课件版1计算机网络

1、COMPUTER NETWORKSChapter 5Signal Encoding TechniquesEncoding TechniquesDigital data, digital signalAnalog data, digital signalDigital data, analog signalAnalog data, analog signalDigital Data, Digital SignalDigital signalDiscrete, discontinuous voltage pulsesEach pulse is a signal elementBinary data

2、 encoded into signal elementsTermsUnipolarAll signal elements have same signPolarOne logic state represented by positive voltage the other by negative voltageData rateRate of data transmission in bits per secondModulation rateRate at which the signal level changesMeasured in baud = signal elements p

3、er secondMark and SpaceBinary 1 and Binary 0 respectively(波特率=比特率/每符号含的比特数)Interpreting SignalsNeed to knowTiming of bits - when they start and endSignal levelsFactors affecting successful interpreting of signalsData rate Signal to noise ratio(SNR)BandwidthComparison of Encoding SchemesSignal Spectr

4、umLack of high frequencies reduces required bandwidthLack of dc component allows ac coupling via transformer, providing isolationConcentrate power in the middle of the bandwidthClockingSynchronizing transmitter and receiverExternal clockSync mechanism based on signalError detectionCan be built in to

5、 signal encodingSignal interference and noise immunitySome codes are better than others (BER)Cost and complexityHigher signal rate (& thus data rate) lead to higher costsSome codes require signal rate greater than data rateEncoding SchemesNonreturn to Zero-Level (NRZ-L)Nonreturn to Zero Inverted (NR

6、ZI)Bipolar -AMIPseudoternaryManchesterDifferential ManchesterB8ZSHDB3Nonreturn to Zero-Level (NRZ-L)Two different voltages for 0 and 1 bitsVoltage constant during bit intervalno transition i.e. no return to zero voltageAbsence of voltage for one, constant positive voltage for zeroNonreturn to Zero I

7、nvertedNonreturn to zero inverted on onesConstant voltage pulse for duration of bitData encoded as presence or absence of signal transition at beginning of bit timeTransition (low to high or high to low) denotes a binary 1No transition denotes binary 0An example of differential encodingNRZ 0 1 0 0 1

8、 1 0 0 0 1 1NRZ pros and consProsEasy to engineerMake good use of bandwidthConsdc componentLack of synchronization capabilityUsed for magnetic recordingNot often used for signal transmissionMultilevel BinaryUse more than two levelsBipolar-AMIzero represented by no signalone represented by positive o

9、r negative pulseone pulses alternate in polarityNo loss of sync if a long string of ones (zeros still a problem)No dc componentLower bandwidthEasy error detectionPseudoternaryOne represented by absence of line signalZero represented by alternating positive and negativeNo advantage or disadvantage ov

10、er bipolar-AMIBipolar-AMI and Pseudoternary 0 1 0 0 1 1 0 0 0 1 1Trade Off for Multilevel BinaryNot as efficient as NRZEach signal element only represents one bitA 3 level system could represent log23 = 1.58 bitsReceiver must distinguish between three levels (+A, -A, 0)Requires approx. 3dB more sign

11、al power for same probability of bit errorBiphaseManchesterTransition in middle of each bit periodTransition serves as clock and dataLow to high represents oneHigh to low represents zeroUsed by IEEE 802.3Differential ManchesterMidbit transition is clocking onlyTransition at start of a bit period rep

12、resents zeroNo transition at start of a bit period represents oneNote: this is a differential encoding schemeUsed by IEEE 802.5Data represented by changes rather than levelsMore reliable detection of transition rather than levelIn complex transmission layouts it is easy to lose sense of polarityManc

13、hester & Differential EncodingBiphase Pros and ConsProsSynchronization on mid bit transition (self clocking)No dc componentError detectionAbsence of expected transitionConsAt least one transition per bit time and possibly twoMaximum modulation rate is twice NRZRequires more bandwidthScramblingUse sc

14、rambling to replace sequences that would produce constant voltageFilling sequence Must produce enough transitions to syncMust be recognized by receiver and replace with originalSame length as originalNo dc componentNo long sequences of zero level line signalNo reduction in data rateError detection c

15、apabilityB8ZS(双极性8零替换码)Bipolar With 8 Zeros SubstitutionBased on bipolar-AMIIf octet of all zeros and last voltage pulse preceding was positive encode as 000+-0-+If octet of all zeros and last voltage pulse preceding was negative encode as 000-+0+-Causes two violations of AMI codeReceiver detects an

