# Calculating the Fiber-Optic Cable Power Margin for EX Series Devices

Before calculating the power margin:

Calculate the power budget (see Calculating the Fiber-Optic Cable Power Budget for EX Series Devices).

Calculate the 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*) has been 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 (

*P*) 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.

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