IEEE 802.11因其廣大的市場占有率，是目前無線區域網路的主流，在可預見的未來，無線網路相關規範的發展仍將以802.11佔居主導地位，但是802.11所使用的通訊傳輸協定是屬於共享式頻寬的存取方式，沒有提供差異式品質服務(QoS)的機制，因此如何有效利用無線網路中有限的頻寬，來達到差異式分類服務的目的，是現今熱門的研究主題之一。
無線網路可以區分為兩種不同的架構：具基礎架構(infrastructure)以及無基礎架構(Ad hoc)的網路系統。現今無線網路的佈署以具基礎架構的網路系統為主，所以本研究以此架構為主要的探討對象。在此無線網路架構下，為了要連結上網際網路，必須架設無線存取點(Access Point, AP)或是基地台(Base Station, BS)，所有無線端的網路傳輸都必須經由存取點，因此在此架構下，無線端的傳輸路徑只是整段傳輸路徑中的其中一段，為了能保證傳輸的服務品質，如何在無線環境中提供服務品質的保證，就變的非常重要。
為了在802.11中提供差異式的服務，有三個因子可以作為提供優先權的考慮因素，分別是(1) Interframe Space (IFS) (2) Backoff interval (3) Maximum frame length，藉由調整這些因子，可以達到提供QoS服務的目的。本研究利用(1)與(2)兩個因子，提出Wireless Differentiation (WD)的方法，藉由動態的改變IFS與Backoff interval，達到提供無線網路差異式服務的目的，並且利用軟體模擬的方式，進行新方法、原始802.11標準以及802.11e標準所提出方法的效能評估。從模擬結果可以得知新方法可以有效的提供分類服務，達到較好的服務品質。

The increasing popularity of wireless networks over the last years indicates that there will be a demand for communicating devices providing high capacity communication together with QoS requirements. There are two types of wireless networks, infrastructure and Ad Hoc networks. The variation of topology caused by the mobility of hosts in the Ad Hoc networks results in a long latency, large jitter and low throughput. In infrastructure wireless networks, a base station (BS) or an Access Point (AP) is in charge of the data transmission. Therefore, the wireless hop can be considered as another hop of the transmission path. With the rapid growth of wireless traffics, the future wireless network is expected to provide services for heterogeneous data traffics with different quality of service requirements. Most proposed schemes do not have mechanisms to adapt to environment changes. In real situation, bandwidths, error rates, and loss rates of wireless links vary frequently.
The QoS issues are very important in modern networks. There are many proposed service models and mechanisms to support QoS in wireline networks. Most of these QoS mechanisms are not suitable for direct application to the wireless network because of the characteristics of wireless communication which includes: 1) high error rates and bursty errors, 2) location-dependent and time-varying wireless channel capacity, 3) scarce bandwidth, 4) user mobility, and 5) power constraints of the mobile hosts. All of these above characteristics make the development of QoS in wireless networks very difficult and challenging.
We try to cope with the bandwidth variations caused by the high error rate and bursty errors in wireless links, and the location-dependent and time-varying natures of wireless channel capacity. Furthermore, we expect to utilize the scarce wireless bandwidth more efficiently. In our proposed scheme, the higher priority flow is capable of broadcasting a message to inform the lower priority flows to change their priorities to adapt to environment variations. We will base on the differentiated service model and propose a Wireless Differentiation (WD) scheme for UDP flows and a Wireless Differentiation with Prioritized ACK (WDPA) scheme for connections with TCP flows which provide QoS support for IEEE 802.11b and do not change the basic access mechanism of IEEE 802.11b.

List of Figures iii
List of Tables vii
Chapter 1 Introduction 1
1.1 Research Motivation 1
1.2 Recent Related Research 3
1.3 Organization of the Dissertation 5
Chapter 2 IEEE 802.11 MAC Overview 6
2.1 Distributed Coordination Function 9
2.2 Point Coordination Function 11
Chapter 3 Quality of Service in IEEE 802.11 16
3.1 Centralized Mechanisms 17
3.1.1 PCF-Based Mechanisms 18
3.1.2 Wireless Rether 20
3.2 Distributed Mechanisms 22
3.3 IEEE 802.11e 28
3.3.1 Enhanced Distributed Channel Access (EDCA) 29
3.3.2 HCF Controlled Channel Access (HCCA) 32
Chapter 4 Wireless Differentiation (WD) Scheme and Wireless Differentiation Scheme with Prioritized ACK (WDPA) 35
4.1 Priority Table 36
4.2 Packet Receiving Process 37
4.3 Packet Transmission Process 38
4.4 Estimation of Currently Obtained Bandwidth 40
4.5 Priority Adaptation 41
Chapter 5 Simulation 53
5.1 UDP Traffics 55
5.2 TCP Traffics 64
5.3 One UDP and two TCP traffics 68
5.4 Impact of number of flows 72
Chapter 6 The impact of initial contention window size 79
6.1 Observation on the transmission process in IEEE 802.11 79
6.2 The Adaptive Initial Contention Window scheme 82
6.3 Analysis of the impact of selection of initial contention window size 86
Chapter 7 Conclusion and Future Works 96
Reference 99

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