Wireless Networking in the Developing World

Bandwidth is measured as a bit rate over a time interval. This means that over time, bandwidth available on any link approaches infinity. Unfortunately, for any given period of time, the bandwidth provided by any given network connection is not infinite. You can always download (orupload) as much traffic as you like; you need only wait long enough. Of course, human users are not as patient as computers, and are not willing to wait an infinite amount of time for their information to traverse the network. For this reason, bandwidth must be managed and prioritized much like any other limited resource. You will significantly improve response time and maximize available throughput by eliminating unwanted and redundant traffic from your network. This section describes many common techniques for making sure that your network carries only the traffic that must traverse it.

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The most critical component to building long distance network links is line of sight (often abbreviated as LOS). Terrestrial microwave systems simply cannot tolerate large hills, trees, or other obstacles in the path of a long distance link. You must have a clear idea of the lay of the land between two points before you can determine if a link is even possible.

But even if there is a mountain between two points, remember that obstacles can sometimes be turned into assets. Mountains may block your signal, but assuming power can be provided they also make very good repeater sites.

Repeaters are nodes that are configured to rebroadcast traffic that is not destined for the node itself. In a mesh network, every node is a repeater. In a traditional infrastructure network, nodes must be configured to pass along traffic to other nodes.

A repeater can use one or more wireless devices. When using a single radio (called a one-arm repeater), overall efficiency is slightly less than half of the available bandwidth, since the radio can either send or receive data, but never both at once. These devices are cheaper, simpler, and have lower power requirements. A repeater with two (or more) radio cards can operate all radios at full capacity, as long as they are each configured to use non-overlapping channels. Of course, repeaters can also supply an Ethernet connection to provide local connectivity.

Repeaters can be purchased as a complete hardware solution, or easily assembled by connecting two or more wireless nodes together with Ethernet cable. When planning to use a repeater built with 802.11 technology, remember that nodes must be configured for master, managed, or ad-hoc mode. Typically, both radios in a repeater are configured for master mode, to allow multiple clients to connect to either side of the repeater. But depending on your network layout, one or more devices may need to use ad-hoc or even client mode.

Typically, repeaters are used to overcome obstacles in the path of a long distance link. For example, there may be buildings in your path, but those buildings contain people. Arrangements can often be worked out with building owners to provide bandwidth in exchange for roof rights and electricity. If the building owner isn’t interested, tenants on high floors may be able to be persuaded to install equipment in a window.

If you can’t go over or through an obstacle, you can often go around it. Rather than using a direct link, try a multi-hop approach to avoid the obstacle.

Finally, you may need to consider going backwards in order to go forwards. If there is a high site available in a different direction, and that site can see beyond the obstacle, a stable link can be made via an indirect route.

Repeaters in networks remind me of the “six degrees of separation” principle. This idea says that no matter who you are looking for, you need only contact five intermediaries before finding the person. Repeaters in high places can “see” a great deal of intermediaries, and as long as your node is in range of the repeater, you can communicate with any node the repeater can reach.

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The unlicensed ISM and U-NII bands represent a very tiny piece of the known electromagnetic spectrum. Since this region can be utilized without paying license fees, many consumer devices use it for a wide range of applications. Cordless phones, analog video senders, Bluetooth, baby monitors, and even microwave ovens compete with wireless data networks for use of the very limited 2.4GHz band. These signals, as well as other local wireless networks, can cause significant problems for long range wireless links. Here are some steps you can use to reduce reception of unwanted signals.

• Increase antenna gain on both sides of a point-to-point link. Antennas not only add gain to a link, but their increased directionality tends to reject noise from areas around the link. Two high gain dishes that are pointed at each other will reject noise from directions that are outside the path of the link. Using omnidirectional antennas will receive noise from all directions.

• Don’t use an amplifier. As we will see in chapter four, amplifiers can make interference issues worse by indiscriminately amplifying all received signals, including sources of interference. Amplifiers also cause interference problems for other nearby users of the band.

• Use sectorials instead of using an omnidirectional. By making use of several sectorial antennas, you can reduce the overall noise received at a distribution point. By staggering the channels used on each sectorial, you can also increase the available bandwidth to your clients.

