Cisco 12008, Installation And Configuration Manual, Page: 124 / 382

Site Wiring Guidelines
Cisco 12008 Gigabit Switch Router Installation and Configuration Guide 2-22
Power Budget
To design an efficient optical data link, you must evaluate the power budget.
The power budget represents the amount of light that must be available to overcome
attenuation in the optical link and to exceed the minimum power required by the receiver
to operate within specifications. Proper operation of an optical data link depends on
modulated light reaching the receiver with enough power to be correctly demodulated.
The following variables reduce the power of the signal (light) transmitted to the receiver in
multimode transmission:
•
Attenuation losses—Losses incurred by passive media components, such as cables,
cable splices, and connectors. Such losses are common in both multimode and single-
mode data transmission.
•
Chromatic dispersion losses—Losses incurred by the spreading of the signal in time due
to the different speeds of light wavelengths.
•
Modal dispersion losses—Losses incurred by the spreading of the signal in time due to
the different propagation modes in the fiber.
Attenuation is significantly lower for optical fiber than for other media.
For multimode transmission, chromatic and modal dispersion reduce the available power
of the system by what is referred to as the combined dispersion penalty (in decibels [dB]).
The power lost over the data link is the sum of the attenuation losses, dispersion losses, and
modal losses.
Table 2-4 lists the attenuation and dispersion limits for typical fiber-optic cable.
Table 2-4 Typical Fiber-Optic Link Attenuation and Dispersion Limits
Factor Single-Mode Multimode
Attenuation 0.5 dB 1.0 dB/km
Dispersion limit No limit 500 MHz/km
1
1. The product of bandwidth and distance must be less than 500 MHz/km.