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    Data and ComputerCommunications

    DATA ENCODING

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    Introduction

    Either form of data can be encoded into

    either form of signal

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    Introduction

    For DIGITAL SIGNALING, Data source g(t)which may be either digitalor analog, is encoded into a digital signal x(t).

    The actual form of x(t) depends on the encoding technique, and is chosento optimize use of the transmission medium. For example, the encoding

    may be chosen to either conserve bandwidth or to minimize errors.

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    Introduction

    The backbone for ANALOG SIGNALING is a continuous, constant-frequency signal known asthe CARRIER SIGNAL. The frequency of the carrier signal is chosen to be compatiblewith the transmission medium being used.Data may be transmitted using a carrier signal by modulation.

    MODULATION is the process of encoding source data onto a carrier signal with frequency fc.All modulation techniques involve operation on one or more of the three fundamental

    frequency-domain parameters:AmplitudeFrequencyPhase

    The input signal m(t) may be analog or digital and is calledthe MODULATING SIGNAL or BASEBAND SIGNAL.The result of modulating the carrier signal is called theMODULATED SIGNAL.The location of the bandwidth on the spectrum is related tofcand is often centered onfc.Again, the actual form of the encoding is chosen to optimize

    some characteristic of the transmission.

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    Introduction

    DIGITAL DATA DIGITAL SIGNALEquipment for encoding is less complex and less expensive compared to digital-to-analogmodulation equipment

    ANALOG DATA DIGITAL SIGNALConversion of analog data to digital form permits the use of modern digital transmissionand switching equipment. Advantages of using the digital approach are summarized bythe following: Digital Technology-Data Integrity-Capacity Utilization-Security andPrivacy-Integration

    DIGITAL DATA ANALOG SIGNALSome transmission media such as optical fiber and the unguided media will onlypropagate analog signal

    ANALOG DATA ANALOG SIGNALAnalog data in electrical form can be transmitted as basebandsignals easily and cheaply by voice transmission over voice-gradelines.One common use of modulation is to shift the bandwidth of abaseband signal to another portion of the spectrum. In this way,multiple signals, each at a different position on the spectrum, canshare the same transmission medium; this is known as FREQUENCY

    DIVISION MULTIPLEXING.

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    Digital Data, DigitalSignal

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    DIGITAL SIGNAL Sequence of discrete, discontinuous voltage pulses

    Each pulse is a SIGNAL ELEMENT

    Binary data are encoded into signal elements

    Simplest case has one-to-one correspondence

    Digital Signal

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    Unipolar: signal elements all have the same algebraic sign(all + or all -)

    Polar: one logic state is represented by a positive voltagelevel, and the other by a negative voltage level

    Data Signaling Rate or Data Rate: rate in bits per second

    that data is transmitted (R) Duration or length of a bit: amount of time it takes for the

    transmitter to emit the bit (1/R)

    Modulation/Signaling Rate: rate at which the signal level ischanged, this will depend on the nature of digital encodingand is expressed in BAUD (signal elements per second)

    Mark and Space: refer to binary digit 1 and 0

    Definition of Terms

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    Need to Know Timing of bits - when they start

    and end

    Signal levels

    Factors Affecting Signal

    Interpretation SNR or Eb/No

    Data Rate

    Bandwidth

    Encoding Scheme

    Interpreting Signals

    An increase in data rate

    increases bit error rate (theprobability that a bit is receivedin error).

    An increase in SNR decreasesbit error rate.

    An increase in bandwidth allowsan increase in data rate.

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    Signal Spectrum Lack of HF component reduces W requirement

    Lack of DC is desirable because ac-coupling via transformer ispossible thus providing excellent electrical isolation and reductionof interference

    Good signal design should concentrate the transmitted power in the

    middle of the transmission W Clocking

    Expensive approach is to provide a separate clock-lead to synch theTX and RX and the alternative is to provide some synch mechanismthat is based on the encoding scheme of the transmitted signal

    Ways of Evaluating EncodingSchemes

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    Error Detection Responsibility of the DATA LINK CONTROL

    Useful to have some error-detection capability built into thephysical signaling-encoding scheme permitting errors to be detectedmore quickly

    Signal Interference and Noise Immunity Certain codes exhibit superior performance in the presence of noise

    This ability is usually expressed in terms of a BIT ERROR RATE

    Cost and Complexity The higher the signaling rate to achieve a given data rate, the

    greater the cost

    Some codes require a signaling rate that is greater than the actualdata rate

    Ways of Evaluating EncodingSchemes

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    Digital Signal Encoding Formats

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    NRZ

    NRZ-L and NRZI Advantage:

    Easy to engineer

    Makes efficient use of W

    Disadvantage:

    Presence of DC component

    Lack of synchronization Differential Encoding

    NRZI and Differential Manchester

    The signal is decoded by comparing the polarity of adjacent signal elements ratherthan determining the absolute value of a signal element

    Advantage:

    more reliable to detect a transition in the presence of noise than to compare a

    value to a threshold Disadvantage:

    Presence of DC component

    Lack of synchronization

    Digital Signal Encoding Formats

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    Multilevel Binary

    Bipolar-AMI and Pseudoternary Advantage:

    No loss of synchronization on long strings of 1s (B-AMI) or 0s (Pseudoternary)

    No net DC component

    Less W compared to NRZ

    Pulse alternation provides some means of error detection

    Disadvantage: Receiver needs to distinguish between 3 levels instead of 2

    Needs higher signal power for the same level of bit error rate

    Biphase

    Manchester and Differential Manchester

    Midbit transition

    Advantage:

    No net DC component

    Synchronization

    Error detection (absence of midbit transition)

    Disadvantage:

    W is considerably greater since modulation rate is 2x the data rate

    Digital Signal Encoding Formats

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    Modulation/Signaling Rate vsData Rate

    Data rate (expressed in bits per second) Modulation rate (expressed in baud).

    The data rate, or bit rate, is l/tB, where tB = bitduration.

    The modulation rate is the rate at which signal elementsare generated.

    D = modulation rate, baud

    R = data rate, bps

    N = number of bits per signalingelement

    M = number of discrete signal

    D = R / N = R /log2M

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    Process of replacing sequences that would produce a long string ofconstant voltage that could result to loss of synchronization

    These filling sequences must: produce enough transitions to sync

    be recognized by receiver & replaced with original

    be same length as original

    Design Goals: no dc component

    no long sequences of zero level line signal

    no reduction in data rate

    give error detection capability

    Scrambling

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    B8ZS and HDB3 B8ZS (bipolar with 8-zeros substitution)

    Common in North America Based in Bipolar-AMI (drawback is on long strings of 0)

    This technique forces two code violations (signal patterns not allowed in AMI) of theAM1 code, an event unlikely to be caused by noise or other transmission impairment.

    The receiver recognizes the pattern and interprets the octet as consisting of allzeros.

    HDB3 (high-density bipolar-3 zeros)

    Common in Europe and Japan

    Based in Bipolar-AMI (drawback is on long strings of 0)

    In this case, the scheme replaces strings of four zeros with sequences containing oneor two pulses.

    In each case, the fourth zero is replaced with a code violation.

    In addition, a rule is needed to ensure that successive violations are of alternate

    polarity so that no dc component is introduced. Thus, if the last violation was positive, this violation must be negative, and vice versa.

    -

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    B8ZS and HDB3

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    Seatwork (20min)

    Encode the following bit stream using (+) pulse = 5V and (-) pulse =-5V NRZ-L NRZI last pulse prior the bit stream is (+)

    Bipolar-AMI

    start with (+) pulse Pseudoternary start with (+) pulse Manchester Differential Manchester last pulse prior the bit stream is

    low-to-high

    1 0 1 0 0 0 1 1 1 0 0 1

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    Seatwork (20min)

    Give the B8ZS equivalent of the ff. bit stream. Start with the (+)pulse. Label the violation.

    1110000000011101101000 Give the HDB3 equivalent of the ff. bit stream. Start with the (+)

    pulse Label the violation and stuffing bit.

    1010000011000011000000

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    Seatwork - Answer

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    Seatwork - Answer

    Give the B8ZS equivalent of the ff. bit stream. Start with the (+) pulse.Label the violation.

    1110000000011101101000+-+000+-0-+-+-0+-0+000Red violation

    Give the HDB3 equivalent of the ff. bit stream. Start with the (+) pulseLabel the violation and stuffing bit.

    1010000011000011000000+0-000-0+-+00+-+-00-00Red

    violationOrange stuffing bit

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    Digital Data, Analog

    Signal

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    Main use is transmitting data through the publictelephone system

    has freq range of 300Hz to 3400Hz

    use modem (modulator-demodulator)

    Modulation involves operation on one or more ofthe 3 characteristics of the carrier signal. In allthese cases the resulting signal occupies the Wcentered on the carrier frequency

    Amplitude Shift Keying (ASK) Frequency Shift Keying (FSK)

    Phase Shift Keying (PK)

