CWNA Guide to Wireless LANs, Second Edition

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

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Analog vs. Digital Transmissions
Analog signal Continuous
Digital signal Discrete
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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 (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)
Sine wave
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Frequency (continued)
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)
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)
Amplitude
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Analog Modulation (continued)
Amplitude modulation (AM)
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Analog Modulation (continued)
Frequency modulation (FM)
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Analog Modulation (continued)
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)
Amplitude shift keying (ASK)
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Digital Modulation (continued)
Frequency shift keying (FSK)
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Digital Modulation (continued)
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)
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)
Absorption RF signal is soaked up by certain
materials such as concrete, wood, and asphalt
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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.

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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.

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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.

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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
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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
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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 and hail do start to cause significant
  attenuation.

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Radio Frequency Behavior Loss (continued)
Scattering
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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.
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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)
• 1mW 0 dB

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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

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RF Measurement RF Math (continued)
The 10s and 3s 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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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
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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.

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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. 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

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Antenna Concepts

• Radio waves transmitted/received using antennas

Antennas are required for sending and receiving
radio signals
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Characteristics of RF Antenna Transmissions
(continued)

• Wave propagation Pattern of wave dispersal
• Read More on Ionosphere

Sky wave propagation
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Characteristics of RF Antenna Transmissions
(continued)
RF Line of Sight (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

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)
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)
Omni-directional antenna
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Antenna Types and Their Installations (continued)
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