1. Physical Layer Propagation: UTP and Optical Fiber Chapter 3 Updated January 2009 XU Zhengchuan Fudan University

2. 2 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, and topologies –Wireless transmission is covered in Chapter 5

3. 3 Figure 3-1: Signal and Propagation Sender Receiver Transmission Medium Propagation Transmitted Signal Received Signal (Attenuated & Distorted) 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. 4 Test Your Understanding P 141

5. Data Representation

6. 6 Binary-Encoded Data Computers store and process data in binary representations –Binary means “two” –There are only ones and zeros –Called bits 1101010110001110101100111

7. 7 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…

8. 8 Binary-Encoded Data Some data are inherently binary –48-bit Ethernet addresses –32-bit IP addresses –Need no further encoding

9. 9 Figure 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”

10. 10 Figure 3-2: Arithmetic with Binary Numbers, Continued Binary Arithmetic for Binary Numbers 1 0 0 1 1 +1 +0+1+0 +1 +1 =0=1=1=10=11 Basic Rules

11. 11 Figure 3-2: Arithmetic with Binary Numbers, Continued Binary Decimal 1000 8 +1+1 =1001=9 +1+1 =1010 =10 +1+1 =1011 =11 +1 +1 =1100 =12 Examples

12. 12 Figure 3-3: Binary Encoding for Alternatives Encoding Alternatives (Product number, region, gender, etc.) (N bits can represent 2N Alternatives ) Number of Bits In Field (N) 1 2 3 4 8 16 … Number of Alternatives That Can be Encoded with N bits 2 (2 1 ) 4 (2 2 ) 8 (2 3 ) 16 (2 4 ) 256 (2 8 ) 65,536 (2 16 ) … Each added bit doubles the number of alternatives that can be represented

13. 13 Figure 3-3: Binary Encoding for Alternatives BitsAlternativesExamples 12 1 =2Male = 0, Female = 1 22 2 =4Spring = 00, Summer = 01, Autumn = 10, Winter = 11 82 8 =256Keyboard characters for U.S. keyboards. Space=00000000, etc. ASCII code actually uses 7 bits

14. 14 Powers of 2 BitsAlternatives 12 24 38 416 532 664 7128 8256 101,024 1665,536 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. Memorize for 1, 4, 8, and 16 bits

15. 15 Figure 3-3: Binary Encoding for Alternatives Quiz –How many flavors of ice cream can you represent in half a byte of storage? –How many bits do you need to represent 64 flavors of ice cream? –How many bits do you need to represent 6 sales districts?

16. 16 Figure 3-4: ASCII and Extended ASCII ASCII Code to Represent Text –ASCII is the traditional binary code to represent text data –Seven bits per character 2 7 (128) characters possible –Sufficient for all keyboard characters (including shifted values) Capital letters (A is 1000001) Lowercase letters (a is 1100001) –Each character is stored in a byte The 8 th bit in a byte normally is not used

17. 17 Figure 3-4: ASCII and Extended ASCII, Continued Extended ASCII –Used on PCs –Uses a full 8 bits per character –2 8 (256) characters possible –Extra characters can represent formatting in word processing, etc. Converters –Text-to-ASCII and Text-to-Extended ASCII Converters are Readily Available on the Internet

18. 18 Figure 3-5: Binary Coding for Graphics Image Pixels –1. Screen is divided into small squares called pixels (picture elements) –2. Each pixel has three dots—red, green, and blue. Sometimes a black dot too 3. JPEG stores one byte per color (24 bits total) This gives 256 intensity levels for each color or 16.8 million colors overall (256 3 )

19. 19 Test Your Understanding P 146 3 c

20. Signaling

21. 21 Figure 3-6: Data Encoding and Signaling Data “Now is the …” Male or Female Graphics Human Voice Binary- Encoded Data 1101010 Binary Encoding Signaling 1. First, data must be converted to binary, as we have just seen 2. Second, bits must be covered Into signals (voltage changes, etc.). Voltage change, etc.

22. 22 Figure 3-7: On/Off Binary Signaling Off= 0 On= 1 On= 1 Off= 0 On= 1 Off= 0 On= 1 Light Source Optical Fiber Clock Cycle During each clock cycle, light is turned on for a one or off for a zero.

