1. © 2009 Pearson Education, Inc. Publishing as Prentice Hall Physical Layer Propagation: UTP and Optical Fiber Chapter 3 Panko’s Business Data Networks and Telecommunications, 7th edition © 2009 Pearson Education, Inc. Publishing as Prentice Hall May only be used by adopters of the book

2. © 2009 Pearson Education, Inc. Publishing as Prentice Hall 3-22 Orientation Chapter 2 –Data link, internet, transport, and application layers –Characterized by message exchanges Chapter 3 –Physical layer (Layer 1) –There are no messages—bits are sent individually –Concerned with transmission media, plugs, signaling methods, propagation effects –Chapter 3: Signaling, UTP, optical fiber, radio, and topologies 1

3. © 2009 Pearson Education, Inc. Publishing as Prentice Hall 3-3 3-1: Signal and Propagation 3 A signal is a disturbance in the media that propagates (travels) down the transmission medium to the receiver If propagation effects are too large, the receiver will not be able to read the received signal

4. © 2009 Pearson Education, Inc. Publishing as Prentice Hall Binary Data Representation

5. © 2009 Pearson Education, Inc. Publishing as Prentice Hall 3-55 Binary-Encoded Data Computers store and process data in binary representations –Binary means “two” –There are only ones and zeros –Called bits 1101010110001110101100111

6. © 2009 Pearson Education, Inc. Publishing as Prentice Hall 3-66 Binary-Encoded Data Non-Binary Data Must be Encoded into Binary –Text –Integers (whole numbers) –Decimal numbers –Alternatives (North, South, East, or West, etc.) –Graphics –Human voice –etc. Hello11011001…

7. © 2009 Pearson Education, Inc. Publishing as Prentice Hall 3-77 Binary-Encoded Data Some data are inherently binary –48-bit Ethernet addresses –32-bit IP addresses –Need no further encoding

8. © 2009 Pearson Education, Inc. Publishing as Prentice Hall 3-88 3-2: Arithmetic with Binary Numbers Binary Arithmetic for Whole Numbers (Integers) (Counting Begins with 0, not 1) Integer 0 1 2 3 4 5 6 7 8 Binary 0 1 10 11 100 101 110 111 1000 “There are 10 kinds of people— those who understand binary and those who don’t”

9. © 2009 Pearson Education, Inc. Publishing as Prentice Hall 3-99 3-2: Arithmetic with Binary Numbers Binary Arithmetic for Binary Numbers 1 0 0 1 1 +1 +0+1+0 +1 +1 =0=1=1=10=11 Basic Rules

10. © 2009 Pearson Education, Inc. Publishing as Prentice Hall 3-1010 3-2: Arithmetic with Binary Numbers Binary Decimal 1000 8 +1+1 =1001=9 +1+1 =1010 =10 +1+1 =1011 =11 +1 +1 =1100 =12 Examples

11. © 2009 Pearson Education, Inc. Publishing as Prentice Hall 3-1111 Powers of 2 BitsAlternatives 12 24 38 416 532 664 7128 8256 101,024 1665,536 An N-bit field can represent 2 N alternatives. Each additional bit doubles the number of possibilities Start with one you know and double or halve until you have what you need E.g., if you know 8 is 256, 10 must be 4 times as large or 1,024.

12. © 2009 Pearson Education, Inc. Publishing as Prentice Hall 3-12 3-3: Binary Encoding for a Number of Alternatives 12 Number of Bits in Field Number of Alternatives that Can Be Encoded 1 Specific Bit Sequences Example 12 1 = 20, 1Yes or No, Male or Female, etc. 22 2 = 400, 01, 10, 11North, South, East, West 42 4 = 160000, 0001, 0010, … Top 10 security threats (6 values go unused) 82 8 = 25600000000, 00000001, … ASCII text representation (128 values go unused) 1 There are 2 N alternatives with N bits.

