Cisco 7000 Hardware Installation And Maintenance page 83

Multimode Power Budget Example of Dispersion Limit
Following is an example with the same parameters as the previous example, but with a multimode
link distance of 4 km:
PB = 11.5 dB – 4 km (1.0 dB/km) – 4 (0.5 dB) – 3 (0.5 dB) – 0.5 dB (HOL) – 1 dB (CRM)
PB = 11.5 dB – 4 dB – 2 dB – 1.5 dB – 0.5 dB – 1 dB
PB = 1.5 dB
The value of 1.5 dB indicates that this link would have sufficient power for transmission. However,
due to the dispersion limit on the link (4 km x 155.52 MHz > 500 MHzkm), this link would not work
with multimode fiber. In this case, single-mode fiber would be the better choice.
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 dBm. 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 a 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.
Statistical Models for Estimating 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 scope of this document. For
further information, refer to UNI Forum specifications, ITU-T standards, and your equipment
specifications.
Site Requirements
Preparing for Installation 2-79