The handbook of ad hoc wireless networks

THE HANDBOOK OF

AD HOC
WIRELESS
NETWORKS

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THE HANDBOOK OF

AD HOC
WIRELESS
NETWORKS
Edited by

Mohammad Ilyas
Florida Atlantic University
Boca Raton, Florida

CRC PR E S S
Boca Raton London New York Washington, D.C.

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The Electrical Engineering Handbook Series
Series Editor

Richard C. Dorf
University of California, Davis

Titles Included in the Series
The Handbook of Ad Hoc Wireless Networks, Mohammad Ilyas
The Avionics Handbook, Cary R. Spitzer
The Biomedical Engineering Handbook, 2nd Edition, Joseph D. Bronzino
The Circuits and Filters Handbook, Second Edition, Wai-Kai Chen
The Communications Handbook, 2nd Edition, Jerry Gibson
The Computer Engineering Handbook, Vojin G. Oklobdzija
The Control Handbook, William S. Levine
The Digital Signal Processing Handbook, Vijay K. Madisetti & Douglas Williams
The Electrical Engineering Handbook, 2nd Edition, Richard C. Dorf
The Electric Power Engineering Handbook, Leo L. Grigsby
The Electronics Handbook, Jerry C. Whitaker
The Engineering Handbook, Richard C. Dorf
The Handbook of Formulas and Tables for Signal Processing, Alexander D. Poularikas
The Handbook of Nanoscience, Engineering, and Technology, William A. Goddard, III,
Donald W. Brenner, Sergey E. Lyshevski, and Gerald J. Iafrate
The Industrial Electronics Handbook, J. David Irwin
The Measurement, Instrumentation, and Sensors Handbook, John G. Webster
The Mechanical Systems Design Handbook, Osita D.I. Nwokah and Yidirim Hurmuzlu
The Mechatronics Handbook, Robert H. Bishop
The Mobile Communications Handbook, 2nd Edition, Jerry D. Gibson
The Ocean Engineering Handbook, Ferial El-Hawary
The RF and Microwave Handbook, Mike Golio
The Technology Management Handbook, Richard C. Dorf
The Transforms and Applications Handbook, 2nd Edition, Alexander D. Poularikas
The VLSI Handbook, Wai-Kai Chen

Forthcoming Titles
The CRC Handbook of Engineering Tables, Richard C. Dorf

The Engineering Handbook, Second Edition, Richard C. Dorf
The Handbook of Optical Communication Networks, Mohammad Ilyas and
Hussein T. Mouftah

© 2003 by CRC Press LLC

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Library of Congress Cataloging-in-Publication Data
The handbook of ad hoc wireless networks / edited by Mohammad Ilyas.
p. cm. -- (The electrical engineering handbook series)
Includes bibliographical references and index.
ISBN 0-8493-1332-5 (alk. paper)
1. Wireless LANs. I. Ilyas, Mohammad, 1953- II. Series
TK5105.78 .H36 2002
621.382--dc21

2002031316

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Preface

To meet the need for fast and reliable information exchange, communication networks have become an
integral part of our society. The success of any corporation largely depends upon its ability to communicate. Ad hoc wireless networks will enhance communication capability significantly by providing
connectivity from anywhere at any time. This handbook deals with wireless communication networks
that are mobile and do not need any infrastructure. Users can establish an ad hoc wireless network on
a temporary basis. When the need disappears, so will the network.
As the field of communications networks continues to evolve, a need for wireless connectivity and
mobile communication is rapidly emerging. In general, wireless communication networks provide wireless (and hence) mobile access to an existing communication network with a well-defined infrastructure.

Ad hoc wireless networks provide mobile communication capability to satisfy a need of a temporary
nature and without the existence of any well-defined infrastructure. In ad hoc wireless networks, communication devices establish a network on demand for a specific duration of time. Such networks have
many potential applications including the following:
•
•
•
•
•

Disaster recovery situations
Defense applications (army, navy, air force)
Healthcare
Academic institutions
Corporate conventions/meetings

This handbook has been prepared to fill the need for comprehensive reference material on ad hoc
wireless networks. The material presented in this handbook is intended for professionals who are designers and/or planners for emerging telecommunication networks, researchers (faculty members and graduate students), and those who would like to learn about this field.
The handbook is expected to serve as a source of comprehensive reference material on ad hoc wireless
networks. It is organized in the following nine parts:
•
•
•
•
•
•
•
•
•

Introduction

Wireless transmission techniques
Wireless communication systems and protocols
Routing techniques in ad hoc wireless networks — part I
Routing techniques in ad hoc wireless networks — part II
Applications of ad hoc wireless networks
Power management in ad hoc wireless networks
Connection and traffic management in ad hoc wireless networks
Security and privacy aspects of ad hoc wireless networks

© 2003 by CRC Press LLC

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The handbook has the following specific salient features:
• It serves as a single comprehensive source of information and as reference material on ad hoc
wireless networks.
• It deals with an important and timely topic of emerging communication technology of tomorrow.
• It presents accurate, up-to-date information on a broad range of topics related to ad hoc wireless
networks.
• It presents material authored by experts in the field.
• It presents the information in an organized and well-structured manner.
Although the handbook is not precisely a textbook, it can certainly be used as a textbook for graduate
courses and research-oriented courses that deal with ad hoc wireless networks. Any comments from
readers will be highly appreciated.
Many people have contributed to this handbook in their unique ways. The first and foremost group
that deserves immense gratitude is the group of highly talented and skilled researchers who have contributed 32 chapters. All of them have been extremely cooperative and professional. It has also been a
pleasure to work with Nora Konopka, Helena Redshaw, and Susan Fox of CRC Press, and I am extremely
grateful for their support and professionalism. My wife Parveen and my four children Safia, Omar, Zakia,
and Maha have extended their unconditional love and strong support throughout this project, and they

all deserve very special thanks.

