Cisco Systems Cisco 12008 manual Multimode Power Budget Example of Dispersion Limit

Site Wiring Guidelines

Multimode Power Budget Example of Dispersion Limit

Below is a multimode power budget example based on the same parameters as in the previous example, but with a multimode link distance of 4 km:

PB = 13 dB – 4 km (1.0 dB/km) – 4 (0.5 dB) – 3 (0.5 dB) – 0.5 dB (HOL) – 1 dB (CRM)

PB = 13 dB – 4 dB – 2 dB – 1.5 dB – 0.5 dB – 1 dB

PB = 4 dB

The resulting power budget (PB) value of 4 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 MHz/km), 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 for fiber-optic transmission is an injection laser diode.

Single-mode transmission is useful for longer distances because a single transmission path within the fiber is used and smear does not occur. In addition, chromatic dispersion is reduced because laser light is essentially monochromatic.

The maximum overload limit on the single-mode receiver is –14 dBm. The single-mode receiver can be overloaded when short lengths of fiber are used because the transmitter can transmit up to –8 dB. The receiver could be overloaded at –14 dB, but no damage will result.

To prevent overloading the receiver when you are interconnecting 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 for two buildings, 11 kilometers apart, that are connected through a patch panel in an intervening building. The entire link is made up of 12 connectors.

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Length of single-mode link = 11 km

10 connectors

Preparing for Installation 2-25