Optical Media 149 Fiber-optic patch panels, as shown in Figure 3-31, are similar to the patch panels used with copper cable. These panels increase the flexibility of an optical network by allow- ing quick changes to the connection of devices like switches or routers with various available fiber runs (cable links). Figure 3-31 Fiber-Optic Patch Panels Signals and Noise in Optical Fiber Fiber-optic cable is not affected by the sources of external noise that cause problems on copper media. Why? Because external light cannot enter the fiber except at the transmitter end. The cladding is covered by a buffer and an outer jacket that stops light from entering or leaving the cable. Furthermore, the transmission of light on one fiber in a cable does not generate inter- ference that disturbs transmission on any other fiber, which means that fiber does not have the problem with crosstalk that copper media does. In fact, the quality of fiber-optic links is so good that the recent standards for Gigabit and 10-Gigabit Ethernet specify transmission distances that far exceed the traditional 2-kilometer reach of the original Ethernet. (You learn more about the Ethernet technologies in Chapter 6, “Ethernet Technologies and Ethernet Switching”.) Fiber-optic transmission allows the Ethernet protocol to be used on metropolitan-area networks (MANs) and WANs. Although fiber is the best of all the transmission media at carrying large amounts of data over long distances, fiber is not without problems. When light travels through fiber, some of the light energy is lost. The farther a light signal travels through a fiber, 1102.book Page 149 Tuesday, May 20, 2003 2:53 PM 150 Chapter 3: Networking Media the more it loses strength. This attenuation of the signal is due to several factors involving the nature of fiber itself. The most important factor is scattering. The scat- tering of light in a fiber is caused by microscopic non-uniformity (distortions) in the fiber that reflects and scatters some of the light energy, as shown in Figure 3-32. Figure 3-32 Scattering Absorption is another cause of light energy loss. When a light ray strikes some types of chemical impurities in a fiber, the impurities absorb part of the ray’s energy. This light energy is converted to a small amount of heat energy. Absorption makes the light signal a little dimmer. Another factor that causes attenuation of the light signal is manufacturing irregularities or roughness in the core-to-cladding boundary. Power is lost from the light signal as a result of the less than perfect total internal reflection in that rough area of the fiber. If there are any microscopic imperfections in the thickness or symmetry of the fiber, it cuts down on total internal reflection, and some light energy is absorbed by the cladding. Dispersion of a light flash limits transmission distances on a fiber. Dispersion is the technical term for the spreading of pulses of light as they travel down the fiber, as shown in Figure 3-33. Figure 3-33 Dispersion Graded index multimode fiber is designed to compensate for the different distances the various modes of light have to travel in the large diameter core. Single-mode fiber does not have the problem of multiple paths that the light signal can follow. Chromatic 1102.book Page 150 Tuesday, May 20, 2003 2:53 PM Optical Media 151 dispersion, however, is a characteristic of both multimode and single-mode fiber. Some wavelengths of light travel at slightly different speeds through glass than do other wave- lengths. This discrepancy causes chromatic dispersion. That is why a prism separates the wavelengths of light. Ideally, an LED or laser light source emits light of just one frequency. Then, chromatic dispersion is not a problem. Unfortunately, lasers and, especially, LEDs generate a range of wavelengths so chromatic dispersion limits the distance you can transmit on a fiber. If you try to transmit a signal too far, what started as a bright pulse of light energy is spread out, separated, and dim when it reaches the receiver. The receiver is not able to distinguish a 1 from a 0. Installation, Care, and Testing of Optical Fiber A major cause of too much attenuation in fiber-optic cable is improper installation. If the fiber is stretched or curved too tightly, it can cause tiny fissures (cracks) in the core that scatter the light rays. Bending the fiber in too tight a curve can