在車載網路上，基礎設施到車輛的群組訊息傳送是一種常見的手法，被各式各樣的車載應用程式所使用。然而，由於車載網路中車輛的高動態及高移動性，從基礎建設傳送群組訊息到一群散布在整個網路中移動的車輛成為了一個困難的問題。這篇論文的目的與貢獻是提出一個在車載網路上以車輛軌跡為基礎的群組訊息傳送協定，利用車輛的軌跡來改善群組訊息傳送的效能。此通訊協定包含兩個階段，在第一個階段，伺服器決定了一些交會點讓成員車輛可以收到群組訊息。接著在第二階段，伺服器利用一個似群播的傳送機制將群組訊息傳送到選定的交會點，避免在重複的路段上傳送多餘的封包。最後，訊息會被保留在這些交會點上等待成員車輛的經過，並且完成最後階段的訊息傳送。此通訊協定的目標是在一個較為寬鬆的延遲限制下，將群組訊息傳送所需的網路層封包數量最小化。我們的模擬結果顯示，與以車輛軌跡為基礎的單播訊息傳送機制相比，我們的通訊協定明顯地節省訊息傳送所需要的封包數量（網路頻寬），同時達到90%的高傳送成功率。

Infrastructure-to-vehicle (I2V) group message delivery is a common operation required by a wide variety of vehicular ad hoc network (VANET) applications.
However, due to the highly dynamic mobility in VANETs, I2V group message delivery remains a difficult problem to deliver messages to a group of moving vehicles scattering over the whole network.
The purpose and contribution of this paper is to propose the first trajectory-based I2V group message delivery protocol, TGMD, that exploits vehicle trajectories to improve the group-message delivery performance.
TGMD contains two phases.
The first phase determines a small set of rendezvous points from which the member vehicles can receive the message.
Then the second phase transmits the group message over multiple hops to these selected rendezvous points using a multicast-like forwarding scheme to avoid redundant packet transmissions over the overlapped road segments.
The message is kept at the rendezvous points for later last-hop delivery when the member vehicles pass through.
TGMD aims to minimize the required number of network-layer packet transmissions under a relatively loose delay constraint.
Our extensive simulation results indicate that in comparison with the trajectory-based single-destination I2V message delivery scheme (through N-unicasting), TGMD significantly reduces the required number of packet transmissions (and thus bandwidth consumption) while achieving a high delivery ratio around 90\%.

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