TAILIEUCHUNG - CCNA Wireless Official Exam Certification Guide part 14

CCNA Wireless Official Exam Certification Guide part 14 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. | 102 CCNA Wireless Official Exam Certification Guide Key Topic Table 6-4 The Protocol Ratified June 2003 RF Technology DSSS and OFDM Frequency Spectrum GHz Coding Barker 11 and CCK Modulation DBPSK and DQPSK Data Rates 1 2 11 Mbps with DSSS 6 9 12 18 24 36 48 54 Mbps with OFDM Nonoverlapping Channels 1 6 11 How Interacts with One interesting point about is that although it is backward compatible with clients you probably do not want it to be because if you must support clients the entire cell suffers. In fact if the average bandwidth is 22 Mbps in an cell and an client shows up the cell performance could degrade. This degradation in performance is because clients do not understand OFDM. If an client sends when an client is sending a collision will occur and both clients will have to resend. However protection mechanisms are built in. To understand how this protection works examine Figure 6-1. Access Point Figure 6-1 Cell with No Clients Assume that initially clients do not exist. The default behavior of an AP is to send beacons that include information about the AP and the wireless cell. Without clients the AP sends the following information in a beacon NON_ERP present no Use Protection no ERP is Extended Rate Physical. These are devices that have extended data rates. In other words NON_ERP is talking about clients. If they were ERP that would support the higher data rates making them clients. Now going back to Figure 6-1 with no clients the AP tells everyone that clients are unavailable and that they do not need to use protection mechanisms. Chapter 6 Overview of the WLAN Protocols 103 After an client associates with the AP things change. In Figure 6-2 the AP alerts the rest of the network about the NON_ERP client. This is done in the beacon that the AP sends. Client C

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