Juniper ACX7348 - How to Calculate Power Margin for Fiber-Optic Cables

of an actual system do not operate at the worst-case levels. To calculate the worst-case esmate of
power budget (P
B
), you assume minimum transmier power (P
T
) and minimum receiver sensivity (P
R
):
P
B
= P
T
– P
R
The following hypothecal power budget equaon uses values measured in decibels (dB) and decibels
referred to one milliwa (dBm):
P
B
= P
T
– P
R
P
B
= –15 dBm – (–28 dBm)
P
B
= 13 dB
How to Calculate Power Margin for Fiber-Opc Cables
Aer calculang a link's power budget, you can calculate the power margin (P
M
), which represents the
amount of power available aer subtracng aenuaon or link loss (LL) from the power budget (P
B
). A
worst-case esmate of P
M
assumes maximum LL:
P
M
= P
B
– LL
P
M
greater than zero indicates that the power budget is sucient to operate the receiver.
Factors that can cause link loss include higher-order mode losses, modal and chromac dispersion,
connectors, splices, and ber aenuaon. Table 28 on page 64 lists an esmated amount of loss for the
factors used in the following sample calculaons. For informaon about the actual amount of signal loss
caused by equipment and other factors, refer to vendor documentaon.
Table 28:
Esmated Values for Factors Causing Link Loss
Link-Loss Factor Esmated Link-Loss Value
Higher-order mode losses Single mode—None
Mulmode—0.5 dB
Modal and chromac dispersion Single mode—None
Mulmode—None, if product of bandwidth and distance is less than 500 MHz-
km
Faulty connector 0.5 dB
64