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					              Data Transmission and
              Computer Networks

     Data   Encoding




1                            Sami Al-Wakeel
         Data Encoding


       Analog and digital data can be encoded
        into either digital or analog signal, creating
        four possible combinations:
           1- Digital Data, Digital Signal.
           2- Analog Data, Digital Signal.
           3- Digital Data, Analog Signal.
           4- Analog Data, Analog Signal.



2                                         Sami Al-Wakeel
       Data Encoding

    1. Digital Data, Digital Signals:
     Binary data are transmitted by encoding
      each data bit into signal element.
     Factors determine how successful the
      receiver will interpret the incoming signal:
      –   An increase in data rate increases bit error
          rate.
      –   An increase in S/N decreases bit error rate.
      –   An increase in bandwidth allows an increase in
          data rate.

3                                         Sami Al-Wakeel
    Data Encoding

        1. Digital Data, Digital Signals (Continued):
                          Digital Signal Encoding



                 Polar                                    Bipolar




     NRZ           RZ       Biphase       AMI              B8ZS             HDB3



    NRZ-L   NRZI         Manchester        Differential
                                           Manchester
4                                                          Sami Al-Wakeel
         Data Encoding


    1. Digital Data, Digital Signals (Continued):




5                                         Sami Al-Wakeel
                Data Encoding


    1. Digital Data, Digital Signals (Continued):




6                                                   Sami Al-Wakeel
         Data Encoding

    1. Digital Data, Digital Signals (Continued):

    Digital signal Encoding Formats:

    I. NonReturn-to-Zero-Level (NRZ-L) Encoding:
     A negative voltage is equated with binary 1 and a positive voltage
        with binary 0.

    II. NonReturn to Zero Inverted (NRZI) Encoding:
     Binary 0 is represented by no transition at the beginning of bit
        interval, and binary 1 is represented by a transition at beginning
        of bit interval.



7                                                   Sami Al-Wakeel
              Data Encoding
1. Digital Data, Digital Signals (Continued):

Advantages of NRZ:
   – The NRZ codes are simple and make efficient use of
     bandwidth.
Disadvantages of NRZ:
   – Lack of synchronization capability. Consider a long
     string of 1’s or 0’s for NRZ-L, or a long string of 0’s for
     NRZI, the output is a constant voltage over a long
     period of time.




8                                               Sami Al-Wakeel
         Data Encoding
    1. Digital Data, Digital Signals (Continued):

    III. Bipolar-AMI Encoding:
     A binary 0 is represented by no line signal, and a binary 1
         is represented by a positive or negative pulse. The binary
         1 pulse must alternate in polarity.

    IV. Pseudoternary Encoding:
     A binary 1 is represented by no line signal, and a binary 0
       by alternating positive or negative pulses.




9                                                   Sami Al-Wakeel
        Data Encoding
     1. Digital Data, Digital Signals (Continued):
      Advantages of Bipolar-AMI or Pseudoternary:
         – No loss of synchronization if long string of binary
            1’s occurs in the case of AMI or 0’s in the case of
            Pseudoternary.
         – The pulse alternation property provides a simple
            means of error detection.
     Disadvantages of Bipolar-AMI or Pseudoternary:
         – Long string of binary 0’s in the case of AMI or 1’s in
            the case of Pseudoternary still present a problem.
         – Multilevel binary signal requires approximately 3 dB
            more signal power than a two-valued signal for the
            same probability of bit error.


10                                              Sami Al-Wakeel
         Data Encoding

     1. Digital Data, Digital Signals
       (Continued):
     V. Manchester Encoding:
      There is a transition at the middle of each bit period.
      The mid-bit transition serves as a clocking mechanism
        and also as data.
      A low-to high transition represents a binary 1, and a
        high-to-low transition represents a binary 0.




11                                             Sami Al-Wakeel
        Data Encoding
     1. Digital Data, Digital Signals (Continued):
     VI. Differential Manchester Encoding:
      There is a transition at the middle of each bit period.
      The mid-bit transition is used only to provide clocking.
      A binary 0 is represented by the presence of a transition at
        the beginning of a bit period, and a binary 1 is represented
        by the absence of a transition at the beginning of a bit
        period




12                                             Sami Al-Wakeel
          Data Encoding
     1. Digital Data, Digital Signals
       (Continued):
     Advantages of Manchester and Differential Manchester Encoding:
        – Synchronization: Because this is a transition at the middle
           of each bit period.
        – Error Detection: The absence of the expected transition
           can be used to detect errors.

