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Networking Fundamentals
Instead of transmitting the data packets over a single channel, the data
packets hop from one channel to another in a set pattern determined by a
software algorithm. None of the 79 channels are occupied for more than 0.4
seconds. Since the data packets switch from channel to channel, or rather
frequency to frequency, this transmission technique is called frequency hopping.
Many people assume that because data packets hop to various channels,
this transmission technique was designed as a security measure. This
assumption is false. The reason that the frequency hopping technique is used
is to avoid interference. If any interference matches the same frequency as
the wireless networking devices, the data would be corrupted. If only one
frequency was used and it encountered interference, then the network would
be useless. By using the frequency hopping technique, interference can be
tolerated. The interference would likely only affect one or two of the available
frequencies in the entire 79 1-MHz channels. This allows more than an ample
number of channels to ensure continuous transmission between wireless
networking devices.
The frequency hopping technique is limited to a maximum of a 2-Mbps data
rate. A much higher data rate can be accomplished using direct sequencing.
Direct sequencing
Direct sequencing divides the 2.4-GHz frequency band into eleven
overlapping channels of 83 MHz each. Within the eleven channels are three
channels with a 22-MHz bandwidth. The three channels do not overlap and can
be used simultaneously. Using three channels at the same time results in higher
data rates than frequency hopping. The data rates for direct sequencing are 11 Mbps
and 33 Mbps. The 33 Mbps is a result of using all three 22-Mbps channels at the
same time, Figure 4-10.
One disadvantage of direct sequencing is that a much larger portion of the
transmitted data is affected by electromagnetic interference than with frequency
hopping. The data rate of direct sequencing, therefore, is drastically affected by
interference.
Direct sequencing is also referred to as direct sequencing spread spectrum (DSSS).
Most vendors use DSSS technology at 11 Mbps for wireless network systems.
Figure 4-9.
In frequency
hopping, data is
transmitted over 79
1-MHz channels. The
data transmissions
continuously use
different channels in
short increments of
less than 0.4 seconds
each.
2.4-GHz
frequency
band
Data packet
Frequency Hopping Spread Spectrum (FHSS)
79 1-MHz channels