tailieunhanh - CCNA Wireless Official Exam Certification Guide part 11

CCNA Wireless Official Exam Certification Guide part 11 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. | 72 CCNA Wireless Official Exam Certification Guide Figure 5-1 Vertical and Horizontal Polarization Horizontal Polarization Figure 5-2 Circular Polarization Here is how it works The two antennas are placed one wavelength apart. When the AP hears a preamble of a frame it switches between the two antennas and uses an algorithm to determine which antenna has the better signal. After an antenna is chosen it is used for the rest of that frame. You can switch antennas and listen to the preamble because it has no real data. As soon as the real data gets there it uses only one of the antennas. Chapter 5 Antenna Communications 73 Most of the time this happens with a single radio in the AP and two antennas connected to it. This is important because the two antennas cover the same area. You wouldn t try to cover two different areas with the same radio. Additionally the antennas need to be the same. If you used a weaker antenna on one side versus the other the coverage area would not be the same. Common Antenna Types The two main types of antennas are directional and omnidirectional. In this section you will learn the difference between the two types and look at some of the antennas that Cisco offers. Both send the same amount of energy the difference is in how the beam is focused. To understand this imagine that you have a flashlight. By twisting the head of the light you can make the beam focus in a specific area. When the beam has a wider focus it doesn t appear to be as bright. While you twist the head of the light you never change its output. The batteries are the same. The power is the same. The light is the same. You simply focus it in different ways. The same goes for wireless antennas. When you look at a directional antenna it appears to be a stronger signal in one direction but it s still emitting the same amount of energy. To increase power in a particular direction you add gain. The angles of coverage are fixed with each antenna. When you buy high-gain antennas it .