wireless networking in developing worlds wndw

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First Edition, January 2006 Second Edition, December 2007 ird Edition, February 2013
Many designations used by manufacturers and vendors to distinguish their products are claimed as trademarks. Where those designations appear in this book, and the authors were aware of a trademark claim, the designations have
been printed in all caps or initial caps. All other trademarks are property of theirrespective owners. e authors and publisher have taken due care in preparation of this book, but make no expressed or implied warranty of any kind and assume no responsibility for errors or omissions. No liability is assumed for incidental or consequential damages in connection with or arising out of the use of the information contained herein.
As we have discovered the developing world of wireless networking is all around us, the authors of this book have included projects in North America, Europe, and in Asia, South America, India and Africa. So we have come to the conclusion
that most places have the potential to fi
nd a ff
ordable indoor and outdoor wirelessnetworks useful. We hope you enjoy reading this book and use it as the starting point of a wireless project in your community. e book and PDFfile are published under aCreative Commons Attribution- ShareAlike 3.0license. is allows anyone to make copies, and even sell them for a profit, as long as proper attribution is given to the authors and any derivative works are made available under the same terms.
Any copies or derivative works must include a prominent link to our website,
http://wndw.net/See http://creativecommons.org/licenses/by-sa/3.0/ for more information about these terms.
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Wireless Networking in the Developing World by e WNDW Authors is licensed under a Creative Commons Attribution -ShareAlike 3.0 01234567 89:61;6<
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is third version of this book was started as a BookSprint in September 2011 in the beautiful city of Copenhagen hosted by Sebastian Buettrich, who is one of the authors.
A core team of eight people thenfinished this version over the following months leading to publication in March 2013.
roughout the project, the core group has actively solicited contributions and feedback from the wireless networking community worldwide. You can provide your own feedback or post technical questions to the authors at our Facebook page: https://www.facebook.com/groups/wirelessu
is book is available as an eBook for your mobile device, or it is
downloadable from the website http://wndw.net/ for free (high and lowresolution available), or it can be ordered as a printed book from http://www.lulu.com/
We do give out a copy to every student who attends a wireless training course given by all of the Institutions we work with such as the International Centre for eoretical Physics (ICTP), the Network Startup Resource Center (NSRC), the Asian Institute of Technology (AIT), e Internet Society (ISOC) and AirJaldi, to name just a few. And we would all strongly encourage you to sign up for a local course.
For information about upcoming courses or if you would like to arrange a course in your region, please contact the editor, Jane Butler [email protected]
If you are planning a wireless project and you need a copy of this book and can neither download it as you have limited bandwidth nor afford to order it online, please send an email to Jane or send a message on Facebook and we'll mail a printed copy to you.
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Jane Butler, who is lead editor of this version of the book. Jane iscurrently President of the private Foundation called networktheworld.org which promotes and supports the growth of Internet connectivity around the world mainly by supporting wireless projects and training http://wirelessu.org. Jane is also head of industrial collaboration and outreach at University College London. Jane holds an Honours Degree in Engineering, is a Chartered Engineer and Fellow of the Institution of Electronics and Technology.
Jane can be reached at [email protected]
e editor would like to acknowledge and thank the core group of contributors who are listed below -
Ermanno Pietrosemoli. Ermanno is currently a researcher at the Telecommunications/ICT for Development Lab of the International Centre for eoretical Physics in Trieste, Italy, and President of Fundación
Escuela Latinoamericana de Redes "EsLaRed", a non- pro fi
t organizationthat promotes ICT in Latin America through training and development projects. EsLaRed was awarded the 2008 Jonathan B. Postel Service Award by the Internet Society. Ermanno has been deploying wireless data communication networks focusing on low cost technology, and has participated in the planning and building of wireless data networks in Argentina, Colombia, Ecuador, Italy, Lesotho, Malawi, Mexico, Morocco, Nicaragua, Peru, Trinidad, U.S.A. and Venezuela. He has presented in many conferences and published several papers related to wireless data communication and is coauthor and technical reviewer of the book "Wireless Networking for the Developing World" freely available from http://wndw.net. Ermanno holds a Master's Degree from Stanford University and was Professor of Telecommunications at Universidad de los Andes in Venezuela from 1970 to 2000. Ermanno can be reached at [email protected]
Marco Zennaro. Marco received his M.Sc. Degree in Electronic
Engineering from University of Trieste in Italy. He defended his PhD thesis on “Wireless Sensor Networks for Development: Potentials and Open Issues” at KTH-Royal Institute of Technology, Stockholm, Sweden. His research interest is in ICT4D, the use of ICT for Development.
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In particular, he is interested in Wireless Networks and in Wireless Sensor Networks in developing countries. He has been giving lectures on Wireless
technologies in more than 20 different countries. When not travelling, he is the editor of wsnblog.com. Marco can be reached at [email protected]
Carlo Fonda is a member of the Radio Communications Unit at the Abdus Salam International Center for eoretical Physics in Trieste, Italy. Carlo can be reached at [email protected]
Stephen Okay.Steve is a geek-of-all-trades with over 20 years of
experience in systems/network programming and administration with a particular passion for free/open networks and software. He has deployed wireless networks in Laos, Malawi, Italy, and the United States. He is an Inveneo co-founder and has taught workshops on VoIP and wireless networking at Institutions around the world. He lives and hacks in San Francisco, California. Steve can be reached at [email protected]
Corinna Elektra Aichele
. Elektra has been busy working on mesh
networking protocols for the Freifunk community in Germany. Beforeinventing the B.A.T.M.A.N. routing protocol for wireless mesh networks in 2006, she was working on improving the OLSR routing protocol. She is one of the people behind the Mesh-Potato device, a rugged outdoor open-source and open-hardware WiFi router with an FXS port. She is part of the Villagetelco community, that strives to deploy mesh networks for VOIP and data. She lives in a solar-powered home in Berlin, Germany. e philosophy behind her ideas about ubiquitous
communication for everyone is: "e fact that you talk in your head doesn'tmean that you think - but only that you speak with yourself".Elektra can be reached at [email protected] http://villagetelco.org http://open-mesh.net/
Sebastian Buettrich.
Sebastian is Research Lab Manager at the IT University of Copenhagen, http://pit.itu.dk
He works with embedded/pervasive systems, wireless technology, opensource / free software and solar energy to build networks, systems, skills and capacity as a manager, developer, architect, consultant and teacher. is work focused on (but not limited to) developing countries and communities, especially in Asia and Africa. One current focus is to help
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develop campus networks for research and education, with emphasis on global integration and sustainability. His current side affiliations are:
http://www.nsrc.org - the Network Startup Resource Center http://wire.less.dk - NGO and company co-founded with Tomas Krag http://wirelessU.org - a group of dedicated professionals working towards a world-wide, people-centered, inclusive Information Society http://wndw.net/ - Co-author of the Wireless Networking in the Developing World book. Sebastian holds a Ph.D. in Quantum Physics from the Technical University of Berlin in Germany, with a focus on optics, radio spectroscopy, photovoltaic systems and scientific
programming. He loves and plays music, is fascinated and engaged with text, language and poetry in many forms. Sebastian can be reached at [email protected]
Jim Forster. Jim is passionate about extending the Internet. He started at Cisco in 1988 when it was quite small and spent 20 years there, mostly in IOS Software Development and System Architecture, and becoming a Distinguished Engineer. While at Cisco he started working on projects
and policies to improve Internet access in developing countries. Now he isengaged in both for-profit and non-profit efforts to extend communications in Africa and India. He founded networktheworld.org, a foundation dedicated to improving communications and Internet, especially in Africa and India. He is on several Board of Directors, including Range Networks / OpenBTS and Inveneo in the US, Esoko Networks in Ghana, and AirJaldi in India. Jim can be reached at [email protected]
Klaas Wierenga.
