Cisco NM-1A-OC3MM-EP= Datasheet download pdf (Page 15)

Connecting Cisco ATM Network Modules to the Network

ATM OC-3 Network Modules

Connecting Cisco ATM Network Modules to the Network

Power Budget and Power Margin

Proper operation of an optical data link depends on modulated light reaching the receiver with enough

power to be demodulated. The power budget (PB) is the difference between transmitter power (PT) and

receiver sensitivity (PR). For instance, if transmitter power is –20 dB and receiver sensitivity is –30 dB,

the power budget is 10 dB:

PB = PT – PR

PB = –20 dB – (–30 dB)

PB = 10 dB

The SONET specification requires that the signal meet the worst-case requirements listed in Table 8.

The difference between the power budget and the link loss (LL) is called the power margin (PM). If the

power margin is zero or positive, the link should work. If it is negative, the signal may not arrive with

enough power to operate the receiver.

Link Loss

Power loss over a fiber-optic link arises from the following causes:

• Passive components—Attenuation caused by cables, cable splices, and connectors is common to

both multimode and single-mode transmission. Attenuation is significantly lower for optical fiber

than for other media.

• Chromatic dispersion—The signal spreads in time because of differing speeds of the different

wavelengths of light.

• Modal dispersion—In multimode fiber, the signal spreads in time because of the different

propagation modes.

• Higher-order mode loss (HOL)—This loss results from light radiated into the fiber cladding.

• Clock recovery at the receiver—This recovery consumes a small amount of power.

The power lost over the data link is the sum of all these losses. Table 9 gives an estimate of the amount

of loss attributable to each cause.

Table 8 SONET Signal Requirements

MM SMI SML

Transmitter power –20 dBm –15 dBm –5 dBm

Receiver sensitivity –30 dBm –31 dBm –34 dBm

Power budget 10 dBm 16 dBm 29 dBm

Table 9 Link Loss Causes and Amounts

Cause Amount of Loss

Fiber attenuation SM: 0.5 dB/km

MM: 1 dB/km

Splice 0.5 dB

Connector 0.5 dB

Modal and chromatic dispersion Depends on fiber and wavelength

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Cisco NM-1A-OC3MM-EP= Datasheet Page 15