CWNA Guide to Wireless
LANs, Second Edition
Chapter Three
How Wireless Works
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
2
Radio Wave Transmission Principles
• Understanding principles of radio wave
transmission is important for:
– Troubleshooting wireless LANs
– Creating a context for understanding wireless
terminology
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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
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4
What Are Radio Waves? (continued)
Table 3-1: Comparison of wave characteristics
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5
Analog vs. Digital Transmissions
Figure 3-2: Analog signal
Figure 3-4: Digital signal
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6
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
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7
Radio Frequency
• Radio frequency, (RF) is a term that refers to
alternating current, (AC) having characteristics
such that, if the current is input to an antenna, an
electromagnetic (EM) field/wave is generated
suitable for wireless communications.
AC Signal
EM Wave
Transmission Line
CWNA Guide to Wireless LANs, Second Edition
Antenna
and
Tower
8
Designation
RF Spectrum
Abbreviation
Frequencies
UHF
300 MHz - 3 GHz
Ultra High Frequency
Super High
Frequency
SHF
Very Low Frequency Extremely High
Frequency
VLF - EHF
CWNA Guide to Wireless LANs, Second Edition
3 GHz - 30 GHz
9 kHz – 300 GHz
9
US Frequency Allocation Chart
• National Telecommunications and Information
Administration. http://www.ntia.doc.gov/osmhome/allochrt.html
300 GHz
9 kHz
AM
Radio
535-1605
kHz
FM
Radio
88-108
MHz
CWNA Guide to Wireless LANs, Second Edition
802.11
a, b, g
10
Frequency
Figure 3-5: Long waves
Figure 3-6: Short Waves
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11
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
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12
Frequency (continued)
Figure 3-7: Sine wave
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13
Frequency (continued)
Table 3-2: Electrical terminology
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14
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
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15
Frequency (continued)
Figure 3-8: Lower and higher frequencies
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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
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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
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Analog Modulation (continued)
Figure 3-9: Amplitude
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19
Analog Modulation (continued)
Figure 3-10: Amplitude modulation (AM)
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20
Analog Modulation (continued)
Figure 3-11: Frequency modulation (FM)
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21
Analog Modulation (continued)
Figure 3-12: Phase modulation (PM)
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22
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
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23
Digital Modulation (continued)
Figure 3-13: Amplitude shift keying (ASK)
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Digital Modulation (continued)
Figure 3-14: Frequency shift keying (FSK)
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25
Digital Modulation (continued)
Figure 3-15: Phase shift keying (PSK)
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26
Amplification and Attenuation
• Amplification/Gain - An increase in signal level,
amplitude or magnitude of a signal. A device that
does this is called an amplifier.
• Attenuation/Loss - A decrease in signal level,
amplitude, or magnitude of a signal. A device that
does this is called an attenuator.
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27
AmplificationOUTPUT
INPUT
100 mW
Signal
Source
Antenna
1W
RF Amplifier
The power gain of the RF amplifier is a power ratio.
Power Gain =
Power Output
=
Power Input
CWNA Guide to Wireless LANs, Second Edition
1W
100 mW
= 10 no units
28
Attenuation
INPUT
OUTPUT
100 mW
Signal
Source
Antenna
50 mW
RF Attenuator
The power loss of the RF attenuator is a power ratio.
