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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
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
Analog vs. Digital Transmissions
Analog signal: Continuous
Digital signal: Discrete
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
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
Frequency (continued)
Sine wave
Frequency (continued)
Electrical terminology
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
Frequency (continued)
Lower and higher frequencies
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
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
Analog Modulation (continued)
Amplitude
Analog Modulation (continued)
Amplitude modulation (AM)
Analog Modulation (continued)
Frequency modulation (FM)
Analog Modulation (continued)
Phase modulation (PM)
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
Digital Modulation (continued)
Amplitude shift keying (ASK)
Digital Modulation (continued)
Frequency shift keying (FSK)
Digital Modulation (continued)
Phase shift keying (PSK)
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
Radio Frequency Behavior: Gain
(continued)
Gain
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
Radio Frequency Behavior: Loss
(continued)
Absorption: RF signal is soaked up by certain materials such as
concrete, wood, and asphalt
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.
25
• Each time the signal is reflected, the amplitude is reduced.
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.
26
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.
27
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
28
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
Diffracted
called diffraction. It is markedly
Signal
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.
Reflection, Refraction, and Diffraction 29
Weather - Precipitation
Precipitation: Rain, snow, hail, fog, and sleet.
• Rain, Snow and Hail
– Wavelength of 2.4 GHz 802.11b/g signal is 4.8 inches
– Wavelength of 5.7 GHz 802.11a 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 and30
hail do start to cause significant attenuation.
Radio Frequency Behavior: Loss
(continued)
Scattering
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.
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
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
34
RF Measurement: RF Math
(continued)
The 10’s and 3’s Rules of RF Math
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
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
37
10
Point A
Inverse
square
law
20
30
40
50
3 times the distance
1/9 the power of Point A
2 times the distance
¼ the power of Point A
100
10 times the distance
1/100 the power of 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.
38
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 802.11b
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
Antenna Concepts
• Radio waves transmitted/received using antennas
Antennas are required for sending and receiving radio signals
Characteristics of RF Antenna
Transmissions (continued)
•
•
Wave propagation: Pattern of wave dispersal
Read More on Ionosphere
Sky wave propagation
Characteristics of RF Antenna
Transmissions (continued)
RF Line of Sight (LOS) propagation
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
Characteristics of RF Antenna
Transmissions (continued)
• Determining extent of “late” multipath signals can
be done by calculating Fresnel zone
Fresnel zone
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
Characteristics of RF Antenna
Transmissions (continued)
Free space path loss for IEEE 802.11b and 802.11g WLANs
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
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
Antenna Types and Their Installations
(continued)
Omni-directional antenna
Antenna Types and Their Installations
(continued)
Semi-directional antenna
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
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
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
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