1. CWNA Guide to Wireless LANs, Second Edition (Modified Spring 2007)
Chapter Three
How Wireless Works

2. CWNA Guide to Wireless LANs, Second EditionCCRI J. Bernardini
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
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 EditionCCRI J. Bernardini

3. CWNA Guide to Wireless LANs, Second EditionCCRI J. Bernardini
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 EditionCCRI J. Bernardini

4. Analog vs. Digital Transmissions
Analog Signal = A signal that has continuously varying voltages, frequencies, or phases. All amplitude values are present from minimum to maximum signal levels.
Digital Signal = A signal in which information is carried in a limited number of different discrete states or levels; High/Low, One/Zero, 1/0
CWNA Guide to Wireless LANs, Second EditionCCRI J. Bernardini

5. Analog vs. Digital Transmissions
Analog Transmission use analog carrier signals and analog modulation.
Digital Transmission use analog carrier signals and digital modulation.
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 modulation of analog signals (carrier signal)
CWNA Guide to Wireless LANs, Second EditionCCRI J. Bernardini

6. CWNA Guide to Wireless LANs, Second EditionCCRI J. Bernardini
Frequency
Frequency: f= Rate at which an event occurs f =1/T
Cycle: Changing event that creates different radio frequencies
When wave completes trip and returns back to starting point it has finished one cycle
Period: T=The time to complete one cycle T=1/f
Hertz (Hz): Cycles per second
Kilohertz (KHz) = thousand hertz
Megahertz (MHz) = million hertz
Gigahertz (GHz) = billion hertz
CWNA Guide to Wireless LANs, Second EditionCCRI J. Bernardini

7. CWNA Guide to Wireless LANs, Second EditionCCRI J. Bernardini
Frequency and Period
CWNA Guide to Wireless LANs, Second EditionCCRI J. Bernardini

8. CWNA Guide to Wireless LANs, Second EditionCCRI J. Bernardini
Frequency
CWNA Guide to Wireless LANs, Second EditionCCRI J. Bernardini

9. Electrical terminology
CWNA Guide to Wireless LANs, Second EditionCCRI J. Bernardini

10. CWNA Guide to Wireless LANs, Second EditionCCRI J. Bernardini
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 EditionCCRI J. Bernardini

11. CWNA Guide to Wireless LANs, Second EditionCCRI J. Bernardini
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 EditionCCRI J. Bernardini

12. CWNA Guide to Wireless LANs, Second EditionCCRI J. Bernardini
Analog Modulation
Amplitude modulation (AM) – Carrier frequency varies in amplitude
Frequency modulation (FM) – Carrier frequency varies in frequency
Phase modulation (PM) – Carrier varies in phase
CWNA Guide to Wireless LANs, Second EditionCCRI J. Bernardini

13. CWNA Guide to Wireless LANs, Second EditionCCRI J. Bernardini
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
Amplitude shift keying (ASK)
CWNA Guide to Wireless LANs, Second EditionCCRI J. Bernardini

14. CWNA Guide to Wireless LANs, Second EditionCCRI J. Bernardini
Digital Modulation
Frequency shift keying (FSK)
Phase shift keying (PSK)
CWNA Guide to Wireless LANs, Second EditionCCRI J. Bernardini

15. 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 EditionCCRI J. Bernardini

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

17. 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 EditionCCRI J. Bernardini

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

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

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

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

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

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

24. 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 EditionCCRI J. Bernardini

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

26. 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 EditionCCRI J. Bernardini

27. 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 EditionCCRI J. Bernardini

28. CWNA Guide to Wireless LANs, Second EditionCCRI J. Bernardini
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 EditionCCRI J. Bernardini

29. 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 EditionCCRI J. Bernardini

30. Antenna Characteristics
CWNA Guide to Wireless LANs, Second EditionCCRI J. Bernardini

31. Characteristics of RF Antenna Transmissions (continued)
Wave propagation: Pattern of wave dispersal
Figure 3-26: Sky wave propagation
CWNA Guide to Wireless LANs, Second EditionCCRI J. Bernardini

32. Characteristics of RF Antenna Transmissions (continued)
Figure 3-27: RF LOS propagation
CWNA Guide to Wireless LANs, Second EditionCCRI J. Bernardini

33. 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 EditionCCRI J. Bernardini

34. 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 EditionCCRI J. Bernardini

35. 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 EditionCCRI J. Bernardini

36. 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 EditionCCRI J. Bernardini

37. 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 EditionCCRI J. Bernardini

38. Antenna Types and Their Installations
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 EditionCCRI J. Bernardini

39. CWNA Guide to Wireless LANs, Second EditionCCRI J. Bernardini
Antenna Issues (cont.)
Antennas have gain in particular directions
Direction other than the main intended radiation pattern, are typically related to the main lobe gain
CWNA Guide to Wireless LANs, Second EditionCCRI J. Bernardini

40. Antenna Gain
If the gain of an antenna goes up, the coverage area or angle goes down
Coverage areas or radiation patterns are measured in degrees
Angles are referred to as beamwidth
Horizontal measurement
Vertical measurement
CCRI J. Bernardini

41. Antenna Theory
A theoretical isotropic antenna has a perfect 360º vertical and horizontal beamwidth
This is a reference for ALL antennas
CCRI J. Bernardini

42. Antenna Theory- Dipole
Energy lobes are ‘pushed in’ from the top and bottom
Higher gain
Smaller vertical beamwidth
Larger horizontal lobe
Typical dipole pattern
Side View
(Vertical Pattern)
Top View
(Horizontal Pattern)
New Pattern (with Gain)
Vertical Beamwidth
CCRI J. Bernardini

43. High Gain Omni-Directionals
More coverage area in a circular pattern
Energy level directly above or below the antenna will become lower
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44. Directional Antennas
Lobes are pushed in a certain direction, causing the energy to be condensed in a particular area
Very little energy is in the back side of a directional antenna
Side View
(Vertical Pattern)
Top View
(Horizontal Pattern)
CCRI J. Bernardini

45. 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 EditionCCRI J. Bernardini

46. CWNA Guide to Wireless LANs, Second EditionCCRI J. Bernardini
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 EditionCCRI J. Bernardini

47. CWNA Guide to Wireless LANs, Second EditionCCRI J. Bernardini
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 EditionCCRI J. Bernardini

48. CWNA Guide to Wireless LANs, Second EditionCCRI J. Bernardini
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 EditionCCRI J. Bernardini