1. CWNA Guide to Wireless LANs, Second Edition Chapter Three How Wireless Works

2. CWNA Guide to Wireless LANs, Second Edition2 Objectives Explain the principals of radio wave transmissions Describe RF loss and gain, and how it can be measured List some of the characteristics of RF antenna transmissions Describe the different types of antennas

3. CWNA Guide to Wireless LANs, Second Edition3 Radio Wave Transmission Principles Understanding principles of radio wave transmission is important for: –Troubleshooting wireless LANs –Creating a context for understanding wireless terminology

4. CWNA Guide to Wireless LANs, Second Edition4 What Are Radio Waves? Electromagnetic wave: Travels freely through space in all directions at speed of light Radio wave: When electric current passes through a wire it creates a magnetic field around the wire –As magnetic field radiates, creates an electromagnetic radio wave Spreads out through space in all directions –Can travel long distances –Can penetrate non-metallic objects

5. CWNA Guide to Wireless LANs, Second Edition5 What Are Radio Waves? (continued) Table 3-1: Comparison of wave characteristics

6. CWNA Guide to Wireless LANs, Second Edition6 Analog vs. Digital Transmissions Figure 3-4: Digital signal Figure 3-2: Analog signal

7. CWNA Guide to Wireless LANs, Second Edition7 Analog vs. Digital Transmissions (continued) Analog signals are continuous Digital signals are discrete Modem (MOdulator/DEModulator): Used when digital signals must be transmitted over analog medium –On originating end, converts distinct digital signals into continuous analog signal for transmission –On receiving end, reverse process performed WLANs use digital transmissions

8. CWNA Guide to Wireless LANs, Second Edition8 Radio Frequency Radio frequency, (RF) is a term that refers to alternating current, (AC) having characteristics such that, if the current is input to an antenna, an electromagnetic (EM) field/wave is generated suitable for wireless communications. AC Signal Transmission Line Antenna and Tower EM Wave

9. CWNA Guide to Wireless LANs, Second Edition9 RF Spectrum DesignationAbbreviationFrequencies Ultra High FrequencyUHF300 MHz - 3 GHz Super High Frequency SHF 3 GHz - 30 GHz Very Low Frequency - Extremely High Frequency VLF - EHF9 kHz – 300 GHz

10. CWNA Guide to Wireless LANs, Second Edition10 US Frequency Allocation Chart National Telecommunications and Information Administration. http://www.ntia.doc.gov/osmhome/allochrt.html 9 kHz 300 GHz 802.11 a, b, g AM Radio FM Radio 535-1605 kHz 88-108 MHz

11. CWNA Guide to Wireless LANs, Second Edition11 Frequency Figure 3-5: Long waves Figure 3-6: Short Waves

12. CWNA Guide to Wireless LANs, Second Edition12 Frequency (continued) Frequency: Rate at which an event occurs Cycle: Changing event that creates different radio frequencies –When wave completes trip and returns back to starting point it has finished one cycle Hertz (Hz): Cycles per second –Kilohertz (KHz) = thousand hertz –Megahertz (MHz) = million hertz –Gigahertz (GHz) = billion hertz

13. CWNA Guide to Wireless LANs, Second Edition13 Frequency (continued) Figure 3-7: Sine wave

14. CWNA Guide to Wireless LANs, Second Edition14 Frequency (continued) Table 3-2: Electrical terminology

15. CWNA Guide to Wireless LANs, Second Edition15 Frequency (continued) Frequency of radio wave can be changed by modifying voltage Radio transmissions send a carrier signal –Increasing voltage will change frequency of carrier signal

16. CWNA Guide to Wireless LANs, Second Edition16 Frequency (continued) Figure 3-8: Lower and higher frequencies

17. CWNA Guide to Wireless LANs, Second Edition17 Modulation Carrier signal is a continuous electrical signal –Carries no information Three types of modulations enable carrier signals to carry information –Height of signal –Frequency of signal –Relative starting point Modulation can be done on analog or digital transmissions

18. CWNA Guide to Wireless LANs, Second Edition18 Analog Modulation Amplitude: Height of carrier wave Amplitude modulation (AM): Changes amplitude so that highest peaks of carrier wave represent 1 bit while lower waves represent 0 bit Frequency modulation (FM): Changes number of waves representing one cycle –Number of waves to represent 1 bit more than number of waves to represent 0 bit Phase modulation (PM): Changes starting point of cycle –When bits change from 1 to 0 bit or vice versa

