CCNA Wireless Official Exam Certification Guide part 8 is a best of breed Cisco® exam study guide that focuses specifically on the objectives for the CCNA® Wireless IUWNE exam. Senior instructor Brandon Carroll shares preparation hints and test-taking tips, helping you identify areas of weakness and improve both your conceptual knowledge and hands-on skills. Material is presented in a concise manner, focusing on increasing your understanding and retention of exam topics. | 42 CCNA Wireless Official Exam Certification Guide Determining Signal Strength Influences Although it might seem hopeless to deploy a network that is susceptible to various forms of interference you can fight back. To fight back you need to know what tools are available out there for you to determine signal strength noise levels and potential sources of interference in the path. This section discusses these tools. The Fresnel Zone To give you a little background Augustin-Jean Fresnel was a French physicist and civil engineer who lived from 1788 to 1827. He correctly assumed that light moved in a wavelike motion transverse to the direction of propagation. His assumption or claim was correct. Because of his work a method for determining where reflections will be in phase and out of phase between sender and receiver is based on his name. This method determines what is called the Fresnel zone. Here is how Fresnel did it. First he divided the path into zones. The first zone should be at least 60 percent clear of obstructions. To visualize this you can think of the shape of a football which is wider in the middle. However with the Fresnel zone calculation you use an equation to determine what the size of the ball is at the middle. This helps to determine the width that a wave will be so you can make sure that no obstacles are in the path. Note While at the CCNA wireless level you should not need to calculate the Fresnel zone. You can find the actual formula at http wiki Fresnel_zone Determining_ Fre snel_zone_clearance. Also you can find an online Fresnel zone clearance calculator at http . com support calculations . Note Indoor signals are too short to be affected. Figure 3-10 illustrates the height an antenna would need to be at different distances to overcome this. For example for a system at 7 miles you need to have the antennas mounted at 45 to 50 feet. Although this is just an example the numbers are pretty close
