# Calculating Power Budget for Fiber-Optic Cable for M Series, MX Series, and T Series Routers

To ensure that fiber-optic connections have sufficient power for
correct operation, you need to calculate the link's power budget,
which is the maximum amount of power it 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 of
power budget (P_{B}), you assume minimum
transmitter power (P_{T}) and minimum
receiver sensitivity (P_{R}):

P_{B} = P_{T} –
P_{R}

The following hypothetical power budget equation uses values measured in decibels (dB) and decibels referred to one milliwatt (dBm):

P_{B} =
P_{T }– P_{R}

P_{B} = –15 dBm – (–28
dBm)

P_{B} = 13 dB