CCNA 1 and 2 Companion Guide, Revised (Cisco Networking Academy Program) part 18 ppsx

Optical Media 139 Figure 3-18 Refraction Example Total Internal Reflection A light ray that is being turned on and off to send data 1s and 0s into an optical fiber must stay inside the fiber until it reaches the far end. The ray must not refract into the material wrapped around the outside of the fiber because such refraction causes the loss of part of the ray’s light energy. A design for the fiber that makes the outside sur- face of the fiber act like a mirror to the light ray moving through the fiber must be achieved. If any light ray that tries to move out through the side of the fiber is reflected back into the fiber at an angle that sends it towards the far end of the fiber, this is a good pipe or wave guide for the light waves, as illustrated in Figure 3-19. Figure 3-19 Total Internal Reflection The laws of reflection and refraction tell how to design a fiber that guides the light waves through the fiber with a minimum energy loss. Two conditions are needed to cause light rays in a fiber to be reflected back into the fiber with out any loss due to refrac- tion. These two conditions are as follows: ■ The core (the inside) of the optical fiber has to have a larger (a higher) index of refraction than the material that surrounds it. The material that surrounds the core of the fiber is called the cladding. ■ The angle of incidence of the light ray is greater than the critical angle for the core and its cladding. When both of these conditions are met, all the incident light in the fiber is reflected back inside the fiber. This condition is called total internal reflection, which is the foundation on which optical fiber is constructed. Total internal reflection causes the light rays in the 1102.book Page 139 Tuesday, May 20, 2003 2:53 PM 140 Chapter 3: Networking Media fiber to bounce off the core-cladding boundary and continue their journey toward the far end of the fiber. The light follows something of a zigzag path through the core of the fiber. A fiber that meets the first condition (a core with a higher index of refraction than the cladding) can be easily created. Also, the angle of incidence of the light rays that enter the core can be controlled. Restricting two factors controls the angle of incidence: ■ The numerical aperture of the fiber. The numerical aperture of a core is the range of angles of incident light rays entering the fiber that are totally internally reflected, as illustrated in Figure 3-20. ■ The paths (called the modes) that a light ray can follow when traveling down a fiber. Figure 3-20 Numerical Aperture Controlling conditions 1 and 2 creates a fiber with total internal reflection, consequently giveing a light wave path that can be used for data communications, as shown in Figure 3-21. Figure 3-21 Light Wave Path 1102.book Page 140 Tuesday, May 20, 2003 2:53 PM Optical Media 141 Fiber-Optic Cables Fiber-optic cable is a networking medium that uses modulated light for data transmis- sions through thin strands of glass. Signals that represent data bits are converted into beams of light. It is important to recognize that while electricity is required to generate and interpret the fiber-optic signals at the end devices, no electricity is in the cable itself as there is with copper media. In fact, fiber-optic cable components are very good insu- lators. Many characteristics of fiber-optic media are superior to copper. Every fiber-optic cable used for networking consists of two glass fibers encased in sep- arate sheaths. One fiber carries transmitted data from device A to device B; the second fiber carries data from device B to device A. A fiber for data goes in each direction, similar to two one-way streets going in opposite directions. This arrangement provides a full-duplex communication link. Just as copper twisted-pair uses separate wire pairs to transmit (Tx) and receive (Rx), fiber-optic circuits use one fiber strand to transmit and one to receive, as illustrated in Figure 3-22. Typically, these two fiber cables are in a single outer jacket until they reach the point at which connectors are attached. Figure 3-22 Duplex Fiber At this point, the two fiber cables are separated. No need for twisting or shielding exists because no light escapes when it is inside a fiber, which means no crosstalk issues exist with fiber. It is common to see multiple fiber pairs encased in the same cable. This arrangement allows a single cable to be run between data