Datasheet

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
Related
Documentation
Fiber-Optic and Network Cable Specifications for the M7i Router on page 178•
• Signal Loss in Multimode and Single-Mode Fiber-Optic Cable for the M7i Router on
page 179
• Attenuation and Dispersion in Fiber-Optic Cable on M Series, MX Series, and T Series
Routers on page 179
• Calculating Power Margin for Fiber-Optic Cable for M Series, MX Series, and T Series
Routers on page 181
Calculating Power Margin for Fiber-Optic Cable for M Series, MX Series, and T Series Routers
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 26 on page 181
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 26: 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
181Copyright © 2012, Juniper Networks, Inc.
Appendix E: Cable and Wire Guidelines and Specifications for the M7i Router