cwna guide to wireless lans, second edition
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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 - PowerPoint PPT PresentationTRANSCRIPT
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CWNA Guide to Wireless LANs, Second Edition
Chapter ThreeHow Wireless Works
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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 3-4: Digital signal
Figure 3-2: Analog 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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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)
• 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
• 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