Disadvantages of Fiber-Optic Cabling

Transcription

1 Disadvantages of Fiber-Optic Cabling 329 Disadvantages of Fiber-Optic Cabling With all of its advantages, many people use fiber-optic cabling. However, fiber-optic cabling does have a couple of major disadvantages, including higher cost and a potentially more difficult installation. Higher Cost It s ironic, but the higher cost of fiber-optic cabling has little to do with the cable these days. Increases in available fiber-optic-cable manufacturing capacity have lowered cable prices to levels comparable to high-end UTP on a per-foot basis, and the cables are no harder to pull. Modern fiber-optic connector systems have greatly reduced the time and labor required to terminate fiber. At the same time, the cost of connectors and the time it takes to terminate UTP have increased because Category 5e and Category 6 require greater diligence and can be harder to work with than Category 5. So the installed cost of the basic link, patch panel to wall outlet, is roughly the same for fiber and UTP. Here s where the costs diverge. Ethernet hubs, switches, routers, NICS, and patch cords for UTP are very (almost obscenely) inexpensive. A good-quality 10/100 auto-sensing Ethernet NIC for a PC can be purchased for less than $20. A fiber-optic NIC for a PC costs several times as much. Hubs, routers, and switches have similar differences in price, UTP vs. fiber. For an IT manager who s got several hundred workstations to deploy and support, that translates to megabucks and keeps UTP a viable solution. The cost of network electronics keeps fiber more expensive than UTP, and ultimately, it is preventing the mass stampede to fiber to the desk.

2 330 Chapter 10 Fiber-Optic Media Difficult to Install Depending on the connector system you select, the other main disadvantage of fiber-optic cabling is that it can be more difficult to install. Copper-cable ends simply need a mechanical connection, and those connections don t have to be perfect. Most often, the plug connectors for copper cables are crimped on (as discussed in Chapter 8) and are punched down in an IDC connection on the jack and patch-panel ends. Fiber-optic cables can be much trickier to make connections for, mainly because of the nature of the glass or plastic coreof the fiber cable. When you cut or cleave (in fiber-optic terms) the inner core, the end of the core consists of many very small shards of glass that diffuse the light signal and prevent it from hitting the receiver correctly. The end of the core must be polished with a special polishing tool to make it perfectly flat so that the light will shine through correctly. Figure 10.3 illustrates the difference between a polished and a nonpolished fiber-optic cable-core end. The polishing step adds extra complexity to the installation of cable ends and amounts to a longer, and thus more expensive, cabling-plant installation. Connector systems are available for multimode fiber-optic cables that don t require the polishing step. Using specially designed guillotine cleavers, a clean-enough break in the fiber is made to allow a good end-to-end mate when the connector is plugged in. And, instead of using epoxy or some other method to hold the fiber in place, the fibers are positioned in the connector so that dynamic tension holds them in proper position. The use of an index-matching gel in such connectors further improves the quality of the connection. Such systems greatly reduce the installation time and labor required to terminate fiber cables. FIGURE 10.3 The difference between a freshly cut and a polished end Before polishing Jacket After polishing Jacket

3 Types of Fiber-Optic Cables 331 Types of Fiber-Optic Cables Fiber-optic cables come in many configurations. The fiber strands can be either single mode or multimode, step index or graded index, and tight buffered or loose-tube buffered. In addition to these options, a variety of core diameters exist for the fiber strands. Most often, the fiber strands are glass, but plastic optical fiber (POF) exists as well. Finally, the cables can be strictly for outdoor use, strictly for indoor use, or a universal type that works both indoors and out. Composition of a Fiber-Optic Cable A typical fiber-optic cable consists of several components: Optical-fiber strand Buffer Strength members Optional shield materials for mechanical protection Outer jacket Each of these components has a specific function within the cable to help ensure that the data gets transmitted reliably. Optical Fiber An optical-fiber strand (also called an optical waveguide) is the basic element of a fiber-optic cable. All fiber strands have at least three components to their cross sections: the core, the cladding, and the coating. Figure 10.4 depicts the three layers of the strand. FIGURE 10.4 Elemental layers in a fiber-optic strand Coating Cladding