Mohammad Ilyas
Boca Raton, Florida

© 2003 by CRC Press LLC

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The Editor

Mohammad Ilyas is a professor of computer science and engineering at Florida Atlantic University, Boca
Raton, Florida. He received his B.Sc. degree in electrical engineering from the University of Engineering
and Technology, Lahore, Pakistan, in 1976. In 1978, he was awarded a scholarship for his graduate studies,
and he completed his M.S. degree in electrical and electronic engineering in June 1980 at Shiraz University,
Shiraz, Iran. In September 1980, he joined the doctoral program at Queen’s University in Kingston,
Ontario. He completed his Ph.D. degree in 1983. His doctoral research was about switching and flow
control techniques in computer communication networks. Since September 1983, he has been with the
College of Engineering at Florida Atlantic University. From 1994 to 2000, he was chair of the Department
of Computer Science and Engineering. During the 1993–94 academic year, he spent a sabbatical leave
with the Department of Computer Engineering, King Saud University, Riyadh, Saudi Arabia.
Dr. Ilyas has conducted successful research in various areas including traffic management and congestion control in broadband/high-speed communication networks, traffic characterization, wireless
communication networks, performance modeling, and simulation. He has published one book and more
than 120 research articles. He has supervised several Ph.D. dissertations and M.S. theses to completion.
He has been a consultant to several national and international organizations. Dr. Ilyas is an active
participant in several IEEE technical committees and activities and is a senior member of IEEE.

© 2003 by CRC Press LLC

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List of Contributors

George N. Aggélou
Institute of Technology
Athens, Greece

Roberto Baldoni
Universita’ di Roma, “La Sapienza”
Roma, Italy

Roberto Beraldi
Universita’ di Roma, “La Sapienza”
Roma, Italy

Ezio Biglieri
Politecnico di Torino
Torino, Italy

Satyabrata Chakrabarti
Sylvaine Algorithmics
Aurora, Illinois

Chaou-Tang Chang
National Chiao Tung University
Hsinchu, Taiwan

Chih Min Chao

National Central University
Chung-Li, Taiwan

Xiao Chen
Southwest Texas State University
San Marcos, Texas

Chua Kee Chaing
National University of Singapore
Singapore, Singapore

Marco Conti
Consiglio Nazionale delle Ricerche
Pisa, Italy

José Ferreira de
Rezende
Federal University of Rio de Janeiro
Rio de Janeiro, Brazil

Pei-Kai Hung
National Central University
Chung-Li, Taiwan

Aditya Karnik
Nelson Fonseca
State University of Campinas
Campinas, Brazil

Indian Institute of Science

Bangalore, India

Won-Ik Kim
Holger Füßler
University of Mannheim
Mannheim, Germany

ETRI
Taejon, South Korea

Anurag Kumar
Silvia Giordano
LCA-IC-EPFL
Lausanne, Switzerland

Indian Institute of Science
Bangalore, India

Dong-Hee Kwon
Zygmunt J. Haas
Cornell University
Ithaca, New York

POSTECH
Pohang, South Korea

Chiew-Tong Lau
Hannes Hartenstein
NEC Europe Ltd.
Heidelberg, Germany

Nanyang Technological University
Singapore, Singapore

Ben Lee
Xiao Hannan
National University of Singapore
Singapore, Singapore

Oregon State University
Corvallis, Oregon

Bu-Sung Lee
Hossam S. Hassanein
Queen's University
Kingston, Ontario, Canada

Nanyang Technological University
Singapore, Singapore

Bo Li
Chih-Shun Hsu
National Central University
Chung-Li, Taiwan

Hong Kong University of Science
and Technology
Kowloon, Hong Kong

Cheng-Ta Hu

Michele Lima

National Central University
Chung-Li, Taiwan

© 2003 by CRC Press LLC

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State University of Parana West
Cascavel, Brazil


Ting-Yu Lin

Matthew Sadiku

Lei Wang

National Chiao-Tung University
Hsinchu, Taiwan

Prairie View A&M University
Prairie View, Texas

Tianjin University
Tianjin, People’s Republic of China

Jiang Chuan Liu

Ahmed M. Safwat

Jörg Widmer

Hong Kong University of Science
and Technology
Kowloon, Hong Kong

Queen's University
Kingston, Ontario, Canada

University of Mannheim
Mannheim, Germany

Prince Samar

Seah Khoon Guan
Winston

Pascal Lorenz
Universtiy of Haute Alsace
Colmar, France

Martin Mauve
University of Mannheim
Mannheim, Germany

Amitabh Mishra
Virginia Polytechnic Institute and

State University
Blacksburg, Virginia

Cornell University
Ithaca, New York

National University of Singapore
Singapore, Singapore

Boon-Chong Seet
Nanyang Technological University
Singapore, Singapore

Jie Wu
Florida Atlantic University
Boca Raton, Florida

Jang-Ping Sheu
National Central University
Chung-Li, Taiwan

Oliver Yang
University of Ottawa
Ottawa, Ontario, Canada

Yantai Shu
Sal Yazbeck

Sangman Moh

Tianjin University
Tianjin, People’s Republic of China

ETRI
Taejon, South Korea

Kazem Sohraby

Hussein T. Mouftah

Lucent Technologies
Lincroft, New Jersey

Queen's University
Kingston, Ontario, Canada

Ivan Stojmenovic

Sungkyunkwan University
Jangangu Chunchundong, South
Korea

Ketan M. Nadkarni

University of Ottawa
Ottawa, Ontario, Canada

Virginia Polytechnic Institute and
State University
Blacksburg, Virginia

Chansu Yu

Young-Joo Suh

Cleveland State University
Cleveland, Ohio

POSTECH
Pohang, South Korea

Qian Zhang

Yu-Chee Tseng

Microsoft Research
Beijing, People’s Republic of China

National Chiao-Tung University
Hsinchu, Taiwan

Dan Zhou

Kuochen Wang

Florida Atlantic University
Boca Raton, Florida

National Chiao Tung University
Hsinchu, Taiwan

Wenwu Zhu

Panagiotis
Papadimitratos
Cornell University
Ithaca, New York

Marc R. Pearlman
Cornell University
Ithaca, New York

Barry University
Palm Beach Gardens, Florida

Hee Yong Youn

Microsoft Research
Beijing, People’s Republic of China

© 2003 by CRC Press LLC

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Table of Contents

1

B ody, Personal, and Local Ad Hoc Wireless Networks Marco Conti

2

Multicasting Techniques in Mobile Ad Hoc Networks Xiao Chen
and Jie Wu

3

Qualit y of Service in Mobile Ad Hoc Networks
Sat yabrata Chakrabarti and Amitabh Mishra

4

Power-Conservative Designs in Ad Hoc Wireless Networks Yu-Chee Tseng
and Ting-Yu Lin

5

Performance Analysis of Wireless Ad Hoc Networks Anurag Kumar
and Aditya Karnik

6

C oding for the Wireless Channel Ezio Big lieri

7

Unicast Routing Techniques for Mobile Ad Hoc Networks Roberto Beraldi
and Roberto Baldoni

