or trans- mission of light pulses at 10 Gbps, or an even higher data rate at 100-GHz spacing, and, with the help of DWDM technology, a pair of fibers can pro- vide data capacity of several hundreds gigabits per second. Most DWDM systems support standard SONET/SDH optical inter- faces. Often short-haul STM-16 (2.4 Gbps) at the 1310-nm wavelength is used as an input signal for DWDM systems but also other interfaces, such as OC-192 for 10-Gb Ethernet, can be supported. The basic structure of a DWDM system is shown in Figure 4.34. Only one transmission direction is shown in the figure. Transponders in Figure 4.34 convert incoming optical signals into ITU-standard wavelengths. Each transponder is designed to sup- port a certain interface, for example, STM-16, and it carries out optical-to- electrical conversion, signal regeneration, and electrical-to-optical conversion and transmits signals to the optical multiplexer at one wavelength specified by ITU. DWDM technology has improved, and will continue to further improve, utilization of fiber bandwidth close to the huge capacity of optical fibers that will be achieved in the future by coherent radio-like optical technology. Optical Optical standard DWDM terminal DWDM terminal standard transmitter Optional interfaces, receiver interfaces, postamplifier such as Transponder such as STM-16 Optional interfaces STM-16 preamplifier Demux at 1,310 nm at 1,310 nm AAA. An Optional line amplifier Figure 4.34 DWDM system (one transmission direction only). Transmission 4.8.8 Optical Amplifiers The section length of a long-haul optical system from optical transmitter to receiver is limited to some tens of kilometers depending on the transmission data rate although attenuation of a fiber is quite low. In the case of longer systems, regeneration or amplification is required. Regeneration of optical DWDM signals is very expensive because it requires an optical demultiplexer and demultiplexer, regeneration of each signal in electrical form, and optical receivers and transmitters for each 