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
Radio Wave Transmission Principles

• Understanding principles of radio wave
  transmission is important for
• Troubleshooting wireless LANs
• Creating a context for understanding wireless
  terminology

4
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

5
What Are Radio Waves? (continued)
Table 3-1 Comparison of wave characteristics
6
Analog vs. Digital Transmissions
Figure 3-2 Analog signal
Figure 3-4 Digital signal
7
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

8
Frequency
Figure 3-5 Long waves
Figure 3-6 Short Waves
9
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

10
Frequency (continued)
Figure 3-7 Sine wave
11
Frequency (continued)
Table 3-2 Electrical terminology
12
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

13
Frequency (continued)
Figure 3-8 Lower and higher frequencies
14
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

15
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

16
Analog Modulation (continued)
Figure 3-9 Amplitude
17
Analog Modulation (continued)
Figure 3-10 Amplitude modulation (AM)
18
Analog Modulation (continued)
Figure 3-11 Frequency modulation (FM)
19
Analog Modulation (continued)
Figure 3-12 Phase modulation (PM)
20
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)
22
Digital Modulation (continued)
Figure 3-14 Frequency shift keying (FSK)
23
Digital Modulation (continued)
Figure 3-15 Phase shift keying (PSK)
24
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

25
Radio Frequency Behavior Gain (continued)
Figure 3-16 Gain
26
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
28
Radio Frequency Behavior Loss (continued)
Figure 3-19 Reflection
29
Radio Frequency Behavior Loss (continued)
Figure 3-20 Scattering
30
Radio Frequency Behavior Loss (continued)
Figure 3-21 Refraction
31
Radio Frequency Behavior Loss (continued)
Figure 3-22 Diffraction
32
Radio Frequency Behavior Loss (continued)
Figure 3-23 VSWR
33
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)
• 10s and 3s Rules of RF Math Basic rule of
  thumb in dealing with RF power gain and loss

34
RF Measurement RF Math (continued)
Table 3-3 The 10s and 3s Rules of RF Math
35
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

36
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
37
Antenna Concepts

• Radio waves transmitted/received using antennas

Figure 3-24 Antennas are required for sending
and receiving radio signals
38
Characteristics of RF Antenna Transmissions

• Polarization Orientation of radio waves as they
  leave the antenna

Figure 3-25 Vertical polarization
39
Characteristics of RF Antenna Transmissions
(continued)

• Wave propagation Pattern of wave dispersal

Figure 3-26 Sky wave propagation
40
Characteristics of RF Antenna Transmissions
(continued)
Figure 3-27 RF LOS propagation
41
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

42
Characteristics of RF Antenna Transmissions
(continued)

• Determining extent of late multipath signals
  can be done by calculating Fresnel zone

Figure 3-28 Fresnel zone
43
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

44
Characteristics of RF Antenna Transmissions
(continued)
Table 3-5 Free space path loss for IEEE 802.11b
and 802.11g WLANs
45
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

46
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

47
Antenna Types and Their Installations (continued)
Figure 3-29 Omni-directional antenna
48
Antenna Types and Their Installations (continued)
Figure 3-30 Semi-directional antenna
49
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

50
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

51
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

52
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