Wireless communications make use of electromagnetic waves to send signals
across long distances. From a users 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, its 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 youve found it! You can also be confident that running many Ethernet
cables alongside each other wont 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.
What is a wave?
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, ) 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, you need to understand electromagnetic forces.
