(As seen in "Gigabit Ethernet Fiber Restrictions," page 90, for a summary of the most up-to-date distance limitations and fiber requirements.)

The DMD problem was significant enough to spur a change in the Gigabit Ethernet standard. The change, implemented in March, was the last outstanding issue for that standard, which was ratified on June 25 as IEEE 802.3z. The changes solved the DMD problem by limiting the distance allowances for 1000BASE-SX and -LX over multimode fiber. This affects all users migrating from FDDI to Gigabit Ethernet. Specifically, the maximum recommended distance over typical FDDI-style fiber was reduced to 220 meters from 260 meters. Fortunately, other (though more expensive) options are available using 1000BASE-LX technology, which allows for longer distances over existing fibers. To overcome the DMD effects associated with 1000BASE-LX, a special "lens," called a launch cable, disperses laser light across the entire fiber, eliminating the possibility of DMD.

It's important to recognize that problems associated with DMD have been resolved. For 1000BASE-SX, this meant shortening the maximum distance and "tweaking" the standard to allow for slightly more jitter in the signal. For 1000BASE-LX, it was specified that a launch cable or other means of optical conditioning be used to disperse the laser light into the fiber. The only reason that DMD is still an issue is that it is possible to exceed the recommended distances for Gigabit Ethernet if your fiber plant can support it.

You may have installed your fiber plant to the TIA 568 building wiring standard, which specifies fiber with a modal bandwidth of 160/500 MHz-km for multimode fiber. If your records indicate what type of fiber was installed, you can easily verify the modal bandwidth by calling the manufacturer. The MHz-km rating indicates the bandwidth of your fiber and is proportional to the frequency of the technology you run over it and the distance of the fiber. Thus, a higher-frequency technology such as Gigabit Ethernet has a much shorter maximum distance than traditional Ethernet or Fast Ethernet. While it may be possible to measure modal bandwidth with high-end optical time-domain reflectometry, a device to do so costs more than $40,000, and trial and error will yield essentially the same results.

If all of these issues feel overwhelming, or if Gigabit Ethernet just seems too unproven for your enterprise, the next best thing is switched Fast Ethernet, which lets you build backbones of up to 400 Mbps (800 Mbps full-duplex) using the same trunking technologies available in Gigabit Ethernet. Cisco Systems, Extreme, Foundry, Lucent Technologies and 3Com all have load-balancing algorithms that enable multiple 100-Mbps connections to be trunked together into a single fat pipe. These solutions are proprietary, though work is under way in the IEEE to develop a standard. If you go this route, you can reap the same Layer 3 benefits available in the Gigabit Ethernet space.