Juniper QFX5220, Руководство по оборудованию, Страница: 93 / 247

Calculating Power Budget for Fiber-Optic Cable
To ensure that fiber-optic connections have sufficient power for correct operation, you need to calculate
the link's power budget, which is the maximum amount of power it can transmit. When you calculate the
power budget, you use a worst-case analysis to provide a margin of error, even though all the parts of an
actual system do not operate at the worst-case levels. To calculate the worst-case estimate of power
budget (P
B
), you assume minimum transmitter power (P
T
) and minimum receiver sensitivity (P
R
):
P
B
= P
T
– P
R
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 33 on page 96 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 33: 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
96