Ch05 e

Notas del editor

• #5: Since skilled technicians are needed to terminate the fiber-optic connectors and the process is time consuming, labor is generally the most expensive element of fiber-optic installation. Despite its expense, fiber is not susceptible to EMI or RFI, has higher data transmission rates, significantly greater transmission distances, no grounding concerns, and better resistance to environmental factors. This may make fiber a more attractive choice over copper in some implementations. Since there are no crosstalk issues with fiber, it is very common to see multiple fiber pairs encased in the same cable when working with backbone cabling This allows a single cable to be run between floors or data closets and yet supports 2, 4, 8, 12, 24 or more circuits.
• #6: Surrounding the cladding is a buffer material used to help shield the core and cladding from damage. A strength material surrounds the buffer, preventing the fiber cable from being stretched when it is being pulled. The material used is often the same material used to produce bulletproof vests. The final element, the outer jacket, is added to protect the fiber against abrasion, solvents, and other contaminants. This outer jacket composition can vary depending on the cable usage. Fire codes may dictate plenum or riser grade materials
• #9: In many ways fiber-optic systems are similar to copper wire systems. The biggest difference is that fiber-optics use light pulses to transmit information through fiber circuits instead of using electronic pulses through copper circuits. The type of device the cable connects to will determine the actual processing, but in general terms the receiver will convert the light pulses into electrical signals that can then be used by the device or transmitted via copper circuits to other devices.
• #10: Multimode fiber uses LEDs as the light source, while single-mode fiber generally uses laser (Light Amplification by Stimulated Emission of Radiation) light sources. A laser is a device that produces a very intense beam of light. This type of light is much stronger than that which is emitted by an LED. This allows single mode fiber using lasers to transmit data for greater distances.
• #11: The core in single-mode fiber is approximately 10 times larger than the wavelength of the light it is carrying. This leaves very little room for the light to bounce around. As a result the data carrying light pulses in single-mode fiber are essentially transmitted in a straight line through the core. Single-mode is often used in exterior segments and to connect buildings in larger campus environments.
• #12: Multimode uses a type of glass, called graded index glass, which has a lower index of refraction towards the outer edge of the core. This causes the light to slow down when passing through the center of the core and accelerate when passing through the outer areas of the core, ensuring that all modes of light reach the end at approximately the same time. Because the diameter of the cladding is considerably larger than the wavelength of the light being transmitted, the light bounces around (reflects) inside the core as it is propagated along the transmission line.
• #13: Fiber-optics require special handling due to the delicate nature of the thin glass fibers that transmit the light signals. The enclosure systems prevent the cables from being twisted or cut which leads to signal loss. The connectors used in fiber-optic enclosures should provide for a 5 cm bend radius minimum. This will ensure an effective signal transmission while utilizing minimum space.
• #14: Compared to copper, optical fiber is relatively small in diameter and much lighter in weight. These characteristics have made it desirable as intra-floor conduits and wiring duct space has become increasing plugged with expanded copper cable installation.
• #15: Since fiber-optics use light to transmit a signal, it is not subject to EMI, RFI, or voltage surges Since fiber does not use electrical impulses and therefore cannot produce or transmit electric sparks, it becomes the logical solution for passing through flammable environments . The non-conductive nature of fiber-optic makes it a great choice for areas of high lightning-strike incidence and even running through liquids, such as running under the oceans . A fiber-optic connection avoids the problem of differing ground potentials and eliminates the danger ground loops pose to personnel and equipment. Signals sent on copper wires can be intercepted by devices placed in close proximity to the cable. The only way to tap a fiber circuit is to actually access the optical fiber itself, which requires intervention that is easily detectable by security surveillance.
• #17: Fiber-optic cable needs fewer boosting devices than copper cable. Long, continuous segment lengths of fiber-optic cable also provide advantages for manufacturers, installers, and end-users.
• #18: Copper standards such as 100 Mbps and Gigabit have evolved and added much needed copper capacity within LANs. The much shorter maximum lengths of copper necessitate fiber for longer runs for LAN backbones and campus inter-building connectivity.
• #19: A 1 cm, 24-strand fiber cable operating at 140 Mbps carries the same number of voice channels as a 7.5 cm 900-pair copper cable. One kilometer (.6 miles) of this 24-strand fiber cable weighs approx. 60 kg (132 lbs.) while the same length of 900-pair copper cable weighs approx. 7250 kg (16000 lbs.). One single strand of single-mode fiber can now carry up to 5 million phone calls simultaneously.
• #20: The Telecommunications Act of 1996 furthered this effort by allowing television and telephone companies to compete in each other's markets. Fiber-optics will become critical in supplying the bandwidth needed to provide the all-in-one service with television, telephone, interactive multimedia, and Internet access in every home.
• #21: The biggest disadvantage of fiber-optic systems alluded to in the economics section is its incompatibility with the current electronic hardware systems that make up today's telecommunications world. Organizations often solve situations by retrofitting current hardware systems to the fiber-optic networks. Much of the speed that is gained through fiber transmission can be lost at the fiber/copper conversion points
• #23: Fiber produced using the OVD process is purely synthetic, exhibits enhanced reliability, and allows for precise geometrical and optical consistency. The OVD process produces a fiber with very consistent dimensions.
