Chapter 5 Signal Encoding Techniques - University of Sydney

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Chapter 5 Signal Encoding Techniques - University of Sydney Powered By Docstoc
					Signal Encoding
     Lesson 05
   NETS2150/2850
   http://www.ug.cs.usyd.edu.au/~nets2150/




  School of IT, The University of Sydney     1
Lecture Outline
   Encoding schemes for digital data to
    transmit in digital transmission systems
    – NRZ schemes
    – Manchester schemes in LANs
    – AMI schemes
       • With scrambling for WANs use
   Encoding schemes for digital data to
    transmit in analog transmission systems
    – ASK Scheme
    – FSK Scheme
    – PSK Scheme
                                               2
Various Encoding Techniques

 Encoding is the conversion of
  streams of bits into a signal (digital or
  analog).
 Categories of Encoding techniques:
    – Digital data, digital signal    Digital transmission
    – Analogue data, digital signal
    – Digital data, analog signal     Analog transmission
    – Analogue data, analog signal

                                                      3
      Digital Data, Digital Signal
                     (Digital to Digital)


   Digital signal
    – Discrete, discontinuous voltage pulses
    – Each pulse is a signal element
    – Binary data encoded into signal elements




                                                 4
Interpreting Signals

   Need to know
    – Timing of bits - when they start and end
    – Signal levels
   Factors affecting interpretation of
    signals
    – SNR
    – Data rate
    – Bandwidth

                                                 5
Comparison of Encoding Schemes

    Error detection
     – Can be built into signal encoding
    Cost and complexity
     – Higher signal rate (& thus data rate) lead to higher
       costs
    Clocking
     – Synchronizing transmitter and receiver
    Signal spectrum
     – Bandwidth requirement
     – Presence of dc component
                                                          6
Digital-to-Digital Encoding Schemes
 3 Broad Categories: Unipolar, Polar,
  and Bipolar
-Nonreturn to Zero-Level (NRZ-L)       Magnetic
                                       Recording
-Nonreturn to Zero Inverted (NRZI)
-Manchester
                              LAN
-Differential Manchester
-Bipolar -AMI
-B8ZS                         WAN
-HDB3

                                               7
Nonreturn to Zero-Level (NRZ-L)

 Polar Encoding
 Two different voltages for 0 and 1 bits
 Voltage constant during bit interval
    – no transition i.e. no return to zero voltage
   Negative voltage for one value and
    positive for the other


                                                     8
Nonreturn to Zero Inverted (NRZI)

 Polar
 Transition (low to high or high to low)
  denotes a binary 1
 No transition denotes binary 0
 This is an example of differential
  encoding


                                            9
NRZ




      10
Differential Encoding

 Polar
 Better encoding technique
 Data represented by changes rather
  than levels
 More reliable detection of bit in noisy
  channels rather than level


                                            11
NRZ pros and cons

   Pros
    – Easy to engineer
    – Make good use of bandwidth
   Cons
    – Lack of synchronisation capability
    – Presence of a dc component
 Used for digital magnetic recording
 Not often used for signal transmission
                                           12
Biphase Schemes
   Polar- signal elements have opposite voltage
    level (-ve and +ve)

   Overcomes the limitations on NRZ codes

   Two biphase techniques are commonly used:
    – Manchester
    – Differential Manchester
   Heavily used in LAN applications


                                                   13
Biphase Scheme1: Manchester

   Transition in middle of each bit interval

 Low to high represents one
 High to low represents zero
 Used by IEEE 802.3 (Ethernet LAN)




                                                14
Biphase Scheme 2: Differential
Manchester

   Midbit transition is clocking only
   Transition at start of a bit interval represents
    zero
   No transition at start of a bit interval
    represents one
   Note: this is a differential encoding scheme
   Used by IEEE 802.5 (Token Ring LAN)
                                                       15
16
Biphase Pros and Cons
   Cons
    – At least one transition per bit time and possibly
      two
    – Maximum baud rate is twice NRZ
    – Requires more bandwidth
   Pros
    – Synchronization on mid bit transition (self
      clocking)
    – Error detection
       • Absence of expected transition
    – No dc component
                                                          17
Multilevel Binary (Bipolar)

 Use more than two voltage levels
 Bipolar-AMI (Alternate Mark Inversion)
    – zero represented by no line signal
    – one represented by positive or negative pulse
    – „one‟ pulses alternate in polarity
    – No loss of sync if a long string of ones (zeros still a
      problem)
    – Lower bandwidth
    – Easy error detection
                                                           18
Bipolar-AMI Encoding




                       19
Trade Off for Multilevel Binary

   Not as efficient as NRZ
    – Receiver must distinguish between three
      levels
      (+A, -A, 0)




                                                20
Scrambling Technique
   Used to replace sequences that would produce
    constant voltage
   Produce “filling” sequence that:
    – Must produce enough transitions to sync
    – Must be recognized by receiver and replace with original
    – Same length as original
 Avoid long sequences of zero level line signal
 No reduction in data rate
 Error detection capability
 Two commonly used techniques are: B8ZS, and
  HDB3
 Used for long distance transmission (WAN)


