Calculating The Fiber-Optic Cable Power Budget For A Qfx Series Device - Juniper QFX5100 Hardware Manual

Related
Documentation

Calculating the Fiber-Optic Cable Power Budget for a QFX Series Device

Copyright © 2016, Juniper Networks, Inc.
Chromatic dispersion, which is the spreading of the signal over time caused by the
different speeds of light rays.
Modal dispersion, which is the spreading of the signal over time caused by 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 limits the 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 within the limits specified for the type of link in the 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.
Calculating the Fiber-Optic Cable Power Budget for a QFX Series Device on page 151
Calculating the Fiber-Optic Cable Power Margin for a QFX Series Device on page 152
Calculate the link's power budget when planning fiber-optic cable layout and distances
to ensure that fiber-optic connections have sufficient power for correct operation. The
power budget is the maximum amount of power the link 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 for the fiber-optic cable power budget (P
link:
Determine values for the link's minimum transmitter power (P
1.
sensitivity (P ). For example, here, (P
R
are referenced to 1 milliwatt (dBm):
P = –15 dBm
T
P = –28 dBm
R
NOTE: See the specifications for your transmitter and receiver to find the
minimum transmitter power and minimum receiver sensitivity.
Calculate the power budget (P
2.
–15 dBm – (–28 dBm) = 13 dBm
) and (P ) are measured in decibels, and decibels
T R
B
) by subtracting (P ) from (P
R
Chapter 9: Cable Specifications
B
) for the
) and minimum receiver
T
):
T
151