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研究生:謝明熹
研究生(外文):Ming-Hsi Hsieh
論文名稱:802.11無線網路之適應性公平增強分散式協調機制
論文名稱(外文):Adaptive Fair Enhanced Distributed Coordination Function in 802.11 Wireless LAN
指導教授:廖俊睿
指導教授(外文):Jan-Ray Liao
學位類別:碩士
校院名稱:國立中興大學
系所名稱:電機工程學系
學門:工程學門
學類:電資工程學類
論文種類:學術論文
論文出版年:2004
畢業學年度:92
語文別:中文
論文頁數:91
中文關鍵詞:802.11無線區域網路媒介存取控制分散式協調機制服務品質保證
外文關鍵詞:Wireless LANmedium access controldistributed coordination functionQuality of Service
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隨著即時性多媒體應用在無網路上的廣泛使用,使得服務品質的支援是個關鍵性問題。在802.11無線區域網路中,分散式協調機制 (Distributed Coordination Function,DCF) 是主要的媒體接取控制機制。DCF採用避免碰撞之載波偵測多重存取(Carrier-Sense Multiple Access/Collision Avoidance,CSMA/CA)的方式進行資料的傳輸,在一般的情況下,雖然可以讓大部份的訊框避免不必要的碰撞,不過當DCF在高負載時,將明顯的造成資料流量降低與較高的延遲。原因在於浪費了太多的頻寛用於決定通道的存取。此外,DCF只適合非即時性的服務,並不能提供任何服務品質。因此802.11工作小組制訂了一個可以支援服務品質的標準,名為802.11e EDCF。
在此論文中,我們可以發現EDCF容易因為工作站變多而嚴重的降低其效能,因此我們使用名為適應性公平EDCF的方法改良EDCF。這個方法主要是利用適應性快速後退機制來改善整體的資料流量。方法為考量通道當時環境並針對不同等級的優先權,分別計算進入快速後退的臨界點,再搭配計算平均碰撞率的方式去調整快速後退的程度。並針對為了使得相同優先權的不同資料流具有較高程度的公平性,特別採用了當通道媒介繁忙時就視為發生碰撞似的增加競爭視窗值。
最後我們使用NS2針對不同的網路情形,實際的模擬分析所提的適應性公平EDCF、EDCF與其它的增強機制的效能差異結果。

With the emerging use of real-time multimedia applications over wireless networks, support of Quality of Service (QoS) becomes a key problem. The distributed coordination function (DCF) scheme is the primary medium access control (MAC) scheme for IEEE 802.11 wireless LANs. The DCF mechanism employs the Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) strategy to provide multiplexed data transmission. If the network load, the majority of frames transmitted via CSMA/CA strategy will not cause collision. However, DCF suffers from significant throughput degradation and high delay under heavy load. The reason is that bandwidth is wasted in the increased time used for negotiating channel access. Moreover, DCF mechanism is only appropriate for non-real time data service and can not provide any Quality of Service (QoS). Thus, the IEEE 802.11 working group is currently working on the support of QoS in a new standard, called IEEE 802.11e EDCF.
In this thesis, we found that EDCF performed poorly when the medium was heavily loaded. So we used a mechanism named Adaptive Fair EDCF that extended EDCF. This new method used an adaptive fast backoff mechanism to improve the total throughput. Our scheme computed an adaptive backoff threshold for each priority level by taking into account the channel load and used the average collision rate to adjust fast backoff. Besides, it provided a higher degree of fairness among different flows of the same priority. By increasing the contention window during deferring periods when channel was busy.
Finally, we used Network Simulator Version 2 (NS2) to simulate adaptive fair EDCF, EDCF and other enhanced schemes. We experimented it with different ad-hoc network topologies to evaluate the performance of multimedia flows with different channel loads.

