Syngress - Eleventh Hour Network+ Exam N10-004 Study Guide (11 ...

56 CHAPTER 4 Wireless Networking
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signal and some signal will be lost due to absorption and scattering. When
a wave is refracted, it passes through a medium and changes course with
some of the original wave being reflected away from the original wave’s
path. Refraction is a particular problem for long-range outdoor point-topoint
links due to changing atmospheric conditions, notably differing air
densities due to changes in air temperature.
Diffraction When a radio wave meets an obstacle, it has a tendency to bend
around the obstacle, which is called diffraction.
Absorption and scattering Absorption and scattering can absolutely
destroy an electromagnetic signal wave and prevent it from reaching its
intended destination. Absorption occurs when the RF signal has been completely
absorbed because it has impacted an object that does not pass it on
through any means (reflection or refraction). In this case, no signal is left
and the data contained in it is lost. Scattering is when an incoming electromagnetic
wave hits a surface that is small compared with its wavelength.
The resultant effect causes many lower magnitude waves to be sent off at
various angles relative to the path of the original wave. Typical sources of
scattering include trees, street signs, and atmospheric conditions.
Antenna characteristics include the following:
Line of sight and Fresnel zone With light waves, if a straight line exists,
it’s implied that the line of sight (LOS) exists. Once you have LOS, the
light waves will be able to travel from point to point. This is true for RF
as well. However, RF waves are also subject to a phenomenon known as
the Fresnel zone (pronounced “fray-nell”), which is an elliptical region
extending outward from the visual LOS. When dealing with optics, visual
LOS is enough to ensure good signal transmission from point to point,
but with electromagnetic waves, objects that extend into the Fresnel zone
can cause signal loss through methods such as reflection, refraction, and
scattering. Objects including buildings and trees can create a blockage, and
to overcome any blockage, you must either remove the object causing the
blockage or raise one or both antennas in the link.
WIRELESS NETWORK CONCEPTS
In the past 5 years, two wireless network technologies have seen considerable
deployment: wireless application protocol (WAP) networks and wireless local
area network (WLAN).
Overview of Wireless Communication in a Wireless Network
Wireless networks rely on the manipulation of an electrical charge to enable
communication between devices. A network adapter can decode and encode the
electric current to and from meaningful information (bits) that can subsequently
be sent or received.