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  LOSS BUDGETS “NOT TO EXCEED” LOSS BUDGETS Written by: Larry Johnson In our last issue, I wrote about what a loss budget is, what it consists of and how designers use it to assure that the system to be installed will operate correctly at the data rate and distance being installed. In this article, the focus is on how installers can create their own “Not to Exceed” loss budget which addresses both component and span attenuation requirements which contractors and installers are responsible for achieving. Installers are only responsible for the spans they are installing. Therefore, the attenuation of the cables, connectors, and splices will be our primary focus. What is the difference between multimode vs single-mode loss budgets? Outside plant (OSP) single-mode loss budgets have tighter attenuation specifications for splices, connectors and fibers. Multimode loss budgets typically have more connections with higher attenuation values and much shorter spans. Below are typical values for premises (Enterprise) networks. Notice that the OSP single-mode fibers using the ITU designations have lower attenuation values. Once the installation is completed, OTDR and optical loss testing occurs to verify that the component and span attenuation values are less than specified by the designer. Testing is always performed at two wavelengths as longer wavelengths can easily identify if macro and micro bending losses are present. In this example, a single-mode fiber span of 10 kilometer (6.2 miles) of ITU-T G.652 conventional single-mode fiber will be installed with splices placed at periodic points and with each of the fibers connectorized at each end. At 1310 nanometers, the fiber has an attenuation value of 0.4 dB per kilometer, giving us 4.0 dB in fiber loss. And .3 dB/km at 1550 nm, the span will have patch panels at each end for a total of two connections with each having an ITU G.671 standard specified value as 0.5 dB each for a total of 1 dB. In addition, in the span there are ten splices. The TIA-758 Customer-owned Outside Plant standard specifies maximum splice loss as 0.1 dB per splice. Of these splices, eight are in-line and two are for terminating the single-mode pigtails on the end of each fiber. The sum of the fiber (4.0 dB), connections (1 dB) and splices (1 dB) now provide a “not to exceed” value of 6.0 dB at 1310nm. And 5.0 at 1550 nm. In the case of Fiber to the Home (FTTH) Passive Optical Networks (PONs), which use optical splitters, an additional line is required. In the below chart, a 1:32 splitter is used and instead of using the 15 dB (theoretical split value) we are using the average 15.8 dB value as specified in the G.671 passive components standard value. Not to exceed loss budgets provide a valuable tool for installers with individual component values specified along with span values. These help quickly to identify any fiber(s) out of specification. Larry Johnson was founder of The Light Brigade and is President of FiberStory and specializes in fiber optic training.      4 | highTECH NEWS | NOVEMBER/DECEMBER 2022  

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