Router; Attenuation And Dispersion In Fiber-Optic Cable On M Series, Mx Series, And T Series Routers - Juniper M40E Hardware Manual

M40e Multiservice Edge Router Hardware Guide
Signal Loss in Multimode and Single-Mode Fiber-Optic Cable for the M40e Router
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Attenuation and Dispersion in Fiber-Optic Cable on M Series, MX Series, and T Series Routers

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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 to 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.
The router uses optical lasers for SONET/SDH PIC single-mode interfaces. These optics
comply with IR-1 of Bellcore GR-253-CORE Issue 2, December 1995 and ANSI TI.105.06.
For information about the maximum transmission distances and wavelength ranges
supported by multimode and single-mode (ATM and SONET/SDH) PIC interfaces, see
the M40e Multiservice Edge Router PIC Guide. Exceeding the maximum transmission
distances can result in significant signal loss, which causes unreliable transmission.
Fiber-Optic and Network Cable Specifications for the M40e Router on page 283
Attenuation and Dispersion in Fiber-Optic Cable on M Series, MX Series, and T Series
Routers on page 284
Calculating Power Budget for Fiber-Optic Cable for M Series, MX Series, and T Series
Routers on page 285
Calculating Power Margin for Fiber-Optic Cable for M Series, MX Series, and T Series
Routers on page 286
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
significantly lower for optical fiber than for other media, it still occurs in both multimode
and single-mode transmission. An efficient optical data link must have enough light
available to overcome attenuation.
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