13. © 2009 Pearson Education, Inc. Publishing as Prentice Hall 3-13 3-3: Binary Encoding for a Number of Alternatives Examples: –1. You have 4 bits. How many alternatives can your represent? –2. You need to represent 8 things. How many bits must you use? –3. You need to represent 6 things. How many bits must you use? 13

14. © 2009 Pearson Education, Inc. Publishing as Prentice Hall 3-14 3-4: ASCII and Extended ASCII (Study Purpose –To represent text (A, a, 3, $, etc.) as binary data for transmission ASCII –Traditional code to represent text data in binary –Seven bits per character –2 7 (128) characters possible –Sufficient for all keyboard characters (including shifted values) 14

15. © 2009 Pearson Education, Inc. Publishing as Prentice Hall 3-15 3-4: ASCII and Extended ASCII ASCII –Sufficient for all keyboard characters 15 CategoryMeaningASCII Capital lettersA1000001 Lower-case lettersa1100001 Digits30110011 Punctuation.0101110 Special characters@1000000 Space0100000 Printing controlCarriage Return0001101 Printing controlLine feed0001010

16. © 2009 Pearson Education, Inc. Publishing as Prentice Hall 3-16 3-4: ASCII and Extended ASCII Each ASCII Character is Sent in a Byte –8 th Bit in Data Bytes Normally is Not Used 16 10100111 Data Byte ASCII Code for Character Unused. Value does not matter.

17. © 2009 Pearson Education, Inc. Publishing as Prentice Hall 3-17 3-4: ASCII and Extended ASCII To send “Hello world!” (without the quotes), how many bytes will you have to transmit? 17

18. © 2009 Pearson Education, Inc. Publishing as Prentice Hall 3-18 3-4: ASCII and Extended ASCII Extended ASCII –Used on PCs –8 bits per character –2 8 (256) characters possible –Extra characters can represent formatting in word processing, etc. Text-to-ASCII and Text-to-Extended ASCII Calculators –Readily available on the Internet 18

19. © 2009 Pearson Education, Inc. Publishing as Prentice Hall 3-19 3-5: Graphics Image and Conversion to Binary 19 Example 2: Screen Resolution: 1000 x 500, so 500,000 pixels per screen If 24 bits/pixel, then 500,000 pixels/screen x 24 bits/pixel = 12,000,000 bits/screen or 1,500,000 bytes/screen 2 Example 1: 8 bits per base color gives 256 levels per base color (2 8 ). Three base colors gives 256 3 or over 16 million colors

20. © 2009 Pearson Education, Inc. Publishing as Prentice Hall 3-20 3-6: Data Encoding and Signals 20 We have just seen this We will now see this Before transmission, two things must happen. First, data must be converted into a bit stream. We have already seen this. Second, the 1s and 0s need to be converted into signals—disturbances that travel down the medium.

21. © 2009 Pearson Education, Inc. Publishing as Prentice Hall Signaling

22. © 2009 Pearson Education, Inc. Publishing as Prentice Hall 3-22 Figure 3-7: On/Off Signaling 22 On/off signaling is used in optical fiber. The light is turned on during a clock cycle for a 1. The light is turned off during a clock cycle for a 0. There are two signaling states—on and off. This is the simplest type of signaling.

23. © 2009 Pearson Education, Inc. Publishing as Prentice Hall 3-23 3-8: Binary Voltage Signaling in 232 Serial Ports 23 The high state (0) is anything from +3 to +15 volts. The low state (1) is anything from -3 to -15 volts. 1

24. © 2009 Pearson Education, Inc. Publishing as Prentice Hall 3-24 3-9: Relative Immunity to Errors in Binary Signaling 24

25. © 2009 Pearson Education, Inc. Publishing as Prentice Hall 3-25 3-10: Four-State Digital Signaling 25 As you add more states/clock cycle, you can send more bits/clock cycle. 2 states/clock cycle = 1 bit/clock cycle (binary) 4 states/clock cycle = 2 bits/clock cycle. However, the states are closer together as you add more states. This reduces immunity to error. 2

