Chapter3a-DataTransmission

1. Chapter3a-DataTransmission 1 Data TransmissionandTransmission Media

2. Chapter3a-DataTransmission 2 Data Transmission

3. Chapter3a-DataTransmission 3 Three Components of Data Communication Data Analog: Continuous value data (sound, light, temperature) Digital: Discrete value (text, integers, symbols) Signal Analog: Continuously varying electromagnetic wave Digital: Series of voltage pulses (square wave) Transmission Analog: Works same for analog or digital signals Digital: Used only with digital signals

4. Chapter3a-DataTransmission 4 Electromagnetic Signals Function of time Analog (varies smoothly over time) Digital (constant level over time, followed by change to another level) Function of frequency Spectrum (range of frequencies) Bandwidth (width of spectrum)

5. Chapter3a-DataTransmission 5 Periodic Signal Characteristics Amplitude (A): signal value, measured in volts Frequency (f ): repetition rate, cycles per second or Hertz Period (T): amount of time it takes for one repetition, T=1/f Phase (f): relative position in time, measured in degrees

6. Chapter3a-DataTransmission 6 Bandwidth Width of spectrum of frequencies that can be transmitted if spectrum=300 to 3400Hz, bandwidth=3100Hz Greater bandwidth leads to greater costs Limited bandwidth leads to distortion

7. Chapter3a-DataTransmission 7 Transmission Choices Analog transmission only transmits analog signals, without regard for data content attenuation overcome with amplifiers signal is not evaluated or regenerated Digital transmission transmits analog or digital signals uses repeaters rather than amplifiers switching equipment evaluates and regenerates signal

8. Chapter3a-DataTransmission 8 Why Study Analog inData Comm? Much of data begins in analog form; must understand it in order to properly convert it Telephone system is primarily analog rather than digital (designed to carry voice signals) Low-cost, ubiquitous transmission medium If we can convert digital information (1s and 0s) to analog form (audible tone), it can be transmitted inexpensively

9. Chapter3a-DataTransmission 9 Why Use Analog Transmission? Already in place Significantly less expensive Lower attentuation rates Fully sufficient for transmission of voice signals

10. Chapter3a-DataTransmission 10 Data vs Signals Analog data Voice Images Digital data Text Digitized voice or images

11. Chapter3a-DataTransmission 11 Analog Data-->Signal Options Analog data to analog signal Inexpensive, easy conversion (eg telephone) Data may be shifted to different part of available spectrum (multiplexing) Used in traditional analog telephony Analog data to digital signal Requires codec (encoder/decoder) Allows use of digital telephony, voice mail

12. Chapter3a-DataTransmission 12 Analog Signaling represented by sine waves

13. Chapter3a-DataTransmission 13 Voice/Audio Analog Signals Easily converted from sound frequencies (measured in loudness/db) to electromagnetic frequencies, measured in voltage Human voice has frequency components ranging from 20Hz to 20kHz For practical purposes, telephone system has narrower bandwidth than human voice, from 300 to 3400Hz

14. Chapter3a-DataTransmission 14 Image/Video: Analog Data to Analog Signals Image is scanned in lines; each line is displayed with varying levels of intensity Requires approximately 4Mhz of analog bandwidth Since multiple signals can be sent via same channel, guardbands are necessary, raising bandwidth requirements to 6Mhz per signal

15. Chapter3a-DataTransmission 15 Digital Data-->Signal Options Digital data to analog signal Requires modem (modulator/demodulator) Allows use of PSTN to send data Necessary when analog transmission is used Digital data to digital signal Requires CSU/DSU (channel service unit/data service unit) Less expensive when large amounts of data are involved More reliable because no conversion is involved

16. Chapter3a-DataTransmission 16 Advantages of Digital Transmission The signal is exact Signals can be checked for errors Noise/interference are easily filtered out A variety of services can be offered over one line Higher bandwidth is possible with data compression

17. Chapter3a-DataTransmission 17 Methods of Modulation Amplitude modulation (AM) or amplitude shift keying (ASK) Frequency modulation (FM) or frequency shift keying (FSK) Phase modulation or phase shift keying (PSK)

18. Chapter3a-DataTransmission 18 Amplitude Shift Keying (ASK) In radio transmission, known as amplitude modulation (AM) The amplitude (or height) of sine wave varies to transmit ones and zeros Major disadvantage is that telephone lines are very susceptible to variations in transmission quality that can affect amplitude

19. Chapter3a-DataTransmission 19 ASK Illustration

20. Chapter3a-DataTransmission 20 Frequency Shift Keying (FSK) In radio transmission, known as frequency modulation (FM) Frequency of carrier wave varies in accordance with signal to be sent Signal transmitted at constant amplitude More resistant to noise than ASK Less attractive because it requires more analog bandwidth than ASK

