臺灣博碩士論文加值系統

隨著IEEE 802.11無線網路應用的增加，使用者對於無線網路的特色與功能上有更多的需求。到目前為止，IEEE 802.11 Task group E為了改善無線網路在QoS的能力而提出了IEEE 802.11e，這個新的協定將會在MAC層提供QoS，而且允許無線端更有效率地傳送語音及影像資料。其中，一個新存取機制被提出來擴充原本IEEE 802.11協定中的DCF機制，此機制稱之為Enhanced DCF (EDCF)，它引進了4種存取類別 (access category, AC)用來實現具有優先權的QoS。每一個AC都有自己的傳送佇列與參數設定，而在不同AC之間的優先權差別是利用設定不同的參數值來達成，換句話說，個別的AC為了存取無線媒介而相互競爭並且藉由不同的AIFS、TXOPLimit與CW來產生差異化。

在本論文中，我們在NS-2上所進行的模擬採用了不同的AC參數及資料類型。也就是說，我們會比較不同參數間在數個組合之下的效能。以CW來看，CW值愈大愈能減少碰撞發生的機率，不過，如此一來便會造成平均等待的時間拉長；在AIFS方面，就我們的模擬結果來看，並非將AC間的區隔加大才能增進高優先權AC的效能；就TXOPLimit來說，在模擬中我們發現TXOPLimit並不是愈大愈好，因為TXOPLimit的時間如果增加，反而會佔用無線媒介太長的時間。

With applications over IEEE 802.11 WLANs increasing, customers need more and more new features and functions. Up to now, Task group E of the IEEE 802.11 working group have proposed an extension to the IEEE 802.11 standard called IEEE 802.11e to improve the QoS capabilities provided on WLAN links. This new standard will provide QoS enhancements at the MAC layer that allow WLAN systems to efficiently stream audio and video data. A new access mechanism called Enhanced DCF (EDCF), is also proposed to extend the basic DCF mechanism used in the original standard 802.11 Prioritized QoS is realized through the introduction of four access categories (AC). Each AC has its own transmit queue and its own set of AC parameters. The differentiation in priority between AC is realized by setting different values for the AC parameters. In other words, individual AC contend for access to the medium and are differentiated by different arbitration inter-frames spaces (AIFS) values, transmission opportunity (TXOP) limits and contention windows (CWmin and CWmax).

Based on NS-2 platform, simulation study was conducted by using various AC parameters and traffic patterns. We will compare the performance between various combinations among different parameters. As for CW, the larger CW values always reduce the probability of collision, but it may extend average waiting time. In terms of AIFS, as we observe from result of our simulation, larger period between each AC may not always improve performance of high-priority station. Regarding TXOPLimit, we observe the fact that it is not always the best way to have larger TXOPLimit because larger one would occupy longer time over wireless medium.

List of Figures viii
List of Tables IX
Chapter 1 Introduction 1
1.1 Background 1
1.2 Motivation and Objective 2

Chapter 2 Related Work 4
2.1 Legacy IEEE 802.11 4
2.1.1 IEEE 802.11 MAC Schemes 4
2.1.2 Distributed Coordination Function 5
2.1.3 RTS/CTS 8
2.1.4 Fragmentation 9
2.1.5 Point Coordination Function 10
2.1.6 Unsolved Problems of PCF 11
2.2 Mechanisms of IEEE 802.11e 12
2.2.1 IEEE 802.11e MAC for QoS Support 12
2.2.2 EDCF 12
2.3 Performance Analysis of EDCF 15
2.3.1 Network Architecture 15
2.3.2 Simulation Results and Analysis 16

Chapter 3 Simulation Environments 20
3.1 Model Description 20
3.1.1 Network Simulator 20
3.1.2 Scenarios 20
3.2 Simulations for the influence of parameters on network performance 21
3.2.1 Contention Windows (CW) and Arbitration Inter Frame Space (AIFS) 22
3.2.2 Persistence Factor (PF) 23
3.2.3 Transmission Opportunity (TXOP) 23
3.2.4 RTS Threshold 23
3.3 Performance Metrics 24
3.3.1 Throughput Analysis 24
3.3.2 Delay Analysis 24

Chapter 4 Simulation Result and Analysis 26
4.1 Performance of the Combinations among Contention Window 26
4.2 Performance of the Combinations among AIFS 31
4.3 Performance of the Combinations among TXOPLimit 36
Chapter 5 Conclusion and Future Work 42
REFERENCES 43

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