ABSTRACT
Distributed coordination Function is the most crucial component of
IEEE 802.11 standard. It is a media access control (MAC) scheme
used by the IEEE 802.11. Because the channels used by wireless
devices is a time-varying broadcast medium, these devices need to
have multi-rate and rate adaptive capability to adapt to the
changing channel so that better performance can be achieved. In this
work, an analytical model is presented to study the throughput and
delay variation of IEEE 802-11b with number of mobiles in a multirate
WLAN using the DCF protocol to contend for data
transmissions in a slowly-varying channel. Auto Rate Fall back
(ARF) protocol was used to adapt rates for different channel
qualities and the best configurations and parameter values for the
ARF in correspondence to network load and topology to get best
performance was discussed.
9
TABLE OF CONTENTS
Title page – – – – – – – – i
Certification – – – – – – – – ii
Approval page – – – – – – – – iii
Dedication – – – – – – – – – vi
Acknowledgement – – – – – – – v
Abstract – – – – – – – – – vii
Table of Contents – – – – – – – viii
List of Tables – – – – – – – – xiv
List of Figures – – – – – – – – xiii
CHAPTER ONE: INTRODUCTION
1.1 Background – – – – – – – 1
1.2 Motivation – – – – – – – 3
1.3 Objectives – – – – – – – 4
1.4 Justification – – – – – – – 5
1.5 Scope of Work – – – – – – – 5
1.6 Project Report Organization – – – – – 6
10
CHAPTER TWO: REVIEW OF RELATED WORKS
2.1 Principles – – – – – – – – 8
2.2 IEEE 802.11 WLAN Standard – – – – 10
2.2.1 IEEE 802.11a – – – – – – – 11
2.2.2 IEEE 802.11b. – – – – – – – 12
2.2.3 IEEE 802.11e – – – – – – – 12
2.2.4 IEEE 802.11g – – – – – – – 13
2.2.5 IEEE 802.11n – – – – – – – 14
2.3 IEEE 802.11 Service Sets – – – – – 16
2.4 IEEE 802.11 Physical Layer – – – – – 18
2.5 IEEE 802.11 Medium Access Control (MAC) Sub-Layer 19
2.6 IEEE 802.11 Distributed Coordination Function (DCF). 20
2.7 Point Co-Ordination Function – – – – 25
2.8 IEEE 802.11b Standard – – – – – – 26
2.9 Multi-Rate Wireless LAN Technology – – – 28
2.10 Single – Rate WLAN Technology – – – – 30
2.11 Limitation Of Single-Rate IEEE 802.11 WLAN – – 31
11
CHAPTER THREE: METHODOLOGY AND SYSTEM DESIGN
3.1 Introduction – – – – – – – 32
3.2 Experimental Environment – – – – – 34
3.3 Description of the experimental environments- – 35
3.3.1 Access Point Environment – – – – – 35
3.3.2 Testbed Environment – – – – – – 37
3.4 An Overview of the Three Regions of the Network – 43
3.5 Assumptions – – – – – – – 45
3.6 Signal Measurements – – – – – – 46
3.7 Rate Adaptation Using Auto Rate Fallback (ARF)
Algorithm – – – – – – – – 48
3.8 Network throughput – – – – – – 51
CHAPTER FOUR: IMPLEMENTATION, RESULTS AND
ANALYSIS
4.1 Throughput Measurement and Analysis – – – 53
4.1.1 Stage 1: All mobile hosts are moving only in region 1 of
the AP. – – – – – – – – 53
4.1.2 Stage 2: All mobile hosts are moving only in region 2 of
the AP. – – – – – – – – 57
12
4.1.3 Stage 3: All mobile hosts moving only in region 3 of
the AP. – – – – – – – – 60
4.1.4 Stage 4: The General Scenario – – – – – 63
4.2 Delay Analysis – – – – – – – 67
4.2.1 Stage 1; all mobile hosts moving in region I of the AP 67
4.2.2 Stage 2: all mobile hosts in region2 of the APs – – 70
4.2.3 Stage 3: all mobile hosts in region 3 of the APs. – – 73
4.2.4 Stage 4: The general scenario – – – – 75
4.3 Deductions – – – – – – – 79
CHAPTER FIVE: SUMMARY AND CONCLUSION
