Approximating The Line Card Power Margin - Cisco Gigabit Switch Router 12008 Installation And Configuration Manual

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 possible 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.
Table 2-5 lists the factors that contribute to link loss and the estimate of the link loss value
attributable to the link loss factors.
Table 2-5
Link Loss Factor
Higher order mode losses
Clock recovery module
Modal and chromatic dispersion Depends on fiber and wavelength used
Connector
Splice
Fiber attenuation
Estimating Link Loss
Estimate of Link Loss Value
0.5 dB
1 dB
0.5 dB
0.5 dB
1 dB/km
Site Wiring Guidelines
Preparing for Installation 2-23