                                                                 21
Bipolar With 8 Zeros
Substitution (B8ZS)
 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 - intentional
    – Unlikely to occur as a result of noise
 Receiver detects and interprets as octet of all
  zeros
 HDB3 – similar but based on 4 zeros
                                                    22
B8ZS




       23
HDB3

 High Density Bipolar 3 Zeros
 Based on bipolar-AMI
 String of four zeros replaced with one or
  two pulses




                                          24
HDB3 Substitution Rules
              # of Bipolar Pulses (ones) since Last Substitution

Polarity of    Odd                           Even
Preceding
Pulse
-              000-                          +00+

+              000+                          -00-


                                                                   25
B8ZS and HDB3




          Change of polarity   26
Recap of Digital Signal Encoding
Formats
                              0                             1
NRZL                          High level                    Low level

NRZI                          No transition at start of     transition
                              interval

Bipolar-AMI                   No line signal                +ve line signal
Manchester                    Transition from high to low   Transition from low to high
                              in the middle of interval     in the middle of interval
Diff Manchester               Tran at start of interval     No transition at start of
(always a Transition in the                                 interval
middle of interval)
HDB3                          Same as bipolar-AMI, except that any string of four
                              zeros is replaced by a string with one code violation
B8ZS                          Same as bipolar-AMI, except that any string of eight
                              zeros are replaced by a string of two code violations


                                                                                        27
 Digital Data, Analog Signal
 Some transmission media only transmit
  analog signals.
 Public telephone system
    – 300Hz to 3400Hz (voice frequency range)
    – Use modem (modulator-demodulator)




                                                28
Digital to Analog modulation
techniques:
Modulation involves operation on signal
characteristics: frequency, phase, amplitude.

   Amplitude shift keying (ASK)

   Frequency shift keying (FSK)

   Phase shift keying (PSK)

                                                29
Modulation Techniques (digital data,
analog signal)




                                   30
Amplitude Shift Keying
 Values represented by different
  amplitudes of carrier
 Usually, one amplitude is zero
    – i.e. presence and absence of carrier is
      used
 Susceptible to sudden gain changes
 Inefficient
 Up to 1200bps on voice grade lines
 Used over optical fiber                       31
ASK




      32
Relationship between baud rate
and bandwidth in ASK




                                 33
Example 1
Find the minimum bandwidth for an ASK signal
transmitting at 2000 bps. The transmission mode is
half-duplex.

  Solution
In ASK, baud rate and bit rate are the
  same. The baud rate is therefore 2000.
  An ASK signal requires a minimum
  bandwidth equal to its baud rate.
  Therefore, the minimum bandwidth is
  2000 Hz.
                                                     34
Example 2

   Given a bandwidth of 5000 Hz for an ASK signal, what
   are the baud rate and bit rate?


Solution
In ASK the baud rate is the same as the bandwidth,
which means the baud rate is 5000. But because the baud
rate and the bit rate are also the same for ASK, the bit
rate is 5000 bps.


                                                           35
Example 3

  Given a bandwidth of 10,000 Hz (1000 to 11,000 Hz),
  draw the full-duplex ASK diagram of the system. Find the
  carriers and the bandwidths in each direction. Assume
  there is no gap between the bands in the two directions.

Solution
For full-duplex ASK, the bandwidth for each direction is
      BW = 10000 / 2 = 5000 Hz
The carrier frequencies can be chosen at the middle of
each band (see Fig. 5.5).
      fc (forward) = 1000 + 5000/2 = 3500 Hz
      fc (backward) = 11000 – 5000/2 = 8500 Hz             36
Solution to Example 3




                        37
Binary Frequency Shift Keying
 Most common form is binary FSK
  (BFSK)
 Two binary values represented by two
  different frequencies (near carrier)
 Less susceptible to error than ASK
 Up to 1200bps on voice grade lines
 High frequency radio
 Even higher frequency on LANs using
  co-ax                                  38
FSK




      39
Multiple FSK

 More than two frequencies used
 More bandwidth efficient
 More prone to error
 Each signalling element represents
  more than one bit



                                       40
FSK on Voice Grade Line




                          41
Phase Shift Keying

 Phase of carrier signal is shifted to
  represent data
 Binary PSK
    – Two phases represent two binary digits
   Differential PSK
    – Phase shifted relative to previous
      transmission rather than some reference
      signal
                                                42
PSK




      43
Differential PSK




                   44
Performance of Digital to Analog
Modulation Schemes
   Bandwidth
    – ASK and PSK bandwidth directly related to
      bit rate
    – FSK bandwidth related to data rate for
      lower frequencies, but to offset of
      modulated frequency from carrier at high
      frequencies
    – (See Stallings for math)
   In the presence of noise, bit error rate of
    PSK and QPSK are about 3dB superior
                                              45
    to ASK and FSK
Summary

 Various encoding schemes
 Some used in LANs
 Others more suitable in WAN with
  scrambling
 Read Stallings Section 5.1
 Next: Data link layer functions.


                                     46

				
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