Cisco ATM Cable Interface Processor ACIP-SM(=) Руководство по установке и настройке - Страница 6
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ACIP Description
Table 2
Limits
Attenuation
Dispersion
1. The product of bandwidth and
distance must be less than 500
MHzkm.
Approximating the ACIP 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:
The power margin calculation is derived from the power budget minus the link loss, as follows:
If the power margin is positive, as a rule, the link will work.
Table 3 lists the factors that contribute to link loss and the estimate of the link loss value attributable
to those factors.
Table 3
Link Loss Factor
Higher order mode losses
Clock recovery module
Modal and chromatic dispersion Dependent on fiber and wavelength used
Connector
Splice
Fiber attenuation
After calculating the power budget minus the data link loss, the result should be greater than zero.
Results less than zero may have insufficient power to operate the receiver.
6 ATM Cable Interface Processor (ACIP) Installation and Configuration
Typical Fiber-Optic Link Attenuation and Dispersion Limits
Multimode
1.0 dB/km
1
500 MHzkm
PB = PT – PR
PB = –18.5 dBm – (–30 dBm)
PB = 11.5 dB
PM = PB – LL
Estimating Link Loss
Estimate of Link Loss Value
0.5 dB
1 dB
0.5 dB
0.5 dB
1 dB/km