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Communication Security: Wireless • Chapter 4 171

wave (in phase), the wave is amplified.When the peak of a wave meets a trough (out
of phase), the wave is effectively cancelled. Multipath interference can be the source
of hard-to-troubleshoot problems. In planning for a wireless network, administrators
should consider the presence of common sources of multipath interference.These
include metal doors, metal roofs, water, metal vertical blinds, and any other source
that is highly reflective to radio waves.Antennas may help to compensate for the
effects of multipath interference, but must be carefully chosen. Many wireless access
points (APs) have two antennas for precisely this purpose. However, a single omnidi-
rectional antenna may be of no use at all for this kind of interference.
Another source of signal loss is the presence of obstacles.While radio waves can
travel through physical objects, they are degraded according to the properties of the
object they travel through. For example, a window, is fairly transparent to radio
waves, but may reduce the effective range of a wireless network by between 50
percent and 70 percent, depending on the presence and nature of the coatings on
the glass.A solid core wall can reduce the effective range of a wireless network by
up to 90 percent or greater.
EM fields are also prone to interference and signal degradation by the presence
of other EM fields. In particular, 802.11 wireless networks are prone to interference
produced by cordless phones, microwave ovens, and a wide range of devices that
use the same unlicensed Industrial, Scientific and Medical (ISM) or Unlicensed
National Information Infrastructure (UNII) bands.To mitigate the effects of inter-
ference from these devices and other sources of electromagnetic interference, RF-
based wireless networks employ spread spectrum technologies. Spread spectrum
provides a way to “share” bandwidth with other devices that may be operating in
the same frequency range. Rather than operating on a single, dedicated frequency
such as is the case with radio and television broadcasts, wireless networks use a
“spectrum” of frequencies for communication.

Spread Spectrum Technology
Conceived of by Hedy Lamarr and George Antheil in 1940 as a method of
securing military communications from jamming and for eavesdropping during
WWII, spread spectrum defines methods for wireless devices to use to send a
number of narrowband frequencies over a range of frequencies simultaneously for
communication.The narrowband frequencies used between devices change
according to a random-appearing, but defined pattern, allowing individual frequen-
cies to contain parts of the transmission. Someone listening to a transmission using
spread spectrum would hear only noise, unless their device understood in advance
what frequencies were used for the transmission and could synchronize with them.

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