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

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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.

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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

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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.

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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