TAILIEUCHUNG - CCNA 1 and 2 Companion Guide, Revised (Cisco Networking Academy Program) part 19

Cisco Networking Academy Program CCNA 1 and 2 Companion Guide, Revised part 19 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 149 Tuesday May 20 2003 2 53 PM Optical Media 149 Fiber-optic patch panels as shown in Figure 3-31 are similar to the patch panels used with copper cable. These panels increase the flexibility of an optical network by allowing quick changes to the connection of devices like switches or routers with various available fiber runs cable links . Figure 3-31 Fiber-Optic Patch Panels Signals and Noise in Optical Fiber Fiber-optic cable is not affected by the sources of external noise that cause problems on copper media. Why Because external light cannot enter the fiber except at the transmitter end. The cladding is covered by a buffer and an outer jacket that stops light from entering or leaving the cable. Furthermore the transmission of light on one fiber in a cable does not generate interference that disturbs transmission on any other fiber which means that fiber does not have the problem with crosstalk that copper media does. In fact the quality of fiber-optic links is so good that the recent standards for Gigabit and 10-Gigabit Ethernet specify transmission distances that far exceed the traditional 2-kilometer reach of the original Ethernet. You learn more about the Ethernet technologies in Chapter 6 Ethernet Technologies and Ethernet Switching . Fiber-optic transmission allows the Ethernet protocol to be used on metropolitan-area networks MANs and WANs. Although fiber is the best of all the transmission media at carrying large amounts of data over long distances fiber is not without problems. When light travels through fiber some of the light energy is lost. The farther a light signal travels through a fiber Page 150 Tuesday May 20 2003 2 53 PM 150 Chapter 3 Networking Media the more it loses strength. This attenuation of the signal is due to several factors involving the nature of fiber itself. The most important factor is scattering. The scattering of light in a fiber is caused by microscopic non-uniformity distortions in the fiber that .

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