![2 Adaptation and Cross Layer Design in Wireless Networks
1.1
Introduction
The IEEE 802.11 protocol [1] has become the predominant technology for wireless local
area networks (WLANs). One of the most important elements of the 802.11 in terms of
performance is the medium access control (MAC). The MAC protocol is used to provide
arbitrated access to a shared medium, in which several terminals access and compete for
the radio spectrum. The design of the MAC protocols is often application dependent,
and it is closely linked to the characteristics of the medium in which it operates. It also
determines the performance and quality of service (QoS) metrics of the network, such as
throughput, stability, and delay. The IEEE 802.11 wireless networks employ the distributed
coordination function (DCF) as a primary medium access mechanism. It is based on the
carrier-sensing multiple-access with collision avoidance (CSMA/CA) protocol and binary
exponential backoff [2, 3].
Several studies have shown that the DCF protocol is very sensitive to the number of
competing terminals that access the wireless channel [2–7], and that a way to optimize
the network performance is to make the parameters of the backoff window depend on
the number of terminals competing for the medium. However, estimating the number
of competing terminals is not an easy task. While a terminal could cache the identity
of the past senders in the network, the number of competing terminals is the number of
terminals that have data to send at any given time, so a simple list of neighbors is not suf-
ficient. The estimation of the number of competing terminals faces two problems. First,
the number of competing terminals is a non-Gaussian nonlinear dynamic system that
is difficult to track accurately with conventional filters. Advanced estimators such as the
extended Kalman filter (EKF)-based one from [3] provide better results, but they are
subject to critics due to their complexity [8]. Second, the performance of the IEEE 802.11
DCF is extremely sensitive to the number of competing terminals [2], particularly in the
typical operating point of one to fifteen terminals. This makes approximate methods
such as [5, 9, 10] to yield suboptimal operation of the protocol compared with the theo-
retical optimum. Hence, there is a need for an accurate estimation algorithm that is able
to efficiently track the number of competing terminals in an IEEE 802.11 network and,
at the same time, is easy to implement. As we will see, sequential Monte Carlo methods
are appropriate for this purpose.
Bayesian Monte Carlo signal processing techniques [11, 12] offer a paradigm for tack-
ling challenging signal processing problems for which traditional methods are difficult
to apply. Among them, the sequential Monte Carlo (SMC) methodology [12] has been
shown to be extremely powerful in dealing with filtering problems in non-Gaussian
and nonlinear complex dynamic systems, where conventional approaches fail to work.
In order to tackle the above estimation of the competing terminals, we develop several
SMC-based adaptive estimators that outperform the existing classical estimator, such as
those based on the extended Kalman filter (EKF) [3]. In particular, we develop a deter-
ministic variant of the SMC estimator that is simpler to implement and offers superior
performance, by including a set of sufficient statistics about the unknown parameters in
the deterministic sample filter setting proposed in [13]. The use of sufficient statistics is
pushed one step further than in [14] because this information about the parameters is](https://image.slidesharecdn.com/25840-250317130042-698d6876/85/Adaptation-and-Cross-Layer-Design-in-Wireless-Networks-1st-Edition-Mohamed-Ibnkahla-24-320.jpg)
![Adaptive Optimization of CSMA/CA MAC Protocols 3
now integrated out so that no Monte Carlo approximation needs to be performed. We
further extend it into a maximum a posteriori (MAP) estimator whose computational
load and memory requirements are equivalent to those of the well-known Viterbi algo-
rithm. We will see that our SMC-based algorithms overcome both of the problems men-
tioned above: accuracy and ease of implementation. We show that the accuracy of the
Bayesian algorithms is particularly good at small timescales, which makes our proposal
attractive to optimize the protocol when the terminals are in a nonsaturation regime,
a problem usually not addressed in the literature. Finally, we propose an optimization
mechanism that uses the predictive distribution of the number of competing terminals
to adapt the parameters of the IEEE 802.11 DCF protocol and maximize its throughput.
1.2
The IEEE 802.11 Distributed
Coordination Function
1.2.1
The DCF Protocol
The DCF defines two distinct techniques to access the medium: basic access and
RTS/CTS access.
1.2.1.1
Basic Access
In basic access, the terminals implement a two-way handshake mechanism (Figure 1.1).
A terminal senses the channel to be idle before starting a transmission. If the channel
is idle for at least a period of distributed interframe space (DIFS), then the terminal is
allowed to transmit. If during this sensing time the channel appears to be busy at any
time, the terminal defers the transmission and enters into the collision avoidance (CA)
mode. In CA mode the terminal generates a random backoff interval during which it
waits before attempting another transmission. This random backoff is used to minimize
the probability of collision between terminals accessing the medium. The idle time after
waiting a DIFS interval is slotted, and the terminals are only allowed to transmit at the
beginning of the slot time. The slot time size σ accounts for the time the signal is propa-
gating, and is set equal to the time needed for any terminal to detect the transmission of
a packet from any other terminal [2]. If this time were not accounted for, a terminal could
assess the channel as idle when the data sent by another terminal have not yet arrived.
