 Computer Networks: The Physical Layer2.3 Wireless Transmission
Highlights
2.3.1 The Electromagnetic SpectrumWireless transmission can provide special services and conveniences to people, such as connection to the Internet or other networks without connecting to a wire directly. It can also facilitate the creation of networks in special situations, such as terrain that is unfriendly to ground cables. Wireless communication began, in fact, in the Hawaiian Islands precisely to solve the problem of terrain separated by large stretched of ocean.Political organizations determine which sections of the spectrum of light are allotted to which wireless transmission purposes. Because the existing organizations are not always in agreement with current practice or with each other, some products manufactured for one country may not work in another. 2.3.2 Radio TransmissionRadio waves are easy to generate and are omnidirectional, but have low transmission rates. Also, depending on their frequency, radio waves either cannot travel very far, or are absorbed by the earth. In some cases, though, High Frequency (HF) waves are reflected back to earth by the Ionosphere (a layer of the atmosphere).2.3.3 Microwave TransmissionMicrowave transmission is popular for its ability to travel in straight lines. A source can be directly focused on its destination without interfering with neighboring transmissions. Because they travel in straight lines, though, the curvature of the earth can interfere with the microwave transmitters; the solution to this is the addition of repeaters inbetween the source and destination to redirect the data path. Microwaves are used for long distance communication (Microwave Communications, Inc.=MCI), cellular phones, garage door openers, and much more.2.3.4 Infrared and Millimeter WavesInfrared light is used for close-range communication, such as remote controls, because it does not pass through objects well. This is also a plus because infrared communications in one room do not interfere with the infrared communications in another room. Infrared communication is more secure than other options, such as radio, but it cannot be used outside due to interference by the Sun.2.3.5 Lightwave TransmissionLasers can be used for wireless communication. It is a relatively low cost way to connect two buildings' LANs, but it has drawbacks. The laser is difficult to target on the destination's receiver because the beam is so small. Laser light also diffuses easily in poor atmospheric conditions, such as rain, fog, or intense heat.
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