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170 Chapter 4 • Communication Security: Wireless

electrical properties of the communications medium being used.A network adapter
can decode changes in the electric current it senses on a wire and convert them to
meaningful information (bits) that can subsequently be sent to higher levels for
processing. Likewise, a network adapter can encode information (bits) by manipu-
lating the properties of the electric current for transmission on the communications
medium (in the case of wired networks, this would be the cable).

Radio Frequency Communications

The primary difference between wired and wireless networks is that wireless net-
works use a special type of electric current known as radio frequency (RF), which
is created by applying alternating current (AC) to an antenna to produce an elec-
tromagnetic field (EM). Devices for broadcasting and reception use the resulting
RF field. In the case of wireless networks, the medium for communications is the
EM field, the region of space that is influenced by electromagnetic radiation.
(Unlike audio waves, radio waves do not require a medium such as air or water to
propagate.) As with wired networks, amplitude decreases with distance, resulting in
the degradation of signal strength and the ability to communicate. However, the
EM field is also dispersed according to the properties of the transmitting antenna,
and not tightly bound as is the case with communication over a wire.The area over
which the radio waves propagate from an electromagnetic source is known as the
fresnel zone.

NOTE
A fresnel zone calculator is available at www.firstmilewireless.com/
calc_fresnel.html.

Like the waves created by throwing a rock into a pool of water, radio waves are
affected by the presence of obstructions and can be reflected, refracted, diffracted, or
scattered, depending on the properties of the obstruction and its interaction with the
radio waves. Reflected radio waves can be a source of interference on wireless net-
works.The interference created by bounced radio waves is called multipath
interference.
When radio waves are reflected, additional wave fronts are created.These dif-
ferent wave fronts may arrive at the receiver at different times and be in phase or
out of phase with the main signal.When the peak of a wave is added to another

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