connect distributed computersand sensors to support human life attract attention internationally. In order to realize theubiquitous networks, following two steps are required: spreading seamless networks into everycorner of our life, then making the established networks easy to use for people.Ad-hoc networks are one of the key technologies to realize the first step. Ad-hoc networksare composed of mobile stations that communicate with each other wirelessly. Ad-hocnetworks have some promising features, such as their absence of infrastructure and theirhigh scalability. However, ad-hoc networks have their drawbacks also, such as the difficultyof packet forwarding between mobile stations, since network topology changes dynamically.Therefore, routing protocols for ad-hoc networks are one of the most important researchsubjects.The standardization of routing protocols for ad-hoc networks is discussed in mobile adhocnetworks (MANET) working group (WG) of Internet Engineering Task Force (IETF).However, these routing protocols give their weight to general versatility. Consequently, theycannot achieve high throughput in particular ad-hoc networks environments. From this pointof view, this thesis focuses on vehicle ad-hoc networks as a concrete network environment andpresents routing protocols that increase communication performance.This thesis consists of six chapters. Chapter 1 is the introduction; the background, subvjects, and purpose of this study are organized. Chapter 2 describes network model that is thepremise condition of this study.Chapter 3 describes routing protocols for short distance inter-vehicle communications. Apoint to consider with respect to vehicle ad-hoc networks is that dynamic changes in topologyfrequently occur. Unless a stable communications route with long life is constructed inintervehicle communications, the transmission of control packets generated when the communicationsroute is disconnected will increase the overhead. In order to solve this problem,this thesis proposes relative speed-based routing (RSR) for vehicle ad-hoc networks in whicha stable communications route is constructed without using beacons or similar means. RSRis a routing protocol that uses the relative speed and the cumulative change of the distanceto neighboring vehicles as the metric. The protocol is realized by including the relative speedand intervehicle distance information as elements in the route request packets used for reactiverouting. RSR is compared by simulation to ad-hoc on-demand distance vector (AODV),which is a typical reactive routing protocol, and flow-oriented routing protocol (FORP), whichis a reactive routing protocol emphasizing stability. It is shown that RSR is better in termsof packet arrival ratio and stability.Chapter 4 describes routing protocols for inter-vehicle communications via establishednetworks. In such network environments as vehicle ad-hoc networks combined with establishedwired networks, two types of routes exist between vehicles: routes using wireless links onlyand routes via access points and the established networks. This thesis presents a new routingprotocol that uses simple formulas to decide which route is effective, a route using wirelesslinks only, or a route via the established networks. The formulas are derived from the resultsof simulation on the communication performance of some routes that uses IEEE802.11b aswireless links and Ethernet of 100 Mbit/s as wired links. The performance of our proposedrouting protocol, which is realized as an extension of AODV, is evaluated by simulation. Theresults show that the proposed routing protocol is effective especially under a high data ratevisituation compared with AODV.Chapter 5 describes flooding methods that are needed to realize long distance inter-vehiclecommunications. In order to realize communication from a vehicle to another vehicle in adistance, application of mobile IP protocol is a realistic approach. There exist many reportsas for applying mobile IP to ad-hoc networks. However, most of them require periodicalflooding of agent advertisement packets. Periodical flooding of the packets wastes preciouswireless bandwidth much. Therefore, an efficient flooding method is needed to reduce wirelessbandwidth waste. This thesis presents sector-based flooding (SBF) and adaptive sectorbasedflooding (ASBF) that are flooding methods for vehicle ad-hoc networks using positioninformation. SBF is divided into two methods, SBF-1 and SBF-2; the difference is the numberof criteria used to decide whether to re-broadcast or to drop the packet. In ASBF, each nodeselects a flooding method from among SBF-1, SBF-2, and pure flooding, depending on itslocal node density. The node density is obtained from the distance between the sender nodeand the sector representative position. Simulation results show that SBF reduces the numberof packet transmissions generated in flooding and ASBF has high packet reachability withfew packet transmissions.Chapter 6 concludes the achievement of this study. The future works and internationaltrends of vehicle ad-hoc networks are described, too.
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