    Digital Data, Analog Signal

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    Bandwidth ASK/PSK bandwidth directly relates to

    bit rate Multilevel PSK gives significantimprovements

    In presence of noise: Bit error rate of PSK are about 3dB

    superior to ASK and FSK

    Performance of Digital toAnalog Modulation Schemes

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    Analog Data, Digital

    Signal

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    DIGITIZATION is conversion of analog data intodigital data which can then:

    1. Be transmitted using NRZ-L. In this case, we have gone directlyfrom analog data to a digital signal.

    2. Be encoded as a digital signal using a code other than NRZ-L.Thus, an extra step is required.

    3. Be converted into an analog signal, using one of the modulationtechniques discussed previously.

    Analog Data, Digital Signal

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    Digitizing Analog Data

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    Based on the sampling theorem:

    If a signal is sampled at regular intervals at a rate higherthan twice the highest signal frequency, the samples containall information in original signal

    eg. 4000Hz voice data, requires 8000 sample per sec (NOTE:These are analog samples)

    Samples are represented as narrow pulses whose amplitude isproportional to the value of the original signal PULSEAMPLITUDE MODULATION (PAM)

    PAM data are quantized to produced PCM DATA.

    The amplitude of each PAM pulse is approximated by an n-bit

    integer.

    Pulse Code Modulation (PCM)

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    Pulse Code Modulation (PCM)

    In the example, n = 3.

    Thus, 8 = 23 levels are availablefor approximating the PAM

    pulses.

    k (10 )

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    Seatwork (10min)

    Give the quantized code and the PCM equivalent of the given signal.

    PAM value is given.

    k A

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    Seatwork - Answer

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    QUANTIZING ERROR The error caused by quantizing the PAM pulse

    The original signal is now only approximated and cannot berecovered exactly.

    The signal-to-noise ratio for quantizing noise can be expressed

    as:

    Pulse Code Modulation (PCM)

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    Non-Linear Coding Quantization levels are not equally spaced.

    The problem with equal spacing is that the mean absolute errorfor each sample is the same, regardless of signal level. Consequently, lower amplitude values are relatively more

    distorted. By using a greater number of quantizing steps for signals of low

    amplitude, and a smaller number of quantizing steps for signalsof large amplitude, a marked reduction in overall signal distortionis achieve.

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    Companding The same effect can be achieved by using uniform quantizing but

    companding (compressing-expanding) the input analog signal.

    COMPANDING is a process that compresses the intensity range of asignal by imparting more gain to weak signals than to strong signals oninput.

    At output, the reverse operation is performed. Note that the effect onthe input side is to compress the sample so that the higher values are

    reduced with respect to the lower values. ]Thus, with a fixed number of quantizing levels, more levels are available

    for lower-level signals. On the output side, the compander expands thesamples so the compressed values are restored to their original values.

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    Companding

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    Popular alternative to PCM

    Analog input is approximated by a staircase function that

    moves up or down by one quantizing level() at eachsampling interval

    Behavior is binary At each sampling time, the function only moves up or

    down at a constant level

    Thus, the output can be encoded as a single binary digitfor each sample

    1 for up or 0 for down

    Delta Modulation

    Two Important ParametersSize of the step ()

    Sampling size

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    Delta Modulation Example

    The transition (up or down) that occurs at eachsampling interval is chosen so that the staircasefunction tracks the original analog waveform asclosely as possible.For transmission, the following occurs: At eachsampling time, the analog input is compared to themost recent value of the approximating staircasefunction.

    If the value of the sampled waveform exceedsthat of the staircase function, a 1 is generated;otherwise, a 0 is generated.The output of the DM process is therefore a binarysequence that can be used at the receiver toreconstruct the staircase function.The staircase function can then be smoothed by sometype of integration process or by passing it through a

    low-pass filter to produce an analog approximation of

    the analog input signal.

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    DM has simplicity compared to PCM buthas worse SNR

    PCM preferred to DM for digitizing

    analog signals that represent digitaldata

    PCM verses Delta Modulation

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    Analog Data, Analog

    Signal

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    Modulation has been defined as the process of combining an inputsignal m(t)and a carrier at frequency f, to produce a signal s(t)whose bandwidth is (usually) centered on f,.

    For digital data, the motivation for modulation should be clear: Whenonly analog transmission facilities are available, modulation is

    required to convert the digital data to analog form. For analog data, the principal reasons for modulation are as follows:

    A higher frequency may be needed for effective transmission.For unguided transmission, it is virtually impossible to transmitbaseband signals. The required antennas would be manykilometers in diameter.

    Modulation permits frequency-division multiplexing

    Analog Data, Analog Signals

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    AmplitudeModulation

    FrequencyModulation

    PhaseModulation

    Analog

    ModulationTechniques

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    END