在水底感測器網路上高傳播延遲對於機率式路由影響之探討與設計

Abstract

因為在水底感測器網路(underwater sensor networks)中是利用聲波訊號(acoustic signals)進行資料傳輸，所以水中的獨特性質讓水底感測器網路衍生許多挑戰。尤其是聲波訊號的高傳播延遲(high propagation delay)與不可靠的無線通道(unreliable wireless link)，使得現有的陸上感測器網路機制無法在水中順利運行。 本篇論文的主題，是針對水底環境的特性，探討機率式路由(Opportunistic Routing)受到的影響，並且設計一個適用於水底環境的網路通訊協定。首先，因為機率式路由對於不可靠的通道有較好的表現，我們將其概念結合到媒體存取控制協定的設計。考量水中高傳播延遲帶來的影響，我們建構一個事件關係圖(Event Relational Graph)，以下簡稱為ERG，其利用時空不確定性(Spatial-Temporal Uncertainty)來描述機率式路由中，封包之間的衝突關係，用來避免封包碰撞(collision)的發生以有效率地減少能源浪費。我們透過ERG設計出一個高效能的機率式路由式媒體存取控制協定(OR-based MAC protocol)。不但保留了原本機率式路由的優點，我們也針對其缺點進行改進，以有效率的能量消耗達成更高的吞吐量。 從實驗結果中可以發現，相較於現有的媒體存取控制協定，我們提出的協定在吞吐量(throughput)、網路生命期(network lifetime)、封包期限(packet deadlines)前收到的有效吞吐量(goodput)都能有較佳的表現。

Because data is transmitted by acoustic signals in Underwater Sensor Networks (UWSNs), the characteristics of underwater environments are different from those of a terrestrial sensor network. Specifically, due to the high propagation delay of acoustic signals and the unreliable wireless link in UWSNs, current terrestrial approaches don’t work well in UWSNs. The thesis is to study the opportunistic routing (OR) impacted by the characteristics of underwater environments, and to design a novel communication protocol for UWSNs. First, because OR works well in unreliable wireless link environments, we exploit the idea to our protocol design. Second, to reduce the impact of high propagation delay, we construct an Event Relationship Graph (ERG), which describes the conflict relationship between packets by considering the Spatial-Temporal Uncertainty, and it is able to avoid collisions to reduce energy consumption. Based on ERG, we design a throughput and energy efficiency OR-based MAC protocol for UWSNs. We not only keep the advantages of OR, but also improves its drawback. The simulation results demonstrate that, the proposed protocol is able to achieve higher system throughput, prolong network lifetime, and improve the end-to-end goodput under deadline constraints.