the first of the three major communications carriers to completely convert their intercity commu- nications from microwave to fiber, quickly followed by other long- distance carriers. Fiber in the WAN Although the initial long-haul fiber systems were based on the use of multimode fiber and LEDs, the demand for higher operating rates resulted in a trend toward the use of single-mode fiber and lasers. As previously noted in this book, LEDs have a maximum data transmission capability near 300 Mbits/s, which is significantly below the capability of lasers. The use of single-mode fiber and lasers resulted in a significant increase in the transmission capacity of optical systems. Today data transmission rates of 10 Gbits/s, are commonly employed in the back- bone infrastructure of many long-distance communications carriers, and 40-Gbit/s systems are being used in field trials and may be in com- mercial service by the time you read this book. Rationale for Use The growth in the use of fiber for intercity communications can be summed up in three words: "capacity" and "error rate." CAPACITY Concerning capacity, as noted earlier, the AT&T FT3C lightwave system supported 240,000 simultaneous voice conversations. To put this capacity in perspective, let's convert it to its equivalent number of T1 and T3 circuits. A T1 circuit is used to transport 24 voice conversations. Thus, the AT&T FT3C fiber system has the capacity equivalent of 240,000/24 or 10,000 T1 circuits! Yes, that's 10,000 T1 circuits! Concerning T3 circuits, first note that a T3 circuit transports 28 T1s. Since a T1 transports 24 voice conversations, this means that a T3 transports 28 X 24 or 4672 voice conversations. Thus, the AT&T FT3C lightwave system has the capacity of 240,000/672 or 357 T3 circuits! In the previous computations we can note that the AT&T FT3C fiber system represented by a thin piece of glass surrounded by shielding replaces 10,000 copper pair wires used to provide a T1 transmission capa- bility or 357 coaxial cables which are used when T3 c