8

S atellite Communications Matthew N.O. Sadiku

9

Wireless Communication Protocols Pascal Lorenz

10

An Integrated Platform for Ad Hoc GSM Cellular Communications
George N. Aggélou

11

IEEE 802.11 and B luetooth: An Architectural Overview Sal Yazbeck

12

Position-Based Routing in Ad Hoc Wireless Networks Jörg Widmer, Martin Mauve,
Hannes Hartenstein, and Holger Füßler

© 2003 by CRC Press LLC

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13

St ructured Proactive and Reactive Routing for Wireless Mobile Ad Hoc

Networks Ahmed M. Safwat, Hossam S. Hassanein, and Hussein T. Mouftah

14

Hybrid Routing: The Pursuit of an Adaptable and Scalable Routing Framework
for Ad Hoc Networks Prince Samar, Marc R. Pearlman, and Zygmunt J. Haas

15

Adaptive Routing in Ad Hoc Networks Yantai Shu, Oliver Yang,
and Lei Wang

16

Position-Based Ad Hoc Routes in Ad Hoc Networks S ilvia Giordano
and Ivan Stojmenovic

17

Route Discovery Optimization Techniques in Ad Hoc Networks
B oon-Chong Seet, Bu-Sung Lee, and Chiew-Tong Lau

18

L ocation-Aware Routing and Applications of Mobile Ad Hoc Networks
Yu-Chee Tseng and Chih-Sun Hsu

19

Mobility over Transport Control Protocol/Internet Protocol (TCP/IP)

José Ferreira de Rezende, Michele Mara de Araújo Espíndula Lima, and
Nelson Luis Saldanha da Fonseca

20

An Intelligent On-Demand Multicast Routing Protocol in Ad Hoc Networks

21

GPS-Based Reliable Routing Algorithms for Ad Hoc Networks

Kuochen Wang and Chaou-Tang Chang

Young-Joo Suh, Won-Ik Kim, and Dong-Hee Kwon

22

Power-Aware Wireless Mobile Ad Hoc Networks Ahmed Safwat, Hossam S. Hassanein,

23

Energy Efficient Multicast in Ad Hoc Networks Hee Yong Youn, Chansu Yu, Ben Lee,

24

Energy-Conserving Grid Routing Protocol in Mobile Ad Hoc Networks

and Hussein T. Mouftah

and Sangman Moh

Jang-Ping Sheu, Cheng-Ta Hu, and Chih-Min Chao

25

Routing Algorithms for Balanced Energy Consumption in Ad Hoc Networks
Hee Yong Youn, Chansu Yu, and Ben Lee

© 2003 by CRC Press LLC


26

Resource Discovery in Mobile Ad Hoc Networks Jiangchuan Liu, Kazem Sohraby,
Qian Zhang, Bo Li, and Wenwu Zhu

27

An Integrated Platform for Quality-of-Service Support in Mobile Multimedia
Clustered Ad Hoc Networks George N. Aggélou

28

Quality of Service Models for Ad Hoc Wireless Networks Xiao Hannan, Chua
Kee Chaing, and Seah Khoon Guan Winston

29

Scheduling of Broadcasts in Multihop Wireless Networks Jang-Ping Sheu,
Pei-Kai Hung, and Chih-Shun Hsu

30

Security in Wireless Ad Hoc Networks — A Survey Amitabh Mishra
and Ketan M. Nadkarni

31

Securing Mobile Ad Hoc Networks Panagiotis Papadimitratos
and Zygmunt J. Haas

32

Security Issues in Ad Hoc Networks Dan Zhou

© 2003 by CRC Press LLC


1
Body, Personal, and
Local Ad Hoc Wireless
Networks
Abstract
1.1 Introduction
1.2 Mobile Ad Hoc Networks
Body Area Network • Personal Area Network • Wireless Local
Area Network

1.3
1.4

Technologies for Ad Hoc Networks
IEEE 802.11 Architecture and Protocols
EEE 802.11 DCF • EEE 802.11 RTS/CTS

1.5

A Technology for WBAN and WPAN: Bluetooth
A Bluetooth Network • Bluetooth Data Transmission

Marco Conti
Consiglio Nazionale delle Ricerche

Acknowledgment
References

Abstract
A mobile ad hoc network (MANET) represents a system of wireless mobile nodes that can freely and
dynamically self-organize into arbitrary and temporary network topologies, allowing people and devices
to seamlessly internetwork in areas without any preexisting communication infrastructure. While many
challenges remain to be resolved before large scale MANETs can be widely deployed, small-scale mobile
ad hoc networks will soon appear. Network cards for single-hop ad hoc wireless networks are already on
the market, and these technologies constitute the building blocks to construct small-scale ad hoc networks
that extend the range of single-hop wireless technologies to few kilometers. It is therefore important to
understand the qualitative and quantitative behavior of single-hop ad hoc wireless networks. The first
part of this chapter presents the taxonomy of single-hop wireless technologies. Specifically, we introduce
the concept of Body, Personal, and Local wireless networks, and we discuss their applicative scenarios.
The second part of the chapter focuses on the emerging networking standards for constructing smallscale ad hoc networks: IEEE 802.11 and Bluetooth. The IEEE 802.11 standard is a good platform to
implement a single-hop local ad hoc network because of its extreme simplicity. Furthermore, multi-hop
networks covering areas of several square kilometers could be built by exploiting the IEEE 802.11

technology. On smaller scales, the Bluetooth technologies can be exploited to build ad hoc wireless
Personal and Body Area Networks, i.e., networks that connect devices placed on a person’s body or inside
a small circle around it. The chapter presents the architectures and protocols of IEEE 802.11 and
Bluetooth. In addition, the performance of these two technologies is discussed.

© 2003 by CRC Press LLC


1.1 Introduction
In recent years, the proliferation of mobile computing devices (e.g., laptops, handheld digital devices,
personal digital assistants [PDAs], and wearable computers) has driven a revolutionary change in the
computing world. As shown in Fig. 1.1, we are moving from the Personal Computer (PC) age (i.e., one
computing device per person) to the Ubiquitous Computing age in which individual users utilize, at the
same time, several electronic platforms through which they can access all the required information
whenever and wherever they may be [47]. The nature of ubiquitous devices makes wireless networks the
easiest solution for their interconnection. This has led to rapid growth in the use of wireless technologies
for the Local Area Network (LAN) environment. Beyond supporting wireless connectivity for fixed,
portable, and moving stations within a local area, wireless LAN (WLAN) technologies can provide a
mobile and ubiquitous connection to Internet information services [10]. It is foreseeable that in the notso-distant future, WLAN technologies will be utilized largely as means to access the Internet.
WLAN products consume too much power and have excessive range for many personal consumer
electronic and computer devices [40]. A new class of networks is therefore emerging: Personal Area
Networks. A Personal Area Network (PAN) allows the proximal devices to dynamically share information
with minimum power consumption [49].
LANs and PANs do not meet all the networking requirements of ubiquitous computing. Situations
exist where carrying and holding a computer are not practical (e.g., assembly line work). A wearable
computer solves these problems by distributing computer components (e.g., head-mounted displays,
microphones, earphones, processors, and mass storage) on the body [21,49]. Users can thus receive jobcritical information and maintain control of their devices while their hands remain free for other work.
A network with a transmission range of a human body, i.e., a Body Area Network (BAN), constitutes the
best solution for connecting wearable devices. Wireless connectivity is envisaged as a natural solution
for BANs.

