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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

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cwna guide to wireless lans second edition

CWNA Guide to Wireless LANs, Second Edition

Chapter Three

How Wireless Works

objectives
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

CWNA Guide to Wireless LANs, Second Edition

radio wave transmission principles
Radio Wave Transmission Principles
  • Understanding principles of radio wave transmission is important for:
    • Troubleshooting wireless LANs
    • Creating a context for understanding wireless terminology

CWNA Guide to Wireless LANs, Second Edition

what are radio waves
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

CWNA Guide to Wireless LANs, Second Edition

what are radio waves continued
What Are Radio Waves? (continued)

Table 3-1: Comparison of wave characteristics

CWNA Guide to Wireless LANs, Second Edition

analog vs digital transmissions
Analog vs. Digital Transmissions

Figure 3-2: Analog signal

Figure 3-4: Digital signal

CWNA Guide to Wireless LANs, Second Edition

analog vs digital transmissions continued
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

CWNA Guide to Wireless LANs, Second Edition

frequency
Frequency

Figure 3-5: Long waves

Figure 3-6: Short Waves

CWNA Guide to Wireless LANs, Second Edition

frequency continued
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

CWNA Guide to Wireless LANs, Second Edition

frequency continued10
Frequency (continued)

Figure 3-7: Sine wave

CWNA Guide to Wireless LANs, Second Edition

frequency continued11
Frequency (continued)

Table 3-2: Electrical terminology

CWNA Guide to Wireless LANs, Second Edition

frequency continued12
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

CWNA Guide to Wireless LANs, Second Edition

frequency continued13
Frequency (continued)

Figure 3-8: Lower and higher frequencies

CWNA Guide to Wireless LANs, Second Edition

modulation
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 Edition

analog modulation
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 Edition

analog modulation continued
Analog Modulation (continued)

Figure 3-9: Amplitude

CWNA Guide to Wireless LANs, Second Edition

analog modulation continued17
Analog Modulation (continued)

Figure 3-10: Amplitude modulation (AM)

CWNA Guide to Wireless LANs, Second Edition

analog modulation continued18
Analog Modulation (continued)

Figure 3-11: Frequency modulation (FM)

CWNA Guide to Wireless LANs, Second Edition

analog modulation continued19
Analog Modulation (continued)

Figure 3-12: Phase modulation (PM)

CWNA Guide to Wireless LANs, Second Edition

digital modulation
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

CWNA Guide to Wireless LANs, Second Edition

digital modulation continued
Digital Modulation (continued)

Figure 3-13: Amplitude shift keying (ASK)

CWNA Guide to Wireless LANs, Second Edition

digital modulation continued22
Digital Modulation (continued)

Figure 3-14: Frequency shift keying (FSK)

CWNA Guide to Wireless LANs, Second Edition

digital modulation continued23
Digital Modulation (continued)

Figure 3-15: Phase shift keying (PSK)

CWNA Guide to Wireless LANs, Second Edition

radio frequency behavior gain
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 Edition

radio frequency behavior gain continued
Radio Frequency Behavior: Gain (continued)

Figure 3-16: Gain

CWNA Guide to Wireless LANs, Second Edition

radio frequency behavior loss
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 Edition

radio frequency behavior loss continued
Radio Frequency Behavior: Loss (continued)

Figure 3-18: Absorption

CWNA Guide to Wireless LANs, Second Edition

radio frequency behavior loss continued28
Radio Frequency Behavior: Loss (continued)

Figure 3-19: Reflection

CWNA Guide to Wireless LANs, Second Edition

radio frequency behavior loss continued29
Radio Frequency Behavior: Loss (continued)

Figure 3-20: Scattering

CWNA Guide to Wireless LANs, Second Edition

radio frequency behavior loss continued30
Radio Frequency Behavior: Loss (continued)

Figure 3-21: Refraction

CWNA Guide to Wireless LANs, Second Edition

radio frequency behavior loss continued31
Radio Frequency Behavior: Loss (continued)

Figure 3-22: Diffraction

CWNA Guide to Wireless LANs, Second Edition

radio frequency behavior loss continued32
Radio Frequency Behavior: Loss (continued)

Figure 3-23: VSWR

CWNA Guide to Wireless LANs, Second Edition

rf measurement rf math
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 Edition

rf measurement rf math continued
RF Measurement: RF Math (continued)

Table 3-3: The 10’s and 3’s Rules of RF Math

CWNA Guide to Wireless LANs, Second Edition

rf measurement rf math continued35
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 Edition

rf measurement wlan measurements
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 Edition

antenna concepts
Antenna Concepts
  • Radio waves transmitted/received using antennas

Figure 3-24: Antennas are required for sending and receiving radio signals

CWNA Guide to Wireless LANs, Second Edition

characteristics of rf antenna transmissions
Characteristics of RF Antenna Transmissions
  • Polarization: Orientation of radio waves as they leave the antenna

Figure 3-25: Vertical polarization

CWNA Guide to Wireless LANs, Second Edition

characteristics of rf antenna transmissions continued
Characteristics of RF Antenna Transmissions (continued)
  • Wave propagation: Pattern of wave dispersal

Figure 3-26: Sky wave propagation

CWNA Guide to Wireless LANs, Second Edition

characteristics of rf antenna transmissions continued40
Characteristics of RF Antenna Transmissions (continued)

Figure 3-27: RF LOS propagation

CWNA Guide to Wireless LANs, Second Edition

characteristics of rf antenna transmissions continued41
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 Edition

characteristics of rf antenna transmissions continued42
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 Edition

characteristics of rf antenna transmissions continued43
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 Edition

characteristics of rf antenna transmissions continued44
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 Edition

antenna types and their installations
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 Edition

antenna types and their installations continued
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

CWNA Guide to Wireless LANs, Second Edition

antenna types and their installations continued47
Antenna Types and Their Installations (continued)

Figure 3-29: Omni-directional antenna

CWNA Guide to Wireless LANs, Second Edition

antenna types and their installations continued48
Antenna Types and Their Installations (continued)

Figure 3-30: Semi-directional antenna

CWNA Guide to Wireless LANs, Second Edition

wlan antenna locations and installation
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 Edition

summary
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 Edition

summary continued
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 Edition

summary continued52
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 Edition