Juniper MX2010, Hardware Manual

Page: 174 / 825

The following hypothetical power budget equation uses values measured in decibels (dB) and decibels
referred to one milliwatt (dBm):
P
B
= P
T
– P
R
P
B
= –15 dBm – (–28 dBm)
P
B
= 13 dB
Calculating Power Margin for Fiber-Optic Cable
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
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 46 on page 152 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 46: Estimated Values for Factors Causing Link Loss
Estimated Link-Loss Value Link-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 dB Connector
0.5 dB Splice
Single mode—0.5 dB/km
Multimode—1 dB/km
Fiber attenuation
The following sample calculation for a 2-km-long multimode link with a power budget (P
B
) of 13 dB uses
the estimated values from Table 46 on page 152 to calculate link loss (LL) as the sum of fiber attenuation
152