Cisco 7603 - Modular Expansion Base Installation Manual page 73

Chapter 2 Preparing for Installation
Determining Cable Distances
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 Figure 2-3.)
Figure 2-3 LED Transmission Compared to Laser Transmission
LED transmission
LED
Laser transmission
Laser
Lasers launch light in a more concentrated fashion. A laser transmitter couples light into only a fraction
of the existing modes or optical pathways present in the fiber-optic cable. (See Figure 2-3.)
The solution is to condition the laser light launched from the source (transmitter) so it spreads the light
evenly across the diameter of the fiber-optic cable making the launch look more like an LED source to
the cable. The objective is to scramble the modes of light to distribute the power more equally in all
modes and prevent the light from being concentrated in just a few modes.
An unconditioned launch, in the worst case, might concentrate all of its light in the center of the
fiber-optic cable, exciting only two or more modes equally.
Book Title
2-15
OL-5077-7