Juniper Networks M10i manual Calculating Power Budget for Fiber-Optic Cable

Appendix D: Cable Specifications

transmit enough power to saturate the receivers on PICs that have a shorter reach. Specifically, LR transceivers can saturate IR PICs, and both IR and LR transceivers can saturate SR PICs. Transceivers in the same reach class can also saturate one another.

To prevent saturation, you might need to attenuate power at the PIC receiver, particularly if you know that it has a shorter reach than the transceiver that is sending the signal. Determine the amount of attenuation needed by measuring the power level at each receiver. Attenuate the power to bring it within the allowable range; for short lengths of fiber, with fiber and connector loss close to zero, an attenuator of 5 to 10 dB should be sufficient.

For specifications of minimum and maximum input level (receiver sensitivity and receiver saturation) and minimum and maximum output level (average launch power) for the SONET/SDH PICs supported on the M10i router, see the M10i Internet Router PIC Guide.

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 (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 185), 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.

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