User Manual

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Fiber-Optic and Network Cable Guidelines
51
When chromatic dispersion is at the maximum allowed, its effect can be considered as a
power penalty in the power budget. The optical power budget must allow for the sum of
component attenuation, power penalties (including those from dispersion), and a safety
margin for unexpected losses. For more information calculating the power budget, see
“Power Budget Calculation” on page 51.
Power Budget Calculation
A link’s power budget 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
B
), you assume minimum transmitter power (P
T
) and
minimum receiver sensitivity (
P
R
).
Table 15 lists equations for calculating the power budget for SONET/SDH PIC interfaces. The
values are measured in decibels (dB) and decibels referred to one milliwatt (dBm).
Table 15: Calculating Power Budget for SONET/SDH PIC Interfaces
Power Margin Calculation
After calculating a link’s power budget (using the equation described in “Power Budget
Calculation” on page 51), you can calculate the power margin (
P
M
), which estimates 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
A P
M
greater than zero indicates that the power budget is sufficient to operate the receiver.
PIC Interface Power Budget Equation
Multimode P
B
= P
T
– P
R
P
B
= –15 dBm – (–28 dBm)
P
B
= 13 dB
OC-12 single-mode P
B
= P
T
– P
R
P
B
= –15 dBm – (–28 dBm)
P
B
= 13 dB
OC-48 single-mode P
B
= P
T
– P
R
P
B
= –5 dBm – (–18 dBm)
P
B
= 13 dB