Wireless Networking in the Developing World

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, is swinging in a periodic manner, with a certain number of cycles per unit of time. This kind of wave is sometimes called a mechanical wave, 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. These oscillations periodically compress and decompress the air, and this periodic change of air pressure then leaves the singers mouth and travels, at the speed of sound. A stone plunging into a lake causes a disturbance, which then travels across the lake as a wave.

A wave has a certain speed, frequency, and wavelength. These are connected by a simple relation:

Speed = Frequency * Wavelength

The wavelength (sometimes referred to as lambda, λ) is the distance measured from a point on one wave to the equivalent part of the next, for example from the top of one peak to the next. The frequency is the number of whole waves that pass a fixed point in a period of time. Speed is measured in meters/second, frequency is measured in cycles per second (or Hertz, abbreviated Hz), and wavelength is measured in meters.

For example, if a wave on water travels at one meter per second, and it oscillates five times per second, then each wave will be twenty centimeters long:

1 meter/second = 5 cycles/second * W
W = 1 / 5 meters
W = 0.2 meters = 20 cm

Waves also have a property called amplitude. This is the distance from the center of the wave to the extreme of one of its peaks, and can be thought of as the “height” of a water wave.. The relationship between frequency, wavelength, and amplitude are shown in Figure 2.1.

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 part that might be hardest to understand is: “What is it that is oscillating?”

In order to understand that, we need to understand electromagnetic forces.

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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 your computer, 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 card are going? What happens when these waves bounce off of 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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If you are new to wireless networking, you likely have a number of questions about what the technology can do and what it will cost. Here are some commonly asked questions, which will be available in future postings.

Power

• How can I supply power to my radio equipment, if there is no power available?

• Do I need to run a power cable all the way up the tower?

• How can I use solar panel to power my wireless node while keeping it online overnight?

• How long will my access point run on a battery?

Management

• How can I monitor and manage remote access points from my office?

• What do I do when the network breaks?

• What are the most common problems encountered on wireless networks, and how do I fix them?

Distance

• How good is the range of my access point?

• Is there any formula I can use to know how far I can go with a given access point?

• How can I know if a remote place can be connected to Internet using a wireless link?

• The manufacturer says my access point has a range of 300 meters. Is that true?

• How can I provide wireless connectivity to many remote clients, spread all around the city?

• Is it true that I can reach a much greater distance adding a tin can or aluminum foil to my AP’s antenna?

• Can I use wireless to connect to a remote site and share a single central Internet connection?

• My wireless link looks like it will be too long. Can I put a repeater in the middle to make it better?

• Should I use an amplifier instead?

Installation

• How can I install my indoor AP on the top of a mast on my roof?

• Is it really useful to add a lightning protector and proper grounding to my antenna mast, or can I go without them?

• Can I build an antenna mast by myself? How high can I go?

• Why does my antenna work much better when I mount it “sideways”?

• Which channel should I use?

• Will radio waves travel through buildings and trees? What about people?

• Will radio waves travel through a hill that is in the way?

• How do I build a mesh network?

• What kind of antenna is the best one for my network?

• Can I build an access point using a recycled PC?

• How can I install Linux on my AP? Why should I do so?

Money

• How can I know if a wireless link is achievable with a limited amount of money?

• Which is the best AP with the lowest price?

• How can I track and bill customers for using my wireless network?

Partners and Customers

• If I am supplying connectivity, do I still need service from an ISP? Why?

• How many customers do I need to cover my costs?

• How many customers will my wireless network support?

• How do I make my wireless network go faster?

• Is my Internet connection as fast as it can be?

Security

• How can I protect my wireless network from unauthorized access?

• Is it true that a wireless network is always insecure and open to attacks by hackers?

• Is it true that the use of open source software makes my network less secure?

• How can I see what is happening on my network?

Information and Licensing

• What other books should I read to improve my wireless networking skills?

• Where can I find more information online?

• Can I use parts of this book for my own teaching? Can I print and sell copies of this book? Yes. See About This Book for more details.

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The primary technology used for building low-cost wireless networks is currently the 802.11 family of protocols, also known in many circles as Wi-Fi. The 802.11 family of radio protocols (802.11a, 802.11b, and 802.11g) have enjoyed an incredible popularity in the United States and Europe. By implementing a common set of protocols, manufacturers world wide have built highly interoperable equipment. This decision has proven to be a significant boon to the industry and the consumer. Consumers are able to use equipment that implements 802.11 without fear of “vendor lock-in”. As a result, consumers are able to purchase low-cost equipment at a volume which has benefitted manufacturers. If manufacturers had chosen to implement their own proprietary protocols, it is unlikely that wireless networking would be as inexpensive and ubiquitous as it is today.

While new protocols such as 802.16 (also known as WiMax) will likely solve some difficult problems currently observed with 802.11, they have a long way to go to match the popularity and price point of 802.11 equipment. As this equipment that supports WiMax is just becoming available at the time of this writing, we will focus primarily on the 802.11 family.