• Use the best available channel. Remember that 802.11b/g channels are 22Mhz wide, but are only separated by 5MHz. Perform a site survey (as detailed in chapter eight), and select a channel that is as far as possible from existing sources of interference. Remember that the wireless landscape can change at any time as people add new devices (cordless phones, other networks, etc.) If your link suddenly has trouble sending packets, you may need to perform another site survey and pick a different channel.

• Use smaller hops and repeaters, rather than a single long distance shot. Keep your point-to-point links as short as possible. While it may be possible to create a 12km link that cuts across the middle of a city, you will likely have all kinds of interference problems. If you can break that link into two or three shorter hops, the link will likely be more stable. Obviously this isn’t possible on long distance rural links where power and mounting structures are unavailable, but noise problems are also unlikely in those settings.

• If possible, use 5.8GHz, 900MHz, or another unlicensed band. While this is only a short term solution, there is currently far more consumer equipment installed in the field that uses 2.4GHz. Using 802.11a or a 2.4GHz to 5.8GHz step-up device will let you avoid this congestion altogether. If you can find it, some old 802.11 equipment uses unlicensed spectrum at 900MHz (unfortunately at much lower bit rates). Other technologies, such as Ronja (http://ronja.twibright.com/) use optical technology for short distance, noise-free links.

• If all else fails, use licensed spectrum. There are places where all available unlicensed spectrum is effectively used. In these cases, it may make sense to spend the additional money for proprietary equipment that uses a less congested band. For long distance point-to-point links that require very high throughput and maximum uptime, this is certainly an option. Of course, these features come at a much higher price tag compared to unlicensed equipment.

To identify sources of noise, you need tools that will show you what is happening in the air at 2.4GHz. We will see some examples of these tools in chapter six.

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While calculating a link budget by hand is straightforward, there are a number of tools available that will help automate the process. In addition to calculating free space loss, these tools will take many other relevant factors into account as well (such as tree absorption, terrain effects, climate, and even estimating path loss in urban areas). In this section, we will discuss two free tools that are useful for planning wireless links: Green Bay Professional Packet Radio’s online interactive network design utilities, and RadioMobile.

Interactive design CGIs

The Green Bay Professional Packet Radio group (GBPRR) has made a variety of very useful link planning tools available for free online. You can browse these tools online at http://www.qsl.net/n9zia/wireless/page09.html . Since the tools are available online, they will work with any device that has a web browser and Internet access.

We will look at the first tool, Wireless Network Link Analysis, in detail. You can find it online at http://my.athenet.net/~multiplx/cgi-bin/wireless.main.cgi .

To begin, enter the channel to be used on the link. This can be specified in MHz or GHz. If you don’t know the frequency, consult the table in Appendix B. Note that the table lists the channel’s center frequency, while the tool asks for the highest transmitted frequency. The difference in the ultimate result is minimal, so feel free to use the center frequency instead. To find the highest transmitted frequency for a channel, just add 11MHz to the center frequency.

Next, enter the details for the transmitter side of the link, including the transmission line type, antenna gain, and other details. Try to fill in as much data as you know or can estimate. You can also enter the antenna height and elevation for this site. This data will be used for calculating the antenna tilt angle. For calculating Fresnel zone clearance, you will need to use GBPRR’s Fresnel Zone Calculator.

The next section is very similar, but includes information about the other end of the link. Enter all available data in the appropriate fields.

Finally, the last section describes the climate, terrain, and distance of the link. Enter as much data as you know or can estimate. Link distance can be calculated by specifying the latitude and longitude of both sites, or entered by hand.

Now, click the Submit button for a detailed report about the proposed link. This includes all of the data entered, as well as the projected path loss, error rates, and uptime. These numbers are all completely theoretical, but will give you a rough idea of the feasibility of the link. By adjusting values on the form, you can play “what-if?” to see how changing various parameters will affect the connection.

