1. Data and Computer Communications Eighth Edition by William Stallings Lecture slides by Lawrie Brown Chapter 4 –Transmission Media

2. Transmission Media Communication channels in the animal world include touch, sound, sight, and scent. Electric eels even use electric pulses. Ravens also are very expressive. By a combination voice, patterns of feather erection and body posture ravens communicate so clearly that an experienced observer can identify anger, affection, hunger, curiosity, playfulness, fright, boldness, and depression. Communication channels in the animal world include touch, sound, sight, and scent. Electric eels even use electric pulses. Ravens also are very expressive. By a combination voice, patterns of feather erection and body posture ravens communicate so clearly that an experienced observer can identify anger, affection, hunger, curiosity, playfulness, fright, boldness, and depression. —Mind of the Raven, Bernd Heinrich

3. 3 Transmission Media - Overview  Transmission Medium Physical path between transmitter and receiver Physical path between transmitter and receiver  Guided Media Waves are guided along a solid medium Waves are guided along a solid medium e.g., copper twisted pair, copper coaxial cable, optical fibere.g., copper twisted pair, copper coaxial cable, optical fiber  Unguided Media Provides means of transmission but does not guide electromagnetic signals Provides means of transmission but does not guide electromagnetic signals Employ an antenna for transmission Employ an antenna for transmission e.g., atmosphere, outer spacee.g., atmosphere, outer space

4. 4 Transmission Media - Overview  Characteristics and quality determined by medium and signal  For guided Medium is more important Medium is more important  For unguided Bandwidth produced by the antenna is more important Bandwidth produced by the antenna is more important  Key concerns are Data rate and Distance Data rate and Distance The greater the BETTER! The greater the BETTER!

5. 5 Design Factors  Bandwidth higher bandwidth gives higher data rate higher bandwidth gives higher data rate  Transmission impairments Attenuation limits distance. Attenuation limits distance.  Interference Overlapping frequency bands in unguided media. Overlapping frequency bands in unguided media. Nearby cables. Nearby cables.  Number of receivers In guided media In guided media More receivers (multi-point) introduce more attenuation More receivers (multi-point) introduce more attenuation

6. Electromagnetic Spectrum

7. Transmission Characteristics of Guided Media Frequency Range Typical Attenuation Typical Delay Repeater Spacing Twisted pair (with loading) 0 to 3.5 kHz0.2 dB/km @ 1 kHz 50 µs/km2 km Twisted pairs (multi-pair cables) 0 to 1 MHz0.7 dB/km @ 1 kHz 5 µs/km2 km Coaxial cable0 to 500 MHz7 dB/km @ 10 MHz 4 µs/km1 to 9 km Optical fiber186 to 370 THz 0.2 to 0.5 dB/km 5 µs/km40 km

8. 8 Transmission Media  Guided media Twisted-pair Twisted-pair Coaxial Cable Coaxial Cable Optical Fiber Optical Fiber  Unguided media Satellites Satellites Terrestrial Microwave Terrestrial Microwave Broadcast Radio Broadcast Radio

9. 9 Guided Transmission Media  Transmission Capacity Either in terms of Either in terms of Bandwidth, orBandwidth, or Data RateData Rate Depends critically on Depends critically on DistanceDistance Type of mediumType of medium Point-to-point Point-to-point Mutipoint Mutipoint

10. 10 Twisted Pair  Most common medium  Two insulated wires twisted together in a helical manner (like DNA)  Advantages Cheap Cheap Easy to work with Easy to work with  Disadvantages Low data rate Low data rate Short range Short range

11. Twisted Pair

12. 12 Twisted Pair - Applications  Telephone network Between house and local exchange Between house and local exchange  Within buildings To private branch exchange (PBX) To private branch exchange (PBX)  For local area networks (LAN) 10 Mbps or 100 Mbps 10 Mbps or 100 Mbps

13. 13 Twisted Pair - Transmission Characteristics  Analog Amplifiers every 5 km to 6 km Amplifiers every 5 km to 6 km  Digital Use either analog or digital signals Use either analog or digital signals Repeater every 2 km or 3 km Repeater every 2 km or 3 km  Limited in Distance Distance Bandwidth (1 MHz) Bandwidth (1 MHz) Data rate (100 Mbps) Data rate (100 Mbps)  Susceptible to interference and noise