26. © 2009 Pearson Education, Inc. Publishing as Prentice Hall 3-26 3-10: Four-State Digital Signaling 26 The baud rate is the number of clock cycles per second. The bit rate is the number of bits sent per second. Example: With four states and a clock cycle of 1/1,000,000 second, The baud rate will be 1 Mbaud (not bauds). The bit rate will be 2 bits/clock cycle * 1 million clock cycles/second= 2 Mbps. In a box but shown here

27. © 2009 Pearson Education, Inc. Publishing as Prentice Hall 3-2727 Quiz Which Is Binary? Which Is Digital? 1. Calendar 5. Number of Fingers 3. Gender: Male or Female 4. On/Off Switch 2. Day of the Week

28. © 2009 Pearson Education, Inc. Publishing as Prentice Hall Box: Multistate Digital Signaling

29. © 2009 Pearson Education, Inc. Publishing as Prentice Hall 3-29 3-11: Multistate Digital Signaling Digital Signaling –Clock cycles –Signal is held fixed during each clock cycle –Binary signaling: two states –Digital signaling: a few states (two or more) –Up to about 256 states 29 Box

30. © 2009 Pearson Education, Inc. Publishing as Prentice Hall 3-30 3-11: Multistate Digital Signaling Why Use More than Two States? –With more than two states, can send more than one bit per clock cycle –Two states = 1 bit per clock cycle (1 or 0) –Four states = 2 bits per clock cycle (00, 01, 10, 11) –Eight states = 3 bits per clock cycle, etc. –Each doubling of states gives one more bit per clock cycle 30 Box

31. © 2009 Pearson Education, Inc. Publishing as Prentice Hall 3-31 3-11: Multistate Digital Signaling Problem of Multiple States –As the number of states increases, the difference between states decreases –There is less tolerance for changes in the signal –This is why there is a limit of a few states (256 maximum and usually much less) 31 Box

32. © 2009 Pearson Education, Inc. Publishing as Prentice Hall 3-32 3-11: Multistate Digital Signaling Concepts –Bit rate: Number of bits sent per second –Baud rate: Number of clock cycles per second If 1,000 clock cycles per second, 1 kbaud If each clock cycle is 1/1,000 second = 1,000 clock cycles/second = 1 kbaud 32 Box

33. © 2009 Pearson Education, Inc. Publishing as Prentice Hall UTP Propagation Unshielded Twisted Pair wiring

34. © 2009 Pearson Education, Inc. Publishing as Prentice Hall 3-3434 3-12: Unshielded Twisted Pair (UTP) Wiring UTP Characteristics –Inexpensive and to purchase and install –Dominates media for access links between computers and the nearest switch

35. © 2009 Pearson Education, Inc. Publishing as Prentice Hall 3-35 3-12: Unshielded Twisted Pair (UTP) Wiring Standards –The TIA/EIA-568 standard governs UTP wiring in the United States –In Europe, the comparable standard is ISO/IEC 11801 35

36. © 2009 Pearson Education, Inc. Publishing as Prentice Hall 3-3636 3-13: 4-Pair UTP Cord with RJ45 Connector 3. 8-pin RJ-45 Connector 2. 8 Wires organized as 4 twisted pairs Industry standard pen 1. UTP cord

37. © 2009 Pearson Education, Inc. Publishing as Prentice Hall 3-37 3-12: Unshielded Twisted Pair (UTP) Wiring Cord Organization –A length of UTP wiring is a cord –Each cord has eight copper wires Each wire is covered with dielectric (nonconducting) insulation. –The wires are organized as four pairs Each pair’s two wires are twisted around each other several times per inch –There is an outer plastic jacket that encloses the four pairs 37

38. © 2009 Pearson Education, Inc. Publishing as Prentice Hall 3-38 3-12: Unshielded Twisted Pair (UTP) Wiring Connector –RJ-45 connector is the standard connector –Plugs into an RJ-45 jack in a NIC, switch, or wall jack 38 RJ-45 Jack RJ-45 Jack 8-pin RJ-45 connectors