23. 23 Figure 3-8: Binary Signaling in 232 Serial Ports 0 Volts -15 Volts 15 Volts Clock Cycle 00 1 3 Volts -3 Volts 0 1 This type of signaling is used in 232 serial ports. In a clock cycle, 3 to 15 volts represents a zero -3 to -15 volts is a ONE

24. 24 Figure 3-9: Relative Immunity to Errors in Binary Signaling 0 Volts -15 Volts 15 Volts Transmitted Signal (12 Volts) Received Signal (6 volts) 3 Volts -3 Volts 0 1 Despite a 50% drop in voltage, the receiver will still know that the signal is a zero

25. 25 Binary and Binary Signaling In binary signaling, there are two states –This can represent a single bit per clock cycle. In digital signaling, there are a few bits per clock cycle—2, 4, 8, 16, 32, … With more states, several bits to be sent per clock cycle Note that all binary transmission (2 states) is digital (few states) But not all digital transmission is binary 11 10 01 00 10 11 00 01 Clock Cycle

26. 26 Test Your Understanding P 149 4 a, b, c

27. 27 Figure 3-10: 4-State Digital Signaling 11 10 01 00 Client PCServer 10 11 00 01 Clock Cycle Digital signaling has a FEW possible states per clock cycle (4 in this slide) This allows it to send multiple bits per clock cycle This increases the bit transmission rate per clock cycle It reduces error resistance because differences between states are smaller Box

28. 28 Quiz Which Is Binary? Which Is Digital? 1. Calendar 2. Number of Fingers 5. Gender Male or Female 3. On/Off Switch 4. Day of the Week Box

29. 29 Figure 3-10: 4-State Digital Signaling, Continued Equation 3-1: Bit rate = Baud rate * Bits sent per clock cycle –Baud rate is the number of clock cycles per second If the clock cycle is 1/1000 of a second, the baud rate is 1,000 baud –Bit rate is then the number of clock cycles per second times the number of bits sent per clock cycle If the three bits are sent per clock cycle, the bit rate is 3,000 bps or 3 kbps Box

30. 30 Figure 3-10: 4-State Digital Signaling, Continued Equation 3-2: States = 2 Bits –Bits is the number of bits to be sent per clock cycle –States is the number of states needed to send that many bits Doubling the number of states transmits one more bit per clock cycle. Box Bits to be sent per clock cycle Number of states required 12 24 38 416

31. 31 Figure 3-10: 4-State Digital Signaling, Continued Example: –The clock cycle is 1/100,000 second The baud rate is 100 kbaud (not kbauds) –You want a bit rate of 500,000 bps Solution: –You have to send 5 bits per clock cycle (baud) –This will require 32 states States = 2 bits States = 2 5 States = 32 Box

32. 32 Figure 3-10: 4-State Digital Signaling, Continued Example: –Suppose there a system has 8 states –Suppose that the clock cycle is 1/10,000 second –How fast can the system transmit? Solution: –With four states, 3 information bits can be sent per clock cycle (8=2 X ) [Equation 3-2] X=3 –With a clock cycle of 1/10,000, baud rate is 10,000 baud –The bit rate will be 30 kbps (3 bits/clock cycle times 10,000 clock cycles per second). [Equation 3-1] Box

33. 33 Test Your Understanding P 151

34. UTP Propagation Unshielded Twisted Pair wiring

35. 35 Figure 3-12: 4-Pair UTP Cord with RJ45 Connector 3. RJ-45 Connector 2. 8 Wires Organized as 4 Twisted Pairs Industry Standard Pen 1. UTP Cord

36. 36 RJ-45 Jacks and Connectors RJ-45 Jack RJ-45 Jack RJ-45 Jack RJ-45 Connectors

37. 37 Figure 3-11: Unshielded Twisted Pair (UTP) Wiring, Continued UTP Characteristics –Inexpensive and to purchase and install –Dominates media for access links between computers and the nearest switch

38. 38 Test Your Understanding P 154

39. 39 Figure 3-13: Attenuation and Noise Power Distance 1. Signal Signals in UTP attenuate with propagation distance. If attenuation is too great, the signal will not be readable by the receiver.

40. 40 Figure 3-14: Decibels Attenuation is Sometimes Expressed in Decibels (dB) The equation for decibels is –dB = 10 log 10 (P2/P1) –Where P1 is the initial power and P2 is the final power after transmission –If P2 is smaller than P1, then the answer will be negative

41. 41 Figure 3-14: Decibels, Continued Example –Over a transmission link, power drops to 37% of its original value –P2/P1 = 37/100 =.37 (37%/100%) –LOG 10 (0.37) = -0.4318 –10*LOG 10 (0.37) = -4.3 dB (negative, reflecting power reduction through attenuation) –In calculations, the Excel LOG10 function can be used