摘要......................................................i
ABSTRACT..................................................iii
致謝......................................................v
目錄......................................................vi
圖目錄....................................................x
表目錄....................................................xiii
第一章 緒論...............................................1
1.1 簡介.................................................1
1.2 動機與目的...........................................2
1.3論文架構..............................................4
第二章 802.11的架構與分散式協調機制介紹...................5
2.1 無線區域網路基本架構.................................5
2.2 無線網路環境與有線網路環境的差異性...................8
2.2.1無線網路中隱藏節點的問題.........................8
2.2.2 碰撞偵測........................................10
2.3 802.11 MAC協定.......................................13
2.4 分散式協調機制.......................................15
2.4.1 訊框間隔........................................16
2.4.2 後退機制(backoff scheme)........................17
2.4.3 DCF訊框傳送範例.................................19
2.5 總結.................................................24
第三章 無線區域網路的優先權機制介紹.......................25
3.1 驗證機制的環境介紹...................................25
3.2 Differentiation Mechanisms...........................26
3.3 改善TCP在Differentiation Mechanisms中優先權的方法....37
3.4 總結.................................................39
第四章 802.11e與AEDCF介紹.................................40
4.1 802.11e的服務品質保證機制............................40
4.1.1 增強分散式協調機制(EDCF)........................41
4.1.2 HCF.............................................43
4.2 AEDCF................................................45
4.2.1 AEDCF演算法架構介紹.............................45
4.2.2 平均碰撞率的計算................................46
4.2.3 傳送資料成功後如何適應性地重設競爭視窗值........47
4.2.4 佇列傳送資料失敗後如何設定其競爭視窗值..........48
4.3 總結.................................................49
第五章 適應性公平EDCF介紹.................................50
5.1 分析討論.............................................50
5.2 適應性快速後退機制...................................52
5.2.1 適應性快速後退機制介紹..........................52
5.2.2 適應性快速後退機制的臨界點決定..................53
5.2.3 適應性公平EDCF機制演算法........................56
5.3 各媒介存取機制比較..................................60
5.4 總結.................................................61
第六章 模擬結果與分析.....................................62
6.1 網路模擬軟體NS2與平台介紹............................62
6.2 模擬環境假設.........................................64
6.3 利用模擬決定出演算法參數.............................66
6.3.1 更新週期........................................66
6.3.2 平滑因子........................................69
6.3.3 AEDCF之DF上限值的決定...........................70
6.4 各機制的效能評估.....................................72
6.4.1 平均延遲比較....................................74
6.4.2 碰撞率與媒介使用率比較..........................76
6.4.3 平均傳輸頻寬比較................................78
6.4.4 各資料流的傳輸頻寬比較.........................79
6.4.5 jitter比較.....................................81
6.4.6 公平性比較......................................83
6.5 總結.................................................86
第七章 結論與未來展望.....................................87
參考文獻..................................................89

[1] Imad Aad and Claude Castelluccia, “Priorities in WLANs”, Computer Networks, 2003.
[2] Jong-Deok Kim and Chong-Kwon Kim, “Performance analysis and evaluation of IEEE 802.11e EDCF”, WIRELESS COMMUNICATIONS AND MOBILE COMPUTING, 2004
[3] Lamia Romdhani, Qiang Ni, and Thierry Turletti, “AEDCF: Enhanced Service Differentiation for IEEE 802.11 Wireless Ad-Hoc Networks”, Wireless Communications and Networking, 2003
[4] “The ns Manual”, http://www.isi.edu/nsnam/ns/doc/ns_doc.pdf.
[5] Qiang Ni, Imad Aad, Chadi Barakat, and Thierry Turletti, “Modeling and Analysis of Slow CW Decrease for IEEE 802.11 WLAN”, IEEE 2003 International Symposium on Personal, Indoor and Mobile Radio Communication Proceedings
[6] Federico Cali, Marco Conti, and Enrico Gregori, “IEEE 802.11 Protocol: Design and Performance Evaluation of an Adaptive Backoff Mechanism”, IEEE JOURNALL ON SELECTED AREAS IN COMMUNICATIONS, VOL. 18, NO. 9, SEPTEMBER 2000.
[7] Younggoo Kwon, Yuguang Fang and Haniph Latchman, “A Novel MAC Protocol with Fast Collision Resolution for Wireless LANs”, IEEE INFOCOM, 2003
[8] Giuseppe Bianchi, “Performance Analysis of the IEEE 802.11 Distributed Coordination Function”, IEEE JOURNAL ON SELECTED AREAS IN COMMUNICATIONS, VOL. 18, NO. 3, MARCH 2000.
[9] Kanghee Kim, Aftab Ahmad, and Kiseon Kim, “A Wireless Multimedia LAN Architecture Using DCF With Shortened Contention Window for QoS Provisioning”, IEEE COMMUNICATIONS LETTERS, VOL. 7,NO. 2, FEBRUARY 2003.
[10] Mohammad Malli, Qiang Ni, Thierry Turletti, Chadi Barakat, “Adaptive Fair Channel Allocation for QoS Enhancement in IEEE 802.11 Wireless LANs”
[11] Sanjay Shakkottai, Theodore S. Rappaport, and Peter Karlesson,“Cross-layer Design for Wireless Networks”, IEEE Communications Magazine, October 2003.
[12] 黃能富, “區域網路與高速網路”, 維科出版社, 1996.
[13] Mattbew S. Gast, “802.11無線網路技術通論”, O’REILLY, 2003.
[14] Larry L. Peterson and Bruce S. Davie, “Computer Networks”, Morgan Kaufmann Publishers, 3nd Ed.
[15] “Network Simlator (ns), version 2”, http://www.isi.edu/nsnam/ns/.
[16] William Stallings, “HIGH — SPEED NETWORS AND INTERNETS-Performance and Quality of Service”, Prentice Hall International Editions, 2nd Ed.
[17] IEEE 802.11 WG, “Reference number ISO/IEC 8802-11: 1999(E) IEEE Std 802.11, 1999 edition. International Standard [for] Information Technology-Telecommunications and information exchange between systems-Local and metropolitan area networks-Specific Requirements — Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications,” 1999.
[18] IEEE 802.11 WG, “Draft Supplement to STANDARD FOR Telecommunications and Information Exchange Between Systems — LAN/MAN Specific Requirements — Part 11: Wireless Medium Access Control (MAC) and Physical layer (PHY) specifications: Medium Access Control (MAC) Enhancements for Quality of Service (QoS)”, IEEE 802.11e/D3.2, Aug. 2002.

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