39. © 2009 Pearson Education, Inc. Publishing as Prentice Hall 3-39 3-12: Unshielded Twisted Pair (UTP) Wiring Characteristics –Inexpensive and easy to purchase and install –Rugged: Can be run over with chairs, etc. –Dominates media for access links Connections to the workgroup switch 39

40. © 2009 Pearson Education, Inc. Publishing as Prentice Hall 3-4040 3-14: Attenuation and Noise Power Distance 3. Noise Floor (Average Noise level) 2. Noise 4. Noise Spike 1. Signal 2. Signal- to-Noise Ratio (SNR) 5. Error 1.The signal attenuates (falls in power) as it propagates. 2.There is noise (random energy) in the wire that adds to the signal. 3.The average noise level is called the noise floor. 4.Noise is random. Occasionally, there will be large noise spikes. 5.Noise spikes as large as the signal cause errors. 6.You want to keep the signal-to-noise ratio high.

41. © 2009 Pearson Education, Inc. Publishing as Prentice Hall 3-4141 Limiting UTP Cord Length Limit UTP cord length to 100 meters –This keeps the signal-to-noise ration (SNR) high. –This makes attenuation and noise problems negligible –Note that limiting cord lengths limits BOTH noise and attenuation problems 100 Meters Maximum Cord Length

42. © 2009 Pearson Education, Inc. Publishing as Prentice Hall Expressing Power Ratios in Decibels 42

43. © 2009 Pearson Education, Inc. Publishing as Prentice Hall 3-43 3-15: Expressing Power Ratios in Decibels Power Ratios (Such as S/N Ratios) Are Encountered Frequently in Networking. Power Ratios for Attenuation –The signal power ratio is P 2 /P 1 P 1 is the initial power and P 2 is the received power. –The final received power is P 2 /P 1 of the original power –Example. Power starts (P 1 ) at 200 milliwatts (mW) and falls to (P 2 ) 100 mW P 2 /P 1 = 100 / 200 = 0.5 = 50% 43

44. © 2009 Pearson Education, Inc. Publishing as Prentice Hall 3-44 3-15: Power Ratios in Decibels Power Ratios Expressed in Decibels –Power reduction ratios vary widely in attenuation When there is a wide range of values, engineers often express them in logarithms –The equation for power ratios in decibels is dB = 10 log 10 (P 2 /P 1 ) Where P 1 is the initial power and P 2 is the final power after transmission If P 2 is smaller than P 1, then the answer will be negative –In calculations, the Excel LOG10 function can be used 44

45. © 2009 Pearson Education, Inc. Publishing as Prentice Hall 3-45 3-15: Power Ratios in Decibels Example –Over a transmission link, power drops by 63 percent. –Therefore it drops to 37% of its original value. –P 2 /P 1 = 37% / 100% =.37 –From Excel: LOG10(0.37) = -0.4318 –10*LOG10(0.37) = -4.3 dB –The negative indicates power reduction through attenuation 45

46. © 2009 Pearson Education, Inc. Publishing as Prentice Hall 3-46 3-15: Power Ratios in Decibels You Can Always Use the Formula. But There are Two Useful Sets of Figures –-3 dB loss is a power ratio of 1/2 (precisely -3.0103) -6 dB is a power ratio of 1/4 (precisely -6.0206) -9 dB is a power ratio of 1/8 (precisely -9.0309)  –-10 dB loss is a power ratio of 1/0 (exactly) -20 dB is a power ratio of 1/100 (exactly) -30 dB is a power ratio of 1/1000 (exactly)  –So a power ratio of.4 is about …. 46

47. © 2009 Pearson Education, Inc. Publishing as Prentice Hall 3-4747 UTP Wiring Electromagnetic Interference (EMI) –Electromagnetic interference is electromagnetic energy from outside sources that adds to the signal From fluorescent lights, electrical motors, microwave ovens, etc. –The problem is that UTP cords are like long radio antennas. They pick up EMI energy nicely When they carry signals, they also send EMI energy out from themselves