change the incident angle of light rays striking the core-to-cladding boundary. Then, the ray’s incident angle becomes less than the critical angle for total internal reflection. Instead of reflecting around the bend, some light rays refract into the cladding and are lost. There are two types of bending: ■ Macrobending—A macrobend is a bend you can see. When you bend fiber, you can cause some of the light rays to exceed the critical angle, allowing light to leak out of the core and into the cladding. When light is in the cladding, it cannot easily get back into the core; it then leaks out through the buffer, as shown in Figure 3-34. ■ Microbending—Microbending produces the same effect as macrobending; it causes the light to exceed the critical angle and leak out of the core, as shown in Figure 3-34. It occurs on a microscopic scale and is not visible to the eye. Figure 3-34 Macrobending and Microbending NOTE Microbending can also be caused by extreme temperature swings in installed cable when the differ- ent materials in the cable structure expand and contract at different rates. This expansion and con- traction causes the fiber to be squeezed or stretched, which causes microbending. 1102.book Page 151 Tuesday, May 20, 2003 2:53 PM 152 Chapter 3: Networking Media To prevent fiber bends that are too sharp, fiber is usually pulled through a type of installed pipe called interducting. The interducting is much stiffer than fiber and can- not be bent so sharply that the fiber inside the interducting has too tight a curve. The interducting protects the fiber, makes it easier to pull the fiber, and ensures that the bending radius (curve limit) of the fiber is not exceeded. When the fiber has been pulled, the ends of the fiber must be cleaved (cut) and prop- erly polished to ensure that the ends are smooth. Figure 3-35 illustrates the problems with improper fiber end face finishes, and Figure 3-36 illustrates the proper fiber end face polishing techniques. Figure 3-35 Fiber End Face Finishes Figure 3-36 Fiber End Face Polishing Techniques 1102.book Page 152 Tuesday, May 20, 2003 2:53 PM Optical Media 153 A microscope or test instrument with a built-in magnifier is used to examine the end of the fiber and verify that it is properly polished and shaped. Then, the connector is care- fully attached to the fiber end. Improperly installed connectors, improper splices, or the splicing of two cables with different core sizes dramatically reduces the strength of a light signal. Figure 3-37 illustrates the splicing of a 62.5 micron fiber to a 50 micron fiber. Figure 3-37 Splicing of Different Types of Fiber After the fiber-optic cable and connectors are installed, the connectors and the ends of the fibers must be kept spotlessly clean. The ends of the fibers should be covered with protective covers to prevent damage to the fiber ends. When these covers are removed prior to connecting the fiber to a port on a switch or router, the fiber ends must be cleaned. Clean the fiber ends with lint-free lens tissue moistened with pure isopropyl alcohol. The fiber ports on a switch or router should also be kept covered when not in use and cleaned with lens tissue and isopropyl alcohol before a connection is made. Dirty ends on a fiber cause a big drop in the amount of light that reaches the receiver. All these factors, scattering, absorption, dispersion, improper installation, and dirty fiber ends diminish the strength of the light signal and are referred to as fiber noise. Before using a fiber-optic cable, it must be tested to ensure that enough light actually reaches the receiver for it to detect the 0s (off) and 1s (on) in the signal. When a fiber-optic link is being planned, the amount of signal power loss that can be tolerated must be calculated. This tolerance is referred to as the optical link loss bud- get. It is like your monthly financial budget. After all your expenses (attenuations) are subtracted from your initial income, enough money must be left to get you through the month. The decibel (dB) is the unit used to measure the amount of power loss. It tells what percent of the power that leaves the transmitter actually enters the receiver. Testing fiber links is extremely important, and records of the results of these tests must be kept. Several types of fiber-optic test equipment are used. Two of the most important instruments are Optical Loss Meters and Optical Time Domain Reflectometers (OTDRs). 