Juniper Networks MX480 - Manual (Page 331)

APPENDIX E
Cable and Wire Guidelines and
Specifications for the MX480 Router
• Understanding Fiber-Optic Cable Signal Loss, Attenuation, and Dispersion on page 307
• Calculating Power Budget and Power Margin for Fiber-Optic Cables on page 308
• Routing Engine Interface Cable and Wire Specifications for MX Series Routers on page 310
Understanding Fiber-Optic Cable Signal Loss, Attenuation, and Dispersion
• Signal Loss in Multimode and Single-Mode Fiber-Optic Cable on page 307
• Attenuation and Dispersion in Fiber-Optic Cable on page 307
Signal Loss in Multimode and Single-Mode Fiber-Optic Cable
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. LEDs are not coherent sources, however. 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 (HOL)
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. It is consequently more expensive.
Exceeding the maximum transmission distances can result in significant signal loss, which
causes unreliable transmission.
Attenuation and Dispersion in Fiber-Optic Cable
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. While attenuation is
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