19. CWNA Guide to Wireless LANs, Second Edition19 Analog Modulation (continued) Figure 3-9: Amplitude

20. CWNA Guide to Wireless LANs, Second Edition20 Analog Modulation (continued) Figure 3-10: Amplitude modulation (AM)

21. CWNA Guide to Wireless LANs, Second Edition21 Analog Modulation (continued) Figure 3-11: Frequency modulation (FM)

22. CWNA Guide to Wireless LANs, Second Edition22 Analog Modulation (continued) Figure 3-12: Phase modulation (PM)

23. CWNA Guide to Wireless LANs, Second Edition23 Digital Modulation Advantages over analog modulation: –Better use of bandwidth –Requires less power –Better handling of interference from other signals –Error-correcting techniques more compatible with other digital systems Unlike analog modulation, changes occur in discrete steps using binary signals –Uses same three basic types of modulation as analog

24. CWNA Guide to Wireless LANs, Second Edition24 Digital Modulation (continued) Figure 3-13: Amplitude shift keying (ASK)

25. CWNA Guide to Wireless LANs, Second Edition25 Digital Modulation (continued) Figure 3-14: Frequency shift keying (FSK)

26. CWNA Guide to Wireless LANs, Second Edition26 Digital Modulation (continued) Figure 3-15: Phase shift keying (PSK)

27. CWNA Guide to Wireless LANs, Second Edition27 Amplification and Attenuation Amplification/Gain - An increase in signal level, amplitude or magnitude of a signal. A device that does this is called an amplifier. Attenuation/Loss - A decrease in signal level, amplitude, or magnitude of a signal. A device that does this is called an attenuator.

28. CWNA Guide to Wireless LANs, Second Edition28 Amplification 100 mW RF Amplifier 1 W Signal Source Antenna INPUT OUTPUT The power gain of the RF amplifier is a power ratio. Power Gain = = = 10 no units Power Output Power Input 1 W 100 mW

29. CWNA Guide to Wireless LANs, Second Edition29 Attenuation 100 mW RF Attenuator 50 mW Signal Source Antenna INPUT OUTPUT The power loss of the RF attenuator is a power ratio. Power Loss = = = 0.5 no units Power Output Power Input 50 mW 100 mW

30. CWNA Guide to Wireless LANs, Second Edition30 Radio Frequency Behavior: Gain Gain: Positive difference in amplitude between two signals –Achieved by amplification of signal –Technically, gain is measure of amplification –Can occur intentionally from external power source that amplifies signal –Can occur unintentionally when RF signal bounces off an object and combines with original signal to amplify it

31. CWNA Guide to Wireless LANs, Second Edition31 Radio Frequency Behavior: Gain (continued) Figure 3-16: Gain

32. CWNA Guide to Wireless LANs, Second Edition32 Radio Frequency Behavior: Loss Loss: Negative difference in amplitude between signals –Attenuation –Can be intentional or unintentional –Intentional loss may be necessary to decrease signal strength to comply with standards or to prevent interference –Unintentional loss can be cause by many factors

33. CWNA Guide to Wireless LANs, Second Edition33 Radio Frequency Behavior: Loss (continued) Figure 3-18: Absorption

34. CWNA Guide to Wireless LANs, Second Edition34 Radio Frequency Behavior: Loss (continued) Figure 3-19: Reflection

35. CWNA Guide to Wireless LANs, Second Edition35 Radio Frequency Behavior: Loss (continued) Figure 3-20: Scattering

36. CWNA Guide to Wireless LANs, Second Edition36 Radio Frequency Behavior: Loss (continued) Figure 3-21: Refraction

37. CWNA Guide to Wireless LANs, Second Edition37 Radio Frequency Behavior: Loss (continued) Figure 3-22: Diffraction

38. CWNA Guide to Wireless LANs, Second Edition38 Radio Frequency Behavior: Loss (continued) Figure 3-23: VSWR

39. CWNA Guide to Wireless LANs, Second Edition39 RF Measurement: RF Math RF power measured by two units on two scales: –Linear scale: Using milliwatts (mW) Reference point is zero Does not reveal gain or loss in relation to whole –Relative scale: Reference point is the measurement itself Often use logarithms Measured in decibels (dB) 10’s and 3’s Rules of RF Math: Basic rule of thumb in dealing with RF power gain and loss

40. CWNA Guide to Wireless LANs, Second Edition40 RF Measurement: RF Math (continued) Table 3-3: The 10’s and 3’s Rules of RF Math

