Installation guide

4-100 Cisco 4000 Series Installation Guide

Network Connection Considerations

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

the combined dispersion penalty, measured in decibels (dB). The power lost over the data

link is the sum of the component, dispersion, and modal losses.

Table 4-11 lists the factors of attenuation and dispersion limit for typical ﬁber-optic cable.

Table 4-11 Typical Fiber-Optic Link Attenuation and Dispersion Limits

Approximating the Power Margin

The LED used for a multimode transmission light source creates multiple propagation

paths of light, each with a different path length and time requirement to cross the optical

ﬁber, causing signal dispersion (smear). Higher order mode loss results from light from the

LED entering the ﬁber and being radiated into the ﬁber cladding. A worst-case estimate of

power margin (M) for multimode transmissions assumes minimum transmitter power (T),

maximum link loss (LL), and minimum receiver sensitivity (PR). The worst-case analysis

provides a margin of error, although not all of the parts of an actual system will operate at

the worst-case levels.

The power budget (PB) is the maximum possible amount of power transmitted. The

following equation lists the calculation of the power budget:

PB = T – PR

PB = –18.5 decibels per milliwatt (dBm) – (–30 dBm )

PB = 11.5 dB

1. The product of bandwidth and distance must be less than

500 MHz/km.

Limits Single-Mode Multimode

Attenuation 0.5 dB 1.0 dB/km

Dispersion No limit 500

MHz/km