ICSA 411: Week 2 Data Communication

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ICSA 411 Week 2Data Communication

• Elizabeth Lane Lawley, Instructor

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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-govt 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)
• an 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)

• a member of ITU-T
• 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

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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

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Data Communication Basics

• Analog or Digital
• Three Components
• Data
• Signal
• Transmission

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Analog Data Choices
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Digital Data Choices
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Transmission Choices

• Analog transmission
• only transmits analog signals, without regard for
  data content
• attenuation overcome with amplifiers
• Digital transmission
• transmits analog or digital signals
• uses repeaters rather than amplifiers

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Data, Signals, and Transmission
A
Data
D
D
Transmission System
A
D
A
Signal
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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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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
• telephone lines are very susceptible to
  variations in transmission quality that 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)
• the frequency of the carrier wave varies in
  accordance with the signal to be sent
• signal is transmitted at constant amplitude
• more immune to noise than ASK
• 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 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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Complex Modulations

• Combining modulation techniques allows us to
  transmit multiple bit values per signal change
  (baud)
• Increases information-carrying capacity of a
  channel without increasing bandwidth
• Increased combinations also leads to increased
  likelihood of errors
• Typically, amplitude and phase modulation are
  combined

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Quadrature Amplitude Modulation (QAM)

• the most common 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
45
amplitude 1
0
180
amplitude 2
225
315
270
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Quadrature Amplitude Modulation Uses

• CCITT V.22 bis modem
• the "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
• the 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
• a Forward Error Correcting (FEC)
• used in the V.32 modem (9600 bps) and all the
  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

• allows transport of asynchronous or synchronous
  data up to 14400 bps
• the modulation rate is 2400 baud
• uses the Trellis coding with QAM
• uses groupings of seven bits
• only six of these bits contain actual user data,
  the remaining bit is the convolutional coded,
  redundant bit generated from the previous bits

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V.34 Modems

• capable of supporting full-duplex synchronous or
  asynchronous data over 4-Wire leased lines or
  2-Wire circuits up to 28.8 kbps
• the 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
• 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
• no official standard yet
• two incompatible systems
• U.S. Robotics (56K x2)
• Rockwell (56K flex)

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Digital Encoding of Analog Data

• Primarily used in retransmission devices
• Uses 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.
• 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
• e.g., hi-fi music, television pictures, the
  output of copying machine, videoconferencing

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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 Schemes
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Asynchronous Synchronous Transmission

• Concerned with timing issues
• How does the receiver know when the bit period
  begins and ends?
• Small timing difference become more significant
  over time if no synchronization takes place
  between sender and receiver

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Asynchronous Transmission

• Data transmitted 1 character at a time
• Character format is 1 start 1 stop bit, plus
  data of 5-8 bits
• Character may include parity bit

• Timing needed only within each character
• Resynchronization each start bit
• Uses simple, cheap technology
• Wastes 20-30 of bandwidth

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Synchronous Transmission

• Large blocks of bits transmitted without
  start/stop codes
• Synchronized by clock signal or clocking data
• Data framed by preamble and postamble bit patterns

• More efficient than asynchronous
• Overhead typically below 5
• Used at higher speeds than asynchronous
• Requires error checking, usually provided by HDLC