車載網路環境中以覆蓋節點作機率型轉送決定

Abstract

隨著車用行動通訊網路進步，帶動智慧型運輸系統的發展。車載網路以無線通訊與資料傳輸技術互相交換訊息，如果訊息能有效率地散佈，將有助於提供駕駛人一個舒適又安全的行車環境。例如路況資訊可提供駕駛人避開擁擠或車禍的路段，除此之外，還能分享其他多媒體服務資訊、廣告，給有興趣的駕駛人接收資訊。 最能實現快速與廣度散佈訊息的方式即為廣播。藉由多躍式重複廣播，盡可能的將訊息告知網路上所有的節點。由於傳統的廣播方式容易造成多餘訊息與網路擁塞問題，故如何選擇適當的節點將訊息重複傳送，是廣播時需考慮的重點。 在已提出的廣播方法中，偏好選擇邊界節點為轉送者，不僅減少多餘訊息，更能提高轉送後的覆蓋範圍。但車用網路有著高速移動的特性，邊界節點容易因移動而脫離傳輸範圍，特別是多段距離以外的鄰居節點。 針對拓樸環境變動迅速的車用網路，本篇論文提出Nodes Covered Forward (NCF)，藉由標計已傳送訊息的鄰居與邊界覆蓋節點的移動性，轉送者可決定是否重傳訊息。此方法可提高邊界結點的接受率，穩定邊界覆蓋邊界節點。同時以機率方式減少冗贅資訊，並且保持一定的傳達度。比起其他的廣播方法，我們的方法能夠減少97%以上的多餘訊息，以及平均93%的傳達率。

The growing of Vehicular Ad Hoc Network (VANET) improves the development of Intelligent Transport System. With wireless transmissions and data dissemination technology in vehicular network, vehicles can exchange information to each other. If the information can be transmitted efficiently, it will be helpful for drivers to get secure and comfortable driving environment. For example, the road condition can help drivers to avoid traffic congestion or accident path. Furthermore, multimedia data and advertisement can be shared by on-road services to the interested drivers. The most efficient and fast method to transmit data is broadcast. By multi-hop rebroadcast, messages can be disseminated to entire network nodes as many as possible. Because conventional broadcast could cause many redundant messages and network congestion problem, how to select appropriate nodes to rebroadcast messages is a key point when broadcast data. Many proposed broadcast method favor border nodes to be forwarders. In the case, not only redundant messages can be reduced, but also increase the message coverage. However, because of the high mobility characteristic of vehicular network, border nodes will easily move out the transmission range of forwarder, especially many hops away neighbors. For the high mobility nature of vehicular network, the nodes covered forward (NCF) method is proposed in this thesis. With marked neighbor nodes which send messages and the mobility of border nodes, forwarders can decide whether to rebroadcast messages. In the forward method, we can increase the acceptation ratio of borders and cover the border nodes stably. In the meantime, NCF can also reduce redundant messages with probability and keep a good notified ratio. Comparing with the other broadcast protocol, our NCF method can reduce about 97% redundant messages and achieve more 93% on average notified ratio.