In addition to the basic link analysis tool, GBPRR provides a “super edition” that will produce a PDF report, as well as a number of other very useful tools (including the Fresnel Zone Calculator, Distance & Bearing Calculator, and Decibel Conversion Calculator to name just a few). Source code to most of the tools is provided as well.

RadioMobile

Radio Mobile is a tool for the design and simulation of wireless systems. It predicts the performance of a radio link by using information about the equipment and a digital map of the area. It is public domain software that runs on Windows, or using Linux and the Wine emulator.

Radio Mobile uses a digital terrain elevation model for the calculation of coverage, indicating received signal strength at various points along the path. It automatically builds a profile between two points in the digital map showing the coverage area and first Fresnel zone. During the simulation, it checks for line of sight and calculates the Path Loss, including losses due to obstacles. It is possible to create networks of different topologies, including net master/slave, point-to-point, and point-to-multipoint.

The software calculates the coverage area from the base station in a point-to-multipoint system. It works for systems having frequencies from 20 kHz to 200 GHz. Digital elevation maps (DEM) are available for free from several sources, and are available for most of the world. DEMs do not show coastlines or other readily identifiable landmarks, but they can easily be combined with other kinds of data (such as aerial photos or topographical charts) in several layers to obtain a more useful and readily recognizable representation. You can digitize your own maps and combine them with DEMs. The digital elevation maps can be merged with scanned maps, satellite photos and Internet map services (such as Mapquest) to produce accurate prediction plots.

Download Radio Mobile here: http://www.cplus.org/rmw/download.html

The main Radio Mobile webpage, with examples and tutorials, is available at: http://www.cplus.org/rmw/english1.html

RadioMobile under Linux

Radio Mobile will also work using Wine under Ubuntu Linux. While the application runs, some button labels may run beyond the frame of the button and can be hard to read.

We were able to make Radio Mobile work with Linux using the following environment:

• IBM Thinkpad x31

• Ubuntu Breezy (v5.10), http://www.ubuntu.com/

• Wine version 20050725, from the Ubuntu Universe repository

There are detailed instructions for installing RadioMobile on Windows at http://www.cplus.org/rmw/download.html. You should follow all of the steps except for step 1 (since it is difficult to extract a DLL from the VBRUN60SP6.EXE file under Linux). You will either need to copy the MSVBVM60.DLL file from a Windows machine that already has the Visual Basic 6 run-time environment installed, or simply Google for MSVBVM60.DLL, and download the file.

Now continue with step 2 at from the above URL, making sure to unzip the downloaded files in the same directory into which you have placed the downloaded DLL file. Note that you don’t have to worry about the stuff after step 4; these are extra steps only needed for Windows users.

Finally, you can start Wine from a terminal with the command:

 # wine RMWDLX.exe

You should see RadioMobile running happily in your XWindows session.

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To calculate the link budget, simply approximate your link distance, then fill in the following tables:

Free Space Path Loss at 2.4GHz

Antenna Gain:

Losses:

Link Budget for Radio 1 -> Radio 2:

Link Budget for Radio 2 -> Radio 1:

If the received signal is greater than the minimum received signal strength in both directions of the link, then the link is feasible.

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As an example, we want to estimate the feasibility of a 5km link, with one access point and one client radio. The access point is connected to an omnidirectional antenna with 10dBi gain, while the client is connected to a sectorial antenna with 14dBi gain. The transmitting power of the AP is 100mW (or 20dBm) and its sensitivity is -89dBm. The transmitting power of the client is 30mW (or 15dBm) and its sensitivity is -82dBm. The cables are short, with a loss of 2dB at each side.

Adding up all the gains and subtracting all the losses for the AP to client link gives:

   20 dBm (TX Power Radio 1)
 + 10 dBi (Antenna Gain Radio 1)
 -  2 dB  (Cable Losses Radio 1)
 + 14 dBi (Antenna Gain Radio 2)
 -  2 dB  (Cable Losses Radio 2)
———————————
40 dB = Total Gain

The path loss for a 5km link, considering only the free space loss is:

 Path Loss = 40 + 20log(5000) = 113 dB

Subtracting the path loss from the total gain

 40 dB - 113 dB = -73 dB

Since -73dB is greater than the minimum receive sensitivity of the client radio (-82dBm), the signal level is just enough for the client radio to be able to hear the access point. There is only 9dB of margin (82dB - 73dB) which will likely work fine in fair weather, but may not be enough to protect against extreme weather conditions.