42. 42 Figure 3-14: Decibels, Continued There are two useful approximations 3 dB loss is a reduction to very nearly 1/2 the original power –6 dB loss is a decrease to 1/4 the original power –9 dB loss is a decrease to 1/8 the original power –…–… 10 dB loss is a reduction to very nearly 1/10 the original power –20 dB loss is a decrease to 1/100 the original power –…–…

43. 43 Figure 3-13: Attenuation and Noise, Continued Power Distance Noise Floor Noise Noise SpikeSignal Signal- to-Noise Ratio (SNR) Noise is random unwanted energy within the wire Its average is called the noise floor ( 噪声基底 ) Random noise spikes ( 噪声毛刺 ) cause errors -- A high signal-to-noise ratio reduces noise error problems As a signal attenuates with distance, damaging noise spikes become more common Error

44. 44 Limiting UTP Cord Length Limit UTP cord length to 100 meters –Limits attenuation to being a negligible problem –Limits noise problems being a negligible problem –Note that limiting cord lengths limits BOTH noise and attenuation problems 100 Meters Maximum Cord Length

45. 45 Test Your Understanding P 157-158

46. 46 Figure 3-11: Unshielded Twisted Pair (UTP) Wiring, Continued Electromagnetic Interference (EMI) (Fig. 3-15) –Electromagnetic interference ( 电磁干扰 ) 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

47. 47 Figure 3-15: Electromagnetic Interference (EMI) and Twisting Interference on the Two Halves of a Twist Cancels Out Twisted Wire Electromagnetic Interference (EMI)

48. 48 Figure 3-16: Crosstalk Interference and Terminal Crosstalk Interference ( 交互干扰 ) Untwisted at Ends Signal Terminal Crosstalk Interference Crosstalk Interference Terminal crosstalk interference Normally is the biggest EMI problem for UTP

49. 49 Figure 3-11: Unshielded Twisted Pair (UTP) Wiring, Continued Electromagnetic Interference (EMI) (Fig. 3-15) –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

50. 50 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

51. 51 Test Your Understanding P 160

52. 52 Figure 3-17: Serial Versus Parallel Transmission One Clock Cycle 1. Serial Transmission (1 bit per clock cycle) 2. Parallel Transmission (1 bit per clock cycle per wire pair) 4 bits per clock cycle on 4 pairs 1 bit Parallel transmission increases speed. But it is only workable over short distances. Parallel is not 4. It is more than one.

53. 53 Test Your Understanding P 161

54. 54 Figure 3-18: Wire Quality Standards Wiring Quality Standards –Rated by Category (Cat) Numbers Category Standards are Set by ANSI/TIA/EIA and ISO/IEC –In the United States, the TIA/EIA/ANSI-568 governs UTP and optical fiber standards –In Europe and many other parts of the world, the standard is ISO/IEC 11801 –The two sets of standards are close but not identical

55. 55 Figure 3-18: Wire Quality Standards UTP Categories 3 and 4 –Early data wiring, which could only handle Ethernet speeds up to 10 Mbps UTP Categories 5 and 5e –Most wiring installed today is Category 5e (enhanced) –Cat 5e and Cat 5 can handle Ethernet up to 1 Gbps –Most wiring sold today is Cat 5e

56. 56 Figure 3-18: Wire Quality Standards UTP Category 6 –Relatively new –No better than Cat 5 or Cat 5e at 1 Gbps –Developed for higher Ethernet speeds of 10 Gbps But can only span 55 meters at that speed Book says cannot be used. This is an error. Category 6A (Augmented) –Able to carry Ethernet signals at 10 Gbps up 100 meters –The book said 55 meters, but this is an error Errors

57. 57 Figure 3-18: Wire Quality Standards Category 7 STP –Shielded twisted pair (STP) rather than unshielded twisted pair (UTP) Metal foil shield around each pair to reduce crosstalk interference Metal mesh around all four pairs to reduce crosstalk from other cords –STP is expensive and awkward to lay –Can 10 Gbps Ethernet to 100 meters

58. 58 Test Your Understanding P 164

59. Optical Fiber Transmission Light through Glass Better than UTP: More Easily Spans Longer Distances at High Speeds

60. 60 Figure 3-19: UTP in Access Lines and Optical Fiber in Trunk Lines Workgroup Switch UTP Access Line UTP Access Line UTP Access Line 2. UTP dominates access lines between stations and their workgroup switches 1. Workgroup Switches Link Computers to the Network

61. 61 Figure 3-19: UTP in Access Lines and Optical Fiber in Trunk Lines, Continued 1. Core switches connect other switches Core Switch CoreFiber Trunk Fiber Trunk Fiber Trunk Fiber Trunk 2. Fiber dominates trunk lines between switches

