# Calculating Power Budget and Power Margin for Fiber-Optic Cables

Use the information in this topic and the specifications for your optical interface to calculate the power budget and power margin for fiber-optic cables.

**Tip**

You can use the Hardware Compatibility Tool to find information about the pluggable transceivers supported on your Juniper Networks device.

To calculate the power budget and power margin, perform the following tasks:

## How to Calculate Power Budget for Fiber-Optic Cable

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

## How to Calculate Power Margin for Fiber-Optic Cable

After calculating a link's power budget, you can calculate the
power margin (P_{M}), which represents the amount
of power available after subtracting attenuation or link loss (LL)
from the power budget (P_{B}). A worst-case estimate
of P_{M} assumes maximum LL:

P_{M} = P_{B} –
LL

P_{M} greater than zero indicates that the
power budget is sufficient to operate the receiver.

Factors that can cause link loss include higher-order mode losses, modal and chromatic dispersion, connectors, splices, and fiber attenuation. Table 1 lists an estimated amount of loss for the factors used in the following sample calculations. For information about the actual amount of signal loss caused by equipment and other factors, refer to vendor documentation.

Table 1: Estimated Values for Factors Causing Link Loss

Link-Loss Factor | Estimated Link-Loss Value |
---|---|

Higher-order mode losses | Single mode—None Multimode—0.5 dB |

Modal and chromatic dispersion | Single mode—None Multimode—None, if product of bandwidth and distance is less than 500 MHz-km |

Connector | 0.5 dB |

Splice | 0.5 dB |

Fiber attenuation | Single mode—0.5 dB/km Multimode—1 dB/km |

The following sample calculation for a 2-km-long multimode link
with a power budget (P_{B}) of 13 dB uses
the estimated values from Table 1 to calculate link loss (LL) as the sum of fiber attenuation (2 km
@ 1 dB/km, or 2 dB) and loss for five connectors (0.5 dB
per connector, or 2.5 dB) and two splices (0.5 dB per splice,
or 1 dB) as well as higher-order mode losses (0.5 dB). The
power margin (P_{M}) is calculated as follows:

P_{M} = P_{B} –
LL

P_{M} = 13 dB – 2 km (1 dB/km) –
5 (0.5 dB) – 2 (0.5 dB) – 0.5 dB

P_{M} = 13 dB – 2 dB – 2.5 dB
– 1 dB – 0.5 dB

P_{M} = 7 dB

The following sample calculation for an 8-km-long single-mode
link with a power budget (P_{B}) of 13 dB
uses the estimated values from Table 1 to calculate link loss (LL) as the sum of fiber attenuation
(8 km @ 0.5 dB/km, or 4 dB) and loss for seven connectors
(0.5 dB per connector, or 3.5 dB). The power margin (P_{M}) is calculated as follows:

P_{M} = P_{B} –
LL

P_{M} = 13 dB – 8 km (0.5 dB/km) –
7(0.5 dB)

P_{M} = 13 dB – 4 dB – 3.5 dB

P_{M} = 5.5 dB

In both examples, the calculated power margin is greater than zero, indicating that the link has sufficient power for transmission and does not exceed the maximum receiver input power.