Juniper QFX5100, Hardware Manual

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performance level. The power margin (P
M
) is the amount of power available after
attenuation or link loss (LL) has been subtracted from the power budget (P
B
).
When you calculate the power margin, 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 worst-case
levels. A power margin (P
M
) greater than zero indicates that the power budget is sufficient
to operate the receiver and that it does not exceed the maximum receiver input power.
This means the link will work. A (P
M
) that is zero or negative indicates insufficient power
to operate the receiver. See the specification for your receiver to find the maximum
receiver input power.
Before you begin to calculate the power margin:
•
Calculate the power budget. See “Calculating the Fiber-Optic Cable Power Budget for
a QFX Series Device” on page 93 .
To calculate the worst-case estimate for the power margin (P
M
) for the link:
1.
Determine the maximum value for link loss (LL) by adding estimated values for
applicable link-loss factors—for example, use the sample values for various factors
as provided in
Table 37 on page 94 (here, the link is 2 km long and multimode, and
the (P
B
) is 13 dBm):
Table 37: Estimated Values for Factors Causing Link Loss
Sample (LL) Calculation Values Estimated Link-Loss Value Link-Loss Factor
0.5 dBm Multimode—0.5 dBm Higher-order mode losses
(HOL)
0 dBm Single-mode—None
0 dBm Multimode—None, if product of
bandwidth and distance is less
than 500 MHz/km Modal and chromatic
dispersion
0 dBm Single-mode—None
This example assumes five connectors. Loss for five
connectors: 5 (0.5 dBm) = 2.5 dBm 0.5 dBm Connector
This example assumes two splices. Loss for two splices:
2 (0.5 dBm) = 1 dBm 0.5 dBm Splice
This example assumes the link is 2 km long. Fiber
attenuation for 2 km: 2 km (1 dBm/km) = 2 dBm Multimode—1 dBm/km Fiber attenuation
This example assumes the link is 2 km long. Fiber
attenuation for 2 km: 2 km (0.5 dBm/km) = 1 dBm Single-mode—0.5 dBm/km
1 dBm 1 dBm Clock Recovery Module
(CRM)
Copyright © 2014, Juniper Networks, Inc. 94
QFX5100 Switch Hardware Guide