在大型無線感測網路上利用小世界現象做時間同步

Abstract

近來，無線感測器網路是一個相當熱門的研究主題，由於體積小、低成本、佈置簡單，已經有了一些實際應用，例如：物體偵測、戰場情資收集...等。然而，這些應用都需要時間同步機制的支援，如何發展一個有效的時間同步機制是本論文的重點。本論文改良現有的機制來完成single-hop時間同步，加上了小世界的概念，來縮短multihop時間同步所需的時間，如此一來，不但更快完成無線感測網路的時間同步，也可以提升時間同步的準確度。由本論文模擬結果可知，我們所使用的方法以更快速度完成時間同步，並顯示出其方法完成multihop時間同步的效能優於現有的方法。有許多人開始在水中發展無線感測網路，現今已有許多的實際應用，例如：洋流觀測、能源探勘…等。這些應用也都需要時間同步機制的支援。然而，在水中發展一套時間同步機制，最大的不同在於利用聲波傳輸其速度較慢，會造成很長的傳播延遲時間(Propagation Delay)，然而現有的時間同步機制大多假設傳播延遲時間極短可以忽略，因此無法套用在水中無線感測網路的時間同步問題上。在本論文，我們利用Underwater Positioning System (UPS)[15]完成位置定位，利用位置資訊與time difference的技巧完成水中時間同步。模擬結果可知，我們使用的時間同步機制，準確度可以到達 ，並且可以在應用層(Application Layer)就完成時間同步，不必再對其它層(Layers)做任何修改。

Recently, the development of the wireless sensor nodes is a hot research topic. Due to its small size, low-cost and easy deployment, the sensor nodes already have many practical applications such as object detection, information gathering in battlefields…etc. However, these applications need sensor nodes have consistence in time. In this thesis, we develop several effective mechanisms for time synchronization in large-scale wireless sensor networks. Based on the existing time synchronization mechanism, coupled with the concept of small world, this thesis proposed a mechanism that can greatly reduce the time for the time synchronization in the wireless sensor networks. The result shows that our schemes can greatly reduce the time spending in the time synchronization, and shows that this methodology used in the multihop time synchronization is better than existing methods. There are so many people trying to develop a wireless sensor network for an underwater environment. There are many applications for an underwater wireless sensor network, such as environmental monitoring, ocean current observation, energy exploration…etc. These applications need a time synchronization scheme also. However, the transmission medium is an acoustic wave that causes a long propagation time in the underwater. Nevertheless, the existing time synchronization schemes neglect the propagation time, because the propagation time is short with the electronic wave on the land. Therefore, the existing time synchronization schemes are not suitable for the underwater wireless sensor networks. In this thesis, we use the Underwater Positioning System (UPS)[15] and the technique of time difference to finish the time synchronization in a single-hop for the underwater. The simulation shows that the accuracy level is and the time synchronization we proposed can finish the procedure in the application layer without any modification in other layers.