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Networks - NRZ - POLAR - BIPOLAR - MANCHESTER Schemes

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Networks - NRZ - POLAR - BIPOLAR - MANCHESTER Schemes Powered By Docstoc
					                        Data Encoding

This project implement by using graphical user interface in
MATLAB.




About project:

This project can be use to
   1- Read any speech signal (*.wav) and you can hear the original
       sound.
   2- The program will plot the speech signal in time domine as shown
       in Fig.1.
                     Fig.1. the original speech signal

3-Find the Fourier transforms of the speech signal and plots the speech
signal in frequency domine as shown in Fig.2.




                  Fig.2. the spectrum of speech signal .

4-Implement the ideal filter at a specific cutoff frequency (you can use
practical filter or a window function) as shown in Fig.3.
In this part you can select the cutoff frequency of the filter.




                        Fig.3. Spectrum of ideal filter
                      Fig.4. Impulse response of ideal filter.

5- Quantize the speech signal using PCM and you can select the number
of bits per sample.
6- Dequantize the speech signal
7-Hear the speech signal after dequantization.




                   Fig.5.The speech signal after dequantization.


8- convert any binary sequence to ( Manchester code, polar NRZ code, unipolar NRZ
code, unipolar RZ code, bipolar RZ code or differential Manchester code).
Fig.6.unipolar NRZ.




 Fig.7. polar NRZ.
Fig.8.unipolar RZ.




 Fig.9.polar RZ.
                            Fig.10.MANCHESTER.




                       Fig.11.Differential MANCHESTER.


9- generate random sequence of ones and zeros and converted to any code
(Manchester code, polar NRZ code, unipolar NRZ code, unipolar RZ code, bipolar
RZ code or differential Manchester code).
10- convert speech signal to binary sequence and then select small frame and
converted to any code ( Manchester code, polar NRZ code, unipolar NRZ code,
unipolar RZ code, bipolar RZ code or differential Manchester code).

11- Find and plot the power spectrum density of any of this code (Manchester code,
polar NRZ code, unipolar NRZ code, unipolar RZ code, bipolar RZ code or
differential Manchester code).




                           Fig.12. PSD of unipolar NRZ




                             Fig.13. PSD of polar NRZ
 Fig.14. PSD of unipolar RZ




  Fig.15. PSD of bipolar RZ




Fig.16. PSD of MANCHESTER
                       Fig.17. PSD of MANCHESTER

Encoding and Modulation




For digital signal, a data source g(t), which may be either digital or
analog, is encoded into a digital signal x(t).
Analog transmission uses a continuous constant-frequency signal known
as the carrier signal. The frequency of the carrier signal is chosen to be
compatible with the transmission medium. Data is transmitted using a
Carrier signal by modulation, which is the process of encoding source
data onto a carrier signal with frequency f c
Why encoding?

Three factors determine successfulness of receiving signal
       S/N
       data rate
       bandwidth

More factor can be used to improve


encoding scheme

With other factors held constant, the following statements are true.
An increase in data rate increases bit error rate
An increase S/N decreases bit error rate
An increase in bandwidth allows an increase in data rate [Stalling, p98]

Encoding evaluation factors

Signal spectrum
Clocking
Error detection
Signal interference& noise immunity
Cost and complexity


Five factors are used to evaluate the various encoding scheme:

     Signal spectrum A lack of high-frequency components means that
      less Bandwidth is required for transmission. No dc component is
      desirable.
     Clocking Suitable encoding provides some synchronization
      mechanism to determine the beginning and end of each bit
      position.
     Error detection Some error detection can be built into the encoding
      scheme.
     Signal interference & noise immunity Some encoding scheme has
      superior performance in the presence of noise
     Cost and complexity Higher signaling rate to achieve a greater data
      rate results expensive devices.

Digital data, Digital signal
     Unipolar
        o All signal elements have same sign
  Polar
     o One logic state represented by positive voltage the
         other by negative voltage
  Data rate
     o Rate of data transmission in bits per second
  Duration or length of a bit
     o Time taken for transmitter to emit the bit
  Modulation rate
     o Rate at which the signal level changes
     o Measured in baud = signal elements per second
  Mark and Space
     o Binary 1 and Binary 0 respectively

Comparison of Encoding Schemes:

  Signal Spectrum
      o Lack of high frequencies reduces required bandwidth
      o Lack of dc component allows ac coupling via
         transformer, providing isolation
      o Concentrate power in the middle of the bandwidth
  Clocking
      o Synchronizing transmitter and receiver
      o External clock
      o Sync mechanism based on signal
  Error detection
      o Can be built in to signal encoding
  Signal interference and noise immunity
      o Some codes are better than others
  Cost and complexity
      o Higher signal rate (& thus data rate) lead to higher
         costs
      o Some codes require signal rate greater than data rate

Encoding Schemes
    Nonreturn to Zero-Level (NRZ-L)
    Nonreturn to Zero Inverted (NRZI)
    Bipolar -AMI
    Pseudoternary
    Manchester
    Differential Manchester
Nonreturn to Zero-Level (NRZ-L

