The Study on the Performance Improvement of IEEE 802.11 Distributed Coordination Function

在IEEE 802.11的效能上，Backoff程序扮演非常重要的角色，尤其是對吞吐量(Throughput)和碰撞機率(Collision)效能的影響，舉凡大部份的文獻，都以此二項為效能改善的指標，而本研究主要針對Backoff程序的改善，提出分析和NS2 (Network Simulator 2)模擬，以驗證分析和模擬是一致的。 目前在IEEE 802.11廣播效能的研究上，都是假設傳輸訊框成功所花費的時間等於傳輸訊框失敗的時間。並沒有考量EIFS (Extended InterFrame Space)時間的影響，以至於分析與模擬的結果發生不一致的現象。本篇考慮EIFS時間的影響，修正Hu所提出的模型，利用吞吐量(Throughput)和訊息可靠性(Message Reliability)為效能指標。模擬實驗顯示我們提出的方法與模擬的結果最為接近，代表我們的分析的方法較接近於實際的情況。 針對改善IEEE 802.11的論文不計其數，但有考量到EIFS影響的論文，就作者的記憶，還未有相關論文出現，本論文考量到EIFS (Extended InterFrame Space)的影響，透過理論分析，求解最大吞吐量(Maximum Throughput)的問題，提出一個更接近最佳解的分析和演算法，有別於其它論文是採用數值方法去逼近最佳解，本篇以二元三次方程式即可獲得更接近最佳解的演算法，另外透過對本篇演算法的特性，分析出CFP (Backoff Counter Consecutive Freeze Process)的影響是可以忽略，甚至可以不去考慮。 另外在IEEE 802.11的DCF (Distributed Coordination Function)模式下，CSMA/CA (Carrier Sense Multiple Access/Collision Avoidance)是使用二元指數後退演算法來避免封包碰撞的發生。至於二元指數後退演算法是使用均勻分布的機率模型，來選取可能的後退值，而本研究提出一個創新的二元指數後退演算法，即是在二元指數後退演算法中使用二項式分配取代均勻分布的機率模型，來選取可能的後退值，並且透過使用NS2模擬，驗證二項式分配的方法可以獲得可觀的效能改善。The backoff procedure plays a very important role in IEEE 802.11 WLANs. In particular, both throughput and collision are not only widely affected by the backoff procedure, but also act as two of the most important performance indicators for performance improvement in most of the literatures. In this thesis, we focus on the improvement of backoff procedure that uses proposed analytical model and NS2 (network simulator 2) simulations to confirm the accuracy of proposed theoretical analysis. The current researches on the performance of IEEE 802.11 broadcast have all assumed that the time taken for a successful frame transmission is the same as the time taken for an unsuccessful frame transmission. However, none of these studies have considered the effects of an extended interframe space (EIFS). Therefore, their analysis and simulation results are not consistent with each other. Based on this observation, we have taken the effects of EIFS into consideration and modified the model proposed by Hu. We use throughput and message reliability as the performance benchmark in our simulation to show that our proposed method yields results that are closest to the simulation results. From the definition of throughput, we have taken such observation further to perform theoretical analyses to provide a solution to the maximum throughput problem for the IEEE 802.11 distributed coordination function (DCF), and an algorithm using a binary cubic equation to obtain a much closer approximation of the optimal solution than previous algorithms. Moreover, by studying and analyzing the characteristics of the proposed algorithm, we found that the effects of backoff counter consecutive freeze process (CFP) could be neglected or even disregarded. From the simulation result, we not only showed that the proposed theoretical analysis complied with the simulated results, but also verified that the proposed approach outperformed others in achieving a much closer approximation to the optimal solution. Apart from EIFS, the carrier sense multiple access/collision avoidance (CSMA/CA) in IEEE 802.11 DCF uses binary exponential backoff (BEB) algorithm to select a random backoff number from a uniform probability distribution to avoid the problem of packet collision. In this thesis, we present a novel backoff algorithm that uses a binominal distribution rather than a uniform distribution to determine the backoff value. In our simulations, the results have shown that the proposed algorithm outperforms the original IEEE 802.11 DCF algorithm.