Klaas works in the Research and Advanced Development group at Cisco Systems where he focuses on Identity, Security and Mobility topics, often in collaboration with the Research and Education Community. He is co-author of the Cisco Press book "Building the Mobile Internet". Prior to joining Cisco he worked at SURFnet, the Dutch Research and Education Network, where he created the global WiFi roaming service in academia called eduroam. He is also the Chair of
the Mobility Task Force of TERENA, the European association of R&ENetworks. Klaas participates in a number of IETF working groups in the fields of identity, security and mobility and chairs the abfab working group that deals with federated identity for non-web applications. He can be reached at [email protected]
Eric Vyncke.
Since 1997, Eric has worked as a Distinguished Engineer at Cisco in thefield of security by assisting customers to deploy secure
networks. Since 2005, Eric has also been active in the IPv6 area, he is notably the co-chair of the Belgian IPv6 Council and has a well-known site for monitoring IPv6 deployments: http://www.vyncke.org/ipv6status/ He is also Associate Professor at the University of Liège in Belgium. He participates in several IETF working groups related to security or to IPv6. Eric can be reached at [email protected]
Bruce Baikie. Bruce is a member of the Broadband for Good team at
Inveneo as Senior Director Broadband Initiatives. He is leveraging his extensive experience in the energy and telecom industries, and 16 years at Sun Microsystems as telecom industry expert to advise on implementing solar powered ICT4D projects. His areas of expertise include: wireless networking, eco-data centers, DC telecom power systems, and solar power. Bruce has published numerous white papers and articles on green data center operations and solar power in ICT4D. His educational background includes a B.S. in Mechanical Engineering from Michigan
Technological University and advanced studies in International Businessfrom the University of Wisconsin. Bruce is also a guest lecturer on solar powered ICT4D at the Abdus Salam International Centre for eoretical Physics in Trieste, Italy. During past two years, Bruce has been mentoring engineering students from Illinois Institute of Technology, University of Colorado-Boulder, San Francisco State University, and San Jose State University in ICT4D design and projects in Haiti, West Africa, and Micronesia. Bruce can be reached at [email protected]
Laura Hosman
. Laura is Assistant Professor of Political Science at Illinois Institute of Technology. Prior to IIT, Professor Hosman held postdoctoral research fellow positions at the University of California, Berkeley and the University of Southern California (USC). She graduated with a PhD in Political Economy and Public Policy from USC. Her current research focuses on the role of information and communications technology (ICT) in developing countries, particularly in terms of its potential effects on
socio-cultural factors, human development, and economic growth.Her work focuses on two main areas: Public-Private Partnerships and ICT-in-education, both with a focus on the developing world. Her blog, giving insights on herfieldwork experiences, is at http://ict4dviewsfromthefield.wordpress.com
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e editorial team would especially like to recognise the support of ourtechnical illustrator, Paolo Atzori, who has over several months worked tirelessly to ensure the book has some wonderful, as well as accurate, easy to read illustrations. He has also ensured that we've been able to publish successfully several versions of the book in high and low resolution format.
Paolo Atzori. Paolo studied Architecture in Venice and Rome and Media Arts in Cologne. After working as an architect in Vienna, Paolo
collaborated with the Cologne Academy of Media Arts (KHM); AtNABA, Milan he was named the Director of the Master Digital Environment Design and Advisor of the PhD program of the Planetary Collegium, M- Node. He has created many theatrical and artistic projects, introducing new representations of space characterised by the dynamics of pervasiveness and interaction. Paolo has also curated exhibitions dedicated to digital arts, directed educational programs, and published articles and essays on digital culture. He has lived and worked in Venice, Rome, New York, Vienna, Cologne, Brussels, Tel Aviv. Since 2005 he has lived with his partner Nicole and their children Alma and Zeno in Trieste, Italy. In 2011 he founded with Nicole Leghissa the Agency "Hyphae”. http://hyphae.org http://vimeo.com/groups/xtendedlab/videos http://www.xtendedlab.com/ http://www.khm.de/~Paolo
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Rob Flickenger.
Rob has written and edited several books about wireless networking and Linux, including Wireless Hacks (O’Reilly) and How To Accelerate Your Internet(http://bwmo.net/). He is proud to be a hacker, amateur mad scientist, and proponent of free networks everywhere.
Laura M. Drewett
is a Co-Founder of Adapted Consulting Inc., a social
enterprise that specialises in adapting technology and business solutionsfor the developing world. Since Laurafirst lived in Mali in the 1990s and wrote her thesis on girls’ education programs, she has strived tofind sustainable solutions for development. Laura holds a Bachelors of Arts
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with Distinction in Foreign Affairs and French from the University of Virginia and a Master’s Certificate in Project Management from the
George Washington University School of Business.
Alberto Escudero-Pascual andLouise Berthilson are the founders of IT +46, a Swedish consultancy company with focus on information technology in developing regions. More information can be found at http://www.it46.se/
Ian Howard. Afterflying around the world for seven years as a paratrooper
in the Canadian military, Ian Howard decided to trade his gun for a computer. Afterfinishing a degree in environmental sciences at the University of Waterloo he wrote in a proposal, "Wireless technology has the opportunity to bridge the digital divide. Poor nations, who do not have the infrastructure for interconnectivity as we do, will now be able to create a wireless infrastructure." As a reward, Geekcorps sent him to Mali as the Geekcorps Mali Program Manager, where he led a team equipping radio stations with wireless interconnections and designed content sharing
systems.
Tomas Krag
spends his days working with wire.less.dk, a registered non- profit, based in Copenhagen, which he founded with his friend and colleague Sebastian Büttrich in early 2002. wire.less.dk specialises in community wireless networking solutions, and has a special focus on low-
cost wireless networks for the developing world. Tomas is also an associateof the Tactical Technology Collective http://www.tacticaltech.org, an Amsterdam-based non-profit “to strengthen social technology movements and networks in developing and transition countries, as well as promote civil society’s effective, conscious and creative use of new technologies.” Currently most of his energy goes into the Wireless Roadshow (http://www.thewirelessroadshow.org), a project that supports civil society partners in the developing world in planning, building and sustaining
connectivity solutions based on license-exempt spectrum, open technologyand open knowledge.