41. CWNA Guide to Wireless LANs, Second Edition41 RF Measurement: RF Math (continued) dBm: Reference point that relates decibel scale to milliwatt scale Equivalent Isotropically Radiated Power (EIRP): Power radiated out of antenna of a wireless system –Includes intended power output and antenna gain –Uses isotropic decibels (dBi) for units Reference point is theoretical antenna with 100 percent efficiency

42. CWNA Guide to Wireless LANs, Second Edition42 RF Measurement: WLAN Measurements In U.S., FCC defines power limitations for WLANs –Limit distance that WLAN can transmit Transmitter Power Output (TPO): Measure of power being delivered to transmitting antenna Receive Signal Strength Indicator (RSSI): Used to determine dBm, mW, signal strength percentage Table 3-4: IEEE 802.11b and 802.11g EIRP

43. CWNA Guide to Wireless LANs, Second Edition43 Parameters & Units of Measure Power - The rate at which work is done, expressed as the amount of work per unit time. Watt - An International System unit of power equal to one joule per second. The power dissipated by a current of 1 ampere flowing between 1 volt of differential.

44. CWNA Guide to Wireless LANs, Second Edition44 Parameters & Units of Measure Current - a flow of electric charge; The amount of electric charge flowing past a specified circuit point per unit time. Ampere – Unit of current.

45. CWNA Guide to Wireless LANs, Second Edition45 Parameters & Units of Measure Voltage - electric potential or potential difference expressed in volts. Volt - a unit of potential equal to the potential difference between two points on a conductor carrying a current of 1 ampere when the power dissipated between the two points is 1 watt.

46. CWNA Guide to Wireless LANs, Second Edition46 Decibels The decibel is defined as one tenth of a bel where one bel is a unit of a logarithmic power scale and represents a difference between two power levels where one is ten times greater than the other. dB = 10 log 10 PXPX P Ref

47. CWNA Guide to Wireless LANs, Second Edition47 Relative and Absolute dB Relative dB is selecting any value for P Ref dB Absolute dB is selecting a standard value for P Ref and identifying the standard value with one or more letter following the dB variable. dBmdBWdBVdBspl

48. CWNA Guide to Wireless LANs, Second Edition48 dB Sample Problem 100 mW RF Amplifier 1 W Signal Source Antenna INPUT OUTPUT Compute the relative power gain of the RF Amplifier in dB. dB = 10 log 10 ( 1W / 100 mW) = 10 log 10 ( 10 ) = 10 ( 1 ) = 10 dB P Ref

49. CWNA Guide to Wireless LANs, Second Edition49 dB Sample Problem 100 mW RF Attenuator 50 mW Signal Source Antenna INPUT OUTPUT Compute the relative power loss of the RF Amplifier in dB. dB = 10 log 10 ( 50 mW / 100 mW) = 10 log 10 (.5 ) = 10 ( -0.3 ) = -3.0 dB P Ref

50. CWNA Guide to Wireless LANs, Second Edition50 dB Sample Problem dBm = 10 log 10 ( 2W / 1 mW) = 10 log 10 ( 2000 ) = 10 ( 3.3 ) = 33 dBm P Ref 50 mW RF Amplifier 2 W Signal Source Antenna INPUT OUTPUT Compute the absolute dBm power level at the output of the RF Amplifier.

51. CWNA Guide to Wireless LANs, Second Edition51 dB Sample Problem 36 dBm = 10 log 10 ( P X / 1 mW) 3.6 = log 10 ( P X / 1 mW) antilog (3.6) = antilog log 10 ( P X / 1 mW) 3,980 = ( P X / 1 mW) 3,980 x 1 mW = P X P X = 3.98 W  4 W RF Amplifier Signal Source Antenna Compute the power level in watts at the output of the RF Amplifier. 36 dBm RF Power Meter

52. CWNA Guide to Wireless LANs, Second Edition52 dB Sample Problem Access Point 20 dBm Output Point APoint B L Antenna Cable loss = - 1.3 dB Power at point A is 20 dBm = 100 mW Power at point B is 20 dBm – 1.3 dB = 18.7 dBm = 74.1 mW

53. CWNA Guide to Wireless LANs, Second Edition53 Antenna Concepts Radio waves transmitted/received using antennas Figure 3-24: Antennas are required for sending and receiving radio signals

54. CWNA Guide to Wireless LANs, Second Edition54 Antenna Gain Antenna Gain - is a measure of the ability of the antenna to focus radio waves in a particular direction. It is the ratio of the power required at the input of a reference antenna to the power supplied to the input of the given antenna to produce the same field strength at the same location.

