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of an actual system do not operate at the worst-case levels. To calculate the worst-case estimate of
power budget (P
), you assume minimum transmitter power (P
B
P
= P
– P
B
T
R
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
How to Calculate Power Margin for Fiber-Optic Cables
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 28: Estimated Values for Factors Causing Link Loss
Link-Loss Factor
Higher-order mode losses
Modal and chromatic dispersion
Faulty connector
assumes maximum LL:
M
Table 28 on page 64
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
) and minimum receiver sensitivity (P
T
), which represents the
M
lists an estimated amount of loss for the
64
):
R
). A
B
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