Gina Kupfermann
is graduate engineer in energy management and holds a degree in engineering and business. Besides her profession asfinancial
controller she has worked for various self-organised community projects and non-profit organisations. Since 2005 she is member of the executive board of the development association for free networks, the legal entity of freifunk.net
Adam Messer.
Originally trained as an insect scientist, Adam Messer metamorphosed into a telecommunications professional after a chance conversation in 1995 led him to start one of Africa'sfirst ISPs. Pioneering
wireless data services in Tanzania, Messer worked for 11 years in eastern and southern Africa in voice and data communications for startups and multinational cellular carriers. He now resides in Amman, Jordan.
Juergen Neumann (http://www.ergomedia.de) started working with information technology in 1984 and since then has been looking for ways to deploy ICT in useful ways for organizations and society. As a consultant for ICT strategy and implementation, he worked for major
German and international companies and many non-profit projects. In2002 he co-foundedwww.freifunk.net, for spreading knowledge and social networking about free and open networks. Freifunk is globally regarded as one of the most successful community-projects in thisfield.
Frédéric Renet is a co-founder of Technical Solutions at Adapted Consulting, Inc. Frédéric has been involved in ICT for more than 10 years and has worked with computers since his childhood. He began his ICT
career in the early 1990s with a bulletin board system (BBS) on an analogmodem and has since continued to create systems that enhance communication. Most recently, Frédéric spent more than a year at IESC/Geekcorps Mali as a consultant. In this capacity, he designed many innovative solutions for FM radio broadcasting, school computer labs and lighting systems for rural communities.
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(5%8#=$+%(#5 is book aims to empower people to build DIY networks using wireless technologies. It has been compiled by a bunch of networking geeks that have been busy designing, deploying and operating wireless networks for quite some time, all of them actively participating in expanding the reach of the Internet all over the world.
We believe that people can have a significant stake in building their own
communications infrastructure and also influence the wider communityaround them to make sure networks become affordable and available. We hope to not only convince you that this is possible, but also show how we have done it, and to give you the information and tools you need to start a network project in your local community.
By providing people in your local community with cheaper and easier access to information, they will directly benefit from what the Internet has
to o ff
e time and e ff
ort saved by having access to the global networkof information translates into value on a local scale. Likewise, the network becomes all the more valuable as more people are connected to it. Communities connected to the Internet at high speed have a voice in a global marketplace, where transactions happen around the world at the speed of light. People all over the world arefinding that Internet access gives them a voice to discuss their problems, politics, and whatever else is important in their lives, in a way that the telephone and television simply cannot compete. What has until recently sounded like sciencefiction is now becoming a reality, and that reality is being built on wireless networks.
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Now lets for a moment look at afictional country called 'Aipotu', in the developing world. Aipotu has been connected to the Internet merely by expensive VSAT links for a long time.
A brand new optical submarine telecommunications connection hasfinally arrived at the shore of Aipotu. e challenge for Aipotu is now to roll out a complete communication infrastructure for a whole country from scratch.
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e method of choice today is likely a three tier strategy. First and foremost Aipotu should try to roll out opticalfibre lines wherever possible.
Fibre lines offer the capability to transport a "sea of bandwidth". e cost of opticalfibre is very low, considering the capacity. By upgrading the optical transceivers the capacity of a opticalfibre line can be upgraded without laying new cable. If Aipotu can afford to establish afibre connection to every household there is no reason not to go for it. is would make our three tier model obsolete and we could stop here. However, there are probably areas in Aipotu that cannot affordfibre lines.
e second tier that the people of Aipotu can use in order to connect remote villages or small cities are high speed point-to-point links between high points. It is possible to establish high speed links (40 Mbps) of 30 km or more between towers of 30 metre height onflat terrain. If mountain tops, high buildings or hill tops are available, even longer links are possible. e network technology experts of Aipotu don't have to worry too much about the wireless technology that they are mounting on top of their towers - the cost lies mostly in building the towers, proper
lightning protection, power supplies, power back-up and theft protection,rather than in the actual wireless equipment and antennas. Like the technology of optical transceivers, wireless transceivers also keep advancing, but a wireless link will always be orders of magnitude slower than the capacity of opticalfibre.
e third challenge for Aipotu is to solve the problem of the last mile(s): Distributing access to all the individual households, offices, production
facilities and so on. Not too long ago the method of choice was to runcopper wires but now there is a better choice. is third tier of our network model is clearly the domain of wireless networking technology.
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e overall goal of this book is to help you build affordable communication technology in your local community by making best use of whatever resources are available.
Using inexpensive off-the-shelf equipment, you can build high speed data networks that connect remote areas together, provide broadband network access in areas where even dialup does not exist, and ultimately connect you and your neighbours to the global Internet.
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By using local sources for materials and fabricating parts yourself, you can build reliable network links with very little budget. And by working with
your local community, you can build a telecommunications infrastructure that benefits everyone who participates in it. is book is not a guide to configuring wireless for your laptop or choosing consumer grade gear for your home network. e emphasis is on building infrastructure links intended to be used as the backbone for wide area wireless networks as well as solving the last mile problem.
With those goals in mind, information is presented from many points of
view, including technical, social, andfinancial factors. e extensive collection of case studies included present various groups’ attempts at building these networks, the resources that were committed to them, and the ultimate results of these attempts.
It is also important to note that all of the resources, techniques and design methodologies described in this book are valid in any part of the world. ere are many rural parts all over the globe that remain unconnected to
the Internet for reasons of cost, geography, politics and so on.Deploying wireless networking can often lead to these problems being solved thereby extending connectivity to those who as yet have not achieved it. ere are many community based networking projects emerging everywhere. So whether you live in the United Kingdom, Kenya, Chile or India or anywhere else, this book can be a useful practical guide.
Since thefirst spark gap experiments at the turn of the XIX century,
wireless has been a rapidly evolving area of communications technology. While we provide specific examples of how to build working high speed data links, the techniques described in this book are not intended to replace existing wired infrastructure (such as telephone systems orfibre optic backbone).
Rather, these techniques are intended to augment existing systems, and provide connectivity in areas where runningfibre or other physical cable
would be impractical. We hope youfind this book useful for solving your communication challenges.
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If you are a network administrator, you may wonder how wireless might fit into your existing network infrastructure. Wireless can serve in many capacities, from a simple extension (like a several kilometre Ethernet cable) to a distribution point (like a large hub). Here just a few examples of how your network can benefit from wireless technology.
Figure I 1: Some wireless networking examples.
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Physics
Networking
.
