Data and Computer Communications

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					 Data and Computer
  Communications
Chapter 3 – Data Transmission


           Ninth Edition
        by William Stallings
       Data Transmission

What we've got here is failure to communicate.


                Paul Newman in Cool Hand
Luke
         Data Transmission

                  •   quality of the signal
                      being transmitted



The successful
transmission of   •   characteristics of the
data depends on       transmission medium
two factors:
  Transmission Terminology
Data transmission occurs between transmitter
and receiver over some transmission medium.


Communication                        Unguided
is in the form of   Guided
                                       media
electromagnetic     media
                                     (wireless)
     waves.
                     twisted pair,
                                       air, vacuum,
                    coaxial cable,
                                        seawater
                     optical fiber
Transmission Terminology
  Transmission Terminology
 Simplex

     signals transmitted in one direction
       • eg. Television


 Half   duplex
     both stations transmit, but only one at a time
       • eg. police radio


 Full   duplex
     simultaneous transmissions
       • eg. telephone
Frequency, Spectrum and
      Bandwidth
          Time Domain Concepts

  analog signal
    • signal intensity varies smoothly with no breaks
  digital signal
    • signal intensity maintains a constant level and
      then abruptly changes to another level
  periodic signal
    • signal pattern repeats over time
  aperiodic signal
    • pattern not repeated over time
Analog and Digital Signals
Periodic
Signals
                       Sine Wave
                 (periodic continuous signal)
   peak amplitude (A)
       maximum strength of signal
       typically measured in volts
   frequency (f)
       rate at which the signal repeats
       Hertz (Hz) or cycles per second
       period (T) is the amount of time for one repetition
       T = 1/f
   phase ()
       relative position in time within a single period of signal
Varying Sine Waves
 s(t) = A sin(2ft +)
            Wavelength ()
   the wavelength of
     a signal is the
   distance occupied
    by a single cycle


can also be stated as the   especially when v=c
  distance between two
 points of corresponding    • c = 3*108 ms-1
phase of two consecutive      (speed of light in
          cycles              free space)


     assuming signal               or
   velocity v, then the
  wavelength is related       equivalently
 to the period as  = vT         f = v
Frequency Domain Concepts
 signals are made up of many frequencies
 components are sine waves
 Fourier analysis can show that any signal
  is made up of components at various
  frequencies, in which each component is a
  sinusoid
 can plot frequency domain functions
 Addition of
 Frequency
Components
   (T=1/f)

c is sum of f & 3f
  Frequency
    Domain
Representations

 frequency domain
  function of Fig 3.4c
 frequency domain
  function of single
  square pulse
Spectrum & Bandwidth
Signal with dc Component
    Data Rate and Bandwidth
      any transmission              this limits the data
        system has a                  rate that can be
       limited band of                 carried on the
         frequencies              transmission medium




                                             square waves
   limiting
                     most energy in           have infinite
 bandwidth
                        first few           components and
   creates
                      components            hence an infinite
 distortions
                                               bandwidth



There is a direct relationship between
      data rate and bandwidth.
      Analog and Digital Data
          Transmission
 data
     entities that convey information
 signals
     electric or electromagnetic representations of
      data
   signaling
     physically propagates along a medium
 transmission
     communication of data by propagation and
      processing of signals
Acoustic Spectrum (Analog)
Analog and Digital
  Transmission
      Digital Data

        Examples:




IRA                  Text



         Character
          strings
Advantages & Disadvantages
     of Digital Signals
             Audio Signals
 frequency range of typical speech is 100Hz-7kHz
 easily converted into electromagnetic signals
 varying volume converted to varying voltage
 can limit frequency range for voice channel to
  300-3400Hz
           to produce a video signal a TV
            camera is used
           USA standard is 483 lines per
 Video      frame, at a rate of 30 complete
Signals     frames per second
                 actual standard is 525 lines but 42
                  lost during vertical retrace
           horizontal scanning frequency is
            525 lines x 30 scans = 15750 lines
            per second
           max frequency if line alternates
            black and white
           max frequency of 4.2MHz
Conversion of PC Input to
     Digital Signal
Analog Signals
Digital Signals
 Analog and
   Digital
Transmission
  Transmission Impairments
       received may differ from signal
 signal
 transmitted causing:
     analog - degradation of signal quality
     digital - bit errors
 most   significant impairments are
     attenuation and attenuation distortion
     delay distortion
     noise
                Equalize                 Received signal
          attenuation across             strength must be:
              the band of                •strong enough to be
           frequencies used               detected
            by using loading             •sufficiently higher than
                                          noise to be received
          coils or amplifiers.            without error




                            Strength can be
                            increased using
                              amplifiers or
                               repeaters.




           ATTENUATION
 signal strength falls off with distance over any
transmission medium
 varies with frequency
Attenuation Distortion
           Delay Distortion
 occurs  because propagation velocity of a
  signal through a guided medium varies
  with frequency
 various frequency components arrive at
  different times resulting in phase shifts
  between the frequencies
 particularly critical for digital data since
  parts of one bit spill over into others
  causing intersymbol interference
Noise
unwanted signals
inserted between
transmitter and
receiver

   is the major limiting
   factor in
   communications
   system performance
Categories of Noise




               Intermodulation noise

         • produced by nonlinearities in the
           transmitter, receiver, and/or
           intervening transmission medium
         • effect is to produce signals at a
           frequency that is the sum or
           difference of the two original
           frequencies
           Categories of Noise
                                      Crosstalk:
                                           a signal from one line is
                                            picked up by another
                                           can occur by electrical
                                            coupling between nearby
                                            twisted pairs or when
Impulse Noise:                              microwave antennas pick
                                            up unwanted signals
     caused by external
      electromagnetic interferences
     noncontinuous, consisting of
      irregular pulses or spikes
     short duration and high
      amplitude
     minor annoyance for analog
      signals but a major source of
      error in digital data
Channel Capacity
             Nyquist Bandwidth
In the case of a channel that is noise free:
 if rate of signal transmission is 2B then can carry
  signal with frequencies no greater than B
       given bandwidth B, highest signal rate is 2B
 for binary signals, 2B bps needs bandwidth B Hz
 can increase rate by using M signal levels
 Nyquist Formula is: C = 2B log2M
 data rate can be increased by increasing signals
       however this increases burden on receiver
       noise & other impairments limit the value of M
    Shannon Capacity Formula
   considering the relation of data rate, noise and
    error rate:
       faster data rate shortens each bit so bursts of noise
        corrupts more bits
       given noise level, higher rates mean higher errors
 Shannon developed formula relating these to
  signal to noise ratio (in decibels)
 SNRdb=10 log10 (signal/noise)
 capacity C = B log2(1+SNR)
       theoretical maximum capacity
       get much lower rates in practice
                  Summary
 transmission    concepts and terminology
          guided/unguided media
 frequency, spectrum and bandwidth
 analog vs. digital signals
 data rate and bandwidth relationship
 transmission impairments
          attenuation/delay distortion/noise
 channel   capacity
          Nyquist/Shannon
hannon developed formula relating these to
  signal to noise ratio (in decibels)
 SNRdb=10 log10 (signal/noise)
 capacity C = B log2(1+SNR)
       theoretical maximum capacity
       get much lower rates in practice
                  Summary
 transmission    concepts and terminology
          guided/unguided media
 frequency, spectrum and bandwidth
 analog vs. digital signals
 data rate and bandwidth relationship
 transmission impairments
          attenuation/delay distortion/noise
 channel   capacity
          Nyquist/Shannon
enuation/delay distortion/noise
 channel   capacity
          Nyquist/Shannon

				
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