Data Transmission

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Data Transmission

• EECE 542 Fall 2003

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Time/Frequency Relationships

• The relationship between time and frequency
  domain representation of signals is defined by
  Fourier analysis.
• Unmodulated (non-sinusoidal) signals have their
  frequency domain spectra centered about 0 Hz.
  (i.e. baseband transmission)
• General rule
• A faster (shorter period) signal in the time
  domain results in a wider (larger bandwidth)
  signal in the frequency domain

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Ex A random sequence of 0s and 1s
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Baseband Data Transmission

• Most physical layer transmission systems rely on
  baseband transmission.
• Almost exclusively use a type of cable or fiber
• Supports only one current transmission
• No parallel transmissions on the same wire unless
  multiple wires are used (both tx and rx)
• Exception some fiber optic systems
• Transmission involves a mapping of binary data to
  analog waveforms.

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Baseband Data Reception

• Line components typically block the transmission
  in the vicinity of 0 Hz (DC).
• The received signal is first filtered and
  amplified to reduce the effects of noise and line
  attenuation.
• Correct decisions on the data being a 0 or 1
  requires knowledge of the bit transition edges or
  boundaries.
• Requires a bit clock which is not typically sent
  with the data

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Bit Synchronization

• A bit (data) clock must be generated at the
  receiver for the data being received.
• The generation of this clock and the alignment
  (phase adjustment) of its edges with the edges
  of the received data is performed by a bit
  synchronizer.
• A bit synch is basically a Phase Lock Loop (PLL)
• PLLs work best if bit transitions occur at most
  if not all data bit boundaries

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Line Coding

• Data embedded in a layer 2 frame may easily
  contain long strings of 0s or 1s
• Few bit transitions for the PLL to work well
• Line coding is the translation of the binary data
  into a new digital stream
• Good line coding schemes guarantee bit
  transitions
• The spectral shape of the transmission is often
  affected

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Types of Line Coding

• Unipolar
• Polar
• NRZ (Nonreturn to Zero)
• RZ (Return to Zero)
• Biphase
• Bipolar

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Unipolar Line Coding

• Simple
• Binary 1 high voltage
• Binary 0 low (zero) voltage
• Properties
• No edge transitions when the original data
  doesnt change
• No change in the spectral shape (still has DC
  component)

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Unipolar cont.
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NRZ (Nonreturn to Zero) Coding

• A type of polar (two non-zero voltage levels)
  coding
• Removes the DC component
• NRZ-I
• NRZ-L
• 0 -gt positive (or neg.) voltage
• 1 -gt negative (or pos.) voltage
• NRZ-I
• 0 -gt voltage remains the same
• 1 -gt causes an inversion in the voltage
• creates bit transitions in long strings of 1s
  (but not 0s)

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NRZ Cont.
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RZ (Return to Zero) Coding

• Another type of polar encoding
• The first half of each bit is mapped as in NRZ-L
• The second half of each bit is set to 0 volts
• Guarantees bit transitions
• Removes the DC component
• The width of the transmitted pulse is cut in half
  so the spectral bandwidth increases

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RZ Cont.
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Biphase Coding

• Another type of polar
• Like RZ, transitions are created in the middle of
  the bit periods
• Most common methods used in LANs
• Manchester
• Middle transition ? if bit 1, ? if bit 0
• Ethernet
• Differential Manchester
• Middle transition always present, but a
  transition at the beginning of a bit only occurs
  if the bit 0
• Token Ring

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Manchester Diff. Manchester
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Line Coding Spectra
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Block Coding

• Enhances the performance of line coding while
  also introducing some error-detecting capability
• Based on substituting a block of n bits for a
  block of m bits, where n gt m
• A dictionary contains the mapping. Some of the
  n-bit blocks are not used in the one-to-one
  mapping

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Block Coding cont.
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Block coding subsitution (m4, n5)
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Block coding cont.

• Errors can be detected if the received n-bit word
  is invalid
• Also called mBnB coding
• Used in some of the newer Ethernet standards
• 100Base-TX (2-wire twisted pair)
• 100Base-FX (Fiber)
• 1000Base-T (2-wire Gigabit Ethernet)

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RF Transmission

• Not baseband
• Requires modulation
• The placement of data onto a cosinusoidal signal
• Multiple bits may be mapped into one modulation
  symbol
• Baud rate modulation symbol rate
• Traditional schemesASK, FSK, PSK, QAM

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ASK Amplitude Shift Keying

• Susceptible to channel degradations

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FSK Frequency Shift Keying
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PSK Phase Shift Keying

• BPSK bit rate baud rate, 0 or 180 deg. phase
• QPSK bit rate 2 baud rate, 45, 135, 225,
  315 deg.

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QAM Quadrature Amplitude Modulation

• Combined ASK and PSK
• Higher-order modulation scheme that lowers the
  symbol rate
• More susceptible to noise and nonlinearities
• Used in most modern phone modems

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8-QAM
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Multiplexing

• Transmission resources are usually limited in
  either time, frequency, or both
• Normally two separate signals cannot share the
  same time and frequency space
• As multiple users or segments become necessary, a
  method of sharing the these resources is critical
• Multiplexing allows this sharing

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FDM Frequency Division Multiplexing

• The frequency channel is divided and each user
  receives one portion of the spectrum
• Requires at least one non-baseband signal
• Guard bands are used to limit the effect of
  adjacent channel interference (ACI)

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FDM cont.
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FDM cont.
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Time-division Multiplexing (TDM)

• Dividing by time
• Supports any combination of baseband and
  modulated signals
• Two types of TDM
• Synchronous TDM
• Asynchronous TDM

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Synchronous TDM

• Each user (1, 2, n) is allocated a time slot
• A frame consists of one full cycle of a time slot
  from every user
• Requires framing bits for time slot
  synchronization
• Inefficient if data is not always being sent by
  ALL users

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Synch. TDM Cont.
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Asynchronous TDM

• m time slots for n users, m lt n
• Time slots are not reserved for each user
• Scans user input lines for available data
• Tries to fill all time slots during each frame
• Requires addressing overhead for correct
  de-multiplexing
• Typically more efficient that synch. TDM

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Asynch. TDM cont.
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Limits on Data Throughput

• Nyquist Bit Rate
• Noiseless, bandlimited channels
• Bit Rate (bps) 2 x B x log2(L)
• L of signal levels used to represent the data
• B frequency bandwidth available (Hz)
• Shannons Capacity Theorem
• Bandlimited channels with noise
• C (bps) B x log2(1 SNR)
• SNR signal-to-noise ratio of the channel

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Nyquist Example

• A noiseless channel with a 5 kHz bandwidth and
  binary transmission (2 levels) can deliver
• Bit Rate 2 x 5000 x log2(2) 10,000 bit/sec.
• If transmission using 4 bits/symbol is used (16
  levels) then
• Bit Rate 2 x 5000 x log2(16) 40,000 bit/sec.

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Shannon Capacity Example

• A modem operating over a telephone line has a
  maximum useful bandwidth of about 3400 Hz (300 Hz
  to 3700 Hz). The maximum SNR of the channel is
  39 dB. What is the maximum capacity?
• First, un-dB the SNR
• SNR 10(39/10) 7943
• C 3400 x log2(17943) 44 kbps