closets, floors, or buildings. One cable can contain 2, 4, 8, 12, 24, 48, or more separate fibers. With copper, one UTP cable has to be pulled for each circuit. Fiber can carry many more bits per second and carry them farther than copper can. As illustrated in Figure 3-23 and Figure 3-24, five parts typically make up each fiber- optic cable: ■ The core ■ The cladding ■ A buffer ■ A strengthening material ■ An outer jacket 1102.book Page 141 Tuesday, May 20, 2003 2:53 PM 142 Chapter 3: Networking Media Figure 3-23 Five Elements of a Fiber-Optic Cable Figure 3-24 Cross Section Showing the Five Elements of a Fiber-Optic Cable The core is the light transmission element at the center of the optical fiber, and all the light signals travel through the core. This core is typically glass made from a combina- tion of silica (silicon dioxide) and other elements. Surrounding the core is the cladding, also made of silica but with a lower index of refraction than the core. Light rays travel- ing through the fiber core reflect off this core-to-cladding interface where the core and cladding meet, which keeps light in the core as it travels down the fiber. Surrounding the cladding is a buffer material, usually plastic, that helps shield the core and cladding from damage. The strengthening material surrounds the buffer, preventing the fiber cable from being stretched when installers pull it. The material used is often Kevlar, the same material used to produce bulletproof vests. The final element, the outer jacket, surrounds the cable to protect the fiber against abrasion, solvents, and other contaminants. This outer jacket composition can vary depending on the cable usage. 1102.book Page 142 Tuesday, May 20, 2003 2:53 PM Optical Media 143 The part of an optical fiber through which light rays travel is called the core of the fiber. Light rays cannot enter the core of an optical fiber at all angles. The rays can enter the core only if their angle is inside the fiber’s numerical aperture. Likewise, once the rays have entered the fiber’s core, a limited number of optical paths exist that a light ray can follow through the fiber. These optical paths are called modes. If the diameter of a fiber’s core is large enough so that many paths exist that light can take as it passes through the fiber, the fiber is called multimode fiber. Single-mode fiber has a much smaller core that allows light rays to travel along only one path (one mode) inside the fiber. Figure 3-25 illustrates the differences between multimode and single-mode fibers. Figure 3-25 Single-Mode Versus Multimode Table 3-3 compares the features of single-mode and multimode fiber. The next two sections cover the two basic types of optical fiber, multimode and single- mode, in more detail. Table 3-3 Features of Single-Mode and Multimode Fiber Single-Mode Fiber Multimode Fiber Core Features Small core (10 microns or less) Larger core than single-mode cable (50 or 62.5 microns or greater) Dispersion Characteristics Less dispersion Allows greater dispersion and, therefore, loss of signal Distance Characteristics Suited for long-distance applications (up to 3 kilo- meters [9,842 feet]) Used for long-distance application, but shorter than single-mode (up to 2 kilometers [6,560 feet]) Light Source Uses lasers as the light source often within campus back- bones for distances of several thousand meters Uses LEDs as the light source, often within LANs or distances of a couple hundred meters within a campus network 1102.book Page 143 Tuesday, May 20, 2003 2:53 PM 144 Chapter 3: Networking Media Multimode Fiber Multimode fiber allows multiple modes (paths) of light to propagate through the fiber- optic core, as compared to single-mode fiber, which allows only one mode. Multiple modes of light propagating through fiber might travel different distances, depending on their entry angles. This angle causes them to arrive at the destination (receiving end of the cable) at slightly different times—a phenomenon called modal dispersion. 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 glass 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. This design is used because a light ray following a mode that goes straight down the center of the core does not have to go as far as a ray following a mode that bounces around in the fiber. All rays should arrive at the end of the fiber together. Then, the receiver at the end of the fiber receives a strong flash of light rather than a long, dim pulse. A standard multimode fiber-optic cable (the most common type of fiber-optic cable used in LANs) uses an optical fiber with either a 62.5- or a 50-micron core and a 125-micron diameter cladding. This cable is commonly designated as 62.5/125 or 50/125 micron optical fiber. A micron is one millionth of a meter. 