CWNA Guide to Wireless LANs,
Second Edition
(Modified Spring 2007)
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
• Understanding principles of radio wave transmission is
important for:
– Troubleshooting wireless LANs
– Creating a context for understanding wireless terminology
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Second EditionCCRI J. Bernardini
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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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Second EditionCCRI J. Bernardini
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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
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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)
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Second EditionCCRI J. Bernardini
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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
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Frequency and Period
CWNA Guide to Wireless LANs,
Second EditionCCRI J. Bernardini
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Frequency
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Second EditionCCRI J. Bernardini
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Electrical terminology
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Second EditionCCRI J. Bernardini
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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
CWNA Guide to Wireless LANs,
Second EditionCCRI J. Bernardini
10
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
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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
12
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
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Digital Modulation
Frequency shift keying (FSK)
Phase shift keying (PSK)
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Second EditionCCRI J. Bernardini
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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
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Radio Frequency Behavior: Gain
(continued)
Figure 3-16: Gain
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Second EditionCCRI J. Bernardini
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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
17
Radio Frequency Behavior: Loss
(continued)
Figure 3-18: Absorption
CWNA Guide to Wireless LANs,
Second EditionCCRI J. Bernardini
18
Radio Frequency Behavior: Loss
(continued)
Figure 3-19: Reflection
CWNA Guide to Wireless LANs,
Second EditionCCRI J. Bernardini
19
Radio Frequency Behavior: Loss
(continued)
Figure 3-20: Scattering
CWNA Guide to Wireless LANs,
Second EditionCCRI J. Bernardini
20
Radio Frequency Behavior: Loss
(continued)
Figure 3-21: Refraction
CWNA Guide to Wireless LANs,
Second EditionCCRI J. Bernardini
21
Radio Frequency Behavior: Loss
(continued)
Figure 3-22: Diffraction
CWNA Guide to Wireless LANs,
Second EditionCCRI J. Bernardini
22
Radio Frequency Behavior: Loss
(continued)
Figure 3-23: VSWR
CWNA Guide to Wireless LANs,
Second EditionCCRI J. Bernardini
23
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
24
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
25
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
26
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
CWNA Guide to Wireless LANs,
Second EditionCCRI J. Bernardini
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Antenna Concepts
• Radio waves transmitted/received using antennas
Figure 3-24: Antennas are required for sending and receiving
radio signals
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Second EditionCCRI J. Bernardini
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Characteristics of RF Antenna
Transmissions
• Polarization: Orientation of radio waves as they
leave the antenna
Figure 3-25: Vertical polarization
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Second EditionCCRI J. Bernardini
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Antenna Characteristics
CWNA Guide to Wireless LANs,
Second EditionCCRI J. Bernardini
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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
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Characteristics of RF Antenna
Transmissions (continued)
Figure 3-27: RF LOS propagation
CWNA Guide to Wireless LANs,
Second EditionCCRI J. Bernardini
32
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
33
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
34
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
35
Characteristics of RF Antenna
Transmissions (continued)
Table 3-5: Free space path loss for IEEE 802.11b and 802.11g
WLANs
CWNA Guide to Wireless LANs,
Second EditionCCRI J. Bernardini
36
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
37
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
38
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
39
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
Antenna Theory
• A theoretical isotropic
antenna has a perfect
360º vertical
and horizontal
beamwidth
• This is a reference for
ALL antennas
CCRI
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Antenna Theory- Dipole
• Energy lobes are
‘pushed in’ from the top
and bottom
• Higher gain
Side View
(Vertical Pattern)
– Smaller vertical beamwidth
– Larger horizontal lobe
Vertical Beamwidth
New Pattern (with Gain)
• Typical dipole pattern
Top View
(Horizontal Pattern)
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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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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
CCRI
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Side View
(Vertical Pattern)
Top View
(Horizontal Pattern)
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
45
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
46
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
47
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
48