Differential Mode Delay - Cisco Catalyst 6807-XL Hardware Installation Manual

Mode-Conditioning Patch Cord
Procedure
Step 1 Plug the end of the patch cord labeled To Equipment into the GBIC. See
on page
Step 2
Plug the end labeled To Cable Plant into the patch panel. See
The patch cord is 9.8-feet (3-meters) long and has duplex SC male connectors at each end.
Figure 45: Patch Cord Installation

Differential Mode Delay

When an unconditioned laser source designed for operation on an SMF cable is directly coupled with an MMF
cable, differential mode delay (DMD) might occur. DMD may degrade the modal bandwidth of the fiber-optic
cable. This degradation causes a decrease in the link span (the distance between the transmitter and the receiver)
that can be reliably supported.
The Gigabit Ethernet specification (IEEE 802.3z) outlines parameters for Ethernet communications at a
gigabit-per-second rate. The specification offers a higher-speed version of Ethernet for backbone and server
connectivity using existing deployed MMF cable by defining the use of laser-based optical components to
propagate data over MMF cable.
Lasers function at the baud rates and longer distances required for Gigabit Ethernet. The 802.3z Gigabit
Ethernet Task Force has identified the DMD condition that occurs with particular combinations of lasers and
MMF cable. The results create an additional element of jitter that can limit the reach of Gigabit Ethernet over
MMF cable.
With DMD, a single laser light pulse excites a few modes equally within an MMF cable. These modes, or
light pathways, then follow two or more different paths. These paths might have different lengths and
transmission delays as the light travels through the cable. With DMD, a distinct pulse propagating down the
cable no longer remains a distinct pulse, or in extreme cases, might become two independent pulses. Strings
of pulses may interfere with each other making it difficult to recover data.
DMD does not occur in all deployed fibers; it occurs with certain combinations of worst-case fibers and
worst-case transceivers. Gigabit Ethernet experiences this problem because of its very high baud rate and its
long MMF cable lengths. SMF cable and copper cable are not affected by DMD.
MMF cable has been tested for use only with LED sources. LEDs can create an overfilled launch condition
within the fiber-optic cable. The overfilled launch condition describes the way LED transmitters couple light
into the fiber-optic cable in a broad spread of modes. Similar to a light bulb radiating light into a dark room,
Cisco Catalyst 6807-XL Switch Hardware Installation Guide
96
96.
Transceivers, Module Connectors, and Cable Specifications
Figure 45: Patch Cord Installation,
Figure 45: Patch Cord Installation, on page 96.
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