One target of the ubiquitous computing revolution is the ability of the technology to adapt itself to the
user without requiring that users modify their behavior and knowledge. PCs provided to their users a
large set of new services that revolutionized their lives. However, to exploit the PC’s benefits, users have
had to adapt themselves to PC standards. The new trend is to help users in everyday life by exploiting
technology and infrastructures that are hidden in the environment and do not require any major change
in the users’ behavior. This new philosophy is the basis of the ambient intelligence concept [1]. The objective
of ambient intelligence is the integration of digital devices and networks into the everyday environment.
This will render accessible, through easy and “natural” interactions, a multitude of services and applications. Ambient intelligence places the user at the center of the information society. This view heavily relies
on wireless and mobile communications [36,37]. Specifically, advances in wireless communication will

Personal
Computing

Ubiquitous
Computing

FIGURE 1.1 From PC age (one-to-one) to ubiquitous computing (one-to-many).
© 2003 by CRC Press LLC


BAN

PAN

~1m ~10m

LAN

WAN

~500m

Range

FIGURE 1.2 Ad hoc networks taxonomy.

enable a radical new communication paradigm: self-organized information and communication systems
[17]. In this new networking paradigm, the users’ mobile devices are the network, and they must cooperatively provide the functionality that is usually provided by the network infrastructure (e.g., routers,
switches, and servers). Such systems are sometimes referred to as mobile ad hoc networks (MANETs) [33]
or as infrastructure-less wireless networks.

1.2 Mobile Ad Hoc Networks
A Mobile Ad hoc NETwork (MANET) is a system of wireless mobile nodes that dynamically self-organize
in arbitrary and temporary network topologies. People and vehicles can thus be internetworked in areas
without a preexisting communication infrastructure or when the use of such infrastructure requires
wireless extension [17].
As shown in Fig. 1.2, we can classify ad hoc networks, depending on their coverage area, into four
main classes: Body, Personal, Local, and Wide Area Networks.
Wide area ad hoc networks are mobile multi-hop wireless networks. They present many challenges
that are still to be solved (e.g., addressing, routing, location management, security, etc.), and they are
not likely to become available for some time. On smaller scales, mobile ad hoc networks will soon appear
[6]. Specifically, ad hoc, single-hop BAN, PAN, and LAN wireless networks are beginning to appear on
the market. These technologies constitute the building blocks to construct small multi-hop ad hoc
networks that extend the range of the ad hoc networks’ technologies over a few radio hops [16,17].

1.2.1 Body Area Network
A Body Area Network is strongly correlated with wearable computers. The components of a wearable
computer are distributed on the body (e.g., head-mounted displays, microphones, earphones, etc.), and
a BAN provides the connectivity among these devices. Therefore, the main requirements of a BAN are
[18,19]:

1. The ability to interconnect heterogeneous devices, ranging from complete devices (e.g., a mobile
phone) to parts of a device (microphone, display, etc.)
2. Autoconfiguration capability (Adding or removing a device from a BAN should be transparent to
the user.)
3. Services integration (Isochronous data transfer of audio and video must coexist with non–real
time data, e.g., Internet data traffic.)
4. The ability to interconnect with the other BANs (to exchange data with other people) or PANs
(e.g., to access the Internet)
The communicating range of a BAN corresponds to the human body range, i.e., 1–2 meters. As wiring
a body is generally cumbersome, wireless technologies constitute the best solution for interconnecting
wearable devices.
© 2003 by CRC Press LLC


One of the first examples of BAN is the prototype developed by T.G. Zimmerman [48], which could
provide data communication (with rates up to 400,000 bits per second) by exploiting the body as the
channel. Specifically, Zimmerman showed that data can be transferred through the skin by exploiting a
very small current (one billionth of an amp). Data transfer between two persons (i.e., BAN interconnection) could be achieved through a simple handshake.
A less visionary BAN prototype was developed in the framework of the MIThril project where a wired
Ethernet network was adopted to interconnect wearable devices [35].
Marketable examples of BANs have just appeared (see [19], [32], and [38]). These examples consist
of a few electronic devices (phone, MP3 player, headset, microphone, and controller), which are directly
connected by wires integrated within a jacket. In the future, it might be expected that more devices (or
parts of devices) will be connected using a mixture of wireless and wired technologies.

1.2.2 Personal Area Network
Personal area networks connect mobile devices carried by users to other mobile and stationary devices.
While a BAN is devoted to the interconnection of one-person wearable devices (see part a of Fig. 1.3),
a PAN is a network in the environment around the person. A PAN communicating range is typically up
to 10 meters, thus enabling (see part b of Fig. 1.3):

1. The interconnection of the BANs of people close to each other
2. The interconnection of a BAN with the environment around it
Wireless PAN (WPAN) technologies in the 2.4 GHz ISM band are the most promising technologies
for widespread PAN deployment. Spread spectrum is typically employed to reduce interference and utilize
the bandwidth.
The technologies for WPANs offer a wide space for innovative solutions and applications that could
create radical changes in everyday life. We can foresee that a WPAN interface will be embedded not only
in such devices as cellular phones, mobile computers, PDAs, and so on, but in every digital device. Obvious
applications arise from the possibility of forming ad hoc networks with our workspace electronic devices,
e.g., a PDA that automatically synchronizes with the desktop computer to transfer e-mail, files, and
schedule information. Moreover, PANs make possible the design of innovative pervasive applications. For
example, let us imagine a PDA (with a PAN interface) that upon your arrival at a location (e.g., home,
office, airport, etc.) automatically synchronizes (via the PAN network interface) with all the electronic
devices within its 10-meter range. For example, when you arrive at home, your PDA can automatically
unlock the door, turn on the house lights while you are getting in, and adjust the heat or air conditioning
to your preset preferences. Similarly, when you arrive at the airport you can avoid the line at the checkin desk by using your handheld device to present an electronic ticket and automatically select your seat.

