# Calculating the Fiber-Optic Cable Power Budget for a QFX Series Device

Calculate the 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`

) for the link:^{B}

- Determine values for the link's minimum transmitter power
(
`P`

) and minimum receiver sensitivity (_{T}`P`

). For example, here, (_{R}`P`

) and (_{T}`P`

) are measured in decibels, and decibels are referenced to 1 milliwatt (dBm):_{R}`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.

- Calculate the power budget (
`P`

) by subtracting (^{B}`P`

) from (_{R}`P`

):_{T}–15 dBm – (–28 dBm) = 13 dBm