Juniper QFX5240 - Calculating Power Budget and Power Margin for Fiber-Optic Cables; How to Calculate Power Budget for Fiber-Optic Cables

For mulmode transmission, modal dispersion, rather than chromac dispersion or aenuaon, 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, chromac dispersion limits the maximum
link length.
An ecient opcal data link must have enough light to exceed the minimum power that the receiver
requires to operate within its specicaons. In addion, the total dispersion must be within the limits
specied for the type of link in the Telcordia Technologies document GR-253-CORE (Secon 4.3) and
Internaonal Telecommunicaons Union (ITU) document G.957.
When chromac dispersion is at the maximum allowed, its eect can be considered as a power penalty
in the power budget. The opcal power budget must allow for the sum of component aenuaon,
power penales (including those from dispersion), and a safety margin for unexpected losses.
Calculang Power Budget and Power Margin for Fiber-Opc Cables
IN THIS SECTION
How to Calculate Power Budget for Fiber-Opc Cables | 51
How to Calculate Power Margin for Fiber-Opc Cables | 52
Use the informaon in this topic and the specicaons for your opcal interface to calculate the power
budget and power margin for ber-opc cables.
TIP: You can use the Hardware Compability Tool to nd informaon about the pluggable
transceivers supported on your Juniper Networks device.
To calculate the power budget and power margin, perform the following tasks:
How to Calculate Power Budget for Fiber-Opc Cables
To ensure that ber-opc connecons have sucient power for correct operaon, 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 esmate of
power budget (P
B
), you assume minimum transmier power (P
T
) and minimum receiver sensivity (P
R
):
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