CWNA Guide to Wireless LANs, Second Edition Chapter Three How Wireless Works

2 Objectives Explain the principals of radio wave transmissions Explain the principals of radio wave transmissions Describe RF loss and gain, and how it can be measured Describe RF loss and gain, and how it can be measured List some of the characteristics of RF antenna transmissions List some of the characteristics of RF antenna transmissions Describe the different types of antennas Describe the different types of antennas

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

4 What Are Radio Waves? Electromagnetic wave: Travels freely through space in all directions at speed of light 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 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 waveAs magnetic field radiates, creates an electromagnetic radio wave Spreads out through space in all directions Spreads out through space in all directions Can travel long distancesCan travel long distances Can penetrate non-metallic objectsCan penetrate non-metallic objects

5 What Are Radio Waves? (continued) Table 3-1: Comparison of wave characteristics

6 Analog vs. Digital Transmissions Figure 3-4: Digital signal (on/off) Figure 3-2: Analog signal (continuous)

7 Analog vs. Digital Transmissions (continued) Analog signals are continuous Analog signals are continuous Digital signals are discrete Digital signals are discrete Modem (MOdulator/DEModulator): Used when digital signals must be transmitted over analog medium 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 transmissionOn originating end, converts distinct digital signals into continuous analog signal for transmission On receiving end, reverse process performedOn receiving end, reverse process performed WLANs use digital transmissions WLANs use digital transmissions

8 Frequency Figure 3-5: Long waves Figure 3-6: Short Waves (e.g. short wave radio)

9 Frequency (continued) Frequency: Rate at which an event occurs Frequency: Rate at which an event occurs Cycle: Changing event that creates different radio frequencies Cycle: Changing event that creates different radio frequencies When wave completes trip and returns back to starting point it has finished one cycleWhen wave completes trip and returns back to starting point it has finished one cycle Hertz (Hz): Cycles per second Hertz (Hz): Cycles per second Kilohertz (KHz) = thousand hertzKilohertz (KHz) = thousand hertz Megahertz (MHz) = million hertzMegahertz (MHz) = million hertz Gigahertz (GHz) = billion hertzGigahertz (GHz) = billion hertz

10 Frequency (continued) Figure 3-7: Sine wave

11 Frequency (continued) Table 3-2: Electrical terminology

12 Frequency (continued) Frequency of radio wave can be changed by modifying voltage Frequency of radio wave can be changed by modifying voltage Radio transmissions send a carrier signal Radio transmissions send a carrier signal Increasing voltage will change frequency of carrier signalIncreasing voltage will change frequency of carrier signal

13 Frequency (continued) Figure 3-8: Lower and higher frequencies

14 Modulation Carrier signal is a continuous electrical signal Carrier signal is a continuous electrical signal Carries no informationCarries no information Three types of modulations enable carrier signals to carry information Three types of modulations enable carrier signals to carry information Height of signalHeight of signal Frequency of signalFrequency of signal Relative starting pointRelative starting point Modulation can be done on analog or digital transmissions Modulation can be done on analog or digital transmissions

15 Analog Modulation Amplitude: Height of carrier wave 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 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 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 bitNumber of waves to represent 1 bit more than number of waves to represent 0 bit Phase modulation (PM): Changes starting point of cycle Phase modulation (PM): Changes starting point of cycle When bits change from 1 to 0 bit or vice versaWhen bits change from 1 to 0 bit or vice versa

16 Analog Modulation (continued) Figure 3-9: Amplitude

17 Analog Modulation (continued) Figure 3-10: Amplitude modulation (AM)

18 Analog Modulation (continued) Figure 3-11: Frequency modulation (FM)

19 Analog Modulation (continued) Figure 3-12: Phase modulation (PM)

20 Digital Modulation Advantages over analog modulation: Advantages over analog modulation: Better use of bandwidthBetter use of bandwidth Requires less powerRequires less power Better handling of interference from other signalsBetter handling of interference from other signals Error-correcting techniques more compatible with other digital systemsError-correcting techniques more compatible with other digital systems Unlike analog modulation, changes occur in discrete steps using binary signals Unlike analog modulation, changes occur in discrete steps using binary signals Uses same three basic types of modulation as analogUses same three basic types of modulation as analog

