Transcript CWNA Guide to Wireless LANs, Second Edition Chapter Three How Wireless Works

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
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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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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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What Are Radio Waves? (continued)
Table 3-1: Comparison of wave characteristics
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Analog vs. Digital Transmissions
Figure 3-2: Analog signal
Figure 3-4: Digital signal
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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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Frequency
Figure 3-5: Long waves
Figure 3-6: Short Waves
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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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Frequency (continued)
Figure 3-7: Sine wave
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Frequency (continued)
Table 3-2: Electrical terminology
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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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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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Analog Modulation (continued)
Figure 3-10: Amplitude modulation (AM)
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Analog Modulation (continued)
Figure 3-11: Frequency modulation (FM)
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Analog Modulation (continued)
Figure 3-12: Phase modulation (PM)
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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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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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Digital Modulation (continued)
Figure 3-15: Phase shift keying (PSK)
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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
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Radio Frequency Behavior: Gain
(continued)
Figure 3-16: Gain
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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
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Radio Frequency Behavior: Loss
(continued)
Figure 3-18: Absorption
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Radio Frequency Behavior: Loss
(continued)
Figure 3-19: Reflection
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Radio Frequency Behavior: Loss
(continued)
Figure 3-20: Scattering
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Radio Frequency Behavior: Loss
(continued)
Figure 3-21: Refraction
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Radio Frequency Behavior: Loss
(continued)
Figure 3-22: Diffraction
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Radio Frequency Behavior: Loss
(continued)
Figure 3-23: VSWR
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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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RF Measurement: RF Math
(continued)
Table 3-3: The 10’s and 3’s Rules of RF Math
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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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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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Antenna Concepts
• Radio waves transmitted/received using antennas
Figure 3-24: Antennas are required for sending and receiving
radio signals
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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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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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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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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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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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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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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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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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Antenna Types and Their Installations
(continued)
Figure 3-29: Omni-directional antenna
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Antenna Types and Their Installations
(continued)
Figure 3-30: Semi-directional antenna
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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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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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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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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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