雙階層車載隨意網路暨同儕網路之資訊擷取系統研究

Abstract

資訊與通訊科技的進步讓車輛可合作分享與擷取有用的資訊，以提供智慧型運輸系統之服務。為了提供這類服務，需要設計一套資訊擷取系統，有效率地在移動的車輛和路邊設施之間擷取資訊。在這篇論文中，我們首先分析與歸納於車載環境提供資訊擷取服務之系統架構。當車輛密度高時，採用短距離的車間隨意通訊（intervehicle ad hoc communication）之單階層系統可於短時間內取得所需資訊。另一方面，採用長距離的基礎建設無線通訊（infrastructure-based communication）與同儕網路（peer-to-peer networking）之單階層系統則可提供高成功率的資訊擷取，而不受限於車輛的數量。然而，對於提供資訊擷取服務，單階層系統可能遭遇較低的擷取成功率或需要較長的通訊時間。為了善用兩種無線通訊系統，我們提出雙階層車載隨意網路（VANET）暨同儕網路（P2P）之系統架構，此雙階層系統整合一個低階的車載隨意網路與另個高階的同儕網路，於車載環境提供資訊擷取服務。我們分析比較單階層系統與所提出之雙階層系統，並透過模擬實驗的方式評估效能。實驗結果顯示，相較於單階層車載隨意網路之系統，雙階層系統可明顯提高資訊擷取的成功率；相較於單階層基礎建設通訊式同儕網路之系統，雙階層系統可降低資訊擷取所需的時間與訊息負擔。最後，我們提出適應性搜尋協定以提升雙階層系統於資訊擷取的效能。此搜尋協定採用Bloom filter以收集路段的可到達狀況，評估路段的可到達機率而調整資訊擷取時的訊息傳送方式。對於雙階層資訊擷取系統，模擬實驗結果顯示，相較於原有的搜尋機制，此適應性搜尋協定可減少資訊擷取所需的時間與訊息負擔，亦可達到資訊擷取的高成功率。

With advance in information and communication technologies, vehicles on roads can cooperatively share and retrieve information in a distributed manner to support Intelligent Transportation Systems (ITS) services such as traffic management and infotainment services. To support such services, a system is needed to retrieve information and data from moving vehicles and roadside facilities in an efficient manner. In this dissertation, we first classify system architectures for information retrieval services in a vehicular environment. Single-tier systems based on short-range intervehicle ad hoc communication can achieve the shortest latency if vehicle density is sufficient. On the other hand, single-tier systems based on long-range infrastructure-based wireless communication and peer-to-peer (P2P) networking technology can provide a high success rate without the need for a sufficient density. However, the single-tier systems relying on either vehicular ad hoc networks (VANETs) or an application-layer P2P overlay over infrastructure-based networks may suffer from low success rate or long latency in information retrieval. To take advantage of both ad hoc and infrastructure-based communications, we propose a two-tier VANET/P2P architecture that integrates low-tier VANETs and a high-tier infrastructure-based P2P overlay network for providing information retrieval services in the vehicular environment. We provide a qualitative analysis of the single-tier and the proposed two-tier architectures. The performance of different system architectures is evaluated and analyzed through simulation. Our results demonstrate that an information retrieval system based on the proposed two-tier VANET/P2P architecture can significantly improve success rate compared to the single-tier VANET-based systems while reducing lookup latency and message overhead compared to the single-tier infrastructure-based P2P systems. In the last part of this dissertation, we propose an adaptive lookup protocol to improve the efficiency of information retrieval in the two-tier VANET/P2P system. The proposed protocol uses the concept of the Bloom filter to collect reachability information of road segments. Therefore, adaptive routing of lookup queries between low-tier and high-tier networks according to reachability probability can be employed. Simulation results show that compared to the conventional two-tier lookup mechanism, the adaptive lookup protocol can reduce the lookup latency and lookup overhead, and also achieve a high success rate in information lookups.