Cisco 7000 Series Hardware Installation And Maintenance Manual page 70

Site Requirements
multimode fiber. The maximum distances for single-mode and multimode transmissions, as defined
by SONET, are in Table 2-8. If the distance between two connected stations is greater than these
maximum distances, significant signal loss can result, making transmission unreliable.
Table 2-8
Transceiver
Type
Single-mode
Multimode
1. This table gives typical results. You should use power budget
calculations to determine the actual distances.
Power Budget
To design an efficient optical data link, evaluate the power budget. The power budget is the amount
of light available to overcome attenuation in the optical link and to exceed the minimum power that
the receiver requires to operate within its specifications. Proper operation of an optical data link
depends on modulated light reaching the receiver with enough power to be correctly demodulated.
Attenuation, caused by the passive media components (cables, cable splices, and connectors), is
common to both multimode and single-mode transmission.
The following variables reduce the power of the signal (light) transmitted to the receiver in
multimode transmission:
•
•
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 (in decibels [dB]). The power lost over the data link is the sum of the
component, dispersion, and modal losses. Table 2-9 lists the factors of attenuation and dispersion
limit for typical fiber-optic cable.
Table 2-9
Attenuation
Dispersion Limit
1. The product of bandwidth and distance must be less than
500 MHz/km.
Approximating the AIP 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
2-10 Cisco 7010 Hardware Installation and Maintenance
SONET Maximum Fiber-Optic Transmission Distances
Maximum Distance between
1
Stations
Up to 9 miles (14.8 kilometers)
Up to 1.5 miles (2.4 kilometers)
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)
Typical Fiber-Optic Link Attenuation and Dispersion Limits
Single-Mode
0.5 dB
No limit
Multimode
1.0 dB/km
1
500 MHz/km