Radio Waves
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.
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, A) 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 below.
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