TAILIEUCHUNG - CCNA 1 and 2 Companion Guide, Revised (Cisco Networking Academy Program) part 23

Cisco Networking Academy Program CCNA 1 and 2 Companion Guide, Revised part 23 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 189 Tuesday May 27 2003 9 01 AM Noise in Time and Frequency 189 Figure 4-4 Fourier Synthesis Oscilloscope Add Wave j Complex Wave Results Are Normalized 8 Spectrum Analyzer I 0 Frequency 1 Electromagnetic signals use different frequencies for transmission so that different signals do not interfere with each other. For example frequency modulation FM radio signals use frequencies that are different than television or satellite signals. When listeners change the station on a radio they are changing the frequency that the radio is receiving. Noise in Time and Frequency An important concept in communications systems including LANs is noise as shown in Figure 4-5. Although noise usually signifies undesirable sounds the definition of noise that is related to communications is undesirable signals. Noise can originate from natural and technological sources and is added to the data signals in communications systems. All communications systems have noise. Even though noise cannot be completely eliminated its effects can be minimized if the sources of the noise are understood. Sources of noise include the following Nearby cables that carry data signals Radio frequency interference RFI which is noise from other signals being transmitted nearby Electromagnetic interference EMI which is noise from nearby sources such as motors and lights Laser noise at the transmitter or receiver of an optical signal Page 190 Tuesday May 27 2003 9 01 AM 190 Chapter 4 Cable Testing and Cabling LANs and WANs Figure 4-5 Recognizing and Defining Noise -NEXT-A - Thermal Noise - Impulse I Refference Ground Noise -EMI RFI - NEXT -B Ground I Noise that affects all transmission frequencies equally is called white noise. Noise that affects only small ranges of frequencies is called narrowband interference. When detected on a radio receiver white noise interferes with all radio stations while narrowband interference affects only a few stations whose frequencies are close .

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