     Disadvantages of Manchester and Differential Manchester
        Encoding:
         – High Signaling Rate: At least one transition per bit time is
           needed, and may have at maximum two transitions.
           Therefore, the maximum modulation rate (rate at which
           signal level is changed) is twice that for NRZ; this means
           the required bandwidth is greater.


13                                                   Sami Al-Wakeel
            Data Encoding


      Modulation Rate:Minimum                   101010              Maximum
                                                  …
          Modulation rate is the rate at which signal elements are generated.

                          0 (all 0’s or
     NRZ-L                                      1.0      1.0
                          1’s)
     NRZI                 0 (all 0’s)           0.5      1.0 (al1’s)
     Bipolar-AMI          0 (all 0’s)           1.0      1.0
     Pseudoternary        0 (all 1’s)           1.0      1.0
                          1.0 (101010
     Manchester                                 1.0      2.0 (all 0’s or 1’s)
                          …)

     Differential
                          1.0 (all 1’s)         1.5      2.0 (all 0’s)
     Manchester


14                                                          Sami Al-Wakeel
         Data Encoding

     1. Digital Data, Digital Signals (Continued):
      VII. Bipolar with 8-zeros substitution (B8ZS):
      The coding scheme is based on a bipolar-AMI. The
        encoding is updated with the following rules:
         – If an octet of all zeros occurs and the last voltage
            pulse preceding this octet was positive, then the
            eight zeros of the octet are encoded as 0 0 0 + - 0 - +
            .
         – If an octet of all zeros occurs and the last voltage
            pulse preceding this octet was negative, then the
            eight zeros of the octet are encoded as 0 0 0 - + 0+ -
            .


15                                                Sami Al-Wakeel
 Data Encoding

          1. Digital Data, Digital Signals (Continued):
              VII. Bipolar with 8-zeros substitution (B8ZS):


     Polarity of                                           Polarity of
     previous bit                                          previous bit




     +    0     0      0   0   0      0    0 0         -        0     0      0    0    0   0      0   0



     +    0     0      0 +     -      0        -   +   -        0     0      0     -   + 0 +          -


           Violation               Violation                     Violation              Violation

16                                                                               Sami Al-Wakeel
              Data Encoding



     1. Digital Data, Digital Signals (Continued):

     VIII. High-density Bipolar-3 zeros (HDB3):
      HDB3 is based on the AMI encoding.
      HDB3 replaces strings of 4 zeros with sequences
        containing one or two pulses.
      In each case, the fourth zero is replaced with a code
        violation.
      In addition, successive violations are of alternate
        polarity. Thus, if the last violation was positive, this
        violation must be negative, and vice versa.


17                                                   Sami Al-Wakeel
          Data Encoding

     1. Digital Data, Digital Signals (Continued):

     VIII. HDB3(Continued):
      The following table shows the HDB3 substitution rules:

         Polarity of      Number of Bipolar Pulses
         Preceding           (Ones) Since Last
           Pulse               Substitution
                            Odd           Even
               -             000-              +00+
              +              000+              -00-

18                                                   Sami Al-Wakeel
      Data Encoding

     1. Digital Data, Digital Signals (Continued):

     VIII. HDB3(Continued):

           +   0 0 0 0              -   0 0 0 0

           +   0 0 0 +              -   0 0 0 -


           +   0 0 0 0              -   0 0 0 0

           +   - 0 0 -              -   + 0 0 +
19                                                   Sami Al-Wakeel
     Data Encoding




20                   Sami Al-Wakeel
         Data Encoding

     2. Digital Data, Analog Signals:
      The most familiar of use of this transformation is for
        transmitting digital data through the public telephone
        network.
      The telephone network is designed to transmit, switch,
        and receive analog signals in the voice-frequency range
        of about 300 to 3400 Hz.
      A telephone line will not pass low-frequency signals
        that could occur if the data stream is made up of a
        continuous string of binary 1s or 0s.
      Thus digital devices are attached to the network via a
        modem (Modulator-demodulator) which coverts digital
        data to analog signals, and vice versa.



21                                              Sami Al-Wakeel
           Data Encoding


     2. Digital Data, Analog Signals (Continued):




22                                      Sami Al-Wakeel
     Definitions




23                 Sami Al-Wakeel
        Data Encoding

     2. Digital Data, Analog Signals (Continued):

     Encoding Techniques:
      There are three basic encoding or modulation
       techniques for transforming digital data into analog
       signals:
        – Amplitude-Shift Keying (ASK).
        – Frequency-Shift Keying (FSK).
        – Phase-Shift Keying (PSK).