(a)

(b)

FIGURE 1.3 Relationship between a Body (part a) and a Personal Area Network (part b).
© 2003 by CRC Press LLC


Serv er

Legend:

(a)

(b)

FIGURE 1.4 WLAN configurations: (a) ad hoc networking; (b) infrastructure-based.

access point
wired network

1.2.3 Wireless Local Area Network
In the last few years, the use of wireless technologies in the LAN environment has become more and
more important, and it is easy to foresee that the wireless LANs (WLANs) — as they offer greater flexibility
than wired LANs — will be the solution for home and office automation.
Like wired LANs, a WLAN has a communication range typical of a single building or a cluster of
buildings, i.e., 100–500 meters.
A WLAN should satisfy the same requirements typical of any LAN, including high capacity, full
connectivity among attached stations, and broadcast capability. However, to meet these objectives,
WLANs should be designed to face some issues specific to the wireless environment, such as security on
the air, power consumption, mobility, and bandwidth limitation of the air interface [39].
Two different approaches can be followed in the implementation of a WLAN (see Fig. 1.4): an infrastructure-based approach or an ad hoc networking approach [39]. An infrastructure-based architecture
imposes the existence of a centralized controller for each cell, often referred to as the Access Point (see
Fig. 1.4b). The Access Point is normally connected to the wired network, thus providing Internet access
to mobile devices. In contrast, an ad hoc network is a peer-to-peer network formed by a set of stations
within the range of each other that dynamically configure themselves to set up a temporary network (see
Fig. 1.4a). In the ad hoc configuration, no fixed controller is required, but a controller is dynamically
elected among all the stations participating in the communication.

1.3 Technologies for Ad Hoc Networks
The success of a network technology is connected to the development of networking products at a competitive price. A major factor in achieving this goal is the availability of appropriate networking standards.
Currently, two main standards are emerging for ad hoc wireless networks: the IEEE 802.11 standard for
WLANs [25] and the Bluetooth specifications1 for short-range wireless communications [3,4,34].

The IEEE 802.11 standard is a good platform for implementing a single-hop WLAN ad hoc network
because of its extreme simplicity. Multi-hop networks covering areas of several square kilometers could
also be built by exploiting the IEEE 802.11 technology. On smaller scales, technologies such as Bluetooth
can be used to build ad hoc wireless Body and Personal Area Networks, i.e., networks that connect devices
on the person, or placed around the person inside a circle with a radius of 10 meters.

1

The Bluetooth specifications are released by the Bluetooth Special Interest Group [12].

© 2003 by CRC Press LLC


Here we present the architecture and protocols of IEEE 802.11 and Bluetooth. In addition, the performances of these technologies are analyzed. Two main performance indices will be considered: the
throughput and the delay.
As far as throughput is concerned, special attention will be paid to the Medium Access Control (MAC)
protocol capacity [15,30], defined as the maximum fraction of channel bandwidth used by successfully
transmitted messages. This performance index is important because the bandwidth delivered by wireless
networks is much lower than that of wired networks, e.g., 1–11 Mb/sec vs. 100–1000 Mb/sec [39]. Since
a WLAN relies on a common transmission medium, the transmissions of the network stations must be
coordinated by the MAC protocol. This coordination can be achieved by means of control information
that is carried explicitly by control messages traveling along the medium (e.g., ACK messages) or can be
provided implicitly by the medium itself using the carrier sensing to identify the channel as either active
or idle. Control messages or message retransmissions due to collision remove channel bandwidth from
that available for successful message transmission. Therefore, the capacity gives a good indication of the
overheads required by the MAC protocol to perform its coordination task among stations or, in other
words, of the effective bandwidth that can be used on a wireless link for data transmission.
The delay can be defined in several forms (access delay, queuing delay, propagation delay, etc.) depending on the time instants considered during its measurement (see [15]). In computer networks, the
response time (i.e., the time between the generation of a message at the sending station and its reception
at the destination station) is the best value to measure the Quality of Service (QoS) perceived by the

users. However, the response time depends on the amount of buffering inside the network, and it is not
always meaningful for the evaluation of a LAN technology. For example, during congested periods, the
buffers fill up, and thus the response time does not depend on the LAN technology but it is mainly a
function of the buffer length. For this reason, hereafter, the MAC delay index is used. The MAC delay
of a station in a LAN is defined as the time between the instant at which a packet comes to the head of
the station transmission queue and the end of the packet transmission [15].

1.4 IEEE 802.11 Architecture and Protocols
In 1997, the IEEE adopted the first wireless local area network standard, named IEEE 802.11, with data
rates up to 2 Mb/sec [27]. Since then, several task groups (designated by letters) have been created to
extend the IEEE 802.11 standard. Task groups 802.11b and 802.11a have completed their work by
providing two relevant extensions to the original standard [25]. The 802.11b task group produced a
standard for WLAN operations in the 2.4 GHz band, with data rates up to 11 Mb/sec. This standard,
published in 1999, has been very successful. Currently, there are several IEEE 802.11b products available
on the market. The 802.11a task group created a standard for WLAN operations in the 5 GHz band,
with data rates up to 54 Mb/sec. Among the other task groups, it is worth mentioning task group 802.11e
(which attempts to enhance the MAC with QoS features to support voice and video over 802.11 networks)
and task group 802.11g (which is working to develop a higher-speed extension to 802.11b).
The IEEE 802.11 standard specifies a MAC layer and a physical layer for WLANs (see Fig. 1.5). The
MAC layer provides to its users both contention-based and contention-free access control on a variety
of physical layers. Specifically, three different technologies can be used at the physical layer: infrared,
frequency hopping spread spectrum, and direct sequence spread spectrum [27].
The basic access method in the IEEE 802.11 MAC protocol is the Distributed Coordination Function
(DCF), which is a Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) MAC protocol.
Besides the DCF, the IEEE 802.11 also incorporates an alternative access method known as the Point
Coordination Function (PCF). The PCF operates similarly to a polling system [15]; a point coordinator
provides (through a polling mechanism) the transmission rights at a single station at a time. As the PCF
access method cannot be adopted in ad hoc networks, in the following we will concentrate on the DCF
access method only.

© 2003 by CRC Press LLC


contention free
services

contention
services

Point Coordination
Function
Distributed Coordination
Function

Physical Layer
FIGURE 1.5 IEEE 802.11 architecture.
Source
DIFS

EIFS

DIFS

DATA

LA

SI

FS

A
ACK

Destination

B
DIFS

C

Other
NAV

(a)

CW

LB

CWA
CW B

LC

CWC

Collision Length =L A

(b)

FIGURE 1.6 IEEE 802.11 DCF: (a) a successful transmission; (b) a collision.