Power Loss =
Power Output
=
Power Input
CWNA Guide to Wireless LANs, Second Edition
50 mW
100 mW
= 0.5 no units
29
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
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30
Radio Frequency Behavior: Gain
(continued)
Figure 3-16: Gain
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31
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
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32
Radio Frequency Behavior: Loss
(continued)
Figure 3-18: Absorption
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33
Radio Frequency Behavior: Loss
(continued)
Figure 3-19: Reflection
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34
Radio Frequency Behavior: Loss
(continued)
Figure 3-20: Scattering
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35
Radio Frequency Behavior: Loss
(continued)
Figure 3-21: Refraction
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36
Radio Frequency Behavior: Loss
(continued)
Figure 3-22: Diffraction
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37
Radio Frequency Behavior: Loss
(continued)
Figure 3-23: VSWR
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38
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
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39
RF Measurement: RF Math
(continued)
Table 3-3: The 10’s and 3’s Rules of RF Math
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40
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
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41
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
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42
Parameters & Units of Measure
• Power - The rate at which work is done, expressed
as the amount of work per unit time.
• Watt - An International System unit of power equal
to one joule per second. The power dissipated by a
current of 1 ampere flowing between 1 volt of
differential.
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43
Parameters & Units of Measure
•
•
Current - a flow of electric charge; The amount of
electric charge flowing past a specified circuit
point per unit time.
Ampere – Unit of current.
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44
Parameters & Units of Measure
• Voltage - electric potential or potential difference
expressed in volts.
• Volt - a unit of potential equal to the potential
difference between two points on a conductor
carrying a current of 1 ampere when the power
dissipated between the two points is 1 watt.
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45
Decibels
• The decibel is defined as one tenth of a bel where
one bel is a unit of a logarithmic power scale and
represents a difference between two power levels
where one is ten times greater than the other.
dB = 10 log10
P
PXRef
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46
Relative and Absolute dB
• Relative dB is selecting any value for PRef
dB
• Absolute dB is selecting a standard value for PRef
and identifying the standard value with one or more
letter following the dB variable.
dBm
dBW
dBV
dBspl
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47
OUTPUT
dB Sample Problem
INPUT
100 mW
Signal
Source
Antenna
1W
RF Amplifier
Compute the relative power gain of the RF
Amplifier in dB.
dB = 10 log10 ( 1W / 100 mW) = 10 log10 ( 10 ) = 10 ( 1 ) = 10 dB
PRef
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48
dBINPUT
Sample Problem
Antenna
OUTPUT
100 mW
Signal
Source
50 mW
RF Attenuator
Compute the relative power loss of the RF Amplifier in dB.
dB = 10 log10 ( 50 mW / 100 mW) = 10 log10 ( .5 ) = 10 ( -0.3 ) = -3.0 dB
PRef
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49
dB Sample Problem
OUTPUT
INPUT
50 mW
Signal
Source
Antenna
2W
RF Amplifier
Compute the absolute dBm power level at the output
of the RF Amplifier.
dBm = 10 log10 ( 2W / 1 mW) = 10 log10 ( 2000 ) = 10 ( 3.3 ) = 33 dBm
PRef
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50
dB Sample Problem
Antenna
36 dBm
Signal
Source
RF Amplifier
RF Power
Meter
Compute the power level in watts at the output of
the RF Amplifier.
36 dBm = 10 log10 ( PX / 1 mW)
3.6 = log10 ( PX / 1 mW)
antilog (3.6) = antilog log10( PX / 1 mW)
3,980 = ( PX / 1 mW)
3,980 x 1 mW = PX
PX = 3.98 W
CWNA Guide to Wireless LANs, Second Edition