Next we calculate the link from the client back to the access point:

   15 dBm (TX Power Radio 2)
 + 14 dBi (Antenna Gain Radio 2)
 -  2 dB  (Cable Losses Radio 2)
 + 10 dBi (Antenna Gain Radio 1)
 -  2 dB  (Cable Losses Radio 1)
———————————
      35 dB = Total Gain

Obviously, the path loss is the same on the return trip. So our received signal level on the access point side is:

 35 dB - 113 dB = -78 dB

Since the receive sensitivity of the AP is -89dBm, this leaves us 11dB of fade margin (89dB - 78dB). Overall, this link will probably work but could use a bit more gain. By using a 24dBi dish on the client side rather than a 14dBi sectorial antenna, you will get an additional 10dBi of gain on both directions of the link (remember, antenna gain is reciprocal). A more expensive option would be to use higher power radios on both ends of the link, but note that adding an amplifier or higher powered card to one end does not help the overall quality of the link.

Online tools can be used to calculate the link budget. For example, the Green Bay Professional Packet Radio’s Wireless Network Link Analysis (http://my.athenet.net/~multiplx/cgi-bin/wireless.main.cgi) is an excellent tool. The Super Edition generates a PDF file containing the Fresnel zone and radio path graphs. The calculation scripts can even be downloaded from the website and installed locally. We will look at one excellent online tool in more detail in the next section, Link planning software.

The Terabeam website also has excellent calculators available online (http://www.terabeam.com/support/calculations/index.php).

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The power available in an 802.11 system can be characterized by the following factors:

• Transmit Power. It is expressed in milliwatts or in dBm. Transmit Power ranges from 30mW to 200mW or more. TX power is often dependent on the transmission rate. The TX power of a given device should be specified in the literature provided by the manufacturer, but can sometimes be difficult to find. Online databases such as the one provided by SeattleWireless (http://www.seattlewireless.net/HardwareComparison) may help.

• Antenna Gain. Antennas are passive devices that create the effect of amplification by virtue of their physical shape. Antennas have the same characteristics when receiving and transmitting. So a 12 dBi antenna is simply a 12 dBi antenna, without specifying if it is in transmission or reception mode. Parabolic antennas have a gain of 19-24 dBm, omnidirectional antennas have 5-12 dBi, sectorial antennas have roughly a 12-15 dBi gain.

• Minimum Received Signal Level, or simply, the sensitivity of the receiver. The minimum RSL is always expressed as a negative dBm (- dBm) and is the lowest power of signal the radio can distinguish. The minimum RSL is dependent upon rate, and as a general rule the lowest rate (1 Mbps) has the greatest sensitivity. The minimum will be typically in the range of -75 to -95 dBm. Like TX power, the RSL specifications should be provided by the manufacturer of the equipment.

• Cable Losses. Some of the signal’s energy is lost in the cables, the connectors and other devices, going from the radios to the antennas. The loss depends on the type of cable used and on its length. Signal loss for short coaxial cables including connectors is quite low, in the range of 2-3 dB. It is better to have cables as short as possible.

When calculating the path loss, several effects must be considered. One has to take into account the free space loss, attenuation and scattering. Signal power is diminished by geometric spreading of the wavefront, commonly known as free space loss. Ignoring everything else, the further away the two radios, the smaller the received signal is due to free space loss. This is independent from the environment, depending only on the distance. This loss happens because the radiated signal energy expands as a function of the distance from the transmitter.

Using decibels to express the loss and using 2.45 GHz as the signal frequency, the equation for the free space loss is

 Lfsl = 40 + 20*log(r)

where Lfsl is expressed in dB and r is the distance between the transmitter and receiver, in meters.

