2. Calculate the power budget (PB ) by subtracting (PR ) from (PT ):–15 dBm – (–28 dBm) = 13 dBmRelatedDocumentationCalculating the J-EX8200 Switch Fiber-Optic Cable Power Margin on page 85•• Optical Interface Support in J-EX8200 Switches on page 45• Understanding J-EX8200 Switch Fiber-Optic Cable Signal Loss, Attenuation, andDispersion on page 72Calculating the J-EX8200 Switch Fiber-Optic Cable Power MarginCalculate the link's power margin when planning fiber-optic cable layout and distancesto ensure that fiber-optic connections have sufficient signal power to overcome systemlosses and still satisfy the minimum input requirements of the receiver for the requiredperformance level. The power margin (PM ) is the amount of power available afterattenuation or link loss (LL) has been subtracted from the power budget (PB ).When you calculate the power margin, you use a worst-case analysis to provide a marginof error, even though all the parts of an actual system do not operate at worst-caselevels. A power margin (PM ) greater than zero indicates that the power budget is sufficientto operate the receiver and that it does not exceed the maximum receiver input power.This means the link will work. A (PM ) that is zero or negative indicates insufficient powerto operate the receiver. See the specification for your receiver to find the maximumreceiver input power.Before you begin to calculate the power margin:• Calculate the power budget. See “Calculating the J-EX8200 Switch Fiber-Optic CablePower Budget” on page 84.To calculate the worst-case estimate for the power margin (PM ) for the link:1. Determine the maximum value for link loss (LL) by adding estimated values forapplicable link-loss factors—for example, use the sample values for various factorsas provided in Table 39 on page 85 (here, the link is 2 km long and multimode, andthe (PB ) is 13 dBm):Table 39: Estimated Values for Factors Causing Link LossSample (LL) Calculation ValuesEstimated Link-Loss ValueLink-Loss Factor• 0.5 dBm• 0 dBm• Multimode—0.5 dBm• Single mode—NoneHigher-order mode losses (HOL)• 0 dBm• 0 dBm• Multimode—None, if product ofbandwidth and distance is less than500 MHz/km• Single mode—NoneModal and chromatic dispersion85Chapter 7: Planning Power Requirements
