IEEE 802.15.4是一個主要用於低功耗、低速率之無線感測網路的標準。此外，它用於設計低功率無線個人區域網路，例如無線監測和控制燈光、安全報警器、動作感應器、恆溫器和煙霧探測器。IEEE802.15.4為了最佳化並滿足這類應用的要求所以規定了實體層和媒介存取控制層。使用此標準的裝置，在兩個時期間傳輸數據：競爭存取週期（CAP）其使用碰撞避免式載波多重存取（CSMA/ CA）來存取通道，以及非競爭存取週期（CFP），其包含保證時槽（GTS）由網路協調者分配給各個裝置。

在競爭存取週期（CAP）中，CSMA/CA用於增進IEEE802.15.4的效能，其方法為不容許當其中一個節點在進行無線傳輸時，另一個節點也同時傳輸，從而減少了因為使用隨機後退時間而發生碰撞的機率。然而，在它內部的後退過程中仍然有問題存在。由於裝置在進行資料傳輸前都會執行盲目的後退程序，因此可能有些空閒的通道時間都浪費掉了。基於此問題，我們提出了早期偵測與早期傳輸演算法(ESET)來解決IEEE 802.15.4CSMA/CA的後退演算法的問題。

ESET演算法將會基於由上一次標準所選擇之隨機後退延遲值而多計算一個隨機後退延遲時間，並記住它們之間的區別。每次當裝置要發送資料時，其必須執行此過程。因此，CSMA /CA將倒數由ESET演算法計算之後退延遲值以及感測通道。如果通道是空閒的，它將會傳送一個頻道空閒評估（CCA）。但是，如果通道為忙碌，那麼它將繼續拖延從標準與ESET之間差異的後退值。之後，該裝置執行了連續兩個跟標準一樣的頻道空閒評估（CCA）。這其中的構想是：我們如何在完成後退延遲前檢查出是否有未使用的空閒通道時間以及做第一次頻道空閒評估（CCA）。

在這篇論文中，將綜合地分析ESET演算法，完成在IEEE 802.15.4中三個重要的衡量標準，如有效流量，能源效率（在模擬時間中總能源消耗與已完成傳輸封包總數之間的比例）和平均封包延遲將會被發表。藉由使用castaliaV3.2，我們證明了ESET演算法可以勝過於上述的標準指標。

此外，我們提出的方法，最重要的是與標準向後兼容。並且沒有對標準做很多變化。因此，它是在事後研究標準時很好的指南。

IEEE 802.15.4 is a leading standard for low power and low data rate wireless sensor networks. Additionally, it is designed for low rate-low power wireless personal area networks such as wireless monitoring and control of lights, security alarms, motion sensors, thermostats and smoke detectors. IEEE 802.15.4 specifies physical and media access control layers that have been optimized to satisfy the requirement of such applications. The device using this standard, transmit its data during two periods: Contention Access Period (CAP) by accessing the channel using carrier sensing multiple access with collision avoidance (CSMA/CA) and Contention Free Period (CFP), which consists of Guaranteed Time Slots (GTS) allocated to individual devices by the network coordinator.

In CAP, CSMA/CA is used to improve IEEE 802.15.4 performance by not allowing wireless transmission of a node if another node is transmitting, thus reducing the probability of collision due to the use of a random backoff time. However, it still has a problem inside its backoff procedure. Since blind backoff procedure is performed before the device transmits its data, possibly some portions of idle channel time are wasted. Knowing about this issue, we propose Early Sensing and Early Transmit algorithm to solve the problem of IEEE 802.15.4 CSMA/CA backoff algorithm.

ESET algorithm will compute one more random backoff delay based on the value of random backoff delay which has been chosen by the standard and memorize the difference between them. Every time the device has data to be transmitted, it must do this procedure. Thus, the CSMA/CA will countdown the backoff delay value computed by ESET algorithm and sense the channel. If the channel is idle, then it will transmit with just one clear channel assessment (CCA). But, if the channel is busy, then it continues to delay for the remaining difference of backoff value chosen by the standard and by the ESET. Next, the device does two consecutive clear channel assessments like in the standard manner. The idea behind this is how we can check if there are idle channel time left unused before finishing the backoff delay and do 1st CCA in the standard.

In this thesis, a comprehensive analysis of ESET algorithm implementation on three important metrics of IEEE 802.15.4 LR-WPANs such as goodput, energy efficiency (the ratio between total energy spent over the total number of delivered packet during the simulation time) and average packet delay will be delivered. By using castalia v3.2, we prove that ESET algorithm can outperform the standard in term of the mentioned metrics.

Furthermore, the most important thing for our proposed method is not many changes that have been done from the standard. So, it can be good companion for the standard itself to be used in the next research.

Abstracts (Chinese)…………………………………………………………………iv
Abstracts (English)…………………………………………………………………vi
Acknowledgements…………………………………………………………………vii
Table of Contents……………………………………………………………………ix
List of Figures……………………………………………………………………xi
List of Tables………………………………………………………………………xiii
Chapter 1. Introduction………………………………………………………………1
1.1 General………………………………………………………………………1
1.2 Motivation……………………………………………………………………2
1.3 Objective and Scope…………………………………………………………3
1.4 Thesis Outline……………………………………………………………4
Chapter 2. Theoretical Background and Related Works……………………………5
2.1 Overview of IEEE 802.15.4 MAC Layer…………………………………5
2.1.1 Device Architecture and Network Topologies………………………6
2.1.2 Superframe Structure……………………………………………………8
2.1.3 Data Transfer Models……………………………………………………10
2.1.4 Data Frame Format………………………………………………………13
2.1.5 Interframe spacing (IFS) ………………………………………………13
2.1.6 CSMA-CA Mechanism………………………………………………14
2.2 Related Works………………………………………………………………18
Chapter 3. The Early Sensing and Early Transmit (ESET) Algorithm………………20
3.1 Description of ESET Algorithm………………20
3.1.1 The calculation of ESET Range………………21
3.1.2 The procedure of ESET Algorithm………………22
3.2 Examples of using ESET Algorithm………………24
3.3 The bottleneck in ACK modes………………26
3.4 Performance Metrics…………………………………………………………27

Chapter 4. Performance Evaluation…………………………………………………28
4.1 Simulation Model……………………………………………………………28
4.2 Simulation Results……………………………………………………………30
4.2.1 Unsaturated Traffic, SO≠BO………………30
4.2.2 Unsaturated Traffic, SO=BO………………37
4.2.3 Saturated Traffic, SO=BO………………43
Chapter 5. Conclusion and Future Works……………………………………………46
References……………………………………………………………………………47

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