B-15
Cisco 7609 Internet Router Installation Guide
OL-5079-04
Appendix B Connector and Cable Specifications
Cable Specifications
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
Figure B-9
.)
Figure B-9
LED Transmission Compared to Laser Transmission
LED transmission
Laser transmission
12871
LED
Laser