There are many protocols in the 802.11 family, and not all are directly related to the radio protocol itself. The three wireless standards currently implemented in most readily available gear are:

• 802.11b. Ratified by the IEEE on September 16, 1999, 802.11b is probably the most popular wireless networking protocol in use today. Millions of devices supporting it have shipped since 1999. It uses a modulation called Direct Sequence Spread Spectrum (DSSS) in a portion of the ISM band from 2.412 to 2.484GHz. It has a maximum rate of 11Mbps, with actual usable data speeds up to about 5Mbps.

• 802.11g. As it wasn’t finalized until June 2003, 802.11g is a relative late-comer to the wireless marketplace. Despite the late start, 802.11g is now the de facto standard wireless networking protocol as it now ships as a standard feature on virtually all laptops and most handheld devices. 802.11g uses the same ISM range as 802.11b, but uses a modulation scheme called Orthogonal Frequency Division Multiplexing (OFDM). It has a maximum data rate of 54Mbps (with usable throughput of up to 25Mbps), and can fall back to 11Mbps DSSS or slower for backwards compatibility with the hugely popular 802.11b.

• 802.11a. Also ratified by the IEEE on September 16, 1999, 802.11a uses OFDM. It has a maximum data rate of 54Mbps, with actual throughput of up to 27Mbps. 802.11a operates in the ISM band between 5.745 and 5.805GHz, and in a portion of the UNII band between 5.170 and 5.320GHz. This makes it incompatible with 802.11b or 802.11g, and the higher frequency means shorter range compared to 802.11b/g at the same power. While this portion of the spectrum is relatively unused compared to 2.4GHz, it is unfortunately only legal for use in a few parts of the world. Check with your local authorities before using 802.11a equipment, particularly in outdoor applications. 802.11a equipment is still quite inexpensive, but is not nearly as popular as 802.11b/g.

In addition to the above standards, there are a number of vendor-specific extensions to equipment, touting speeds of 108Mbps, stronger encryption, and increased range. Unfortunately these extensions will not operate between equipment from different manufacturers, and purchasing them will effectively lock you into that vendor for every part of your network. New equipment and standards (such as 802.11n, 802.16, MIMO, and WiMAX) promise significant increases in speed and reliability, but this equipment is just starting to ship at the time of this writing, and availability and vendor interoperability is unclear.

Due to the ubiquity of equipment, better range, and unlicensed nature of the 2.4GHz ISM band, this book will concentrate building networks using 802.11b and 802.11g.

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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 kilometer 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 1.1: Some wireless networking examples.

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The 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 that even dialup does not exist, and ultimately connect you and your neighbors to the global Internet. 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.

This book is not a guide to configuring a radio card in your laptop or choosing consumer grade gear for your home network. The emphasis is on building infrastructure links intended to be used as the backbone for wide area wireless networks. With that goal in mind, information is presented from many points of view, including technical, social, and financial factors. The extensive collection of case studies present various groups’ attempts at building these networks, the resources that were committed to them, and the ultimate results of these attempts.

Since the first spark gap experiments at the turn of the last 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 or fiber optic backbone). Rather, these techniques are intended to augment existing systems, and provide connectivity in areas where running fiber or other physical cable would be impractical.

We hope you find this book useful for solving your particular communication challenges.

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This book was created by a team of individuals who each, in their own field, are actively participating in the ever-expanding Internet by pushing its reach farther than ever before. The massive popularity of wireless networking has caused equipment costs to continually plummet, while equipment capabilities continue to sharply increase. We believe that by taking advantage of this state of affairs, people can finally begin to have a stake in building their own communications infrastructure. 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.

Wireless infrastructure can be built for very little cost compared to traditional wired alternatives. But building wireless networks is only partly about saving money. By providing people in your local community with cheaper and easier access to information, they will directly benefit from what the Internet has to offer. The time and effort saved by having access to the global network of information translates into wealth on a local scale, as more work can be done in less time and with less effort.

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 are finding that Internet access gives them a voice to discuss their problems, politics, and whatever else is important to their lives, in a way that the telephone and television simply cannot compete with. What has until recently sounded like science fiction is now becoming a reality, and that reality is being built on wireless networks.

But even without access to the Internet, wireless community networks have tremendous value. They allow people to collaborate on projects across wide distances. Voice communications, email, and other data can be exchanged for very little cost. By involving local people in the construction of the network, knowledge and trust are spread throughout the community, and people begin to understand the importance of having a share in their communications infrastructure. Ultimately, they realize that communication networks are built to allow people to connect with each other.

In this book we will focus on wireless data networking technologies in the 802.11 family. While such a network can carry data, voice, and video (as well as traditional web and Internet traffic), the networks described in this book are data networks. We specifically do not cover GSM, CDMA, or other wireless voice technologies, since the cost of deploying these technologies is well beyond the reach of most community projects.

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