# Fiber optic communication
An optical fiber is a thin (2 to 125 pm), flexible medium capable of conducting an optical ray. Various glasses and plastics can be used to make optical fibers. The lowest losses have been obtained using fibers of ultra pure fused silica. Ultra pure fiber is difficult to manufacture; higher-loss multi component glass fibers are more economical and still provide good performance. Plastic fiber is even less costly and can be used for short-haul links, for which moderately high losses are acceptable.
An optical fiber cable has a cylindrical shape and consists of three concentric sections: the core, the cladding, and the jacket (Figure 13.4)T. he core is the innermost section and consists of one or more very thin strands, or fibers, made of glass or plastic. Each fiber is surrounded by its own cladding, a glass or plastic coating that has optical properties different from those of the core. The outermost layer, surrounding one or a bundle of cladded fibers, is the jacket. The jacket is composed of plastic and other material layered to protect against moisture, abrasion, crushing, and other environmental dangers.
Fig 13.4 : Optical fiber structure .
One of the most significant technological breakthroughs in data transmission has been the development of practical fiber optic communications systems. Optical fiber already enjoys considerable use in long-distance telecommunications, and its use in military applications is growing. The continuing improvements in performance and decline in prices, together with the inherent advantages of optical fiber, have made it increasingly attractive for local area networking.
# The following characteristics distinguish optical fiber from twisted pair or coaxial cable:
The potential bandwidth, and hence data rate, of optical fiber is immense; data rates of 2 Gbps over tens of kilometers have been demonstrated. Compare this capability to the practical maximum of hundreds of Mbps over about 1 km for coaxial cable and just a few Mbps over 1 km or up to 100 Mbps over a few tens of meters for twisted pair.
2)Smaller size and lighter weight:
Optical fibers are considerably thinner than coaxial cable or bundled twisted-pair cable-at least an order of magnitude thinner for comparable information-transmission capacity. For cramped conduits in buildings and underground along public rights-of-way, the advantage
of small size is considerable. The corresponding reduction in weight reduces structural support requirements.
3)Lower attenuation: Attenuation is significantly lower for optical fiber than for coaxial cable or twisted pair and is constant over a wide range.
Optical fiber systems are not affected by external electromagnetic fields. Thus, the system is not vulnerable to interference, impulse noise, or crosstalk. By the same token, fibers do not radiate energy, thereby causing little interference with other equipment and thus providing a
high degree of security from eavesdropping. In addition, fiber is inherently difficult to tap.
5)Greater repeater spacing:
Fewer repeaters means lower cost and fewer sources of error. The performance of optical fiber systems from this point of view has been steadily improving. For example, AT&T has developed a fiber transmission system that achieves a data rate of 3.5 Gbps over a distance of
318 km [PARK921 without repeaters. Coaxial and twisted-pair systems generally
have repeaters every few kilometers.
Five basic categories of application have become important for optical fiber:
- Long-haul trunks
- Metropolitan trunks
- Rural-exchange trunks
- Subscriber loops
- Local area networks
Long-haul fiber transmission is becoming increasingly common in the telephone network. Long-haul routes average about 900 miles in length and offer high capacity (typically 20,000 to 60,000 voice channels). These systems compete economically with microwave and have so underpriced coaxial cable in many developed countries that coaxial cable is rapidly being phased out of the telephone network in such areas.
Metropolitan trunking circuits have an average length of 7.8 miles and may have as many as 100,000 voice channels in a trunk group. Most facilities are installed in underground conduits and are repeaterless, joining telephone exchanges in a metropolitan or city area. Included in this category are routes that link long-haul microwave facilities that terminate at a city perimeter to the main telephone exchange building downtown.
Rural exchange trunks have circuit lengths ranging from 25 to 100 miles that link towns and villages. In the United States, they often connect the exchanges of different telephone companies. Most of these systems have fewer than 5,000 voice channels. The technology in these applications competes with microwave facilities. Subscriber loop circuits are fibers that run directly from the central exchange to a subscriber. These facilities are beginning to displace twisted pair and coaxial cable links as the telephone networks evolve into full-service networks capable of handling not only voice and data, but also image and video. The initial penetration of optical fiber in this application is for the business subscriber, but fiber transmission into the home will soon begin to appear.
A final important application of optical fiber is for local area networks. Recently, standards have been developed and products introduced for optical fiber networks that have a total capacity of 100 Mbps and can support hundreds or even thousands of stations in a large office building or in a complex of buildings. The advantages of optical fiber over twisted pair and coaxial cable become more compelling as the demand for all types of information (voice, data, image, and video) increases.