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Tutorial
Wireless LAN Technology and
Network Implementation
Carriers
If you tune the radio in
your home to 103.9 FM, you will receive the same station all
the time. In the
US, this is because the FCC regulates this range of
frequencies. However,
the frequency band used for wireless – both the 2 and 5 GHz
ranges – are unregulated.
There is no ownership of any one frequency.
Interference could become a problem if fixed carrier
frequencies were used. To overcome this problem, carrier frequencies are
consistently changed via several approaches. The major
approach used in wireless is called spread spectrum.
The height of the carrier is reduced (suppressed
carrier), and the carrier frequency is consistently
changed within a predefined range and with a pattern known by
both the receiver and the transmitter.
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Spread Spectrum Methods
Frequency
Hopping Spread Spectrum (FHSS)
uses a pseudo-random carrier hop method. In theory, FHSS
is more secure because of the difficulty involved in
predicting and capturing carriers generated in
pseudo-random patterns.
Direct
Sequence Spread Spectrum (DSSS) uses a sequence
generator to produce carrier frequencies, and is
therefore less secure than FHSS. |
Figure
2: Spread Spectrum Methods
In
Figure 3, we see the output of a spread spectrum system.
Notice how the carrier moves back and forth. There are several
approaches to spread spectrum; these approaches comprise
different 802.11 standards.
Figure
3: Spread Spectrum Transmitter Block Diagram
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