21 Digital Modulation (continued) Figure 3-13: Amplitude shift keying (ASK)

22 Digital Modulation (continued) Figure 3-14: Frequency shift keying (FSK)

23 Digital Modulation (continued) Figure 3-15: Phase shift keying (PSK)

24 Radio Frequency Behavior: Gain Gain: Positive difference in amplitude between two signals Gain: Positive difference in amplitude between two signals Achieved by amplification of signalAchieved by amplification of signal Technically, gain is measure of amplificationTechnically, gain is measure of amplification Can occur intentionally from external power source that amplifies signalCan 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 itCan occur unintentionally when RF signal bounces off an object and combines with original signal to amplify it

25 Radio Frequency Behavior: Gain (continued) Figure 3-16: Gain

26 Radio Frequency Behavior: Loss Loss: Negative difference in amplitude between signals Loss: Negative difference in amplitude between signals AttenuationAttenuation Can be intentional or unintentionalCan be intentional or unintentional Intentional loss may be necessary to decrease signal strength to comply with standards or to prevent interferenceIntentional loss may be necessary to decrease signal strength to comply with standards or to prevent interference Unintentional loss can be cause by many factorsUnintentional loss can be cause by many factors

27 Radio Frequency Behavior: Loss (continued) Figure 3-18: Absorption

28 Radio Frequency Behavior: Loss (continued) Figure 3-19: Reflection

29 Radio Frequency Behavior: Loss (continued) Figure 3-20: Scattering

30 Radio Frequency Behavior: Loss (continued) Figure 3-21: Refraction

31 Radio Frequency Behavior: Loss (continued) Figure 3-22: Diffraction

32 Radio Frequency Behavior: Loss (continued) Figure 3-23: VSWR

33 RF Measurement: RF Math RF power measured by two units on two scales: RF power measured by two units on two scales: Linear scale:Linear scale: Using milliwatts (mW) Using milliwatts (mW) Reference point is zero Reference point is zero Does not reveal gain or loss in relation to whole Does not reveal gain or loss in relation to whole Relative scale:Relative scale: Reference point is the measurement itself Reference point is the measurement itself Often use logarithms Often use logarithms Measured in decibels (dB) 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 10’s and 3’s Rules of RF Math: Basic rule of thumb in dealing with RF power gain and loss

34 RF Measurement: RF Math (continued) Table 3-3: The 10’s and 3’s Rules of RF Math

35 RF Measurement: RF Math (continued) dBm: Reference point that relates decibel scale to milliwatt scale dBm: Reference point that relates decibel scale to milliwatt scale Equivalent Isotropically Radiated Power (EIRP): Power radiated out of antenna of a wireless system Equivalent Isotropically Radiated Power (EIRP): Power radiated out of antenna of a wireless system Includes intended power output and antenna gainIncludes intended power output and antenna gain Uses isotropic decibels (dBi) for unitsUses isotropic decibels (dBi) for units Reference point is theoretical antenna with 100 percent efficiency Reference point is theoretical antenna with 100 percent efficiency

36 RF Measurement: WLAN Measurements In U.S., FCC defines power limitations for WLANs In U.S., FCC defines power limitations for WLANs Limit distance that WLAN can transmitLimit distance that WLAN can transmit Transmitter Power Output (TPO): Measure of power being delivered to transmitting antenna 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 Receive Signal Strength Indicator (RSSI): Used to determine dBm, mW, signal strength percentage Table 3-4: IEEE b and g EIRP