4W
51
dB Sample Problem
Point A
Point B
Cable loss = - 1.3 dB
L
Access Point
20 dBm Output
Antenna
Power at point A is 20 dBm = 100 mW
Power at point B is 20 dBm – 1.3 dB = 18.7 dBm = 74.1 mW
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52
Antenna Concepts
• Radio waves transmitted/received using antennas
Figure 3-24: Antennas are required for sending and receiving
radio signals
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53
Antenna Gain
• Antenna Gain - is a measure of the ability of
the antenna to focus radio waves in a
particular direction. It is the ratio of the power
required at the input of a reference antenna to
the power supplied to the input of the given
antenna to produce the same field strength at
the same location.
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54
Antenna Gain
The light analogy. Reference device
Eye
Lamp
1 Watt
Omni-directional
Radiation Pattern
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55
Antenna Gain
The light analogy. Focus/Field Strength
Reflector
Eye
Lamp
1 Watt
Directional
Radiation Pattern
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56
Two reference Antennas
• Isotropic Antenna - A hypothetical antenna that radiates or
receives energy equally in all directions.
dBi or Gi
• Dipole Antenna - a straight, center-fed, one-half wavelength
antenna.
dBd or Gd
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57
Characteristics of RF Antenna
Transmissions
• Polarization: Orientation of radio waves as they
leave the antenna
Figure 3-25: Vertical polarization
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58
Characteristics of RF Antenna
Transmissions (continued)
• Wave propagation: Pattern of wave dispersal
Figure 3-26: Sky wave propagation
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Characteristics of RF Antenna
Transmissions (continued)
Figure 3-27: RF LOS propagation
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60
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
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61
Characteristics of RF Antenna
Transmissions (continued)
• Determining extent of “late” multipath signals can
be done by calculating Fresnel zone
Figure 3-28: Fresnel zone
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62
Line of Sight (LOS)
• An unobstructed path between sending and receiving
antennas.
Transmitters
Lake
Receivers
Mountain Range
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63
Fresnel Zone
• Fresnel Zone - one of a (theoretically infinite)
number of a concentric ellipsoids of revolution
centered around the LOS path.
Provides a technique to determine the required
clearance between the signal and any obstacles
along the transmission path.
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64
Fresnel Zone
D3
D1
D2
WISP Building
Client Condos
Water Tower
D3 = 72.1
CWNA Guide to Wireless LANs, Second Edition
(D1) (D2)
f (D1 + D2)
65
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
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66
Characteristics of RF Antenna
Transmissions (continued)
Table 3-5: Free space path loss for IEEE 802.11b and 802.11g
WLANs
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67
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
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68
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
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69
Antenna Types and Their Installations
(continued)
Figure 3-29: Omni-directional antenna
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70
Antenna Types and Their Installations
(continued)
Figure 3-30: Semi-directional antenna
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71
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
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72
Attenuation of an EM wave
• Attenuation/Loss - A decrease in signal level, amplitude,
or magnitude of a signal.
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73
Basic Properties of EM waves
•
Reflection – cast off or turn back, (bouncing).
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74
Basic Properties of EM waves
•
Refraction - deflection from a straight path,
(bending).
Atmosphere
Straight-Line Wave Path
Sky Wave
Refracted Wave Path
Antenna
CWNA Guide to Wireless LANs, Second Edition
Earth
75
Basic Properties of EM waves
• Diffraction – Change in the directions and
intensities of a group of waves when they pass
near the edge of an EM opaque object,
(scattering).
Transmitter
Building
Diffracted Signal
CWNA Guide to Wireless LANs, Second Edition
Shadow
Zone
Receiver
76
Basic Properties of EM waves
•
Interference - hinders, obstructs, or impedes.
When two or more wave fronts meet, (colliding).
Interference
Multipath
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77
•
EIRP
EIRP - The product of the power supplied to the
antenna and the antenna gain in a given direction
relative to a reference antenna.
EIRP = Pin X Gi
1.58 W = 100 mW x 15.8
AP
100 mW
CWNA Guide to Wireless LANs, Second Edition
Antenna
12 dBi = 15.8
78
EIRP
Point A
Point B
Point C
Cable loss = - 1.3 dB
Access Point
20 dBm Output
Parabolic Antenna
24 dbi
Power at point A is 20 dBm = 100 mW
Power at point B is 20 dBm – 1.3 dB = 18.7 dBm = 74.1 mW
EIRP at point C is 74.1 mW x 251 = 18.6 W
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79
System Problem
Find the EIRP given:
AP Power Output 100 mW
N-connector insertion loss 0.2 dB max
Lightning Surge Arrester insertion loss 0.4 dB max
RG-8/U Coax cable loss 6.7 dB/100 feet. There is a
total cable run of 43 feet in this problem.
Antenna gain 24 dBi
AP
CWNA Guide to Wireless LANs, Second Edition
Antenna
Lightning
Surge
Arrester
80
Voltage Standing Wave Ratio
• VSWR - is a measure of how well the components
of the RF system are matched in impedance.
VSWR is the ratio of the maximum voltage to the
minimum voltage in a standing wave. For maximum
power transfer the ideal VSWR is 1.
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81
Voltage Standing Wave Ratio
50 
50 
50 
Output impedance of AP is 50 
Impedance of cable is 50 
Input impedance of antenna is 50 
The impedances are matched so the VSWR = 1
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82
Voltage Standing Wave Ratio
1.0
50 
50 
VSWR
50 
VSWR Meter
2.0
50 
VSWR
25 
50 
VSWR =
Z2
Z1
50 
=
25 
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= 2 no units
83
Frequency and Wavelength
• Frequency - The number of repetitions per unit
time of a complete waveform, measured in Hertz.
The number of complete oscillations per second of
electromagnetic radiation.
• Wavelength –The distance between any two
successive identical points on the wave.
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84
Sine Wave Cycle

Amplitude
Period,
F=
1

1 Cycle
Time
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85
Wavelength
1 Wavelength,



300,000,000 m/s
2.45 GHz

300,000,000 m/s
=
Frequency (Hz)
= 0.122 m = 12.2 cm

=
=
984,000,000 f/s
Frequency (Hz)
In a Vacuum
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86
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
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87
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
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88
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
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89