>>
Within the 4 main sections there are chapters written by key experts with theoretical and practical hands on experience of the topics. ere are a broad range of topics in the chapters which have been selected as being key to enabling you to start and grow a real wireless deployment in your own community. Another resource you mayfind useful is here - http://wtkit.org/groups/wtkit/wiki/820cb/download_page.html
It is the set of presentation materials used by these same key experts to deliver wireless networking training classes around the world. In addition all of the key experts who have written this book regularly
check our Facebook page. So as you plan your deployment please do ask questions on our page - we do answer quickly. https://www.facebook.com/groups/wirelessu
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Wireless communications make use of electromagnetic waves to send signals across long distances. From a user’s perspective, wireless connections are not particularly different from any other network connection: your web browser, email, and other applications all work as you would expect. But radio waves have some unexpected properties compared to Ethernet cable. For example, it’s very easy to see the path
that an Ethernet cable takes: locate the plug sticking out of yourcomputer, follow the cable to the other end, and you’ve found it! You can also be confident that running many Ethernet cables alongside each other won’t cause problems, since the cables effectively keep their signals contained within the wire itself. But how do you know where the waves emanating from your wireless device are going? What happens when these waves bounce off objects in the room or other buildings in an outdoor link? How can several wireless cards be used in the same area without interfering with each other? In order to build stable high-speed wireless links, it is important to understand how radio waves behave in the real world.
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We are all familiar with vibrations or oscillations in various forms: a pendulum, a tree swaying in the wind, the string of a guitar - these are all examples of oscillations.
What they have in common is that something, some medium or object, isswinging in a periodic manner, with a certain number of cycles per unit of time. is kind of wave is sometimes called amechanicalwave, since it is defined by the motion of an object or its propagating medium. When such oscillations travel (that is, when the swinging does not stay bound to one place) then we speak of waves propagating in space. For example, a singer singing creates periodic oscillations in his or her vocal cords. ese oscillations periodically compress and decompress the air, and
this periodic change of air pressure then leaves the singers mouth andtravels, at the speed of sound. A stone plunging into a lake causes a disturbance, which then travels across the lake as awave.
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A wave has a certainspeed,frequency, andwavelength. ese are connected by a simple relation:
Speed = Frequency * Wavelength
e wavelength (sometimes referred to aslambda ) is the distance measured from a point on one wave to the equivalent part of the next (or, in a more technical way, to the next point that is in the same phase), for example from the top of one peak to the next. e frequency is the number of whole waves that pass afixed point in a
period of time. Speed is measured in metres/second, frequency is measuredin cycles per second (or Hertz, represented by the symbolHz), and wavelength is measured in metres. For example, if a wave on water travels at one metre per second, and it oscillatesfive times per second, then each wave will be twenty centimetres long:
1 metre/second = 5 cycles/second *W W = 1 / 5 metres
W = 0.2 metres = 20 cm Waves also have a property calledamplitude. is is the distance from the centre of the wave to the extreme of one of its peaks, and can be thought of as the “height” of a water wave. Frequency, wavelength, and amplitude are shown in Figure RP 1.
Figure RP 1: Wavelength, amplitude, and frequency. For this wave, the frequency is 2 cycles per second, or 2 Hz, while the speed is 1 m/s.
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Waves in water are easy to visualize. Simply drop a stone into the lake and you can see the waves as they move
across the water over time. In the case of electromagnetic waves, the partthat might be hardest to understand is: “What is it that is oscillating?” In order to understand that, you need to understand electromagnetic forces.
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Electromagnetic forces are the forces between electrical charges and currents. Our most direct access to those is when our hand touches a door handle after walking on synthetic carpet, or brushing up against an electrical fence. A more powerful example of electromagnetic forces is the lightning we see during thunderstorms. eelectrical forceis the force between electrical charges. emagnetic forceis the force between electrical currents. Electrons are particles that carry a negative electrical charge. ere are other charged particles too, but it is the electrons that are responsible for most of what we need to know about how radio behaves. Let us look at what is happening in a piece of straight vertical wire, in which we push the electrons from one end to the other and back, periodically. At one moment, the top of the wire is negatively charged - all the negative electrons are gathered there. is creates an electricfield from the positively charged end to the negatively charged one along the wire. e next moment, the electrons have all been driven to the other side, and the electricfield points the other way. As this happens again and again,
the electricfield vectors (represented by arrows from plus to minus) are leaving the wire, so to speak, and are radiated out into the space around the wire. What we have just described is known as a dipole (because of the two differently charged poles, plus and minus, that are created in the straight vertical wire), or more commonly adipole antenna. is is the simplest form of an omnidirectional antenna. e moving electricfield is commonly referred to as an electromagnetic wave because
there is also an associated magneticfield. A moving electricfield, such as a wave, always comes together with a magneticfield - you will notfind one with out the other. Why is this the case? An electricfield is caused by electrically charged objects.
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A moving electricfield is produced by moving electrically charged objects, such as we have just described above in a dipole antenna.
Wherever electrical charges are moving, they induce a magneticfield. Mathematically, this is formulated in Maxwell's equations: https://en.wikipedia.org/wiki/Electromagnetic_field#Mathematical_description
Since the electrical and magnetic components are tied together in this way, we speak of an electromagneticfield. In practical wireless networking, we focus in the electrical component but
there be always a magnetic component as well.
Let us come back to the relation:
Speed = Frequency * Wavelength
In the case of electromagnetic waves, the speed isc, the speed of light.
c = 300,000 km/s = 300,000,000 m/s = 3*108m/s
c = f *!
Electromagnetic waves differ from mechanical waves in that they require no medium in which to propagate. Electromagnetic waves will even propagate through perfect vacuum. e light from the stars is a good example: it reaches us through the vacuum of space.
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In physics, maths, and engineering, we often express numbers by powers of ten. We will meet these terms again, and the symbols used to represent them, e.g. gigahertz (GHz), centimetres (cm), microseconds (µs), and so on.
ese symbols are part of the international system of measurementSI(http://www.bipm.org/utils/common/pdf/si_brochure_8_en.pdf), they are not abbreviations and should not be changed. e case is significant and should not be altered.
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Knowing the speed of light, we can calculate the wavelength for a given frequency. Let us take the example of the frequency of 802.11b wireless networking, which is:
f = 2.4 GHz = 2,400,000,000 cycles / second
wavelength (! ) = c / f = 3*108 / 2.4*109= 1.25*10-1 m = 12.5 cm
Frequency and therefore wavelength determine most of an electromagnetic wave’s behaviour. It governs the dimensions of the antennas that we build as well as the effect of the interactions with objects that are in the propagation path, including the biological effects in living beings.
Wireless standards of course are distinguished by more than just the frequency they are working at - for example, 802.11b, 802.11g, 802.11n and 802.16 can all work at 2.4 GHz -, yet they are very different from one another.
e chapter calledTelecommunications Basicswill discuss modulation techniques, media access techniques, and other relevant features of wireless
communications standards. However, the basic capabilities ofelectromagnetic waves to penetrate objects, to go long distances, and so forth - these are determined by physics alone. e electromagnetic wave "does not know or care" what modulation or standard or technique you put on top of it. So, while different standards may implement advanced techniques to deal with NLOS (Non Line of Sight), multipath and so forth - they still cannot make a wave go through a wall, if that wall is absorbing the respective frequency. erefore, an understanding of the basic
ideas of frequency and wavelength helps a lot in practical wireless work.