4 332 Chapter 10 Fiber-Optic Media NOTE WARNING Fiber strands have elements so small that it is hard to imagine the scale. You re just not used to dealing with such tiny elements in everyday life. The components of a fiber strand are measured in microns. A micron is one thousandth of a millimeter, or about inches. A typical single-mode fiber strand has a core only 8 microns, or inches, in diameter. A human hair is huge by comparison. The core of a commonly used multimode fiber is 62.5 microns, or inches in diameter. For both single mode and multimode, the cladding usually has a diameter of 125 microns, or inches. And finally, commonly used single- and multimode fiber strands have a coating layer that is 250 microns, or 0.01 inches, in diameter. Now we re getting somewhere, huh? That s all the way to one hundredth of an inch. The tiny diameter of fiber strands makes them extremely dangerous. When stripped of their coating layer, as must be done for some splicing and connectorizing techniques, the strands can easily penetrate the skin. Shards, or broken pieces of strand, can even be carried by blood vessels to other parts of the body (or the brain) where they could wreak serious havoc. They are especially dangerous to the eye because small pieces can pierce the eyeball, doing damage to the eye s surface and possibly getting trapped inside. Safety glasses and special shard-disposal containers are a must when connecting or splicing fibers. The fiber core is usually made of some type of plastic or glass. Several types of materials make up the glass or plastic composition of the optical fiber core. Each material differs in its chemical makeup and cost as well as its index of refraction, which is a number that indicates how much light will bend when passing through a particular material. The number also indicates how fast light will travel through a particular material. A fiber-optic strand s cladding is a layer around the central core that is the first, albeit the smallest, layer of protection around the glass or plastic core. It also reflects the light inside the core because the cladding has a lower index of refraction than the core. The cladding thus permits the signal to travel in angles from source to destination it s like shining a flashlight onto one mirror and having it reflect into another, then another, and so on. The protective coating around the cladding protects the fiber core and cladding from damage. It does not participate in the transmission of light but is simply a protective material. It protects the cladding from abrasion damage, adds additional strength to the core, and builds up the diameter of the strand. The most basic differentiation of fiber optic cables is whether the fiber strands they contain are single mode or multimode. A mode is a path for the light to take through the cable. The wavelength of the light transmitted, the acceptance angle, and the numerical aperture interact in such a way that only certain paths are available for the light. Single-mode fibers have cores that are so small that only a single pathway for the light is possible. Multimode fibers have larger cores; the options for the angles at which the light can enter the cable are greater, and so multiple pathways are possible.

5 Types of Fiber-Optic Cables 333 Using its single pathway, single-mode fibers can transfer light over great distances with high data-throughput rates. Concentrated (and expensive) laser light sources are required to send data down single-mode fibers, and the small core diameters make connections expensive. Multimode fibers can accept light from less intense and less expensive sources, usually LEDs. In addition, connections are easier to align properly due to larger core diameters. Distance and bandwidth are more limited than with single-mode fibers, but multimode cabling and electronics are generally a less expensive solution. Single-mode fibers are usually used in long-distance transmissions or in backbone cables, so you find them in both outdoor and indoor cables. These applications take advantage of the extended distance and high-bandwidth properties of single-mode fiber. Multimode fibers are usually used in an indoor LAN environment in the horizontal cables. They are also often used in the backbone cabling where great distances are not a problem. Single-mode and multimode fibers come in a variety of flavors. Some of the types of optical fibers, listed from highest bandwidth and distance potential to least, include the following: Single-mode glass Multimode graded-index glass Multimode step-index glass Multimode plastic-clad silica (PCS) Multimode plastic Single-Mode Step-Index Glass A single-mode glass fiber core is very narrow (usually less than 10 microns) and made of silica glass. To keep the cable size manageable, the cladding for a single-mode glass core is usually more than 10 times the size of the core (around 125 microns). Single-mode fibers are expensive, but because of the lack of attenuation (less than 2dB per kilometer), very high speeds are possible in some cases, up to 50Gbps. Figure 10.5 shows a single-mode glass-fiber core. Multimode Graded-Index Glass A graded-index glass-fiber core, made of silica glass, has an index of refraction that changes gradually from the center outward to the cladding. The center of the core has the highest index of refraction, i.e., the light is distorted the least near the center. If the signals travel outside the center of the core, the lower index of refraction will bend them back toward the center, where they will travel faster, with less signal loss. The most commonly used multimode graded-index glass fibers have a core that is either 62.5 microns in diameter or 50 microns in diameter. Figure 10.6 shows a graded-index glass core. Notice that the core is bigger than the single-mode core.

