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
multi-rate 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.
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 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
multi-rate 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.
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 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.
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).
This project is systematically
presented in five chapters to describe the modeling and analysis carried out on
802.11b DCF in a multi-rate 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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