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Later in this chapter, we will talk about concepts like interference, multipath and Fresnel zones. In order to understand these, we will need to know about thephaseof a wave, or rather,phase di erencesbetween waves. Look at the sine wave shown in Fig RP 1 - now imagine we have two such waves moving. ese can be in exactly the same position: Where the one has its peak, the other one also has a peak. en, we would say, they are in phase, or, their phase difference is zero. But one wave could also be displaced from the other, for example it could have its peak where the other wave is at zero. In this case, we have a phase difference. is phase difference can be expressed in fractions of the wavelength, e.g.!//4, or in degrees, e.g. 90 degrees - with one full cycle of the wave being 360 degrees. A phase difference of 360 degrees is the same as that of 0 degrees: no phase difference.
Figure RP 2: Phase Di fference between Two Waves
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Another important quality of electromagnetic waves ispolarization.Polarization describes the direction of the electricalfield vector. If you imagine a vertically aligned dipole antenna (the straight piece of wire), electrons can only move up and down, not sideways (because there is no room to move) and thus electricalfields only ever point up or down, vertically. efield leaving the wire and travelling as a wave has a strict linear (and in this case, vertical) polarization. If we put the antennaflat on the ground, we wouldfind horizontal linear polarization.
Linear polarization is just one special case, and is never quite so perfect: in general, we will always have some component of thefield pointing in
other directions too. If we combine two equal dipoles fed with the same signal, we can generate a circularly polarized wave, in which the electric field vector keeps rotating perpendicularly to the wave's trajectory.
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e most general case is elliptical polarization, in which the electricfield vector maximum value is not the same in the vertical and horizontal
direction. As one can imagine, polarization becomes important whenaligning antennas. If you ignore polarization, you might have very little signal even though you have the best antennas. We call this polarization mismatch. Much in the same way, polarization may also be used in a smart way, to keep two wireless links independent and without interference, even though they might use the same end points (or even share a common reflector) and therefore the same trajectory: if one link is polarized
vertically and the other horizontally, they will not "see" each other. is isa convenient way to double data rates over one link using a single frequency. e antennas used in this kind of application must be carefully built in order to reject the "unwanted" polarization, i.e. an antenna meant for vertical polarization must not receive or transmit any horizontally polarized signal, and vice versa. We say they must have a high "cross polarization" rejection.
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Electromagnetic waves span a wide range of frequencies (and, accordingly, wavelengths). is range of frequencies or wavelengths is called the electromagnetic spectrum. e part of the spectrum most familiar to humans is probably light, the visible portion of the electromagnetic spectrum. Light lies roughly between the frequencies of 7.5*1014 Hz and 3.8*1014 Hz, corresponding to wavelengths from circa 400 nm
(violet/blue) to 800 nm (red). We are also regularly exposed to other regions of the electromagnetic spectrum, including Alternating Current (AC) or grid electricity at 50/60 Hz, AM and FM radio, Ultraviolet (at frequencies higher than those of visible light), Infrared (at frequencies lower than those of visible light), X-Ray radiation, and many others. Radio is the term used for the portion of the electromagnetic spectrum in which waves can be transmitted by applying alternating current to an
antenna. is is true for the range from 30 kHz to 300 GHz, but in the more narrow sense of the term, the upper frequency limit would be about 1 GHz, above which we talk of microwaves and millimetric waves.
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When talking about radio, many people think of FM radio, which uses a frequency around 100 MHz. In between radio and infrared wefind the
region of microwaves - with frequencies from about 1 GHz to 300 GHz,and wavelengths from 30 cm to 1 mm. e most popular use of microwaves might be the microwave oven, which in fact works in exactly the same region as the wireless standards we are dealing with. ese regions lie within the bands that are being kept open for general unlicensed use. is region is called theISM band, which stands for Industrial, Scientific, and Medical. Most other parts of the electromagnetic spectrum are tightly controlled by
licensing legislation, with license values being a huge economic factor. Inmany countries the right to use portions of the spectrum have been sold to communications companies for millions of dollars. In most countries, the ISM bands have been reserved for unlicensed use and therefore do not have to be paid for when used.
Figure RP 4:e electromagnetic spectrum.
e frequencies most interesting to us are 2.400 - 2.495 GHz, which is used by the 802.11b and 802.11g standards (corresponding to wavelengths of about 12.5 cm), and 5.150 - 5.850 GHz (corresponding to wavelengths of about 5 to 6 cm), used by 802.11a. e 802.11n standard can work in either of these bands. See the Chapter calledWiFi Familyfor an overview of standards and
frequencies. In addition you canfind out more about the Radio portion of the electromagnetic spectrum in the Chapter calledRadio Spectrum.
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A term you will meet often in radio physics isbandwidth. Bandwidth issimply a measure of frequency range. If a range of 2.40 GHz to 2.48 GHz is used by a device, then the bandwidth would be 0.08 GHz (or more commonly stated as 80 MHz). It is easy to see that the bandwidth we define here is closely related to the amount of data you can transmit within it - the more room in frequency space, the more data you canfit in at a given moment. e term bandwidth is often used for something we should rather call data rate, as in “my Internet connection has 1 Mbps of bandwidth”, meaning it can transmit data at 1 megabit per second. How much exactly you canfit into a physical signal will depend on the modulation, encoding and other techniques. For example, 802.11g uses the same bandwidth as 802.11b, however itfits more data into those same frequency ranges transmitting up to 5 times more bits per second. Another example we have mentioned: you may double your data rate by adding a second link at perpendicular polarization to an existing radio link. Here, frequency and bandwidth have not changed, however the
data rate is doubled.
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Let us look a bit closer at how the 2.4 GHz band is used in 802.11b. e spectrum is divided into evenly sized pieces distributed over the band as individual channels. Note that channels are 22 MHz wide, but are only separated by 5 MHz.
is means that adjacent channels overlap, and can interfere with eachother. is is represented visually in Figure RP 5.
Figure RP 5: Channels and centre frequencies for 802.11b. Note that channels 1, 6, and 11 do not overlap.

ere are a few simple rules of thumb that can prove extremely usefulwhen makingfirst plans for a wireless network:
" the longer the wavelength, the further it goes; " the longer the wavelength, the better it travels through and around things; " the shorter the wavelength, the more data it can transport.
All of these rules, simplified as they may be, are rather easy to understand
by example.
Longer waves travel further
Waves with longer wavelengths tend to travel further than waves with shorter wavelengths. As an example, AM radio stations have a much greater range than FM stations, which use a frequency 100 times higher. Lower frequency transmitters tend to reach much greater distances than high frequency transmitters at the same power.
Longer waves pass around obstacles
A wave on water which is 5 metres long will not be affected by a 5 mm piece of woodfloating on the water. If instead the piece of wood were 50 metres big (e.g. a ship), it would modify the behavior of the wave.
e distance a wave can travel depends on the relationship between the wavelength of the wave and the size of obstacles in its path of
propagation. It is harder to visualize waves moving “through” solid objects,but this is the case with electromagnetic waves. Longer wavelength (and therefore lower frequency) waves tend to penetrate objects better than shorter wavelength (and therefore higher frequency) waves. For example, FM radio (88-108 MHz) can travel through buildings and other obstacles easily, while shorter waves (such as GSM phones operating at 900 MHz or 1800 MHz) have a harder time penetrating buildings. is effect is partly due to the difference in power levels used for FM radio and GSM, but is also partly due to the shorter wavelength of GSM signals. At much higher frequencies, visible light does not go through a wall or even 1 mm of wood - as we all know, from practical experience. But metal will stop any kind of electromagnetic wave.
