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 38 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 38: Estimated Values for Factors Causing Link Loss

The following example uses the estimated values in Table 38 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 38 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.

- Signal Loss in Multimode and Single-Mode Fiber-Optic Cable for the SRX3400 Services Gateway
- Attenuation and Dispersion in Fiber-Optic Cable for the SRX3400 Services Gateway
- Calculating the Power Budget for Fiber-Optic Cable for the SRX3400 Services Gateway

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