Chapter 16: Data Communication Fundamentals

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Chapter 16Data Communication Fundamentals

• Business Data Communications, 5e

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Data Communication Components

• 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?Signal 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?Signal 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 attenuation 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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Voice Grade Modems
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Cable Modems

• Permits Internet access over cable television
  networks.
• ISP is at or linked by high-speed line to central
  cable office
• Cables used for television delivery can also be
  used to deliver data between subscriber and
  central location
• Upstream and downstream channels are shared among
  multiple subscribers, time-division multiplexing
  technique (see Chapter 17)
• Splitter is used to direct TV signals to a TV and
  the data channel to a cable modem

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Cable Modem Layout
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Asymmetric DigitalSubscriber Line (ADSL)

• New modem technology for high-speed digital
  transmission over ordinary telephone wire.
• Telephone central office can provide support for
  a number of ISPs,
• At central office, a combined data/voice signal
  is transmitted over a subscriber line
• At subscribers site, twisted pair is split and
  routed to both a PC and a telephone
• At the PC, an ADSL modem demodulates the data
  signal for the PC.
• At the telephone, a microfilter passes the 4-kHz
  voice signal.
• The data and voice signals are combined on the
  twisted pair line using frequency-division-multipl
  exing techniques (Chapter 17)

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DSL Modem Layout
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Digital Encoding of Analog Data

• Evolution of telecommunications networks to
  digital transmission and switching requires voice
  data in digital form
• Best-known technique for voice digitization is
  pulse-code modulation (PCM)
• 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.
• Good-quality voice transmission can be achieved
  with a data rate of 8 kbps
• Some videoconference products support data rates
  as low as 64 kbps

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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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Analog Encoding of Analog Information

• Voice-generated sound wave can be represented by
  an electromagnetic signal with the same frequency
  components, and transmitted on a voice-grade
  telephone line.
• Modulation can produce a new analog signal that
  conveys the same information but occupies a
  different frequency band
• A higher frequency may be needed for effective
  transmission
• Analog-to-analog modulation permits
  frequency-division multiplexing (Chapter 17)

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Asynchronous and Synchronous Transmission

• For receiver to sample incoming bits properly, it
  must know arrival time and duration of each bit
  that it receives

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Asynchronous Transmission

• Avoids timing problem by not sending long,
  uninterrupted streams of bits
• Data transmitted one character at a time, where
  each character is 5 to 8 bits in length.
• Timing or synchronization must only be maintained
  within each character the receiver has the
  opportunity to resynchronize at the beginning of
  each new character.
• Simple and cheap but requires an overhead of 2 to
  3 bits per character

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Synchronous Transmission

• Block of bits transmitted in a steady stream
  without start and stop codes.
• Clocks of transmitter and receiver must somehow
  be synchronized
• Provide a separate clock line between transmitter
  and receiver works well over short distances,
• Embed the clocking information in the data
  signal.
• Each block begins with a preamble bit pattern and
  generally ends with a postamble bit pattern
• The data plus preamble, postamble, and control
  information are called a frame

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Error Control Process

• All transmission media have potential for
  introduction of errors
• All data link layer protocols must provide method
  for controlling errors
• Error control process has two components
• Error detection
• Error correction

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Error Detection Parity Bits

• Bit added to each character to make all bits add
  up to an even number (even parity) or odd number
  (odd parity)
• Good for detecting single-bit errors only
• High overhead (one extra bit per 7-bit
  character12.5)

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Error Detection Cyclic Redundancy Check (CRC)

• Data in frame treated as a single binary number,
  divided by a unique prime binary, and remainder
  is attached to frame
• 17-bit divisor leaves 16-bit remainder, 33-bit
  divisor leaves 32-bit remainder
• For a CRC of length N, errors undetected are 2-N
• Overhead is low (1-3)