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In fact, it was this lower-cost, relatively high bandwidth fiber optic technology that fueled the modern wide area communications revolution. In the wide area, the availability of right-of-way for fiber cable installation was the key to building a fiber network. One of the earliest implementers utilized partnerships with railway companies to place fiber along rail lines from coast to coast. International communications also benefited from this technology, as literally thousands of fibers of fiber optic cable were placed along the ocean beds between continents.
The development of this high-volume intercity fiber network required significant advancements in the technology of both optical fiber and optical transceivers. As the technology moved forward, devices and materials became available for use in the bandwidth-hungry local area network domain as well.
Local area networks (LANs) have quickly adopted faster data transmission technologies, during the same period of time. These data topologies, such as Fast and Gigabit Ethernet, require high transmission bandwidths and really put twisted-pair copper to the test. Fiber optic cable, in contrast, has a vastly superior bandwidth capability, particularly at the shorter distances prevalent in LANs.
In this chapter, we will explore the many facets of fiber optic technology as they relate to LAN applications. Naturally, some of these topics are covered in other parts of this book. We will repeat certain key points here, and reference other chapters in the book for supplementation in other instances. The focus will be to provide a concise explanation of the utility of fiber optic technology in the LAN environment.
Fiber Optic Basics
Before we delve into the LAN applications, let's review the basics of fiber optic technology.
Fiber Optic Transmission
The phenomenon of the transmission of light through fiber optic media comes from the optical principles of refraction and reflection. As you know, a beam of light bends when it passes from one medium into another. The most familiar example of this is the air-water transition. Fill a pan with water and place a coin at the midpoint of the pan's bottom. Now, stand back from the pan and place a straw (or pencil) into the water, along your line of vision to the coin. The straw appears to bend upward from the straight-line path at the surface of the water. Place the straw into the water from the right or left side, and you will see the same upward bend. The straw has obviously not bent, but the reflected light from the part of the straw below the water's surface has bent at the air-water interface, making the straw appear to bend. This bending of light as it passes from material to material is called refraction. Materials that transmit light have a property called the index of refraction' that is proportional to the amount of bending that would occur between a given material and a vacuum reference. Both air and water have respective indexes that indicate this behavior.
While the air-water refraction is trivial, the same principle is used in forming lenses, such as those used for vision correction, and can be used to collimate light, as well. As illustrated in Fig. 12.1, the path of a ray of light bends at the interface between two transmissive materials. The amount of the bend is determined by the index of refraction. However, beyond a certain incident angle, called the critical angle, the ray of light is reflected from the interface, rather than refracted into the medium. That is exactly the reason that the coin in the water seems to disappear when you, try to view it from a low angle to the water's surface. By the way, the angle at which the light is bent depends on the wavelength (color) of the light, which is how prisms (and rainbows) work. This phenomenon actually causes lots of problems for multimode fiber, as we will see later.
For years we concentrated on the incident angles that were necessary for the effective transmission and refraction of light from one medium to another, through the interface. This was useful for the design of eyeglasses, binoculars, telescopes, and microscopes. However, if the transmission medium is formed into a solid strand of glass or plastic, we can use the reflective and refractive principles to guide the rays of light within the strand, along its length.
Carefully constructed fibers of glass or plastic can efficiently transfer light for long distances along their length, with very little loss of light outside the fiber. You are probably familiar with the decorative lamps made with hundreds of short plastic fibers. The fiber strands glow very slightly along their length when a light source illuminates the strand ends placed at the base of the lamp, but the fiber ends shine brightly. Similar fibers are used to transmit light along the network pathway of local area networks. We simply modulate (vary) the light beam in order to carry data.
Optical fibers can be either glass or plastic, but for most of this discussion, we will refer to the fiber optic material as glass, since silica compounds are used in most current LAN implementations.
These silica fibers are sometimes called light pipes, because light seems to be conducted through the flexible fiber, just like water through a tube. Optical fiber is not actually a pipe for light, as we have said. Light passes through the glass, not through a pipe formed from the glass. The transmission properties of the glass are used to direct the light through the center portion of the fiber, as shown in Fig. 12.2. Depending on the type of fiber core, the incident light beam reflects or refracts its way down the length of the fiber, staying roughly in the center portion of the core, much as water in a pipe (thus the analogy).
An appropriate light source is used to send a communications signal through an optical fiber. Here is how data transmission over a fiber works. A data-modulated electrical signal, such as a LAN packed is first applied to the light source, which converts it into light. The beam of light from the source is then directed at one end of the fiber, which transmits it through the fiber all the way to the other end...
Posted March 26, 2003
I am a computer engineering student at a local community college. I bought this book as part of my plan to become knowledgable in the area of structured cabling for a prospective employer in the IT field and to work on my semester inmplementation project. Although the book has lots of referential information on the many types of cabling technoilogy available today, if fails by not showing the reader how the parts are physically conected (pictures/diagrams). It's a real shame that Mr. Trulove left this particularly important aspect out of his book. Had I known this, I would have spent my money elsewhere.Was this review helpful? Yes NoThank you for your feedback. Report this reviewThank you, this review has been flagged.
Posted October 14, 2000
If you are responsible for the operation on any data network, then you must get this book! I am in the cabling business specializing in Wireless LAN's. In an industry that changes frequently, it is wonderful to have a reference that puts it all together. Not only has Trulove provided insights to present and future technologies, he supports his work with numerous illustrations and excerpts from existing industry standards. If...you have a network that has copper, fiber, or wireless connectivity...you use token ring, ethernet, ATM, or some other protocol...you are just starting a network and don't have a clue what to do first...YOU NEED THIS BOOK!Was this review helpful? Yes NoThank you for your feedback. Report this reviewThank you, this review has been flagged.