CCNA 1 and 2 Companion Guide, Revised (Cisco Networking Academy Program) part 22 ppsx

Check Your Understanding 179 13. Which fiber-optic cable type is typically used for outside installations? A. Tight-buffered B. Tight-tube C. Loose-buffered D. Loose-tube 14. What is the light source typically used in single mode fiber optics? A. Photo transistor B. Laser C. Photo resistor D. LED 15. Which term describes the angle at which a ray hits a glass surface? A. Angle of reflection B. Angle of refraction C. Angle of incidence D. Angle of attack 16. Modulation is a process of changing amplitude, frequency, or phase. Which acronym does not represent a type of modulation? A. AM B. FM C. PM D. RM 17. Which range does not correctly identify an unlicensed wireless frequency? A. 2.4 GHz B. 5 GHz C. 9 GHz D. 900 MHz 18. Which statement does not describe a benefit of spread spectrum? A. Spread-spectrum transmissions are transmitted at high speeds. B. Spread spectrum is less susceptible to radio noise. C. Spread spectrum has a higher probability of correct reception. D. Spread spectrum creates little interference. 1102.book Page 179 Tuesday, May 20, 2003 2:53 PM 180 Chapter 3: Networking Media 19. Which statement does not describe the features of direct-sequence spread spectrum (DSSS)? A. DSSS is reliable because each bit is represented by a string of 1s and 0s. B. If up to 40 percent of the string is lost, the original transmission can be reconstructed. C. DSSS technology has low throughput of data and short-range access. D. The recently released evolution of the IEEE standard, 802.11b, provides for a full Ethernet-like data rate of 11 Mbps over DSSS. 20. Which of the following is not a feature of wired equivalent privacy (WEP)? A. WEP uses the RC4 stream cipher for encryption. B. WEP is a security mechanism defined within in the 802.3 standards. C. One of the goals of WEP is to deny access to the network by unauthorized users who do not possess the appropriate WEP key. D. None of the above. 1102.book Page 180 Tuesday, May 20, 2003 2:53 PM 1102.book Page 181 Tuesday, May 20, 2003 2:53 PM Objectives Upon completion of this chapter, you will be able to ■ Become familiar with the basic terminology used for frequency-based cable testing ■ Understand what signals and noise impact networking media ■ Properly cable a LAN ■ Properly cable a WAN chpt_04.fm Page 182 Tuesday, May 27, 2003 9:01 AM Chapter 4 Cable Testing and Cabling LANs and WANs This chapter describes issues relating to the testing of media used for physical layer con- nectivity in local-area networks (LANs). In order for the LAN or WAN to function properly, the physical layer medium must meet the industry standards specified for the data rate used to transmit signals over Ethernet (10, 100, 1000, or 10,000 Mbps). The use of signals in this text refers to the data signals that move from the transmitter to the receiver. The signals weaken (attenuate) traveling over the physical media; however, the receiver must still be able to clearly determine the state of each bit of the data (one or zero). Otherwise, the error rate on the network will be too high for the LAN or WAN connections to be useful. Networking media is literally and physically the backbone of a network. Inferior quality network cabling results in network failures and in networks with unreliable performance. All three categories of networking media (copper-based, optical fiber, and wireless) require testing and measurement to determine their quality, and this testing is the primary subject of this chapter. Please be sure to look at this chapter’s associated e-Lab Activities, Videos, and PhotoZooms that you will find on the CD-ROM accompanying this book. These CD elements are designed to supplement the material and reinforce the concepts introduced in this chapter. chpt_04.fm Page 183 Tuesday, May 27, 2003 9:01 AM 184 Chapter 4: Cable Testing and Cabling LANs and WANs Background for Studying Frequency-Based Cable Testing The equipment used to perform quality testing and measurement of copper-based, optical fiber, and wireless networking media involves certain electrical and mathematical concepts and terms, such as signal, wave, frequency, and noise. Understanding this vocabulary is helpful when learning about networking, cabling, and cable testing. Waves A wave is energy traveling from one place to another. Many types of waves exist, but all can be described with similar vocabulary. It