of minimum dispersion is shifted to 1.55 um. At this wavelength, the attenuation can be significantly lower than that at 1.31 um. Single-mode fibers can also be designated as disper- sion-flattened fibers. Dispersion-flattened single-mode fibers are attractive. The dispersion is low in both the 1.31-um window and 1.55-um window. 2.1.6 Multimode Fibers Multimode fibers support dozens of propagating modes, each of which trav- els at a slightly different velocity. These fibers tend to be used in GbE and 10-GbE LAN applications, although they also can utilize single-mode fiber. The core diameter of the multimode fiber normally used is 62.5 um, but other special designs of multimode fibers exist. Multimode fiber can be step- index or graded-index fiber, although most of the fiber deployed is step- index fiber. As we've seen, graded-index multimode fiber reduces pulse distortion by nearly equalizing the velocities of modes propagating in the fiber. With graded-index fiber, the index of refraction decreases almost par- abolically from the center of the core to the boundary between the core and cladding material. However, the equalization of mode velocities is not per- Optics Primer fect, and the information transmission capacity of multimode fibers is sub- stantially less than that of single-mode fibers. Roughly speaking, 850- and 1,310-nm multimode systems can operate at distances from a few hundred meters to a couple of kilometers; see Table 2-26 for examples. A 1,310-nm Fabry-Perot (FP) laser single-mode system can range from 10 to 40 km; Distributed Feedback (DFB) lasers can reach up to 100 km with good fiber. If the distance spanned exceeds these limits, the light must be amplified directly (with an erbium-doped fiber amplifier [EDFA] or converted to electricity, regenerated, amplified, and converted back to light. 2.1.7 Optical Fiber Specs Fiber cable has traditionally been codified using a three-code field as fol- lows: MM2 SS2S3S4S5S6 NNN MM2 is a manufacturer code; NNN is a code indicating the number of working