中文摘要 I SUMMARY II Contents IV List of Tables VII List of Figures VIII 1. Introduction 1 1.1. Research motivation and objectives 1 1.2. Research scope 3 1.3. List of notation 4 1.4. Organization 7 2. Research background 8 2.1. The binary exponential backoff (BEB) algorithm 8 2.2. IEEE 802.11 ad hoc based broadcast 9 2.3. The p-persistent-based model 11 2.4. The maximum throughput problem for the IEEE 802.11 DCF 15 2.5. The non-p-persistent model 16 2.6. The mean contention window size for the IEEE DCF BEB algorithm 20 2.7. Simulation performance indicators 22 3. Performance analysis of IEEE 802.11 ad hoc based broadcast 25 3.1. Proposed method 25 3.2. Experiment 26 3.3. Summary 27 4. The dynamic contention window (DCW) algorithm 29 4.1. Introduction 29 4.2. Analysis of proposed method 30 4.2.1 Analysis of collision probability 30 4.2.2 Analysis of maximum throughput 33 4.3. The dynamic contention window (DCW) algorithm 37 4.3.1 Backoff counter consecutive freeze process (CFP) 39 4.4. Simulations 41 4.4.1 Environmental settings 41 4.4.2 Different data packet lengths 41 4.4.3 Comparison between different algorithms 44 4.5. Summary 50 5. The binominal backoff algorithm 52 5.1. Introduction 52 5.2. The binominal backoff algorithm 52 5.3. Simulations 56 5.3.1 Environmental settings 56 5.3.2 Experiment I: comparison between different distributions 57 5.3.3 Experiment II: changing number of nodes in simulation process with different distributions 59 5.3.4 Experiment III: in comparison with DCF 61 5.3.5 Experiment IV: comparison with DCF for a changing number of nodes in the simulation process 64 5.4. Summary 65 6. Conclusions and future works 66 References 67 Appendix A 71 Appendix B 73 Appendix C 7

The Study on the Performance Improvement of IEEE 802.11 Distributed Coordination Function

在IEEE 802.11的效能上，Backoff程序扮演非常重要的角色，尤其是對吞吐量(Throughput)和碰撞機率(Collision)效能的影響，舉凡大部份的文獻，都以此二項為效能改善的指標，而本研究主要針對Backoff程序的改善，提出分析和NS2 (Network Simulator 2)模擬，以驗證分析和模擬是一致的。 目前在IEEE 802.11廣播效能的研究上，都是假設傳輸訊框成功所花費的時間等於傳輸訊框失敗的時間。並沒有考量EIFS (Extended InterFrame Space)時間的影響，以至於分析與模擬的結果發生不一致的現象。本篇考慮EIFS時間的影響，修正Hu所提出的模型，利用吞吐量(Throughput)和訊息可靠性(Message Reliability)為效能指標。模擬實驗顯示我們提出的方法與模擬的結果最為接近，代表我們的分析的方法較接近於實際的情況。 針對改善IEEE 802.11的論文不計其數，但有考量到EIFS影響的論文，就作者的記憶，還未有相關論文出現，本論文考量到EIFS (Extended InterFrame Space)的影響，透過理論分析，求解最大吞吐量(Maximum Throughput)的問題，提出一個更接近最佳解的分析和演算法，有別於其它論文是採用數值方法去逼近最佳解，本篇以二元三次方程式即可獲得更接近最佳解的演算法，另外透過對本篇演算法的特性，分析出CFP (Backoff Counter Consecutive Freeze Process)的影響是可以忽略，甚至可以不去考慮。 另外在IEEE 802.11的DCF (Distributed Coordination Function)模式下，CSMA/CA (Carrier Sense Multiple Access/Collision Avoidance)是使用二元指數後退演算法來避免封包碰撞的發生。至於二元指數後退演算法是使用均勻分布的機率模型，來選取可能的後退值，而本研究提出一個創新的二元指數後退演算法，即是在二元指數後退演算法中使用二項式分配取代均勻分布的機率模型，來選取可能的後退值，並且透過使用NS2模擬，驗證二項式分配的方法可以獲得可觀的效能改善。The backoff procedure plays a very important role in IEEE 802.11 WLANs. In particular, both throughput and collision are not only widely affected by the backoff procedure, but also act as two of the most important performance indicators for performance improvement in most of the literatures. In this thesis, we focus on the improvement of backoff procedure that uses proposed analytical model and NS2 (network simulator 2) simulations to confirm the accuracy of proposed theoretical analysis. The current researches on the performance of IEEE 802.11 broadcast have all assumed that the time taken for a successful frame transmission is the same as the time taken for an unsuccessful frame transmission. However, none of