CONCEPTS IN TRANSMISSION TRANSPORT
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
Types of Optical Fiber
There are three categories of optical fiber, as distinguished by their modal and physi-
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
). Figure 9.27 shows the fiber construction and refractive index profile for step-
index fiber (Figure 9.27
a) and graded-index fiber (Figure 9.27b). Both step-index and
graded-index light transmission are characterized as multimode because more than one
mode propagates. (Two modes are shown in the figure.) Graded-index fiber has a superior
bandwidthdistance product compared to that of step-index fiber. In other words, it can
transport a higher bit rate further than step index. It is also more expensive. We can
eliminate this cause of dispersion if we use singlemode fiber. Figure 9.28 shows a
typical five-fiber cable for direct burial.
Splices and Connectors
Optical fiber cable is commonly available in 1-km sections; it is also available in longer
sections, in some types up to 10 km or more. In any case there must be some way of