由於鋪設路側單元(如802.11 基地台)的成本昂貴，路側單元的訊號範圍難以涵蓋所有的道路，使得車載網路與外部網路的連結時常會有斷線的情況，當使用者從這些路側單元下載影音檔案或是地圖資訊時，將由於部分路段的路側單元鋪設不足，需要經過較長的時間才能完成下載。本論文利用了檔案的地域性以及MPEG編碼的特性，提出一個車輛之間能夠互助合作下載影片訊框的機制，稱為ZFEP (Zone-based Frame Exchange Protocol)，並透過ns-2 模擬來測量ZFEP 下載機制所能夠提升的使用者影片下載速度，且實驗結果證實當區域內有越多車輛使用ZFEP 下載機制時，系統效能會更加提升。然而，ZFEP 是需要使用者的互助合作為基礎來運作，但現實生活中可能會出現許多自私的使用者，不願意花費下載影片或是不願意分享影片給其餘使用者，影響系統的公平性與效能。因此，本論文在原來的ZFEP 下載機制中加入了自私行為的檢測機制，稱為ZFEP/SD (Selfishness Detection)，而模擬結果也證實了此自私行為檢測機制能夠有效的抑制自私者的下載效能，藉以提升使用者的合作意願。

Due to the limited budget and high maintenance cost, it is difficult to deploy a large amount of roadside units (such as 802.11 base stations) to cover the complicated road areas. Vehicles have to wait a long time to download large size multimedia file, such as video clips regarding local news, through a sparse network infrastructure. In this paper, we propose a Zone-based Frame Exchange Protoco (ZFEP) to utilize the locality and the characteristics of MPEG encoding in the video file. Through ZFEP, the local video downloading performance can be improved by integrating V2I and V2V communications. This paper also investigates how to prevent selfish node in ZFEP. ZFEP/SD is able to exploit user requests locality and to initiate video frames sharing among neighboring vehicles. The extensive simulation results show that our proposed protocol is practical, scalable and cost-effective in vehicular networks.

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