Advances in information and communication technologies enable vehicles on roads to cooperatively share information and data to support intelligent transportation systems (ITS) services, such as vehicle safety [1], traffic management [2], [3], and infotainment services [4], without requiring a centralized server. For example, current traffic conditions in a specific road segment can be obtained by sending queries to the vehicles either driving on the road segment or near the area. In addition, a gas station may advertise its price information in a local area so that nearby vehicles can receive these messages. The growing necessity for sharing and retrieving required information among vehicles has motivated the creation of an information retrieval infrastructure in a vehicular environment.
Information sharing and retrieval require the support of wireless communication to transmit and receive data to and from moving vehicles. Recently, two major types of wireless communication technologies have been considered in the vehicular environment [5]. They are short-range ad hoc communication, such as IEEE 802.11p, also known as intervehicle communication (IVC), and long-range infrastructure-based communication, such as Mobile WiMAX and LTE. IVC provides direct and low-latency communication between vehicles without requiring an infrastructure support [6]. Vehicles communicate with each other in a hop-by-hop manner to establish vehicular ad hoc networks (VANETs).
A number of systems have been proposed to share traffic information [2], [7] and content delivery [4] over VANETs. However, these systems require a sufficient number of vehicles participating in VANETs. VANETs may become disconnected under low vehicle densities, and thus, information and data may not be exchanged among vehicles where low vehicle densities occur.
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On the other hand, infrastructure-based wireless communication, which can offer a wide range of communication to vehicles, does not suffer from a network disconnectivity problem. Recent research [3] has used infrastructure-based cellular communication to implement a cooperative traffic information service among vehicles. In particular, an application-layer service overlay is built over an infrastructure-based network using peer-to-peer (P2P) networking technology [8]. Vehicles cooperatively share and retrieve traffic information over the infrastructure-based P2P overlay network. However, significant service delays may be introduced in this type of system because of limited network bandwidths and high communication latencies through base stations and mobile communication core networks.
The systems described above utilize VANETs, infrastructure-based networks, or an application-layer P2P overlay network to provide information retrieval services in the vehicular environment. We classify these systems as single-tier information retrieval systems. As vehicles are envisioned to support multiple wireless access technologies [5], [9], they can communicate with each other via not only direct ad hoc communication but also infrastructure-based communication. Direct intervehicle communication provides low communication latencies (e.g., a few milliseconds) in connected areas while infrastructure-based communication offers a wide range of communication with longer latencies (e.g., on the order of hundreds of milliseconds). To exploit the two communication systems, we propose a two-tier VANET/P2P architecture that integrates low-tier VANETs and a high-tier infrastructure-based P2P overlay network. In low-tier VANETs, vehicles can directly exchange and collect information using IVC efficiently. In addition, certain vehicles are elected to establish a P2P overlay through infrastructure-based communication to alleviate the disconnectivity problem in VANETs.
Vehicles cooperatively share information with each other and send queries through low-tier
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VANETs and the high-tier P2P overlay to retrieve information of interest. An information retrieval system based on the two-tier VANET/P2P architecture can achieve a high lookup success rate, low lookup latency, and low maintenance overhead compared with the single-tier systems.
The conventional design of the information lookup in the two-tier VANET/P2P system simultaneously performs queries over the low-tier VANETs and high-tier P2P overlay network. Although this increases the lookup success rate and improves the lookup response time, this approach may introduce redundant lookup messages and delays. For example, it is unnecessary to forward and broadcast lookups to the P2P overlay if the lookup message can be delivered to the intended destination through well-connected VANETs. Moreover, a query should be directed from the P2P overlay to VANETs to minimize the lookup latency if the query can be routed to the destination through the VANETs. Making full use of VANETs can improve the lookup speed and reduce redundant lookups in the infrastructure communication network whose radio resources are relatively expensive. Therefore, we propose an adaptive lookup protocol for the two-tier VANET/P2P system. The proposed protocol uses the concept of the Bloom filter [10] to exchange reachability information of road segments among vehicles in VANETs. The reachability of a road segment indicates whether the road segment can be reached through low-tier VANETs. The Bloom filter is a space-efficient probabilistic data structure that can considerably reduce storage and wireless communication overheads for information exchanges between vehicles. Although the reachability information maintained by Bloom filters can only provide an estimation regarding whether a query can be routed to a specific road segment through the VANETs, simulation results show that the design fits and can accommodate the dynamic nature of a vehicular environment. The proposed protocol forwards lookups adaptively between the low-tier VANETs and high-tier P2P overlay network according to the reachability of the
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destination. Therefore, compared with the conventional two-tier lookup mechanism, the latency of information retrieval and the lookup message overheads can be significantly reduced in the two-tier VANET/P2P system by applying the adaptive lookup protocol.
The rest of this dissertation is organized as follows. Chapter 2 presents single-tier architectures and existing systems for information retrieval in a vehicular environment.
Chapter 3 proposes a two-tier VANET/P2P information retrieval system, compares different system architectures, and provides a performance evaluation through simulation.
Chapter 4 proposes an adaptive lookup protocol to improve the performance of the two-tier VANET/P2P system. Finally, Chapter 5 concludes this dissertation and suggests some future directions.
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