21.4.3 Spread Spectrum Technology
Most WLANs use spread spectrum technology, a wideband radio frequency
technique that uses the entire allotted spectrum in a shared
fashion as opposed to
dividing it into discrete private pieces (as with narrowband). The spread spectrum
system (see Chapter 11) spreads the transmission power over the entire usable spec-
trum. This is obviously a less effi cient use of the bandwidth than the narrowband
approach. However, spread spectrum is designed to trade off bandwidth effi ciency
for
reliability, integrity, and security. The bandwidth trade-off produces a signal
that is easier to detect, provided that the receiver
knows the parameters of the
spread spectrum signal being broadcast. If the receiver is not tuned to the right
frequency, a spread spectrum signal looks like background noise.
By operating across a broad range of radio frequencies, a spread spectrum
device could communicate clearly despite interference from
other devices using the
same spectrum in the same physical location. In addition to its relative immunity
to interference, spread spectrum makes eavesdropping and jamming inherently
diffi cult.
In
commercial applications, spread spectrum techniques currently offer data
rates up to 2 Mbps. Because the FCC does not require site licensing for the bands
used by spread spectrum systems, this technology
has become the standard for
high-speed RF data transmission. Two modulation schemes are commonly used
to encode spread spectrum signals: direct sequence spread spectrum (DSSS) and
frequency hopping spread spectrum (FHSS).
FHSS uses a narrowband carrier that changes frequency in a pattern known
to both transmitter and receiver. Properly synchronized,
the net effect is to main-
tain a single logical channel. To an unintended receiver, FHSS appears to be a
short-duration impulse noise.
DSSS generates a redundant bit pattern for each bit to be transmitted. This
bit pattern is called a
spreading code
.
The longer the code, the greater the prob-
ability that the original data can be recovered (and, of course the more band-
width will be required). To an unintended receiver DSSS appears as low-power,
wideband noise and is rejected by most narrowband receivers.
