Cisco Networking Academy Program CCNA 1 and 2 Companion Guide, Revised part 20 is the Cisco approved textbook to use alongside version of the Cisco Networking Academy Program CCNA 1 and CCNA 2 web-based courses. The topics covered provide you with the necessary knowledge to begin your preparation for the CCNA certification exam (640-801, or 640-821 and 640-811) and to enter the field of network administration. | Page 159 Tuesday May 20 2003 2 53 PM Wireless Communications 159 complements the Cisco Aironet 1200 Series Access Point providing a solution that combines performance and mobility with the security and manageability that enterprises require. It will be possible to achieve data rates of greater than 20 Mbps in this frequency range. The drawback of the 5-GHz frequency however is its limited range. The typical range for 5 GHz inside is about 50 feet outside poses a limitation of approximately 2500 feet. Spread-Spectrum Technology Just as the radio in your car has AM and FM bands other radios use certain bands frequencies and types of modulation. Spread spectrum SS is a modulation technique developed in the 1940s that spreads a transmission signal over a broad band of radio frequencies. The term spread spectrum describes a modulation technique that sacrifices bandwidth to gain signal-to-noise performance. This technique is ideal for data communications because it is less susceptible to radio noise and creates little interference. Spread spectrum as illustrated in Figure 3-42 is a system in which the transmitted signal is spread over a frequency much wider than the minimum bandwidth required to send the signal. The fundamental premise is that in channels with narrowband interference increasing the transmitted signal bandwidth results in an increased probability that the received information is correct. NOTE Narrowband interference occurs when two signals are broadcasting at the same frequency in the same geographic area. The term band refers to a grouping of frequencies narrowband would mean a relatively smaller range of frequencies. Narrowband noise might disrupt certain channels or spreadspectrum components. Figure 3-42 Spread-Spectrum Technology To use the unlicensed radio bands you have to use spread-spectrum techniques. Frequency-hopping spread spectrum FHSS and direct-sequence spread spectrum DSSS are two ways of doing spread spectrum. These .
