ECE 766

1
Data Conversion Methods

• Sending data from one place to the next?
  Transform data into signals
• Formats of source vs. medium
• Format of the original data (analog/digital)
• Format used by the communication hardware
  (analog/digital)
• 4 possible combinations
• Digital data / digital signal ? (computers over
  LAN)
• Analog data / digital signal ? (long distance
  phone)
• Digital data / analog signal ? (computers over
  phone lines)
• Analog data / analog signal ? (radio broadcast)

2
Data Encoding / Modulation

• Baseband
• Digitally Encoded
• Resources shared by Time Division Multiplexing

• Broadband
• Analog Modulation
• Resources shared by Frequency Division
  Multiplexing

Should I have called the vertical axis bandwidth?
3
Terminology

• Data rate (bps)
• Baud rate, modulation rate (signal
  elements/sec)
• Mark (1) and space (0) conditions(from
  telegraphy)
• Connection types
• Simplex One way
• Half Duplex Two way, but only one way at a time
• (Full) Duplex Two way simultaneously

4
Criteria for a Good Encoding Scheme

• Signal Spectrum
• Minimize high frequency components
• No DC components
• Synch capability (find bit positions)
• Signal error detection capability
• Signal interference and noise immunity
• Cost and complexity

5
Absolute vs. Differential Encoding / Modulation
Schemes

• Absolute
• Each signal corresponds to a predetermined
  information unit
• The meaning of a signal sequence is fixed, not
  relative.
• Differential
• Information is encoded by difference between
  current and previous signal element
• The meaning of a signal sequence is relative, not
  absolute.

6
Digital Encoding Schemes

• Digital information is converted to a sequence of
  voltage pulses that propagate over the link
• Three subcategories by voltage use
• Unipolar (Zero and Positive)
• Polar (Negative and Positive)
• Bipolar (Negative, Zero, and Positive)

7
Unipolar Encoding

• Uses zero and positive voltage pulses to encode
  binary data
• Not really encoded at all!

8
Polar Encoding

• Polar encoding uses a positive and a negative
  voltage level to represent bits? Solves the DC
  component problem(if balanced)
• Categories
• Nonreturn to Zero (NRZ)
• NRZ-L (LLevel)
• NRZ-I (IInverted)
• Return to Zero (RZ)(as shown in book)
• Biphase
• Manchester
• Differential Manchester

9
Nonreturn to Zero (NRZ)

• The voltage level is constant during a bit
  interval, i.e., no returns to zero
• Absolute and differential versions
• Absolute NRZ NRZ-L (LLevel)(like ntl)
• 0 Positive voltage
• 1 Negative voltage

10
Nonreturn to Zero (NRZ)

• Differential NRZ NRZ-I (IInverted)
• A bit is represented by the transition of the
  voltage level, not the voltage level itself!
• 0 No inversion at beginning of bit interval
• 1 Inversion at beginning of bit interval

11
Nonreturn to Zero (NRZ)

• Evaluation
• No DC component
• Simple
• Few high frequency components
• Synchronization
• No synchronization at large (consider a string
  of the same bit)
• NRZ-I provides synchronization for every 1
  encountered ? can handle strings of 1s(superior
  to NRZ-L)

12
Return to Zero (RZ)(bipolar form)

• Targets to solve the synchronization problem
• A scheme that handles both strings of both 1s and
  0s
• Voltage level change for every bit value ? three
  levels ,-, 0
• 0 Transition from negative to zero
• 1 Transition from positive to zero

13
Return to Zero (RZ)

• Variations used also for magnetic recording (no
  synchronization capability)
• Evaluation
• Solves synchronization problem
• Two signal changes / bit? More transitions ?
  Occupies more bandwidth

14
Biphase

• Signal changes in the middle of the bit interval,
  but does not return to zero
• Signal change ? bit representation ?
  synchronization
• Manchester
• 0 Transition from positive to negative
• 1 Transition from negative to positive

15
Biphase

• Differential Manchester
• 0 Transition at the beginning of bit period
• 1 No transition at the beginning of bit period
• Evaluation
• Not as simple
• Higher frequency components (as RZ)
• Synchronization capability
• No DC component

16
Bipolar

• Like in RZ, three voltage levels are used
• Zero voltage level used for binary 0
• Categories
• Alternate Mark Inversion (AMI)
• Bipolar 8-Zero Substitution (B8ZS) ? North
  America
• High Density Bipolar 3 (HDB3)? Europe and Japan

17
Alternate Mark Inversion (AMI)

• Uses three voltage levels
• 0 Zero volts
• 1 Non-zero voltage, opposite in polarity to
  the last logical 1
• Evaluation
• No DC component
• Synchronized only for 1s, not 0s
• Error detection

18
Bipolar 8-Zero Substitution (B8ZS)

• Adds synchronization for long strings of 0s
• North American system
• Same working principle as AMI except for eight
  consecutive 0s
• Evaluation
• Adds synchronization without changing the DC
  balance
• Error detection possible

10000000001 ? 000-0-01 in general
00000000?000V(-V)0(-V)V
19
High Density Bipolar 3 (HDB3)

• Goal like B8ZS to improve Sync of AMI
• Just like AMI except 4 0s are replaced by code
• For 0000 use 000V or B00V
• Where B and V are or
• And V is AMI violation, B is Balance Bit
• Use 000V if EVEN number of and pulses so far
• Use B00V if ODD, and B is opposite last pulse

20
High Density Bipolar 3 (HDB3)

• Same goal as B8ZS
• Based on AMI
• Replaces every four consecutive 0s based on
• Number of pulses since last substitution
• Polarity of last logical 1

21
High Density Bipolar 3 (HDB3)

• Example (revised 1-13-06)
• Number of 1s since last substitution is even,
  last 1 negative (before this string)
• Encode 100000000001

0
0
0
0
0
0
0
1
0
1
0
0
Amplitude
Time