Chapter 8: Data Communication Fundamentals

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Chapter 8Data CommunicationFundamentals

• Business Data Communications, 4e

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Three Components of Data Communication

• Data
• Analog Continuous value data (sound, light,
  temperature)
• Digital Discrete value (text, integers, symbols)
• Signal
• Analog Continuously varying electromagnetic wave
• Digital Series of voltage pulses (square wave)
• Transmission
• Analog Works the same for analog or digital
  signals
• Digital Used only with digital signals

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Analog Data--gtSignal Options

• Analog data to analog signal
• Inexpensive, easy conversion (eg telephone)
• Data may be shifted to a different part of the
  available spectrum (multiplexing)
• Used in traditional analog telephony
• Analog data to digital signal
• Requires a codec (encoder/decoder)
• Allows use of digital telephony, voice mail

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Digital Data--gtSignal Options

• Digital data to analog signal
• Requires modem (modulator/demodulator)
• Allows use of PSTN to send data
• Necessary when analog transmission is used
• Digital data to digital signal
• Requires CSU/DSU (channel service unit/data
  service unit)
• Less expensive when large amounts of data are
  involved
• More reliable because no conversion is involved

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Transmission Choices

• Analog transmission
• only transmits analog signals, without regard for
  data content
• attenuation overcome with amplifiers
• signal is not evaluated or regenerated
• Digital transmission
• transmits analog or digital signals
• uses repeaters rather than amplifiers
• switching equipment evaluates and regenerates
  signal

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Data, Signal, and Transmission Matrix
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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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Why Use Analog Transmission?

• Already in place
• Significantly less expensive
• Lower attentuation rates
• Fully sufficient for transmission of voice signals

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Analog Encoding of Digital Data

• Data encoding and decoding technique to represent
  data using the properties of analog waves
• Modulation the conversion of digital signals to
  analog form
• Demodulation the conversion of analog data
  signals back to digital form

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Modem

• An acronym for modulator-demodulator
• Uses a constant-frequency signal known as a
  carrier signal
• Converts a series of binary voltage pulses into
  an analog signal by modulating the carrier signal
• The receiving modem translates the analog signal
  back into digital data

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Methods of Modulation

• Amplitude modulation (AM) or amplitude shift
  keying (ASK)
• Frequency modulation (FM) or frequency shift
  keying (FSK)
• Phase modulation or phase shift keying (PSK)

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Amplitude Shift Keying (ASK)

• In radio transmission, known as amplitude
  modulation (AM)
• The amplitude (or height) of the sine wave varies
  to transmit the ones and zeros
• Major disadvantage is that telephone lines are
  very susceptible to variations in transmission
  quality that can affect amplitude

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ASK Illustration
1
0
0
1
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Frequency Shift Keying (FSK)

• In radio transmission, known as frequency
  modulation (FM)
• Frequency of the carrier wave varies in
  accordance with the signal to be sent
• Signal transmitted at constant amplitude
• More resistant to noise than ASK
• Less attractive because it requires more analog
  bandwidth than ASK

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FSK Illustration
1
1
0
1
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Phase Shift Keying (PSK)

• Also known as phase modulation (PM)
• Frequency and amplitude of the carrier signal are
  kept constant
• The carrier signal is shifted in phase according
  to the input data stream
• Each phase can have a constant value, or value
  can be based on whether or not phase changes
  (differential keying)

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PSK Illustration
0
0
1
1
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Differential Phase Shift Keying (DPSK)
0
0
1
1
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Analog Channel Capacity BPS vs. Baud

• Baud of signal changes per second
• BPSbits per second
• In early modems only, baudBPS
• Each signal change can represent more than one
  bit, through complex modulation of amplitude,
  frequency, and/or phase
• Increases information-carrying capacity of a
  channel without increasing bandwidth
• Increased combinations also leads to increased
  likelihood of errors

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Voice Grade Modems
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Cable Modems
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DSL Modems
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Digital Encoding of Analog Data

