隨著IEEE 802.11無線區域網路的發展，許多公眾場所已經將之列為基礎建設的一部份。這類型的無線區域網路提供給使用者一種方便的網路連線方式。雖然無線區域網路允許移動節點在不同的IP網路之間進行漫遊，但是，在切換網路時所產生的換手延遲（Handoff Latency）卻成了最大的阻礙。更進一步地，在移動節點更換所屬的無線基地台時，也將會有鍊結層的換手發生。即使在不討論Mobility Protocol所造成的影響下，這一個鍊結層的換手所造成的延遲也已經使得許多有即時性需求的應用無法正確運作。
在這一份論文中，我們提出了三種針對不同網路階層的解決方案。這些方法不僅能夠順利解決換手時所造成的延遲，並且也能在不違反IEEE 802.11標準以及與現存的網路裝置相容的情況下來達成目標。L2-Optimize與AIL的搭配，能夠將鍊結層的換手延遲縮減到最小；而LASP則是能夠讓在不同的IP網路間切換時所發生的換手延遲降低到一個可以被接受的程度。如此一來，即使是有即時性需求的應用，都能夠在這一類型的網路中不受到漫遊的影響，順利操作。

A growing number of IEEE 802.11-based wireless LANs have been set up in many public places in the recent years. These wireless LANs provide convenient network connectivity to users. Although mobile nodes allowed roaming across wireless LANs, handoff latency becomes an obstacle when mobile nodes migrate between different IP networks. Advanced, the link-layer handoff process disrupts the association when a mobile node moves from one access point to another. Even without discussing the latency of Mobility Protocols, this link-layer handoff latency already made many real time applications can not meet their requirements.
In this dissertation, it is proposed three schemes to solve the problems occurred in the different network layers. These schemes not only reduce the latency of whole handoff procedure but also have no violation to the existing specifications in the IEEE 802.11 standard and compatible with existing devices. L2-Optimize and AIL used to minimize the duration of link-layer handoff. With LASP, Mobility handoff can be reduced to an acceptable situation. Therefore, even real time applications can meet their requirements when users are roaming across wireless LANs.

Content
摘要 iv
Abstract vi
Content ix
List of Figures xii
List of Tables xiv
Chapter 1 Introduction 1
1.1 Motivation and Objectives 1
1.2 Summary of the Dissertation 5
1.3 Organization of the Dissertation 8
Chapter 2 Research Background 9
2.1 Mobility Protocol 9
2.2 Link-Layer Handoff 14
2.2.1 Experiment 14
2.2.2 Analysis 17
Chapter 3 Link-Layer Optimization Scheme 22
3.1 Link-Layer Optimization Scheme 22
3.1.1 Preliminary 22
3.1.2 L2-Optimize 23
3.1.3 Optimized Results 29
3.2 Simulation and Analysis 30
3.2.1 Latency of Probe Response 30
3.2.2 Duration of probe phase 31
3.2.3 Power consumption 33
3.3 Discussion 35
3.3.1 Compatibility 35
3.3.2 Reduction of auth phase 36
Chapter 4 Actively Intelligent Link-Layer Handoff 37
4.1 Actively Intelligent Link-Layer Handoff 37
4.1.1 AIL 37
4.1.2 Overhead of AIL 40
4.2 Simulation and Analysis 43
4.2.1 Signal Quality Experiment 43
4.2.2 UDP Transmission Experiment 46
4.2.3 TCP Transmission Experiment 48
4.3 Discussion 51
4.3.1 Compatibility and Flexibility 51
4.3.2 Advantages and Disadvantages 52
Chapter 5 Link Layer Assisted Seamless Mobility Handoff Protocol 54
5.1 LASP Descriptions 54
5.1.1 Movement detection 55
5.1.2 Fast data forwarding 56
5.1.3 Registrations acceleration 60
5.2 LASP Illustrations 63
5.2.1 Micromobility 64
5.2.2 Macro/Global mobility 65
5.2.3 Macro/Global mobility without L2-Module 67
5.3 Performance Analysis 69
5.3.1 Link-layer Handoff Experiment 70
5.3.2 Micromobility Experiment 71
5.3.3 Macro/Global mobility Experiment 72
5.3.4 TCP Transmission Experiment 75
5.4 Discussion 77
5.4.1 TCP retransmission 77
5.4.2 Message delay 78
Chapter 6 Conclusion and Future Work 80
6.1 Conclusion 80
6.2 Future Work 83
References 84
Appendix 89
Abbreviations 89
Author Biography 91
Publication List 92
Journal Papers 92
Patent....... 94
Conference Papers 95

List of Figures
Figure 1: The overview of handoff relating objects. 3
Figure 2: Experiment network. 15
Figure 3: The link-layer handoff procedure. 17
Figure 4: The designed field appended to beacon. 23
Figure 5: Link-layer handoff with the proposed optimization scheme. 25
Figure 6: Latency of probe response. 31
Figure 7: Duration of probe phase. 32
Figure 8: Number of probe requests be sent. 34
Figure 9: Algorithm for AIL. 40
Figure 10: The simulation environment. 43
Figure 11: Signal quality experiment. 44
Figure 12: UDP transmission experiment. 47
Figure 13: TCP transmission experiment. 49
Figure 14: RUM format. 58
Figure 15: Algorithm for L2-Module. 59
Figure 16: A general network environment. 63
Figure 17: Link-layer handoff latencies. 70
Figure 18: Micromobility experiment. 71
Figure 19: Macro/Global mobility experiment. 73
Figure 20: TCP packet transmission experiment. 75

List of Tables
Table 1: Comparison of different Mobility Protocols. 13
Table 2: The duration of link-layer handoff for selected cards. 16
Table 3: The operations of BA. 62
Table 4: The equipments of the experimental devices. 69
Table 5: Transmission performance comparison. 76

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