The second contribution to the path loss is given by attenuation. This takes place as some of the signal power is absorbed when the wave passes through solid objects such as trees, walls, windows and floors of buildings. Attenuation can vary greatly depending upon the structure of the object the signal is passing through, and it is very difficult to quantify. The most convenient way to express its contribution to the total loss is by adding an “allowed loss” to the free space. For example, experience shows that trees add 10 to 20 dB of loss per tree in the direct path, while walls contribute 10 to 15 dB depending upon the construction.

Along the link path, the RF energy leaves the transmitting antenna and energy spreads out. Some of the RF energy reaches the receiving antenna directly, while some bounces off the ground. Part of the RF energy which bounces off the ground reaches the receiving antenna. Since the reflected signal has a longer way to travel, it arrives at the receiving antenna later than the direct signal. This effect is called multipath, fading or signal dispersion. In some cases reflected signals add together and cause no problem. When they add together out of phase, the received signal is almost worthless. In same cases, the signal at the receiving antenna can be zeroed by the reflected signals. This is known as nulling. There is a simple technique that is used to deal with multipath, called antenna diversity. It consists in adding a second antenna to the radio. Multipath is in fact a very location-specific phenomenon. If two signals add out of phase at one location, they will not add destructively at a second, nearby location. If there are two antennas, at least one of them should be able to receive a usable signal, even if the other is receiving a distorted one. In commercial devices, antenna switching diversity is used: there are multiple antennas on multiple inputs, with a single receiver. The signal is thus received through only one antenna at a time. When transmitting, the radio uses the antenna last used for reception. The distortion given by multipath degrades the ability of the receiver to recover the signal in a manner much like signal loss. A simple way of applying the effects of scattering in the calculation of the path loss is to change the exponent of the distance factor of the free space loss formula. The exponent tends to increase with the range in an environment with a lot of scattering. An exponent of 3 can be used in an outdoor environment with trees, while one of 4 can be used for an indoor environment.

When free space loss, attenuation, and scattering are combined, the path loss is:

 L(dB) = 40 + 10*n*log(r) + L(allowed)

For a rough estimate of the link feasibility, one can evaluate just the free space loss. The environment can bring further signal loss, and should be considered for an exact evaluation of the link. The environment is in fact a very important factor, and should never be neglected.

To evaluate if a link is feasible, one must know the characteristics of the equipment being used and evaluate the path loss. Note that when performing this calculation, you should only add the TX power of one side of the link. If you are using different radios on either side of the link, you should calculate the path loss twice, once for each direction (using the appropriate TX power for each calculation). Adding up all the gains and subtracting all the losses gives

   TX Power Radio 1
 + Antenna Gain Radio 1
 - Cable Losses Radio 1
 + Antenna Gain Radio 2
 - Cable Losses Radio 2
————————
     = Total Gain

Subtracting the Path Loss from the Total Gain:

               Total Gain
             - Path Loss
            ————–
 = Signal Level at one side of the link

If the resulting signal level is greater than the minimum received signal level, then the link is feasible! The received signal is powerful enough for the radios to use it. Remember that the minimum RSL is always expressed as a negative dBm, so -56dBm is greater than -70dBm. On a given path, the variation in path loss over a period of time can be large, so a certain margin (difference between the signal level and the minimum received signal level) should be considered. This margin is the amount of signal above the sensitivity of radio that should be received in order to ensure a stable, high quality radio link during bad weather and other atmospheric disturbances. A margin of error of 10-15 dB is fine. To give some space for attenuation and multipath in the received radio signal, a margin of 20dB should be safe enough.

Once you have calculated the link budget in one direction, repeat the calculation for the other direction. Substitute the transmit power for that of the second radio, and compare the result against the minimum received signal level of the first radio.

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Wireless links can provide significantly greater throughput to users than traditional Internet connections, such as VSAT, dialup, or DSL. Throughput is also referred to as channel capacity, or simply bandwidth (although this term is unrelated to radio bandwidth). It is important to understand that a wireless device’s listed speed (the data rate) refers to the rate at which the radios can exchange symbols, not the usable throughput you will observe. As mentioned earlier, a single 802.11g link may use 54Mbps radios, but it will only provide up to 22Mbps of actual throughput. The rest is overhead that the radios need in order to coordinate their signals using the 802.11g protocol.

