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.
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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
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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
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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
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CHAPTER ONE
INTRODUCTION
1.0 Background of the Study
IEEE 802.11 wireless LAN (WLAN) is one of the most deployed wireless technologies all over
the world and is likely to play a major role in next-generation wireless communication networks
[1]. The main characteristics of the IEEE 802.11 WLAN technologies are simplicity, flexibility
and cost effectiveness. This technology provides people with ubiquitous communication and
computing environment in offices, hospitals, campuses, factories, airports, stock markets, etc [2].
Simultaneously, multimedia applications have experienced an explosive growth. People are now
requiring high-speed video, audio, voice and Web services even when they are moving in offices
or travelling around campuses [2]. However, multimedia applications require some QoS support
such as guaranteed bandwidth, delay, and jitter and error rate [3]. Guaranteeing these QoS
requirements in IEEE 802.11 WLAN is very challenging due to the QoS unaware functions of its
media access layer (MAC) and the variable physical (PHY) layer characteristics.
However, in WLAN, stations are connected wirelessly to a common access point (AP) that
makes use of distributed multiplexing algorithm for sharing an access channel referred to as
MAC. In such networking, a media access control protocol is needed to coordinate access to the
link. With respect to this, a number of network protocols have been devised to handle access to a
wireless shared link; this includes distributed coordination function (DCF), point coordination
function (PCF), enhanced distributed coordination function (EDCF) and hybrid coordination
control function (HCCF).
The DCF protocol is the fundamental channel access mechanism based on Carrier Sense
Multiple Access with Collision avoidance (CSMA/CA). It provides fair access to the channel for
all devices or stations with no room for prioritization [4]. The second protocol; PCF, is an
optional capability that can be used to provide contention free services by allowing poll station to
transmit without contending for the channel. These two protocols do not guarantee QoS
requirements for real-time services. This is because all traffic undergo through the same queuing
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and transmission processes. As a result of the limitation of DCF and PCF protocols in providing
guaranteed QoS for multimedia applications, EDCF and HCCF media access techniques were
introduced in 2007 by IEEE 802.11e task group. These two protocols are hybrid coordination
functions (HCF) and an enhancement of DCF and PCF protocols respectively
In this thesis, performance analysis of WLAN access protocol “EDCA” was presented. The
objective is to establish the influence of differentiating channel access parameters such as
arbitration inter-frame space number (AIFSN) and contention window (CW) size for each access
category on QoS parameters. This is intended to guide network service providers in Network
planning and also a stepping stone for the development of WLAN access protocol that will
guarantee QoS. Wireless LAN network model was developed and converted into a computer
simulation model using MATLAB Simevent environment.
The enhanced distributed channel access is a potential improvement over DCF protocol for IEEE
802.11 MAC protocol. It is as well a contention based CSMA/CA protocol with traffic classifier,
four access category, exponential back-off algorithm and scheduler. This protocol was therefore
modelled and simulated. The simulation results generated with MATLAB (SimEvent) simulation
package were collected and analyzed to establish the sort for influence. The relationship between
traffic intensity (number of users of a given traffic pattern) and frame loss rate, delay, as well as
throughput were established by the results.
The EDCF MAC protocol of IEEE 802.11 WLAN is a unique way of providing priority scheme
to different traffic classes such as background, data, video, and voice services using
differentiated channel parameters with an intension of meeting QoS requirements. This protocol
assigns high priority to voice traffic while background is allotted the list priority. The effect is
that in priority queuing, packets in the highest-priority queue are processed first. Packets in the
lowest-priority queue are processed last. Note that the system does not stop serving a queue until
it is empty. With this mechanism, higher priority traffic, such as multimedia, can reach the
destination with less delay. It is also obvious that collision in this medium is resolved by granting
transmission opportunity to higher priority traffic in every collision. However, there is a potential
drawback. If there is a continuous flow in a high-priority queue, the packets in the lower-priority
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queues will never have a chance to be processed. This is a condition called starvation [5]. At this
point of saturation the amount of access delay, traffic congestion and frame loss rate experienced
is high, as the number of bursty packets transmitted through the access protocol (traffic intensity)
increases with number of contending stations.
1.1 Purpose of the Study
This work is most essential to the Educational trust Fund (ETF) Laboratory, Nigeria
Communication Commission (NCC) and the department of Electronic Engineering, University
of Nigeria Nsukka (UNN). The purpose of this work is to analyze the service differentiation
impact of IEEE 802.11e contention based WLAN on networks QoS. This will assist wireless
LAN vendors to satisfy the QoS requirements of real-time services such as voice and video
traffic and also remedy the problem of channel access starvation that affects lower priority
traffics as a result of poor management of the channel access parameters. Therefore, a state-ofthe-
art WLAN model that handles service differentiation will be designed and converted into a
computer model in MATLAB Simevet environment. The MATLAB model will be simulated and
results collected and analyzed to establish the effect of differentiating services on networks QoS
parameters.
1.2 Scope of the Study
The scope of this work covers the IEEE 802.11 WLAN standards, the physical architecture and
topologies. It also covers the IEEE 802.11 WLAN medium access control (MAC) layer
architecture which includes the legacy distributed Coordinate function (DCF), and the point
coordinate function (PCF). The state-of-the-art media access control protocol (IEEE 802.11e
WLAN) comprising of the contention based EDCF protocol also referred to as enhanced
distributed channel access (EDCA) protocol and contentionless hybrid coordination function
controlled access. The IEEE 802.11 basic MAC frame formats, the MAC physical (PHY) layer
and WLAN modulation techniques are also addressed. The analysis covered the modelling of
contention based EDCA MAC protocol while the legacy Coordinate Function and Centralized
hybrid coordinate function were not considered in the model. Also, the Power management and
security are not considered
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1.3 Methodology
The system of collecting data for this research work was based on simulation approach. An
isolated WLAN AP that uses EDCA mechanism to support multiple AC’s on a shared wireless
medium was modelled and implemented in MATLAB simevent computer programme. Insight
into the system’s performance was provided by the probes strategically positioned to collect data
for the calculation of QoS parameters (mean throughput, delay and packet loss rate). The data
was analysed by relating the points of the responses to the objectives of the study. Because some
important factors in research methodology includes validity of research data, ethics and the
reliability of design, the information collected in the simulation model was validated using Bahi
Hour etal’s simulation parameters. The designed model was also simulated and the result used to
analyse the service differentiation ability of channel access parameters under study.
1.4 Thesis outline
The rest of this thesis is organized as follows: In Chapter Two, the evolution of WLAN and the
MAC specification of WLAN were presented. In chapter Three, the architecture, models and
simulations of the WLAN protocol were equally, presented. In Chapter Four, simulation result
and analysis were presented. In Chapter Five, conclusions were drawn and recommendations
made.
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