Differential Mode Delay - Cisco 6503 - Catalyst Firewall Security Sys Installation Manual

Appendix B Transceivers, Module Connectors, and Cable Specifications
Patch Cord Installation
Warning Invisible laser radiation may be emitted from disconnected fibers or connectors. Do not stare into
beams or view directly with optical instruments. Statement 1051
Plug the end of the patch cord labeled "To Equipment" into the GBIC. (See
labeled "To Cable Plant" into the patch panel. The patch cord is 9.8 feet (3 meters) long and has duplex
SC male connectors at each end.
Figure B-22
To equipment

Differential Mode Delay

When an unconditioned laser source designed for operation on an SMF cable is directly coupled to an
MMF cable, differential mode delay (DMD) might occur. DMD can 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 can 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, the generated light that shines in multiple directions can overfill the
existing cable space and excite a large number of modes. (See
OL-5781-08
Patch Cord Installation
Figure
To cable plant
Figure B-23.)
Catalyst 6500 Series Switches Installation Guide
Cables
B-22.) Plug the end
B-37