is helpful to think of waves as disturbances. A bucket of water that is completely still, with no disturbances, does not have waves. Conversely, the ocean always has some sort of detectable waves due to disturbances such as wind and tide. Ocean waves can be described in terms of their height, or amplitude, which can be measured in meters. They can also be described in terms of how frequently the waves reach the shore. This feature can be described in two similar ways: period and frequency. The period of the waves is the amount of time between each wave and is measured in seconds. The frequency is the number of waves that reach the shore each second. The amplitude of an electrical signal represents its height just as in ocean waves, but it is measured in volts instead of meters. If the signal repeats itself regularly, then the period of the signal is the amount of time to complete one cycle of the signal, and is measured in seconds, just as the period of ocean waves is the amount of time for one wave to complete. The frequency of an electrical signal is the number of complete cycles (or waves) per second and is measured in hertz. If a disturbance is deliberately caused and involves a fixed, predictable duration, it is called a pulse. Pulses are important in electrical signals because they determine the value of the data being transmitted. Figure 4-1 shows a representation of the concepts of amplitude and frequency. Networking professionals are interested in specific types of waves: ■ Voltage waves on copper media ■ Light waves in optical fiber ■ Alternating electric and magnetic fields called electromagnetic waves used in the wireless environment chpt_04.fm Page 184 Tuesday, May 27, 2003 9:01 AM Background for Studying Frequency-Based Cable Testing 185 Figure 4-1 Amplitude and Frequency Sine Waves and Square Waves Sine waves, or sinusoids, are graphs of mathematical functions as shown in Figure 4-2. Figure 4-2 Analog Signals Sine waves have certain characteristics: ■ They are periodic (which means that they repeat the same pattern at regular intervals). ■ They are continuously varying (which means that no two adjacent points on the graph have the same value). A = Amplitude (Height or Depth of Wave) T = Period (Time to Complete 1 Wave Cycle) F = Frequency (Cycles Per Second) = 1/T V T A A t • Continuous Voltage • “Wavy” Voltage as Time Progresses • Many Encodings Possible chpt_04.fm Page 185 Tuesday, May 27, 2003 9:01 AM 186 Chapter 4: Cable Testing and Cabling LANs and WANs Sine waves are graphical representations of many natural occurrences that change reg- ularly over time, such as the distance from the earth to the sun, the distance from the ground while riding a Ferris wheel, and the time of day that the sun rises. Since sine waves are continuously varying, they are examples of analog waves. Square waves, like sine waves, are periodic. However, square wave graphs do not con- tinuously vary with time. The values remain the same for some time, then suddenly change, then remain the same, and then suddenly return to the initial value, as shown in Figure 4-3. Square waves represent digital signals, or pulses. Square waves, like all waves, can be described in terms of amplitude, period, and frequency. Figure 4-3 Digital Signals Exponents and Logarithms As discussed in Chapter 1, “Introduction to Networking,” in networking, remember three important number systems: ■ Base 2 (binary) ■ Base 10 (decimal) ■ Base 16 (hexadecimal) Recall that the base of a number system refers to the number of different symbols that can occupy one place. For example, binary numbers have only two different place- holders (the numbers 0 and 1), decimal numbers have 10 different place holders (the numbers 0–9), and hexadecimal numbers have 16 different placeholders (the numbers 0–9 and the letters A through F). Remember that 10 * 10 can be written as 10 2 (10 “squared,” or 10 raised to the second power, or 10 multiplied by itself 2 times. 