Cisco CISCO2650 - 2650 Router Hardware Installation Manual page 192

ATM OC-3 Network Modules
Table 11-10 Link Loss Causes and Amounts
Cause
Fiber attenuation
Splice
Connector
Modal and chromatic dispersion Depends on fiber and wavelength
Higher-order mode losses
Clock recovery
1. Dispersion is usually negligible for single-mode fiber. For multimode fiber, the product of bandwidth and distance should be
Estimating the Power Margin
The following example calculates a multimode power margin based on these values:
•
•
•
•
•
•
The power margin is:
The positive result means this link should have enough power for transmission. The product of
bandwidth and distance is 155 MHz x 3 km = 465 MHz-km; this is within the dispersion limit of
500 MHz-km.
Single-Mode Transmission
Single-mode transmission is useful for longer distances, because there is a single transmission path
within the fiber and modal dispersion does not occur.
The maximum receive power for SML is –10 dBm, and the maximum transmit power is 0 dBm. The
SML receiver can be overloaded when using short lengths of fiber. Overloading the receiver does not
damage it, but can cause unreliable operation. To prevent overloading an SML receiver, insert a
minimum 10-dB attenuator on the link between any SML transmitter and the receiver.
The SMI receiver cannot be overloaded by the SMI transmitter and does not require a minimum fiber
cable length or loss.
Cisco Network Modules Hardware Installation Guide
11-16
less than 500 MHz-km.
Power budget 10 dB (SONET worst-case specification for multimode fiber)
Link length 3 km
Four connectors
Three splices
Higher-order loss (HOL)
Clock recovery
PM = PB – LL
= 10 dB – [3 km x (1.0 dB/km) + 4 x (0.5 dB) + 3 x (0.5 dB) + 0.5 dB + 1 dB] = 2 dB
Amount of Loss
SM: 0.5 dB/km
MM: 1 dB/km
0.5 dB
0.5 dB
0.5 dB
1 dB
Chapter 11 Connecting ATM Network Modules
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