Network Router User Manual

Factors that can cause link loss include higher-order mode losses, modal and
chromatic dispersion, connectors, splices, and fiber attenuation. Table 23 on page
186 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 23: Estimated Values for Factors Causing Link Loss
Estimated Link-Loss ValueLink-Loss Factor
Single-mode—None
Multimode—0.5 dB
Higher-order mode losses
Single-mode—None
Multimode—None, if product of bandwidth and distance
is less than 500 MHz–km
Modal and chromatic dispersion
0.5 dBConnector
0.5 dBSplice
Single-mode—0.5 dB/km
Multimode—1 dB/km
Fiber attenuation
The following example uses the estimated values in Table 23 on page 186 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 23 on page 186 to calculate
link loss (LL) as the sum of fiber attenuation (8 km @ 0.5 dB/km, or 4 dB) and loss
186 ■ Network Cable Specifications and Guidelines
M10i Internet Router Hardware Guide