Calculating Power Budget For Fiber-Optic Cable For M Series, Mx Series, And T Series Routers - Juniper M40E Hardware Manual

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Calculating Power Budget for Fiber-Optic Cable for M Series, MX Series, and T Series Routers

Copyright © 2010, Juniper Networks, Inc.
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Appendix E: Cable and Wire Guidelines and Specifications for the M40e Router
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.
Fiber-Optic and Network Cable Specifications for the M40e Router on page 283
Signal Loss in Multimode and Single-Mode Fiber-Optic Cable for the M40e Router 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
To ensure that fiber-optic connections have sufficient power for correct operation, you
need to calculate the link's power budget, which is the maximum amount of power it
can transmit. When you calculate the power budget, you use a worst-case analysis to
provide a margin of error, even though all the parts of an actual system do not operate
at the worst-case levels. To calculate the worst-case estimate of power budget (
you assume minimum transmitter power (
P = P – P
B T R
) and minimum receiver sensitivity (
P
T
P ),
B
):
P
R
285