ABSTRACT
With recent advances in wireless technology, users of wireless network now expect quality of service and performance comparable to what is available in fixed networks. The IEEE 802.11e Medium Access Control (MAC) is an emerging supplement to the IEEE 802.11 Wireless Local Area Network (WLAN) standard that supports Quality-of-Service (QoS) requirements of both data and real-time applications. The IEEE802.11e MAC is based on both centrally-controlled and contention-based channel accesses. One of the most important functions in this MAC is the contention-based channel access mechanism called enhanced distributed coordination function (EDCF), which provides a priority scheme by differentiating the arbitration inter-frame spacing (AIFS), transmission opportunity (TXOP), and contention window parameters (CWmin and CWmax), for each access category. In this research work the EDCA priority scheme was evaluated to ascertain the effect of differentiating frames with different priorities on QoS using a MatLab computer simulation model. This model was used to compute the optimal performance, maximum sustainable throughput, loss rate and service delay distribution for each priority class under saturation load. Insight obtained from the analysis shows that the acquisition of the radio channel by the higher priority traffic is much more aggressive than for the lower priority traffic, causing the packets in the lower priority queue to be starved. Considering the performance of different access parameters, arbitration inter-frame space number (AIFSN) has more influence on the QoS performance of IEEE 802.11e EDCA Protocol, than the CW size. It was also observed that small CW values generate higher packet drops and collision rate probability. As a consequence, the EDCA mechanism suffers significantly. It is recommended that small values of AIFS should be cautiously used in order not to starve the lower priority traffics while CW size has to be tuned dynamically in response to varying load. Finally, larger CW size is advised to reduce the chances of collision.
TABLE OF CONTENTS
CHAPTER ONE: INTRODUCTION- - - - - - - 1
1.0 Background - - - - - - - - - - 1
1.1 Purpose of Study- - - - - - - - - 3
1.2 Scope - - - - - - - - - - 3
1.3 Methodology - - - - - - - - - 4
1.4 Thesis outline - - - - - - - - - 4
CHAPTER TWO: LITERATURE REVIEW- - - - - - 5
2.0 Introduction - - - - - - - - - - 5
2.1 Evolution of WLAN Standards - - - - - - 6
2.1.1 Original IEEE 802.11 - - - - - - - 6
2.1.2 IEEE 802.11a Standard - - - - - - 7
2.1.3 IEEE 802.11b Standard - - - - - - 8
2.1.4 IEEE 802.11g Standard - - - - - - 9
2.1.5 IEEE 802.11n Standard - - - - - - 10
2.1.6 IEEE 802.11e Standard - - - - - - 11
2.1.7 Other IEEE 802.11 Standards Suite - - - - - 11
2.2 WLAN IEEE 802.11 Physical Architecture - - - - - 14
2.2.1 Basic Service Set (BSS) - - - - - - 16
2.2.2 Extended Service Set (ESS) - - - - - - 17
2.2.3 Independent Basic Service Set (IBSS) - - - - 19
2.3 Protocol Architecture of IEEE 802.11 - - - - - 19
2.3.1 IEEE 802.11 Media Access Control (MAC) Sublayer - - 20
2.3.1.1 Distributed Coordinate Function (DCF) - - - 21
2.3.1.2 Point Coordinate Function (PCF) - - - - 24
2.3.1.3 Enhanced Distributed Channel Access (EDCA) - - 26
2.3.1.4 Hybrid coordination function controlled access (HCCA) - 27
2.3.1.5 Limitations of the IEEE802.11e Protocols- - - - 27
2.3.2 IEEE 802.11 Basic MAC Frame Formats - - - - 28
2.3.2.1 Control Frames - - - - - - 29
2.3.2.2 Management Frame - - - - - - 29
2.3.2.3 Data Frame - - - - - - - 30
2.3.3 IEEE 802.11 Physical Layer (PHY) - - - - - 30
2.3.3.1 Physical Layer Convergence Protocol (PLCP) Sublayer - 30
2.3.3.2 Physical Medium Dependent (PMD) Sublayer - - 31
2.4 IEEE 802.11 Modulation Techniques- - - - - - 31
2.4.1 Spread spectrum Technique - - - - - - 32
2.4.1.1 Frequency Hopping Spread Spectrum - - - 32
2.4.1.2 Direct Sequence Spread Spectrum - - - - 34
2.4.2 Infrared Technology - - - - - - - 36
2.4.3 Orthogonal Frequency Division Multiplexing - - - 36
2.5 Related Works- - - - - - - - - 37
2.6 Conclusion - - - - - - - - - - 40
CHAPTER THREE: IEEE 802.11 EDCA MODEL DESIGN - - - 41
3.0 Introduction - - - - - - - - - 41
3.1 WLAN Simulation Architecture - - - - - - 41
3.2 MAC sub-layer functional description - - - - - 43
3.3 EDCA (Enhanced Distributed Channel Access) - - - - 44
3.3.1 Access Categories (ACs) - - - - - - 43
3.3.2 EDCA Parameters - - - - - - - 45
3.3.2.1 Arbitration Inter-frame Space (AIFS) - - - 45
3.3.2.2 Contention Window Minimum and Maximum - - 45
3.3.2.3 Transmission Opportunity Limit (TXOP) - - 46
3.4 EDCA Model Operation Mechanism - - - - - - 46
3.4.1 Internal Collision - - - - - - - 49
3.4.2 External Collision - - - - - - - 49
3.5 MATLAB Simevent EDCA Model - - - - - - 51
3.6 Performance Metrics - - - - - - - - 54
3.6.1 Throughput - - - - - - - - 54
3.6.2 Average End-to-End Delay - - - - - - 54
3.6.3 Packet Loss - - - - - - - - 55
CHAPTER FOUR: MODEL SIMULATION AND RESULTS ANALYSIS - 56
4.0 Introduction - - - - - - - - - 56
4.1 Traffic Distribution Process - - - - - - - 58
4.1.1 Background (AC0) and Best-effort Data Traffic (AC1) generation process - - - - - - - 58
4.1.2 Video Traffic (AC2) generation process - - - - 59
4.1.3 Voice Traffic (AC3) generation process - - - - 60
4.2 Simulation Scenarios - - - - - - - - 62
4.2.1 Scenarios One: Proposed Model simulation - - - - 62
4.2.2 Scenario Two: Proposed Model Validation Simulation - - 62
4.2.3 Scenario Three and Four: Differentiation Effect Simulation - - 63
4.3 Simulation Results and Analysis - - - - - - 64
4.3.1 Scenario One and Two: Simulation Result and Analysis - - 65
4.3.1.1 Throughput Analysis - - - - - - - 65
4.3.1.2 Delay Analysis - - - - - - - 67
4.3.1.3 Loss Rate Analysis - - - - - - - 68
4.3.2 Scenario Three: Simulation Result and Analysis - - - 70
4.3.2.1 Throughput Analysis - - - - - - - 70
4.3.2.2 Delay Analysis - - - - - - - 75
4.3.2.3 Packet Loss Rate Analysis - - - - - - 78
CHAPTER FIVE: CONCLUSION AND RECOMMENDATION- - - 83
5.0 Introduction - - - - - - - - - 83
5.1 Conclusion - - - - - - - - - 83
5.2 Observations - - - - - - - - - 83
5.3 Recommendation - - - - - - - - 84
References - - - - - - - - - 86
