CWNA Guide to Wireless LANs, Second Edition

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


Radio Wave Transmission Principles

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

terminology


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


What Are Radio Waves? (continued)

Table 3-1: Comparison of wave characteristics


Analog vs. Digital Transmissions

Figure 3-4: Digital signal

Figure 3-2: Analog signal


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


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


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


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


Frequency

Figure 3-5: Long waves

Figure 3-6: Short Waves


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



Figure 3-7: Sine wave



Table 3-2: Electrical terminology



• Frequency of radio wave can be changed by modifying voltage

• Radio transmissions send a carrier signal– Increasing voltage will change frequency of carrier

signal



Figure 3-8: Lower and higher frequencies


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


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


Analog Modulation (continued)

Figure 3-9: Amplitude



Figure 3-10: Amplitude modulation (AM)



Figure 3-11: Frequency modulation (FM)



Figure 3-12: Phase modulation (PM)


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


Digital Modulation (continued)

Figure 3-13: Amplitude shift keying (ASK)



Figure 3-14: Frequency shift keying (FSK)



Figure 3-15: Phase shift keying (PSK)


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.


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


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


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


Radio Frequency Behavior: Gain (continued)

Figure 3-16: Gain


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


Radio Frequency Behavior: Loss (continued)

Figure 3-18: Absorption



Figure 3-19: Reflection



Figure 3-20: Scattering



Figure 3-21: Refraction



Figure 3-22: Diffraction



Figure 3-23: VSWR


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


RF Measurement: RF Math (continued)

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


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


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


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.



• Current - a flow of electric charge; The amount of electric charge flowing past a specified circuit point per unit time.

• Ampere – Unit of current.



• 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.


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


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


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


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


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.


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


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


Antenna Concepts

• Radio waves transmitted/received using antennas

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


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.


Antenna GainThe light analogy. Reference device

Omni-directionalRadiation Pattern

Lamp1 Watt

Eye


Antenna GainThe light analogy. Focus/Field Strength

DirectionalRadiation Pattern

Lamp1 Watt

Eye

Reflector


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


Characteristics of RF Antenna Transmissions

• Polarization: Orientation of radio waves as they leave the antenna

Figure 3-25: Vertical polarization


Characteristics of RF Antenna Transmissions (continued)

• Wave propagation: Pattern of wave dispersal

Figure 3-26: Sky wave propagation



Figure 3-27: RF LOS propagation



• 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



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

Figure 3-28: Fresnel zone


Line of Sight (LOS)

• An unobstructed path between sending and receiving antennas.

Line of Sight

Transmitters

Mountain Range

ReceiversLake


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.


Fresnel Zone

D2D1

(D1) (D2)

f (D1 + D2)72.1D3 =

D3

WISP Building Client Condos

Water Tower



• 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



Table 3-5: Free space path loss for IEEE 802.11b and 802.11g WLANs


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


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



Figure 3-29: Omni-directional antenna



Figure 3-30: Semi-directional antenna


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


Attenuation of an EM wave

• Attenuation/Loss - A decrease in signal level, amplitude, or magnitude of a signal.


Basic Properties of EM waves

• Reflection – cast off or turn back, (bouncing).



• Refraction - deflection from a straight path, (bending).

Earth

Atmosphere

Refracted Wave Path

Straight-Line Wave PathSky Wave

Antenna



• 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



• Interference - hinders, obstructs, or impedes. When two or more wave fronts meet, (colliding).

Direct WaveMultipathInterferenceReflected Wave


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


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


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


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.


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


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


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.


Sine Wave CycleA

mp

litu

de

Time

1 Cycle

Period,

F = 1


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


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


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


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

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