Shorter waves can carry more data
e faster the wave swings or beats, the more information it can carry - every
beat or cycle could for example be used to transport a digital bit, a '0' or a'1', a 'yes' or a 'no'. So the data rate scales with bandwidth, and can be further enhanced by advanced modulation and media access techniques such as OFDM, and MIMO (Multiple Input, Multiple Output).
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ere is another principle that can be applied to all kinds of waves, and which is extremely useful for understanding radio wave propagation. is principle is known as theHuygens Principle, named after Christiaan Huygens, Dutch mathematician, physicist and astronomer, 1629 - 1695.
Imagine you are taking a little stick and dipping it vertically into a still lake's surface, causing the water to swing and dance. Waves will leave the centre of the stick - the place where you dip in - in circles. Now, wherever water particles are swinging and dancing, they will cause their neighbor
particles to do the same: from every point of disturbance, a new circular wave will start. is is, in simple form, the Huygens principle. In the words of wikipedia.org: “e Huygens' principle is a method of analysis applied to problems of wave propagation in the far field limit. It recognizes that each point of an advancing wave front is in fact the centre of a fresh disturbance and the source of a new train of waves; and that the advancing wave as a whole may be regarded as the sum of all the secondary waves arising from points in the
medium already traversed”.
is view of wave propagation helps better understand a variety of wave phenomena, such as diffraction.” is principle holds true for radio waves as well as waves on water, for sound as well as light, but for light the wavelength is far too short for human beings to actually see the effects directly. is principle will help us to understand diffraction as well as Fresnel
zones, and the fact that sometimes we seem to be able to transmit aroundcorners, with no line of sight. Let us now look into what happens to electromagnetic waves as they travel.
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When electromagnetic waves go through 'something' (some material),they generally get weakened or dampened. How much they lose in power will depend on their frequency and of course the material. Clear window glass is obviously transparent for light, while the glass used in sunglassesfilters out quite a share of the light intensity and most of the ultraviolet radiation. Often, an absorption coefficient is used to describe a material’s impact on radiation.
For microwaves, the two main absorbent materials are:
Metal. Electrons can move freely in metals, and are readily able to swing and thus absorb the energy of a passing wave. Water. Microwaves cause water molecules to jostle around, thus taking away some of the wave’s energy.
For the purpose of practical wireless networking, we may well consider metal and water perfect absorbers: we will not be able to go through them (although thin layers of water will let some power pass). ey are to microwave what a brick wall is to light.
When talking about water, we have to remember that it comes in different forms: rain, fog and mist, low clouds and so forth, all will be in the way of radio links. ey have a strong influence, and in many circumstances a
change in weather can bring a radio link down.
When talking about metal, keep in mind that it may be found in unexpected places: it may be hidden in walls (for example, as metal grids in concrete) or be a thin coat on modern types of glass (tinted glass, colored glass). However thin the layer of metal, it might be enough to significantly absorb a radio wave.
ere are other materials that have a more complex effect on radio absorption. For trees and wood, the amount of absorption depends on how much water they contain.
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Old dead dry wood is more or less transparent, wet fresh wood will absorb a lot. Plastics and similar materials generally do not absorb a lot of radio
energy, but this varies depending on the frequency and type of material.Lastly, let us talk about ourselves: humans (as well as other animals) are largely made out of water.
As far as radio networking is concerned, we may well be described as big bags of water, with the same strong absorption. Orienting an office access point in such a way that its signal must pass through many people is a key mistake when building office networks.
e same goes for hotspots, cafe installations, libraries, and outdoorinstallations.
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Just like visible light, radio waves are reflected when they come in contact with materials that are suited for that: for radio waves, the main sources of reflection are metal and water surfaces. e rules for reflection are quite simple: the angle at which a wave hits a surface is the same angle at which it gets deflected.
Note that in the eyes of a radio wave, a dense grid of bars acts just the same as a solid surface, as long as the distance between bars is small compared to the wavelength.
At 2.4 GHz, a one cm metal grid will act much the same as a metal plate.
Although the rules of reflection are quite simple, things can become very complicated when you imagine an office interior with many many small metal objects of various complicated shapes. e same goes for urban situations: look around you in city environment and try to spot all of the metal objects.
is explains whymultipath e ects(i.e. signal reaching their target along different paths, and therefore at different times) play such an important
role in wireless networking. Water surfaces, with waves and ripples changing all the time, effectively make for a very complicated reflection object which is more or less impossible to calculate and predict precisely.
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Figure RP 6: Re flection of radio waves.e angle of incidence is always equal to the angle of re flection. A metal parabolic surface uses this e ffect to concentrate
radio waves spread out over it in a common direction.
We should also add that polarization has an impact: waves of different polarization in general will be reflected differently. We use reflection to our advantage in antenna building: e.g. we put huge parabolas behind our radio transmitter/receiver to collect and bundle the radio signal into a single point, the focal point.
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Diffraction is the apparent bending of waves when hitting an object. It is the effect of “waves going around corners”. Imagine a wave on water traveling in a straight wave front, just like a wave that we see rolling onto an ocean beach. Now we put a solid barrier, say a wooden solid fence, in its way to block it. We cut a narrow slit opening into that wall, like a small door. From this opening, a circular wave will start, and it will of course reach points that are not in a direct line behind this opening, but also on either
side of it. If you look at this wavefront - and it might just as well be an electromagnetic wave - as a beam (a straight line), it would be hard to explain how it can reach points that should be hidden by a barrier. When modelled as a wavefront, the phenomenon makes sense.
Figure RP 7: Di ffraction through a narrow slit.
e Huygens Principle provides one model for understanding this behavior. Imagine that at any given instant, every point on a wavefront can be considered the starting point for a spherical “wavelet”.
is idea was later extended by Fresnel, and whether it adequatelydescribes the phenomenon is still a matter of debate. But for our purposes, the Huygens model describes the effect quite well.
Figure RP 8:e Huygens Principle.
rough means of the effect of diffraction, waves will “bend” around corners or spread through an opening in a barrier.
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e wavelengths of visible light are far too small for humans to observe this effect directly.
Microwaves, with a wavelength of several centimeters, will show theeffects of diffraction when waves hit walls, mountain peaks, and other obstacles. It seems as if the obstruction causes the wave to change its direction and go around corners.
Figure RP 9: Di ffraction over a mountain top.
Note that diffraction comes at the cost of power: the energy of the diffracted wave is significantly less than that of the wavefront that caused it. But in some very specific applications, you can take advantage of the diffraction effect to circumvent obstacles.