DATA
ACK
SIFS
Defer Access
NAV
Source
Destination
Other
DIFS Contention
Contention
DATA
ACK
SIFS
Defer Access
Data NAV
RTS NAV
DIFS
RTS
SIFS
CTS
SIFS
CTS NAV
Basic Access RTS/CTS Access
Fig u r e 1.1 IEEE 802.11 MAC access mechanisms.](https://image.slidesharecdn.com/25840-250317130042-698d6876/85/Adaptation-and-Cross-Layer-Design-in-Wireless-Networks-1st-Edition-Mohamed-Ibnkahla-25-320.jpg)
Adaptation and Cross Layer Design in Wireless Networks 1st Edition Mohamed Ibnkahla
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- 5. Adaptation and Cross Layer Design in Wireless Networks 1st Edition Mohamed Ibnkahla Digital Instant Download Author(s): Mohamed Ibnkahla ISBN(s): 9781420046038, 1420046039 Edition: 1 File Details: PDF, 16.23 MB Year: 2008 Language: english
- 7. Adaptation in Wireless Communications Edited by Mohamed Ibnkahla ADAPTIVE SIGNAL PROCESSING in WIRELESS COMMUNICATIONS ADAPTATION and CROSS LAYER DESIGN in WIRELESS NETWORKS
- 9. THE ELECTRICAL ENGINEERING AND APPLIED SIGNAL PROCESSING SERIES Edited by Alexander Poularikas The Advanced Signal Processing Handbook: Theory and Implementation for Radar, Sonar, and Medical Imaging Real-Time Systems Stergios Stergiopoulos The Transform and Data Compression Handbook K.R. Rao and P.C. Yip Handbook of Multisensor Data Fusion David Hall and James Llinas Handbook of Neural Network Signal Processing Yu Hen Hu and Jenq-Neng Hwang Handbook of Antennas in Wireless Communications Lal Chand Godara Noise Reduction in Speech Applications Gillian M. Davis Signal Processing Noise Vyacheslav P. Tuzlukov Digital Signal Processing with Examples in MATLAB® Samuel Stearns Applications in Time-Frequency Signal Processing Antonia Papandreou-Suppappola The Digital Color Imaging Handbook Gaurav Sharma Pattern Recognition in Speech and Language Processing Wu Chou and Biing-Hwang Juang Propagation Handbook for Wireless Communication System Design Robert K. Crane Nonlinear Signal and Image Processing: Theory, Methods, and Applications Kenneth E. Barner and Gonzalo R. Arce Smart Antennas Lal Chand Godara Mobile Internet: Enabling Technologies and Services Apostolis K. Salkintzis and Alexander Poularikas Soft Computing with MATLAB® Ali Zilouchian Wireless Internet: Technologies and Applications Apostolis K. Salkintzis and Alexander Poularikas Signal and Image Processing in Navigational Systems Vyacheslav P. Tuzlukov
- 10. Medical Image Analysis Methods Lena Costaridou MIMO System Technology for Wireless Communications George Tsoulos Signals and Systems Primer with MATLAB® Alexander Poularikas Adaptation in Wireless Communications - 2 volume set Mohamed Ibnkahla
- 11. ADAPTATION AND CROSS LAYER DESIGN IN WIRELESS NETWORKS Edited by Mohamed Ibnkahla CRC Press is an imprint of the Taylor & Francis Group, an informa business Boca Raton London New York
- 12. CRC Press Taylor & Francis Group 6000 Broken Sound Parkway NW, Suite 300 Boca Raton, FL 33487-2742 © 2009 by Taylor & Francis Group, LLC CRC Press is an imprint of Taylor & Francis Group, an Informa business No claim to original U.S. Government works Printed in the United States of America on acid-free paper 10 9 8 7 6 5 4 3 2 1 International Standard Book Number-13: 978-1-4200-4603-8 (Hardcover) This book contains information obtained from authentic and highly regarded sources. Reasonable efforts have been made to publish reliable data and information, but the author and publisher can- not assume responsibility for the validity of all materials or the consequences of their use. The authors and publishers have attempted to trace the copyright holders of all material reproduced in this publication and apologize to copyright holders if permission to publish in this form has not been obtained. If any copyright material has not been acknowledged please write and let us know so we may rectify in any future reprint. Except as permitted under U.S. Copyright Law, no part of this book may be reprinted, reproduced, transmitted, or utilized in any form by any electronic, mechanical, or other means, now known or hereafter invented, including photocopying, microfilming, and recording, or in any information storage or retrieval system, without written permission from the publishers. For permission to photocopy or use material electronically from this work, please access www.copy- right.com (http://www.copyright.com/) or contact the Copyright Clearance Center, Inc. (CCC), 222 Rosewood Drive, Danvers, MA 01923, 978-750-8400. CCC is a not-for-profit organization that pro- vides licenses and registration for a variety of users. For organizations that have been granted a photocopy license by the CCC, a separate system of payment has been arranged. Trademark Notice: Product or corporate names may be trademarks or registered trademarks, and are used only for identification and explanation without intent to infringe. Library of Congress Cataloging-in-Publication Data Adaptation and cross layer design in wireless networks / editor, Mohamed Ibnkahla. p. cm. -- (Electrical engineering and applied signal processing series ; 21) Includes bibliographical references and index. ISBN 978-1-4200-4603-8 (alk. paper) 1. Wireless communication systems. I. Ibnkahla, Mohamed. II. Title. III. Series. TK5103.2.A355 2008 621.384--dc22 2008025417 Visit the Taylor & Francis Web site at http://www.taylorandfrancis.com and the CRC Press Web site at http://www.crcpress.com
- 13. vii Contents 1. Adaptive Optimization of CSMA/CA MAC Protocols Based on Bayesian State Estimation Alberto Lopez Toledo, Tom Vercauteren, Xiaodong Wang.................................................1 2. A Survey of Medium Access Control Protocols for Wireless Local and Ad Hoc Networks Tiantong You, Hossam Hassanein, Chi-Hsiang Yeh............................................39 3. Adaptive Scheduling for Beyond 3G Cellular Networks Sameh Sorour, Shahrokh Valaee...................................................85 4. Adaptive Resource Allocation in CDMA Cellular Wireless Mobile Networks under Time-Varying Traffic: A Transient Analysis-Based Approach Dusit Niyato, Ekram Hossain....... 121 5. Utility-Based Bandwidth Adaptation for Multimedia Wireless Networks Ning Lu, John Bigham, Nidal Nasser....... 149 6. An Extensive Survey and Taxonomy of MAC Protocols for Vehicular Wireless Networks Hamid Menouar, Fethi Filali, Massimiliano Lenardi............................................. 183 7. Network Coding for Wireless Networks Yunnan Wu............ 213 8. A Survey of Wireless Sensor Networks: Technologies, Challenges, and Future Trends Ali Alemdar, Mohamed Ibnkahla. ....................................................................243 9. Adaptive Routing in Wireless Sensor Networks Hong Luo, Guohua Zhang, Yonghe Liu, Sajal K. Das..................................263 10. Coverage and Connectivity in Wireless Sensor Networks: Lifetime Maximization Ananthram Swami, Qing Zhao......... 301
- 14. viii Contents 11. Routing in Wireless Self-Organizing Networks Marcelo Dias de Amorim, Farid Benbadis, Mihail S. Sichitiu, Aline Carneiro Viana, Yannis Viniotis......................................325 12. Selfishness in MANETs Younghwan Yoo, Dharma P. Agrawal.................................................................... 355 13. Mobile-Relay Forwarding in Opportunistic Networks Giuseppe Anastasi, Marco Conti, Andrea Passarella, Luciana Pelusi. ............................................................................389 14. Adaptive Techniques in Wireless Networks Yuxia Lin, Vincent W.S. Wong..................................................................... 419 15. Tunable Security Services for Wireless Networks Stefan Lindskog, Anna Brunstrom, Zoltán Faigl. ....................... 451 Index................................................................................................... 481
