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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

  3. Radio Wave Transmission Principles • Understanding principles of radio wave transmission is important for: • Troubleshooting wireless LANs • Creating a context for understanding wireless terminology

  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

  5. What Are Radio Waves? (continued) Table 3-1: Comparison of wave characteristics

  6. Analog vs. Digital Transmissions Figure 3-2: Analog signal (continuous) Figure 3-4: Digital signal (on/off)

  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

  8. Frequency Figure 3-5: Long waves Figure 3-6: Short Waves (e.g. short wave radio)

  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

  10. Frequency (continued) Figure 3-7: Sine wave

  11. Frequency (continued) Table 3-2: Electrical terminology

  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

  13. Frequency (continued) Figure 3-8: Lower and higher frequencies

  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

  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

  16. Analog Modulation (continued) Figure 3-9: Amplitude

  17. Analog Modulation (continued) Figure 3-10: Amplitude modulation (AM)

  18. Analog Modulation (continued) Figure 3-11: Frequency modulation (FM)

  19. Analog Modulation (continued) Figure 3-12: Phase modulation (PM)

  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

  21. Digital Modulation (continued) Figure 3-13: Amplitude shift keying (ASK)

  22. Digital Modulation (continued) Figure 3-14: Frequency shift keying (FSK)

  23. Digital Modulation (continued) Figure 3-15: Phase shift keying (PSK)

  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

  25. Radio Frequency Behavior: Gain (continued) Figure 3-16: Gain

  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

  27. Radio Frequency Behavior: Loss (continued) Figure 3-18: Absorption

  28. Radio Frequency Behavior: Loss (continued) Figure 3-19: Reflection

  29. Radio Frequency Behavior: Loss (continued) Figure 3-20: Scattering

  30. Radio Frequency Behavior: Loss (continued) Figure 3-21: Refraction

  31. Radio Frequency Behavior: Loss (continued) Figure 3-22: Diffraction

  32. Radio Frequency Behavior: Loss (continued) Figure 3-23: VSWR

  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

  34. RF Measurement: RF Math (continued) Table 3-3: The 10’s and 3’s Rules of RF Math

  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

  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

  37. Antenna Concepts • Radio waves transmitted/received using antennas Figure 3-24: Antennas are required for sending and receiving radio signals

  38. Characteristics of RF Antenna Transmissions • Polarization: Orientation of radio waves as they leave the antenna Figure 3-25: Vertical polarization

  39. Characteristics of RF Antenna Transmissions (continued) • Wave propagation: Pattern of wave dispersal Figure 3-26: Sky wave propagation

  40. Characteristics of RF Antenna Transmissions (continued) Figure 3-27: RF LOS propagation

  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

  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

  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

  44. Characteristics of RF Antenna Transmissions (continued) Table 3-5: Free space path loss for IEEE 802.11b and 802.11g WLANs

  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

  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

  47. Antenna Types and Their Installations (continued) Figure 3-29: Omni-directional antenna

  48. Antenna Types and Their Installations (continued) Figure 3-30: Semi-directional antenna

  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

  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