Note that throughput is a measurement of bits over time. 22Mbps means that in any given second, up to 22 megabits can be sent from one end of the link to the other. If users attempt to push more than 22 megabits through the link, it will take longer than one second. Since the data can’t be sent immediately, it is put in a queue, and transmitted as quickly as possible. This backlog of data increases the time needed for the most recently queued bits to the traverse the link. The time that it takes for data to traverse a link is called latency, and high latency is commonly referred to as lag. Your link will eventually send all of the queued traffic, but your users will likely complain as the lag increases.

How much throughput will your users really need? It depends on how many users you have, and how they use the wireless link. Various Internet applications require different amounts of throughput.

To estimate the necessary throughput you will need for your network, multiply the expected number of users by the sort of application they will likely use. For example, 50 users who are chiefly browsing the web will likely consume 2.5 to 5Mbps or more of throughput at peak times, and will tolerate some latency. On the other hand, 50 simultaneous VoIP users would require 5Mbps or more of throughput in both directions with absolutely no latency. Since 802.11g wireless equipment is half duplex (that is, it only transmits or receives, never both at once) you should accordingly double the required throughput, for a total of 10Mbps. Your wireless links must provide that capacity every second, or conversations will lag.

Since all of your users are unlikely to use the connection at precisely the same moment, it is common practice to oversubscribe available throughput by some factor (that is, allow more users than the maximum available bandwidth can support). Oversubscribing by a factor of 2 to 5 is quite common. In all likelihood, you will oversubscribe by some amount when building your network infrastructure. By carefully monitoring throughput throughout your network, you will be able to plan when to upgrade various parts of the network, and how much additional resources will be needed.

Expect that no matter how much capacity you supply, your users will eventually find applications that will use it all. As we’ll see at the end of this chapter, using bandwidth shaping techniques can help mitigate some latency problems. By using bandwidth shaping, web caching, and other techniques, you can significantly reduce latency and increase overall network throughput.

To get a feeling for the lag felt on very slow connections, the ICTP has put together a bandwidth simulator. It will simultaneously download a web page at full speed and at a reduced rate that you choose. This demonstration gives you an immediate understanding of how low throughput and high latency reduce the usefulness of the Internet as a communications tool. It is available at http://wireless.ictp.trieste.it/simulator/

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As long as the WiFi-cards can ’see’ each other directly with their radios, doing a ping will work whether olsrd is running or not. This works because the large netmasks effectively make every node link-local, so routing issues are sidestepped at the first hop. This should be checked first if things do not seem to work as expected. Most headaches people face with WiFi in Ad-Hoc mode are caused by the fact that the ad-hoc mode in drivers and cards are implemented sloppily. If it is not possible to ping nodes directly when they are in range it is most likely a card/driver issue, or your network settings are wrong.

If the machines can ping each other, but olsrd doesn’t find routes, then the IP-addresses, netmask and broadcast address should be checked.

Are you running a firewall? Make sure it doesn’t block UDP port 698.

Have fun!

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Compile the olsr plugins separately and install them. To load the plugin add the following lines to /etc/olsrd.conf

 LoadPlugin "olsrd_dot_draw.so.0.3"
 {
       PlParam “accept” “192.168.0.5″
       PlParam “port” “2004″
 }

The parameter “accept” specifies which host is accepted to view the Topology Information (currently only one) and is “localhost” by default. The parameter “port” specifies the TCP port.

Then restart olsr and check if you get output on TCP Port 2004

 telnet localhost 2004

After a while you should get some text output.

Now you can save the output graph descriptions and run the tools dot or neato form the graphviz package to get images.

Bruno Randolf has written a small perl script which continuously gets the topology information from olsrd and displays it using the graphviz and ImageMagick tools.

First install the following packages on your workstation:

• graphviz, http://www.graphviz.org/

• ImageMagick, http://www.imagemagick.org/

Download the script at: http://meshcube.org/nylon/utils/olsr-topology-view.pl

Now you can start the script with ./olsr-topology-view.pl and view the topology updates in near-realtime.

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