37 Antenna Concepts Radio waves transmitted/received using antennas Radio waves transmitted/received using antennas Figure 3-24: Antennas are required for sending and receiving radio signals

38 Characteristics of RF Antenna Transmissions Polarization: Orientation of radio waves as they leave the antenna Polarization: Orientation of radio waves as they leave the antenna Figure 3-25: Vertical polarization

39 Characteristics of RF Antenna Transmissions (continued) Wave propagation: Pattern of wave dispersal Wave propagation: Pattern of wave dispersal Figure 3-26: Sky wave propagation

40 Characteristics of RF Antenna Transmissions (continued) Figure 3-27: RF LOS propagation

41 Characteristics of RF Antenna Transmissions (continued) Because RF LOS propagation requires alignment of sending and receiving antennas, ground-level objects can obstruct signals Because RF LOS propagation requires alignment of sending and receiving antennas, ground-level objects can obstruct signals Can cause refraction or diffractionCan cause refraction or diffraction Multipath distortion: Refracted or diffracted signals reach receiving antenna later than signals that do not encounter obstructionsMultipath 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 Antenna diversity: Uses multiple antennas, inputs, and receivers to overcome multipath distortion

42 Characteristics of RF Antenna Transmissions (continued) Determining extent of “late” multipath signals can be done by calculating Fresnel zone Determining extent of “late” multipath signals can be done by calculating Fresnel zone Figure 3-28: Fresnel zone

43 Characteristics of RF Antenna Transmissions (continued) As RF signal propagates, it spreads out As RF signal propagates, it spreads out Free space path loss: Greatest source of power loss in a wireless systemFree space path loss: Greatest source of power loss in a wireless system Antenna gain: Only way for an increase in amplification by antennaAntenna gain: Only way for an increase in amplification by antenna Alter physical shape of antenna Alter physical shape of antenna Beamwidth: Measure of focusing of radiation emitted by antennaBeamwidth: Measure of focusing of radiation emitted by antenna Measured in horizontal and vertical degrees Measured in horizontal and vertical degrees

44 Characteristics of RF Antenna Transmissions (continued) Table 3-5: Free space path loss for IEEE b and g WLANs

45 Antenna Types and Their Installations Two fundamental characteristics of antennas: Two fundamental characteristics of antennas: As frequency gets higher, wavelength gets smallerAs frequency gets higher, wavelength gets smaller Size of antenna smaller Size of antenna smaller As gain increases, coverage area narrowsAs gain increases, coverage area narrows High-gain antennas offer larger coverage areas than low-gain antennas at same input power level 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 Omni-directional antenna: Radiates signal in all directions equally Most common type of antennaMost common type of antenna

46 Antenna Types and Their Installations (continued) Semi-directional antenna: Focuses energy in one direction Semi-directional antenna: Focuses energy in one direction Primarily used for short and medium range remote wireless bridge networksPrimarily used for short and medium range remote wireless bridge networks Highly-directional antennas: Send narrowly focused signal beam Highly-directional antennas: Send narrowly focused signal beam Generally concave dish-shaped devicesGenerally concave dish-shaped devices Used for long distance, point-to-point wireless linksUsed for long distance, point-to-point wireless links

47 Antenna Types and Their Installations (continued) Figure 3-29: Omni-directional antenna

48 Antenna Types and Their Installations (continued) Figure 3-30: Semi-directional antenna

49 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 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 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 If high-gain omni-directional antenna used, must determine that users located below antenna area still have reception

50 Summary A type of electromagnetic wave that travels through space is called a radiotelephony wave or radio wave 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 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 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 The carrier signal sent by radio transmissions is simply a continuous electrical signal and the signal itself carries no information

51 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 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 Gain is defined as a positive difference in amplitude between two signals Loss, or attenuation, is a negative difference in amplitude between 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 RF power can be measured by two different units on two different scales

52 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 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 There are a variety of characteristics of RF antenna transmissions that play a role in properly designing and setting up a WLAN