21. Chapter3a-DataTransmission 21 FSK Illustration

22. Chapter3a-DataTransmission 22 Phase Shift Keying (PSK) Also known as phase modulation (PM) Frequency and amplitude of carrier signal are kept constant Carrier signal is shifted in phase according to input data stream Each phase can have constant value, or value can be based on whether or not phase changes (differential keying)

23. Chapter3a-DataTransmission 23 PSK Illustration

24. Chapter3a-DataTransmission 24 Differential Phase Shift Keying (DPSK)

25. Chapter3a-DataTransmission 25 Analog Channel Capacity: BPS vs. Baud Baud=# of signal changes per second BPS=bits per second In early modems only, baud=BPS Each signal change can represent more than one bit, through complex modulation of amplitude, frequency, and/or phase Increases information-carrying capacity of channel without increasing bandwidth Increased combinations leads to increased likelihood of errors

26. Chapter3a-DataTransmission 26 Digital Signaling represented by square waves or pulses

27. Chapter3a-DataTransmission 27 Digital Text Signals Transmission of electronic pulses representing binary digits 1 and 0 Earliest example: Morse code (dots and dashes) Most common current form: ASCII

28. Chapter3a-DataTransmission 28 Digital Image Signals Analog facsimile similar to video scanning Digital facsimile, bitmapped graphics uses pixelization Object-oriented graphics image represented using library of objects e.g. Postscript, TIFF

29. Chapter3a-DataTransmission 29 Pixelization and Binary Representation Used in digital fax, bitmapped graphics

30. Chapter3a-DataTransmission 30 Analog Encoding of Digital Data Data encoding and decoding technique to represent data using properties of analog waves Modulation: conversion of digital signals to analog form Demodulation: conversion of analog data signals back to digital form

31. Chapter3a-DataTransmission 31 Digital Encoding of Analog Data Primarily used in retransmission devices Sampling theorem: If signal is sampled at regular intervals of time and at rate higher than twice significant signal frequency, samples contain all information of original signal. 8000 samples/sec sufficient for 4000hz

32. Chapter3a-DataTransmission 32 Converting Samples to Bits Quantizing Similar concept to pixelization Breaks wave into pieces, assigns value in particular range 8-bit range allows for 256 possible sample levels More bits means greater detail, fewer bits means less detail

33. Chapter3a-DataTransmission 33 Codec Coder/Decoder Converts analog signals into digital form and converts it back to analog signals Where do we find codecs? Sound cards Scanners Voice mail Video capture/conferencing

34. Chapter3a-DataTransmission 34 Digital Encodingof Digital Data Most common, easiest method is different voltage levels for two binary digits Typically, negative=1 and positive=0 Known as NRZ-L, or nonreturn-to-zero level, because signal never returns to zero, and voltage during bit transmission is level

35. Chapter3a-DataTransmission 35 Differential NRZ Differential version is NRZI (NRZ, invert on ones) Change=1, no change=0 Advantage of differential encoding is that it is more reliable to detect change in polarity than it is to accurately detect specific level

36. Chapter3a-DataTransmission 36 Problems With NRZ Difficult to determine where one bit ends and next begins In NRZ-L, long strings of ones and zeroes would appear as constant voltage pulses Timing is critical, because any drift results in lack of synchronization and incorrect bit values being transmitted

37. Chapter3a-DataTransmission 37 Biphase Alternatives to NRZ Require at least one transition per bit time, and may even have two Modulation rate is greater, so bandwidth requirements are higher Advantages Synchronization due to predictable transitions Error detection based on absence of transition

38. Chapter3a-DataTransmission 38 Manchester Code Transition in middle of each bit period Transition provides clocking and data Low-to-high=1 , high-to-low=0 Used in Ethernet

39. Chapter3a-DataTransmission 39 Differential Manchester Midbit transition is only for clocking Transition at beginning of bit period=0 Transition absent at beginning=1 Has added advantage of differential encoding Used in token-ring

40. Chapter3a-DataTransmission 40 Digital Encoding Illustration

41. Chapter3a-DataTransmission 41 Digital Interfaces The point at which one device connects to another Standards define what signals are sent, and how Some standards also define physical connector to be used

42. Chapter3a-DataTransmission 42 Generic CommunicationsInterface Illustration

43. Chapter3a-DataTransmission 43 DTE and DCE

44. Chapter3a-DataTransmission 44 RS-232C (EIA 232C) EIA’s “Recommended Standard” (RS) Specifies mechanical, electrical, functional, and procedural aspects of interface Used for connections between DTEs and voice-grade modems, and many other applications

45. Chapter3a-DataTransmission 45 EIA-232-D new version of RS-232-C adopted in 1987 improvements in grounding shield, test and loop-back signals prevalence of RS-232-C made it difficult for EIA-232-D to enter marketplace

