Data Encoding

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Data Encoding

• Data Encoding refers the various techniques of
  impressing data (0,1) or information on an
  electrical, electromagnetic or optical signal
  that would propagate through the physical medium
  making up the communication link between the two
  devices.

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Plan for the Lecture

• Why Encoding, Encoding Issues
• Digital Data and Digital Signals
• Analog Data and Digital Signals
• Digital Data and Analog Signals
• Analog Data and Analog Signals

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Data and Signals

• Two types of data Analog and Digital
• Two types of Signals (transmission techniques)
  Analog and Digital

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Advantages of Digital Transmission

• The signal is exact
• Signals can be checked for errors
• Noise/interference are easily filtered out
• A variety of services can be offered over one
  line
• Higher bandwidth is possible with data compression

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Advantages of Analog Transmission

• Most mediums support analog transmission - used
  for wireless communication
• The telephone infrastructure provides a
  relatively cheap individual point-to-point
  transmission

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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
• Bandwidth

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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
• Sync mechanism based on signal

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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 than data
  rate

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Analog Signals Carrying Analog and Digital Data
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Digital Signals Carrying Analog and Digital Data
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Digital Data - Digital Signal

• It is logical to represent digital data with a
  digital signal
• Digital signal
• Discrete, discontinuous voltage pulses
• Each pulse is a signal element
• Binary data encoded into signal elements
• Signal changes value as the data changes value
  from 0 to 1 and 1 to 0
• Several line encoding schemes are possible. Each
  has pros and cons

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Encoding Schemes

• Nonreturn to Zero-Level (NRZ-L)
• Nonreturn to Zero Inverted (NRZI)
• Bipolar -AMI
• Pseudoternary
• Manchester
• Differential Manchester

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Digital Data - Digital Signal
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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
• Not often used for signal transmission

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Problems With NRZ

• Difficult to determine where one bit ends and the
  next begins
• In NRZ-L, long strings of ones and zeroes would
  appear as constant voltage pulses
• Timing is critical, because any drift results in
  lack of synchronization and incorrect bit values
  being transmitted

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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
• Used by IEEE 802.5

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Digital Data - Digital Signal
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Digital Data, Analog Signal

• Amplitude shift keying (ASK)
• Frequency shift keying (FSK)
• Phase shift keying (PK)

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Modulation Techniques
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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
• Up to 1200bps on voice grade lines
• Used over optical fiber