en the latest TDM option STM-64/OC-192. However, you must recognize that there are instances in which TDM may offer a better solution than DWDM. The recently intro- duced fiber NZDSF, for example, is flexible enough for the latest TDM equipment, but it is expensive and may limit the carriers' ability to migrate to the greater bandwidth available through DWDM at STM-16/ OC-48 rates. Chapter 5 OC-192 Figure 5-25 Example of interfaces. OC-12/12c 1310-nm Optics with 1+1 Protection OC-192 STM-4/4c 1310-nm Optics with 1+1 Protection Network STS/OC-48/48c 1310/1557-nm Optics with 1+1/ Element 1:1 Protection STM-16/16c 1310/1557-nm Optics with 1+1/1:1 Protection Many other service interfaces such as DS1/T1, DS3/T3, OC-3/3c, 10/100BaseT Ethernet, and MPEG2 digital video are also available in conjunction with subtending shelves. Tributaries Supported on OC-192 Network Elements 5.2.4 Standardization Efforts The wavelengths of the light output by laser diodes (LDs) have been chosen to minimize transmission loss across the fiber cable span and to utilize the highest sensitivity ranges of receivers. The most commonly used wave- lengths employed by laser diodes are centered on 850, 1,310, and 1,550 nm. The ITU has recently standardized a grid of wavelengths for use in DWDM systems. This grid defines a minimum channel spacing of 100 GHz in the frequency domain corresponding to approximately 0.8 nm in the wave- length domain. 13 The ITU has defined in standard G.692 a channel optical frequency grid based on 100-GHz spacing from a reference frequency of 193.1 THz = 1,552.52 nm (see Figure 5-26). Figure 5-27 shows the grid from a different graphical perspective. A channel plan describes a number of factors that need to be taken into account in an optical communications network includ- ing the number of channels, channel spacing and width, and channel center wavelengths. Although this grid defines the standard, users are free to use wavelengths in arbitrary ways and choose from any part of the spectrum. Users can also deviate from t