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 b and g 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 b and g 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

51. 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
CWNA Guide to Wireless LANs, Second Edition

52. 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
CWNA Guide to Wireless LANs, Second Edition