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CCNA4.com
CCNA Wireless Study Guide
Waves, Frequencies, and RF
Waves – Wireless starts and ends with waves, specifically radio waves. There are different
modulation techniques to encode data onto a carrier wave signal. These techniques differ
between the 3 (now 4 with N) flavors of wireless, A, B, and G. DSSS (Direct Sequence
Spread Spectrum) is the modulation technique used by 802.11b, which uses “chipping
codes” to send redundant data to allow for interference. OFDM (Orthogonal Frequency
Division Multiplexing) is the modulation technique used by 802.11a and 802.11g. This
technique divides a channel into multiple subcarriers, similar to how a T1 is divided up. Data
is sent simultaneously over these subchannels to achieve redundancy and a combined higher
data rate. MIMO (Multiple-Input Multiple-Output) is the modulation technique used by
802.11n and allows a device to use more than 1 antenna for sending data and 1 antenna for
receiving data. This is the main thing that helps N products achieve such higher data rates
than a and b/g, along with many other advances in how signals are processed.
Frequencies – All wireless devices use unlicensed frequencies, meaning that you do not have
to apply for a license from the FCC to use them and they are subject to interference from
other devices. Within the frequencies assigned to a and b/g there are also “channels,” which
are the portion of the frequency that an individual device can use. This is less important for
802.11a devices as AP’s using 802.11a will automatically sense and choose a channel that is
less likely to conflict with the AP’s around it. A also has a lot more non-overlapping channels
to choose from – 23 within the 5GHz range of 802.11a. 802.11b only has 3 non-overlapping
channels to choose from within the 2.4Ghz range 802.11b uses. If 2 AP’s next to each other
are transmitting at on the same channel, the signal to noise ratio will rise and the bandwidth
available will decrease.
RF – Radio Frequency waves behave lik