C-22
Cisco 7100 Series VPN Router Installation and Configuration Guide
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Appendix C Cable Specifications
Cisco 7120-AT3, Cisco 7140-2AT3, Cisco 7120-AE3, Cisco 7140-2AE3, Cisco 7120-SMI3, and Cisco 7140-2MM3 Cables
Note A single fiber link should not mix 62.5- and 50-micron cable.
Fiber-Optic Transmission Specifications
The following sections describe the SONET specifications for fiber-optic transmissions, define the
power budget, and help you approximate the power margin for multimode and single-mode
transmissions.
For more information on determining attenuation and power budget, see the following publications:
• T1E1.2/92-020R2 ANSI, the Draft American National Standard for Telecommunications entitled
Broadband ISDN Customer Installation Interfaces: Physical Layer Specification.
• Power Margin Analysis, AT&T Technical Note, TN89-004LWP, May 1989
SONET Distance Limitations
The SONET specification for fiber-optic transmission defines two types of fiber: single mode and
multimode. Modes can be thought of as bundles of light rays entering the fiber at a particular angle.
Single-mode fiber allows only one mode of light to propagate through the fiber, whereas multimode fiber
allows multiple modes of light to propagate through the fiber. Because multiple modes of light
propagating through the fiber travel different distances depending on the entry angles, causing them to
arrive at the destination at different times (a phenomenon called modal dispersion), single-mode fiber is
capable of higher bandwidth and greater cable run distances than multimode fiber.
The typical maximum distances for single-mode and multimode transmissions, as defined by SONET,
are in Table C-20. If the distance between two connected stations is greater than this maximum distance,
significant signal loss can result, making transmission unreliable.
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:
Table C-20 SONET Maximum Fiber-Optic Transmission Distances
Transceiver Type Maximum Distance between Stations
1
1. Table C-20 gives typical results. Use the power budget calculations described in the following sections to determine the
actual distances.
Single-mode long reach (SML) Up to 24.8 miles (40 kilometers)
Single-mode intermediate reach (SMI) Up to 9.3 miles (15 kilometers)
Multimode (MM) Up to 1.2 miles (2 kilometers)
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