Calculating the Fiber-Optic Cable Power Budget for QFX Series Switches
Calculate the fiber-optic data link's power budget when planning fiber-optic cable layout and distances to
ensure that fiber-optic connections have sufficient power for correct operation. The power budget is the
maximum amount of power the link 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 for the fiber-optic cable power budget (P
B
) for the link:
1. Determine values for the link's minimum transmitter power (P
T
) and minimum receiver sensitivity (P
R
).
For example, here, (P
T
) and (P
R
) are measured in decibels, and decibels are referenced to 1 milliwatt (dBm):
P
T
= –15 dBm
P
R
= –28 dBm
NOTE: See the specifications for your transmitter and receiver to find the minimum transmitter
power and minimum receiver sensitivity.
2. Calculate the power budget (P
B
) by subtracting (P
R
) from (P
T
):
–15 dBm – (–28 dBm) = 13 dBm
Calculating the Fiber-Optic Cable Power Margin for QFX Series Switches
Calculate the fiber-optic data link's power margin when planning fiber-optic cable layout and distances to
ensure that fiber-optic connections have sufficient signal power to overcome system losses and still satisfy
the minimum input requirements of the receiver for the required performance level. The power margin
(P
M
) is the amount of power available after attenuation or link loss (LL) is subtracted from the power
budget (P
B
).
When you calculate the power margin, 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 worst-case levels. A power margin (P
M
) greater
than zero indicates that the power budget is sufficient to operate the receiver and that it does not exceed
the maximum receiver input power. This means the link will work. A power margin (P
M
) that is zero or
negative indicates insufficient power to operate the receiver. See the specification for your receiver to
find the maximum receiver input power.
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