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Chapter 1
Overview: PA-A2 ATM CES Port Adapter
Use the power budget calculations in the following section to determine actual distances. If the distance
between two connected stations is greater than this maximum distance, significant signal loss can result,
making transmission unreliable.
Power Budget
To design an efficient optical data link, evaluate the power budget. The power budget is the amount of
light available to overcome attenuation in the optical link and to exceed the minimum power that the
receiver requires to operate within its specifications. Proper operation of an optical data link depends on
modulated light reaching the receiver with enough power to be correctly demodulated.
Attenuation, caused by the passive media components (cables, cable splices, and connectors), is
common to single-mode transmission.
The following variables reduce the power of the signal (light) transmitted to the receiver in single-mode
transmission:
•
•
Attenuation is significantly lower for optical fiber than for other media. For fiber-optic transmission,
chromatic and modal dispersion reduce the available power of the system by the combined dispersion
penalty (measured in decibels). The power lost over the data link is the sum of the component,
dispersion, and modal losses. However, at OC-3 speeds on single-mode fiber, dispersion and modal
losses are minimal.
The following sections offer more detail on power budgeting:
•
•
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Approximating the ATM CES Port Adapter Power Margin
The single-mode signal source is an injection laser diode. Single-mode transmission is useful for longer
distances, because there is a single transmission path within the fiber and smear does not occur. In
addition, chromatic dispersion is also reduced because laser light is essentially monochromatic.
The maximum overload specification on the single-mode receiver is –8 dBm. The single-mode receiver
can be overloaded when you are using short lengths of fiber, because the transmitter can transmit up to
–8 dB, but no damage to the receiver will result from an overload.
If you are connecting to a non-Cisco single-mode SONET receiver or transmitter, insert a 5- to 10-dB
Note
attenuator on the link to prevent overloading the receiver that connects short fiber links.
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 single-mode transmissions assumes
minimum transmitter power (PT), maximum link loss (LL), and minimum receiver sensitivity (PR). The
worst case analysis provides a margin of error; not all of the parts of an actual system will operate at the
worst case levels.
OL-3460-01
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)
Approximating the ATM CES Port Adapter Power Margin, page 1-21
Using Statistics to Estimate the Power Budget, page 1-23
For Further Reference, page 1-23
Fibre-Optic Transmission Specifications
PA-A2 ATM CES Port Adapter Installation and Configuration
1-21
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