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Overview of Wireless Communication in a ... > Overview of Wireless Communication i... - Pg. 56

56 CHAPTER 4 Wireless Networking 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-to- point 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 com- pletely 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 electro- magnetic 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.