Wireless Networking in the Developing World

A wave on water which is 5 meters long will not be stopped by a 5 mm piece of wood sticking out of the water. If instead the piece of wood were 50 meters big (e.g. a ship), it would be well in the way of the wave. The 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 (88-108MHz) can travel through buildings and other obstacles easily, while shorter waves (such as GSM phones operating at 900MHz or 1800MHz) have a harder time penetrating buildings. This 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.

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Assuming equal power levels, waves with longer wavelengths tend to travel further than waves with shorter wavelengths. This effect is often seen in FM radio, when comparing the range of an FM transmitter at 88MHz to the range at 108MHz. Lower frequency transmitters tend to reach much greater distances than high frequency transmitters at the same power.

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There are a few simple rules of thumb that can prove extremely useful when making first 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.

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Let us look a bit closer at how the 2.4GHz band is used in 802.11b. The spectrum is divided into evenly sized pieces distributed over the band as individual channels. Note that channels are 22MHz wide, but are only separated by 5MHz. This means that adjacent channels overlap, and can interfere with each other. This is represented visually in Figure 2.4.

For a complete list of channels and their center frequencies for 802.11b/g and 802.11a, see Appendix A, once it’s posted.

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A term you will meet often in radio physics is bandwidth. Bandwidth is simply 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 80MHz).

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 can fit in at a given moment. The term bandwidth is often used for something we should rather call a data rate, as in “my Internet connection has 1 Mbps of bandwidth”, meaning it can transmit data at 1 megabit per second.

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Electromagnetic waves span a wide range of frequencies (and, accordingly, wavelengths). This range of frequencies and wavelengths is called the electromagnetic spectrum. The 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 AC (Alternating Current) or grid electricity, at 50/60 Hz, X-Rays / Roentgen radiation, Ultraviolet (on the higher frequencies side of visible light), Infrared (on the lower frequencies side of visible light) and many others. Radio is the term used for the portion of the electromagnetic spectrum in which waves can be generated by applying alternating current to an antenna. This is true for the range from 3 Hz to 300 GHz, but in the more narrow sense of the term, the upper frequency limit would be 1 GHz.

When talking about radio, many people think of FM radio, which uses a frequency around 100 MHz. In between radio and infrared we find the region of microwaves - with frequencies from about 1 GHz to 300 GHz, and wavelengths from 30 cm to 1 mm.

The 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. These regions lie within the bands that are being kept open for general unlicensed use. This region is called the ISM 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. This goes especially for those parts of the spectrum that are suitable for broadcast (TV, radio) as well as voice and data communication. In most countries, the ISM bands have been reserved for unlicensed use.

The frequencies most interesting to us are 2.412 - 2.484 GHz, which is used by the 802.11b and 802.11g radio standards (corresponding to wavelengths of about 12.5 cm). Other commonly available equipment uses the 802.11a standard, which operates at 5.170 - 5.805 GHz (corresponding to wavelengths of about 5 to 6 cm).

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Another important quality of electromagnetic waves is polarization. Polarization describes the direction of the electrical field vector.

If you imagine a vertically aligned dipole antenna (the straight piece of wire), electrons only move up and down, not sideways (because there is no room to move) and thus electrical fields only ever point up or down, vertically. The field leaving the wire and traveling as a wave has a strict linear (and in this case, vertical) polarization. If we put the antenna flat on the ground (horizontal, we would find 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 the field pointing other directions too. The most general case is elliptic polarization, with the extremes of linear (only one direction) and circular polarizations (both directions at equal strength).

As one can imagine, polarization becomes important when aligning antennas. If you ignore polarization, you might have very little signal even though you have the strongest antennas. We call this polarization mismatch.

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In physics, math, and engineering, we often express numbers by powers of ten. We will meet these terms again, e.g. in Giga-Hertz (GHz), Centi-meters (cm), Micro-seconds (µs), and so on.

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 lambda (λ) = c / f
                      = 3*10^8 / 2.4*10^9
                      = 1.25*10^-1 m
                      = 12.5 cm

Frequency and wavelength determine most of an electromagnetic wave’s behaviour, from antennas that we build to objects that are in the way of the networks we intend to run. They are responsible for many of the differences between different standards we might be choosing. Therefore, an understanding of the basic ideas of frequency and wavelength helps a lot in practical wireless work.

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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. The electrical force is the force between electrical charges. The magnetic force is the force between electrical currents.

Electrons are particles that carry a negative electrical charge. There are other particles too, but electrons 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 wire, in which we push the electrons from one and to the other and back, periodically. At one moment, the top of the wire is negatively charged - all the negative electrons are gathered there. This creates an electric field from plus to minus along the wire. The next moment, the electrons have all been driven to the other side, and the electric field points the other way. As this happens again and again, the electric field vectors (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 poles, plus and minus), or more commonly a dipole antenna. This is the simplest form of omnidirectional antenna. The motion of the electric field is commonly referred to as an electromagnetic wave.

Let us come back to the relation:

Speed = Frequency * Wavelength

In the case of electromagnetic waves, the speed is c, the speed of light.

c = 300,000 km/s = 300,000,000 m/s = 3*10^8 m/s
c = f * λ

Electromagnetic waves differ from mechanical waves in that they require no medium in which to propagate. Electromagnetic waves will even propagate through the vacuum of space.

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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, 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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