(Data Transmission (chapter 3

(Data Transmission (chapter 3

اسلاید 1: William Stallings Data and Computer CommunicationsChapter 3Data Transmission

اسلاید 2: Terminology (1)TransmitterReceiverMediumGuided mediume.g. twisted pair, optical fiberUnguided mediume.g. air, water, vacuum

اسلاید 3: Terminology (2)Direct linkNo intermediate devicesPoint-to-pointDirect link Only 2 devices share linkMulti-pointMore than two devices share the link

اسلاید 4: Terminology (3)SimplexOne directione.g. TelevisionHalf duplexEither direction, but only one way at a timee.g. police radioFull duplexBoth directions at the same timee.g. telephone

اسلاید 5: Frequency, Spectrum and BandwidthTime domain conceptsContinuous signalVarious in a smooth way over timeDiscrete signalMaintains a constant level then changes to another constant levelPeriodic signalPattern repeated over timeAperiodic signalPattern not repeated over time

اسلاید 6: Continuous & Discrete Signals

اسلاید 7: Periodic Signals

اسلاید 8: Sine WavePeak Amplitude (A)maximum strength of signalvoltsFrequency (f)Rate of change of signalHertz (Hz) or cycles per secondPeriod = time for one repetition (T)T = 1/fPhase ()Relative position in time

اسلاید 9: Varying Sine Waves

اسلاید 10: WavelengthDistance occupied by one cycleDistance between two points of corresponding phase in two consecutive cyclesAssuming signal velocity v = vTf = vc = 3*108 ms-1 (speed of light in free space)

اسلاید 11: Frequency Domain ConceptsSignal usually made up of many frequenciesComponents are sine wavesCan be shown (Fourier analysis) that any signal is made up of component sine wavesCan plot frequency domain functions

اسلاید 12: Addition of Frequency Components

اسلاید 13: Frequency Domain

اسلاید 14: Spectrum & BandwidthSpectrumrange of frequencies contained in signalAbsolute bandwidthwidth of spectrumEffective bandwidthOften just bandwidthNarrow band of frequencies containing most of the energyDC ComponentComponent of zero frequency

اسلاید 15: Signal with DC Component

اسلاید 16: Data Rate and BandwidthAny transmission system has a limited band of frequenciesThis limits the data rate that can be carried

اسلاید 17: Analog and Digital Data TransmissionData Entities that convey meaningSignalsElectric or electromagnetic representations of dataTransmissionCommunication of data by propagation and processing of signals

اسلاید 18: DataAnalogContinuous values within some intervale.g. sound, videoDigitalDiscrete valuese.g. text, integers

اسلاید 19: Acoustic Spectrum (Analog)

اسلاید 20: SignalsMeans by which data are propagatedAnalogContinuously variableVarious mediawire, fiber optic, spaceSpeech bandwidth 100Hz to 7kHzTelephone bandwidth 300Hz to 3400HzVideo bandwidth 4MHzDigitalUse two DC components

اسلاید 21: Data and SignalsUsually use digital signals for digital data and analog signals for analog dataCan use analog signal to carry digital dataModemCan use digital signal to carry analog data Compact Disc audio

اسلاید 22: Analog Signals Carrying Analog and Digital Data

اسلاید 23: Digital Signals Carrying Analog and Digital Data

اسلاید 24: Analog TransmissionAnalog signal transmitted without regard to contentMay be analog or digital dataAttenuated over distance Use amplifiers to boost signalAlso amplifies noise

اسلاید 25: Digital TransmissionConcerned with contentIntegrity endangered by noise, attenuation etc.Repeaters usedRepeater receives signalExtracts bit patternRetransmitsAttenuation is overcomeNoise is not amplified

اسلاید 26: Advantages of Digital TransmissionDigital technologyLow cost LSI/VLSI technologyData integrityLonger distances over lower quality linesCapacity utilizationHigh bandwidth links economicalHigh degree of multiplexing easier with digital techniquesSecurity & PrivacyEncryptionIntegrationCan treat analog and digital data similarly

اسلاید 27: Transmission ImpairmentsSignal received may differ from signal transmittedAnalog - degradation of signal qualityDigital - bit errorsCaused byAttenuation and attenuation distortionDelay distortionNoise

اسلاید 28: AttenuationSignal strength falls off with distanceDepends on mediumReceived signal strength:must be enough to be detectedmust be sufficiently higher than noise to be received without errorAttenuation is an increasing function of frequency

اسلاید 29: Delay DistortionOnly in guided mediaPropagation velocity varies with frequency

اسلاید 30: Noise (1)Additional signals inserted between transmitter and receiverThermalDue to thermal agitation of electronsUniformly distributedWhite noiseIntermodulationSignals that are the sum and difference of original frequencies sharing a medium

اسلاید 31: Noise (2)CrosstalkA signal from one line is picked up by anotherImpulseIrregular pulses or spikese.g. External electromagnetic interferenceShort durationHigh amplitude

اسلاید 32: Channel CapacityData rateIn bits per secondRate at which data can be communicatedBandwidthIn cycles per second of HertzConstrained by transmitter and medium

اسلاید 33: Required ReadingStallings chapter 3