Calculating The Fiber-Optic Cable Power Margin For Qfx Series Switches - Juniper QFX5120 Series Hardware Manual

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
1. Determine values for the link's minimum transmitter power (P
For example, here, (P
(dBm):
P = –15 dBm
T
P = –28 dBm
R
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
–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 ) is the amount of power available after attenuation or link loss (LL) has been subtracted from the
M
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
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
negative indicates insufficient power to operate the receiver. See the specification for your receiver to
find the maximum receiver input power.
Before you begin to calculate the power margin, calculate the power budget. See
Cable Power Budget for QFX Series Switches" on page
) and (P ) are measured in decibels, and decibels are referenced to 1 milliwatt
T R
B
) by subtracting (P
) and minimum receiver sensitivity (P
T
) from (P ):
R T
78.
B
) for the link:
) greater
M
) that is zero or
M
"Calculating the Fiber-Optic
79
).
R