Preparing for Installation 2-23
Site Wiring Guidelines
Approximating the Line Card Power Margin
The LED used for a multimode transmission light source creates multiple propagation paths
of light, with each path having a different path length and time requirement to cross the
optical fiber. This causes signal dispersion (smear).
Higher order loss (HOL) results from light from the LED entering the fiber and being
radiated into the fiber cladding. A worst-case estimate of power margin (PM) for
multimode transmissions is based on assumptions of minimum transmitter power (PT),
maximum link loss (LL), and minimum receiver sensitivity (PR). The worst-case analysis
provides a margin of error, because not all parts of an actual system will operate at worst-
case levels.
The power budget (PB) is defined as the maximum possi ble amount of power transmitted.
The following equation shows the calculation of the power budget:
PB = PT – PR
PB = –18.5 dBm – (– 30 dBm)
PB = 11.5 dB
The power margin is equal to the power budget minus the link loss:
PM = PB – LL
If the power margin is positive, as a rule, the fiber-optic link will work satisfactorily.
Table2-5 lists the factors that contribute to link loss and the estimate of the link loss value
attributable to the link loss factors.
Table2-5 Estimating Link Loss
Link Loss Factor Estimate of Link Loss Value
Higher order mode losses 0 .5 dB
Clock recovery module 1 dB
Modal and chromatic dispersion Depends on fiber and wavelength used
Connector 0.5 dB
Splice 0.5 dB
Fiber attenuation 1 dB/km