Calculating the Power Margin for Fiber-Optic Cable for the SRX3400 Services Gateway

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

PM = PB – LL

A PM 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 42 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.

Link-Loss Factor

Estimated Link-Loss Value

Higher-order mode losses


Multimode—0.5 dB

Modal and chromatic dispersion


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


0.5 dB


0.5 dB

Fiber attenuation

Single-mode—0.5 dB/km

Multimode—1 dB/km

The following example uses the estimated values in Table 42 to calculate link loss (LL) for a 2 km-long multimode link with a power budget (PB) of 13 dB:

The power margin (PM) is calculated as follows:

PM = PB – LL

PM = 13 dB – 2 km (1.0 dB/km) – 5 (0.5 dB) – 2 (0.5 dB) – 0.5 dB [HOL] – 1 dB [CRM]

PM = 13 dB – 2 dB – 2.5 dB – 1 dB – 0.5 dB – 1 dB

PM = 6 dB

The following sample calculation for an 8 km-long single-mode link with a power budget (PB) of 13 dB uses the estimated values from Table 42 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 (PM) is calculated as follows:

PM = PB – LL

PM = 13 dB – 8 km (0.5 dB/km) – 7 (0.5 dB)

PM = 13 dB – 4 dB – 3.5 dB

PM = 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.

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