discrete ones and zeros. Light energy, like other forms of energy, attenuates as it moves over a distance, SO it has to run though an amplification or repeating process. As mentioned earlier, until about 1994, electronic repeaters were used with fiber, so the optical signal would have to stop; be converted into electri- Laser Diode Pulses of Light Strand of Fiber-Optic Glass Avalanche Photo Detector Figure 2.9 Fiber-optic transmission Fiber Optics cal energy; be resynchronized, retimed, and regenerated; and then be converted back into optical energy to be passed to the next point in the network. This was a major problem because it limited the data rate to 2.5Gbps. But some developments introduced in the early 1990s dramatically changed long-distance communications over fiber. The next section talks about those innovations. Innovations in Fiber Optics: EDFAs and WDM As mentioned in Chapter 1, erbium-doped fiber amplifiers (EDFAs) are optical repeaters made of fiber doped with erbium metal at periodic intervals (normally every 30 to 60 miles [50 to 100 km]). The introduction of EDFAs made it possible for fiber-optic systems to operate at 10Gbps. EDFAs also opened the way for Wave- length Division Multiplexing (WDM), the process of dividing the optical transmis- sion spectrum into a number of nonoverlapping wavelengths, with each wavelength supporting a single high-speed communications channel. Today, undersea cables need to be designed with WDM in mind; until 1998 or so, most were not, which means they have inappropriate repeater spacing. So, again, for the next generation of fiber communications over undersea cables, many systems will have to be replaced or upgraded. Since the development of WDM, several categories have been introduced, including Dense Wavelength Division Multiplexing (DWDM) and Coarse Wave- length Division Multiplexing (CWDM). There have also been developments in new generations of fiber and amplifiers; all these are discussed in Chapter 11. Fiber All Around the World Some years ago there was a f