Calculating Power Margin For Fiber-Optic Cable - Juniper MX2010 Hardware Manual

The following hypothetical power budget equation uses values measured in decibels (dB) and decibels
referred to one milliwatt (dBm):
P = P – P
B T R
P = –15 dBm – (–28 dBm)
B
P = 13 dB
B

Calculating Power Margin for Fiber-Optic Cable

After calculating a link's power budget, you can calculate the power margin (P
amount of power available after subtracting attenuation or link loss (LL) from the power budget (P
worst-case estimate of P
P = P – LL
M B
P greater than zero indicates that the power budget is sufficient to operate the receiver.
M
Factors that can cause link loss include higher-order mode losses, modal and chromatic dispersion,
connectors, splices, and fiber attenuation.
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 46: Estimated Values for Factors Causing Link Loss
Link-Loss Factor
Higher-order mode losses
Modal and chromatic dispersion
Connector
Splice
Fiber attenuation
The following sample calculation for a 2-km-long multimode link with a power budget (P
the estimated values from
assumes maximum LL:
M
Table 46 on page 152
Estimated Link-Loss Value
Single mode—None
Multimode—0.5 dB
Single mode—None
Multimode—None, if product of bandwidth and distance is less than
500 MHz-km
0.5 dB
0.5 dB
Single mode—0.5 dB/km
Multimode—1 dB/km
Table 46 on page 152
lists an estimated amount of loss for the
to calculate link loss (LL) as the sum of fiber attenuation
), which represents the
M
). A
B
) of 13 dB uses
B
152