1.4.1 IEEE 802.11 DCF
The DCF access method, hereafter referred to as Basic Access, is summarized in Fig. 1.6. When using the
DCF, before a station initiates a transmission, it senses the channel to determine whether another station
is transmitting. If the medium is found to be idle for an interval that exceeds the Distributed InterFrame
Space (DIFS), the station continues with its transmission.2 The transmitted packet contains the projected
length of the transmission. Each active station stores this information in a local variable named Network
Allocation Vector (NAV). Therefore, the NAV contains the period of time the channel will remain busy
(see Fig. 1.6a).3
The CSMA/CA protocol does not rely on the capability of the stations to detect a collision by hearing
their own transmissions. Hence, immediate positive acknowledgments are employed to ascertain the
successful reception of each packet transmission. Specifically, the receiver after the reception of the data
frame (1) waits for a time interval, called the Short InterFrame Space (SIFS), which is less than the DIFS,
and then (2) initiates the transmission of an acknowledgment (ACK) frame. The ACK is not transmitted
if the packet is corrupted or lost due to collisions. A Cyclic Redundancy Check (CRC) algorithm is
adopted to discover transmission errors. Collisions among stations occur when two or more stations
start transmitting at the same time (see Fig. 1.6b). If an acknowledgment is not received, the data frame
is presumed to have been lost, and a retransmission is scheduled.

2

To guarantee fair access to the shared medium, a station that has just transmitted a packet and has another
packet ready for transmission must perform the backoff procedure before initiating the second transmission.
3 This prevents a station from listening to the channel during transmissions. This feature is useful to implement
(among others) power-saving policies.
© 2003 by CRC Press LLC

After an erroneous frame is detected (due to collisions or transmission errors), the channel must
remain idle for at least an Extended InterFrame Space (EIFS) interval before the stations reactivate the
backoff algorithm to schedule their transmissions (see Fig. 1.6b).
To reduce the collision probability, the IEEE 802.11 uses a mechanism (backoff mechanism) that
guarantees a time spreading of the transmissions.
When a station S, with a packet ready for transmission, observes a busy channel, it defers the
transmission until the end of the ongoing transmission. At the end of the channel busy period, the
station S initializes a counter (called the backoff timer) by selecting a random interval (backoff interval)
for scheduling its transmission attempt. The backoff timer is decreased for as long as the channel is
sensed as idle, stopped when a transmission is detected on the channel, and reactivated when the channel
is sensed as idle again for more than a DIFS. The station transmits when the backoff timer reaches zero.
Specifically, the DCF adopts a slotted binary exponential backoff technique. The time immediately
following an idle DIFS or EIFS is slotted, and a station is allowed to transmit only at the beginning of
each Slot Time.4 The backoff time is uniformly chosen in the interval (0, CW – 1), defined as the Backoff
Window, also referred to as the Contention Window. At the first transmission attempt CW = CWmin, and
then CW is doubled at each retransmission up to CWmax. The CWmin and CWmax values depend on the
physical layer adopted. For example, for the frequency hopping, CWmin and CWmax are 16 and 1024,
respectively [27].
An IEEE 802.11 WLAN can be implemented with the access points (i.e., infrastructure based) or with
the ad hoc paradigm. In the IEEE 802.11 standard, an ad hoc network is called an Independent Basic
Service Set (IBSS). An IBSS enables two or more IEEE 802.11 stations to communicate directly without
requiring the intervention of a centralized access point or an infrastructure network. Due to the flexibility
of the CSMA/CA algorithm, synchronization (to a common clock) of the stations belonging to an IBSS
is sufficient for correct receipt or transmission of data. The IEEE 802.11 uses two main functions for the
synchronization of the stations in an IBSS: (1) synchronization acquisition and (2) synchronization
maintenance.
Synchronization Acquisition — This functionality is necessary for joining an existing IBSS. The
discovery of existing IBSSs is the result of a scanning procedure of the wireless medium. During the
scanning, the station receiver is tuned on different radio frequencies, searching for particular control
frames. Only if the scanning procedure does not result in finding any IBSS may the station initialize a

new IBSS.
Synchronization Maintenance — Because of the lack of a centralized station that provides its own
clock as common clock, the synchronization function is implemented via a distributed algorithm that
shall be performed by all of the members of the IBSS. This algorithm is based on the transmission of
beacon frames at a known nominal rate. The station that initialized the IBSS decides the beacon interval.
1.4.1.1 IEEE 802.11 DCF Performance
In this section we present a performance analysis of the IEEE 802.11 basic access method by analyzing
the two main performance indices: the capacity and the MAC delay. The physical layer technology
determines some network parameter values relevant for the performance study, e.g., SIFS, DIFS, backoff,
and slot time. Whenever necessary, we choose the values of these technology-dependent parameters by
referring to the frequency hopping spread spectrum technology at a transmission rate of 2 Mb/sec.
Specifically, Table 1.1 reports the configuration parameter values of the IEEE 802.11 WLAN analyzed in
this chapter [27].
1.4.1.1.1 Protocol Capacity
The IEEE 802.11 protocol capacity was extensively investigated in [14]. The main results of that analysis
are summarized here. Specifically, in [14] the theoretical throughput limit for IEEE 802.11 networks was
4

A slot time is equal to the time needed at any station to detect the transmission of a packet from any other station.
© 2003 by CRC Press LLC

www.allitebooks.com


TABLE 1.1
Parameter
Value

IEEE 802.11 Parameter Values
tslot

50 µsec

τ
≤1 µsec

DIFS
2.56 tslot

EIFS
340 µsec

SIFS
0.56 tslot

ACK
240 bits

CWmin

CWmax

8 tslot

256 tslot

Bit Rate
2 Mb/sec

analytically derived,5 and this limit was compared with the simulated estimates of the real protocol
capacity. The results showed that, depending on the network configuration, the standard protocol can

