127
is, in effect, louder and thus easier to detect, provided that the
receiver knows the parameters of the spread-spectrum signal bein
g
broadcast. If a receiver is not tuned to the ri
g
ht frequency, a
spread-spectrum si
g
nal looks like back
g
round noise. There are two
main alternatives, Direct Sequence Spread Spectrum (DSSS) and
Frequency Hopping Spread Spectrum (FHSS).
Q
uestion What is DSSS? What is FHSS? And what are their differences?
A
nswer Frequency-Hoppin
g
Spread-Spectrum (FHSS) uses a narrowband
carrier that chan
g
es frequency in a pattern that is known to both
transmitter and receiver. Properly synchronized, the net effect is to
maintain a sin
g
le lo
g
ical channel. To an unintended receiver, FHSS
appears to be short-duration impulse noise. Direct-Sequence
Spread-Spectrum (DSSS)
g
enerates a redundant bit pattern for each
bit to be transmitted. This bit pattern is called a chip (or chippin
g
code). The lon
g
er the chip, the
g
reater the probability that the
ori
g
inal data can be recovered. Even if one or more bits in the chip
are dama
g
ed durin
g
transmission, statistical techniques embedded
in the radio can recover the original data without the need for
retransmission. To an unintended receiver, DSSS appears as low
power wideband noise and is re
j
ected (i
g
nored) by most
narrowband receivers.
Q
uestion Would the information be intercepted while transmittin
on air?
A
nswer WLAN features two-fold protection in security. On the hardware
side, as with Direct Sequence Spread Spectrum technolo
g
y, it has
the inherent security feature of scramblin
g
. On the software side,
WLAN offers the encryption function (WEP) to enhance security and
access control.
Q
uestion What is WEP?
A
nswer WEP is Wired Equivalent Privacy, a data privacy mechanism based
on a 64-bit or 128-bit shared key al
g
orithm, as described in the IEEE
802.11 standard.