|  | 5.1 | Most Common LAN Media
| 5.1.4 | Optical fiber

Fiber-optic cable is a networking medium capable of conducting modulated light transmissions. Compared to other networking media, it is more expensive; however, it is not susceptible to electromagnetic interference and is capable of higher data rates than any of the other types of networking media discussed here. Fiber-optic cable does not carry electrical impulses, as other forms of networking media that employ copper wire do. Instead, signals that represent bits are converted into beams of light. Even though light is an electromagnetic wave, light in fibers is not considered wireless because the electromagnetic waves are guided in the optical fiber. The term wireless is reserved for radiated, or unguided, electromagnetic waves.

Fiber-optic communication is rooted in a number of inventions made in the 19th century. It was not until the 1960s, when solid-state laser light sources and high-quality impurity-free glasses were introduced, that fiber-optic communication became practical. Its use on a widespread basis was pioneered by telephone companies, who saw its benefits for long-distance communication.

Fiber-optic cable used for networking consists of two fibers encased in separate sheaths. If viewed in cross section, you would see that each optical fiber is surrounded by layers of protective buffer material, usually a plastic such as Kevlar, and an outer jacket. The outer jacket provides protection for the entire cable. Usually made of plastic, it conforms to appropriate fire and building codes. The purpose of the Kevlar is to furnish additional cushioning and protection for the fragile hair-thin glass fibers. Wherever buried fiber-optic cables are required by codes, a stainless steel wire is sometimes included for added strength.

The light-guiding parts of an optical fiber are called the core and the cladding. The core is usually very pure glass with a high index of refraction. When the core glass is surrounded by a cladding layer of glass or plastic with a low index of refraction, light can be trapped in the fiber core. This process is called total internal reflection, and it allows the optical fiber to act like a light pipe, guiding light for tremendous distances, even around bends.

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