55. CWNA Guide to Wireless LANs, Second Edition55 Antenna Gain The light analogy. Reference device Omni-directional Radiation Pattern Lamp 1 Watt Eye

56. CWNA Guide to Wireless LANs, Second Edition56 Antenna Gain The light analogy. Focus/Field Strength Directional Radiation Pattern Lamp 1 Watt Eye Reflector

57. CWNA Guide to Wireless LANs, Second Edition57 Two reference Antennas Isotropic Antenna - A hypothetical antenna that radiates or receives energy equally in all directions. dBi or G i Dipole Antenna - a straight, center-fed, one-half wavelength antenna. dBd or G d

58. CWNA Guide to Wireless LANs, Second Edition58 Characteristics of RF Antenna Transmissions Polarization: Orientation of radio waves as they leave the antenna Figure 3-25: Vertical polarization

59. CWNA Guide to Wireless LANs, Second Edition59 Characteristics of RF Antenna Transmissions (continued) Wave propagation: Pattern of wave dispersal Figure 3-26: Sky wave propagation

60. CWNA Guide to Wireless LANs, Second Edition60 Characteristics of RF Antenna Transmissions (continued) Figure 3-27: RF LOS propagation

61. CWNA Guide to Wireless LANs, Second Edition61 Characteristics of RF Antenna Transmissions (continued) Because RF LOS propagation requires alignment of sending and receiving antennas, ground-level objects can obstruct signals –Can cause refraction or diffraction –Multipath distortion: Refracted or diffracted signals reach receiving antenna later than signals that do not encounter obstructions Antenna diversity: Uses multiple antennas, inputs, and receivers to overcome multipath distortion

62. CWNA Guide to Wireless LANs, Second Edition62 Characteristics of RF Antenna Transmissions (continued) Determining extent of “late” multipath signals can be done by calculating Fresnel zone Figure 3-28: Fresnel zone

63. CWNA Guide to Wireless LANs, Second Edition63 Line of Sight (LOS) An unobstructed path between sending and receiving antennas. Line of Sight Transmitters Mountain Range Receivers Lake

64. CWNA Guide to Wireless LANs, Second Edition64 Fresnel Zone Fresnel Zone - one of a (theoretically infinite) number of a concentric ellipsoids of revolution centered around the LOS path. Provides a technique to determine the required clearance between the signal and any obstacles along the transmission path.

65. CWNA Guide to Wireless LANs, Second Edition65 Fresnel Zone D2D2 D1D1 (D 1 ) (D 2 ) f (D 1 + D 2 ) 72.1 D 3 = D3D3 WISP Building Client Condos Water Tower

66. CWNA Guide to Wireless LANs, Second Edition66 Characteristics of RF Antenna Transmissions (continued) As RF signal propagates, it spreads out –Free space path loss: Greatest source of power loss in a wireless system –Antenna gain: Only way for an increase in amplification by antenna Alter physical shape of antenna –Beamwidth: Measure of focusing of radiation emitted by antenna Measured in horizontal and vertical degrees

67. CWNA Guide to Wireless LANs, Second Edition67 Characteristics of RF Antenna Transmissions (continued) Table 3-5: Free space path loss for IEEE 802.11b and 802.11g WLANs

68. CWNA Guide to Wireless LANs, Second Edition68 Antenna Types and Their Installations Two fundamental characteristics of antennas: –As frequency gets higher, wavelength gets smaller Size of antenna smaller –As gain increases, coverage area narrows High-gain antennas offer larger coverage areas than low-gain antennas at same input power level Omni-directional antenna: Radiates signal in all directions equally –Most common type of antenna

69. CWNA Guide to Wireless LANs, Second Edition69 Antenna Types and Their Installations (continued) Semi-directional antenna: Focuses energy in one direction –Primarily used for short and medium range remote wireless bridge networks Highly-directional antennas: Send narrowly focused signal beam –Generally concave dish-shaped devices –Used for long distance, point-to-point wireless links

70. CWNA Guide to Wireless LANs, Second Edition70 Antenna Types and Their Installations (continued) Figure 3-29: Omni-directional antenna

71. CWNA Guide to Wireless LANs, Second Edition71 Antenna Types and Their Installations (continued) Figure 3-30: Semi-directional antenna

72. CWNA Guide to Wireless LANs, Second Edition72 WLAN Antenna Locations and Installation Because WLAN systems use omni-directional antennas to provide broadest area of coverage, APs should be located near middle of coverage area Antenna should be positioned as high as possible If high-gain omni-directional antenna used, must determine that users located below antenna area still have reception

73. CWNA Guide to Wireless LANs, Second Edition73 Attenuation of an EM wave Attenuation/Loss - A decrease in signal level, amplitude, or magnitude of a signal.

