tailieunhanh - CompTIA Network+ Certification Study Guide part 20

CompTIA’s Network+ certification Study Guide part 20 is a globally-recognized, vendor neutral exam that has helped over 235,000 IT professionals reach further and higher in their careers. The 2009 Network+ exam (N10-004) is a major update with more focus on security and wireless aspects of networking. Our new study guide has been updated accordingly with focus on network, systems, and WAN security and complete coverage of today’s wireless networking standards. | 176 CHAPTER 5 Wireless Networking Radio Frequency Behaviors The following radio frequency behaviors are important in your basic understanding of how wireless networks operate and interact with their environments. Gain and Loss Gain occurs when a signal has its strength increased such as by passing through an amplifier. As you can see in the left-hand side of Figure the lower wave represents the signal before passing through the amplifier and the higher wave represents the signal after passing through the amplifier. The difference in amplitudes between both signal strengths is the gain. Loss is the exact opposite of gain and occurs when a signal has its strength decreased either intentionally through the use of a device such as an attenuator or unintentionally such as through resistance losses in a transmission cable. As you can see in the right-hand side of Figure the higher wave represents the preloss signal strength and the lower wave represents the wave signal strength after the loss has occurred. Reflection and Refraction Some RF behaviors are similar to those of light waves such as in the case of reflection and refraction. As you can see in the left-hand side of Figure reflection has occurred because our electromagnetic RF wave has impacted upon a surface that has a much larger cross-section than that of the wave itself. Many things cause reflection such as doors walls floors ceilings buildings and the curvature of the earth to name a few. Assuming that the surface being impacted is relatively flat and smooth a large majority of the original signal will be reflected off at a different angle than the entry signal. FIGURE Comparing Gain and Loss. Radio Frequency and Antenna Behaviors and Characteristics 177 Some signal will be lost due to absorption and scattering discussed later in this chapter . Surfaces that are less smooth or flat will cause a larger percentage of the original wave to be lost. The right-hand side of Figure depicts the .