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CWNA Guide to Wireless LANs, Second Edition

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

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

  9. 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

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

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

  12. 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

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

  14. 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

  15. 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

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

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

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

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

  20. 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

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

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

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

  24. 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

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

  26. 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

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

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

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

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

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

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

  33. 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

  34. 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

  35. 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

  36. 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

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

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

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

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

  41. 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 CWNA Guide to Wireless LANs, Second Edition

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

  43. 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 CWNA Guide to Wireless LANs, Second Edition

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

  45. 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 CWNA Guide to Wireless LANs, Second Edition

  46. 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 CWNA Guide to Wireless LANs, Second Edition

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

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

  49. 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 CWNA Guide to Wireless LANs, Second Edition

  50. 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 CWNA Guide to Wireless LANs, Second Edition

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