48. © 2009 Pearson Education, Inc. Publishing as Prentice Hall 3-4848 3-16: Electromagnetic Interference (EMI) and Twisting Interference on the Two Halves of a Twist Cancels Out Twisted Wire Electromagnetic Interference (EMI)

49. © 2009 Pearson Education, Inc. Publishing as Prentice Hall 3-4949 3-16: Crosstalk Interference and Terminal Crosstalk Interference Untwisted at Ends Signal Terminal Crosstalk Interference Crosstalk Interference Terminal crosstalk interference Normally is the biggest EMI problem for UTP

50. © 2009 Pearson Education, Inc. Publishing as Prentice Hall 3-5050 Interference Hierarchy EMI is any interference –Signals in adjacent pairs interfere with one another (crosstalk interference). This is a specific type of EMI Crosstalk interference is worst at the ends, where the wires are untwisted. This is terminal crosstalk interference—a specific type of crosstalk EMI EMI Crosstalk Interference Terminal Crosstalk Interference

51. © 2009 Pearson Education, Inc. Publishing as Prentice Hall 3-5151 Terminal Crosstalk Interference Terminal crosstalk interference dominates interference in UTP –Terminal crosstalk interference is limited to an acceptable level by not untwisting wires more than a half inch (1.25 cm) at each end of the cord to fit into the RJ-45 connector –This reduces terminal crosstalk interference to a negligible level. 1.25 cm or 0.5 inches

52. © 2009 Pearson Education, Inc. Publishing as Prentice Hall 3-5252 UTP Limitations Limit cords to 100 meters –Limits BOTH noise AND attenuation problems to an acceptable level Do not untwist wires more than 1.25 cm (a half inch) when placing them in RJ-45 connectors –Limits terminal crosstalk interference to an acceptable level Neither completely eliminates the problems but they usually reduce the problems to negligible levels 2

53. © 2009 Pearson Education, Inc. Publishing as Prentice Hall 3-53 3-18: Serial versus Parallel Transmission 53

54. © 2009 Pearson Education, Inc. Publishing as Prentice Hall 3-54 Figure 3-19: Wire Quality Standards 54 CategoryTechnologyMaximum SpeedMaximum Ethernet Distance at this Speed 1UTPNever definedNot Applicable 2UTPNever definedNot Applicable 3UTP10 Mbps100 meters 4UTP10 Mbps100 meters 5UTP1 Gbps100 meters 5eUTP1 Gbps100 meters 6UTP10 Gbps55 meters 6AUTP10 Gbps100 meters 7STP 1 10 Gbps+100 meters Category numbers indicate wire quality. STP is shielded twisted pair. There is foil around each pair and a metal mesh around the four pairs.

55. © 2009 Pearson Education, Inc. Publishing as Prentice Hall Optical Fiber Transmission Light through Glass More Easily Spans Longer Distances than UTP

56. © 2009 Pearson Education, Inc. Publishing as Prentice Hall 3-56 3-20: Optical Fiber Transceiver and Strand 56 An optical fiber strand has a thin glass core. This core is 8.3, 50, or 62.5 microns in diameter. This glass core is surrounded by a tubular glass cladding. The outer diameter of the cladding is 125 microns, regardless of the core’s diameter. The transceiver injects laser light into the core.

57. © 2009 Pearson Education, Inc. Publishing as Prentice Hall 3-57 3-20: Optical Fiber Transceiver and Strand 57 When a light wave ray hits the core/cladding boundary, there is perfect internal reflection. There is no signal loss.

58. © 2009 Pearson Education, Inc. Publishing as Prentice Hall 3-5858 Two-Strand Full-Duplex Optical Fiber Cord with SC and ST Connectors A fiber cord has two-fiber strands for full-duplex (two- way) transmission SC Connectors ST Connectors Two Strands Cord

59. © 2009 Pearson Education, Inc. Publishing as Prentice Hall 3-59 3-22: Full-Duplex Optical Fiber Cord with SC and ST Connectors 59 SC Connectors (push and click) ST Connectors (bayonet connectors: push and click) In contrast to UTP, which always uses RJ-45 connectors, there are several optical fiber connector types. SC and ST are the most popular.

