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Communication Security: Wireless • Chapter 4 173

Direct Sequence Spread Spectrum

DSSS works somewhat differently.With DSSS, the data is divided and simultane-
ously transmitted on as many frequencies as possible within a particular frequency
band (the channel). DSSS adds redundant bits of data known as chips to the data to
represent binary 0s or 1s.The ratio of chips to data is known as the spreading ratio:
the higher the ratio, the more immune to interference the signal is, because if part
of the transmission is corrupted, the data can still be recovered from the remaining
part of the chipping code.This method provides greater rates of transmission than
FHSS, which uses a limited number of frequencies, but fewer channels in a given
frequency range.And, DSSS also protects against data loss through the redundant,
simultaneous transmission of data. However, because DSSS floods the channel it is
using, it is also more vulnerable to interference from EM devices operating in the
same range. In the 2.4 to 2.4835 GHz frequency range employed by 802.11b,
DSSS transmissions can be broadcast in any one of 14 22 MHz-wide channels.The
number of center-channel frequencies used by 802.11 DSSS devices depends on
the country. For example, North America allows 11 channels operating in the 2.4
to 2.4835 GHz range, Europe allows 13, and Japan allows 1. Because each channel
is 22-MHz-wide, they may overlap each other. Of the 11 available channels in
North America, only a maximum of three (1, 6, and 11) may be used concurrently
without the use of overlapping frequencies.

TEST DAY TIP

When comparing FHSS and DSSS technologies, it should be noted that
FHSS networks are not inherently more secure than DSSS networks, con-
trary to popular belief. Even if the relatively few manufacturers of FHSS
devices were not to publish the hopping sequence used by their devices,
a sophisticated hacker armed with a spectrum analyzer and a computer
could easily determine this information and eavesdrop on the communi-
cations.

Wireless Network Architecture

The seven-layer open systems interconnect (OSI) networking model defines the
framework for implementing network protocols.Wireless networks operate at the
physical and data link layers of the OSI model.The PHY layer is concerned with the

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