Calculating Power Budget And Power Margin For Fiber-Optic Cables; Calculating Power Budget For Fiber-Optic Cable - Juniper M320 Hardware Manual

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Calculating Power Budget and Power Margin for Fiber-Optic Cables

Calculating Power Budget for Fiber-Optic Cable

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power of the light signal as it is transmitted. Attenuation is caused by passive media
components, such as cables, cable splices, and connectors. 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 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.
Use the information in this topic and the information in Supported Network Interface
Standards by Transceiver for ACX, M, MX, and T Series Routers or Supported Network
Interface Standards by Transceiver for PTX Series Routers to calculate the power budget
and power margin for fiber-optic cables.
To calculate the power budget and power margin, perform the following tasks:
Calculating Power Budget for Fiber-Optic Cable on page 112
1.
2. Calculating Power Margin for Fiber-Optic Cable on page 113
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
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