operate very far from the theoretical throughput limit. These results, summarized in Fig. 1.7a, indicate
that the distance between the IEEE 802.11 and the analytical bound increases with the number of active
stations, M. In the IEEE 802.11 protocol, due to its backoff algorithm, the average number of stations
that transmit in a slot increases with M, and this causes an increase in the collision probability. A
significant improvement of the IEEE 802.11 performance can thus be obtained by controlling the number
of stations that transmit in the same slot.
Several works have shown that an appropriate tuning of the IEEE 802.11 backoff algorithm can
significantly increase the protocol capacity [2,13,46]. In particular, in [13], a distributed algorithm to
tune the size of the backoff window at run time, called Dynamic IEEE 802.11 Protocol, was presented and
evaluated. Specifically, by observing the status of the channel, each station gets an estimate of both the
number of active stations and the characteristics of the network traffic. By exploiting these estimates,
each station then applies a distributed algorithm to tune its backoff window size in order to achieve the
theoretical throughput limit for the IEEE 802.11 network.
The Dynamic IEEE 802.11 Protocol is complex due to the interdependencies among the estimated
quantities [13]. To avoid this complexity, in [7] a Simple Dynamic IEEE 802.11 Protocol is proposed and
evaluated. It requires only simple load estimates for tuning the backoff algorithm. An alternative and
interesting approach for tuning the backoff algorithm, without requiring complex estimates of the
network status, has been proposed in [5]. In this work a distributed mechanism is defined, called
Asymptotically Optimal Backoff (AOB), which dynamically adapts the backoff window size to the current
load. AOB guarantees that an IEEE 802.11 WLAN asymptotically (i.e., for a large number of active
stations) achieves its optimal channel utilization. The AOB mechanism adapts the backoff window to
the network contention level by using two load estimates: the slot utilization and the average size of
transmitted frames. These estimates are simple and can be obtained with no additional costs or overheads.
It is worth noting that the above mechanisms that tune the IEEE 802.11 protocol to optimize the
protocol capacity also guarantee quasi-optimal behavior from the energy consumption standpoint (i.e.,
minimum energy consumption). Indeed, in [11] it is shown that the optimal capacity state and the
optimal energy consumption state almost coincide.
1.4.1.1.2 MAC Delay
The IEEE 802.11 capacity analysis presented in the previous section is performed by assuming that the
network operates in asymptotic conditions (i.e., each LAN station always has a packet ready for transmission). However, LANs normally operate in normal conditions, i.e., the network stations generate an

aggregate traffic that is lower (or slightly higher) than the maximum traffic the network can support. In
these load conditions, the most meaningful performance figure is the MAC delay (see Section 1.3 and
[15]). Two sets of MAC delay results are presented here, corresponding to traffic generated by 50 stations,
made up of short (two slots) and long (100 slots) messages, respectively. Stations alternate between idle
and busy periods. In the simulative experiments, the channel utilization level is controlled by varying
the idle periods’ lengths.
Figure 1.7b (which plots the average MAC delay vs. the channel utilization) highlights that, for light
load conditions, the IEEE 802.11 exhibits very low MAC delays. However, as the offered load approaches
5 That is, the maximum throughput that can be achieved by adopting the IEEE 802.11 MAC protocol and using
the optimal tuning of the backoff algorithm.

© 2003 by CRC Press LLC


1

Capacity

M=5
0.6

M=10

0.4

M=50

0.2

M=100

MAC delay (µsec)

2E+04

Theoretical bound

0.8

2E+04

2 slots

2E+04

100 slots

1E+04
5E+03
0E+00

0

0

0

25

50

75

Packet size (slots)

100

0.1

0.2

0.3

0.4

0.5

Channel utilization

(a)

(b)

FIGURE 1.7 IEEE 802.11 performance: (a) protocol capacity; (b) average MAC delay

the capacity of the protocol (see Fig. 1.7a), the MAC delay sharply increases. This behavior is due to the
CSMA/CA protocol. Under light-load conditions, the protocol introduces almost no overhead (a station
can immediately transmit as soon as it has a packet ready for transmission). On the other hand, when
the load increases, the collision probability increases as well, and most of the time a transmission results
in a collision. Several transmission attempts are necessary before a station is able to transmit a packet,

and hence the MAC delay increases. It is worth noting that the algorithms discussed in the previous
section (i.e., SDP, AOB, etc.) for optimizing the protocol capacity also help prevent MAC delays from
becoming unbounded when the channel utilization approaches the protocol capacity (see [5] and [7]).

1.4.2 IEEE 802.11 RTS/CTS
The design of a WLAN that adopts a carrier-sensing random access protocol [24], such as the IEEE
802.11, is complicated by the presence of hidden terminals [42]. A pair of stations is referred to as being
hidden from each other if a station cannot hear the transmission from the other station. This event makes
the carrier sensing unreliable, as a station wrongly senses that the wireless medium has been idle while
the other station (which is hidden from its standpoint) is transmitting. For example, as shown in Fig. 1.8,
let us assume that two stations, say S1 and S2, are hidden from each other, and both wish to transmit to
a third station, named Receiver. When S1 is transmitting to Receiver, the carrier sensing of S2 does not
trigger any transmission, and thus S2 can immediately start a transmission to Receiver as well. Obviously,
this event causes a collision that never occurs if the carrier sensing works properly.
The hidden stations phenomenon may occur in both infrastructure-based and ad hoc networks.
However, it may be more relevant in ad hoc networks where almost no coordination exists among the
stations. In this case, all stations may be transmitting on a single frequency, as occurs in the WaveLAN
IEEE 802.11 technology [45].
To avoid the hidden terminal problem, the IEEE 802.11 basic access mechanism was extended with a
virtual carrier sensing mechanism, called Request To Send (RTS)/Clear To Send (CTS).
In the RTS/CTS mechanism, after access to the medium is gained and before transmission of a data
packet begins, a short control packet, called RTS, is sent to the receiving station announcing the upcoming
transmission. The receiver replies to this with a CTS packet to indicate readiness to receive the data. RTS
and CTS packets contain the projected length of the transmission. This information is stored by each
active station in its NAV, the value of which becomes equal to the end of the channel busy period.
Therefore, all stations within the range of at least one of the two stations (receiver and transmitter) know
how long the channel will be used for this data transmission (see Fig. 1.9).
The RTS/CTS mechanism solves the hidden station problem during the transmission of user data. In
addition, this mechanism can be used to capture the channel control before the transmission of long
packets, thus avoiding “long collisions.” Collisions may occur only during the transmissions of the small

RTS and CTS packets. Unfortunately, as shown in the next section, other phenomena occur at the physical
layer making the effectiveness of the RTS/CTS mechanism quite arguable.

© 2003 by CRC Press LLC


Sender S1

Receiver
Sender S2

FIGURE 1.8 The hidden stations phenomenon.
SIFS

DIFS
Source

RTS

DATA
SIFS

Destination

SIFS
ACK

CTS

DIFS

Other stations
NAV RTS

Contention Window

NAV CTS
NAV DATA

BACKOFF

access to the medium is deferred

FIGURE 1.9 The RTS/CTS mechanism.