16、d interprets as octet of all zerosHDB3(高密度双极性3零码)High Density Bipolar 3 ZerosBased on bipolar-AMIString of four zeros replaced with one or two pulsesNumber of bipolar pulses (ones) since last substitutionPolarity of preceding pulseoddeven-000-+00+000+-00-B8ZS and HDB3Digital Data, Analog SignalPubli

17、c telephone system300Hz to 3400HzUse modem (modulator-demodulator)Amplitude shift keying (ASK)Frequency shift keying (FSK)Phase shift keying (PSK)Modulation TechniquesAmplitude Shift KeyingValues represented by different amplitudes of carrierUsually, one amplitude is zeroi.e. presence and absence of

18、 carrier is usedSusceptible to sudden gain changesInefficientUp to 1200bps on voice grade linesUsed over optical fiber2ASK调制解调相干解调Binary Frequency Shift KeyingMost common form is binary FSK (BFSK)Two binary values represented by two different frequencies (near carrier)Less susceptible to error than

19、ASKUp to 1200bps on voice grade linesHigh frequency radioEven higher frequency on LANs using co-axFSK on Voice Grade LineMultiple FSKMore than two frequencies usedMore bandwidth efficientMore prone to errorEach signalling element represents more than one bit2FSK调制: BNRZ信号经过模拟调频器解调鉴频器过零检测相干解调非相干解调Pha

20、se Shift KeyingPhase of carrier signal is shifted to represent dataBinary PSKTwo phases represent two binary digitsDifferential PSKPhase shifted relative to previous transmission rather than some reference signal2PSK调制方法解调方法2DPSKDefinition: 源 0 1 0 0 1 1 0 1 0 1 0 1 0 1 0 1 1 1 12PSK 0 0 0 0 0 0 0 0

21、 2DPSK 0 0 0 0 0 0 0 0 0 差分编码 0 0 1 1 1 0 1 1 0 0 1 1 0 0 1 1 0 1 0 1 2DPSK相干解调与2PSK类似，但判决后需要进行差分译码。非相干解调Quadrature PSKMore efficient use by each signal element representing more than one bitshifts of /2 (90o)Each element represents two bitsOffset QPSK (orthogonal QPSK)Delay one bit in Q streamThe n

22、eighboring phase shift no more than 90oQPSK and OQPSK ModulatorsExamples of QPSK and OQPSK WaveformsPerformance of Digital to Analog Modulation SchemesBandwidthASK and PSK bandwidth directly related to bit rateFSK bandwidth related to data rate for lower frequencies, but to offset of modulated frequ

23、ency from carrier at high frequenciesIn the presence of noise, Eb/N0 of PSK and QPSK are about 3dB superior to ASK and FSKQuadrature Amplitude ModulationQAM used on asymmetric digital subscriber line (ADSL) and some wirelessCombination of ASK and PSKLogical extension of QPSKSend two different signal

24、s simultaneously on same carrier frequencyUse two copies of carrier, one shifted 90Each carrier is ASK modulatedTwo independent signals over same mediumDemodulate and combine for original binary outputQAM ModulatorQAM LevelsTwo level ASKEach of two streams in one of two statesFour state systemEssent

25、ially QPSKFour level ASKCombined stream in one of 16 states64 and 256 state systems have been implementedImproved data rate for given bandwidthIncreased potential error rate (b) 星型16QAM星座QAM的星座图(a) 方型16QAM星座；Analog Data, Digital SignalDigitizationConversion of analog data into digital dataDigital da

26、ta can then be transmitted using NRZ-L and other techniquesDigital data can then be converted to analog signalAnalog to digital conversion done using a codecPulse code modulationDelta modulationDigitizing Analog DataPulse Code Modulation(PCM) (1)Sampling theorem: If a signal is sampled at regular in

27、tervals at a rate higher than twice the highest signal frequency, the samples contain all the information of the original signalVoice data limited to below 4000HzRequire 8000 sample per secondAnalog samples (Pulse Amplitude Modulation, PAM)Each sample assigned digital valuePulse Code Modulation(PCM)

28、 (2)QuantizedQuantizing error or noiseApproximations mean it is impossible to recover original exactly4 bit system gives 16 levels8 bit sample gives 256 levelsQuality comparable with analog transmission8000 samples per second of 8 bits each gives 64kbpsPCM ExampleNonlinear EncodingReduces overall signal distortionQuantization levels not evenly spacedCan also be done by comp