- 15. ix Preface Adaptive techniques play a key role in modern wireless communication systems. The concept of adaptation is emphasized in the Adaptation in Wireless Communications Series across all layers of the wireless protocol stack, ranging from the physical layer to the application layer. This book covers the concept of adaptation at the data link layer, network layer, and application layer. It presents state-of-the-art adaptation techniques and methodologies including cross layer adaptation, joint signal processing, coding and networking, self- ishness in mobile ad hoc networks, cooperative and opportunistic protocols, adaptation techniques for multimedia support, self-organizing routing, and tunable security ser- vices. The book offers several new theoretical paradigms and analytical findings, which are supported by various simulation and experimental results, and contains more than 170 figures, 25 tables, and 650 references. I would like to thank all the contributing authors for their patience and excellent work. The process of editing started in June 2005. Each chapter has been blindly reviewed by at least two reviewers (more than 50% of the chapters received three reviews or more), and I would like to thank the reviewers for their time and valuable contribution to the quality of this book. Finally, a special thank you goes to my parents, my wife, my son, my daughter, and all my family. They all have been of a great support for this project. Mohamed Ibnkahla Queen’s University Kingston, Ontario, Canada
- 17. xi Editor Dr. Mohamed Ibnkahla earned an engineering degree in electronics in 1992, an M.Sc. degree in signal and image processing in 1992, a Ph.D. degree in signal process- ing in 1996, and the Habilitation à Diriger des Recherches degree in 1998, all from the National Polytechnic Institute of Toulouse (INPT), Toulouse, France. Dr. Ibnkahla is currently an associate professor in the Department of Electrical and Computer Engineering, Queen’s University, Kingston, Canada. He previously held an assistant professor position at INPT (1996–1999) and Queen’s University (2000–2004). Since 1996, Dr. Ibnkahla has been involved in several research programs, includ- ing the European Advanced Communications Technologies and Services (ACTS), and the Canadian Institute for Telecommunications Research (CITR). His current research is supported by industry and government agencies such as the Ontario Centers of Excellence (OCE), the Natural Sciences and Engineering Research Council of Canada (NSERC), the Ontario Ministry of Natural Resources, and the Ontario Ministry of Research and Innovation. He is currently leading multidisciplinary projects designing, implementing, and deploying wireless sensor networks for various applications in Canada. Among these applications are natural resources management, ecosystem and forest monitoring, spe- cies at risk tracking and protection, and precision agriculture. Dr. Ibnkahla has published a significant number of journal papers, book chapters, technical reports, and conference papers in the areas of signal processing and wireless communications. He has supervised more than 40 graduate students and postdoctoral fellows. He has given tutorials in the area of signal processing and wireless commu- nications in several conferences, including IEEE Global Communications Conference (GLOBECOM, 2007) and IEEE International Conference in Acoustics, Speech, and Sig- nal Processing (ICASSP, 2008). Dr. Ibnkahla received the INPT Leopold Escande Medal for the year 1997, France, for his research contributions in signal processing; the Prime Minister’s Research Excel- lence Award (PREA), Ontario, Canada in 2000, for his contributions in wireless mobile communications; and the Favorite Professor Award, Queen’s University in 2004 for his excellence in teaching.
- 19. xiii Contributors Dharma P. Agrawal Center for Distributed and Mobile Computing Department of Computer Science University of Cincinnati Cincinnati, Ohio Ali Alemdar Electrical and Computer Engineering Department Queen’s University Kingston, Ontario, Canada Giuseppe Anastasi Department of Information Engineering University of Pisa Pisa, Italy Farid Benbadis Université Pierre et Marie Curie Paris, France John Bigham Department of Electronic Engineering Queen Mary, University of London London, UK Anna Brunstrom Department of Computer Science Karlstad University Karlstad, Sweden Marco Conti IIT-CNR National Research Council Pisa, Italy Sajal K. Das Department of Computer Science and Engineering University of Texas at Arlington Arlington, Texas Marcelo Dias de Amorim CNRS Paris, France Zoltán Faigl Mobile Innovation Center Budapest University of Technology and Economics Budapest, Hungary Fethi Filali Eurecom Institute Sophia-Antipolis, France Hossam Hassanein School of Computing Queen’s University Kingston, Ontario, Canada Ekram Hossain Department of Electrical and Computer Engineering University of Manitoba Winnipeg, Manitoba, Canada Mohamed Ibnkahla Electrical and Computer Engineering Department Queen’s University Kingston, Ontario, Canada
- 20. xiv Contributors Massimiliano Lenardi Hitachi Europe Sophia-Antipolis, France Yuxia Lin Department of Electrical and Computer Engineering University of British Columbia Vancouver, British Columbia, Canada Stefan Lindskog Department of Computer Science Karlstad University Karlstad, Sweden and Centre for Quantifiable Quality of Service in Communication Systems Norwegian University of Science and Technology Trondheim, Norway Yonghe Liu Department of Computer Science and Engineering University of Texas at Arlington Arlington, Texas Ning Lu MPI-QMUL Information Systems Research Centre Macao Polytechnic Institute Macao Hong Luo School of Computer Science and Technology Beijing University of Posts and Telecommunications Beijing, China Hamid Menouar Hitachi Europe Sophia-Antipolis, France Nidal Nasser Department of Computing and Information Science University of Guelph Toronta, Ontario, Canada Dusit Niyato Department of Electrical and Computer Engineering University of Manitoba Winnipeg, Manitoba, Canada Andrea Passerella IIT-CNR National Research Council Pisa, Italy Luciana Pelusi IIT-CNR National Research Council Pisa, Italy Mihail S. Sichitiu North Carolina State University Raleigh, North Carolina Sameh Sorour Department of Electrical and Computer Engineering University of Toronto Toronto, Ontario, Canada Ananthram Swami Communications and Information Sciences Directorate U.S. Army Research Laboratory Adephi, Maryland Alberto Lopez Toledo Telefonica Research Barcelona, Spain Shahrokh Valaee Department of Electrical and Computer Engineering University of Toronto Toronto, Ontario, Canada Tom Vercauteren INRIA Sophia-Antipolis, France Aline Carneiro Viana INRIA Paris, France Yannis Viniotis North Carolina State University Raleigh, North Carolina Xiaodong Wang Department of Electrical Engineering Columbia University New York, New York
- 21. Contributors xv Vincent W. S. Wong Department of Electrical and Computer Engineering University of British Columbia Vancouver, British Columbia, Canada Yunnan Wu Microsoft Research Redmond, Washington Chi-Hsiang Yeh Department of Electrical and Computer Engineering Queen’s University Kingston, Ontario, Canada Younghwan Yoo Ubiquitous Computing Laboratory School of Computer Science and Engineering Pusan National University Busan, Korea Tiantong You School of Computing Queen’s University Kingston, Ontario, Canada Guohua Zhang Department of Computer Science and Engineering University of Texas at Arlington Arlington, Texas Qing Zhao Department of Electrical and Computer Engineering University of California Davis, California