   Two different voltages for 0 and 1 bits
   Voltage constant during bit interval
       o no transition I.e. no return to zero voltage
   e.g. Absence of voltage for zero, constant positive voltage for
    one
   More often, negative voltage for one value and positive for
    the other
   This is NRZ-L

Nonreturn to Zero Inverted

 Nonreturn to zero inverted on ones
 Constant voltage pulse for duration of bit
 Data encoded as presence or absence of signal transition at
  beginning of bit time
 Transition (low to high or high to low) denotes a binary 1
 No transition denotes binary 0
 An example of differential encoding




Differential Encoding

   Data represented by changes rather than levels
   More reliable detection of transition rather than level
   In complex transmission layouts it is easy to lose sense of
    polarity
NRZ pros and cons
   Pros
       o Easy to engineer
       o Make good use of bandwidth
   Cons
       o dc component
       o Lack of synchronization capability
   Used for magnetic recording
   Not often used for signal transmission

Multilevel Binary

   Use more than two levels
   Bipolar-AMI
       o zero represented by no line signal
       o one represented by positive or negative pulse
       o one pulses alternate in polarity
       o No loss of sync if a long string of ones (zeros still a
          problem)
       o No net dc component
       o Lower bandwidth
       o Easy error detection

Pseudoternary

      One represented by absence of line signal
      Zero represented by alternating positive and negative
      No advantage or disadvantage over bipolar-AMI

Bipolar-AMI and Pseudoternary
Trade Off for Multilevel Binary
   Not as efficient as NRZ
   Each signal element only represents one bit
   In a 3 level system could represent log23 = 1.58 bits
   Receiver must distinguish between three levels
    (+A, -A, 0)
   Requires approx. 3dB more signal power for same
    probability of bit error

Biphase

   Manchester
      o Transition in middle of each bit period
      o Transition serves as clock and data
      o Low to high represents one
      o High to low represents zero
      o Used by IEEE 802.3

   Differential Manchester
       o Midbit transition is clocking only
       o Transition at start of a bit period represents zero
       o No transition at start of a bit period represents one
       o Note: this is a differential encoding scheme
       o Used by IEEE 802.5


Biphase Pros and Cons

  Con
             At least one transition per bit time and possibly two
             Maximum modulation rate is twice NRZ
             Requires more bandwidth
  Pros
         o Synchronization on mid bit transition (self clocking)
         o No dc component
         o Error detection
               Absence of expected transition
Scrambling

   Use scrambling to replace sequences that would produce constant
    voltage
   Filling sequence
        o Must produce enough transitions to sync
        o Must be recognized by receiver and replace with original
        o Same length as original
   No dc component
   No long sequences of zero level line signal
   No reduction in data rate
   Error detection capability

B8ZS

      Bipolar With 8 Zeros Substitution
      Based on bipolar-AMI
      If octet of all zeros and last voltage pulse preceding was
       positive encode as 000+-0-+
      If octet of all zeros and last voltage pulse preceding was
       negative encode as 000-+0+-
      Causes two violations of AMI code
      Unlikely to occur as a result of noise
      Receiver detects and interprets as octet of all zeros

HDB3

      High Density Bipolar 3 Zeros
      Based on bipolar-AMI
      String of four zeros replaced with one or two pulses
Definition of Digital Signal Encoding Formats
Nonreturn-to-Zero-Level (NRZ-L)

0 = high level
1 = low level l

Nonreturn to Zero Inverted (NRZI)

0 = no transition at beginning of interval (one bit time)
1 = transition at beginning of interval

Bipolar-AMI

0 = no line signal
1 = positive or negative level, alternating for successive ones

Pseudo ternary

0 = positive or negative level, alternating for successive zeroes
1 = no line signal
Manchester

0 = transition from high to low in middle of interval
1 = transition from high to low in middle of interval

Differential Manchester

Always a transition in middle of interval
0 = no transition at beginning of interval
1 = transition at beginning of interval

B8ZS

Same as bipolar AMI, except that any string of eight zeros is replaced by
a string with two code violations

HDB3

Same as bipolar AMI, except that any string of four zeros is replaced by a
string with one code violation

[Stallings, p99, 100]


                        Scrambling techniques




To maintain synchronization for the receiver’s clock using bipolar.
B8ZS

If an octet of all zeros occur and the last voltage pulse preceding this octet
was positive, the eight zeros of the octet are encode as 00+-0-+
If an octet of all zeros occur and the last voltage pulse preceding this octet
was positive, the eight zeros of the octet are encode as 00-+0+-HD3B
The scheme replace strings of four zeros with the sequence B00V
[Stallings, p106]




Spectral density




NRZ make efficient use of bandwidth. Most of the frequency in NRZ and
NRZI signals are between dc and half the bit rate.
Manchester& Different Manchester has the bulk of the energy between
one-half and one times the bit rate. Thus the bandwidth is reasonably
narrow and contains no dc component.
AMI make use of bandwidth less than the bandwidth of NRZ
[Stallings, p102]


References:
        William Stallings, Data and Computer Communications
        Internet

				
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