5.1 Summary of achievement – – – – – 81
5.2 Problems Encountered and Solution – – – 81
5.3 Suggestions for Further Improvement – – – 82
5.4 Conclusion – – – – – – – 83
REFERENCES – – – – – – – – 84
APPENDIX A – – – – – – – – 92
APPENDIX B – – – – – – – – 94
APPENDIX C – – – – – – – – 96
13
APPENDIX D – – – – – – – – 98
APPENDIX E – – – – – – – – 100
APPENDIX F – – – – – – – – 102
APPENDIX G – – – – – – – – 104
APPENDIX H – – – – – – – – 106
14
LIST OF TABLES
Table 2.1: Summary of IEEE 802.11 standards – – 15
Table 2.2: IEEE 802.11 Physical layer specifications – 19
Table 3.1: The input parameters used during the measurement 47
Table 4.1: Measured throughput values in region 1 of the AP’s 55
Table 4.2: Measured throughput values in region 2 of the AP’s 58
Table 4.3: Measured throughput values in region 3 of the AP’s 62
Table 4.4: Measured throughput values in the general case
scenario – – – – – – – 65
Table 4.5: Measured delay in region 1 of the APs – – 69
Table 4.6: Measured delay in region 2 of the APs – – 72
Table 4.7: Measured delay in region 3 of APs – – – 74
Table 4.8: Measured delay in the general scenario – – 77
15
LIST OF FIGURES
Fig 2.1: Frame exchange in a basic access mechanism
(two-way hand shaking) – – – – – 24
Fig 2.2: Frame exchange in a four-way hand shaking
Mechanism – – – – – – – 25
Fig 3.1: The floor plan of GlobalMicrocom Computers Awka
indicating the positions of the access points. – 36
Fig 3.2: Floor plan of the test bed one: Nnedioramma Extension
Hostel – – – – – – – – 38
Fig 3.3: Test Bed Two: floor plan of the second floor of
Omeokachie Hostel Awka. – – – – – 40
Fig 3.4: Test bed Three: Floor plan of the University computers,
Awka – – – – – – – – 42
Fig 3.5: An overview diagram of the entire network – 44
Fig 3.6: The general architecture of the Auto rate fallback
(ARF) protocol. – – – – – – 50
Fig 4.1: A graph of the mean throughput against the number
of mobile hosts in region 1 – – – – 56
Fig 4.2: A graph of the mean throughput against the number of
mobile hosts in region 2 – – – – – 59
16
Fig 4.3: A graph of the mean throughput against the number of
mobile hosts in region 3. – – – – – 63
Fig 4.4: A graph of the mean throughput against the number of
mobile hosts in general scenario. – – – 66
Fig 4.5: A graph of the average delay against the number of
mobile hosts in region 1 – – – – – – 70
Fig 4.6: A graph of average delay against the number of mobile
hosts in region 2 – – – – – – – 73
Fig 4.7: A graph of average delay against the number of mobile
hosts in regions 3. – – – – – – 75
Fig 4.8: A graph of average delay against number of mobile
hosts in the general scenario. – – – – 78
17
CHAPTER ONE
INTRODUCTION
1.1 Background
The family of IEEE 802.11 protocols has become the most popular
access technologies in the world today. They provide an effective
means of achieving wireless data connectivity in homes, public
places and offices. In 802.11 protocols, the fundamental medium
access method is called DCF (distributed coordination function), a
form of carrier sense multiple access with collision avoidance
(CSMA/CA). The DCF is based on listen before talk procedure
where the terminals first checks to see if the radio link is free before
transmitting and then initiates a random back off procedure to
avoid collisions. The DCF can also use the RTS (request to send) and
CTS (clear to send) technique to further prevent collisions.
