| 摘要: | 近年來眾多將無線網路通訊應用於地面交通運輸系統的相關研究被提出，車輛隨意網路 (VANET, vehicular ad-hoc networks) 便是利用無線區域網路技術所建構而成具備容易實作、富高度價值的應用，例如路面安全檢測、多媒體分享、線上遊戲、車輛網際網路以及各式商業應用。然而在車輛隨意網路上的多點跳躍訊息傳播(Multi-hop information dissemination)將受制於車輛間的移動性以及頻繁的斷線而導致低傳輸成功率。本篇論文提出一個適用於高速公路車輛無線網路環境的路由機制，其中包含以目的節點探索為基礎的單點傳播流程 (unicast destination discovery)、堅固的節點選擇式轉送機制 (forward node selection)以及具有位置性的Hello 機制 (positional hello)。在這篇論文中，為了避免頻繁維護路徑所造成的網路資源浪費，因此沒有設定任何專用的路徑。此外，排除掉洪流法(flooding)以及位置服務將能從本質上減輕控制訊息對網路造成的額外負擔。具有位置性的Hello 機制能夠同時確保節點的連結性與減少控制訊息對網路的額外負擔。模擬結果表現出本論文提出的路由策略(DDOR)相較於過去其他研究所帶來的好處─較高的封包送達比率、減輕路由對網路造成的額外負擔以及縮短延遲時間。現今地理性的路由協定由於並不需要經歷建立與維護路由的階段，因此被廣泛應用在車輛隨意網路上。此外，若加上連接性感知(connectivity awareness)的功能將使得資料傳送更加可靠。但這類協定為了取得目的節點的位置將使用位置服務或是洪流法，然而洪流法將不利於在城市環境中應用，因為其中RTS-CTS 機制無法為探查封包(probe packet)提供安全保護傳送。更進一步，在車輛稀疏甚至淨空的區域，此類協定頻繁的使用復原策略將會不必要的增加傳輸跳躍數(hop-count)。部分地理性的路由協定在距離或連接性上應用最小權重演算法(minimum weighted algorithm)為基礎來選擇中間的交叉點；然而，最短路徑或是擁有較高連接性的路徑將包含許多中間交叉點，這種現象導致這一類協定最終選出較高傳輸跳躍數的路徑。在本論文的第二項研究中，我們提出一個貪婪式跳躍的機制將連接性納入考量來選出擁有最少中間交叉節點的路徑。此外本論文採用了後端骨幹節點的關鍵技術來提供關於交叉點附近的連結狀態，同時追蹤來源端與目的端節點的移動狀況，後端骨幹節點將使得封包能夠往改變後的方向傳送。模擬結果表現出本論文提出的BAHG 路由策略有著較高的封包送達率以及較短的點對點延遲時間。The emerging adoption of wireless communications on surface transportation systems has generated extensive interest among researchers over the last several years. Using advanced WLAN technologies, vehicular ad-hoc networks have become viable and valuable for their wide variety of novel applications such as road-safety, multimedia content sharing, online gaming, internet on vehicles, and commerce on wheels. Multi-hop information dissemination in vehicular ad hoc networks is constrained by high mobility of vehicles and frequent disconnections. We propose a destination discovery oriented routing (DDOR) scheme for Highway/Freeway VANETs. DDOR consists of a unicast destination discovery process, a robust forward node selection mechanism and a positional hello mechanism. In this work, no dedicated path is framed in order to prevent frequent path maintenance. In addition, the elimination of flooding and location services substantially reduces the control overhead. Positional hello scheme ensures connectivity and diminishes control overhead concurrently. Simulation results signify the benefits of the proposed routing strategy which has higher packet delivery ratio, reduced routing overhead and shorter delay compared with existing routing protocols. Currently, the geographic routing protocols are widely adopted for city scenarios as they do not require route construction and route maintenance phases. Again, with connectivity awareness they perform well in terms of reliable delivery. To obtain destination position, such protocols employ location service or flooding. Flooding can be detrimental in city environments as probe packets are not safeguarded by RTS-CTS. Further, in case of sparse and void regions, frequent use of recovery strategy in such protocols elevates hop-count. Some of the geographic routing protocols adopt minimum weighted algorithm based on distance or connectivity to select the intermediate intersections. However, the shortest path or the path with higher connectivity may include numerous intermediate intersections. As a result, these protocols yield routing paths with higher hop-count. We propose a back-bone assisted hop-greedy (BAHG) routing scheme to address these problems by selecting a routing path with minimum number of intermediate intersection nodes while taking connectivity into considerations. Besides, we introduce back bone nodes which play a key role in providing connectivity status around an intersection. Apart from this, by tracking the movement of source as well as destination, the back bone nodes enable a packet to be forwarded in the changed direction. Simulation results demonstrate that the proposed routing strategy outperforms state-of-art geographic routing protocols in terms of packet delivery ratio and end-to-end delay. |
