Juniper Networks MX240 manual Calculating Power Budget for Fiber-Optic Cable

Appendix E: Cable and Wire Guidelines and Specifications

Calculating Power Budget for Fiber-Optic Cable

To ensure that fiber-optic connections have sufficient power for correct operation, 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 (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 181), 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 38 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 38: 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

Network Cable Specifications and Guidelines ■ 181