60. © 2009 Pearson Education, Inc. Publishing as Prentice Hall 3-60 3-23: Frequency and Wavelength 60 Light travels in waves. The amplitude is the intensity of the wave. In sound waves, amplitude is loudness. Amplitude is a measure of power. Wave

61. © 2009 Pearson Education, Inc. Publishing as Prentice Hall 3-61 3-23: Frequency and Wavelength 61 Wavelength is the physical distance between comparable points on adjacent cycles (peak-to-peak, trough-to-trough, start-to-start, etc.). Wavelengths are measured in meters. Light is measured in wavelength. Light wavelengths are in the nanometer range. Wave

62. © 2009 Pearson Education, Inc. Publishing as Prentice Hall 3-62 3-20 Optical Fiber Strand 62 In optical fiber transmission, light is expressed in nanometers. The transceiver transmits at 850 nm, 1,310 nm, or 1,550 nm. Shorter-wavelength (850 nm) transceivers are less expensive. Longer-wavelength (1,310 or 1,550 nm) light travels farther. For LAN fiber, 850 nm provides sufficient distance and dominates.

63. © 2009 Pearson Education, Inc. Publishing as Prentice Hall 3-63 3-23: Frequency and Wavelength 63 Waves can also be measured in frequency. The frequency is the number of complete cycles per second. Hertz (Hz) is the term for cycles per second. Radio transmission is measured in frequency. Radio transmission usually takes place in the MHz or GHz range. Wave

64. © 2009 Pearson Education, Inc. Publishing as Prentice Hall 3-64 3-25: Multimode Fiber and Single-Mode Fiber 64 Multimode fiber has a thick core (50 or 62.5 microns). Light can only enter the core at certain angles, called modes. Modes traveling straight through arrive faster than modes that bounce against the cladding several times.

65. © 2009 Pearson Education, Inc. Publishing as Prentice Hall 3-65 3-25: Multimode Fiber and Single-Mode Fiber 65 Modal dispersion is the difference in time it takes modes to propagate. If modal dispersion is too large, adjacent waves will overlap. That will produce errors. Modal dispersion is the limiting factor for multimode fiber.

66. © 2009 Pearson Education, Inc. Publishing as Prentice Hall 3-66 3-25: Multimode Fiber and Single-Mode Fiber 66 Modal dispersion can be reduced by having a graded index of refraction in the core, decreasing from the center to the cladding. All multimode fiber is graded index multimode fiber today. Modal dispersion is also reduced by better-quality multimode fiber. Modal bandwidth (measured as MHz-km) is the measure of multimode fiber quality.

67. © 2009 Pearson Education, Inc. Publishing as Prentice Hall 3-67 3-26: Wavelength, Core Diameters, Modal Bandwidth, and Maximum Propagation Distance for Ethernet 1000BASE-SX 67 WavelengthCore DiameterModal BandwidthMaximum Propagation Distance 850 nm62.5 microns160 MHz.km220 m 850 nm62.5 microns200 MHz.km275 m 850 nm50 microns500 MHz.km550 m With 850 nm light, distance can be increased by using a smaller core diameter or using better-quality fiber with higher modal bandwidth.

68. © 2009 Pearson Education, Inc. Publishing as Prentice Hall 3-68 3-25: Multimode Fiber and Single- Mode Fiber 68 Single mode fiber has a core diameter that is so small (8.3 microns) that only one mode can propagate. Consequently, there is no modal dispersion. Single mode fiber transmission distance is limited only by absorptive attenuation, which is extremely low. Consequently, single mode fiber can carry signals for kilometers. However, single-mode fiber is more expensive than multimode.

