近幾年，無線感測網路已經成為一個有趣且重要的研究領域。在這樣的無線感測網路中的應用所需環境，是由一群分散、各自獨立的無線感測節點，相互溝通共同合作來實行任務，因此，為了要讓感測網路能正確的提供在各節點事件發生的順序，以及事件移動的狀況，各節點的時鐘是否能達到同步，就是在無線感測網路中一個非常關鍵重要的研究。
由於無線感測網路特有許多能量上的限制，因此在解決同步的問題上面，跟以往傳統的LAN或是IEEE802.11的網路有所不同。此篇論文利用overhearing的概念，在計算各節點之間的時鐘差異時，減少控制訊息的數量而達到節省電量的目的。此方法另一個特別的優點，是在不需要增加其他額外步驟上，而能使用在有經過修改的topology環境內。因此，我們提出一個能有效率節省電量的時間同步方法(METS)，此METS方法是在所有無線感測節點的網路傳輸距離相同的環境內使用。另外，又為了要降低同步的錯誤差異，我們更進一步提議延伸的第二個方法(Heterogeneous METS)，希望能透過增加一定比例的傳輸距離較遠的感測節點，以減少同步過程中的hop count數，得以增加同步的準確率。又經過模擬的結果顯示，我們提出的同步方法確實能提供準確的時間同步效果，並且相較於前人的方法，的確有效的節省大量的電量，達到增加網路lifetime的效果。

In recent years, wireless sensor networks have become an interesting and important research area. The applications for such networks require collaborative execution of a distributed task among a large set of sensor nodes. Therefore, the time synchronization is a critical issue for wireless sensor networks. Because of limited energy for WSN, we utilize the concept of overhearing to decrease the control overhead while calculating the clock offset values to adjust the timers of sensor nodes. Especially, it can use in the modify topology without other extra steps. In this thesis, we propose the message-efficient time synchronization scheme (METS) which aims at providing network-wide time synchronization for a homogeneous sensor network where the transmission ranges of sensor nodes are assumed to be the same. In order to reduce synchronization error, we further propose a Heterogeneous METS to decrease hop counts by taking advanced nodes into consideration. The simulation results show that our proposed synchronization schemes provide accurate time synchronization and are energy efficient comparing to other schemes.

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