1.4.2.1 RTS/CTS Effectiveness in Ad Hoc Networks
The effectiveness of the RTS/CTS mechanism was studied in [44] in a real field trial. The main results
of that study are summarized here. The testbed analyzed the performance of the TCP protocol over an
IEEE 802.11 ad hoc network. To reduce the complexity of the study, static ad hoc networks were
considered, i.e., the network nodes did not change their positions during an experiment. Both indoor
and outdoor scenarios were investigated.
1.4.2.1.1 Indoor Experiments
In this case the experiments were performed in a scenario characterized by hidden stations. The scenario
is shown in Fig. 1.10. Nodes 1, 2, and 3 are transferring data, via ftp, toward node 4. As these data transfers
are supported by the TCP protocol, in the following the data flows will be denoted as TCP #i, where i is
the index of the transmitting station.
In the analyzed scenario, a reinforced concrete wall (the black rectangle in the figure) is located between
node 1 and node 2 and between node 2 and node 3. As a consequence, the three transmitting nodes are
hidden from each other, e.g., nodes 2 and 3 are outside the transmission range of node 1.6 Node 4 is in
the transmission range of all the other nodes.
Two sets of experiments were performed using the DCF mechanism with or without the RTS/CTS

mechanism. In Table 1.2, the results of the experiments are summarized. Two main conclusions can be
reached from these experiments:
1. No significant performance differences exist between adopting the RTS/CTS mechanism vs. the
basic access mechanism only.
2. Due to the additional overheads of the RTS and CTS packets, the aggregate network throughput
with the RTS/CTS mechanism is a bit lower with respect to the basic access mechanism.

6

Specifically, the ping application indicated no delivered packet.

© 2003 by CRC Press LLC


1

2

3

4
FIGURE 1.10 Indoor scenario.

TABLE 1.2

Indoor Results — Throughput (Kbytes/sec)

No RTS/CTS
RTS/CTS

TCP #1

TCP #2

TCP #3

Aggregate

42
34

29.5
27

57
48

128.5
109

These results seem to indicate that the carrier sensing mechanism is still effective even if transmitting
stations are “apparently” hidden from each other. Indeed, a distinction must be made between transmission range, interference range, and carrier sensing range, as follows:
• The Transmission Range (TX_Range) represents the range (with respect to the transmitting
station) within which a transmitted packet can be successfully received. The transmission range
is mainly determined by the transmission power and the radio propagation properties.
• The Physical Carrier Sensing Range (PCS_Range) is the range (with respect to the transmitting
station) within which the other stations detect a transmission.
• The Interference Range (IF_Range) is the range within which stations in receive mode will be
“interfered with” by a transmitter and thus suffer a loss. The interference range is usually larger
than the transmission range, and it is a function of the distance between the sender and receiver

and of the path loss model.
Normally, the following relationship exists between the transmission, carrier sensing, and interference
ranges: TX_Range ≤ IF_Range ≤ PCS_Range.7 The relationship among TX_Range, IF_Range, and PCS_Range
helps in explaining the results obtained in the indoor experiments: even though transmitting nodes are
outside the transmission range of each other, they are inside the same carrier sensing range. Therefore, the
physical carrier sensing is effective, and hence adding a virtual carrier sensing (i.e., RTS/CTS) is useless.
1.4.2.1.2 Outdoor Experiments
The reference scenario for this case is shown in Fig. 1.11. The nodes represent four portable computers,
each with an IEEE 802.11 network interface. Two ftp sessions are contemporary active. The arrows
represent the direction of the ftp sessions.
Several experiments were performed by varying the transmission, the carrier sensing, and the interference ranges. This was achieved by modifying the distance, d, between nodes 2 and 3. In all the
experiments, the receiving node was always within the transmission range of its transmitting node —

7

For example, in NS2 the following values are used: TX_Range = 250 m, IF_Range = PCS_Range = 550 m.

© 2003 by CRC Press LLC


1

TCP #1

TCP #2

2

3

4

d

FIGURE 1.11 Outdoor reference scenario.

TABLE 1.3

Outdoor Results — Throughput (Kbytes/sec)
Exp #1

No RTS/CTS
RTS/CTS

TCP #1
61
59.5

Exp #2
TCP #2
54
49.5

TCP #1
123
81

Exp #3
TCP #2
0.5

6.5

TCP #1
122.5
96

TCP #2
122
100

i.e., node 2 (4) was within the transmitting range of node 1 (3) — while, by varying the distance d, the
other two nodes8 could be:
1. In the same transmitting range (Exp #1)
2. Out of the transmitting range but inside the same carrier sensing range (Exp #2)
3. Out of the same carrier sensing range (Exp #3)
The achieved results, summarized in Table 1.3, show the following:
• Exp #1. In this case (all stations are inside the same TX_Range), a fair bandwidth sharing is almost
obtained: the two ftp sessions achieve (almost) the same throughput. The RTS/CTS mechanism
is useless as (due to its overheads) it only reduces the throughput.
• Exp #3. In this case the two sessions are independent (i.e., outside their respective carrier sensing
ranges), and both achieve the maximum throughput. The RTS/CTS mechanism is useless as (due
to its overheads) it only reduces the throughput.
• Exp #2. In the intermediate situation, a “capture” of the channel by one of the two TCP connections
is observed. In this case, the RTS/CTS mechanism provides a little help in solving the problem.
The experimental results confirm the results on TCP unfairness in ad hoc IEEE 802.11 obtained, via
simulation, by several researchers, e.g., see [43]. As discussed in previous works, the TCP protocol, due
to flow control and congestion mechanisms, introduces correlations in the transmitted traffic that
emphasize/generate the capture phenomena. This effect is clearly pointed out by experimental results
presented in Table 1.4. Specifically, the table reports results obtained in the Exp #2 configuration when
the traffic flows are either TCP or UDP based. As shown in the table, the capture effect disappears when

the UDP protocol is used.
To summarize, measurement experiments have shown that, in some scenarios, TCP connections may
suffer significant throughput unfairness, even capture. The causes of this behavior are the hidden terminal
problem, the 802.11 backoff scheme, and large interference ranges. We expect that the methods discussed
in the section “IEEE 802.11 DCF Performance” for optimizing the IEEE 802.11 protocol capacity area
are moving in a promising direction to solve the TCP unfairness in IEEE 802.11 ad hoc networks. Research
activities are ongoing to explore this direction.

8

That is, the couple (3,4) with respect to the couple (1,2) and vice versa.

© 2003 by CRC Press LLC