 At that time a technique referred to as wavelength division multiplexing (WDM), which allowed multiple communications over a com- mon fiber, was unknown. Thus, the ability to obtain two-way transmis- sion over an optical fiber required the use of one fiber for transmission and a second for reception. Figure 3.13 illustrates how a pair of fibers are used to obtain bidirec- tional transmission capability. Note that at each end of the cable a trans- mitter in the form of a light source and a receiver in the form of a light detector are connected to each fiber in the cable. The cable shown in Figure 3.13 consists of two fiber-optic cables, each containing a single fiber, encased in a common jacket. Over the years numerous types of fiber-optic cables have been manufactured that build TABLE 3.2 Characteristic Multimode Fiber Single-mode Fiber Comparison of Operational Char- Application acteristics and Uti- Attenuation -3 to - -1 dB/km -0.35 to -0.20 dB/km lization of Multi- mode and Wavelength 850/1300 nm 1310/1550 nm Single-Mode Fiber Light Source LED, laser Laser Bandwidth 160-550 MHz/km 10 GHz/100 km Understanding Optical Fiber Figure 3.13 Most fiber-optic cables include one or more optical fiber/core/jacket pairs of fibers, with each pair supporting two-way full-duplex optical fiber/core/jacket communications. on this design. Some cables contain two, four, six, eight, or more individ- ual fiber-optic cables, each containing one or more fibers. Cable Architecture One example of the cable architecture described above is shown in Fig- ure 3.14. In this example a common cable housing holds eight cables, with each cable containing eight fibers. Thus, this cable provides 8 X 8, or 64 optical fibers. By increasing the number of fibers per cable and the number of fiber-optic cables within a common cable jacket, it becomes possible to rapidly increase the total number of fibers. Because the cost of cable installation can easily exceed the cost of the cable, most communications carriers install a large number of fibers even