Approximating The Line Card Power Margin - Cisco GSR12/60 - 12012 Router -AnyLAN, SONET/SDH Installation And Configuration Manual

Site Requirement Guidelines
Table 2-6
Attenuation
Dispersion Limit
1. The product of bandwidth and distance must be less than
500 MHzkm.

Approximating the Line Card 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, 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 = PT – PR
PB = –18.5 dBm –(– 30 dBm)
PB = 11.5 dB
The power margin calculation is derived from the power budget and subtracts the link loss,
as follows:
PM = PB – LL
If the power margin is positive, as a rule, the link will work.
Table 2-7 lists the factors that contribute to link loss and the estimate of the link loss value
attributable to those factors.
2-18 Cisco 12012 Gigabit Switch Router Installation and Configuration Guide
Typical Fiber-Optic Link Attenuation and Dispersion Limits
Single-Mode Multimode
0.5 dB 1.0 dB/km
No limit 500 MHz/km
1