ircuits are installed on copper conductors. If you think about the physical wiring required for those T1 or T3 circuits, their power requirements, and the need to install repeaters with power and to shelter approximately every 6000 ft, you can begin to appreciate the cost savings associated with the use of optical fiber. Chapter Six Although the preceding computations are accurate, they do not repre- sent the true state of the present capacity of fiber in comparison to T1 and T3 copper circuits. This is because there have been significant advances in the design of lasers that pulse at higher rates, permitting an increase in transmission rates at a given frequency. As we noted earlier in this chapter, optical systems based on the use of single-mode fiber and lasers commonly operate at 10 Gbits/s, and field trials of systems operating at 40 Gbits/s are now being conducted. When coupled with advances in wavelength division multiplexing (WDM) and dense wave- length division multiplexing (DWDM), the capacity of an optical fiber has increased by several magnitudes and explains the popular Qwest Communications commercial in which a person entering a store is told that that company can transmit the contents of every book written in every language in a small amount of time. BIT ERROR RATE Earlier in this book we noted that one of the key advantages obtained from the use of optical fiber is the immunity of light transmission to electromagnetic radiation. This not only allows fiber-optic cable to be routed through locations close to machinery but also significantly lowers the error rate resulting from transmission occurring on optical fiber in comparison to the use of copper cable. Many sources of impairments (interference) adversely affect copper cable, ranging in scope from crosstalk to impulse noise caused by light- ning and machinery. Optical fiber is immune to those impairments, which commonly results in an error rate several orders of magnitude lower than on copper cable. Perhaps the only common impairment that affect