ICSA 411: Week 2b Data Communication

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ICSA 411 Week 2bData Communication

• Elizabeth Lane Lawley, Instructor

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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?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 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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Quadrature Amplitude Modulation (QAM)

• Commonly used method for quadbit transfer
• Combination of 8 different angles in phase
  modulation and two amplitudes of signal
• Provides 16 different signals, each of which can
  represent 4 bits

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Quadrature Amplitude Modulation Illustration
90
135
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amplitude 1
0
180
amplitude 2
225
315
270
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QAM Example CCITT V.22bis Modem

• Uses QAM
• "bis" qualifier is a French term for "duo" or
  "twice"
• Supports transmission of full-duplex 2400 bps
  synchronous or asynchronous data over a switched,
  2-Wire, voice circuit
• Modulation rate is 600 baud, with each baud
  representing four data bits

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Trellis Coded Modulation (TCM)

• Sophisticated mathematics are used to predict the
  best fit between the incoming signal and a large
  set of possible combinations of amplitude and
  phase changes
• Forward Error Correcting (FEC)
• Used in V.32 (9600 bps) and higher speed modems

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CCITT V-Series Modem Recommendations

• V.22 1200 bps duplex modem standardized for use
  in the PSTN and on leased circuits
• V.29 9600 bps modem standardized for use on
  point-to-point 4-wire leased telephone circuits
• V. 32 2-wire, duplex modems operating at data
  rate of up to 9600 bps for use on the PSTN and on
  leased circuits

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V.32 bis Modems

• Uses Trellis coding with QAM
• Allows transport of data up to 14400 bps
• Modulation rate is 2400 baud

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V.34 Modems

• Capable transmission up to 28.8 kbps
• Modulation rate (baud rate) and carrier frequency
  can vary
• Multi-dimensional Trellis-coding is employed

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V.34 Modems

• Data rate up to 33.6 kbps over dial-up circuits
• Can achieve the above data rate only over
  extremely clean lines (see class handout from
  dbTechnology site)
• Use a range of adaptive techniques that enable a
  modem to learn and adjust to line conditions.

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56kbps Modems

• Asymmetrical can download at 56kbps but upload
  at 33.6kbps only
• Requires digital T-1 or ISDN PRI connection at
  central site or ISP, single hop between sender
  and receiver
• Two incompatible systems, no official standard
• U.S. Robotics (56K x2)
• Rockwell (56K flex)

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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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Telecommunications Standards

• Where do they come from?
• Standard setting bodies
• Governments
• Two types
• Market-driven and voluntary
• Government-regulated and mandatory

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Advantages

• Assures a large market, which encourages mass
  production and often lowers costs
• Encourages vendors to enter market because
  investment is protected
• Allows products from multiple vendors to
  communicate, providing consumers with wider
  selection

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Disadvantages

• Standards process can freeze technology too
  early, due to the length of the standards-setting
  process and the speed with which technology
  changes
• Current process allows for multiple standards for
  the same thing

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Institute of Electrical and Electronics Engineers
(IEEE)

• The largest professional society in the world
• Develops standards in the area of electrical
  engineering and computing
• Publishes scores of journals and runs numerous
  conferences each year
• e.g. IEEE 802.x network standards

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American National Standards Institute (ANSI)

• Non-governmental and nonprofit organization
• Members are U.S. manufacturers and other interest
  groups
• Sets a variety of a standards, not just
  computer-related
• ANSI proposals are usually approved by ISO as
  international standards
• e.g. 802.x, created by IEEE, approved by ANSI,
  passed on and approved by ISO

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National Institute of Standards and Technology
(NIST)

• Formerly known as the National Bureau of
  Standards (NBS)
• Agency of the U.S. Dept.. of Commerce
• Issues standards that are mandatory for purchases
  made by the U.S. Government except the Department
  of Defense

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Industry Associations

• Electronic Industries Association (EIA)
• Telecommunication Industry Association (TIA)
• e.g. EIA-232 (formerly RS-232-C)

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Intl Telecommunications Union (ITU)

• Formerly known as Consultative Committee on
  International Telegraph and Telephone (CCITT)
• Standardize techniques and operations in the
  telecommunications field
• e.g.
• CCITT Group 3 Fax
• CCITT V.x modem standards

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ISO (International Standards Organization)

• Founded in 1946
• Issues standards on a vast number of subjects,
  ranging from nuts and bolts to telephone pole
  coatings
• Has almost 200 Technical Committees
• A member of ITU-T

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Internet Engineering Task Force (IETF)

• Part of the Internet Architecture Board (IAB)
• IETF proposes and published Internet RFCs
• IAB determines which RFCs become standards, based
  on IETF recommendations

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RFC? Internet Standard

• Stable and well-understood
• Technically competent
• Numerous independent and interoperable
  implementations in operation
• Significant public support
• Recognizably useful
• Differs from other standards processes because of
  the emphasis on operational experience