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CWNA Guide to Wireless LANs, Second Edition

CWNA Guide to Wireless LANs, Second Edition

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CWNA Guide to Wireless LANs, Second Edition

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

  2. 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 CWNA Guide to Wireless LANs, Second Edition

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

  4. 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 CWNA Guide to Wireless LANs, Second Edition

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

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

  7. 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 CWNA Guide to Wireless LANs, Second Edition

  8. 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 EM Wave Transmission Line Antenna and Tower CWNA Guide to Wireless LANs, Second Edition

  9. RF Spectrum CWNA Guide to Wireless LANs, Second Edition

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

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

  12. 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 CWNA Guide to Wireless LANs, Second Edition

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

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

  15. 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 CWNA Guide to Wireless LANs, Second Edition

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

  17. 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 CWNA Guide to Wireless LANs, Second Edition

  18. 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 CWNA Guide to Wireless LANs, Second Edition

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

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

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

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

  23. 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 CWNA Guide to Wireless LANs, Second Edition

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

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

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

  27. 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. CWNA Guide to Wireless LANs, Second Edition

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

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

  30. 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 CWNA Guide to Wireless LANs, Second Edition

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

  32. 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 CWNA Guide to Wireless LANs, Second Edition

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

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

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

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

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

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

  39. 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 CWNA Guide to Wireless LANs, Second Edition

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

  41. 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 CWNA Guide to Wireless LANs, Second Edition

  42. 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 CWNA Guide to Wireless LANs, Second Edition

  43. 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. CWNA Guide to Wireless LANs, Second Edition

  44. 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. CWNA Guide to Wireless LANs, Second Edition

  45. 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. CWNA Guide to Wireless LANs, Second Edition

  46. 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 log10 PX PRef CWNA Guide to Wireless LANs, Second Edition

  47. Relative and Absolute dB • Relative dB is selecting any value for PRef dB • Absolute dB is selecting a standard value for PRef and identifying the standard value with one or more letter following the dB variable. dBm dBW dBV dBspl CWNA Guide to Wireless LANs, Second Edition

  48. dB Sample Problem OUTPUT Antenna INPUT 100 mW 1 W Signal Source RF Amplifier Compute the relative power gain of the RF Amplifier in dB. dB = 10 log10 ( 1W / 100 mW) = 10 log10 ( 10 ) = 10 ( 1 ) = 10 dB PRef CWNA Guide to Wireless LANs, Second Edition

  49. dB Sample Problem INPUT Antenna OUTPUT 100 mW 50 mW Signal Source RF Attenuator Compute the relative power loss of the RF Amplifier in dB. dB = 10 log10 ( 50 mW / 100 mW) = 10 log10 ( .5 ) = 10 ( -0.3 ) = -3.0 dB PRef CWNA Guide to Wireless LANs, Second Edition

  50. dB Sample Problem OUTPUT Antenna INPUT 50 mW 2 W Signal Source RF Amplifier Compute the absolute dBm power level at the output of the RF Amplifier. dBm = 10 log10 ( 2W / 1 mW) = 10 log10 ( 2000 ) = 10 ( 3.3 ) = 33 dBm PRef CWNA Guide to Wireless LANs, Second Edition