10 * 10 * 10 can be written as 10 3 (ten “cubed,” A = Amplitude (Height of Pulses) A1 1 10 0 • Discrete Pulses (Not Continuous) • Can Only Have One of Two States (1/0, On/Off) • Voltage Jumps Between Levels chpt_04.fm Page 186 Tuesday, May 27, 2003 9:01 AM Background for Studying Frequency-Based Cable Testing 187 or 10 raised to the third power, or 10 multiplied by itself 3 times). When written this way, you say that 10 is the base of the number and 2 or 3 is the exponent of the num- ber. The following example demonstrates the preceding concepts. The base of a number system also refers to the value of each digit. The least significant digit has a value of base 0 (base raised to the zero power), or one. The next digit has a value of base 1 . This is equal to 2 for binary numbers, 10 for decimal numbers, and 16 for hexadecimal numbers. Numbers with exponents are used to more easily represent very large numbers. It is much easier and less error prone to represent one billion numerically as 10 9 than as 1,000,000,000. Many calculations involved in cable testing involve numbers that are very large, so exponents are the preferred format. Decibels An important way of describing networking signals is a unit of measure called the decibel (dB). The decibel is related to the exponents and logarithms described in prior sections. The formulas for calculating decibels are as follows: dB = 10 log 10 (P final / P ref ) or dB = 20 log 10 (V final / V reference ) Typically, light waves on optical fiber and radio waves in the air are measured using the power formula, and electromagnetic waves on copper cables are measured using the voltage formula. In these formulas, ■ dB measures the loss or gain of the power of a wave. Decibels are usually negative numbers representing a loss in power as the wave travels, but can also be positive values representing a gain in power if the signal is amplified. ■ log 10 indicates that the number in parenthesis is transformed using the base 10 logarithm rule. ■ P final is the delivered power measured in watts. ■ P ref is the original power measured in watts. y = 10 x y = 10 x x: 2 x: 3 y: 100 y: 1000 chpt_04.fm Page 187 Tuesday, May 27, 2003 9:01 AM 188 Chapter 4: Cable Testing and Cabling LANs and WANs ■ V final is the delivered voltage measured in volts. ■ V ref is the original voltage measured in volts. The following example illustrates how the dB value is calculated: P final = P ref * 10 (dB/10) dB = 20 P ref = 2 kilowatts P final = 200 kilowatts When you enter values for dB and P ref , the resulting power changes. This calculation can be used to see how much power is left in a radio wave after it has traveled over a distance, through different materials, and through various stages of electronic systems such as a radio. Viewing Signals in Time and Frequency One of the most important facts of the “information age” is that data symbolizing characters, words, pictures, video, or music can be represented electrically by voltage patterns on wires and in electronic devices. The data represented by these voltage patterns can be converted to light waves or radio waves and back to voltage waves. Consider the example of an analog telephone. The sound waves of the caller’s voice enter a microphone in the telephone. The microphone converts the patterns of sound energy into voltage patterns of electrical energy that represent the voice. If the voltage patterns are graphed over time, the distinct patterns representing the voice are displayed. An oscilloscope is an important electronic device used to view electrical signals, such as voltage waves and pulses. The x-axis on the display repre- sents time, and the y-axis represents voltage or current. There are usually two y-axis inputs, so two waves can be observed and measured at the same time. Analyzing signals using an oscilloscope is called time-domain analysis because the x-axis or domain of the mathematical function represents time. Engineers also use frequency-domain analysis to study signals. In frequency-domain analysis, the x-axis represents frequency. An electronic device called a spectrum analyzer creates graphs for frequency-domain analysis. Figure 4-4 illustrates several signals of how the output looks on both the oscilloscope and the spectrum analyzer. chpt_04.fm Page 188 Tuesday, May 27, 2003 9:01 AM . through F). Remember that 10 * 10 can be written as 10 2 (10 “squared,” or 10 raised to the second power, or 10 multiplied by itself 2 times. 10 * 10 * 10 can be written as 10 3 (ten “cubed,” A. power measured in watts. y = 10 x y = 10 x x: 2 x: 3 y: 10 0 y: 10 00 chpt_04.fm Page 18 7 Tuesday, May 27 , 20 03 9: 01 AM 18 8 Chapter 4: Cable Testing and Cabling LANs and WANs ■ V final is the. appropriate WEP key. D. None of the above. 11 02. book Page 18 0 Tuesday, May 20 , 20 03 2: 53 PM 11 02. book Page 18 1 Tuesday, May 20 , 20 03 2: 53 PM Objectives Upon completion of this chapter, you will be able