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ICSA 411: Week 2b Data Communication

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Trellis Coded Modulation (TCM) ... Uses Trellis coding with QAM. Allows transport of data up to 14400 bps ... Multi-dimensional Trellis-coding is employed ... – PowerPoint PPT presentation

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Title: ICSA 411: Week 2b Data Communication


1
ICSA 411 Week 2bData Communication
  • Elizabeth Lane Lawley, Instructor

2
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

3
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

4
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

5
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

6
Data, Signal, and Transmission Matrix
7
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

8
Why Use Analog Transmission?
  • Already in place
  • Significantly less expensive
  • Lower attentuation rates
  • Fully sufficient for transmission of voice signals

9
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

10
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

11
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)

12
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

13
ASK Illustration
1
0
0
1
14
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

15
FSK Illustration
1
1
0
1
16
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)

17
PSK Illustration
0
0
1
1
18
Differential Phase Shift Keying (DPSK)
0
0
1
1
19
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

20
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

21
Quadrature Amplitude Modulation Illustration
90
135
45
amplitude 1
0
180
amplitude 2
225
315
270
22
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

23
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

24
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

25
V.32 bis Modems
  • Uses Trellis coding with QAM
  • Allows transport of data up to 14400 bps
  • Modulation rate is 2400 baud

26
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

27
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.

28
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)

29
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

30
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

31
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

32
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

33
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

34
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

35
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

36
Manchester Code
  • Transition in the middle of each bit period
  • Transition provides clocking and data
  • Low-to-high1 , high-to-low0
  • Used in Ethernet

37
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

38
Digital Encoding Illustration
39
Telecommunications Standards
  • Where do they come from?
  • Standard setting bodies
  • Governments
  • Two types
  • Market-driven and voluntary
  • Government-regulated and mandatory

40
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

41
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

42
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

43
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

44
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

45
Industry Associations
  • Electronic Industries Association (EIA)
  • Telecommunication Industry Association (TIA)
  • e.g. EIA-232 (formerly RS-232-C)

46
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

47
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

48
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

49
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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