Cisco CX-AIP-TM Installation And Configuration Manual page 11

Multimode Power Margin 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:
PM = 11.5 dB – 4 km (1.0 dB/km) – 4 (0.5 dB) – 3 (0.5 dB) – 0.5 dB (HOL) – 1 dB (CRM)
PM = 11.5 dB – 4 dB – 2 dB – 1.5 dB – 0.5 dB – 1 dB
PM = 1.5 dB
The value of 1.5 dB indicates that this link would have sufficient power for transmission; however,
because of 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 reduced because laser light is essentially monochromatic.
The maximum overload specification on the single-mode receiver is –14 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 describes two buildings, 11 kilometers apart
(with a loss of .05 dB/km), connected through a patch panel in an intervening building with a total
of 12 connectors (each with a loss of 0.5 dB).
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 scope of this document. For
further information, refer to UNI Forum specifications, ITU-T standards, and your equipment
specifications.
Additional Power Budget and Attenuation References
The following publications contain information on determining attenuation and power budget:
•
T1E1.2/92-020R2 ANSI, the Draft American National Standard for Telecommunications
entitled "Broadband ISDN Customer Installation Interfaces: Physical Layer Specification"
•
Power Margin Analysis, AT&T Technical Note, TN89-004LWP, May 1989
Asynchronous Transfer Mode Interface Processor (AIP) Installation and Configuration
AIP Installation Prerequisites
11