source. The lower portion of Fig. 6.1 illus- trates the use of three light sources at distinct wavelengths to form three channels within an optical fiber. As we noted earlier in this chap- ter, the AT&T FT3C lightwave system coupled the output of three LEDs onto a common multimode optical fiber, representing the first commercial use of WDM. Advances in Technology Although the AT&T FT3C lightwave transmission system was a pio- neering effort, it was rapidly rendered obsolete by advances in optical technology. For example, while AT&T was constructing its FT3C sys- tem, British Telecom demonstrated the use of a single-mode fiber capa- ble of carrying signals at 10 times the rate of the AT&T system. Within a few years Sprint, AT&T, and MCI were using single-mode fiber in their backbone networks as it provided a transmission capacity much greater than the AT&T pioneering system. As the three major communications carriers wired their infra- structure with "glass" fiber, fiber was extended to link the continents. In 1988 the first submarine fiber cables were laid across the Atlantic, to be followed the next year by fiber cables across the Pacific. Within a few years optical transmission rates reached 2.5 Gbits/s, which appeared to represent a sufficient transmission rate that could pro- vide the necessary capacity to interconnect cities and continents for the foreseeable future. Similar to the play titled A Funny Thing Hap- pened on the Way to the Forum, a pair of events transpired that made the projections of pundits obsolete. First, the digital communications revolution occurred during the mid-1980s, with tens of thousands of bulletin board systems serving millions of PC users. Instead of a typ- ical 3-min voice call, a person accessing a bulletin board system com- monly maintained access for 20 or more minutes, requiring commu- nications carriers to upgrade their facilities. Just when those carriers probably felt they were ahead of the curve, along came the World Wide Web and the Mosaic browser during the mid-1990s. 