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To calculate the power budget and power margin, perform the following tasks:
Calculating Power Budget for Fiber-Optic Cable | 87
1.
Calculating Power Margin for Fiber-Optic Cable | 87
2.
Calculating Power Budget for Fiber-Optic Cable
To ensure that fiber-optic connections have sufficient power for correct operation, 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 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
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 29: Estimated Values for Factors Causing Link Loss
Link-Loss Factor
Higher-order mode losses
assumes maximum LL:
M
Table 29 on page 87
Estimated Link-Loss Value
Single mode—None
Multimode—0.5 dB
) and minimum receiver sensitivity (P
T
), which represents the
M
lists an estimated amount of loss for the
87
):
R
). A
B
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