46. Chapter3a-DataTransmission 46 RS-449 EIA standard improving on capabilities of RS-232-C provides for 37-pin connection, cable lengths up to 200 feet, and data rates up to 2 million bps covers functional/procedural portions of R-232-C electrical/mechanical specs covered by RS-422 & RS-423

47. Chapter3a-DataTransmission 47 Functional Specifications Specifies role of individual circuits Data circuits in both directions allow full-duplex communication Timing signals allow for synchronous transmission (although asynchronous transmission is more common)

48. Chapter3a-DataTransmission 48 Procedural Specifications Multiple procedures are specified Simple example: exchange of asynchronous data on private line Provides means of attachment between computer and modem Specifies method of transmitting asynchronous data between devices Specifies method of cooperation for exchange of data between devices

49. Chapter3a-DataTransmission 49 Mechanical Specifications 25-pin connector with a specific arrangement of leads DTE devices usually have male DB25 connectors while DCE devices have female In practice, fewer than 25 wires are generally used in applications

50. Chapter3a-DataTransmission 50 RS-232 DB-25 Pinouts

51. Breakout Box

52. Chapter3a-DataTransmission 52 Electrical Specifications Specifies signaling between DTE and DCE Uses NRZ-L encoding Voltage < -3V = binary 1 Voltage > +3V = binary 0 Rated for <20Kbps and <15M greater distances and rates are theoretically possible, but not necessarily wise

53. Chapter3a-DataTransmission 53 Transmission Media Physical path between transmitter and receiver (“channel”) Design factors affecting data rate bandwidth physical environment number of receivers (upcoming slide) impairments

54. Chapter3a-DataTransmission 54 Impairments and Capacity Impairments exist in all forms of data transmission Analog signal impairments result in random modifications that impair signal quality Digital signal impairments result in bit errors (1s and 0s transposed)

55. Chapter3a-DataTransmission 55 Transmission Impairments:Guided Media Attenuation loss of signal strength over distance Attenuation Distortion different losses at different frequencies Delay Distortion different speeds for different frequencies Noise distortions of signal caused by interference

56. Chapter3a-DataTransmission 56 Transmission Impairments:Unguided (Wireless) Media Free-Space Loss Signals disperse with distance Atmospheric Absorption Water vapor and oxygen contribute to signal loss Multipath Obstacles reflect signal creating multiple copies Refraction Noise

57. Chapter3a-DataTransmission 57 Types of Noise Thermal (aka “white noise”) Uniformly distributed, cannot be eliminated Intermodulation When different frequencies collide (creating “harmonics”) Crosstalk Overlap of signals Impulse noise Irregular spikes, less predictable

58. Chapter3a-DataTransmission 58 Channel Capacity The rate at which data can be transmitted over a given path, under given conditions Four concepts Data rate Bandwidth Noise Error rate

59. Chapter3a-DataTransmission 59 Transmission Media

60. Chapter3a-DataTransmission 60 Classes of Transmission Media Conducted or guided media Use conductor (wire or a fiber optic) to move signal from sender to receiver Wireless or unguided media Use radio waves of different frequencies and do not need wire or cable conductor to transmit signals

61. Chapter3a-DataTransmission 61 Design Factors for Transmission Media Bandwidth: All other factors remaining constant, greater band-width of signal, higher data rate achieved. Transmission impairments; limits distance signal can travel. Interference: Competing signals in overlapping frequency bands can distort or wipe out signal. Number of receivers: Each attachment introduces some attenuation and distortion, limiting distance and/or data rate.

62. Chapter3a-DataTransmission 62 Electromagnetic Spectrum for Transmission Media

63. Chapter3a-DataTransmission 63 Guided Transmission Media Transmission capacity depends on distance and whether medium is point-to-point or multi-point Examples twisted pair wires coaxial cables optical fiber

64. Chapter3a-DataTransmission 64 Twisted Pair Wires Consists of two insulated copper wires arranged in regular spiral pattern to minimize electromagnetic interference between adjacent pairs Often used at customer facilities and over distances to carry voice as well as data communications Low frequency transmission medium

65. Chapter3a-DataTransmission 65 Types of Twisted Pair STP (Shielded Twisted Pair) pair is wrapped with metallic foil or braid to insulate pair from electromagnetic interference UTP (Unshielded Twisted Pair) each wire is insulated with plastic wrap, but pair is encased in outer covering

66. Chapter3a-DataTransmission 66 Ratings of Twisted Pair Category 3 UTP data rates up to 16 mbps are achievable Category 5 UTP data rates up to 100 mbps are achievable more tightly twisted than Cat 3 cables more expensive, but better performance STP More expensive, harder to work handle

67. Chapter3a-DataTransmission 67 Twisted Pair Advantages Inexpensive and readily available Flexible and light weight Easy to work with and install