14. Unshielded vs Shielded TP  unshielded Twisted Pair (UTP) ordinary telephone wire ordinary telephone wire cheapest cheapest easiest to install easiest to install suffers from external EM interference suffers from external EM interference  shielded Twisted Pair (STP) metal braid or sheathing that reduces interference metal braid or sheathing that reduces interference more expensive more expensive harder to handle (thick, heavy) harder to handle (thick, heavy)  in a variety of categories - see EIA-568

15. Unshielded vs Shielded TP  unshielded Twisted Pair (UTP) ordinary telephone wire ordinary telephone wire cheapest cheapest easiest to install easiest to install suffers from external EM interference suffers from external EM interference  shielded Twisted Pair (STP) metal braid or sheathing that reduces interference metal braid or sheathing that reduces interference more expensive more expensive harder to handle (thick, heavy) harder to handle (thick, heavy)  in a variety of categories - see EIA-568

16. 16 UTP Categories  Cat 3 Up to 16 MHz Up to 16 MHz Voice grade found in most offices Voice grade found in most offices Twist length of 7.5 cm to 10 cm Twist length of 7.5 cm to 10 cm  Cat 4 Up to 20 MHz Up to 20 MHz  Cat 5 Up to 100 MHz Up to 100 MHz Commonly pre-installed in new office buildings Commonly pre-installed in new office buildings Twist length 0.6 cm to 0.85 cm Twist length 0.6 cm to 0.85 cm

17. UTP Categories

18. Comparison of Shielded and Unshielded Twisted Pair

19. Near End Crosstalk  coupling of signal from one pair to another  occurs when transmit signal entering the link couples back to receiving pair  ie. near transmitted signal is picked up by near receiving pair  The tighter the twist in the cable, the more effective the cancellation  Twisting is used to balance the noise. http://www.cabletesting.com

20. 20 Coaxial Cable  Most versatile medium *Braided shield is also referred to as the “outer conductor”

21. 21 Coaxial Cable Applications  Television distribution Cable TV Cable TV  Long distance telephone transmission Can carry 10,000 voice calls simultaneously Can carry 10,000 voice calls simultaneously Being replaced by fiber optic Being replaced by fiber optic  Short distance computer systems links  LANs

22. Coaxial Cable - Transmission Characteristics  superior frequency characteristics to TP  performance limited by attenuation & noise  analog signals amplifiers every few km amplifiers every few km closer if higher frequency closer if higher frequency up to 500MHz up to 500MHz  digital signals repeater every 1km repeater every 1km closer for higher data rates closer for higher data rates

23. 23 Optical Fiber  Greater capacity Data rates of hundreds of Gbps Data rates of hundreds of Gbps  Smaller size & weight  Lower attenuation  Electromagnetic isolation  Greater repeater spacing 10s of km at least 10s of km at least

24. 24 Optical Fiber  System components: Transmission medium - fiber optic cable Transmission medium - fiber optic cable Light source - LED or laser diode Light source - LED or laser diode Detector - photodiode Detector - photodiode

25. 25 Optical Fiber - Applications  Telephone Network Applications Long-haul, metropolitan, rural, and subscriber loop circuits Long-haul, metropolitan, rural, and subscriber loop circuits  Local Area Networks Optical fiber networks Optical fiber networks Data rates from 100 Mbps to 1 Gbps Data rates from 100 Mbps to 1 Gbps Support hundreds (or even thousands) of stations Support hundreds (or even thousands) of stations

26. 26 Optical Fiber - Transmission Characteristics  Light Sources Light Emitting Diode (LED) Light Emitting Diode (LED) CheaperCheaper Wider operating temp rangeWider operating temp range Last longerLast longer Injection Laser Diode (ILD) Injection Laser Diode (ILD) More efficientMore efficient Greater data rateGreater data rate  Wavelength Division Multiplexing

27. Optical Fiber

28. 28 Optical Fiber Transmission Modes  Three types of transmission modes: – Step-index multimode: Rays are reflected, absorbed and propagated along the fiber. Rays are reflected, absorbed and propagated along the fiber. Signal elements (light pulses) spread out in time. Signal elements (light pulses) spread out in time. Suited for short distance transmission. Suited for short distance transmission. – Single mode: Fiber core diameter is reduced to a single wavelength (3-10 µm). Fiber core diameter is reduced to a single wavelength (3-10 µm). Single transmission path. Single transmission path. Long distance application (telephone and cable TV). Long distance application (telephone and cable TV). – Graded-index multimode: Intermediate between single mode and step-index multimode. Intermediate between single mode and step-index multimode. Used in LAN. Used in LAN.