Because of signal fading, transmission interference, and user
mobility, wireless channels have time-varying characteristics [1].
This makes different mobile hosts to perceive different channel
18
qualities at the same time. In order to obtain a better throughput, the
hosts in a network need to use different transmission rates for
different channel qualities. This multi-rate support is currently
included in most protocols like IEEE 802.11b, 802.11a, 802.11g, and
Hiper-LAN-11. Some rate adaptation techniques have been
designed for 802.11 WLANs including ARF (Auto Rate fall-back)
and RBAR (Receiver Based Auto Rate).
Thus, it is necessary to evaluate and analyze the performance
of IEEE 802.11 WLAN system under the fundamental access
mechanism for medium access control (MAC) called DCF in a multirate
WLAN. Most of the analysis on DCF in the past were based
on simulation and no particular IEEE 802.11 standard was used.
Thus the need to carry out an empirical analysis of IEEE 802.11b in a
multi-rate WLAN.
19
1.2 Motivation
The performance analysis of IEEE 802.11 networks is an area of
important research interest in the international literature[2].
According to [3], to better understand the performance of WLANs,
a critical challenge is how to analyze IEEE 802.11 DCF. Because the
channel used by wireless devices is a time-varying broadcast
medium, these devices need to have multi-rate and rate-adaptive
capability to adapt to the changing channel so that better
performance can be achieved [4]. This work models and empirically
analyzes the IEEE 802.11b Distributed Coordination Function (DCF)
in a multi-rate wireless LAN. The ARF protocol was used to adapt
rates for different channel qualities and the DCF protocol was
followed to contend for data transmissions in a slowly-varying
channel. There are many factors that affect the performance of a
wireless network such as packet collisions, number of nodes in the
network, the distance between the sender and the receiver ( i.e the
range ), interference by other wireless devices and obstructions like
20
walls e.t.c. But in this work, the main aim is to verify the effects of
the distance between the sender and the receiver (Range) and the
number of nodes in a network on the saturation throughput and
delay performances of the network.
1.3 Objectives
The main objectives of this work are;
(i) To present a model for IEEE 802.11b distributed
coordination function (DCF) in a Multi-rate WLAN.
(ii) An empirical analysis of IEEE 802.11b DCF in a multi-rate
WLAN based on the saturation throughput and delay
using the ARF (auto Rate full back) as a protocol to adapt
rates for different channel qualities.
(iii) To use the results in (ii) to determine the best
configurations and parameter values for the ARF relative to
network load.
21
1.4 Justification
High throughput and low delay is the desire of every WLAN user.
In order to achieve this, a multi rate WLAN is to be used. This work
will go a long way to improve on the user mobility of the wireless
LAN and as well help to reduce the effect of signal fading and
transmission interference on the system.
1.5 Scope of work
This work deals with modeling and empirical analysis of IEEE
802.11b DCF in a multi-rate WLAN. In this work a model is to be
presented for IEEE 802.11b DCF in a Multi-Rate WLAN. An actual
measurement of the saturation throughput and delay is to be carried
out for a given number of mobile hosts and at different ranges. The
ARF parameters to be used in the analysis are S (maximum number
of consecutive successes before switch to higher rate) and F
(maximum number of consecutive failures before switching to lower
rate).
22
1.6 Project report organization
This project is systematically presented in five chapters to describe
the modeling and analysis carried out on 802.11b DCF in a multirate
WLAN. Chapter 1 explains the background scope and
organization of the entire project. Chapter 2 presents a review of
other work already done in IEEE 802.11b multi-rate WLAN, 802.11
standards and Distributed Coordination function (DCF). A
comparative review of single-rate and multi-rate 802.11 WLAN is
then drawn.
Chapter 3 is the methodology and system design. It contains
the assumptions made in the research, throughput measurement
and estimation and delay measurement and estimation.
Chapter 4 is results and analysis. It shows the tables of values
of all the measurement taken and the graphs of the average through
put and delay against number of mobile hosts in each region of the
network using different ARF Sm and Fm values.
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