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)

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 b/g signal is 4.8 inches
Wavelength of 5.7 GHz a 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. Lab 3.1: Performing RF Math Calculations Confirm your answers
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
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
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.

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 b 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 b and g 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

51. 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

52. 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

53. 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

54. 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