Cisco 7000 Hardware Installation And Maintenance page 81

The following variables reduce the power of the signal (light) transmitted to the receiver in
multimode transmission:
•
Chromatic dispersion (spreading of the signal in time because of the different speeds of light
wavelengths)
•
Modal dispersion (spreading of the signal in time because of the different propagation modes in
the fiber)
Attenuation is significantly lower for optical fiber than for other media. For multimode transmission,
chromatic and modal dispersion reduce the available power of the system by the combined
dispersion penalty (in decibels [dB]). The power lost over the data link is the sum of the component,
dispersion, and modal losses. Table 2-9 lists the factors of attenuation and dispersion limit for typical
fiber-optic cable.
Table 2-9 Typical Fiber-Optic Link Attenuation and Dispersion Limits
Single-Mode
Attenuation 0.5 dB
Dispersion Limit No limit
1. The product of bandwidth and distance must be less than
500 MHzkm.
Approximating the AIP Power Margin
The LED used for a multimode transmission light source creates multiple propagation paths of light,
each with a different path length and time requirement to cross the optical fiber, causing signal
dispersion (smear). Higher order mode 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 assumes minimum transmitter power (PT), maximum link loss (LL), and
minimum receiver sensitivity (PR). The worst case analysis provides a margin of error, although not
all of the parts of an actual system will operate at the worst case levels.
The power budget (PB) is the maximum possible amount of power transmitted. The following
equation lists the calculation of the power budget:
PB = PT – PR
PB = –18.5 dBm – (–30 dBm)
PB = 11.5 dB
The power margin calculation is derived from the power budget and subtracts the link loss, as
follows:
PM = PB – LL
If the power margin is positive, as a rule, the link will work.
Multimode
1.0 dB/km
1
500 MHzkm
Site Requirements
Preparing for Installation 2-77