現今以開車當作代步工具已經是普遍的現象，而駕駛人開車最擔心的莫過於碰上一些緊急情況，例如塞車或車禍。如果能將這些訊息提早利用廣播方式送給駕駛人，將可以讓他們有比較充足的時間作出反應。在車載環境中廣播訊息會遇到幾個問題，第一個問題是傳統行動網路常見的問題：隱藏節點和暴露節點，而在行動網路也提出了RTS/CTS機制防止因為這個問題而導致的封包碰撞，但在車載環境中，車輛節點移動性高，環境變動也比較快，如果用這方法，會浪費一些可以傳送的時間，本論文在這部份所採用的是一個時間點選擇一個車輛節點幫忙廣播封包，如此一來就可以不用擔心封包碰撞的問題，如何選擇會在本文敘述之。車載環境中還有第二個問題，就是車流密度不一的情況，有可能前後兩個車輛節點不在彼此的傳輸範圍內，導致後方的車輛節點無法收到封包，本論文把這情況視為車流密度過低，必須透過對向車道的車輛節點和路側單元輔助，將前方車輛節點的訊息帶給後方車輛節點，而會利用路側單元輔助是因為單純透過對向車道的車輛節點無法確定封包的抵達時間，擺設路側單元則不只可以有效提高抵達率也可以確保封包的抵達距離。本論文在路側單元的擺設方式也有提出一套方式，透過前面車流狀況的平均，可以得出後面路側單元該擺放在哪可以有比較好的效果。最後模擬工具採用VanetMobiSim結合Ns-2，利用VanetMobiSim產生的移動情況放入Ns-2進行模擬，分析指標分為二種：抵達相同所花的時間和轉送次數。

On the road the driver’s primary concern is about the emergency information, for example, traffic jam and car accident. In case that emergency message can be broadcast to drivers as soon as possible, the drivers will have more time to react immediately. However, broadcasting in vehicular ad hoc networks (VANETs) has some problems. The first one is traditional in Mobile Ad Hoc Networks (MANETs): the hidden node and exposed node problem. To avoid this problem that will cause packet collision or delay, the RTS/CTS mechanism is used in MANETs But in VANETs, the vehicle nodes have high mobility and change its environmental location quickly. If using RTS/CTS, it will waste the time to transmit packets. In this paper, we choose only one node to broadcast messages at the same time. The second problem in VANETs is of traffic density. For instance, two nodes cannot exchange packet directly. It lets the rare node does not receive messages. And, we consider this case as low traffic density. In this paper, we forward the messages to the vehicle in opposite lane and Road Side Unit (RSU). We use RSU to improve transmission rate and transmission distance. For deployment of RSU, we also provide a mechanism. Through the average traffic density in the limited time, we can have better results. Finally, we use VanetMobiSim to create node mobilities and put this mobility into NS-2 for simulation. Finally, we have two performance metrics: the transmission time at the same distance and relay count.

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