ad 1,300 nm. This optical fiber is called a dispersion-shifted fiber (DSF). This fiber may give significantly longer unrepeated distances at higher infor- mation transmission rates than the unshifted fiber when conventional LDs or LEDs are used. Segmented-core fiber is a fiber that is designed to oper- ate as a low-loss single-mode fiber at 1,300 and 1,550 nm while maintain- ing a zero-dispersion wavelength in the 1,550-nm window. In addition, this fiber performs as a high-bandwidth few-mode fiber near 800 nm where inexpensive light sources are available. Segmented-core fiber enables mode equalization over a large range of wavelengths. This fiber can be used in high-performance long-haul or in subscriber-loop applications at longer wavelengths. Figure 2-16 contains taxonomy of the key fiber types cur- rently available. The six major generations of fiber-optic systems spanning the past 20 years are as follows: First Multimode (step index)-850 nm Second Multimode (graded index)-1,300 nm Third Single mode (pre-SONET/SDH)-1,300 nm Fourth Single mode (dispersion shifted) (SONET)-1,550 nm Fifth WDM systems; also OC-192 (SONET)-1,550 nm Sixth (latest) Dense WDM (DWDM) systems; also OC-768 (SONET) -1,550 nm A number of fiber types and the behaviors of rays as they travel down the fiber are identified in Figure 2-17. Research is continuing on nonsilica glass, which theoretically offers much less loss. Plastic fibers are also available for intrabuilding use in applications such as LANs. The attenuation of this fiber, however, is very high; hence, the majority of commercially available Gigabit Ethernet (GbE) and 10-GbE LANs use multimode silica fiber rather than plastic. In conclusion, chromatic dispersion can be managed. At higher speeds, the issue becomes more pressing (particularly at 40 Gbps on NDSF; oper- ation at OC-768 is 16 times more sensitive to the problem than operation at OC-192). Managing chromatic dispersion imposes a cost and a loss premium. Optics Primer Single mode (DWDM) Figure 2-16 Fiber taxonomy. NZDSF 