02_1587202115_ch01.qxd 9/29/08 2:43 PM Page 9
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.
Bạn đang xem 02_ - CCNA WIRELESS OFFICIAL EXAM CERTIFICATION GUIDE PART 4 PPT