29. Optical Fiber Transmission Modes

30. 30 Optical Fiber Transmission Modes

31. Frequency Utilization for Fiber Applications

32. Attenuation in Guided Media

33. 33 Wireless Transmission  Unguided media (transmission and reception via antenna). – Transmission: the antenna radiates electromagnetic energy into the medium (usually air). – Reception: the antenna pick up electromagnetic waves from the surrounding medium.  Two basic configurations: – Directional: Focused electromagnetic beam. Focused electromagnetic beam. Careful alignment required. Careful alignment required. – Omnidirectional: Signal spreads in all directions. Signal spreads in all directions. Can be received by many antennas. Can be received by many antennas.

34. Wireless Transmission Frequencies  >1 GHz to 40GHz microwave microwave highly directional highly directional point to point point to point satellite satellite  30MHz to 1GHz (radio) omnidirectional omnidirectional broadcast radio broadcast radio  3 x 10 11 to 2 x 10 14 infrared infrared local local

35. Antennas  electrical conductor used to radiate or collect electromagnetic energy  transmission antenna radio frequency energy from transmitter radio frequency energy from transmitter converted to electromagnetic energy by antenna converted to electromagnetic energy by antenna radiated into surrounding environment radiated into surrounding environment  reception antenna electromagnetic energy impinging on antenna electromagnetic energy impinging on antenna converted to radio frequency electrical energy converted to radio frequency electrical energy fed to receiver fed to receiver  same antenna is often used for both purposes

36. 36 Radiation Pattern  Away to characterize the performance of an antenna  A graphical representation of the radiation as a function of space coordinates  Power radiated in all directions  Not same performance in all directions  Isotropic antenna is (theoretical) point in space Radiates in all directions equally Radiates in all directions equally Gives spherical radiation pattern Gives spherical radiation pattern

37. Radiation Pattern

38. 38 Parabolic Reflective Antenna  Used for terrestrial and satellite microwave  Parabola is locus of point equidistant from a line and a point not on that line Fixed point is focus Fixed point is focus Line is directrix Line is directrix  Revolve parabola about axis to get paraboloid Cross section parallel to axis gives parabola Cross section parallel to axis gives parabola Cross section perpendicular to axis gives circle Cross section perpendicular to axis gives circle  Source placed at focus will produce waves reflected from parabola in parallel to axis Creates (theoretical) parallel beam of light/sound/radio Creates (theoretical) parallel beam of light/sound/radio  On reception, signal is concentrated at focus, where detector is placed

39. Parabolic Reflective Antenna

41. Antenna Gain  measure of directionality of antenna  power output in particular direction verses that produced by an isotropic antenna  measured in decibels (dB)  results in loss in power in another direction  effective area relates to size and shape related to gain related to gain

42. Antenna Gain G= Antenna gain  A e = effective area (related to the physical size of the antenna and to its shape) F = carrier frequency C = speed of light  the effective area of an ideal isotropic antenna is with a power gain of 1  effective area of a parabolic antenna with a face area of A is 0.56A

43. Example

44. Terrestrial Microwave  used for long haul telecommunications  alternative to coaxial cable or optical fiber  and short point-to-point links closed-circuit TV or as a data link between local area networks closed-circuit TV or as a data link between local area networks  requires fewer repeaters but line of sight  use a parabolic dish to focus a narrow beam onto a receiver antenna

45. Terrestrial Microwave  Characteristics 1-40GHz frequencies 1-40GHz frequencies  higher frequencies give higher data rates  main source of loss is attenuation  distance, rainfall (above 10 GHz)  also interference  d and lamda have the same uni t

46. Satellite Microwave  satellite is relay station  receives on one frequency, amplifies or repeats signal and transmits on another frequency eg. uplink 5.925-6.425 GHz & downlink 3.7-4.2 GHz eg. uplink 5.925-6.425 GHz & downlink 3.7-4.2 GHz  typically requires geo-stationary orbit height of 35,784km height of 35,784km spaced at least 3-4° apart (angular displacement as measured from the earth) spaced at least 3-4° apart (angular displacement as measured from the earth)  typical uses television television long distance telephone long distance telephone private business networks private business networks global positioning global positioning

