2.7.1.2 Block Coding

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2.7.1.2 Block Coding

• Block coding changes m bits into a block of n
  bits (n gt m)
• Referred as mB/nB encoding technique
• Block coding normally involves 3 steps
• Division a sequence of bits is divided into
  groups of m bits (ex in 4B/5B encoding, the
  original bit sequence is divided into 4-bit
  groups).
• Substitution substitute an m-bit group for an
  n-bit group (ex in 4B/5B encoding, a 4-bit code
  is substituted for a 5-bit group.
• Combination n-bit groups are combined together
  to form a stream.
• The new stream has more bits than the original
  bits

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2.7.1.2 Block Coding

• 4B/5B
• was designed to be used in combination with NRZ-I
  
• to overcome a long 0s synchronization problem,
  4B/5B technique is used prior to encoding with
  NRZ-I to prevent a long stream of 0s.
• In 4B/5B, the 5-bit output that replaces the
  4-bit input has no more than 1 leading (zero and
  2 trailing zeros).
• when different groups are combined to make a new
  sequence, there are never more than 3 consecutive
  0s.

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2.7.1.2 Block Coding
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2.7.1.3 Scrambling

• modified part of bipolar AMI technique
• Narrow bandwidth and does not create a DC
  component problem.
• However long 0s upset the synchronization and not
  suitable for long distance.
• Looking for a solution a technique that does
  not increase the number of bits and providing
  synchronization.

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2.7.1.3 Scrambling

• B8ZS
• Commonly used in North America
• Updated version of AMI with synchronization
• Substitutes eight consecutive zeros with 000VB0VB
  
• V denotes violation, B denotes bipolar
• Letter V (violation) same polarity as the
  polarity of the previous nonzero pulse.
• Letter B (bipolar) polarity opposite to the
  polarity of the previous nonzero pulse.

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2.7.1.3 Scrambling

• HDB3
• High-density bipolar 3-zero
• Commonly used outside of North America
• HDB3 substitutes four consecutive zeros with 000V
  or B00V depending on the number of nonzero pulses
  after the last substitution
• If the number of the nonzero pulses after the
  last substitution is odd, the substitution
  pattern I 000V, which makes the number of nonzero
  pulses even.
• If the number of the nonzero pulses after the
  last substitution is even, the substitution
  pattern is B00V, which make the total number of
  nonzero pulses even

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2.7.2 Analog-To-Digital Conversion

• Analog information (e.g., voice) ? digital signal
  (e.g., 10001011)
• Two techniques
• Pulse Code Modulation (PCM)
• Delta Modulation

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2.7.2.1 Pulse Code Modulation (PCM)

• Three processes
• The analog signal is sampled
• The sampled signal is quantized
• The quantized values are encoded as streams of
  bits

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2.7.2.1 Pulse Code Modulation (PCM)

• Sampling (PAM Pulse Amplitude Modulation)
• According to the Nyquist theorem, the sampling
  rate must be at least 2 times the highest
  frequency contained in the signal.

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2.7.2.1 Pulse Code Modulation (PCM)

• Recovery of a sampled sine wave for different
  sampling rates

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2.7.2.1 Pulse Code Modulation (PCM)

• Ex telephone companies digitize voice by
  assuming a maximum frequency of 4000 Hz. The
  sampling rate therefore is 8000 samples per
  second.
• Ex a complex low-pass signal as a bandwidth of
  200 kHz. What is the minimum sampling rate for
  this signal ?
• Ex a complex bandpass signal has a bandwidth
  has a bandwidth of 200 kHz. What is the minimum
  sampling rate for this signal ?

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2.7.2.1 Pulse Code Modulation (PCM)

• Quantization
• The result of sampling is a series of pulses with
  amplitude values between the maximum and minimum
  amplitude of the signal.
• The set of amplitudes can be infinite with
  nonintegral values between the two limits and the
  values cannot be used in the encoding process.
• Steps in Quantization
• Assume that the original analog signal has
  instantaneous amplitudes between Vmin and Vmax.
• The range is divided into L zones, each of height
  ? (delta).
• Assign quantized values of 0 to L-1 to the
  midpoint of each zone.
• Approximate the value of the sample amplitude to
  the quantized values

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2.7.2.1 Pulse Code Modulation (PCM)

• Quantization (Examples)
• Assume a sampled signal and the sample amplitudes
  are between -20 and 20V. We decide to have eight
  levels (L 8). This means that ? 5V.

Normalized PAM values actual amplitude/?
Normalized quantized values mid value of the
zone
Normalized error different between normalized
values and normalized quantized values
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2.7.2.1 Pulse Code Modulation (PCM)

• Quantization Levels
• Depends on the range of the amplitudes of the
  analog signal and how accurately we want to
  recover the signal.
• Quantization error
• If the input value is at the middle of the zone
  no quantization error
• Otherwise there is an error
• The value of the error for any sample is less
  than ?/2.
• Range of error -?/2 error ?/2
• Contribution of the quantization error to the
  SNRdB of the signal depends on the number of
  quantization levels L, or the bits per sample nb
  as follow

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2.7.2.1 Pulse Code Modulation (PCM)

• Ex what is the SNRdB in the previous example ?
• Ex A telephone subscriber line must have an
  SNRdB above 40. What is the minimum number of
  bits per sample ?