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. Infrared light emitting diodes (LEDs) or vertical cavity surface emitting lasers (VCSELs) are usually the light source used with multimode fiber. LEDs are a little cheaper to build and require somewhat less safety concerns than lasers. However, LEDs cannot transmit light over cable as far as the lasers. Multimode fiber (62.5/125) can carry data distances of up to 2000 meters (6560 feet). Multimode fiber is mainly used in LAN applications including backbone cabling. Lab Activity Optical Fiber Purchase In this lab, you are introduced to the variety and prices of network cabling and related components in the market. This lab looks specifically at fiber-optic patch cables and bulk fiber cable. 1102.book Page 144 Tuesday, May 20, 2003 2:53 PM Optical Media 145 Single-Mode Fiber Single-mode fiber uses only one mode of light to propagate through the fiber-optic core. In single-mode fiber-optic cabling, the core is much smaller than in multimode. The single-mode core is 8 to 10 microns in diameter. Nine micron cores are the most common. A 9/125 marking on a single-mode fiber’s jacket indicates that the core fiber has a diameter of 9 microns and the surrounding cladding is 125 microns in diameter. The size of the core in single-mode fiber leaves very little room for light to bounce around. Furthermore, a very focused infrared laser is used as the light source in single- mode fiber. The ray of light it generates enters the core at a 90-degree angle. As a result the data carrying light ray pulses in single-mode fiber are essentially transmitted in a straight line right down the middle of the core, as shown in Figure 3-26. This greatly increases both the speed and the distance that data can be transmitted. Figure 3-26 Single-Mode Fiber Because of its design, single-mode fiber is capable of higher rates of data transmission (bandwidth) and greater cable run distances than multimode fiber. Single-mode fiber can carry LAN data up to 3000 meters. Multimode is only capable of up to 2000 meters. Lasers and single-mode fibers are more expensive than LEDs and multimode fiber. Because of these characteristics, single-mode fiber is often used for interbuilding con- nectivity or WANs (for example, telephone company network connections). Figure 3-27 compares the relative sizes of the core and cladding for both types of fiber- optic cable in different sectional views. The much smaller and more refined fiber core in single-mode fiber, although it entails more manufacturing costs, is the reason single- mode has a higher bandwidth and cable run distance than multimode fiber. Figure 3-27 Single-Mode and Multimode Fiber WARNING Be aware that the laser light used with single-mode has a longer wavelength than can be seen, and it is so strong that it can seriously damage eyes. Never look at the end of a fiber that is connected to a device at its far end. Never look into the transmit port on a NIC, switch, or router. There is nothing that can be seen anyway. Remem- ber to keep protective covers over the ends of fiber and inserted into the fiber-optic ports of switches and routers. Be very careful! 1102.book Page 145 Tuesday, May 20, 2003 2:53 PM 146 Chapter 3: Networking Media The following summarizes the features of fiber-optic cables: ■ Speed and throughput—More than 1 Gbps ■ Average cost per node—Expensive ■ Media and connector size—Small ■ Maximum cable length—More than 10 kilometers (km) for single mode; up to 2 km for multimode Cable Designs As illustrated in Figure 3-28, two basic cable designs exist: ■ Loose-tube ■ Tight-buffered Figure 3-28 Loose-Tube Versus Tight-Buffer Construction Tight-buffered cables have the buffering material that surrounds the cladding in direct contact with the cladding. The main practical difference between the two designs is the applications for which they are used. Loose-tube cable is primarily used for outside- building installations, while tight-buffered cable is used inside buildings. Most of the fiber used in LANs is tight-buffered multimode cable. 