To calculate the worst-case estimate for the power margin (P
M
) for the link:
1. 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 31 on page 80 (here,
the link is 2 km long and multimode, and the power margin (P
M
) is 13 dBm).
Table 31: Estimated Values for Factors Causing Link Loss
Sample Link Loss Calculation ValuesEstimated Link Loss ValueLink-Loss Factor
0.5 dBmMultimode—0.5 dBmHigher-order mode
losses
0 dBmSingle-mode—None
0 dBmMultimode—None, if the sum of
bandwidth and distance is less
than 500 MHz/km
Modal and chromatic
dispersion
0 dBmSingle-mode—None
This example assumes five connectors. Loss for five
connectors: (5)x(0.5 dBm) = 2.5 dBm.
0.5 dBmConnector
This example assumes two splices. Loss for two
splices: 2 (0.5 dBm) = 1 dBm.
0.5 dBmSplice
This example assumes the link is 2 km long. Fiber
attenuation for 2 km: 2 km (1 dBm/km) = 2 dBm.
Multimode—1 dBm/kmFiber attenuation
This example assumes the link is 2 km long. Fiber
attenuation for 2 km: 2 km (0.5 dBm/km) = 1 dBm.
Single-mode—0.5 dBm/km
1 dBm1 dBmClock Recovery Module
(CRM)
NOTE: For information about the actual amount of signal loss caused by equipment and other
factors, see your vendor documentation for that equipment.
2. Calculate the (P
M
) by subtracting (LL) from (P
B
):
P
B
– LL = P
M
13 dBm – 0.5 dBm [HOL] – 5 x (0.5 dBm) – 2 (0.5 dBm) – 2 km (1.0 dBm/km) – 1 dB [CRM] = P
M
80
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