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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 Presentation
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CWNA Guide to Wireless LANs, Second Edition
Chapter ThreeHow Wireless Works
CWNA Guide to Wireless LANs, Second Edition 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
CWNA Guide to Wireless LANs, Second Edition 3
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 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
CWNA Guide to Wireless LANs, Second Edition 5
What Are Radio Waves? (continued)
Table 3-1: Comparison of wave characteristics
CWNA Guide to Wireless LANs, Second Edition 6
Analog vs. Digital Transmissions
Figure 3-4: Digital signal
Figure 3-2: Analog signal
CWNA Guide to Wireless LANs, Second Edition 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
CWNA Guide to Wireless LANs, Second Edition 8
Radio Frequency
• Radio frequency, (RF) is a term that refers to alternating current, (AC) having characteristics such that, if the current is input to an antenna, an electromagnetic (EM) field/wave is generated suitable for wireless communications.
AC Signal
Transmission Line Antennaand
Tower
EM Wave
CWNA Guide to Wireless LANs, Second Edition 9
RF SpectrumDesignation Abbreviation Frequencies
Ultra High Frequency UHF 300 MHz - 3 GHz
Super High Frequency
SHF3 GHz - 30 GHz
Very Low Frequency -Extremely High
Frequency VLF - EHF 9 kHz – 300 GHz
CWNA Guide to Wireless LANs, Second Edition 10
US Frequency Allocation Chart
• National Telecommunications and Information Administration. http://www.ntia.doc.gov/osmhome/allochrt.html
9 kHz 300 GHz
802.11a, b, g
AMRadio
FMRadio
535-1605kHz
88-108MHz
CWNA Guide to Wireless LANs, Second Edition 11
Frequency
Figure 3-5: Long waves
Figure 3-6: Short Waves
CWNA Guide to Wireless LANs, Second Edition 12
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 13
Frequency (continued)
Figure 3-7: Sine wave
CWNA Guide to Wireless LANs, Second Edition 14
Frequency (continued)
Table 3-2: Electrical terminology
CWNA Guide to Wireless LANs, Second Edition 15
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 16
Frequency (continued)
Figure 3-8: Lower and higher frequencies
CWNA Guide to Wireless LANs, Second Edition 17
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 18
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 19
Analog Modulation (continued)
Figure 3-9: Amplitude
CWNA Guide to Wireless LANs, Second Edition 20
Analog Modulation (continued)
Figure 3-10: Amplitude modulation (AM)
CWNA Guide to Wireless LANs, Second Edition 21
Analog Modulation (continued)
Figure 3-11: Frequency modulation (FM)
CWNA Guide to Wireless LANs, Second Edition 22
Analog Modulation (continued)
Figure 3-12: Phase modulation (PM)
CWNA Guide to Wireless LANs, Second Edition 23
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 24
Digital Modulation (continued)
Figure 3-13: Amplitude shift keying (ASK)
CWNA Guide to Wireless LANs, Second Edition 25
Digital Modulation (continued)
Figure 3-14: Frequency shift keying (FSK)
CWNA Guide to Wireless LANs, Second Edition 26
Digital Modulation (continued)
Figure 3-15: Phase shift keying (PSK)
CWNA Guide to Wireless LANs, Second Edition 27
Amplification and Attenuation
• Amplification/Gain - An increase in signal level, amplitude or magnitude of a signal. A device that does this is called an amplifier.
• Attenuation/Loss - A decrease in signal level, amplitude, or magnitude of a signal. A device that does this is called an attenuator.
CWNA Guide to Wireless LANs, Second Edition 28
Amplification
100 mW
RF Amplifier
1 W
SignalSource
AntennaINPUT
OUTPUT
The power gain of the RF amplifier is a power ratio.
Power Gain = = = 10 no units
Power OutputPower Input
1 W100 mW
CWNA Guide to Wireless LANs, Second Edition 29
Attenuation
100 mW
RF Attenuator
50 mW
SignalSource
AntennaINPUT
OUTPUT
The power loss of the RF attenuator is a power ratio.
Power Loss = = = 0.5 no units
Power OutputPower Input
50 mW100 mW
CWNA Guide to Wireless LANs, Second Edition 30
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 31
Radio Frequency Behavior: Gain (continued)
Figure 3-16: Gain
CWNA Guide to Wireless LANs, Second Edition 32
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 33
Radio Frequency Behavior: Loss (continued)
Figure 3-18: Absorption
CWNA Guide to Wireless LANs, Second Edition 34
Radio Frequency Behavior: Loss (continued)
Figure 3-19: Reflection
CWNA Guide to Wireless LANs, Second Edition 35
Radio Frequency Behavior: Loss (continued)
Figure 3-20: Scattering
CWNA Guide to Wireless LANs, Second Edition 36
Radio Frequency Behavior: Loss (continued)
Figure 3-21: Refraction
CWNA Guide to Wireless LANs, Second Edition 37
Radio Frequency Behavior: Loss (continued)
Figure 3-22: Diffraction
CWNA Guide to Wireless LANs, Second Edition 38
Radio Frequency Behavior: Loss (continued)
Figure 3-23: VSWR
CWNA Guide to Wireless LANs, Second Edition 39
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 40
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 41
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 42
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 43
Parameters & Units of Measure
• Power - The rate at which work is done, expressed as the amount of work per unit time.
• Watt - An International System unit of power equal to one joule per second. The power dissipated by a current of 1 ampere flowing between 1 volt of differential.
CWNA Guide to Wireless LANs, Second Edition 44
Parameters & Units of Measure
• Current - a flow of electric charge; The amount of electric charge flowing past a specified circuit point per unit time.
• Ampere – Unit of current.
CWNA Guide to Wireless LANs, Second Edition 45
Parameters & Units of Measure
• Voltage - electric potential or potential difference expressed in volts.
• Volt - a unit of potential equal to the potential difference between two points on a conductor carrying a current of 1 ampere when the power dissipated between the two points is 1 watt.
CWNA Guide to Wireless LANs, Second Edition 46
Decibels
• The decibel is defined as one tenth of a bel where one bel is a unit of a logarithmic power scale and represents a difference between two power levels where one is ten times greater than the other.
dB = 10 log10
PXPRef
CWNA Guide to Wireless LANs, Second Edition 47
Relative and Absolute dB
• Relative dB is selecting any value for PRef
dB
• Absolute dB is selecting a standard value for PRef and identifying the standard value with one or more letter following the dB variable.
dBm dBW dBV dBspl
CWNA Guide to Wireless LANs, Second Edition 48
dB Sample Problem
100 mW
RF Amplifier
1 W
SignalSource
AntennaINPUT
OUTPUT
Compute the relative power gain of the RF Amplifier in dB.
dB = 10 log10 ( 1W / 100 mW) = 10 log10 ( 10 ) = 10 ( 1 ) = 10 dB
PRef
CWNA Guide to Wireless LANs, Second Edition 49
dB Sample Problem
100 mW
RF Attenuator
50 mW
SignalSource
AntennaINPUT
OUTPUT
Compute the relative power loss of the RF Amplifier in dB.
dB = 10 log10 ( 50 mW / 100 mW) = 10 log10 ( .5 ) = 10 ( -0.3 ) = -3.0 dB
PRef
CWNA Guide to Wireless LANs, Second Edition 50
dB Sample Problem
dBm = 10 log10 ( 2W / 1 mW) = 10 log10 ( 2000 ) = 10 ( 3.3 ) = 33 dBm
PRef
50 mW
RF Amplifier
2 W
SignalSource
AntennaINPUT
OUTPUT
Compute the absolute dBm power level at the output of the RF Amplifier.
CWNA Guide to Wireless LANs, Second Edition 51
dB Sample Problem
36 dBm = 10 log10 ( PX / 1 mW) 3.6 = log10 ( PX / 1 mW)
antilog (3.6) = antilog log10( PX / 1 mW) 3,980 = ( PX / 1 mW)
3,980 x 1 mW = PX PX = 3.98 W 4 W
RF AmplifierSignalSource
Antenna
Compute the power level in watts at the output of the RF Amplifier.
36 dBm
RF PowerMeter
CWNA Guide to Wireless LANs, Second Edition 52
dB Sample Problem
Access Point20 dBm Output
Point A Point B
L
Antenna
Cable loss = - 1.3 dB
Power at point A is 20 dBm = 100 mW
Power at point B is 20 dBm – 1.3 dB = 18.7 dBm = 74.1 mW
CWNA Guide to Wireless LANs, Second Edition 53
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 54
Antenna Gain
• Antenna Gain - is a measure of the ability of the antenna to focus radio waves in a particular direction. It is the ratio of the power required at the input of a reference antenna to the power supplied to the input of the given antenna to produce the same field strength at the same location.
CWNA Guide to Wireless LANs, Second Edition 55
Antenna GainThe light analogy. Reference device
Omni-directionalRadiation Pattern
Lamp1 Watt
Eye
CWNA Guide to Wireless LANs, Second Edition 56
Antenna GainThe light analogy. Focus/Field Strength
DirectionalRadiation Pattern
Lamp1 Watt
Eye
Reflector
CWNA Guide to Wireless LANs, Second Edition 57
Two reference Antennas
• Isotropic Antenna - A hypothetical antenna that radiates or receives energy equally in all directions.
dBi or Gi
• Dipole Antenna - a straight, center-fed, one-half wavelength antenna.
dBd or Gd
CWNA Guide to Wireless LANs, Second Edition 58
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 59
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 60
Characteristics of RF Antenna Transmissions (continued)
Figure 3-27: RF LOS propagation
CWNA Guide to Wireless LANs, Second Edition 61
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 62
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 63
Line of Sight (LOS)
• An unobstructed path between sending and receiving antennas.
Line of Sight
Transmitters
Mountain Range
ReceiversLake
CWNA Guide to Wireless LANs, Second Edition 64
Fresnel Zone
• Fresnel Zone - one of a (theoretically infinite) number of a concentric ellipsoids of revolution centered around the LOS path.
Provides a technique to determine the required clearance between the signal and any obstacles along the transmission path.
CWNA Guide to Wireless LANs, Second Edition 65
Fresnel Zone
D2D1
(D1) (D2)
f (D1 + D2)72.1D3 =
D3
WISP Building Client Condos
Water Tower
CWNA Guide to Wireless LANs, Second Edition 66
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 67
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 68
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 69
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 70
Antenna Types and Their Installations (continued)
Figure 3-29: Omni-directional antenna
CWNA Guide to Wireless LANs, Second Edition 71
Antenna Types and Their Installations (continued)
Figure 3-30: Semi-directional antenna
CWNA Guide to Wireless LANs, Second Edition 72
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 73
Attenuation of an EM wave
• Attenuation/Loss - A decrease in signal level, amplitude, or magnitude of a signal.
CWNA Guide to Wireless LANs, Second Edition 74
Basic Properties of EM waves
• Reflection – cast off or turn back, (bouncing).
CWNA Guide to Wireless LANs, Second Edition 75
Basic Properties of EM waves
• Refraction - deflection from a straight path, (bending).
Earth
Atmosphere
Refracted Wave Path
Straight-Line Wave PathSky Wave
Antenna
CWNA Guide to Wireless LANs, Second Edition 76
Basic Properties of EM waves
• Diffraction – Change in the directions and intensities of a group of waves when they pass near the edge of an EM opaque object, (scattering).
Transmitter Receiver
Build
ing
ShadowZone
Diffracted Signal
CWNA Guide to Wireless LANs, Second Edition 77
Basic Properties of EM waves
• Interference - hinders, obstructs, or impedes. When two or more wave fronts meet, (colliding).
Direct WaveMultipathInterferenceReflected Wave
CWNA Guide to Wireless LANs, Second Edition 78
EIRP• EIRP - The product of the power supplied to the
antenna and the antenna gain in a given direction relative to a reference antenna.
EIRP = Pin X Gi
1.58 W = 100 mW x 15.8
AP100 mW
12 dBi = 15.8
Antenna
CWNA Guide to Wireless LANs, Second Edition 79
EIRP
Access Point20 dBm Output
Point A Point B
Parabolic Antenna24 dbi
Cable loss = - 1.3 dB
Power at point A is 20 dBm = 100 mW
Power at point B is 20 dBm – 1.3 dB = 18.7 dBm = 74.1 mW
EIRP at point C is 74.1 mW x 251 = 18.6 W
Point C
CWNA Guide to Wireless LANs, Second Edition 80
System Problem
AP
Antenna
Find the EIRP given:AP Power Output 100 mWN-connector insertion loss 0.2 dB maxLightning Surge Arrester insertion loss 0.4 dB maxRG-8/U Coax cable loss 6.7 dB/100 feet. There is atotal cable run of 43 feet in this problem.Antenna gain 24 dBi
LightningSurge
Arrester
CWNA Guide to Wireless LANs, Second Edition 81
Voltage Standing Wave Ratio
• VSWR - is a measure of how well the components of the RF system are matched in impedance. VSWR is the ratio of the maximum voltage to the minimum voltage in a standing wave. For maximum power transfer the ideal VSWR is 1.
CWNA Guide to Wireless LANs, Second Edition 82
Voltage Standing Wave Ratio50
50
50
Output impedance of AP is 50 Impedance of cable is 50 Input impedance of antenna is 50
The impedances are matched so the VSWR = 1
CWNA Guide to Wireless LANs, Second Edition 83
Voltage Standing Wave Ratio50
50
50 1.0
VSWR
50
50
25 2.0
VSWR
VSWR Meter
VSWR = Z1
Z2 =
50
25 =2 no units
CWNA Guide to Wireless LANs, Second Edition 84
Frequency and Wavelength
• Frequency - The number of repetitions per unit time of a complete waveform, measured in Hertz. The number of complete oscillations per second of electromagnetic radiation.
• Wavelength –The distance between any two successive identical points on the wave.
CWNA Guide to Wireless LANs, Second Edition 85
Sine Wave CycleA
mp
litu
de
Time
1 Cycle
Period,
F = 1
CWNA Guide to Wireless LANs, Second Edition 86
Wavelength
1 Wavelength,
= 300,000,000 m/sFrequency (Hz)
= 984,000,000 f/sFrequency (Hz)
In a Vacuum
= 300,000,000 m/s2.45 GHz
= 0.122 m = 12.2 cm
CWNA Guide to Wireless LANs, Second Edition 87
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 88
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 89
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