Chapter 5 Data Encoding

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William Stallings

Data and Computer

Communications

Chapter 5

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Encoding Techniques

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Digital Data, Digital Signal

 _{Digital signal}

 _{Discrete, discontinuous voltage pulses}  _{Each pulse is a signal element}

 _{Binary data encoded into signal}

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Terms (1)

 _Unipolar

 _{All signal elements have same sign}

 _Polar

 _{One logic state represented by positive}

voltage the other by negative voltage

 _{Data rate}

 _{Rate of data transmission in bits per}

second

 _{Duration or length of a bit}

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Terms (2)

 _{Modulation rate}

 _{Rate at which the signal level changes}

 _{Measured in baud = signal elements per}

second

 _{Mark and Space}

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Interpreting Signals

 _{Need to know}

 _{Timing of bits - when they start and end}  _{Signal levels}

 _{Factors affecting successful interpreting of}

signals

 _{Signal to noise ratio}  _{Data rate}

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Comparison of Encoding Schemes

(1)

 _{Signal Spectrum}

 _{Lack of high frequencies reduces} required bandwidth

 _{Lack of dc component allows ac coupling} via transformer, providing isolation

 _{Concentrate power in the middle of the} bandwidth

 _Clocking

 _{Synchronizing transmitter and receiver}  _{External clock}

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Comparison of Encoding Schemes

(2)

 _{Error detection}

 _{Can be built in to signal encoding}

 _{Signal interference and noise immunity}  _{Some codes are better than others}  _{Cost and complexity}

 _{Higher signal rate (& thus data rate)}

lead to higher costs

 _{Some codes require signal rate greater}

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Encoding Schemes

 _{Nonreturn to Zero-Level (NRZ-L)}  _{Nonreturn to Zero Inverted (NRZI)}  _{Bipolar -AMI}

 _{Pseudoternary}  _Manchester

 _{Differential Manchester}  _B8ZS

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Nonreturn to Zero-Level (NRZ-L)

 _{Two different voltages for 0 and 1 bits}  _{Voltage constant during bit interval}

 _{no transition I.e. no return to zero}

voltage

 _{e.g. Absence of voltage for zero, constant}

positive voltage for one

 _{More often, negative voltage for one value}

and positive for the other

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Nonreturn to Zero Inverted

 _{Nonreturn to zero inverted on ones}

 _{Constant voltage pulse for duration of bit}  _{Data encoded as presence or absence of}

signal transition at beginning of bit time

 _{Transition (low to high or high to low)}

denotes a binary 1

 _{No transition denotes binary 0}

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Differential Encoding

 _{Data represented by changes rather than}

levels

 _{More reliable detection of transition rather}

than level

 _{In complex transmission layouts it is easy}

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NRZ pros and cons

 _Pros

 _{Easy to engineer}

 _{Make good use of bandwidth}  _Cons

 _{dc component}

 _{Lack of synchronization capability}  _{Used for magnetic recording}

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Multilevel Binary

 _{Use more than two levels}

 _Bipolar-AMI

 _{zero represented by no line signal}

 _{one represented by positive or negative} pulse

 _{one pulses alternate in polarity}

 No loss of sync if a long string of ones (zeros still a problem)

 _{No net dc component}

 _{Lower bandwidth}

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Pseudoternary

 _{One represented by absence of line signal}  _{Zero represented by alternating positive}

and negative

 _{No advantage or disadvantage over}

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Trade Off for Multilevel Binary

 _{Not as efficient as NRZ}

 _{Each signal element only represents one} bit

 In a 3 level system could represent log₂3 = 1.58 bits

 Receiver must distinguish between three levels

(+A, -A, 0)

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Biphase

 _Manchester

 _{Transition in middle of each bit period}

 _{Transition serves as clock and data}

 _{Low to high represents one}

 _{High to low represents zero}

 _{Used by IEEE 802.3}

 _{Differential Manchester}

 _{Midbit transition is clocking only}

 _{Transition at start of a bit period represents zero}

 _{No transition at start of a bit period represents one}

 _{Note: this is a differential encoding scheme}

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Biphase Pros and Cons

 _Con

 _{At least one transition per bit time and}

possibly two

 _{Maximum modulation rate is twice NRZ}  _{Requires more bandwidth}

 _Pros

 _{Synchronization on mid bit transition}

(self clocking)

