s both copper cable and optical fiber is Joe Backhoe, who typically works where he shouldn't. When Joe cuts a cable, then you unfortunate- ly will note another difference between copper cable and optical fiber. That difference is the number of adversely affected users, which is higher when an optical fiber is cut since the fiber supports many more users than does a copper cable. The AT&T FT3C System The AT&T FT3C lightwave transmission system can be considered as a milestone in the deployment of optical fiber. In addition to providing a showcase high-speed transmission system between Boston and Fiber in the WAN Washington, DC, the FT3C system represented the first commercial use of wavelength division multiplexing. To understand how this was accomplished, consider Figure 6.1. Frequency division multiplexing (FDM) is illustrated in the top portion of the figure. Note that FDM repre- sents an analog technology in which the bandwidth of the transmis- sion medium is subdivided by frequency. In the top portion of the figure three distinct frequencies (f, 1' f2, and f3) are used to derive three separate channels. Each channel can be modulated separately from the other channels, permitting three simultaneous transmissions to occur on one analog circuit. In fact, most trunks linking telephone company central offices used FDM through the 1960s to transport multiple voice conversations over one physical line connecting geographically separated offices. Perhaps thinking about the manner by which FDM operated per- mitted engineers insight into developing WDM equipment. Early ver- sions of WDM coupled light from two or more sources at discrete wavelengths into a fiber, with each wavelength in effect becoming a a. Frequency Division Multiplexing Figure 6. 1 Frequency division Channel 3 multiplexing and wavelength division Channel 2 multiplexing. Channel 1 b. Wavelength Division Multiplexing Sheath Cladding Sheath Cladding Chapter Six transmission channel operating at the modulation rate used by the equipment driving the light 