6 334 Chapter 10 Fiber-Optic Media FIGURE 10.5 An example of a singlemode glass-fiber core 8 10-micron 125-micron 250-micron Cladding Coating FIGURE 10.6 A graded-index glassfiber core Coating Cladding Multimode Step-Index Glass A step-index glass core is similar to a single-mode glass core but with a much larger diameter (usually around 62.5 microns, although it can vary largely in size between 50 and 125 microns). It gets its name from the large and abrupt change of index of refraction from the glass core to the cladding. In fact, a step-index glass core has a uniform index of refraction. Because the signal bounces around inside the core, it is less controllable and thus suffers from larger attenuation values and, effectively, lower bandwidths. However, equipment for cables with this type of core is cheaper than other types of cable, so step-index glass cores are found in many cables. Multimode Plastic-Clad Silica (PCS) A plastic-clad silica (PCS) fiber core is made out of glass central core clad with a plastic coating, hence the name. PCS optical fibers are usually very large (200 microns or larger) and thus have

7 Types of Fiber-Optic Cables 335 limited bandwidth availability. However, the PCS-core optical cables are relatively cheap when compared to their glass-clad counterparts. Multimode Plastic Plastic optical fibers (POF) consist of a plastic core of anywhere from 50 microns on up, surrounded by a plastic cladding of a different index of refraction. Generally speaking, these are the lowest-quality optical fibers and are seldom sufficient to transmit light over long distances. Plastic optical cables are used for very short-distance data transmissions or for transmission of visible light in decorations or other specialty lighting purposes not related to data transmission. Recently, POF has been promoted as a horizontal cable in LAN applications. However, the difficulty in manufacturing a graded-index POF, combined with a low bandwidth-for-dollar value, has kept POF from being accepted as a horizontal medium. Buffer The buffer, the second-most distinguishing characteristic of the cable, is the component that provides the most protection for the optical fibers inside the cable. The buffer does just what its name implies; it buffers, or cushions, the optical fiber from the stresses and forces of the outside world. Optical-fiber buffers are categorized as either tight or loose tube. With tight buffers, a protective layer (usually a 900-micron thermoplastic covering) is directly over the coating of each optical fiber in the cable. Tight buffers make the entire cable more durable, easier to handle, and easier to terminate. Figure 10.7 shows tight buffering in a single-fiber (simplex) construction. Tight-buffered cables are most often used indoors because expansion and contraction caused by outdoor temperature swings can exert great force on a cable. Tight-buffered designs tend to transmit the force to the fiber strand, which can damage the strand or inhibit its transmission ability, so thermal expansion and contraction from temperature extremes is to be avoided. There are some specially designed tight-buffered designs for either exclusive outdoor use or a combination of indoor/outdoor installation. A loose-tube buffer, on the other hand, is essentially a tough plastic pipe about inches in diameter. One or several coated fibers can be placed inside the tube, depending on the cable design. The tube is then filled with a protective substance, usually a water-blocking gel, to provide cushioning, strength, and protection from the elements. Sometimes, water-blocking powders and tapes are used to waterproof the cable, either as a replacement for the gel (rare) or in addition to it (more common). A loose-tube design is very effective at absorbing forces exerted on the cable so that the fiber strands are isolated from the damaging stress. For this reason, loose-tube designs are almost always seen in outdoor installations. Multiple tubes can be placed in a cable to accommodate a large fiber count, for highdensity communication areas such as in a large city or as trunk lines for long-distance telecommunications.