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CHAPTER 12
Network Cable and Transceiver Planning
Understanding Fiber-Optic Cable Signal Loss, Attenuation, and Dispersion
Signal Loss in Multimode and Single-Mode Fiber-Optic Cable
Attenuation and Dispersion in Fiber-Optic Cable
Copyright © 2018, Juniper Networks, Inc.
Understanding Fiber-Optic Cable Signal Loss, Attenuation, and Dispersion on page 101
Calculating Power Budget and Power Margin for Fiber-Optic Cables on page 102
This topic describes signal loss, attenuation, and dispersion in fiber-optic cable.
Signal Loss in Multimode and Single-Mode Fiber-Optic Cable on page 101
Attenuation and Dispersion in Fiber-Optic Cable on page 101
Multimode fiber is large enough in diameter to allow rays of light to reflect internally
(bounce off the walls of the fiber). Interfaces with multimode optics typically use LEDs
as light sources. However, LEDs are not coherent sources. They spray varying wavelengths
of light into the multimode fiber, which reflects the light at different angles. Light rays
travel in jagged lines through a multimode fiber, causing signal dispersion. When light
traveling in the fiber core radiates into the fiber cladding, higher-order mode loss results.
Together these factors limit the transmission distance of multimode fiber compared with
single-mode fiber.
Single-mode fiber is so small in diameter that rays of light can reflect internally through
one layer only. Interfaces with single-mode optics use lasers as light sources. Lasers
generate a single wavelength of light, which travels in a straight line through the
single-mode fiber. Compared with multimode fiber, single-mode fiber has higher
bandwidth and can carry signals for longer distances.
Exceeding the maximum transmission distances can result in significant signal loss, which
causes unreliable transmission.
Correct functioning of an optical data link depends on modulated light reaching the
receiver with enough power to be demodulated correctly. Attenuation is the reduction in
power of the light signal as it is transmitted. Attenuation is caused by passive media
components, such as cables, cable splices, and connectors. Although attenuation is
significantly lower for optical fiber than for other media, it still occurs in both multimode
101
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