- 23. 1 1 Adaptive Optimization of CSMA/CA MAC Protocols Based on Bayesian State Estimation 1.1 Introduction...............................................................2 1.2 The IEEE 802.11 Distributed Coordination Function......................................................................3 The DCF Protocol • Analytical Throughput of IEEE 802.11 DCF 1.3 Bayesian Estimation of the Number of Competing Terminals in IEEE 802.11 DCF..........6 Problem Formulation • Sequential Monte Carlo Estimation • Deterministic Sequential Monte Carlo Estimators • Performance of the SMC Estimators 1.4 Adaptive Optimization of IEEE 802.11 DCF Based on the SMC Estimators......................20 Predictive Distribution Based on SMC Samples • Choice of Backoff Window Size Set W 1.5 Simulation Results...................................................24 Simulation Setup • Effect of the Adaptive Choice of Parameters on the DCF Optimization • Instantaneous Network Utilization • Results under Nonsaturated Network Conditions 1.6 Conclusions.............................................................. 31 References............................................................................33 Appendix..............................................................................36 Alberto Lopez Toledo Telefonica Research Barcelona Tom Vercauteren INRIA Xiaodong Wang Columbia University
- 24. 2 Adaptation and Cross Layer Design in Wireless Networks 1.1 Introduction The IEEE 802.11 protocol [1] has become the predominant technology for wireless local area networks (WLANs). One of the most important elements of the 802.11 in terms of performance is the medium access control (MAC). The MAC protocol is used to provide arbitrated access to a shared medium, in which several terminals access and compete for the radio spectrum. The design of the MAC protocols is often application dependent, and it is closely linked to the characteristics of the medium in which it operates. It also determines the performance and quality of service (QoS) metrics of the network, such as throughput, stability, and delay. The IEEE 802.11 wireless networks employ the distributed coordination function (DCF) as a primary medium access mechanism. It is based on the carrier-sensing multiple-access with collision avoidance (CSMA/CA) protocol and binary exponential backoff [2, 3]. Several studies have shown that the DCF protocol is very sensitive to the number of competing terminals that access the wireless channel [2–7], and that a way to optimize the network performance is to make the parameters of the backoff window depend on the number of terminals competing for the medium. However, estimating the number of competing terminals is not an easy task. While a terminal could cache the identity of the past senders in the network, the number of competing terminals is the number of terminals that have data to send at any given time, so a simple list of neighbors is not suf- ficient. The estimation of the number of competing terminals faces two problems. First, the number of competing terminals is a non-Gaussian nonlinear dynamic system that is difficult to track accurately with conventional filters. Advanced estimators such as the extended Kalman filter (EKF)-based one from [3] provide better results, but they are subject to critics due to their complexity [8]. Second, the performance of the IEEE 802.11 DCF is extremely sensitive to the number of competing terminals [2], particularly in the typical operating point of one to fifteen terminals. This makes approximate methods such as [5, 9, 10] to yield suboptimal operation of the protocol compared with the theo- retical optimum. Hence, there is a need for an accurate estimation algorithm that is able to efficiently track the number of competing terminals in an IEEE 802.11 network and, at the same time, is easy to implement. As we will see, sequential Monte Carlo methods are appropriate for this purpose. Bayesian Monte Carlo signal processing techniques [11, 12] offer a paradigm for tack- ling challenging signal processing problems for which traditional methods are difficult to apply. Among them, the sequential Monte Carlo (SMC) methodology [12] has been shown to be extremely powerful in dealing with filtering problems in non-Gaussian and nonlinear complex dynamic systems, where conventional approaches fail to work. In order to tackle the above estimation of the competing terminals, we develop several SMC-based adaptive estimators that outperform the existing classical estimator, such as those based on the extended Kalman filter (EKF) [3]. In particular, we develop a deter- ministic variant of the SMC estimator that is simpler to implement and offers superior performance, by including a set of sufficient statistics about the unknown parameters in the deterministic sample filter setting proposed in [13]. The use of sufficient statistics is pushed one step further than in [14] because this information about the parameters is
- 25. Adaptive Optimization of CSMA/CA MAC Protocols 3 now integrated out so that no Monte Carlo approximation needs to be performed. We further extend it into a maximum a posteriori (MAP) estimator whose computational load and memory requirements are equivalent to those of the well-known Viterbi algo- rithm. We will see that our SMC-based algorithms overcome both of the problems men- tioned above: accuracy and ease of implementation. We show that the accuracy of the Bayesian algorithms is particularly good at small timescales, which makes our proposal attractive to optimize the protocol when the terminals are in a nonsaturation regime, a problem usually not addressed in the literature. Finally, we propose an optimization mechanism that uses the predictive distribution of the number of competing terminals to adapt the parameters of the IEEE 802.11 DCF protocol and maximize its throughput. 1.2 The IEEE 802.11 Distributed Coordination Function 1.2.1 The DCF Protocol The DCF defines two distinct techniques to access the medium: basic access and RTS/CTS access. 1.2.1.1 Basic Access In basic access, the terminals implement a two-way handshake mechanism (Figure 1.1). A terminal senses the channel to be idle before starting a transmission. If the channel is idle for at least a period of distributed interframe space (DIFS), then the terminal is allowed to transmit. If during this sensing time the channel appears to be busy at any time, the terminal defers the transmission and enters into the collision avoidance (CA) mode. In CA mode the terminal generates a random backoff interval during which it waits before attempting another transmission. This random backoff is used to minimize the probability of collision between terminals accessing the medium. The idle time after waiting a DIFS interval is slotted, and the terminals are only allowed to transmit at the beginning of the slot time. The slot time size σ accounts for the time the signal is propa- gating, and is set equal to the time needed for any terminal to detect the transmission of a packet from any other terminal [2]. If this time were not accounted for, a terminal could assess the channel as idle when the data sent by another terminal have not yet arrived. DATA ACK SIFS Defer Access NAV Source Destination Other DIFS Contention Contention DATA ACK SIFS Defer Access Data NAV RTS NAV DIFS RTS SIFS CTS SIFS CTS NAV Basic Access RTS/CTS Access Fig u r e 1.1 IEEE 802.11 MAC access mechanisms.