I&'$#2$#$&7$ Interference is one of the most misunderstood terms and phenonema in wireless networking. Interference often gets the blame when we are too lazy tofind the real problem, or when a regulator wants to shut down someone else's network for business reasons. So, why all the misunderstandings? It is mostly because different people mean different things though they are using the same word. A physicist and a telecommunications engineer will use the word "Interference" in very different ways. e physicists' view will be concerned with the "behaviour of waves". e telecommunications engineer will talk about “... any noise that gets in the way”.
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Both views are relevant in wireless, and it is important to be able to know them both and know the difference. Let us start with the physicists' view:
When working with waves, one plus one does not necessarily equal two. Itcan also result in zero.
Figure RP 10: Constructive and destructive interference.
is is easy to understand when you draw two sine waves and add up the
amplitudes. When the phase difference is zero, peak hits peak, and you will have maximum results (1 + 1 = 2). is is calledconstructive interference. When the phase difference is 180 degrees, or!/2, peak hits valley, and you will have complete annihilation ((1 + (-)1 = 0) -destructive interference. You can actually try this with waves on water and two little sticks to create circular waves - you will see that where two waves cross, there will be areas
of higher wave peaks and others that remain almostflat and calm. In order for whole trains of waves to add up or cancel each other out perfectly, they have to have the exact same wavelength and afixed phase relation.
You can see obvious examples of interference in action when you look at the way that antennas are arranged in what are calledbeamformingarrays, in order to give maximum constructive interference in the directions
where you want the signal, and destructive interference (no signal) whereyou want no signal. Technically, this is achieved by a combination of physical dimensioning and control of phase shifts.
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Simplified, imagine that you have three antennas - and you don't want antenna 3 to pick up signal from antenna 1 and 2. You would then place
antenna 3 at a position where the signals from antennas 1 and 2 canceleach other out. Now let us have a look at the way the word interference is typically used: in a wider sense, for any disturbance through other RF sources, any noise that might get in our way, e.g. from neighboring channels or competing providers. So, when wireless networkers talk about interference they typically talk about all these kinds of disturbance by other networks, and any other sources of microwave, whether it has exactly the same frequency
and afixed phase relation or not. Interference of this kind is one of themain sources of difficulty in building wireless links, especially in urban environments or closed spaces (such as a conference space) where many networks may compete for use of the spectrum. But, interference of this kind is also often overrated: for example, imagine you had to build a point to point link that has to cross a crowded inner city area, before reaching its target on the other side of the city. Such a highly directional beam will cross the "electric smog" of the urban centre without any problem. You may imagine this like a green and a red light beam crossing each other in a 90 degrees angle: while both beams will overlap in a certain area, the one will not have any impact on the other at all. Generally, managing spectrum and coexistence has become a main issue especially in dense indoor environments and urban areas.
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e term line of sight, often abbreviated asLOS, is quite easy to
understand when talking about visible light: if we can see a point B frompoint A where we are, we have line of sight. Simply draw a line from A to B, and if nothing is in the way, we have line of sight.
ings get a bit more complicated when we are dealing with microwaves. Remember that most propagation characteristics of electromagnetic waves scale with their wavelength. is is also the case for the widening of waves as they travel.
Light has a wavelength of about 0.5 micrometres, microwaves as used inwireless networking have a wavelength of a few centimetres. Consequently, their beams are a lot wider - they need more space, so to speak.
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Note that visible light beams widen just the same, and if you let them travel long enough, you can see the results despite their short wavelength.
When pointing a well focussed laser at the moon, its beam will widen towell over 100 metres in radius by the time it reaches the surface. You can see this effect for yourself using an inexpensive laser pointer and a pair of binoculars on a clear night. Rather than pointing at the moon, point at a distant mountain or unoccupied structure (such as a water tower). e radius of your beam will increase as the distance increases. is is due to the diffraction.
e line of sight that we need in order to have an optimal wirelessconnection from A to B is more than just a thin line - its shape is more like that of a cigar, an ellipsoid. Its width can be described by the concept of Fresnel zones - see next section for an explanation. You will alsofind the abbreviationNLOS, for "non line of sight", which is mostly used to describe and advertise technologies that allow for dealing with waves that reach the receiver through multiple trajectories (multipath) or diffraction. It does not indicate that the single electromagnetic beam goes "around
corners" (other than through di ff
raction) or "through obstacles" any betterthan that of other technologies. For example, you might call White Space technology NLOS, as its lower frequencies (longer wavelengths) allow it to permeate objects and utilize diffraction much better than comparable 2.4 GHz or 5 GHz transmissions.
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e exact theory of Fresnel (pronounced “Fray-nell”) zones is quite
complicated. However, the concept is quite easy to understand: we know from the Huygens principle that at each point of a wavefront new circular waves start, we know that microwave beams widen as they leave the antenna, we know that waves of one frequency can interfere with each other. Fresnel zone theory simply looks at a line from A to B, and then at the space around that line that contributes to what is arriving at point B. Some waves travel directly from A to B, while others travel on paths off axis and reach the receiver by reflection.
Consequently, their path is longer, introducing a phase shift between thedirect and indirect beam. Whenever the phase shift is one half wavelength, you get destructive interference: the signals cancel.
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Taking this approach youfind that when the reflected path is less than half a wavelength longer than the direct path, the reflections will add to the
received signal. Conversely, when the reflected path length exceeds thedirect path by more than one half wavelength, its contribution will decrease the received power.
Figure RP 11:e Fresnel zone is partially blocked on this link, although the visual line of sight appears clear.
Note that there are many possible Fresnel zones, but we are chiefly concerned with thefirst zone, because the contributions from the second zone are negative. e contributions from the third zone are positive again, but there is no practical way to take advantage of those without the penalty incurred in going through the second Fresnel Zone. If thefirst Fresnel zone is partially blocked by an obstruction, e.g. a tree or a building, the signal arriving at the far end would be diminished. When building wireless links, we therefore need to be sure that thefirst zone is kept free of obstructions. In practice, it is not strictly necessary that the whole of this zone is clear, in wireless networking we aim to clear about 60 percent of the radius of thefirst Fresnel zone.
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...where r is the radius of the zone in metres, d1 and d2 are distances from the obstacle to the link end points in metres, d is the total link distance in metres, and f is the frequency in MHz.
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with all the variables in metres
It is apparent that the maximum value of thefirst Fresnel zone happens exactly in the middle of the trajectory and its value can be found setting d1=d2=d/2 in the preceding formulas.Note that the formulae give you the
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r =7.84 metres
Assuming both of our towers were ten metres tall, thefirst Fresnel zone would pass just 2.16 metres above ground level in the middle of the link.
But how tall could a structure at that point be to block no more than 60% of thefirst zone?
r = 0.6 * 7.84 metres r = 4.70 metres
Subtracting the result from 10 metres, we can see that a structure 5.3 metres tall at the centre of the link would block up to 40% of thefirst
Fresnel zone. is is normally acceptable, but to improve the situation we would need to position our antennas higher up, or change the direction of the link to avoid the obstacle.
