Juniper Networks M5 - Manual (Page 70)

Network Cable Specifications and Guidelines
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 (PB), you assume minimum transmitter
power (PT) and minimum receiver sensitivity (PR):
PB = PT – PR
The following hypothetical power budget equation uses values measured in decibels (dB) and
decibels referred to one milliwatt (dBm):
PB = PT – PR
PB = –15 dBm – (–28 dBm)
PB = 13 dB
Calculating Power Margin for Fiber-Optic Cable
After calculating a link’s power budget (using the equation described in “Calculating Power
Budget for Fiber-Optic Cable” on page 49), you can calculate the power margin (PM), which
represents the amount of power available after subtracting attenuation or link loss (LL) from
the power budget (PB). A worst-case estimate of PM assumes maximum LL:
PM = PB – LL
A PM 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 12 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 12: Estimated Values for Factors Causing Link Loss
Link-Loss Factor Estimated Link-Loss Value
Higher-order mode losses Single-mode—None
Multimode—0.5 dB
Modal and chromatic dispersion Single-mode—None
Multimode—None, if product of bandwidth and
distance is less than 500 MHz–km
Connector 0.5 dB
Splice 0.5 dB
Fiber attenuation Single-mode—0.5 dB/km
Multimode—1 dB/km
50 M5 and M10 Internet Routers Hardware Guide