 to differ from being step index and SO alter the waveguide dispersion in equation 6.28 to more negative values. As a result, the total chromatic dispersion, D, can be adjusted to the point where 20 is about 1550 nm, resulting in what are known as Dispersion-Shifted Fibers (DSF) (ITU-T G.653) Unfortunately, as will be explained in later sections (Cross-Phase Modulation and Four- Wave Mixing), operation in the region of zero dispersion causes increased nonlinear cross- talk and SO the strategy is not favored by most network operators. However, we can compromise and design fibers for dispersions in the C-band of about 3 to 8 ps/(km.nm); such types, called "nonzero-dispersion-shifted fibers" (NZ-DSF) (ITU-T G.655), have lower dispersion without incurring unacceptable nonlinear crosstalk. A third dispersion accommodation technique, which can be regarded as a type of modulation format, is to provide careful control of transmitted chirp from what is called a "Chirp-Managed Laser" (CML) (Chandrasekhar et al., 2006). The earlier section Single- Channel Transmission, explained that positively chirped pulses from a directly modulated laser experience considerable broadening in positive dispersion fiber. However, when nega- tively chirped pulses are launched, they initially narrow and then broaden again. With careful optimization of both phase and amplitude of the emitted pulses, we can operate over longer distances before pulse broadening becomes unacceptable. By using a combination of alternative modulation formats (which includes CMLs) and lower-dispersion fibers, system spans can be improved by up to one order of magnitude. Thus, with reference to Figure 6.19, the maximum transmission spans for 10- and 40-Gbit/s operation could be around 500 km and 50 km, respectively. While there are applications for such performance, it does not serve the needs of many national and continental trunk routes. Therefore, additional measures are required to overcome dispersive pulse spreading. Optical Dispersion Compensation: Negative-Dis