69. © 2009 Pearson Education, Inc. Publishing as Prentice Hall Radio Propagation

70. © 2009 Pearson Education, Inc. Publishing as Prentice Hall 3-70 Radio Propagation 70 Radio signals also propagate as waves. as noted earlier, radio waves are measured in Hz, which is a measure of frequency. Radio usually operates in the MHz and GHz range.

71. © 2009 Pearson Education, Inc. Publishing as Prentice Hall 3-71 3-27: Omnidirectional and Dish Antennas 71

72. © 2009 Pearson Education, Inc. Publishing as Prentice Hall 3-72 3-28: Wireless Propagation Problems 72 The first propagation problem is electromagnetic interference (EMI) from nearby radio sources.

73. © 2009 Pearson Education, Inc. Publishing as Prentice Hall 3-73 3-28: Wireless Propagation Problems 73 Another problem is inverse square law attenuation. As a signal propagates, its energy spreads out over the Surface of an ever-expanding sphere.

74. © 2009 Pearson Education, Inc. Publishing as Prentice Hall 3-74 3-28: Wireless Propagation Problems An Example of Inverse Square Law Attenuation –P1 = Power at Point A. –P2 = Power at Point B (which is farther from A). –r1 = Distance to Point A. –r2 = Distance 2Point B (which is farther from A). –P2 = P1 * (r1/r2) 2 –If the power is 400 mW (milliwatts) at 100 meters –What is the power at 200 meters? –P2 = 400 mW * (100/200) 2 –P2 = 400 mW * (1/2)2 = 400 mW * 1/4 = 100 mW 74

75. © 2009 Pearson Education, Inc. Publishing as Prentice Hall 3-75 3-28: Wireless Propagation Problems An Example of Inverse Square Law Attenuation –P1 = Power at Point A. –P2 = Power at Point B (which is farther from A). –r1 = Distance to Point A. –r2 = Distance 2Point B (which is farther from A). –P2 = P1 * (r1/r2) 2 –If the power is 900 mW (milliwatts) at 10 meters –What is the power at 30 meters? 75

76. © 2009 Pearson Education, Inc. Publishing as Prentice Hall 3-76 3-28: Wireless Propagation Problems 76 Confusingly, wireless propagation suffers from two forms of attenuation. We have just seen inverse square law attenuation. There is also absorptive attenuation, which is attenuation because power is absorbed by water molecules along the way. Absorptive attenuation increases with frequency.

77. © 2009 Pearson Education, Inc. Publishing as Prentice Hall 3-77 3-28: Wireless Propagation Problems 77 When radio waves hit thick objects, they cannot penetrate. This creates shadow zones, which are also called dead spots. Shadow zones get worse as frequency increases.

78. © 2009 Pearson Education, Inc. Publishing as Prentice Hall Topology Network topology is the physical arrangement of a network’s computers, switches, routers, and transmission lines. It is a physical layer concept.

79. © 2009 Pearson Education, Inc. Publishing as Prentice Hall 3-79 3-29: Major Topologies 79 The simplest topology is the point-to-point topology.

80. © 2009 Pearson Education, Inc. Publishing as Prentice Hall 3-80 3-29: Major Topologies 80 Ethernet uses a star topology. Note that the switch does not have to be in the middle of the star.

81. © 2009 Pearson Education, Inc. Publishing as Prentice Hall 3-81 3-29: Major Topologies 81 Larger Ethernet LANs use an extended star topology. This is better called a hierarchical topology.

82. © 2009 Pearson Education, Inc. Publishing as Prentice Hall 3-82 3-29: Major Topologies 82 In a mesh topology, there are many connections between switches or routers. Consequently, there are many alternative routes between hosts.

83. © 2009 Pearson Education, Inc. Publishing as Prentice Hall 3-83 3-29: Major Topologies 83 In the ring topology, messages travel around a loop.

84. © 2009 Pearson Education, Inc. Publishing as Prentice Hall 3-84 3-29: Major Topologies 84 The bus topology uses broadcasting. The message receives each host at almost the same time. All wireless transmission uses a bus topology.