the fiber plant. To accomplish this, DWDM uses smaller spacing between wavelengths, such as 0.8 nm for a 16-wavelength system and 0.4 nm for a 32-wavelength system. Although there is no clear division between the point where WDM becomes DWDM, ITU standards stipulate DWDM 0.8- nm separations, SO an increase in capacity of 16 times or more is represen- tative of a DWDM system. However, some publications consider anything in excess of four derived optical channels to represent a DWDM system. OPERATION The most common utilization of DWDM systems is to provide additional capacity to existing OC-48 systems. This is because the jump from OC-48 to OC-192 requires either some form of dispersion compensating fiber or new fiber using non-zero dispersion-shifted fiber (NZDSF), which costs approximately 50 percent more than traditional single- mode fiber. Because the vast majority of fiber plant installed during the 1980s and 1990s was single-mode fiber, the ability to operate at OC-192 or STM-64 is limited primarily to relatively new communications carriers that in the late 1990s and through 2001 installed nationwide fiber systems. The most common form of DWDM uses a fiber pair, where one opti- cal fiber is used for transmission while the other fiber functions as a receiver. Figure 6.14 illustrates the use of a 16-channel DWDM system. In examining this figure, it was assumed that amplifiers were required every 200 km and that the span distance between locations was 1000 km. CAPACITY If we assume that the original optical pair shown in Fig- ure 6.14 operated at 2.5 Gbits/s, then the use of a 16-channel DWDM sys- tem would increase the capacity of the system to 2.5 Gbits/s X 16 or 40 Gbits/s. In addition, if separate fiber pairs were installed instead of DWDM equipment, the communications carrier would require 4 X 16 Chapter Six Figure 6.14 A 16-channel DWDM system. Legend: optical amplifier Wavelength Division Multiplexer or 64 amplifiers instead of 4. If the carrier simply bit the bullet and installed all new fiber, such as 