47. Satellite Point to Point Link

48. Satellite Broadcast Link

49. Broadcast Radio  radio is 3kHz to 300GHz  use broadcast radio, 30MHz - 1GHz, for: FM radio FM radio UHF and VHF television UHF and VHF television  is omnidirectional  the ionosphere is transparent to radio waves above 30 MHz  still need line of sight  suffers from multipath interference reflections from land, water, other objects reflections from land, water, other objects

50. Infrared  modulate noncoherent infrared light  end line of sight (or reflection)  are blocked by walls  no licenses required  typical uses TV remote control TV remote control IRD port IRD port

51. 51 Wireless Propagation-Ground wave  Follows contour of earth  Up to 2MHz  AM radio  Reasons  the electromagnetic wave induces a current in the earth’s surface  slow the wavefront near the earth  tilt downward and hence follow the earth’s curvature diffraction, the behavior of electromagnetic waves in the presence of obstacles diffraction, the behavior of electromagnetic waves in the presence of obstacles

52. 52 Wireless Propagation-Sky wave  Amateur radio, BBC world service, Voice of America  Signal reflected from ionosphere layer of upper atmosphere  (Actually refracted)  sky wave signal can travel through a number of hops, bouncing back and forth between the ionosphere and the earth’s surface

53. 53 Wireless Propagation- Line of sight  Above 30Mhz  neither ground wave nor sky wave propagation modes operate communication must be by line of sight communication must be by line of sight  satellite communication, signal is not reflected by the ionosphere  ground-based communication, the transmitting and receiving antennas must be within an effective line of sight of each other

54. Wireless Propagation Ground Wave

55. Wireless Propagation Sky Wave

56. Wireless Propagation Line of Sight

57. 57 Refraction  Velocity of electromagnetic wave is a function of density of material ~3 x 10 8 m/s in vacuum, less in anything else ~3 x 10 8 m/s in vacuum, less in anything else  As wave moves from one medium to another, its speed changes Causes bending of direction of wave at boundary Causes bending of direction of wave at boundary Moving from a less dense to a more dense medium Moving from a less dense to a more dense medium Towards more dense mediumTowards more dense medium

58. 58 Refraction  Index of refraction (refractive index) of one medium relative to another is sin(angle of incidence)/sin(angle of refraction) sin(angle of incidence)/sin(angle of refraction) Varies with wavelength Varies with wavelength  May cause sudden change of direction at transition between media  May cause gradual bending if medium density is varying Density of atmosphere decreases with height Density of atmosphere decreases with height Results in bending towards earth of radio waves Results in bending towards earth of radio waves

59. 59 Optical and Radio Horizons no intervening obstacles, the optical line of sight d = the distance between an antenna and the horizon For radio K is adjustment factor to account for the refraction =4/3

60. Optical and Radio Horizons  the maximum distance between two antennas for LOS  where h1 and h2 are the heights of the two

61. Optical and Radio Horizons

62. Line of Sight Transmission  Free space loss loss of signal with distance loss of signal with distance  Atmospheric Absorption from water vapour and oxygen absorption from water vapour and oxygen absorption  Multipath multiple interfering signals from reflections multiple interfering signals from reflections  Refraction bending signal away from receiver bending signal away from receiver

63. 63 Line of Sight Transmission  Free space loss Signal disperses with distance Signal disperses with distance Greater for lower frequencies (longer wavelengths) Greater for lower frequencies (longer wavelengths)  Free space loss (ideal isotropic)  For other antenna

64. 64 Free Space Loss

65. 65 Line of Sight Transmission  Atmospheric Absorption Water vapour and oxygen absorb radio signals Water vapour and oxygen absorb radio signals Water greatest at 22GHz, less below 15GHz Water greatest at 22GHz, less below 15GHz Oxygen greater at 60GHz, less below 30GHz Oxygen greater at 60GHz, less below 30GHz Rain and fog scatter radio waves Rain and fog scatter radio waves  Multipath Better to get line of sight if possible Better to get line of sight if possible Signal can be reflected causing multiple copies to be received Signal can be reflected causing multiple copies to be received May be no direct signal at all May be no direct signal at all May reinforce or cancel direct signal May reinforce or cancel direct signal  Refraction May result in partial or total loss of signal at receiver May result in partial or total loss of signal at receiver

66. 66 Multipath Interference

67. Free Space Loss

68. Summary  looked at data transmission issues  frequency, spectrum & bandwidth  analog vs digital signals  transmission impairments

69. Problem Assignments  Solve all the review questions  Try the following problems:  4.1, 4.3, 4.6, 4.7, 4.14, 4.15, 4.16, 4.18