WirelessCommunication.PPT

Wireless# Guide to Wireless CommunicationsChapter 2Wireless Data Transmission

Wireless# Guide to Wireless Communications

ObjectivesExplain how network data is represented using binary notationList and explain the two types of wireless transmissionIllustrate the basic concepts and techniques through which data can be transmitted by radio waves


How Data is RepresentedDigital data for wireless communicationsRepresented using the two binary digits 0 and 1


The Decimal Number SystemDecimal or Base 10 number systemThere are 10 different symbolsUsed to represent each digitNo additional symbols (beyond 0-9) are needed to represent any number in decimalExample:


The Binary Number SystemBinary or Base 2 number systemComputers and data transmission equipment are better suited for a base of 2Binary uses a base number of 2 instead of 10Two symbols are used to represent a digit, 0 and 1The digits 0 and 1 are known as bits (BInary digiTS)Eight binary digits grouped together form a byteAmerican Standard Code for Information Interchange (or ASCII code)Represents letters or symbols in a Base 2 system


The Binary Number System (continued)Decimal digits represented in binary:


The Binary Number System (continued)


Wireless SignalsWireless data signals travel on electromagnetic wavesThrough space at the speed of light186,000 miles per second (300,000 kilometers per second)Two basic types of wavesInfrared lightRadio waves


Wireless Signals (continued)


Infrared LightIt is easy to transmit information with lightBecause computers and data communication equipment use binary codeA 1 in binary code could result in a light quickly flashing onLight spectrumTypes of light that travel from the Sun to the EarthInfrared lightAdjacent to visible light (although invisible)A much better medium for data transmissionLess susceptible to interference


Infrared Light (continued)


Infrared Light (continued)Infrared wireless systems require:Emitter that transmits a signal (LED)Detector that receives the signalInfrared wireless systems send data by the intensity of the light waveDetector senses the higher intensity pulse of lightAnd produces a proportional electrical currentInfrared wireless transmission typesDirected transmission (called line-of-sight or LOS)Diffused transmission


Infrared Light (continued)AdvantagesIt does not interfere with other types of communication signalsInfrared light does not penetrate wallsSignals are kept inside a roomLimitationsLack of mobilityRange of coverageCan cover a range of only 50 feet (15 meters)Diffused infrared can only be used indoorsSpeed of transmission


Infrared Light (continued)Some specialized wireless local area networks are based on the infrared methodUsed in situations where radio signals would interfere with other equipmentLight waves cannot penetrate through materials like wood or concreteHeat rays are absorbed by most objectsRadio wavesDo not have the distance limitations of light or infrared


Radio WavesMost common and effective means of wireless communications todayEnergy travels through space or air in electromagnetic wavesRadio (radiotelephony) wavesWhen an electric current passes through a wire, it creates a magnetic fieldIn the space around the wireAs this magnetic field radiates or moves out, it creates radio waves


Radio Waves (continued)


Radio Waves (continued)Advantages of radio wavesCan travel great distancesCan penetrate nonmetallic objectsInvisible


Radio Waves (continued)


How Radio Data is TransmittedRadio waves can be used to transmit dataOver long distancesWithout the need for wiresTypes of dataAnalog dataDigital data


Analog and DigitalAnalog signalThe intensity (voltage or amplitude) variesIt is broadcast continuouslyExamples:AudioVideoVoiceLight


Analog and Digital (continued)


Analog and Digital (continued)Digital signalConsists of discrete or separate pulsesHas numerous starts and stops throughout the signal streamExample:Morse codeComputers operate using digital signalsAnalog signal must be converted into a digital format Before it can be stored and processed or interpreted by a computer


Analog and Digital (continued)


Analog and Digital (continued)Modem (Modulator/DEModulator)Converts the distinct digital signals from a computer Encodes them into a continuous analog signalFor transmission over analog phone linesModulationProcess of encoding the digital signals (bits) onto an analog wave


