cwna guide to wireless lan's second edition - chapter 3

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


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)


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


Antenna Concepts

• Radio waves transmitted/received using antennas

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


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



• 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


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 lan's second edition - chapter 3

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