Juniper QFX10016, Hardware Manual

Page: 196 / 439

How to Calculate Power Margin for Fiber-Optic Cables
After calculating a link's power budget, you can calculate the power margin (P
M
), which represents the
amount of power available after subtracting attenuation or link loss (LL) from the power budget (P
B
). A
worst-case estimate of P
M
assumes maximum LL:
P
M
= P
B
– LL
P
M
greater than zero indicates that the power budget is sufficient to operate the receiver.
Factors that can cause link loss include higher-order mode losses, modal and chromatic dispersion,
connectors, splices, and fiber attenuation.
Table 70 on page 185 lists an estimated amount of loss for the
factors used in the following sample calculations. For information about the actual amount of signal loss
caused by equipment and other factors, refer to vendor documentation.
Table 70: Estimated Values for Factors Causing Link Loss
Link-Loss Factor Estimated Link-Loss Value
Higher-order mode losses Single mode—None
Multimode—0.5 dB
Modal and chromatic dispersion Single mode—None
Multimode—None, if product of bandwidth and distance is less than 500 MHz-
km
Faulty connector 0.5 dB
Splice 0.5 dB
Fiber attenuation Single mode—0.5 dB/km
Multimode—1 dB/km
The following sample calculation for a 2-km-long multimode link with a power budget (P
B
) of 13 dB uses
the estimated values from Table 70 on page 185 . This example calculates link loss (LL) as the sum of
fiber attenuation (2 km @ 1 dB/km, or 2 dB) and loss for five connectors (0.5 dB per connector, or
2.5 dB) and two splices (0.5 dB per splice, or 1 dB) as well as higher-order mode losses (0.5 dB). The
power margin (P
M
) is calculated as follows:
P
M
= P
B
– LL
185