Specifications

C-23
Cisco 7100 Series VPN Router Installation and Configuration Guide
78-6341-03
Appendix C Cable Specifications
Cisco 7120-AT3, Cisco 7140-2AT3, Cisco 7120-AE3, Cisco 7140-2AE3, Cisco 7120-SMI3, and Cisco 7140-2MM3 Cables
• Chromatic dispersion—Spreading of the signal in time because of the different speeds of light
wavelengths
• Modal dispersion—Spreading of the signal in time because of 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 the combined dispersion
penalty (dB). The power lost over the data link is the sum of the component, dispersion, and modal
losses.
Table C-21 lists the factors of attenuation and dispersion for typical fiber-optic cable.
Approximating the MM3 and SMI3 Port 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 (HOL) results from light from the LED entering the fiber
and being radiated into the fiber cladding. A worst-case estimate of power margin (PM) for multimode
transmissions assumes minimum transmitter power (PT), maximum link loss (LL), and minimum
receiver sensitivity (PR). The worst-case analysis provides a margin of error; 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 = PT – PR
PB = –20 dBm – (–30 dBm)
PB = 10 dB
The power margin calculation is derived from the power budget minus the link loss, as follows:
PM = PB – LL
If the power margin is positive, the link will work.
Table C-22 lists the factors that contribute to link loss and the estimate of the link loss value attributable
to those factors.
Table C-21 Typical Fiber-Optic Link Attenuation and Dispersion Limits
Limits Single-mode Multimode
Attenuation 0.5 dB/km 1.0 dB/km
Dispersion No limit 500 MHz/km
1
1. The product of bandwidth and distance must be less than 500 MHz/km.
Table C-22 Link Loss Factors and Values
Link Loss Factor Estimate of Link Loss Value
Higher-order mode losses 0.5 dB
Clock recovery module 1 dB
Modal and chromatic dispersion Dependent on fiber and wavelength used
Connector 0.5 dB