FrequencyFrequencyRate at which a radio circuit creates the wavesThe number of times a cycle occurs within one secondCycleComposed of one top [positive] and one bottom [negative] peakCarrier signalSent by radio transmittersContinuous wave (CW) of constant amplitude (also called voltage) and frequencyAn up-and-down wave called an oscillating signal or a sine wave


Frequency (continued)


Frequency (continued)A change in voltage does not create a change in frequencyThe term Hertz (Hz) is used to measure frequencyA Kilohertz (KHz) is 1,000 HertzA Megahertz (MHz) is 1,000,000 HertzA Gigahertz (GHz) is 1,000,000,000 HertzThe wave measured as 710,000 Hz is referred to as 710 KHz


Frequency (continued)AntennaLength of copper wire, or similar materialWith one end free and the other end connected to a receiver or transmitterElectrical current moves the antennaAt the same frequency as the radio waves


Transmission SpeedSpeed of transmission is usually shown in bits per second (bps)Baud rateAnother term used in measuring the speed of radio transmissionNumber of signal units per second that are required to represent the bits transmittedBaud is a change in the carrier signalIt is possible to have a change in signal (a baud) represent more than 1 bit


Transmission Speed (continued)


Transmission Speed (continued)Analog modems transmit at a rate of 4,800 baudMaximum number of signal changes per second that a phone line can supportCurrent modems can transmit up to 33,600 bpsUsing more complex modulation techniquesAlong with compression of the dataBandwidthRange of frequencies that can be transmitted by a particular system or mediumRefers to the maximum data transmission capacityAccurate only when referring to purely digital systems


Transmission Speed (continued)


Analog ModulationRepresentation of analog information by an analog signalAnalog modulation typesAmplitude modulationFrequency modulationPhase modulationAmplitude modulation (AM)Height of a carrier wave is known as the amplitudeCan be measured in volts (electrical pressure)Height of the carrier wave is changed in accordance with the height of the modulating signal


Analog Modulation (continued)


Analog Modulation (continued)Amplitude modulation (AM)Used by broadcast radio stationsVery susceptible to interference from outside sourcesFrequency modulation (FM)Number of waves that occur in one second change Based on the amplitude of the modulating signalOften used by broadcast radio stationsNot as susceptible to interference from outside sourcesFM carrier has a wider bandwidthAllows it to carry Hi-Fi as well as stereophonic signals


Analog Modulation (continued)Phase modulation (PM)Changes the starting point of the cycleIt is not generally used to represent analog signalsA signal composed of sine waves has a phase associated with itPhase is measured in degreesOne complete wave cycle covers 360 degreesA phase change is always measured with reference to some other signalPM systems almost always use the previous wave cycle as the reference signal


Digital ModulationMethod of encoding a digital signal onto an analog waveFor transmission over a medium that does not support digital signalsIn a digital system, the changes are distinct using binary signalsWhich exist in one of two states, a 1 or a 0For a computer to be able to understand these signalsEach bit must have a fixed duration to represent 1 or 0


Digital Modulation (continued)AdvantagesIt makes better use of the bandwidth availableIt requires less power to transmitIt performs better when the signal experiences interference from other signalsIts error-correcting techniques are more compatible with other digital systemsThere are three basic types of digital modulations:Amplitude, frequency, and phaseUsers demand more transmission speedToday there are dozens of different types of modulation


Digital Modulation (continued)Binary signalsDigital data transmissions are typically sent in bursts of bitsThree types of binary signaling techniques can be used:Return-to-zero (RZ)Non-return-to-zero (NRZ)Polar non-return-to-zero (polar NRZ)A variation on a non-return-to-zero-level is non-return-to-zero, invert-on-ones (NRZ-I)


Digital Modulation (continued)


