D IRECTIONS
In this dissertation, we have classified and investigated various single-tier systems, and proposed a two-tier VANET/P2P system for information retrieval in a vehicular environment. The two-tier system exploits both VANET and P2P technologies. Vehicles form low-tier VANETs via IVC while a portion of vehicles establish a high-tier P2P overlay through infrastructure-based communication. Information can be shared among vehicles and retrieved through lookups preformed in the low-tier VANETs and high-tier P2P overlay. We analyzed design issues of different system architectures and compared their performance through the SUMO traffic simulator and QualNet network simulator.
Simulation results reveal that the two-tier VANET/P2P architecture achieves much higher lookup success rates than single-tier VANET-based systems and outperforms single-tier infrastructure-based P2P systems in terms of success rate, latency, and maintenance cost.
Open research issues for the new two-tier VANET/P2P architecture such as adaptive lookup and routing in between VANET/P2P networks, superpeer election and redundancy, and others are also identified and should be further studied.
We further proposed an adaptive lookup protocol to improve the performance of information retrieval in the two-tier VANET/P2P system. The adaptive lookup protocol exploits reachability information of road segments to perform lookups between low-tier and high-tier networks. To efficiently maintain the reachability information in a dynamic vehicular environment, the Bloom filter and probabilistic information are used to determine the lookup forwarding. Simulation results show that the proposed adaptive lookup protocol significantly improves the performance of lookup latency while it can also
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achieve a high lookup success rate similar to the conventional approach. Although the improvement may lead to an increase in message overhead in VANETs, the proposed protocol can significantly reduce P2P message overhead in the infrastructure-based network by 20%–33%. It is particularly important and valuable for the two-tier VANET/P2P system to make full use of VANETs and to efficiently use the resources of the infrastructure-based wireless communication. We have investigated the effects of the protocol designs on the lookup performance under various network scenarios. Since vehicle density (i.e., network condition) varies in areas and over time, one future enhancement could consider different and/or dynamic thresholds in various areas (e.g., urban or rural area) and during different times (e.g., rush hour or nighttime) according to the vehicle density and network condition that vehicles observe or estimate.
The proposed two-tier VANET/P2P system provides an infrastructure to share and retrieve information in the vehicular environment. This infrastructure can also be used to implement high-level applications and services such as route planning. For example, the route planning application determines what information (i.e., traffic conditions) it needs, performs lookups for the necessary traffic information, and then suggests a suitable route to the driver based on the obtained traffic reports. A possible implementation of the route planning application is to send a number of lookup queries to nearby road segments and areas along the path from the source to the destination. The performance of an application depends on the number of lookup queries it generates and the lookup latencies required to obtain the necessary information. Information can be retrieved in the two-tier VANET/P2P system with a high success rate, low latency, and low overhead. Moreover, compared with the conventional two-tier lookup mechanism, the adaptive lookup protocol further reduces the lookup latency, VANET overhead, and P2P overhead for each lookup query, and achieves a high lookup success rate. Therefore, the adaptive lookup protocol is likely to
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improve the performance of high-level applications. Because of various implementations of applications, the discussion of more application scenarios and an evaluation of high-level services are topics for future research.
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