# How to Calculate the Fiber-Optic Cable Power Margin for QFX Series Switches

Before you calculate the power margin, calculate the power budget. See How to Calculate the Fiber-Optic Cable Power Budget 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.

To calculate the worst-case estimate for the power margin
(P_{M}) for the link: