Installation guide

Making External Connections to Cisco 4000 Series Routers 4-103
Network Connection Considerations
Single-Mode Transmission
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 –14 decibels (dB). The
single-mode receiver can be overloaded when using short lengths of fiber because the
transmitter can transmit up to –8 dB, while the receiver could be overloaded at –14 dB, but
no damage to the receiver will result. To prevent overloading the receiver connecting short
fiber links, insert a 5 to 10 dB attenuator on the link between any single-mode SONET
transmitter and the receiver.
SONET Single-Mode Power Budget Example
The following example of a single-mode power budget is of two buildings, 11 kilometers
apart, connected through a patch panel in an intervening building with a total of 12
connectors.
Length of single-mode link = 11 km
12 connectors
Estimate the power budget as follows:
PB = 11.5 dB – 11 km (0.5 dB/km) – 12 (0.5 dB)
PB = 11.5 dB – 5.5 dB – 6 dB
PB = 2.5 dB
The value of 2.5 dB indicates that this link would have sufficient power for transmission
and is not in excess of the maximum receiver input power.
Using Statistics to Estimate the Power Budget
Statistical models more accurately determine the power budget than the worst-case method.
Determining the link loss with statistical methods requires accurate knowledge of
variations in the data link components. Statistical power budget analysis is beyond the