Juniper Networks M160 manual Calculating Power Margin for Fiber-Optic Cable

Preparing for Router Installation

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 72), 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 18 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 18: 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

The following example uses the estimated values in Table 18 to calculate link loss (LL) for a 2 km-long multimode link with a power budget (PB) of 13 dB:

Fiber attenuation for 2 km @ 1.0 dB/km= 2 dB

Loss for five connectors @ 0.5 dB per connector = 5(0.5 dB) = 2.5 dB

Loss for two splices @ 0.5 dB per splice =2(0.5 dB) = 1 dB

Higher-order loss = 0.5 dB

Clock recovery module = 1 dB

The power margin (PM) is calculated as follows:

Network Cable Specifications and Guidelines 73