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# How to Calculate 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 (PB) for the link:

1. Determine values for the link's minimum transmitter power (PT) and minimum receiver sensitivity (PR). For example, here, (PT) and (PR) are measured in decibels, and decibels are referenced to 1 milliwatt (dBm):

PT = –15 dBm

PR = –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 (PB) by subtracting (PR) from (PT):

–15 dBm – (–28 dBm) = 13 dBm