- 26. Discovering Diverse Content Through Random Scribd Documents
- 27. Having examined the mole some time, Jack threw it down, and went along. About two or three hours afterwards, he was coming back the same way. As he was passing the dead mole, he noticed that it appeared to be sunk in the ground. He stopped, and looked at it attentively. Pretty soon he saw a large black bug, which we call a beetle, creep from under it, and run around it. This attracted Jack’s attention, and kneeling down, he watched carefully to see what was going on. After looking about a little, he noticed that there were four or five beetles, all at work, digging a hole under the mole, into which the creature was gradually sinking. He lifted up the mole a little, so that he might observe them more carefully; but the creatures did not seem to mind him. They went to work again, immediately, digging away the earth, which they threw out at the sides. It was amusing to see how hard they toiled. The heads of the beetles were shaped somewhat like a spade. With these they dug up the earth, and then clawed it away with their feet. Never did a set of men, digging a cellar, appear more active, busy and efficient. Jack had always before hated beetles, as being ugly, disagreeable things; but now he was delighted to see them. He immediately ran off to get his aunt Piper to come and observe what was going on. She was very busy, but Jack persuaded her to go with him. They soon came to the spot, and aunt Betsey now saw that what Jack had told her was all true. The beetles were, indeed, burying the mole. “But what are they doing it for?—are they sextons?” said Jack. “They are called burying beetles,” said the aunt; “but, Jack, I shall leave you to find out yourself what they are burying the mole for.” So, after a time, Jack and his aunt went away. The next day Jack went to the place, when, behold, the mole was not to be seen! There was a little spot of fresh earth where it had lain, but that was all. “So,” thought Jack to himself, “so, neighbor beetles, I guess you have buried the mole,—at any rate, we will see.” So Jack began to
- 28. dig away the earth a little, with his fingers, and about two inches below the surface, there was the mole, sure enough. The beetles were all around the carcass, and every one of them was at work, making a feast. Jack covered up the hole, and left them all to themselves.
- 29. Lucy and Ann. a dialogue. Ann. Tell me, dear Lucy, Tell me true— What is that thing Above, so blue? Lucy. My little Ann, We call it sky— And there, you know, God lives, on high.
- 30. And it is heaven Far up above, Where angels dwell, In peace and love. And there, when dead, Good people go, To dwell with God, And angels too. Ann. ’Tis beautiful! But, sister dear, Is it in heaven As ’tis here? Are there sweet flowers, And blossoms fair, And silver streams, And fragrant air? And pretty birds, And butterflies,— Are there such things Beyond the skies? Lucy. We cannot tell; We only know, ’Tis happier there Than here below. The sweetest thing On earth is Love; That fills each heart In Heaven above. These things amid, The birds and flowers, The butterflies In blooming bowers,— All these are fair,— And yet more blest
- 31. Art thou, dear Ann, On mother’s breast. ’Tis Love, dear child, That makes thee so, And Heaven is Love,— That’s all we know.
- 32. The Little Flower Girl. How beautiful is Spring! See this little girl—she has gone forth into the fields alone, and she has gathered a handful of blossoms— dandelions, buttercups, and pansies. Why should the little girl gather flowers? Because they are beautiful, you will tell me. And who told you they were beautiful? You cannot tell me that, perhaps; then I will tell you. It is God, who made us, that has told us flowers are beautiful. He made us with eyes to see, and hearts to feel their beauty. He made these things, and He formed us so that we might love them, and take pleasure in them. How good and kind is it of our Heavenly Father, to spread pleasant things around us, and give us faculties to enjoy them!
- 33. Surely, when we see so many lovely blossoms, and so many pleasant things, scattered around us, we should think with gratitude of Him who has arranged all these things; and our hearts being filled with love to Him, we should all try to please Him, by doing as He wishes us to do. God has done other kind things for us—He has not only made flowers, but He has made truth, charity, kindness, goodness, and other things, which are called virtues. God looks on these things, as we do upon flowers, as very lovely things; and what he wishes of us, is, that our hearts shall be full of these virtues. He likes to look on a virtuous mind, as we do on a flowery garden. Now, my little reader, you can love these virtues, if you please, and you can adorn yourself with them. When you have learned to love them, it will give you great pleasure to think of them, and if they dwell in your own heart, you will be happy indeed. If you love truth, if you love charity and kindness, if you are fond of everything that is good and amiable, then God will love you, and you will be cheerful and happy.
- 34. Dash won’t learn his Lesson! Here is Dash in his kennel, and Alice by his side. Alice has got a book, and she tries to get Dash to read. Let us listen, and hear what Alice says. “Oh Dash, you are a naughty dog! Why don’t you say a, b, c? Look on the book! Mind! If you don’t mind, I’ll whip you! “A’nt you ashamed, Dash, not to learn your letters? What! be a blockhead all your days, and not learn to read? Shame! Shame on you! Why, father says you are six years old, and you don’t know your letters! Naughty Dash!”
