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Chapter 1: Introduction to Wireless Networking Concepts 9

Note: The FCC is the regulatory body that exists in the United States. The European

Telecommunications Standards Institute (ETSI) is the European equivalent to the FCC.

Other countries have different regulatory bodies.

To achieve bandwidth from RF signals, you need to send data as electrical signals using

some type of emission method. One such emission method is known as Spread Spectrum.

In 1986, the FCC agreed to allow the use of spread spectrum in the commercial market

using what is known as the industry, scientific, and medical (ISM) frequency bands. To

place data on the RF signals, you use a modulation technique. Modulation is the addition

of data to a carrier signal. You are probably familiar with this already. To send music,

news, or speech over the airwaves, you use frequency modulation (FM) or amplitude

modulation (AM). The last time you were sitting in traffic listening to the radio, you were

using this technology.

Unlicensed Frequency Bands Used in WLANs

As you place more information on a signal, you use more frequency spectrum, or band-

width. You may be familiar with using terms like bits, kilobits, megabits, and gigabits

when you refer to bandwidth. In wireless networking, the word bandwidth can mean two

different things. In one sense of the word, it can refer to data rates. In another sense of the

word, it can refer to the width of an RF channel.

Note: This book uses the term bandwidth to refer to the width of the RF channel and not

to data rates.

When referring to bandwidth in a wireless network, the standard unit of measure is the

Hertz (Hz). A Hertz measures the number of cycles per second. One Hertz is one cycle

per second. In radio technology, a Citizens’ Band (CB) radio is pretty low quality. It uses

about 3 kHz of bandwidth. FM radio is generally a higher quality, using about 175 kHz of

bandwidth. Compare that to a television signal, which sends both voice and video over the

air. The TV signal you receive uses almost 4500 kHz of bandwidth.

Figure 1-1 shows the entire electromagnetic spectrum. Notice that the frequency ranges

used in CB radio, FM radio, and TV broadcasts are only a fraction of the entire spectrum.

Most of the spectrum is governed by folks like the FCC. This means that you cannot use

the same frequencies that FM radio uses in your wireless networks.

As Figure 1-1 illustrates, the electromagnetic spectrum spans from Extremely Low Fre-

quency (ELF) at 3 to 30 Hz to Extremely High Frequency (EHF) at 30 GHz to 300 GHz.

The data you send is not done so in either of these ranges. In fact, the data you send using

WLANs is either in the 900-MHz, 2.4-GHz, or 5-GHz frequency ranges. This places you in

the Ultra High Frequency (UHF) or Super High Frequency (SHF) ranges. Again, this is just

a fraction of the available spectrum, but remember that the FCC controls it. You are

locked into the frequency ranges you can use. Table 1-2 lists the ranges that can be used in

the United States, along with the frequency ranges allowed in Japan and Europe.