1102.book Page 146 Tuesday, May 20, 2003 2:53 PM Optical Media 147 Other Optical Networking Components Most of the data sent over a LAN is in the form of electrical signals. But, optical fiber links use light to send data. So, something is needed to convert the electricity to light and, at the other end of the fiber, convert the light back to electricity. This situation means that two devices are always required: a transmitter and a receiver. In addition to transmitters and receivers, this section also discusses different types of fiber-optic cable connectors as well as devices used in optical networking. Transmitters The transmitter receives data to be transmitted from switches and routers. This data is in the form of electrical signals. The transmitter converts the electronic signals into their equivalent light pulses. Two types of light sources encode and transmit the data through the cable. These two sources are ■ Light emitting diodes (LEDs)—An LED produces infrared light with wavelengths of either 850 or 1310 nm. These are used with multimode fiber in LANs. Lenses are used to focus the infrared light on the end of the fiber. LEDs have fewer safety concerns. ■ Light amplification by stimulated emission radiation (Laser)—Laser is a light source producing a thin beam of intense infrared light usually with wavelengths of 1310 or 1550 nm. Lasers are used with single-mode fiber over the longer dis- tances involved in WANs or campus backbones. Exercise extra care to prevent eye injury. Each light source can be lighted and darkened quickly to send data 1s and 0s at a high number of bits per second. Receiver At the other end of the optical fiber from the transmitter is the receiver. It functions something like the photoelectric cell in a solar-powered calculator. When light strikes the receiver, it produces electricity. The first job of the receiver is to detect a light pulse that arrives from the fiber. Then the receiver converts the light pulse back into the orig- inal electrical signal that first entered the transmitter at the far end of the fiber. Now the signal is again in the form of voltage changes. The signal is ready to be sent over copper wire into any receiving electronic device such as a computer, switch, or router. The semiconductor devices that are usually used as receivers with fiber-optic links are called p-intrinsic-n diodes (PIN photodiodes). PIN photodiodes are manufactured to be sensitive to the particular wavelength of light (850, 1310, or 1550 nm) generated by the transmitter at the far end of the fiber. When 1102.book Page 147 Tuesday, May 20, 2003 2:53 PM 148 Chapter 3: Networking Media struck by a pulse of light at the proper wavelength, the PIN photodiode quickly produces an electric current of the proper voltage for the network. It instantly stops producing the voltage when no light strikes the PIN photodiode. This process generates the voltage changes that represent the data 1s and 0s on a copper cable. Connectors Connectors are attached to the fiber ends so that the fibers can be connected to the ports on the transmitter and receiver. The most common type of connector used with multi- mode fiber is the subscriber connector (SC ), as shown in Figure 3-29. On single-mode fiber, the straight tip (ST) connector, as shown in Figure 3-30, is frequently used. With SC and ST, there is one connector for each fiber. Newer connectors combine the send and receive fibers into one modular connector, comparable in size to an RJ-45, to save space. Figure 3-29 SC Connector Figure 3-30 ST Connector Optical Amplifiers and Fiber Patch Panels In addition to the transmitters, receivers, connectors, and fibers that are always required on an optical network, you can sometimes see several other devices on an optical fiber network. Repeaters are optical amplifiers that receive attenuating light pulses traveling long distances and restore them to their original shapes, strengths, and timings. Then the restored signals can be sent on along the journey to the receiver at the far end of the fiber. 1102.book Page 148 Tuesday, May 20, 2003 2:53 PM . communications, as shown in Figure 3- 21 . Figure 3- 21 Light Wave Path 11 02. book Page 14 0 Tuesday, May 20 , 20 03 2: 53 PM Optical Media 14 1 Fiber-Optic Cables Fiber-optic cable is a networking medium that uses. the particular wavelength of light (850, 13 10, or 15 50 nm) generated by the transmitter at the far end of the fiber. When 11 02. book Page 14 7 Tuesday, May 20 , 20 03 2: 53 PM 14 8 Chapter 3: Networking. abrasion, solvents, and other contaminants. This outer jacket composition can vary depending on the cable usage. 11 02. book Page 14 2 Tuesday, May 20 , 20 03 2: 53 PM Optical Media 14 3 The part of an optical