 _{No dc component}  _{Error detection}

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Scrambling

 _{Use scrambling to replace sequences that}

would produce constant voltage  _{Filling sequence}

 Must produce enough transitions to sync

 _{Must be recognized by receiver and}

replace with original

 _{Same length as original}

 _{No dc component}

 _{No long sequences of zero level line signal}  _{No reduction in data rate}

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B8ZS

 _{Bipolar With 8 Zeros Substitution}  _{Based on bipolar-AMI}

 _{If 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 code}  _{Unlikely to occur as a result of noise}

 _{Receiver detects and interprets as octet of all}

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HDB3

 _{High Density Bipolar 3 Zeros}  _{Based on bipolar-AMI}

 _{String of four zeros replaced with one or}

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Digital Data, Analog Signal

 _{Public telephone system}  _{300Hz to 3400Hz}

 _{Use modem (modulator-demodulator)}  _{Amplitude shift keying (ASK)}

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Amplitude Shift Keying

 _{Values represented by different}

amplitudes of carrier

 _{Usually, one amplitude is zero}

 _{i.e. presence and absence of carrier is}

used

 _{Susceptible to sudden gain changes}  _Inefficient

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Frequency Shift Keying

 _{Values represented by different}

frequencies (near carrier)

 _{Less susceptible to error than ASK}  _{Up to 1200bps on voice grade lines}  _{High frequency radio}

 _{Even higher frequency on LANs using}

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Phase Shift Keying

 _{Phase of carrier signal is shifted to}

represent data

 _{Differential PSK}

 _{Phase shifted relative to previous}

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Quadrature PSK

 _{More efficient use by each signal element}

representing more than one bit

 _{e.g. shifts of /2 (90}o)

 _{Each element represents two bits}

 _{Can use 8 phase angles and have more}

than one amplitude

 _{9600bps modem use 12 angles , four of}

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Performance of Digital to Analog

Modulation Schemes

 _Bandwidth

 _{ASK and PSK bandwidth directly related} to bit rate

 _{FSK bandwidth related to data rate for} lower frequencies, but to offset of

modulated frequency from carrier at high frequencies

 _{(See Stallings for math)}

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Analog Data, Digital Signal

 _Digitization

 _{Conversion of analog data into digital data}  _{Digital data can then be transmitted using}

NRZ-L

 _{Digital data can then be transmitted using}

code other than NRZ-L

 _{Digital data can then be converted to}

analog signal

 _{Analog to digital conversion done using a}

codec

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Pulse Code Modulation(PCM) (1)

 _{If a signal is sampled at regular intervals}

at a rate higher than twice the highest

signal frequency, the samples contain all the information of the original signal

 _{(Proof - Stallings appendix 4A)}

 _{Voice data limited to below 4000Hz}  _{Require 8000 sample per second}

 _{Analog samples (Pulse Amplitude}

Modulation, PAM)

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Pulse Code Modulation(PCM) (2)

 _{4 bit system gives 16 levels}  _Quantized

 _{Quantizing error or noise}

 _{Approximations mean it is impossible to}

recover original exactly

 _{8 bit sample gives 256 levels}

 _{Quality comparable with analog}

transmission

 _{8000 samples per second of 8 bits each}

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Nonlinear Encoding

 _{Quantization levels not evenly spaced}  _{Reduces overall signal distortion}

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Delta Modulation

 _{Analog input is approximated by a}

staircase function

 _{Move up or down one level () at each}

sample interval

 _{Binary behavior}

 _{Function moves up or down at each}

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Delta Modulation - Performance

 _{Good voice reproduction}  _{PCM - 128 levels (7 bit)}  _{Voice bandwidth 4khz}

 _{Should be 8000 x 7 = 56kbps for PCM}  _{Data compression can improve on this}

 _{e.g. Interframe coding techniques for}

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Analog Data, Analog Signals

 _{Why modulate analog signals?}

 _{Higher frequency can give more}

efficient transmission

 _{Permits frequency division multiplexing}

(chapter 8)

 _{Types of modulation}  _Amplitude

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Analog

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Spread Spectrum

 _{Analog or digital data}  _{Analog signal}

 _{Spread data over wide bandwidth}

 _{Makes jamming and interception harder}

 _{Frequency hoping}

 _{Signal broadcast over seemingly random}

series of frequencies

 _{Direct Sequence}

 _{Each bit is represented by multiple bits} in transmitted signal

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Required Reading