Optical fiber is similar to coaxial cable in that it has a central core, but that is where all similarities stop. The core of an optical fiber is glass or high quality plastic. Rather than conducting electrons, this core conducts photons (i.e., light). Around the core is a cladding, also made of glass or plastic. Around the cladding is a plastic cover, a layer of padding to protect the glass or plastic interior, and finally—an outer sheath. Most optical cable bundles also include a strength member designed to protect the core, increase tensile strength, and prevent kinking.
The core of an optical fiber is designed to minimally attenuate light at one or more particular wavelengths. The cladding is designed to attenuate light. The different attenuation characteristics of the core and the cladding produces a reflective surface at the boundary between the two, much like the reflection one sees when looking through a window at an angle. At some angles, virtually all light is reflected back to the viewer, who can no longer see through the window. At other angles, virtually all light passes through the window and the viewer can see beyond the pane of glass. This reflective property is called refraction.
Optical fiber comes in two basic flavors: single-mode fiber (SMF) and multimode fiber (MMF). The primary difference between these two types of fiber is the diameter of the core. The SMF core is approximately eight microns, effectively limiting the light to a tightly focused, single mode. MMF has a core that normally ranges from 50 to 125 microns, allowing multiple light modes with differing angles of reflection within the core. Each path through a fiber is called a mode, hence the designations single-mode fiber and multimode fiber.
A problem with MMF fiber is the tendency for the light to lose focus as it moves down the core. Because the differing modes travel at slightly differing rates, the light signal has a tendency to “spread out” as it moves down the fiber. If the fiber is too long, what began as a series of tight pulses of light will arrive as a smooth, virtually continuous pulse. The receiver is unable to extract the original signal. As a result, MMF has shorter maximum run lengths (i.e., 2 kilometers) than SMF (i.e., 60 to 100 kilometers), and typically is used for lower-speed (i.e., hundreds of megabits instead of gigabits per second) or shorter distance transmissions.
Fiber has a number of major advantages. Because it is a closed optical environment, it is interference-free. This closed environment also provides a much greater degree of security because the fiber is extremely difficult to “tap.” Although it is possible to scrape away the sheath and cladding to access the core, doing so permits some of the light to escape the core. The loss of power can be easily detected at the receiver, indicating the security of the cable has been breached. “Tapping” a copper cable plant does not even require the cable itself to be penetrated. Because every copper environment radiates electromagnetic energy, one can simply place the correct detecting equipment around the cable and read the EM fluctuations to read the signal on the cable.
Finally, fiber can support extremely high transmission rates. In fact, the primary limitation on the bandwidth in today's networks is the need for optical-to-electronic conversion at either end of the fiber, and the inability of integrated circuits (IC) to operate at fiber speeds.
The primary disadvantage of fiber is perceived to be cost. Inside buildings, fiber is more expensive to install than unshielded twisted pair (UTP). But only some of the cost of a fiber LAN is the media; much of the cost lies in the optical transceivers required at either end of the fiber and in the installation. SMF is typically more expensive than MMF, largely due to the light source and power needs. The light source for MMF is typically a simple light emitting diode (LED), but SMF requires a laser photodiode.
As a consequence, there is a strong tendency for MMF to be used within buildings and in small campus-scale environments. SMF is more commonly used in the metropolitan and wide area.
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