Tài liệu Chapter 5 Data Encoding docx

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Terms (2)
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Modulation rate
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Rate at which the signal level changes
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Measured in baud = signal elements per second
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Mark and Space
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Binary 1 and Binary 0 respectively

Interpreting Signals
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Need to know
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Timing of bits - when they start and end
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Signal levels
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Factors affecting successful interpreting of
signals
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Signal to noise ratio
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Data rate
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Bandwidth

Comparison of Encoding
Schemes (1)
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Signal Spectrum
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Lack of high frequencies reduces required bandwidth
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Lack of dc component allows ac coupling via
transformer, providing isolation
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Concentrate power in the middle of the bandwidth
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Clocking
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Synchronizing transmitter and receiver
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External clock
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Sync mechanism based on signal

Comparison of Encoding
Schemes (2)
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Error detection
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Can be built in to signal encoding
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Signal interference and noise immunity
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Some codes are better than others
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Cost and complexity
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Higher signal rate (& thus data rate) lead to higher
costs
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Some codes require signal rate greater than data
rate

Encoding Schemes
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Nonreturn to Zero-Level (NRZ-L)
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Nonreturn to Zero Inverted (NRZI)
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Bipolar -AMI
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Pseudoternary
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Manchester
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Differential Manchester
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B8ZS
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HDB3

Nonreturn to Zero-Level (NRZ-L)
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Two different voltages for 0 and 1 bits
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Voltage constant during bit interval
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no transition I.e. no return to zero voltage
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e.g. Absence of voltage for zero, constant
positive voltage for one
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More often, negative voltage for one value and
positive for the other
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This is NRZ-L

Nonreturn to Zero Inverted
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Nonreturn to zero inverted on ones
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Constant voltage pulse for duration of bit
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Data encoded as presence or absence of signal
transition at beginning of bit time
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Transition (low to high or high to low) denotes a
binary 1
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No transition denotes binary 0
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An example of differential encoding

NRZ

Differential Encoding
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Data represented by changes rather than levels
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More reliable detection of transition rather than
level
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In complex transmission layouts it is easy to lose
sense of polarity

NRZ pros and cons
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Pros
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Easy to engineer
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Make good use of bandwidth
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Cons
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dc component
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Lack of synchronization capability
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Used for magnetic recording
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Not often used for signal transmission