臺灣博碩士論文加值系統

為了提供即時性訊務的服務品質保證，IEEE 802.11標準制定團隊引入『混合協調功能』(Hybrid Coordination Function)的通道存取機制，其中又分成兩種模式，以競爭來獲得通道存取權利的方法稱為Enhanced Distributed Channel Access (EDCA)；另一種則為非競爭的通道存取模式稱為 Hybrid Controlled Channel Access (HCCA)。然而，在HCCA排程器中，允入控制與分配傳送機會(TXOP) 的參考設計只適用於傳送固定位元速率(CBR)的訊務，對於傳送變動位元速率(VBR)的訊務來說，則可能會發生嚴重的封包遺失(Packet Loss)。

在這一篇論文中，我們提出了一個簡單的允入控制演算法，其基本設計的思維在於利用高斯分佈(Gaussian Distribution)來近似變動位元速率的訊務。電腦模擬的結果指出我們提出的允入控制演算法，可以來確保擁有服務品質保證的站台(QoS-Enhanced Station)在傳送訊務的過程中，封包遺失的機率在事前保證的範圍以內。再者，當有站台一次要求傳送多個變動位元速率之訊務流時，我們提出的方法可以因為獲得多工增益(Multiplexing Gain)而更有效率的分配傳送機會(TXOP)。

For Quality of Service (QoS) requirements of real-time traffic, IEEE 802.11 working group introduces a QoS-aware channel access mechanism, called Hybrid Coordination Function (HCF), which consists of contention-based Enhanced Distributed Channel Access (EDCA) and contention-free HCF Controlled Channel Access (HCCA). The TXOP allocation and the admission control units of the HCCA reference scheduler are only appropriate for constant bit rate (CBR) flows. It may result in serious packet loss for variable bit rate (VBR) flows.

In this thesis, we propose a simple admission control algorithm which adopts Gaussian distribution to approximate VBR traffic. Numerical results obtained from computer simulations show that our proposed algorithm can effectively and efficiently allocate Transmission Opportunity (TXOP) durations to QoS-enhanced stations (QSTAs) to guarantee a predefined packet loss probability. Moreover, our proposed scheme can easily handle multiple VBR flows of the same QSTA to get the advantage of multiplexing gain.

中文摘要 i

English Abstract ii

誌謝 iii

Contents iv

List of Tables vi

List of Figures vii

Acronym viii

Chapter 1 Introduction 1

Chapter 2 Background 4

2.1 Overview of IEEE 802.11 Protocol Architecture………………….. 4

2.2 Distribution Coordination Function.………………………………. 6

2.3 Point Coordination Function………………………………………. 10

2.4 Hybrid Coordination Function...………………………………....... 11

2.5 Enhanced Distributed Channel Access…………………………...... 12

2.6 HCF Controlled Channel Access………………………….............. 14

2.7 The Reference HCCA Scheduler…………………………............... 16



Chapter 3 Related Works 18

3.1VBR traffic with Constant Packet Size.............................................. 19

3.2 VBR traffic with Variable Packet Size.............................................. 20

Chapter 4 Gaussian Approximation Based Admission Control Algorithm 22

4.1 Motivation…………………………................................................. 22

4.2 Gaussian Approximated VBR traffic................................................ 24

4.3 VBR traffic with Constant Packet Size............................................. 25

4.4 VBR traffic with Variable Packet Size.............................................. 26

4.5 Gaussian Approximation Based Admission Control Algorithm…... 26

4.6 Aggregated TXOP Duration……………………………………….. 28

Chapter 5 Analysis of Packet Loss Probability 29

5.1 Analysis of Packet Loss Probability……………………………….. 29

5.2 Approximation of Packet Loss Probability………………………... 30

Chapter 6 Simulation Results 33

Chapter 7 Conclusions 39

Bibliography 40

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[2] IEEE 802.11 WG: IEEE 802.11e/D8.0, Wireless MAC and Physical Layer Specifications: MAC QoS Enhancements.

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[8] P. Ansel, Q. Ni, and T. Turletti, "FHCF: A Fair Scheduling Scheme for IEEE 802.11e WLAN", INRIA Research Report No 4883, July 2003.

[9] S. Mangold, S. Choi, G. R. Hiertz, et al, “Analysis of IEEE 802.11e for QoS Support in Wireless LANs”, IEEE Wireless Communications, Volume 10, Issue 6, Dec, 2003.

[10] William Stallings,” Data and Computer Communications,” Prentice Hall 2004.

[11] Arthur W. Berger, Ward Whitt, “Extending the Effective Bandwidth Concept to Networks with Priority Classes”, IEEE communication August 1998.

[12] P.O.Borjession and C.E.W. Sundberg, ”Simple Approximation of the Error Function Q(x) for Communication Applications,” IEEE transaction on Communications, March,1979