CWNA Guide to Wireless LANs, Second Edition

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. 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

4. Analog vs. Digital Transmissions Analog signal: Continuous Digital signal: Discrete

5. 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

6. 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

7. Frequency (continued) Sine wave

8. Frequency (continued) Electrical terminology

9. 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

10. Frequency (continued) Lower and higher frequencies

11. 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

12. 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

13. Analog Modulation (continued) Amplitude

14. Analog Modulation (continued) Amplitude modulation (AM)

15. Analog Modulation (continued) Frequency modulation (FM)

16. Analog Modulation (continued) Phase modulation (PM)

17. 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

18. Digital Modulation (continued) Amplitude shift keying (ASK)

19. Digital Modulation (continued) Frequency shift keying (FSK)

20. Digital Modulation (continued) Phase shift keying (PSK)

21. 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

22. Radio Frequency Behavior: Gain (continued) Gain

23. 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

24. Radio Frequency Behavior: Loss (continued) Absorption: RF signal is soaked up by certain materials such as concrete, wood, and asphalt

25. Reflections • Microwave signals: • Frequencies between 1 GHz – 30 GHz (this can vary among experts). • Wavelength between 12 inches down to less than 1 inch. • Microwave signals reflect off objects that are larger than their wavelength, such as buildings, cars, flat stretches of ground, and bodes of water. • Each time the signal is reflected, the amplitude is reduced.

26. Microwave Reflections Multipath Reflection • Advantage: Can use reflection to go around obstruction. • Disadvantage: Multipath reflection – occurs when reflections cause more than one copy of the same transmission to arrive at the receiver at slightly different times.

27. Multipath Reflection • Reflected signals 1 and 2 take slightly longer paths than direct signal, arriving slightly later. • These reflected signals sometimes cause problems at the receiver by partially canceling the direct signal, effectively reducing the amplitude. • The link throughput slows down because the receiver needs more time to either separate the real signal from the reflected echoes or to wait for missed frames to be retransmitted. • Solution discussed later.

28. Multipath Reflection Delay spread is a parameter used to signify Multipath. The delay of reflected signal is measured in nanoseconds (ns). The amount of delay spread varies for indoor home, office, and manufacturing environments. Multipath and Diversity Article from Cisco

29. Diffraction • Diffraction. This occurs when the wave encounters an edge. The wave has the ability to turn the corner of the edge. This ability of waves to turn corners is called diffraction. It is markedly dependent on frequency -- the higher the frequency, the less diffraction. Very high frequencies (light) hardly diffract at all; "light travels in straight lines." • A diffracted signal is usually attenuated so much it is too weak to provide a reliable microwave connection. • Do not plan to use a diffracted signal, and always try to obtain an unobstructed path between microwave antennas. Diffracted Signal Reflection,Refraction, and Diffraction

30. Weather - Precipitation Precipitation: Rain, snow, hail, fog, and sleet. • Rain, Snow and Hail • Wavelength of 2.4 GHz 802.11b/g signal is 4.8 inches • Wavelength of 5.7 GHz 802.11a signal is 2 inches • Much larger than rain drops and snow, thus do not significantly attenuate these signals. • At frequencies 10 GHz and above, partially melted snow and hail do start to cause significant attenuation.

31. Radio Frequency Behavior: Loss (continued) Scattering

32. Radio Frequency Behavior: Loss (continued) Voltage Standing Wave Ratio (VSWR): Caused by the equipment itself. If one part of the equipment has different impedance than another part, the RF signal may be reflected back within the device itself.

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) • 1mW = 0 dB

34. Calculating dB • P(dBm) =10log P(mW) • P(mW) = 10(dBm/10) • Change in Power (dBm) = 10log10 (P(final mw) /P(reference mw)) • dB = The amount of decibels. • This usually represents: • a loss in power such as when the wave travels or interacts with matter, • can also represent a gain as when traveling through an amplifier. • Pfinal = The final power. This is the delivered power after some process has occurred. • Pref = The reference power. This is the original power. • Lab 3.1: Performing RF Math Calculations • Confirm your answers

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

36. 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

37. Inverse square law • “Signal strength does not fade in a linear manner, but inversely as the square of the distance. • This means that if you are at a particular distance from an access point and you move twice as far away, the signal level will decrease by a factor of four.” Twice the distance Point A Point B ¼ the power of Point A

38. Inverse square law 10 20 30 40 50 100 • Double the distance of the wireless link, we receive only ¼ of the original power. • Triple the distance of the wireless link, we receive only 1/9 the original power. • Move 5 times the distance, signal decreases by 1/25. Point A 10 times the distance 1/100 the power of A 3 times the distance 1/9 the power of Point A 2 times the distance ¼ the power of Point A 5 times the distance 1/25 the power of Point A

39. 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. This is generally 100 milliwatts. • When using omni-directional antennas having less than 6 dB gain in this scenario, the FCC rules require EIRP to be 1 watt (1,000 milliwatts) or less. • In most cases, you'll be within regulations using omni-directional antennas supplied by the vendor of your radio NICs and access points. For example, you can set the transmit power in an 802.11b access point or client to its highest level (generally 100 milliwatts) and use a typical 3 dB omni-directional antenna. This combination results in only 200 milliwatts EIRP, which is well within FCC regulations. Read more here. • Receive Signal Strength Indicator (RSSI): Used to determine dBm, mW, signal strength percentage

40. Antenna Concepts • Radio waves transmitted/received using antennas Antennas are required for sending and receiving radio signals

41. Characteristics of RF Antenna Transmissions (continued) • Wave propagation: Pattern of wave dispersal • Read More on Ionosphere Sky wave propagation

42. Characteristics of RF Antenna Transmissions (continued) RF Line of Sight (LOS) propagation

43. 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

44. Characteristics of RF Antenna Transmissions (continued) • Determining extent of “late” multipath signals can be done by calculating Fresnel zone Fresnel zone

45. 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

46. Characteristics of RF Antenna Transmissions (continued) Free space path loss for IEEE 802.11b and 802.11g WLANs

47. 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

48. 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

49. Antenna Types and Their Installations (continued) Omni-directional antenna

50. Antenna Types and Their Installations (continued) Semi-directional antenna