- 36. To Correspondents. We are gratified to find from the letters we get from our friends in all quarters, that the “Little Leaves” are acceptable to our subscribers. We are particularly glad to find that our larger readers do not object that so many pages should be devoted to the amusement of “Little Readers.” To J. L. S., who inquires if the story of Limping Tom is true, we have to reply that if it is not, it sounds very like the truth. As to R——, who discovers that in our April number we have got in a cut which was inserted last year, we beg to observe, that we had noticed this ourselves, but were afraid to speak about it, lest it should be only an April fool trick of the printer’s boys, and we should get the laugh upon us, thereby. We say to L——, who wishes to hear something more about Bill Keeler, that we have inserted an anecdote of him in this number. We are glad to find that the story of Inquisitive Jack is approved of. We shall make Jack out a very clever fellow, before we have done with him.
- 38. Truth Triumphant. In a far-off country, there was once a jeweller who left home with some valuable diamonds, for the purpose of selling them in a city at some distance from his own residence. He took with him his son and a slave. This slave he had purchased when quite a small boy, and had brought him up more like an adopted child than a servant. The merchant at length reached the city whither he was going, and disposed of his diamonds with great advantage. While preparing to return home he was seized with a sudden illness, which in a few hours terminated his life. The merchant was quite a stranger in the city. This his servant knew, and believing himself quite safe, he declared himself to be the son of the deceased jeweller, and entitled to take charge of his property. The real son was filled with great grief, but what could he do? He had no means of establishing his right to the property, for he had no means of proving himself to be the son of the deceased. The servant was loud in his pretensions, and one circumstance served to favor his claims. He was a young man, quite comely in his person, and polished in his manners; whereas the jeweller’s son was mean in his appearance, and had been seriously injured in his education by the indulgence of his parents. It was, therefore, quite natural that strangers should take part with the servant against the son. At length, in order to end the dispute, the latter referred the matter to a court of law. There, however, from a total want of proof, nothing could be decided. Each party was equally positive, but neither could do more than to assert his claim. At length the judge declared his utter inability to determine which was the rightful heir to the property. The novelty of the case, and the great amount of property in question, excited the interest and curiosity of a large part of the city. Divers opinions prevailed, and the subject became a fruitful theme of
- 39. conversation and dispute. It was thought to be a case of so much importance as to merit the attention of the prince of the country. The case was accordingly stated to him; but in like manner he also was confounded, and at a loss how to decide the question. At length a happy thought occurred to the chief judge of the prince, by which to ascertain the real heir. The two claimants were summoned before him. He ordered them to stand behind a curtain prepared for the occasion. Through this curtain two openings were made. They were directed to project their heads through these holes, and then each one might tell his story. When the judge had heard them he was to decide the case, and cut off the head of the one whom he should judge to be the slave. Both agreed to the plan; the son relying upon the honesty and the justice of his cause; the servant, through his confidence in the impossibility of detection. The judge took his seat, and the parties took their stations. An officer with a drawn sword stood in front ready to strike off the head of the one whom the judge should decide to be the impostor. They now told their stories. Just as the last one had finished, the judge cried out in a stern voice to the officer, “Enough! Enough! strike off the villain’s head!” The officer sprung towards the young men with an uplifted sword. The impostor, conscious of his guilt, started back behind the curtain; the son, conscious of the justice of his cause, stood unmoved! The judge immediately decided for the latter, ordered the property to be given to him, and the slave to be punished for his wicked and ungrateful attempt at deception.
- 41. Vol. V. No. 6. M E R R Y ’ S M U S E U M . JUNE, 1843. Louisa Vinning. This little artist of nature, who has recently excited the astonishment of the musical world, and claims the admiration of all, as a remarkable instance of precocious powers, was born on the
- 42. 10th Nov., 1836, at Kingsbridge, Devonshire, England. Her father, John Vinning, is a musician of talent: as violinist and composer, he has acquired celebrity. His two brothers are also musicians, having been induced, by an irresistible inclination for music, to abandon the professions to which they had been bred. When the little warbler who is now the subject of notice, had reached the age of nine months, it was observed that she derived intense delight from music; and if she was ever fretful, the sound of her father’s violin immediately soothed her; at the same time her whole frame would move in unison with the measure, and her face beam with ecstasy. Her father indulged the child in occasionally playing to her, but the excitement was so intense that he became apprehensive it might injure her. He accordingly took the advice of several medical men, who recommended a moderate indulgence only of the child’s passion. Before Louisa could speak words, she began to sing tunes with accuracy and effect; and often while asleep, she would warble forth some soft and sweet melody,—an air she had heard, or the impromptu suggestion of a dream. On one occasion the father was called by the mother, whom he found in tears by the bed-side of the sleeping child, who was singing a beautiful air, in a soft and mellow voice. The father remained at the place, and the little warbler repeated it several times. He immediately took a pen and wrote it down. This song was sold to Mr. Burkley, of London, who wrote some lines to it, and it was published under the title of the “Infant’s Dream.” This composition has been much admired. In the morning after the child had sung this song in her sleep, she said to her mother, “Oh, I have seen such beautiful angels in my sleep—all gold —beautiful gold!” When this interesting prodigy was two years old, she was announced to sing in public in London. Her performances were such as to excite the utmost wonder, even of musicians of the highest standing. The celebrated Thalberg, then in London, Sir G. Smart, and the Misses Moschelles, stated that her singing was astonishingly
- 43. correct, her voice pleasing and of great compass, her ear correct, and her feeling quick and deep. Such was her fame, at this age, that the queen sent for her to come to Buckingham palace. On hearing the child sing, her majesty was so touched, that she clasped her warmly to her breast, and afterwards placed a large diamond pin in her bosom, as an expression of her delight. From this time, this little singer continued to attract public attention, and promises, when she arrives at maturity, fully to realize the hopes her early displays and genius have inspired.
