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

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

A 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 example uses the estimated values
in Table 1 to calculate link loss
(LL) for a 2 km-long multimode link with a power budget
(P_{B}) of 13 dB:

- Fiber attenuation for 2 km @ 1.0 dB/km= 2 dB
- Loss for five connectors @ 0.5 dB per connector = 5(0.5 dB) = 2.5 dB
- Loss for two splices @ 0.5 dB per splice =2(0.5 dB) = 1 dB
- Higher-order loss = 0.5 dB
- Clock recovery module = 1 dB

The power margin (P_{M})
is calculated as follows:

P_{M} = P_{B} –
LL

P_{M} = 13 dB – 2 km (1.0 dB/km)
– 5 (0.5 dB) – 2 (0.5 dB) – 0.5 dB [HOL] –
1 dB [CRM]

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

P_{M} = 6 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.