Differential Mode Delay - Cisco Catalyst 6500 Series Installation Manual

Chapter 2 Preparing for Installation
Determining Cable Distances
Figure 2-2 Patch Cord Installation
To equipment To cable plant

Differential Mode Delay

When an unconditioned laser source designed for operation on a single-mode
fiber (SMF) cable is directly coupled to a multimode fiber (MMF) cable,
differential mode delay (DMD) might occur. DMD can degrade the modal
bandwidth of the fiber-optic cable causing 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. 802.3z identifies 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.
Catalyst 6500 Series Switch Module Installation Guide
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