1102.book Page 153 Tuesday, May 20, 2003 2:53 PM 154 Chapter 3: Networking Media These meters both test optical cable to ensure that the cable meets the TIA standards for fiber. They also test to verify that the link power loss does not fall below the optical link loss budget. OTDRs can provide a lot of detailed diagnostic information about a fiber link and can be used to troubleshoot a link when problems occur. Wireless Communications Wireless signals are electromagnetic waves that can travel through the vacuum of outer space or through a medium such as air. No physical copper-based or fiber-optic medium is necessary for wireless signals, which makes utilizing wireless signals a very versatile way to build a network. Wireless transmissions can cover large distances by using high-frequency signals. Each signal uses a different frequency measured in hertz so that they remain unique from one another. Wireless technologies have been around for many years. Satellite TV, AM/FM radio, cellular phones, remote-control devices, radar, alarm systems, weather radios, cordless phones, and retail scanners are integrated into everyday life. Today, wireless technolo- gies are a fundamental part of business and personal life. Wireless Data Communications The radio spectrum is the part of the electromagnetic spectrum used to transmit voice, video, and data. It uses frequencies from 3 kilohertz (kHz) to 300 gigahertz (GHz). This section considers only the part of the radio spectrum that supports wireless data transmission. Many different types of wireless data communications exist, as illustrated in Figure 3-38. Lab Activity Fiber Optic Purchase In this lab, you are introduced to the variety and prices of cabling and related components in the market. This lab looks specifically at fiber-optic patch cables and bulk fiber cable. 1102.book Page 154 Tuesday, May 20, 2003 2:53 PM Wireless Communications 155 Figure 3-38 Wireless Data Networks Each type of wireless data communication has its advantages and drawbacks, as follows: ■ Infrared (IR)—Very high data rates and lower cost, but very short distance. ■ Narrowband—Low data rates and medium cost. Requires a license and covers a limited distance. ■ Spread spectrum—Medium cost and high data rates. Limited to campus coverage. Cisco Aironet products are spread spectrum. ■ Broadband personal communications service (PCS)—Low data rates, medium cost, and citywide coverage. Sprint is an exception; Sprint PCS provides nation- wide and international coverage. ■ Circuit and packet data (cellular data and Cellular Digital Packet Data [CDPD])—Low data rates, high packet fees, and national coverage. ■ Satellite—Low data rates, high cost, and nationwide or worldwide coverage. Wireless Signal When a signal is transmitted in a data format, you must consider the following three parameters: ■ How fast—What data rate can be achieved? ■ How far—How far can wireless LAN (WLAN) units be placed apart and still get the maximum data rate? ■ How many—How many users can exist without slowing the data rate? 1102.book Page 155 Tuesday, May 20, 2003 2:53 PM 156 Chapter 3: Networking Media These parameters all relate to the ability to receive a good signal as far away as possible. Increasing the amount of data requires the use of more frequency spectra or a different method of placing the data on the radio frequency (RF) signal. RF efficiency is affected by the following three factors, as shown in Figure 3-39: ■ Type of modulation used—More complex modulation techniques provide greater throughput. ■ Distance—The farther the signal must be transmitted, the weaker the signal becomes. ■ Noise—Electronic noise and barriers negatively affect RF. Figure 3-39 Factors Affecting RF Efficiency The following sections discuss these three factors in greater detail. Modulation Modulation is the process by which the amplitude, frequency, or phase of an RF or light wave is altered to transmit data. The characteristics of the carrier wave instanta- neously are varied by another modulating waveform. Modulation blends a data signal (text, voice, and so on) into a carrier for transmission over a network. The most common methods of modulation are as follows (see Figure 3-40): ■ Amplitude modulation (AM)—Modulates the height of the carrier wave ■ Frequency modulation (FM)—Modulates the frequency of the wave ■ Phase modulation (PM)—Modulates the polarity (phase) of the wave 1102.book Page 156 Tuesday, May 20, 2003 2:53 PM Wireless Communications 157 Figure 3-40 Modulation Effects of Distance on a Signal As a receiver moves farther from a transmitter, the signal gets weaker, and the differ- ence between the signal and noise becomes less. Eventually, the signal cannot be distin- guished from the noise, and loss of communication occurs. The amount of compression (or modulation scheme) at which the signal is transmitted determines the amount of signal needed to be heard through the noise. As transmission, or modulation schemes (compression), becomes more complex and data rates increase, immunity to noise less- ens. Therefore, the distance is reduced. Effects of Noise on a Signal Electronic noise and barriers negatively affect RF efficiency. An exact transmission dis- tance for WLAN products cannot be provided without going to the site and actually testing the environment. Walls with internal metal structures, for example, greatly limit RF transmission range. Radio Frequency Bands Most radio frequencies are licensed by government agencies, such as the Federal Com- munications Commission (FCC) in the United States. To broadcast over these frequencies, you need to have a license and to pay a fee. Unlicensed frequency bands are easier to implement and cost less over time because they do not require licenses. Three unlicensed bands exist, as illustrated in Figure 3-41: ■ 900 megahertz (MHz)—The 900-MHz band carries cordless and cellular phones. ■ 2.4 gigahertz (GHz)—The 802.11b standard, the most widely deployed wireless standard, operates in the 2.4-GHz unlicensed radio band, delivering a maximum data rate of 11 Mbps. NOTE To be received cor- rectly, complex modu- lation schemes require optimal signal-to-noise ratios (more signal with less noise). If there is noise on the channel, the line speed is reduced. Noise, speed, and distance are all interrelated. 1102.book Page 157 Tuesday, May 20, 2003 2:53 PM 158 Chapter 3: Networking Media ■ 5 GHz—Recently, the FCC opened up the 5-GHz band for unlicensed use by high-speed data communications devices. Cisco has acquired 5-GHz technology and uses this frequency in new products, such as Cisco Aironet 1200 series, which is dual band, delivering support for both 2.4 GHz (802.11b) and 5 GHz (802.11a) standards. The 802.11a standard can deliver a maximum data rate of 54 Mbps. Figure 3-41 Unlicensed Frequency Bands A relationship exists between the frequency and the amount of data that can be sent. The concept is like that of a pipe. The wider the bandwidth is, the more frequencies are available. The wider the spectrum is, the higher the data rate can be transmitted. The amount of spectrum available determines the data rate. Because the 900-MHz band supports cellular phones and other consumer products, the band has become overcrowded. As a result, users often experience interference or cannot access the network. As a benefit, 900 MHz offers longer range (for the same gain antennas) than 2.4 GHz. The drawback of 900 MHz is that the fastest, most reli- able data rate is only 1 Mbps because of its limited frequency range. The 2.4-GHz frequency range is much wider than 900 MHz, allowing higher data rates with a reliable range of up to 25 miles. The Cisco Aironet 340 Wireless LAN Series can deliver 11-Mbps throughput because it operates in the 2.4-GHz frequency. Cisco has acquired 5-GHz technology and will deliver products for the 5-GHz frequency range because its wider bandwidth allows for faster throughput of data. The Cisco Aironet 5 GHz 54 Mbps Wireless LAN client adapter is an IEEE 802.11a-compliant CardBus adapter that operates in the UNII-1 and UNII-2 bands. The client adapter 1102.book Page 158 Tuesday, May 20, 2003 2:53 PM . and uses this frequency in new products, such as Cisco Aironet 12 00 series, which is dual band, delivering support for both 2. 4 GHz (8 02 .11 b) and 5 GHz (8 02 .11 a) standards. The 8 02 .11 a standard. placed apart and still get the maximum data rate? ■ How many—How many users can exist without slowing the data rate? 11 02. book Page 15 5 Tuesday, May 20 , 20 03 2: 53 PM 15 6 Chapter 3: Networking. Noise, speed, and distance are all interrelated. 11 02. book Page 15 7 Tuesday, May 20 , 20 03 2: 53 PM 15 8 Chapter 3: Networking Media ■ 5 GHz—Recently, the FCC opened up the 5-GHz band for unlicensed