62. 62 Test Your Understanding P 165 14-15

63. 63 Figure 3-20: Optical Fiber Transceiver and Strand Transceiver 5. Perfect internal reflection at core/cladding boundary; No signal loss, so low attenuation 4. Light Ray 3. Cladding （镀层） 125 micron diameter 2. Core 8.3, 50 or 62.5 micron diameter 1. (Transmitter/Receiver) Light Source 850 nm, 1,310 nm, and 1,550 nm Strand （股）

64. 64 Figure 3-22: 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

65. 65 Figure 3-22: Pen and Full-Duplex Optical Fiber Cord with SC and ST Connectors SC Connectors (Push in and Snap) ST Connectors (Bayonet: Push in and Twist)

66. 66 Figure 3-23: Frequency and Wavelengths 1. Amplitude Power, Voltage, etc. 2. Wavelength Distance between comparable points in successive cycles (Measured in nanometers for light) 3. Frequency is the number of cycles per second. 1 Hz = 1 cycle per second In this case, there are two cycles in 1 second, so frequency is two hertz (2 Hz). 1 Second Amplitude Wave

67. 67 Light Wavelengths Light signals are measured by wavelength Light wavelengths measured in nanometers (nm) There are three fiber wavelength “windows” with good propagation characteristics –850 nm –1310 nm –1550 nm Shorter wavelength allows cheaper transceivers Longer-wavelength light travels farther

68. 68 Test Your Understanding P 169 16

69. 69 Figure 3-24: Carrier Fiber and LAN Fiber LAN Fiber –Uses multimode fiber, which has a “thick” core diameter of 50 or 62.5 microns Less expensive than single-mode fiber (later) 62.5 micron fiber is more common in the US but does not carry signals as far as 50 micron fiber –Also uses inexpensive 850 nm transceivers –Multimode fiber （多模光纤） with 850 nm signaling cannot span the kilometer distances needed by carriers, but can span the 200-300 meters needed in LAN fiber cords

70. 70 Figure 3-24: Multimode and Single-Mode Optical Fiber Multimode Fiber In thicker fiber, light only travels in one of several allowed modes. Different modes travel different distances and arrive at different times (See that Mode 1 light takes longer to arrive than Mode 2 light.) If distance is too long, modes from successive light pulses will overlap. This is modal distortion （模态散射）. If it is too large, signals will be unreadable. Modal distortion is the main limitation on distance in multimode fiber. Light Source (Usually Laser) Core Mode 1 Arrives Later Mode 2

71. 71 Test Your Understanding P 172 17

72. 72 Figure 3-24: Carrier Fiber and LAN Fiber LAN Fiber –All multimode fiber today is graded-index multimode fiber The index of refraction （折射级数） decreases from the center of the core to the core’s outer edge. Higher Incidence of Refraction Lower

73. 73 Figure 3-24: Carrier Fiber and LAN Fiber LAN Fiber –Graded-index multimode fiber Light speed increases when the index decreases The central mode (Mode 2) is slowed High-angle modes (Mode 1) are speeded up Mode 2 (Slowed) Mode 1 (Speeded Up Near Edge of Core) Lower Modal Dispersion

74. 74 Figure 3-24: Carrier Fiber and LAN Fiber LAN Fiber –UTP quality is measured by category number. –Multimode Fiber Quality Measured as modal bandwidth (MHz.km or MHz-km) More modal bandwidth is better Increases the speed–distance product –With greater mobile bandwidth, can go faster, farther, or some combination of the two

75. 75 Figure 3-24: Carrier Fiber and LAN Fiber LAN Fiber –Example: 1000BASE-SX Ethernet Uses inexpensive 850 nm light With 62.5 micron fiber and 160 MHz-km modal bandwidth, maximum distance is 220 m With 62.5 micron fiber and 200 MHz-km bandwidth, maximum distance is 275 m Some vendors with higher-than-standard modal bandwidth can carry traffic farther

76. 76 Figure 3-24: Carrier Fiber and LAN Fiber LANs and WAN carriers use different types of fiber Carrier Fiber –Carrier fiber must span long distances –This requires expensive long-wavelength laser light sources (1,310 and 1,550 nm) –It also requires expensive “single-mode” fiber with a very narrow core (8.3 microns)

77. 77 Figure 3-24: Multimode and Single-Mode Optical Fiber, Continued Light Source Single-Mode Fiber Cladding Core Single Mode Light enters only at certain angles called modes Single-mode fiber cores are so thin that only one mode can propagate—the one traveling straight through No modal dispersion (discussed earlier), so can span long distances without this distortion Expensive but necessary in WANs

