Chapter 5 Fiber-Optic Media

Chapter 5 Fiber-Optic Media
Introduction fiber-optics Advantages and disadvantages of fiber-optics Fiber-optic connectors Basics of fiber-optic transmission Copyright 2003 Fundamentals of Voice and Data Cabling 1.2

Fundamentals of Voice and Data Cabling 1.2
Fiber Optics Fiber-optic cable is a communications medium that uses modulated light for transmissions through thin strands of glass. Signals that represent data bits are converted into beams of light. Although the cost of the fiber media is not significantly more than copper cable, the connectors, tools, and labor needed to terminate the fiber are considerably more expensive. 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. Copyright 2003 Fundamentals of Voice and Data Cabling 1.2

Fiber-optic cable construction
Typically there are five elements that make up each fiber-optic cable: Core Cladding Buffer Strength material (Aramid Yarn) Outer jacket 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 Copyright 2003 Fundamentals of Voice and Data Cabling 1.2

How Fiber-Optics Work Incoming light is reflected or refracted off of the cladding depending on what angle it strikes the cladding. It then bounces inside the core and cladding for very great distances. Copyright 2003 Fundamentals of Voice and Data Cabling 1.2

Fiber-Optic Cables Each fiber-optic circuit is actually two fiber cables. One fiber is used for transmission in each direction. Notice that each cable has both a transmit and a receive connector. A pair (Tx/Rx) could plug into a router, switch, termination panel, server, or even workstation. Copyright 2003 Fundamentals of Voice and Data Cabling 1.2

Data Transmission A transmitter converts data into coded light pulses and injects the light pulses into the fiber. A special device known as a Light Emitting Diode (LED) produces the light. This small electronic device lights up very quickly when electricity is applied to it. A sequence of pulses represents the data that is being sent . When the light pulse reaches the destination it is channeled into the optical receiver. 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. Copyright 2003 Fundamentals of Voice and Data Cabling 1.2

Fiber-Optic Transmission Types
Single-mode uses a single mode of light to transmit the signal and multimode uses multiple modes of light to transmit the signal, thus the term multimode. A mode in optical transmission is a ray of light entering the core at a particular angle. Modes can therefore be thought of as bundles of light rays of the same wavelength entering the fiber at a specific angle. 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. Copyright 2003 Fundamentals of Voice and Data Cabling 1.2

Single-mode Fiber Distances up to 3000 meters in Campus / Building backbone LASER light source Very small core Less dispersion Higher Bandwidth 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. Copyright 2003 Fundamentals of Voice and Data Cabling 1.2

Multimode Fiber Distances up to 2000 meters LED light source Larger core than Single- mode Allows more dispersion Lower Bandwidth 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. Copyright 2003 Fundamentals of Voice and Data Cabling 1.2

Fiber-Optic Enclosures
Fiber-optic enclosure systems, consisting of connectors and protective channels, are designed to protect fiber-optic cable. 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. Wall mount enclosure Fiber-Optic Raceway Copyright 2003 Fundamentals of Voice and Data Cabling 1.2

Fiber-Optic Advantages
Electromagnetic immunity including non-conductivity. Nearly impossible to tap providing for greater security than copper cable. Decreased attenuation and increased transmission distance. Increased bandwidth potential. Small diameter and weight. Cost effective in a long-term installation. 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. Copyright 2003 Fundamentals of Voice and Data Cabling 1.2

Fiber-Optic Transmission Properties
Fiber is not subject to EMI, RFI, or voltage surges. Cannot produce or transmit electric sparks. The non-conductive nature of fiber-optics makes it a great choice for areas of high lightning-strike incidence. 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. Copyright 2003 Fundamentals of Voice and Data Cabling 1.2

Fiber-Optic Security Considerations
The use of light in optical fiber makes it difficult to remotely detect the signal being transmitted within 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. Copyright 2003 Fundamentals of Voice and Data Cabling 1.2

Fiber-Optic Attenuation and Transmission Distance
To keep a signal going, it must be boosted every so often along the line. A signal regenerator (repeater) is used to boost the electronic pulse in a copper cable. An optical repeater is used to boost the light pulse in a fiber-optic cable. The advantage of optical fiber is that it performs better with respect to attenuation. 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. Copyright 2003 Fundamentals of Voice and Data Cabling 1.2

Increased Bandwidth of Fiber-Optics
Fiber circuits used in trunk connections between cities and countries carry information at up to 10 gigabits per second (Gbps). This is enough to carry 160,000 telephone circuits or 1000 television channels. Industry experts predict larger bandwidths than this as technology advances. 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. Copyright 2003 Fundamentals of Voice and Data Cabling 1.2

Fiber-Optic Size and Weight
Compared to copper, optical fiber is relatively small in diameter and much lighter in weight. 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 kg (16000 lbs.). One single strand of single-mode fiber can now carry up to 5 million phone calls simultaneously. This multi-pair copper cable could easily be replaced with a single pair of optical fibers Copyright 2003 Fundamentals of Voice and Data Cabling 1.2