68. Chapter3a-DataTransmission 68 Twisted Pair Disadvantages Susceptibility to interference and noise Attenuation problem For analog, repeaters needed every 5-6 km For digital, repeaters needed every 2-3 km Relatively low bandwidth (3000 Hz)

69. Chapter3a-DataTransmission 69 Coaxial Cable (Coax) Cable television, LANs, telephony Has inner conductor surrounded by braided mesh Both conductors share common center axial, hence term “co-axial”

70. Chapter3a-DataTransmission 70 Coax Layers

71. Chapter3a-DataTransmission 71 Coax Advantages Higher bandwidth 400 to 600 Mhz up to 10,800 voice conversations Can be tapped easily (pros and cons) Less susceptible to interference than twisted pair

72. Chapter3a-DataTransmission 72 Coax Disadvantages High attenuation rate makes it expensive over long distance Bulky Difficult with which to work Ends are expensive, and easy to mess up Bad end can bring down segment Machine made ends are more reliable than man made ends

73. Chapter3a-DataTransmission 73 Fiber Optic Cable Relatively new transmission medium used by telephone companies in place of long-distance trunk lines Used by private companies in implementing LANs Require light source with Injection Laser Diode (ILD) or Light-Emitting Diodes (LED)

74. Chapter3a-DataTransmission 74 Fiber Optic Layers Consists of three concentric sections

75. Chapter3a-DataTransmission 75 Fiber Optic Types Multimode step-index fiber Reflective walls of fiber move light pulses to receiver Multimode graded-index fiber Acts to refract light toward center of fiber by variations in density Single mode fiber Light is guided down center of narrow core

76. Chapter3a-DataTransmission 76 Fiber Optic Signals

77. Chapter3a-DataTransmission 77 Fiber Optic Advantages Greater capacity (bandwidth up to 2 Gbps) Smaller size and lighter weight Lower attenuation Immunity to environmental interference Highly secure due to tap difficulty and lack of signal radiation

78. Chapter3a-DataTransmission 78 Fiber Optic Disadvantages expensive over short distance requires highly skilled installers adding additional nodes is difficult

79. Chapter3a-DataTransmission 79 Wireless (Unguided Media) Transmission transmission and reception are achieved by means of antenna directional transmitting antenna puts out focused beam transmitter and receiver must be aligned omnidirectional signal spreads in all directions can be received by many antennas

80. Chapter3a-DataTransmission 80 Wireless Examples terrestrial microwave satellite microwave broadcast radio infrared

81. Chapter3a-DataTransmission 81 Terrestrial Microwave used for long-distance telephone service radio frequency spectrum, from 2 to 40 Ghz parabolic dish transmitter, mounted high

82. Chapter3a-DataTransmission 82 Terrestrial Microwave, Cont’d used by common carriers, and private networks requires unobstructed line of sight between source and receiver curvature of earth requires stations (repeaters) ~30 miles apart

83. Chapter3a-DataTransmission 83 Satellite MicrowaveApplications Television distribution Long-distance telephone transmission Private business networks

84. Chapter3a-DataTransmission 84 Microwave Transmission Disadvantages line of sight requirement expensive towers, and repeaters subject to interference - passing airplanes, and rain

85. Chapter3a-DataTransmission 85 Satellite Microwave Transmission microwave relay station in space can relay signals over long distances geostationary satellites remain above equator at height of 22,300 miles (geosynchronous orbit) travel around earth in time earth rotate

86. Chapter3a-DataTransmission 86 Satellite Transmission Links earth stations communicate by sending signals to satellite on an uplink satellite repeats signals on downlink broadcast nature of downlink makes it attractive for services as distribution of television programming

87. Chapter3a-DataTransmission 87 Satellite Transmission Process

88. Chapter3a-DataTransmission 88 Satellite Transmission Applications television distribution network provides programming from a central location direct broadcast satellite (DBS) long-distance telephone transmission high-usage international trunks private business networks

89. Chapter3a-DataTransmission 89 Principal Satellite Transmission Bands C band: 4(downlink) - 6(uplink) GHz first to be designated Ku band: 12(downlink) -14(uplink) GHz rain interference is major problem Ka band: 19(downlink) - 29(uplink) GHz equipment needed to use band is expensive

90. Chapter3a-DataTransmission 90 Fiber vs Satellite

91. Chapter3a-DataTransmission 91 Radio Radio is omnidirectional and microwave is directional Radio is term used to encompass frequencies in range 3 kHz to 300 GHz. Mobile telephony occupies several frequency bands just under 1 GHz.

92. Chapter3a-DataTransmission 92 Infrared Uses transmitters/receivers (transceivers) that modulate noncoherent infrared light. Transceivers must be within line of sight of each other (directly or via reflection ). Unlike microwaves, infrared does not penetrate walls.