74. CWNA Guide to Wireless LANs, Second Edition74 Basic Properties of EM waves Reflection – cast off or turn back, (bouncing).

75. CWNA Guide to Wireless LANs, Second Edition75 Basic Properties of EM waves Refraction - deflection from a straight path, (bending). Earth Atmosphere Refracted Wave Path Straight-Line Wave Path Sky Wave Antenna

76. CWNA Guide to Wireless LANs, Second Edition76 Basic Properties of EM waves Diffraction – Change in the directions and intensities of a group of waves when they pass near the edge of an EM opaque object, (scattering). Transmitter Receiver Building Shadow Zone Diffracted Signal

77. CWNA Guide to Wireless LANs, Second Edition77 Basic Properties of EM waves Interference - hinders, obstructs, or impedes. When two or more wave fronts meet, (colliding). Direct Wave Multipath Interference Reflected Wave

78. CWNA Guide to Wireless LANs, Second Edition78 EIRP EIRP - The product of the power supplied to the antenna and the antenna gain in a given direction relative to a reference antenna. EIRP = P in X G i 1.58 W = 100 mW x 15.8 AP 100 mW 12 dBi = 15.8 Antenna

79. CWNA Guide to Wireless LANs, Second Edition79 EIRP Access Point 20 dBm Output Point APoint B Parabolic Antenna 24 dbi Cable loss = - 1.3 dB Power at point A is 20 dBm = 100 mW Power at point B is 20 dBm – 1.3 dB = 18.7 dBm = 74.1 mW EIRP at point C is 74.1 mW x 251 = 18.6 W Point C

80. CWNA Guide to Wireless LANs, Second Edition80 System Problem AP Antenna Find the EIRP given: AP Power Output 100 mW N-connector insertion loss 0.2 dB max Lightning Surge Arrester insertion loss 0.4 dB max RG-8/U Coax cable loss 6.7 dB/100 feet. There is a total cable run of 43 feet in this problem. Antenna gain 24 dBi Lightning Surge Arrester

81. CWNA Guide to Wireless LANs, Second Edition81 Voltage Standing Wave Ratio VSWR - is a measure of how well the components of the RF system are matched in impedance. VSWR is the ratio of the maximum voltage to the minimum voltage in a standing wave. For maximum power transfer the ideal VSWR is 1.

82. CWNA Guide to Wireless LANs, Second Edition82 Voltage Standing Wave Ratio 50  Output impedance of AP is 50  Impedance of cable is 50  Input impedance of antenna is 50  The impedances are matched so the VSWR = 1

83. CWNA Guide to Wireless LANs, Second Edition83 Voltage Standing Wave Ratio 50  1.0 VSWR 50  25  2.0 VSWR VSWR Meter VSWR = Z1Z1 Z2Z2 = 50  25  = 2 no units

84. CWNA Guide to Wireless LANs, Second Edition84 Frequency and Wavelength Frequency - The number of repetitions per unit time of a complete waveform, measured in Hertz. The number of complete oscillations per second of electromagnetic radiation. Wavelength –The distance between any two successive identical points on the wave.

85. CWNA Guide to Wireless LANs, Second Edition85 Sine Wave Cycle Amplitude Time 1 Cycle Period,  F = 1 

86. CWNA Guide to Wireless LANs, Second Edition86 Wavelength 1 Wavelength, = 300,000,000 m/s Frequency (Hz) = 984,000,000 f/s Frequency (Hz) In a Vacuum = 300,000,000 m/s 2.45 GHz = 0.122 m = 12.2 cm

87. CWNA Guide to Wireless LANs, Second Edition87 Summary A type of electromagnetic wave that travels through space is called a radiotelephony wave or radio wave An analog signal is a continuous signal with no breaks in it A digital signal consists of data that is discrete or separate, as opposed to continuous The carrier signal sent by radio transmissions is simply a continuous electrical signal and the signal itself carries no information

88. CWNA Guide to Wireless LANs, Second Edition88 Summary (continued) Three types of modulations or changes to the signal can be made to enable it to carry information: signal height, signal frequency, or the relative starting point Gain is defined as a positive difference in amplitude between two signals Loss, or attenuation, is a negative difference in amplitude between signals RF power can be measured by two different units on two different scales

89. CWNA Guide to Wireless LANs, Second Edition89 Summary (continued) An antenna is a copper wire or similar device that has one end in the air and the other end connected to the ground or a grounded device There are a variety of characteristics of RF antenna transmissions that play a role in properly designing and setting up a WLAN