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Comparison between different Encoding Schemes

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Comparison between different Encoding Schemes Powered By Docstoc
					        Comparison between different Encoding Schemes

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

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

Nonreturn to Zero Inverted (NRZ-I)

   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

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
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

Bi phase

     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


Bi phase Pros and Cons

  Cons
                  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
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

Pseudo ternary

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

Bipolar-AMI and Pseudo ternary




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
   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



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

				
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