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2.7.2.1 Pulse Code Modulation (PCM)

• Encoding
• After each sample is quantized and the number of
  bits per sample is decided, each sample is
  changed to an nb-bit codeword.
• If the number of quantization levels is L, the
  number of bits is nb log2L.
• The bit rate can be found as
• Ex we want to digitize the human voice. What is
  the bit rate, assuming 8 bits per sample ?

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2.7.2.1 Pulse Code Modulation (PCM)

• Original Signal Recovery
• The recovery of the original signal requires the
  PCM decoder.
• The decoder first uses circuitry to convert the
  code words into a pulse that holds the amplitude
  until the next pulse.
• Then the complete staircase signal is passed
  through a low pass filter to smooth the staircase
  signal into an analog signal.
• The filter has the same cutoff frequency as the
  original signal at the sender.
• If the signal has been sampled at (or greater
  than) the Nyquist sampling rate and enough
  quantization levels, the original signal will be
  recreated.

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2.7.2.1 Pulse Code Modulation (PCM)

• PCM bandwidth
• Supposed we are given an analog signal with a
  given bandwidth, then digitized the signal, what
  is the new minimum bandwidth of the channel that
  can pass this digital signal ?
• Previously, the minimum bandwidth of a
  line-encoded signal is
• Substitute the value of N (data rate) fs x nb
• When 1/r 1 (for a NRZ or bipolar signal) and c
  ½ (average situation), the minimum bandwidth is

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2.7.2.1 Pulse Code Modulation (PCM)

• Maximum data rate of a channel
• Minimum required bandwidth

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2.7.2.2 Delta Modulation

• Developed to reduce the complexity of PCM.
• PCM finds the value of the signal amplitude for
  each sample Delta Modulation finds the changes
  from the previous sample
• No code words bits are sent one after another.

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2.7.2.2 Delta Modulation

• Modulator
• Modulator is used at the sender to create a
  stream of bits from an analog signal.
• The process records the small positive or
  negative changes, called delta d.
• If the dis positive, the process records 1. If
  the dis negative, the process records 0.
• The modulator builds a second signal that
  resembles a staircase.
• The modulator at each sampling intervals,
  compares the input analog signal with the last
  value of the staircase.
• If the amplitude of the analog signal is larger,
  the next bit in the digital data is 1 otherwise
  0.

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2.7.2.2 Delta Modulation

• Modulator
• The output of the comparator also makes the
  staircase itself if the next bit is 1, the
  staircase maker moves the last point of the
  staircase signal dup, otherwise moves the ddown.
• A delay unit is used to hold the staircase
  function for a period between two comparisons.

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2.7.2.2 Delta Modulation

• Demodulator
• The demodulator takes the digital data, and using
  the staircase maker and the delay unit, creates
  the analog signal.
• A low-pass filter is used to smoothen the signal.

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2.7.3 Transmission Modes

• A transmission of binary data across a link can
  be accomplished in either parallel or serial
  mode.
• In parallel mode multiple bits are sent with
  each clock tick.
• In serial mode 1 bit is sent with each clock
  tick.

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2.7.3 Transmission Modes

• A transmission of binary data across a link can
  be accomplished in either parallel or serial
  mode.
• In parallel mode multiple bits are sent with
  each clock tick.
• In serial mode 1 bit is sent with each clock
  tick.

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2.7.3 Transmission Modes

• Parallel Transmission
• Use n wires to send n bits at one time
  synchronously
• Advantage speed
• Disadvantage cost ? Limited to short distances

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2.7.3 Transmission Modes

• Serial Transmission
• On communication channel
• Advantage reduced cost
• Parallel/serial converter is required
• Three ways asynchronous, synchronous, or
  isochronous

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2.7.3 Transmission Modes

• Serial Transmission Asynchronous transmission
• so named because the timing of a signal is
  unimportant.
• In asynchronous transmission, a sender send 1
  start bit (0) at the beginning and 1 or more stop
  bits (1s) at the end of each byte.
• There may be a gap between each byte.

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2.7.3 Transmission Modes

• Serial Transmission Synchronous transmission
• Bit stream is combined into frames
• In synchronous transmission, bits are sent after
  another without start or stop bits or gaps. It is
  the responsibility of the receiver to group the
  bits.
• Advantage speed ? high-speed transmission
• Accuracy of the received information depends on
  the ability of the receiver to keep an accurate
  count of the bits as they come in.

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2.7.3 Transmission Modes

• Serial Transmission Isochronous transmission
• In real time audio and video, in which uneven
  delays between frames are not acceptable,
  synchronous transmission fails.
• Ex TV images are broadcasted at the rate of 30
  images per second and they must be viewed at
  the same rate.
• The isochronous transmission guarantees that the
  data arrive at a fixed rate.
• Isochronous occurring at the same time

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2.8 Data Flow

• A communication between two devices can be
  simplex, half duplex or full duplex.

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2.8 Data Flow

• Simplex
• Unidirectional
• As on a one-way street
• Ex TV broadcasting
• Half-duplex
• Both transmit and receive possible, but not at
  the same time
• Like a one-lane road with two-directional traffic
• Ex Walkie-talkie
• Full-duplex
• Transmit and receive simultaneously
• Like a two-way street, telephone network
• Channel capacity must be divided between two
  directions
• Ex computer network