- 45. Importance of a Fly. The following facts may give our readers some idea of the vast importance to the British treasury of a little insect, called the hop fly, or Aphis Humelia. In the year 1822, on the 14th of May, the hop duty, on a fair estimate, according to the prospect of the crop, was laid at £100,000; the fly, however, appearing pretty plentifully towards the end of the month, it was estimated as likely to produce only £80,000. The fly increased, and by the end of June the duty had gone down to £60,000; by the end of July, to £30,000; by the end of August, to £22,000, and by the end of December to £14,000. The duty actually paid this year was £15,463 10s. 6d. In 1825, the duty commenced at an estimate of £130,000; but, owing to the excessive increase of the fly, it had in July fallen to £16,000; at the beginning of September it rose to £29,000, but towards the end fell again to £22,000. The amount paid was £24,317 0s. 11d. In the following year, the summer was remarkably dry and hot; the thermometer for several nights continued above seventy degrees all the night through. The crop of hops was immense; scarcely a fly was to be found, and the duty, which was estimated in May at £120,000, rose to £265,000, and the amount actually paid was £262,331 0s. 9d., being the largest amount ever known. From this, it will appear, that in duty alone, a little insignificant- looking fly has a control over £150,000, or 700,000 dollars, annual income to the British treasury! Supposing the hop grounds of England capable of paying this latter duty annually, which they certainly are, it is very manifest that in 1825 these creatures were the means of robbing the treasury of over £100,000; this seems a large sum, but it is only a small part of the amount actually destroyed, for we here only speak of the tax, and not of the entire
- 46. value of the hops destroyed. This, no doubt, is three times as much as we have estimated, for the duty alone. In some years it is probable that the hop fly destroys hops in England to the value of nearly two millions of dollars! Is not this driving business on a large scale?
- 47. June. This first month of summer is said to derive its name from Juno, a Roman goddess, in honor of whom a festival was celebrated at the beginning of the month: its zodiacal sign is Cancer, which the sun enters on the 21st. Summer is now fully established, and even in New England the grumblers say nothing of easterly winds, and confess that it is warm enough. In the southern states, the people are beginning to fear the return of fevers; beside, the days and nights are there too hot for comfort—and, therefore, those who can leave home are about departing for the north. The mower is now in the field with his scythe. At an early hour we hear the merry ringing of the stone upon the steel, as he is sharpening it. How beautiful now is the meadow—the orchard—the wheat field, the maize field, and the forest! How various and how brilliant the flowers; how fragrant the air—how balmy, yet how healthful the breeze!
- 48. William the Conqueror. Henry II. Edward Longshanks. Henry I. Richard of the Lion Heart. Henry IV.
- 49. A Chapter of English Kings. In the year 1041, the Danes were finally driven out of England, and for a short space the Saxon kings were restored to the throne. The last of these, Harold, was defeated by William, Duke of Normandy, or William the Conqueror, in the celebrated battle of Hastings, in the year 1066. This decided the fate of England: Harold was killed upon the field, and the Norman line of kings succeeded to the throne. William reigned about twenty years. He was an able warrior, but he ruled with great severity. He divided the lands of the nobility of England among his followers, and oppressed the people by rigorous laws. They therefore held meetings to devise plots of rebellion, usually in the evening. To suppress them, he ordered a bell to be rung every evening at eight o’clock, at which time they were required to put out their fires and lights. This was called the couvre feu,—French words, signifying cover fire: it is from this that the word curfew has come, which now means, the bell rung at evening, about the time for the people to go to bed. William endeavored to make the English people live like Frenchmen; he required the French language to be used in the courts of law, and it was spoken by himself and all around the palace. William took pains to make his new kingdom thrive; he had a careful survey made of the lands and property, and these were registered in a book called the Dooms-day book, copies of which are still preserved. During William’s reign, the laws were firmly established, and a great deal was done to benefit the people. He was at first hated as a conqueror and oppressor, but he was finally regarded as a
- 50. sagacious, though severe king. He died at Rouen, in France, 1087, aged 63 years. William Rufus, or William the Red, the second son of the Conqueror, succeeded his father; he was addicted to field sports, and one day, while pursuing a deer in the forest, he was killed by an arrow, discharged by Walter Tyrrel. The arrow first hit a tree, but it glanced, and struck the king in the breast. This occurred in the year 1100. Henry First, the third son of the Conqueror, succeeded. He was very despotic, but still he sought to promote the prosperity of his kingdom. He patronised learning, restored the University of Cambridge, and received the title of Beau Clerk, which implies that he was a good scholar. He punished robbers, who were very numerous in his day, with severity; he abolished the hated curfew, established a uniform standard of weights and measures, and granted charters of various kinds, which were the first beginnings of English liberty. He died in Normandy, 1135. Stephen usurped the throne which Henry had bequeathed to his daughter, Matilda, in 1135. After a disturbed reign of nineteen years, Stephen died in 1154, and was succeeded by Henry II., son of Matilda. He held the kingdom of Brittany, in France, and conquered Ireland, which has ever since been attached to the British crown. He was a wise prince, but he had great trouble with the clergy. Thomas á Becket, the primate of England, who was thought to be disposed greatly to extend his power, was murdered by some persons in the cathedral of Canterbury. This crime was laid to King Henry, and he was obliged, by the Pope of Rome, to take a solemn oath that he had nothing to do with it, in order to clear himself. He was also obliged to walk barefoot to the tomb of Becket, and to yield to various penances, imposed by the monks of Canterbury. He was severely whipped with rods by the monks, during his penance, and to this he patiently submitted. He died 1189.
