Data Encoding Reading Assignment : Stallings Chapter 2, pp. 45 - 60, Chapter 4, pp. 96 - 101, 103 - 105, 107 - 111

1
Data Encoding Reading Assignment Stallings
Chapter 2, pp. 45 - 60,Chapter 4, pp. 96 - 101,
103 - 105, 107 - 111

• Terminology
• Data
• entities that convey meaning
• Signals
• electromagnetic representation of data
• Signaling
• the act of propagating the signal along a
  suitable medium
• Transmission
• the communication of data by the propagation and
  processing of signals.
• Encoding
• converting analog or digital data to analog or
  digital signals

2

• Data
• analog
• continuous values on some interval
• examples
• audio / acoustic
• sound waves perceived by human
• 20 Hz - 20 k Hz
• above 700 Hz -- slight intelligibility added
• video
• amount of illumination at each point
• time varying analog signal
• horizontal and vertical retrace
• 483 horizontal lines ? 30 scans / sec
• interlace 241.5 lines ? 60 scans / sec to avoid
  flickering
• digital
• e.g., text or character strings
• ASCII
• 7 bits parity bit

3

• Signals
• analog
• continuously varying electromagnetic wave
• medium twisted pair, coaxial cable, fiber optic
  cable, atmosphere
• examples
• speech spectrum 20 Hz to 20 k Hz
• telephone transmitter 300 to 3400 Hz
• Either high frequencies of speech are cut off, or
• for higher fidelity, convert acoustic signal into
  an electromagnetic signal over 300 - 3400 Hz.
• video signal
• horizontal blanking pulse
• horizontal sync pulse to maintain
  transmitter-receiver synchronization
• vertical blanking pulse
• Digital
• e.g., high voltage for 1, low voltage for 0
• See chapter 4.

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• modem (modulator-demodulator)
• digital ? analog (carrier frequency) ? digital
• codec (coder-decoder)
• analog ? digital ? analog
• methods of transmission
• analog
• amplifier
• digital
• repeater regenerates digital signal
• digital transmission preferred
• cost dropping for integrated circuits (not as
  much for analog equipment)
• Effects of noise, etc. are not cumulative (due to
  the use of repeaters).
• effective use of large capacity of medium
• security and privacy

5

• Transmission impairments
• Attenuation
• The signal strength falls off logarithmically
  with distance for guided medium.
• Attenuation is an increasing function of
  frequency.
• Decibel notation 10 log10 ( P2 / P1 )
• Considerations
• The transmitter generates a signal as strong as
  possible without causing distortion.
• The received signal must be well above noise
  level.
• Delay distortion
• The signal velocity over a guided medium varies
  with frequency.
• critical for digital data
• Intersymbol interference
• Some of the signal components of one bit position
  spill over into other bit positions.

6

• Noise
• unwanted signals inserted between transmitter and
  receiver
• types
• thermal noise
• thermal agitation of electrons in a conductor
• across entire frequency spectrum
• cannot be eliminated
• intermodulation noise
• generated by two or more frequencies sharing the
  same transmission medium
• f1 f2 or f1 - f2
• crosstalk
• electrical coupling between nearby twisted pair
• unwanted signals picked up by microwave antennas
• same order of magnitude as thermal noise
• impulse noise
• noncontinuous, irregular pulses/spikes, short
  duration, relatively high amplitude
• electromagnetic disturbances, lightning
• affects digital signals most

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• Data encoding techniques
• analog signaling
• carrier signal
• analog medium usually band limited
• modulation
• the process of encoding source data onto a
  carrier signal with frequency fc .
• input modulating signal (baseband signal)
• transmitted modulated signal
• four possibilities
• analog/digital data ? analog/digital signal
• digital data, digital signals
• terminology
• signal element
• signaling rate / baud rate / modulation rate
• unipolar signaling
• all voltage levels positive or negative
• polar signaling
• voltage levels both positive and negative

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• Facts
• An increase in data rate increases bit error
  rate.
• An increase in S/N ratio decreases bit error
  rate.
• An increase in bandwidth allows an increase in
  data rate.
• factors affecting receiver in interpreting
  digital signals
• timing of each bit (clocking)
• signal level for each bit position
• S/N ratio, data rate, bandwidth
• encoding scheme
• desired properties in encoding scheme
• signal spectrum
• lack of direct-current (dc) component
• lack of high-frequency components, i.e., lower
  bandwidth
• transmitted power concentrated in the middle of
  the transmission bandwidth
• synchronization mechanism for clocking
• error detection capability
• noise immunity

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• Nonreturn to Zero (NRZ) coding techniques
• Nonreturn-to-Zero-Level (NRZ-L)
• 1 constant positive voltage
• 0 constant negative voltage
• NRZI
• nonreturn to zero, invert on ones
• A transition at the beginning of a bit time
  denotes a binary 1 no transition indicates a
  binary 0.
• Limitations
• presence of dc component
• lack of synchronization capability
• used for signal input, digital magnetic recording
• differential encoding
• signal decoded by comparing the polarity of
  adjacent signal elements
• advantages easier to detect a transition
• E.g., if the leads of a twisted pair is inverted,
  NRZI still gives correct results, but NRZL does
  not.

10

• Biphase coding techniques
• at least one transition per bit time
• Manchester code
• 1 low-to-high transition
• 0 high-to-low transition
• used in Ethernet
• Differential Manchester code
• 1 absence of transition at beginning of bit
  period
• 0 presence of transition at beginning
• differential coding used
• used in token ring
• mid-bit transition clocking
• advantages
• synchronization
• no dc component
• error detection absence of an expected
  transition ? error
• Modulation rate (D)
• rate at which signal elements are generated.
• Data rate (R) bits per second

11

• digital data, analog signals
• carrier frequency fc
• phone lines 300 - 3400 Hz
• modulation techniques
• Amplitude-shift keying (ASK)
• binary 1 A cos( 2 ? fc t )
• binary 0 0
• susceptible to sudden gain change
• 1200 bps on voice-grade phone lines
• ASK is used in transmission over optical fiber.
• Frequency-shift keying (FSK)
• binary 1 A cos( 2 ? f1 t )
• binary 0 A cos( 2 ? f2 t )
• f1 , f2 fc ? ?
• used in 1200 bps modems, high-frequency radio
  transmission
• Phase-shift keying (PSK)
• data represented by shifting phase of carrier
  signal
• A cos( 2 ? fc t s )

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• Two-phase differential PSK
• binary 1 A cos( 2 ? fc t ? )
• binary 0 A cos( 2 ? fc t )
• Quadrature phase-shift keying (QPSK)
• binary 11 A cos( 2 ? fc t ?/4 )
• binary 10 A cos( 2 ? fc t 3?/4 )
• binary 00 A cos( 2 ? fc t 5?/4 )
• binary 01 A cos( 2 ? fc t 7?/4 )
• Combining PSK and ASK
• 9600 bps modem, 12 phase angles, 2 amplitude
  values, on 2400 bps baud line