Juniper Networks MX480 - Manual (Page 333)

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
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 (HOL), modal and
chromatic dispersion, connectors, splices, and fiber attenuation.
Table 44 on page 309
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 44: 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 example uses the estimated values in Table 33 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 mode loss = 0.5 dB
• Clock recovery module = 1 dB
The power margin (P
M
) is calculated as follows:
309 Copyright © 2013, Juniper Networks, Inc.
Appendix E: Cable and Wire Guidelines and Specifications for the MX480 Router