Introduction to Data Communications

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Introduction to Data Communications
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Quantization

• The process of approximating analog signal with a
  digital signal
• The modulus increases with the number of bits. N
  is the number of bits.
• Digital approximation increases in fidelity as
  the number of allowable digital states (modulus)
  increases.

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Modulus 8 Quantizing
Record
Eight States
Digital Playback
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Modulus 16 Quantizing
Record
Digital Playback
Sixteen States
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Time Division Multiplexing

• Serializes digital data.
• A bunch of bits is transmitted one bit at a time
• The LSB is sent first
• The MSB is sent last.
• D7D6D5D4D3D2D1D0
• Bit time is the duration of a bit cell (Tb)

Number of bits sent in one bit cell. (Could be
more than one!)
bps bits per second
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Single Level (m1) Modulation of a
CarrierModulus 2 Transmission

• Carrier is modulated by only one 1 data stream
• There is only 1 bit sent per bit cell.
• Modulus 2 so the carrier must have two
  modulation states.

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Two Level Modulation of a CarrierModulus 4
Transmission
Tb
Level 1
Level 2

• Carrier is modulated by two data streams
• 2 bits are sent per bit cell.
• Modulus 4 so the carrier must have four
  modulation states.

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m-level (Multilevel) Modulation

• Modulation can accommodate N data levels
• There must be 2m distinct modulation states
• Note the two modulation states for single level
  modulation
• Note the four modulation states for two level
  modulation.

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BAUD rate

• The number of carrier transitions per second.
• Where m the modulation level
• If BAUD 1000 then
• For single level modulation bps 1000
• For 2-level modulation bps 2000
• For 4-level modulation bps 4000

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Shannon-Hartley Theorem

• where
• C is the channel capacity in bits per second
• B is the bandwidth of the channel in hertz
• S is the total signal power over the bandwidth
  and
• N is the total noise power over the bandwidth.
• S/N is the signal-to-noise ratio expressed as a
  straight power ratio (not as decibels).

So what is log2? Try this
Who knew?