• Primarily used in retransmission devices
• The sampling theorem If a signal is sampled at
  regular intervals of time and at a rate higher
  than twice the significant signal frequency, the
  samples contain all the information of the
  original signal.
• 8000 samples/sec sufficient for 4000hz

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Converting Samples to Bits

• Quantizing
• Similar concept to pixelization
• Breaks wave into pieces, assigns a value in a
  particular range
• 8-bit range allows for 256 possible sample levels
• More bits means greater detail, fewer bits means
  less detail

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Codec

• Coder/Decoder
• Converts analog signals into a digital form and
  converts it back to analog signals
• Where do we find codecs?
• Sound cards
• Scanners
• Voice mail
• Video capture/conferencing

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Digital Encodingof Digital Data

• Most common, easiest method is different voltage
  levels for the two binary digits
• Typically, negative1 and positive0
• Known as NRZ-L, or nonreturn-to-zero level,
  because signal never returns to zero, and the
  voltage during a bit transmission is level

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Differential NRZ

• Differential version is NRZI (NRZ, invert on
  ones)
• Change1, no change0
• Advantage of differential encoding is that it is
  more reliable to detect a change in polarity than
  it is to accurately detect a specific level

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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 Alternatives to NRZ

• Require at least one transition per bit time, and
  may even have two
• Modulation rate is greater, so bandwidth
  requirements are higher
• Advantages
• Synchronization due to predictable transitions
• Error detection based on absence of a transition

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Manchester Code

• Transition in the middle of each bit period
• Transition provides clocking and data
• Low-to-high1 , high-to-low0
• Used in Ethernet

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Differential Manchester

• Midbit transition is only for clocking
• Transition at beginning of bit period0
• Transition absent at beginning1
• Has added advantage of differential encoding
• Used in token-ring

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Digital Encoding Illustration
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Digital Interfaces

• The point at which one device connects to another
• Standards define what signals are sent, and how
• Some standards also define physical connector to
  be used

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Generic CommunicationsInterface Illustration
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DTE and DCE
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RS-232C (EIA 232C)

• EIAs Recommended Standard (RS)
• Specifies mechanical, electrical, functional, and
  procedural aspects of the interface
• Used for connections between DTEs and voice-grade
  modems, and many other applications

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EIA-232-D

• new version of RS-232-C adopted in 1987
• improvements in grounding shield, test and
  loop-back signals
• the prevalence of RS-232-C in use made it
  difficult for EIA-232-D to enter into the
  marketplace

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RS-449

• EIA standard improving on capabilities of
  RS-232-C
• provides for 37-pin connection, cable lengths up
  to 200 feet, and data rates up to 2 million bps
• covers functional/procedural portions of R-232-C
• electrical/mechanical specs covered by RS-422
  RS-423

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Functional Specifications

• Specifies the role of the individual circuits
• Data circuits in both directions allow
  full-duplex communication
• Timing signals allow for synchronous transmission
  (although asynchronous transmission is more
  common)

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Procedural Specifications

• Multiple procedures are specified
• Simple example exchange of asynchronous data on
  private line
• Provides means of attachment between computer and
  modem
• Specifies method of transmitting asynchronous
  data between devices
• Specifies method of cooperation for exchange of
  data between devices

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Mechanical Specifications

• 25-pin connector with a specific arrangement of
  leads
• DTE devices usually have male DB25 connectors
  while DCE devices have female
• In practice, fewer than 25 wires are generally
  used in applications

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RS-232 DB-25 Connectors
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RS-232 DB-25 Pinouts
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RS-232 DB-9 Connectors

• Limited RS-232

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RS-422 DIN-8

• Found on Macs

DIN-8 Male
DIN-8 Female
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Electrical Specifications

• Specifies signaling between DTE and DCE
• Uses NRZ-L encoding
• Voltage lt -3V binary 1
• Voltage gt 3V binary 0
• Rated for lt20Kbps and lt15M
• greater distances and rates are theoretically
  possible, but not necessarily wise

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RS-232 Signals (Asynch)
Odd Parity
Even Parity
No Parity