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Any electromagnetic wave carries energy - we can feel that when we enjoy(or suffer from) the warmth of the sun. e amount of energy divided by the time during which we measure it is called power. e powerPis measured in W (watts) and is of key importance for a wireless links to work: you need a certain minimum power in order for a receiver to make sense of the signal.
We will come back to details of transmission power, losses, gains and radio sensitivity in the chapter called Antennas/Transmission Lines.
Here we will briefly discuss how the power P is defined and measured. e electricfield is measured in V/m (potential difference per metre), the power contained within it is proportional to the square of the electricfield:
P ~ E 2
Practically, we measure the power in watts by means of some form of
receiver, e.g. an antenna and a voltmetre, power metre, oscilloscope, spectrum analyser or even a radio card and laptop. Looking at the signal’s power directly means looking at the square of the signal in volts and dividing by the electrical resistance.
Calculating with dB
By far the most important technique when calculating power is calculating
with decibels (dB). ere is no new physics hidden in this - it is just aconvenient method which makes calculations a lot simpler.
e decibel is a dimensionless unit, that is, it defines a relationship between two measurements of power. It is defined by:
dB = 10 * Log (P1 / P0 )
where P1 and P0 can be whatever two values you want to compare.Typically, in our case, this will be some amount of power. Why are decibels so handy to use? Many phenomena in nature happen to behave in a way we call exponential.
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For example, the human ear senses a sound to be twice as loud as another one if it has ten times the physical signal power.
Another example, quite close to ourfield of interest, is absorption. Suppose a wall is in the path of our wireless link, and each metre of wall takes away half of the available signal. e result would be:
0 metres = 1 (full signal) 1 metre = 1/2 2 metres = 1/4
3 metres = 1/84 metres = 1/16 n metres = 1/2n = 2-n
is is exponential behaviour.
But once we have used the trick of applying the logarithm (log), things become a lot easier: instead of taking a value to the n-th power, we just
multiply by n. Instead of multiplying values, we just add. Here are some commonly used values that are important to remember:
+3 dB = double power -3 dB = half the power +10 dB = order of magnitude (10 times power) -10 dB = one tenth power
In addition to dimensionless dB, there are a number of definitions that are based on a certain base value P0. e most relevant ones for us are:
dBm relative to P0 = 1 mW dBi relative to an ideal isotropic antenna
An isotropic antenna is a hypothetical antenna that evenly distributes power in all directions.
It is approximated by a dipole, but a perfect isotropic antenna cannot be built in reality. e isotropic model is useful for describing the relative power gain of a real world antenna.
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Another common (although less convenient) convention for expressing power is in milliwatts. Here are equivalent power levels expressed in
milliwatts and dBm:
1 mW = 0 dBm 2 mW = 3 dBm
100 mW = 20 dBm 1 W = 30 dBm
For more details on dB refer to the dB math lecture of the Wireless
Training kit:http://wtkit.org/sandbox/groups/wtkit/wiki/820cb/attachments/ebdac/02 -dB_Math-v1.12_with-notes.pdf
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Don’t worry if the concepts in this chapter seem challenging. Understanding how radio waves propagate and interact with the environment is a complexfield of study in itself. Most peoplefind it difficult to understand phenomenon that they can’t even see with their own eyes. By now you should understand that radio waves don’t travel only in a straight, predictable path. To make reliable communication networks, you will need to be able to calculate how much power is needed to cross a given distance, and predict how the waves will travel along the way.
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e purpose of any telecommunications system is to transferinformation from the sender to the receiver by a means of a communicationchannel. e information is carried by asignal, which is certain physical quantity that changes with time. e signal can be a voltage proportional to the amplitude of the voice, like in a simple telephone, a sequence of pulses of light in an opticalfibre, or a radio-electric wave irradiated by an antenna.
For analog signals, these variations are directly proportional to some physical variable like sound, light, temperature, wind speed, etc. e information can also be transmitted by digital binary signals, that will have only two values, a digitaloneand a digitalzero. Any analog signal can be converted into a digital signal by appropriatelysamplingand then coding it. e sampling frequency must be at least twice the maximum frequency present in the signal in order to carryallthe information contained therein. Random signals are the ones that are unpredictable and can be
described only by statistical means. Noise is a typical random signal,described by its mean power and frequency distribution. A signal can be characterised by its behaviour over time or by its frequency components, which constitute its spectrum. Some examples of signals are shown in Figure TB 1.
Figure TB 1: Examples of signals
Any periodic signal is composed of many sinusoidal components, all of them
multiples of the fundamental frequency, which is the inverse of the period ofthe signal. So a signal can be characterised either by a graph of its amplitude over time, called a waveform, or a graph of of the amplitudes of its frequency components, called a spectrum.
Figure TB 2: Waveforms, Spectrum and filters
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Figure TB 2 shows how the same signal can be seen from two different perspectives. e waveform can be displayed by an instrument called an oscilloscope, while the spectrum can be displayed by what is called a Spectrum Analyzer. e spectrum distribution relays very important information about the signal and allows for the intuitive understanding of the concept offiltering of electrical signals. In the example shown, the
signal is formed by the superposition of three sinusoidal components offrequency f1, f2 and f3. If we pass this signal through a device that will remove f2 and f3, the output is a pure sinusoidal at frequency f1.
We call this operation “ Low Pass ltering
” because it removes the higher frequencies. Conversely, we can apply the signal to a “
High
Pass Filter”, a device that will remove f1 and f2 leaving only a sinusoidal signal at the f3 frequency. Other combinations are
possible, giving rise to a variety of fi
lters. No physical device cantransmit all the infinite frequencies of the radio-electric spectrum, so every device will always perform some extent offiltering to the signal that goes through it. ebandwidthof a signal is the difference between the highest and the lowest frequency that it contains and is expressed in Hz (number of cycles per second). While travelling through the communication channel, the signal is subject tointerferencecaused by other signals and is also affected by the electrical noisealways present in any electrical or optical component.Intra-channel interference originates in the same channel as our signal.Co-channel interference is due to the imperfection of thefilters that will let in signals from adjacent channels. Consequently, the received signal will always be a distorted replica of the
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transmitted signal, from which the original information must be retrieved by appropriate means to combat the effect of interference and noise.
Furthermore, the received signal will be subject toattenuationanddelaythat increase with the distance between the transmitter and the receiver.
Figure TB 3: Attenuation and delay
Although it is relatively simple to restore the amplitude of signal by means of an electricalampli fier, the components of the amplifier will add additional noise to the signal, so at very long distances where the received signal is feeble, the amplifier will produce a signal so garbled with noise that the information originally transmitted will no longer be retrievable.
One way to address this problem consists in converting the continuous quantity carrying the information into a sequence of very simplesymbols which can be easier to recognise even at great distance. For instance, the flag of a ship is a convenient way to distinguish the nationality of the ship even at distances at which the letters on the hull cannot be read. is technique has been extended to carry generalised messages by
assigning different position offlags to every letter of the alphabet, in an early form of long distance telecommunications by means ofdigitalor numericsignals. e limitation of this method is obvious; to be able to distinguish among,
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say, 26 symbols corresponding to each letters of the alphabet, one must be quite close to the commu

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