24                                             Sami Al-Wakeel
                    Data Encoding



     2. Digital Data,
        Analog Signals:




25                                  Sami Al-Wakeel
                Data Encoding


     2. Digital Data, Analog Signals:

        Bit rate is the number of bits per second.
        Baud rate is the number of signal elements per second.
        The baud rate equals the bit rate divided by the number
         of bits represented by each signal element.
        The carrier signal is a high-frequency signal that acts as
         a basis for information signal. The receiving device is
         turned to the frequency of the carrier signal that it
         expects from the sender.




26                                                 Sami Al-Wakeel
            Data Encoding


     2. Digital Data, Analog Signals (Continued):
     Encoding Techniques:

                   Digital/analog Encoding




             ASK            FSK              PSK



                            QAM



27                                                 Sami Al-Wakeel
                    Data Encoding



     2. Digital Data,
        Analog Signals:

     I. Amplitude-Shift Keying:




28                                  Sami Al-Wakeel
           Data Encoding

     2. Digital Data, Analog Signals (Continued):
     I. Amplitude-Shift Keying (ASK):
      We can represent a unipolar periodic signal, vd(t), with unity
         amplitude and fundamental frequency w0 as:
                    1 2          1         1
           vd (t )   {cosw0 t  cos3w0t  cos5w0t  ...}
                    2           3         5
        We can represent the carrier signal as:
                  vc (t )  cos wc t

        ASK can be represented mathematically as:
              v ASK (t )  vc (t ).vd (t )


29                                                   Sami Al-Wakeel
              Data Encoding


             2. Digital Data, Analog Signals (Continued):
             I. Amplitude-Shift Keying (ASK):
              However:
                 1         2                   1                  1
     v ASK (t )  cos wct  {coswc t. cosw0 t  coswc t. cos3w0t  coswc t. cos5w0t  ...}
                 2                            3                  5
              2 cos A. cos B  cos(A  B)  cos(A  B)

                  1          1
      v ASK (t )  cos wc t  {cos(wc  w0 )t cos(wc  w0 )t
                  2          

                  1               1
                  cos(wc 3w0 )t cos(wc 3w0 )t
                  3               3
                  1               1
                  cos(wc 5w0 )t cos(wc 5w0 )t  ...}
                  5               5

30                                                              Sami Al-Wakeel
                  Data Encoding



     2. Digital Data,
        Analog Signals:

     II. Frequency-Shift Keying:




31                                 Sami Al-Wakeel
        Data Encoding

     2. Digital Data, Analog Signals (Continued):
     II. Frequency-Shift Keying (FSK):
     FSK can be represented mathematically as:
          vFSK (t )  vc1 (t ).vd (t )  vc 2 (t ).[1  vd (t )]
           where
          vc1 (t )  cos w1t
          vc 2 (t )  cos w2t

     w1 and w2 are the two carrier frequencies in radians
     per second.



32                                                             Sami Al-Wakeel
        Data Encoding

     2.Digital Data, Analog Signals (Continued):
     II. Frequency-Shift Keying (FSK):
      An example of use of FSK for full-duplex operation
          over the PSTN.
      The PSTN will pass frequencies in the approximate
          range 300 to 3400 Hz.
      To achieve full-duplex, the bandwidth is split at 1700
          Hz.
      In one direction, the frequencies used to represent 1
          and 0 are centered on 1170 Hz. Similarly, for the
          opposite direction, the frequencies used to represent
          1 and 0 are centered on 2125 Hz




33                                             Sami Al-Wakeel
        Data Encoding

     2. Digital Data, Analog Signals:




34                                  Sami Al-Wakeel
        Data Encoding


     2. Digital Data,
        Analog Signals:

     III. Phase-Shift Keying:




35                              Sami Al-Wakeel
       Definitions

     Relationship between different phases:




36                                Sami Al-Wakeel
        Data Encoding

     2. Digital Data, Analog Signals (Continued):
     Multilevel Modulation Methods:
      More efficient use of bandwidth can be achieved
        if each signaling element represents more than
        one bit. For example, instead of a phase shift of
        180o, Quadrature Phase-Shift Keying (QPSK)
        or (4-PSK) technique uses phase shifts of
        multiple of 90o.




