 detector. Essentially, the light is "on" for a binary 1 and "off" for a binary 0. As shown in Figure 9.26, the delayed energy from bit position 1 falls into bit position 2 (and possibly Optical Fiber-optic Optical source transmission medium detector Driver Output circuit Electrical Electrical input output signal signal Figure 9.25 A model of a typical fiber-optic communication link. CONCEPTS IN TRANSMISSION TRANSPORT Dispensed or smeared symbol energy position position position position position Figure 9.26 A simplified sketch of delayed symbol (bit) energy of bit 1 spilling into bit position 2. Alternating 1s and 0s are shown. It should be noted that as the bit rate increases, the bit duration (period) decreases, exacerbating the situation. 3, 4, etc.) confusing the decision circuit. Likewise, delayed energy from bit position 2 falls into bit position 3 (possibly 4, 5, etc.), and SO on. This is aptly called intersymbol interference (ISI), which was previously introduced. One way we can limit the number of modes propagated down a fiber is to make the fiber diameter very small. This is called monomode fiber, whereas the larger fibers are called multimode fibers. For higher bit rate (e.g., >622 Mbps), long-distance fiber-optic links, the use of monomode fiber is mandatory. This, coupled with the employment of the longer wavelengths (e.g., 1330 nm and 1550 nm), allows us to successfully transmit bit rates greater than 622 Mbps, and with certain care the new 10-Gbps rate can be accommodated. 9.4.3 Types of Optical Fiber There are three categories of optical fiber, as distinguished by their modal and physi- cal properties: 1. Step index (multimode) 2. Graded index (multimode) 3. Single mode (also called monomode) Step-index fiber is characterized by an abrupt change in refractive index, and graded index is characterized by a continuous and smooth change in refractive index (i.e., from n1 to n2). Figure 9.27 shows the fiber construction and refractive index profile for step- index fiber (Figure 9.27a) and graded-index