Digital Modulation (continued)Amplitude Shift Keying (ASK)Binary modulation technique similar to amplitude modulationHeight of the carrier signal can be changed to represent a 1 bit or a 0 bitASK uses NRZ codingFrequency Shift Keying (FSK)Binary modulation technique that changes the frequency of the carrier signalMore wave cycles are needed to represent a 1 bit


Digital Modulation (continued)Phase Shift Keying (PSK)Binary modulation technique similar to phase modulationTransmitter varies the starting point of the wavePSK signal starts and stops because it is a binary signalQuadrature amplitude modulation (QAM)Technique of combining amplitude and phase modulationReceivers can detect phase changes much more reliably than a frequency or amplitude changeIn the presence of noise


Digital Modulation (continued)Phase Shift Keying (PSK)PSK-based systems are more attractive for high-speed wireless communicationsQuadrature phase shift keying (QPSK)Combines amplitude modulation with PSK


Spread SpectrumNarrow-band transmissionsEach signal transmits on one radio frequencyOr a very narrow range of frequenciesVulnerable to outside interference from another signalRadio signal transmissions are narrow-bandSpread spectrum transmissionTakes a narrow band signal and spreads it over a broader portion of the radio frequency bandResults in less interference and fewer errorsTwo common methodsFrequency hopping and direct sequence


Spread Spectrum (continued)


Frequency Hopping Spread Spectrum (FHSS)Uses a range of frequenciesChanges frequencies several times during transmissionHopping codeThe sequence of changing frequenciesThe receiving station must also know the hopping codeMultiple radios can each use a different sequence of frequencies within the same areaAnd never interfere with each otherIf interference is encountered on a frequencyOnly a small part of the message is lost


Frequency Hopping Spread Spectrum (FHSS) (continued)


Direct Sequence Spread Spectrum (DSSS)Uses an expanded redundant code to transmit each data bitAnd then a modulation technique such as QPSKA DSSS signal is effectively modulated twiceBarker code (or chipping code)A particular sequence of 1s and 0sIdeal for modulating radio wavesAs well as for being detected correctly by the receiverIt is also called a pseudo-random codeBefore transmission, add the original data bit to the chipping code


Direct Sequence Spread Spectrum (DSSS) (continued)


Direct Sequence Spread Spectrum (DSSS) (continued)DSSS system transmits combinations of multiple chips11 chips are transmitted at a rate 11 times faster than the data rateCharacteristicsFrequency of the digital component of the signal is much higher than that of the original data (chip rate)A plot of the frequency spectrum of this signal would look similar to random noiseAll of the information contained in the original signal (a 0 or a 1 bit) is still there!


Direct Sequence Spread Spectrum (DSSS) (continued)


Direct Sequence Spread Spectrum (DSSS) (continued)AdvantagesDSSS signal appears to an unintended narrow-band receiver to be low-powered noiseNoise can cause some of the chips to change valueReceiver can recover the original data bitUsing statistical techniques and mathematical algorithmsThus avoiding the need for retransmissionDSSS devices are typically higher-end productsBecause they are more expensive to manufacture than FHSS systems


SummaryHumans use the decimal or Base 10 number systemElectrical devices use the binary or Base 2 number system insteadAmerican Standard Code for Information Interchange (ASCII)Coding scheme that uses numbers from 0 to 255 to represent symbolsWireless transmissions do not use wires or any other visible mediaInfrared wireless transmission can be either directed or diffused


Summary (continued)Radio transmissions use a carrier signalA continuous wave (CW) of constant amplitude (voltage) and frequencyCarrier signal can undergo three types of modulation:Amplitude, frequency, and phaseDigital modulation basic techniquesAmplitude, frequency and phaseRadio signals are by nature a narrow-band type of transmissionTransmit on one radio frequency or a very narrow spectrum of frequencies


Summary (continued)Spread spectrumTakes a narrow signal and spreads it over a broader portion of the radio frequency bandSpread spectrum common methodsFrequency hopping spread spectrum (FHSS)Direct sequence spread spectrum (DSSS)


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