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What
is
Spectrum?
What is DSSS? What is
FHSS? And what are
their differences?
Would the information
be
intercepted
transmitting on air?
Spread
Spread Spectrum technology is a wideband radio
frequency technique developed by the military for
use
communications systems. It is designed to trade off
bandwidth efficiency for reliability, integrity, and
security. In other words, more bandwidth is
consumed
transmission, but the trade-off produces a signal
that is, in effect, louder and thus easier to detect,
provided that the receiver knows the parameters of
the spread-spectrum signal being broadcast. If a
receiver is not tuned to the right frequency, a
spread-spectrum signal looks like background noise.
There are two main alternatives, Direct Sequence
Spread Spectrum (DSSS) and Frequency Hopping
Spread Spectrum (FHSS).
Frequency-Hopping Spread-Spectrum (FHSS) uses a
narrowband carrier that changes frequency in a
pattern that is known to both transmitter and
receiver. Properly synchronized, the net effect is to
maintain a single logical channel. To an unintended
receiver, FHSS appears to be short-duration impulse
noise. Direct-Sequence Spread-Spectrum (DSSS)
generates a redundant bit pattern for each bit to be
transmitted. This bit pattern is called a chip (or
chipping code). The longer the chip, the greater the
probability that the original data can be recovered.
Even if one or more bits in the chip are damaged
during
embedded in the radio can recover the original data
without
unintended receiver, DSSS appears as low power
wideband noise and is rejected (ignored) by most
narrowband receivers.
WLAN features two-fold protection in security. On
while
the hardware side, as with Direct Sequence Spread
Spectrum technology, it has the inherent security
feature of scrambling. On the software side, WLAN
offers the encryption function (WEP) to enhance
in
reliable,
than
in
the
transmission,
the
need
for
59
secure,
mission-critical
case
of
narrowband
statistical
techniques
retransmission.
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an
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