The Electromagnetic Spectrum

In guided media, medium is more important in setting transmission parameters, whereas in unguided media, signal characteristics is more important.

Wireless transmission medium typically used where laying a cable is not practical. For example

If running a fiber (or any guided medium) to a building is difficult due to the Terrain (mountains, jungles, swamps etc.)

There are varieties of wireless media such as lasers, radio etc. as follows.

The Electromagnetic Waves

When electrons move, they create electromagnetic waves that can propagate through free space. In vacuum, all electromagnetic waves travel at the same speed, this speed usually called the speed of light, c, is approximately 3 * 108 m/sec. and the speed of light is the ultimate speed limit. 

The electromagnetic spectrum is shown in fig. it indicates the frequencies at which various guided media and unguided transmission techniques run.

The terms LF (low frequency), MF (Medium), HF (High), VHF (Very), UHF (Ultra), SHF (Super High), EHT (Extremely), THF (Tremendously)

The data rate depends on the width of wavelength, the wider the band, the higher the data rate.

To prevent total confusion, there are national and international agreements about who gets to use which frequencies.

Most transmissions use a narrow frequency band to get best reception many watts/Hz.

The transmissions are intentionally spread out over a wide frequency band, this technique is called spread spectrum. This is used in military communication.

True spread spectrum, sometimes called direct sequence spread spectrum and it used in commercial world.

Fig.   The electromagnetic spectrum and its uses for communication.

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