• #24: Although the majority (60%) of fibers are all-glass single-mode used for long distance telephone networks, the low cost of multimode electronics makes multimode fiber popular in local area networking. The total cost of an installation is the combined cost of the cables and the electronics used to communicate over the media.
• #25: Due to the potential exposure to damage by the environment, color-coded plastic buffer tubes house and protect optical fibers. The fibers are floating in a color-coded gel-filled tube to impede water penetration and cushion the fiber. The gel also helps to protect the fibers in low temperature areas. Because the fibers are floating within the tube, there is some slack in the fibers themselves. This extra length helps buffer the fibers during installation. The buffer tubes are often stranded around a dielectric or metal strand to prevent buckling. It is also possible to have armoring around the jacketed cables for additional protection. If the finished cable has metallic components, it may be necessary and beneficial to provide bonding to secure ground.
• #26: Unlike loose tube designs, tight-buffered cables have the buffering material in direct contact with the fiber. Single-fiber, tight-buffered cables are used as patch cords and jumpers to terminate loose-tube cables directly into devices. The fiber is given an acrylate coating prior to the application of  the PVC buffer material, giving each fiber a 900 micron overall diameter. The final bundle provides a rugged cable able to protect individual fibers during handling, routing, and termination. The Aramid jacket helps keep the fiber from being stretched when installers are pulling on the cable.
• #37: There are many different types of connectors in use today. The fiber technician must be sure to match correctly the equipment with the proper connections. Many devices allow for different types of connections. The ST uses a bayonet type connection similar in concept but much smaller than the one used by coaxial cable. Although the ST is still the most widely used because it is relatively easy to terminate, the ST is losing ground to the SC because it requires substantially more room to secure and disconnect. Device manufacturers are typically looking for high port density (closeness) to keep their manufacturing costs and rack space requirements down. SC connectors can be used individually or as part of a duplex connector. Both parts of the SC connector have a key mechanism to help in seating the connection. Some students remember the difference between ST and SC by thinking that "C" stands for "cube", which is the shape of the connector.
• #39: The first step in testing is to affix a jumper to the light source. Using the power meter, obtain a reading of the power level. Record this value as Reference Power Level. Disconnect the power meter from the jumper. Insert a short test lead into the power meter, and using an adapter, connect it to the free end of the jumper extending from the power source. Make a reading, and subtract this value from the Reference Power Level just obtained. The result should be 0.75dB or less. If not, clean all connections, try a new test lead if necessary, and repeat.
• #40: It is important to have a precise knowledge of the speed with which light travels down the particular type of cable. The manufacturer usually provides this figure based on statistical testing. It is called the Nominal Velocity of Propagation (NVP). Usually the operator must key into the OTDR either the NVP or else the manufacturer’s name and type of cable. In some systems, it is impossible to make measurements in the first several meters of fiber. This span, called the “dead zone” occurs because the pulses that come from the OTDR take a certain amount of time to launch. During the time that the transmitter is active, the receiver cannot function well because the transmitter output is so strong. Usually certification includes documentation, and a copy of the OTDR trace is usually included as well.
• #41: Wavelength is the distance that an electromagnetic wave travels in the time it takes to oscillate through a complete cycle. Two other wave phenomena are actually seen. These wave phenomena are the amplitude and attenuation. Amplitude is the height of the wave from the bottom of a valley to the top of the next peak. Attenuation is the diminishing of the wave over time and distance. The wave is at its greatest amplitude near the source and over distance flattens out to the point it is no longer recognizable.
• #43: This reflection of all light back into the source is a phenomenon called total internal reflection. Total internal reflection is a simple property of glass, which causes light hitting the surface at a certain angle to be totally reflected back into the fiber core with little or no loss.    When the angle is greater than the critical angle, then all of the light is reflected and the signal is sent down the fiber.    When the angle is less than the critical angle, at least some of the light escapes or is absorbed into the surface of the second medium, in the case of fiber, the cladding. This can lead to problems such as the signal being distorted or not reaching its destination.
• #44: As the angle of incidence at which the light ray hits the glass surface increases, the emerging light will bend more towards the glass, in fact it will eventually begin to reflect back into the glass. Only when light strikes the surface between the two media at a perpendicular angle will it pass straight through both.
• #45: The speed of light in a vacuum, free of any impurities, is considered perfect, so every index of refraction will be a value greater than 1.0 The refractive index is usually represented by the symbol "η"
• #46: Encoding also means the manipulation of light signals so that they travel in a predetermined pattern reflecting the information it carries. Data is encoded so that 'on' is a binary '1', and 'off' is a binary '0'. Think of coding like old-style Morse code, with 'on' instead of a dot and 'off' instead of dash.
• #47: When this happens, the information once more is in the form of 1s and 0s, ready to be put back into any receiving electronic device such as a computer, video monitor and so on. Depending on the type of service being used on the communications network, transmitters and receivers may be separate units. A device known as a transceiver, which performs both the transmission and reception functions of a transmitter and receiver, can also be used.
• #48: One example of multiplexing is when data from multiple applications is multiplexed into a single data packet. Another example of multiplexing is when data from multiple devices is multiplexed into a single physical channel (using a device called a multiplexer)