西电初试专业课讲义计算机网络

1、COMPUTER NETWORKSChapter 3Data TransmissionTerminology SimplexOne directione.g. TelevisionHalf duplexEither direction, but only one way at a timee.g. police radioFull duplexBoth directions at the same timee.g. telephoneFrequency, Spectrum and BandwidthTime domain conceptsAnalog signalVarious in a sm

2、ooth way over timeDigital signalMaintains a constant level then changes to another constant levelPeriodic signalPattern repeated over timeAperiodic signalPattern not repeated over timeAnalogue & Digital SignalsPeriodicSignalsSine WavePeak Amplitude (A)maximum strength of signalvoltsFrequency (f)Rate

3、 of change of signalHertz (Hz) or cycles per secondPeriod = time for one repetition (T)T = 1/fPhase ()Relative position in timeVarying Sine Wavess(t) = A sin(2ft +)WavelengthDistance occupied by one cycleDistance between two points of corresponding phase in two consecutive cyclesAssuming signal velo

4、city v = vTf = vc = 3*108 ms-1 (speed of light in free space)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 functionsFrequencyDomainRepresentations

5、 Spectrum & BandwidthSpectrumrange of frequencies contained in signalAbsolute bandwidthwidth of spectrumEffective bandwidthOften just bandwidthNarrow band of frequencies containing most of the energyDC ComponentComponent of zero frequencySignal with DC ComponentAnalog and Digital SignalsMeans by whi

6、ch data are propagatedAnalogContinuously variableVarious mediawire, fiber optic, spaceSpeech bandwidth 100Hz to 7kHzTelephone bandwidth 300Hz to 3400HzVideo bandwidth 4MHzDigitalUse two DC componentsAdvantages & Disadvantages of DigitalCheaperLess susceptible to noiseGreater attenuationPulses e roun

7、ded and smallerLeads to loss of informationAcoustic Spectrum (Analog)Components of SpeechFrequency range (of hearing) 20Hz-20kHzSpeech 100Hz-7kHzEasily converted into electromagnetic signal for transmissionSound frequencies with varying volume converted into electromagnetic frequencies with varying

8、voltageConversion of Voice Input into Analog SignalConversion of PC Input to Digital SignalData 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 audioAnal

9、og Signals Carrying Analog and Digital DataDigital Signals Carrying Analog and Digital DataAnalog TransmissionAnalog signal transmitted without regard to contentMay be analog or digital dataAttenuated over distance Use amplifiers to boost signalAlso amplifies noiseDigital TransmissionConcerned with

10、contentIntegrity endangered by noise, attenuation etc.Repeaters usedRepeater receives signalExtracts bit patternRetransmitsAttenuation is eNoise is not amplifiedAdvantages of Digital TransmissionDigital technologyLow cost LSI/VLSI technologyData integrityLonger distances over lower quality linesCapa

11、city utilizationHigh bandwidth links economicalHigh degree of multiplexing easier with digital techniquesSecurity & PrivacyEncryptionIntegrationCan treat analog and digital data similarlyTransmission ImpairmentsSignal received may differ from signal transmittedAnalog - degradation of signal qualityD

12、igital - bit errorsCaused byAttenuation(衰减) and attenuation distortion (失真) Delay distortionNoiseAttenuationSignal 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

13、an increasing function of frequencyDelay DistortionOnly in guided mediaPropagation velocity varies with frequencyNoise (1)Additional signals inserted between transmitter and receiverThermalDue to thermal agitation of electronsUniformly distributedWhite noiseIntermodulationSignals that are the sum an

14、d difference of original frequencies sharing a mediumNoise (2)CrosstalkA signal from one line is picked up by anotherImpulseIrregular pulses or spikese.g. External electromagnetic interferenceShort durationHigh amplitudeChannel CapacityData rateIn bits per secondRate at which data can be communicate

15、dBandwidthIn cycles per second of HertzConstrained by transmitter and mediumAny transmission system has a limited band of frequencies. This limits the data rate that can be carriedNyquist BandwidthIf rate of signal transmission is 2B then signal with frequencies no greater than B is sufficient to ca

16、rry signal rateGiven bandwidth B, highest signal rate is 2BGiven binary signal, data rate supported by B Hz is 2B bpsCan be increased by using M signal levelsC= 2B log2MShannon Capacity FormulaConsider data rate, noiseFaster data rate shortens each bit so burst of noise affects more bitsAt given noise level, high data rate means