these studies have considered the effects of an extended interframe space (EIFS). Therefore, their analysis and simulation results are not consistent with each other. Based on this observation, we have taken the effects of EIFS into consideration and modified the model proposed by Hu. We use throughput and message reliability as the performance benchmark in our simulation to show that our proposed method yields results that are closest to the simulation results. From the definition of throughput, we have taken such observation further to perform theoretical analyses to provide a solution to the maximum throughput problem for the IEEE 802.11 distributed coordination function (DCF), and an algorithm using a binary cubic equation to obtain a much closer approximation of the optimal solution than previous algorithms. Moreover, by studying and analyzing the characteristics of the proposed algorithm, we found that the effects of backoff counter consecutive freeze process (CFP) could be neglected or even disregarded. From the simulation result, we not only showed that the proposed theoretical analysis complied with the simulated results, but also verified that the proposed approach outperformed others in achieving a much closer approximation to the optimal solution. Apart from EIFS, the carrier sense multiple access/collision avoidance (CSMA/CA) in IEEE 802.11 DCF uses binary exponential backoff (BEB) algorithm to select a random backoff number from a uniform probability distribution to avoid the problem of packet collision. In this thesis, we present a novel backoff algorithm that uses a binominal distribution rather than a uniform distribution to determine the backoff value. In our simulations, the results have shown that the proposed algorithm outperforms the original IEEE 802.11 DCF algorithm.中文摘要 I SUMMARY II Contents IV List of Tables VII List of Figures VIII 1. Introduction 1 1.1. Research motivation and objectives 1 1.2. Research scope 3 1.3. List of notation 4 1.4. Organization 7 2. Research background 8 2.1. The binary exponential backoff (BEB) algorithm 8 2.2. IEEE 802.11 ad hoc based broadcast 9 2.3. The p-persistent-based model 11 2.4. The maximum throughput problem for the IEEE 802.11 DCF 15 2.5. The non-p-persistent model 16 2.6. The mean contention window size for the IEEE DCF BEB algorithm 20 2.7. Simulation performance indicators 22 3. Performance analysis of IEEE 802.11 ad hoc based broadcast 25 3.1. Proposed method 25 3.2. Experiment 26 3.3. Summary 27 4. The dynamic contention window (DCW) algorithm 29 4.1. Introduction 29 4.2. Analysis of proposed method 30 4.2.1 Analysis of collision probability 30 4.2.2 Analysis of maximum throughput 33 4.3. The dynamic contention window (DCW) algorithm 37 4.3.1 Backoff counter consecutive freeze process (CFP) 39 4.4. Simulations 41 4.4.1 Environmental settings 41 4.4.2 Different data packet lengths 41 4.4.3 Comparison between different algorithms 44 4.5. Summary 50 5. The binominal backoff algorithm 52 5.1. Introduction 52 5.2. The binominal backoff algorithm 52 5.3. Simulations 56 5.3.1 Environmental settings 56 5.3.2 Experiment I: comparison between different distributions 57 5.3.3 Experiment II: changing number of nodes in simulation process with different distributions 59 5.3.4 Experiment III: in comparison with DCF 61 5.3.5 Experiment IV: comparison with DCF for a changing number of nodes in the simulation process 64 5.4. Summary 65 6. Conclusions and future works 66 References 67 Appendix A 71 Appendix B 73 Appendix C 7