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Calculating Power Budget and Power Margin for Fiber-Optic Cables
Copyright © 2019, Juniper Networks, Inc.
Attenuation and Dispersion in Fiber-Optic Cable
An optical data link functions correctly provided that modulated light reaching the receiver
has enough power to be demodulated correctly. Attenuation is the reduction in strength
of the light signal during transmission. Passive media components such as cables, cable
splices, and connectors cause attenuation. Although 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 transmit enough light to overcome
attenuation.
Dispersion is the spreading of the signal over time. The following two types of dispersion
can affect signal transmission through an optical data link:
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.
Use the information in this topic and the specifications for your optical interface to
calculate the power budget and power margin for fiber-optic cables.
TIP:
You can use the
the pluggable transceivers supported on your Juniper Networks device.
To calculate the power budget and power margin, perform the following tasks:
Calculating Power Budget for Fiber-Optic Cable on page 134
1.
2.
Calculating Power Margin for Fiber-Optic Cable on page 134
Chapter 2: Site Planning, Preparation, and Specifications
Hardware Compatibility Tool
to find information about
133
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