78. 78 Test Your Understanding P 174 18

79. 79 Figure 3-24: Carrier Fiber and LAN Fiber Carrier Fiber –Main propagation effect for single-mode fiber is attenuation, which is very low For 850 nm light, attenuation is around 2.5 dB/km At 1,310 nm, attenuation is lower—about 0.8 dB/km At 1,550 nm, attenuation falls even lower—about 0.2 dB/km –Longer wavelengths carry farther but cost more –Carrier fiber uses wavelengths of 1,310 or 1,550 nm

80. 80 Figure 3-24: Carrier Fiber and LAN Fiber Noise and Electromagnetic Interference (EMI) Are Not Problems for Either LAN or Carrier Fiber –Noise from moving electrons cannot interfere with light signals –EMI would have to be light signals Wrapping the cladding in an opaque covering prevents light from coming in

81. 81 Figure 3-24: Carrier Fiber and LAN Fiber Corporate LAN Multimode Fiber Carrier (WAN) Single-Mode Fiber Needed DistanceOnly 200-300 meters Many kilometers CostMuch Lower ($)Very high ($$$$) Fiber TypeMultimode ($)Single-mode ($$$$) WavelengthUsually 850 nm ($)Usually 1,310 or 1,550 nm ($$$$) Typical Core50/62.5 microns ($)8.3 microns ($$$) Propagation LimitModal DistortionAttenuation Is Modal Bandwidth Important? YesNo. Only attenuation matters

82. 82 Test Your Understanding P 174 19

83. Topology

84. 84 Figure 3-26: Major Topologies Topology –Network topology refers to the physical arrangement of a network’s computers, switches, routers, and transmission lines –Topology is a physical layer concept –Different network (and internet) standards specify different topologies Point-to-Point Topology (Telephone Modem Communication, Private Lines)

85. 85 Figure 3-26: Major Topologies, Continued Star (Modern Ethernet) Example: Pat Lee’s House in Chapter 1a

86. 86 Figure 3-26: Major Topologies, Continued Extended Star or Hierarchy (Modern Ethernet) Only one possible path between any two computers For computers X and Y, the path is XBACDY A B C D E X Y Z

87. 87 Figure 3-26: Major Topologies, Continued Mesh (Routers, Frame Relay, ATM) Multiple alternative paths between two computers A B C D Path ABD Path ACD

88. 88 Figure 3-26: Major Topologies, Continued Ring (SONET/SDH)

89. 89 Test Your Understanding P 176 20

90. 90 Figure 3-26: Major Topologies, Continued Bus Topology (Broadcasting) Used in Wireless LANs

91. Topics Covered

92. 92 Topics Covered Binary Data Encoding Inherently binary data (IP addresses, etc.) Integers (binary arithmetic) Alternatives (N bits can represent 2 N Alternatives) Text (ASCII and Extended ASCII) Graphics (pixels, bits per pixel color) … For transmission the sender converts bits to signals (on/off, voltage levels, etc.)

93. 93 Topics Covered, Continued Digital Transmission (Box) A few states instead of just two states (binary) All binary transmission is digital transmission Only some digital transmission (transmission with two states) is binary In the box: bit rates and baud rates

94. 94 Topics Covered, Continued UTP –4-pair UTP cords and RJ-45 connectors and jacks –Attenuation (often expressed in decibels) and noise Limit UTP cords to 100 meters –Electromagnetic interference, crosstalk interference, and terminal crosstalk interference Limit wire unwinding to 1.25 cm (a half inch) to limit terminal crosstalk interference –Serial versus parallel transmission

95. 95 Topics Covered, Continued Optical Fiber –On/off light pulses from transceiver –Core and cladding; perfect internal reflection –Dominates for trunk lines among core switches –2 fiber strands/fiber cord for full-duplex transmission –SC and ST connectors are the most common –Carriers use single-mode fiber and long wavelengths –LANs use multimode fiber and short wavelengths

96. 96 Topics Covered, Continued Multimode Optical Fiber Distance Increases With … –Greater Wavelength 850 nm < 1310 nm < 1550 nm “windows” But larger-wavelength transceivers cost more –Smaller Core Diameter 50 microns > 62.5 microns –Greater Modal Bandwidth (MHz.km) Measure of multimode fiber quality

97. 97 Topics Covered, Continued Topologies –Organization of devices and transmission links –Physical layer concept –Point-to-point, star, hierarchy, ring, etc.