- 51. Richard the Lion-Hearted, his son, succeeded him, and was crowned 1189. He was a man of great bodily strength, of invincible courage, and possessed many generous qualities. He was seized with the mania of the age, to go and fight against the Turks for the recovery of Jerusalem. In Palestine, he performed prodigies of valor, and on his way back, was taken and imprisoned by the Duke of Austria. He was released after some years, his people paying a heavy ransom. He was afterwards shot by a cross-bow, while besieging a castle in Normandy. He was succeeded by his brother John, who was so poor as to get the surname of Lackland. John was a weak, vicious king; but one good thing happened during his reign. He signed a paper, called Magna Charta, or Great Charter, in which he yielded some portion of the despotic power before claimed by the kings of England. This was a serious abridgement of the king’s authority, and it opened the way for still further enlarging the liberties of the people. He died in 1216. Henry III., though but nine years old, succeeded his father John, and reigned fifty-five years. He was a weak and vicious king, and the people took advantage of his folly and weakness, to extort from the crown various concessions, which extended and secured their liberties. He was succeeded by his son Edward, in 1272, called Longshanks, on account of his uncommonly long legs. He was a famous warrior, and conquered Wales. He made war upon Scotland, and was bravely resisted by the famous William Wallace. While marching with a great army against Robert Bruce, who headed the Scotch, he died, and his son, Edward II., succeeded him in 1307. He led an army of 100,000 men against Scotland, but was defeated with vast slaughter, in the celebrated battle of Bannockburn. He was a foolish king, and his own wife imprisoned him, and caused him to be murdered. His son, Edward III., began to reign in 1327, at the age of eighteen. He was a brave warrior, and beat the Scotch and the French. His son, the celebrated Black Prince, was not less renowned
- 52. as a warrior, while he was also distinguished for his kindness and generosity. This prince died 1376, and his father the year after. Richard II., grandson of Edward III., came to the throne in 1377, being only eleven years old. During his reign, there was a famous rebellion, the leader of which was Walter Tyler, a blacksmith. Walter, in a conference with the king, was so insolent, that William Walworth knocked him down with a club, and one of the soldiers despatched him. Richard was deposed by the duke of Lancaster, and being imprisoned, was either killed or starved to death. The Duke was crowned king in 1400, under the title of Henry IV. It was during his reign, that the wars of the two roses took place; that is, the war between the houses of York and Lancaster. Their partisans were distinguished, the one by red, the other by white roses. He died 1413, aged forty-six, and was succeeded by his son, Henry V. We shall have something more to say of English Kings, in a future number. “Sir, you have had an education, I suppose,” said an illiterate preacher in Maine to a learned clergyman. “Yes, sir,” was the reply. “I am thankful,” rejoined the former, “that the Lord has opened my mouth to preach without any learning.” “A similar event,” replied the latter, “took place in Balaam’s time; but such things are of rare occurrence at the present day.”
- 54. True Stories. Sir Christopher Wren.—When this eminent architect was building St. Paul’s cathedral, he caused the following notice to be affixed to several parts of the structure. “Whereas, among laborers and others, that ungodly custom of swearing is so frequently heard, to the dishonor of God and contempt of his authority; and to the end that such impiety may be utterly banished from these works, which are intended for the service of God and the honor of religion: it is ordered, that profane swearing shall be a sufficient crime to dismiss any laborer that comes to the call; and the clerk of the works, upon a sufficient proof, shall dismiss him accordingly: and that if any master, working by task, shall not, upon admonition, reform the profanation among his apprentices, servants, and laborers, it shall be construed his fault, and he shall be liable to be censured by the commissioners.” Madame de Genlis.—Madame de Genlis relates the following anecdote, which occurred during her residence at Berlin:— “My saloon had two doors, one opening into my chamber, and the other conducting to a private staircase, descending to the court. On the platform of this staircase was a door opposite to mine, belonging to the apartments of an emigrant. This man was of a savage disposition, and never saw any one in the house. Some one had given me two pots of beautiful hyacinths; at night I placed them on this platform, between my neighbor’s door and my own. In the morning, I went to take them again, and had the disagreeable surprise to see my beautiful hyacinths cut into pieces, and scattered round the pots which held them. I easily guessed that my neighbor was the author of this deed, who had been excited to it, doubtless, notwithstanding his French politeness, by the libels which were published against me. Not wishing the affair to be known, I did not
- 55. ask more flowers of the persons who had given me these, but directed a servant to buy me some. Having placed these in the pots, I attached to them a slip of paper, on which I wrote these words: —‘Destroy my works, if you will, but respect the works of God.’ At night I placed them on the platform: in the morning I went with eagerness to see what had been their fate, and saw with great pleasure that some one had been content with simply watering them. I carried them immediately into the saloon, and placing them on the table, perceived that there were attached to them two silk strings, each suspending a cornelian ring.” A Gentleman in America.—“A man of my acquaintance,” says Dr. Dwight, “who was of a vehement and rigid temper, had, many years since, a dispute with a friend of his, a professor of religion, and had been injured by him. With strong feelings of resentment, he made him a visit, for the avowed purpose of quarrelling with him. He accordingly stated the nature and extent of the injury; and was preparing, as he afterwards confessed, to load him with a train of severe reproaches, when his friend cut him short by acknowledging, with the utmost readiness and frankness, the injustice of which he had been guilty; expressing his own regret for the wrong he had done, requesting his forgiveness, and proffering him ample compensation. He was compelled to say that he was satisfied, and withdrew full of mortification that he had been precluded from venting his indignation and wounding his friend with keen and violent reproaches for his conduct. “As he was walking homeward, he said to himself to this effect: ‘There must be something more in religion than I have hitherto suspected. Were any man to address me in the tone of haughtiness and provocation with which I accosted my friend this morning, it would be impossible for me to preserve the equanimity of which I have been a witness; and especially with so much frankness, humility and meekness, to acknowledge the wrong which I had done; so readily ask forgiveness of the man whom I had injured;
- 56. and so cheerfully promise a satisfactory recompense. I should have met his anger with at least equal resentment, paid him reproach for reproach, and inflicted wound for wound. There is something in this man’s disposition which is not mine. There is something in the religion which he professes, and which I am forced to believe he feels; something which makes him so superior, so much better, so much more amiable, than I can pretend to be. The subject strikes me in a manner to which I have hitherto been a stranger. It is high time to examine it more thoroughly, with more candor, and with greater solicitude, also, than I have done hitherto.’ “From this incident, a train of thoughts and emotions commenced in the mind of this man, which terminated in his profession of the Christian religion, his relinquishment of the business in which he was engaged, and his consecration of himself to the ministry of the gospel.” A Quaker.—A gay young man, travelling in a stage coach to London, forced his deistical sentiments on the company, by attempting to ridicule the Scriptures; and, among other topics, made himself merry with the story of David and Goliath, strongly urging the impossibility of a youth like David being able to throw a stone with sufficient force to sink into the giant’s forehead. On this he appealed to the company, and particularly to a grave Quaker gentleman, who sat silent in one corner of the carriage. “Indeed, friend,” replied he, “I do not think it at all impossible, if the Philistine’s head was as soft as thine.” This grave rebuke reduced the young man to silence. An Aged Minister.—A venerable minister at H—— preached a sermon on the subject of future punishment. On the next day it was agreed among some thoughtless young men, that one of them should go to him, and endeavor to draw him into a dispute, with the design of making a jest of him and of his doctrine. The wag
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