Optical Fiber Cost Benefits
The initial cost of changing over to fiber can be quite expensive. Long-term cost effectiveness is due to the relative ease of upgrading fiber-optics to higher speeds and performance since many electronic devices can be upgraded without modifying the fiber circuits. 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. Copyright 2003 Fundamentals of Voice and Data Cabling 1.2

Fiber-Optic Disadvantages
Higher initial cost than copper. Fiber can be less forgiving of abuse than copper cable. Fiber connectors are more delicate than copper connectors. It takes a higher level of training and skill to terminate fiber. The installation tools and meters are more expensive. 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 Copyright 2003 Fundamentals of Voice and Data Cabling 1.2

Loose-Tube Cables Loose-tube cables are typically used for outside-plant installation. A loose-tube cable is made up of six components: 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. Copyright 2003 Fundamentals of Voice and Data Cabling 1.2

Tight Buffered Cables Tight-buffered cables are frequently used for indoor installations. A tight-buffered cable is made up of four components: 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. Copyright 2003 Fundamentals of Voice and Data Cabling 1.2

Connectors Connectors are used to connect fiber to panels or active devices. ANSI/TIA/EIA 568-B.3 standard specifies that the duplex SC is the recommended connector for all terminations, yet the older ST is still acceptable. 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. SC Connector ST Connector Copyright 2003 Fundamentals of Voice and Data Cabling 1.2

Testing Fiber-Optics Testing the cables and connections is perhaps the most important part of the fiber-optic installation. If the fiber does not pass testing, the installation must be repaired, even if new fibers must be pulled. A simple flashlight check will only show that a cable is functional. Two fiber-optic tests: light source and power meter and OTDR. Copyright 2003 Fundamentals of Voice and Data Cabling 1.2

Light Source and Power Meter
Fiber-optic installations are usually tested with a light source and power meter. The power source delivers about one milliwatt (1 mW) on one or more popular fiber-optic wavelengths. The meter detects power and displays it in dB. This test determines the amount of loss exhibited by the fiber. 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. Copyright 2003 Fundamentals of Voice and Data Cabling 1.2

Optical Time Domain Reflectometers (OTDR)
An optical time domain reflectometer (OTDR) works like a radar that shoots pulses down the fiber under test. Every misaligned joint and discontinuity causes a little bit of the light to reflect back up the fiber to where the OTDR monitors for echoes. By charting the strength of the echoes versus time, it is possible to learn a great deal about the condition of the cable. 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. Copyright 2003 Fundamentals of Voice and Data Cabling 1.2

Light waves and wavelengths
Wavelengths of light are measured in nanometers (nm) or microns (µm). The wavelength is the distance between the waves. 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. Copyright 2003 Fundamentals of Voice and Data Cabling 1.2

Reflection When light travels through a medium like air and strikes another medium like glass, the light either reflects off the surface (reflection) or passes into or through the second medium. Copyright 2003 Fundamentals of Voice and Data Cabling 1.2

Critical Angle The angle at which the ray hits the glass surface is called the angle of incidence. When this angle of incidence reaches a certain point, called the critical angle, all the light is reflected back into the original medium. 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. Copyright 2003 Fundamentals of Voice and Data Cabling 1.2

Refraction Refraction is the bending of a beam of light through an interface between two dissimilar media, such as the glass and air. 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. Copyright 2003 Fundamentals of Voice and Data Cabling 1.2

Refractive Index The refractive index is the ratio between the speed of light in a vacuum and lights speed in another medium. The lower the refractive index, the faster the light travels in a material. The refractive index of a material such as those used in fiber-optic cables is an important property that determines how light will behave in that material. Reference 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" Copyright 2003 Fundamentals of Voice and Data Cabling 1.2

Transmitters The encoding side of the fiber-optic communication system is the transmitter. The role of the transmitter is to convert data in the form of electrical signals into equivalent light pulses and send them to the fiber-optic cable for transmission. 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. Copyright 2003 Fundamentals of Voice and Data Cabling 1.2

Optical Receiver At the opposite end of the fiber-optic system, is the receiver or decoder. The role of the receiver is to detect the light pulse arriving at the other end and to convert it back to the original electrical signal containing the information that was impressed on the light at the transmitting end. A device known as a transceiver, which performs both the transmission and reception functions of a transmitter and receiver, can also be used. 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. Copyright 2003 Fundamentals of Voice and Data Cabling 1.2

Multiplexing Multiplexing (MUX) is a process in which multiple data channels are combined into a single data or physical channel at the source. Demultiplexing (DEMUX) is the process of separating multiplexed data channels at the destination. 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) Data Copyright 2003 Fundamentals of Voice and Data Cabling 1.2

Chapter 5 Fiber-Optic Media

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