The third element in this picture is the buffer (coating), which has nothing to do with the
confinement of the light in the fiber. Its purpose is to protect the glass from scratches and
moisture. The fiber-optic cable can be easily scratched and broken, like a glass pane. If
the fiber is scratched, the scratch could propagate and break the fiber. Another important
aspect is the need to keep the fiber dry.
The most significant difference between single-mode fiber (SMF) and multimode fiber
(MMF) is in the ability of the fiber to send light for a long distance at high bit rates. In
general, MMF is used for shorter distances at a lower bit rate than SMF. For long-distance
communications, SMF is preferred. There are many variations of fiber for both MMF and
SMF. Figure 3-20 shows the two fiber types.