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

Calculate the link's power margin when planning fiber-optic
cable layout and distances to ensure that fiber-optic connections
have sufficient signal power to overcome system losses and still satisfy
the minimum input requirements of the receiver for the required performance
level. The power margin (`P`

) is the amount of power available after attenuation or link loss
(_{M}`LL`

) has been subtracted from the power budget
(`P`

)._{B}

When you calculate the power margin, 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 worst-case levels. A power margin (`P`

) greater than zero indicates
that the power budget is sufficient to operate the receiver and that
it does not exceed the maximum receiver input power. This means the
link will work. A (_{M}`P`

)
that is zero or negative indicates insufficient power to operate the
receiver. See the specification for your receiver to find the maximum
receiver input power._{M}

Before you begin to calculate the power margin:

Calculate the power budget. See Calculating the Fiber-Optic Cable Power Budget for a QFX Series Device.

To calculate the worst-case estimate for the power margin
(`P`

) for the link:_{M}

- Determine the maximum value for link loss (
`LL`

) by adding estimated values for applicable link-loss factors; for example, use the sample values for various factors as provided in Table 1 (here, the link is 2 km long and multimode, and the (`P`

) is 13 dBm)._{B}Table 1: Estimated Values for Factors Causing Link Loss

Link-Loss Factor

Estimated Link Loss Value

Sample Link Loss (LL) Calculation Values

Higher-order mode losses

Multimode—0.5 dBm

0.5 dBm

Single-mode—None

0 dBm

Modal and chromatic dispersion

Multimode—None, if product of bandwidth and distance is less than 500 MHz/km

0 dBm

Single-mode—None

0 dBm

Connector

0.5 dBm

This example assumes five connectors. Loss for five connectors: 5 (0.5 dBm) = 2.5 dBm.

Splice

0.5 dBm

This example assumes two splices. Loss for two splices: 2 (0.5 dBm) = 1 dBm.

Fiber attenuation

Multimode—1 dBm/km

This example assumes the link is 2 km long. Fiber attenuation for 2 km: 2 km (1 dBm/km) = 2 dBm.

Single-mode—0.5 dBm/km

This example assumes the link is 2 km long. Fiber attenuation for 2 km: 2 km (0.5 dBm/km) = 1 dBm.

Clock Recovery Module (CRM)

1 dBm

1 dBm

**Note**For information about the actual amount of signal loss caused by equipment and other factors, see your vendor documentation for that equipment.

- Calculate the (
`P`

) by subtracting (_{M}`LL`

) from (`P`

):_{B}`P`

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

_{M}13 dBm – 0.5 dBm – 2.5 dBm – 1 dBm – 2 dBm – 1 dBm = P

_{M}P

_{M}= 6 dBmThe calculated power margin is greater than zero, indicating that the link has sufficient power for transmission. Also, the power margin value does not exceed the maximum receiver input power. Refer to the specifications for your receiver to find the maximum receiver input power.