M120 Attenuation And Dispersion In Fiber-Optic Cable - Juniper M120 Hardware Manual

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M120 Attenuation and Dispersion in Fiber-Optic Cable

Copyright © 2010, Juniper Networks, Inc.
Appendix E: Cable and Wire Guidelines and Specifications for the M120 Router
For specifications of minimum and maximum input level (receiver sensitivity and receiver
saturation) and minimum and maximum output level (average launch power) for the
SONET/SDH PICs supported on the M120 router, see the M120 PIC Guide.
Fiber-Optic and Network Cable Specifications for the M120 Router on page 241
Attenuation and Dispersion in Fiber-Optic Cable on M Series, MX Series, and T Series
Routers on page 243
Calculating Power Budget for Fiber-Optic Cable for M Series, MX Series, and T Series
Routers on page 244
Calculating Power Margin for Fiber-Optic Cable for M Series, MX Series, and T Series
Routers on page 244
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.
Dispersion is the spreading of the signal in time. The following two types of dispersion
can affect an optical data link:
Chromatic dispersion—Spreading of the signal in time resulting from the different
speeds of light rays.
Modal dispersion—Spreading of the signal in time resulting from the different
propagation modes in the fiber.
For multimode transmission, modal dispersion, rather than chromatic dispersion or
attenuation, usually limits the maximum bit rate and link length. For single-mode
transmission, modal dispersion is not a factor. However, at higher bit rates and over longer
distances, chromatic dispersion rather than modal dispersion limits maximum link length.
An efficient optical data link must have enough light to exceed the minimum power that
the receiver requires to operate within its specifications. In addition, the total dispersion
must be less than the limits specified for the type of link in Telcordia Technologies
document GR-253-CORE (Section 4.3) and International Telecommunications Union
(ITU) document G.957.
When chromatic dispersion is at the maximum allowed, its effect can be considered as
a power penalty in the power budget. The optical power budget must allow for the sum
of component attenuation, power penalties (including those from dispersion), and a
safety margin for unexpected losses.
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