37                                         Sami Al-Wakeel
         Data Encoding

     2. Digital Data, Analog Signals (Continued):
     4-PSK:




38                                              Sami Al-Wakeel
              Data Encoding


                Tribit   Phase
     8-PSK:      000       0
                 001      45
                 010      90
                 011      135
                 100      180
                 101      225
                 110      270
                 111      315


39                               Sami Al-Wakeel
         Data Encoding

     2. Digital Data, Analog Signals (Continued):
     Quadrature Amplitude Modulation (QAM).
     Higher bit rates are achieved using 8 and even 16 phase
        changes. In practice, however, there is a limit to how
        many phases can be used.
      Hence to increase the bit rate further, it is more
        common to introduce amplitude as well as phase
        variations of each vector. This type of modulation is
        then known as Quadrature Amplitude Modulation
        (QAM).
      16-QAM has 16 levels per signal element, and hence 4-
        bit symbols.




40                                              Sami Al-Wakeel
        Data Encoding

     2. Digital Data, Analog Signals
        (Continued):
     4-QAM (1 amplitude, 4 phases):




41                                     Sami Al-Wakeel
           Data Encoding


     2. Digital Data, Analog Signals (Continued):
     8-QAM (2 amplitudes, 4 phases):




42                                 Sami Al-Wakeel
         Data Encoding

     2. Digital Data, Analog Signals
        (Continued):
     16-QAM ( 4 amplitudes, 8 phases):




43                                       Sami Al-Wakeel
         Data Encoding
     2. Digital Data, Analog Signals (MODEMS):
      A modem converts the digital signal generated by the
         computer into an analog signal to be carried by a public
         phone line. It is also converts the analog signals
         receiver over a phone line into digital signals usable by
         the computer.
      The term modem is composite word that refers to a
         signal modulator and a signal demodulator.
      A modulator treats a digital signal as a series of 1s and
         0s, and so can transform it into an analog signal by
         using the digital-to-analog mechanisms of ASK, FSK,
         PSK, and QAM.




44                                               Sami Al-Wakeel
        Data Encoding

     2. Digital Data, Analog Signals
        (MODEMS):




45                                     Sami Al-Wakeel
              Data Encoding


     2. Digital Data, Analog Signals (MODEMS):
     Telephone Line Bandwidth:




46                                               Sami Al-Wakeel
                 Data Encoding


     Modem Speeds: Theoretical Bit Rates for Modems:

             Encoding         Half-Duplex      Full-Duplex
       ASK , FSK , 2-PSK         2400              1200
       4-PSK , 4 QAM             4800              2400
       8-PSK , 8-QAM             7200              3600
       16-QAM                    9600              4800
       32-QAM                    12000             6000
       64-QAM                    14400             7200
       128-QAM                   16800             8400
       256-QAM                   19200             9600


47                                           Sami Al-Wakeel
              Data Encoding


     3. Analog Data, Digital Signals:

        A process of converting analog data into digital data,
         which process is known as digitization.
        The device used for converting analog data into digital
         form for transmission, and subsequently recovering the
         original data from the digital is known as a codec
         (coder-decoder)




48                                             Sami Al-Wakeel
               Data Encoding


     3. Analog Data, Digital Signals (Continued):

        Voice transmissions are limited to a maximum bandwidth of
         less than 4 KHz.
        To Convert such signals into digital form, the Nyquest
         sampling theorem states:
              If a signal f(t) is sampled at regular intervals of time and
               at the rate higher than twice the highest frequency
               component, then the samples contain all the information
               of the original signal. The function f(t) may be
               reconstructed from these samples.




49                                                     Sami Al-Wakeel
           Data Encoding

     3. Analog Data, Digital Signals (Continued):

        Hence to convert a 4 KHz voice signal into digital form, it must be
         sampled at rate of 8000 times per second.
        The sampled signal is first converted into a pulse stream, the
         amplitude of each pulse being equal to the amplitude of the original
         analog signal at the sampling instant. The resulting signal is known
         as a pulse amplitude modulated (PAM) signal.
        The PAM signal is still analog since its amplitude can vary over the
         full amplitude range.




50                                                        Sami Al-Wakeel
          Data Encoding

     3. Analog Data, Digital Signals (Continued):

        It is converted into an all-digital form by quantizing each
         pulse into its equivalent binary form.
        If eight bits are used to quantize each PAM signal, then
         256 distinct levels are used.
        The resulting digital signal is known as a pulse code
         modulated (PCM) signal and has a bit rate of 64 kbps –
         8000 sample per second each of 8 bits.




51                                                  Sami Al-Wakeel
                 Data Encoding


     3. Analog Data,
        Digital Signals:




52                               Sami Al-Wakeel
        Data Encoding

     3. Analog Data, Digital Signals:




53                                  Sami Al-Wakeel

				
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