Side Arrows Background

1
Lecture 15
Computer Communication Networks
2

• Todays Menu
• Encoding/Decoding
• Unipolar, Polar and Bipolar encoding

3
Encoding/Decoding

• Digital-to-Digital conversion or
  encoding/decoding is the representation of
  digital information by digital signal
• For example when we transmit data from computer
  to the printer, both original and transmitted
  data have to be digital
• Encoding a digital signal is where 1s and 0s
  generated by the computer are translated into
  voltage pulses that can be propagated over the
  wire

4
Encoding/Decoding
5
Encoding/Decoding

• A digital signal is a sequence of discrete,
  discontinuous voltage pulses, each pulse is a
  signal element
• Binary data are transmitted by encoding each data
  bit into signal elements
• In the simplest case, there is a one-to-one
  correspondence between bits and signal elements
• An example would be in which binary 0 is
  represented by a lower voltage level and binary 1
  by a higher voltage level
• A variety of other encoding schemes are also used

6
Encoding/Decoding

• Types of Encoding
• Unipolar
• Polar
• Bipolar
• 1-Unipolar
• Encoding is simple , with only one technique in
  use
• Simple and primitive
• Almost obsolete today
• Study provides introduction to concepts and
  problems involved with more complex encoding
  systems

7
Unipolar Encoding

• It works by sending voltage pulses on the
  transmission medium
• The signal elements all have the same algebraic
  sign, that is, all positive or negative
• One voltage level stands for binary 0 while the
  other stands for binary 1
• It is called Unipolar because it uses only one
  polarity
• This polarity is assigned to one of the two
  binary states usually a 1
• The other state usually a 0 is represented by
  zero voltage

8
Unipolar Encoding

• Figure shows the idea 1s are encoded as ve
  values, and 0s are encoded as ve values

9
Unipolar Encoding

• Pros and Cons of Unipolar Encoding
• Pros
• Straight forward and simple
• Inexpensive to implement
• Cons
• DC component
• Synchronization

10
Polar Encoding

• 2-Polar
• Polar encoding uses two voltage levels, positive
  and negative
• One logic state is represented by a positive
  voltage level, and the other by a negative
  voltage level
• It has 3 subcategories
• Non Return to Zero (NRZ)
• NRZL
• NRZI
• Return to Zero (RZ)
• Biphase
• Manchester
• Differential Manchester

11
Polar Encoding
12
Non Return to Zero (NRZ)

• In NRZ, the level of signal is either positive or
  negative
• NRZ-L (Non-Return-to-Zero-Level)
• Level of the signal depends on the type of bit it
  represents
• A ve voltage usually means the bit is a 1 and a
  ve voltage means the bit is a 0 (vice versa)

13
Non Return to Zero (NRZ)
Problem with NRZ-L When long streams of 0s
or 1s are there in data, receiver receives a
continuous voltage and should determine how many
bits are sent by relying on its clock, which may
or may not be synchronized with the sender clock
14
Non Return to Zero (NRZ)

• NRZ-I (Non-Return-to-Zero-Invert On One)
• The inversion of the level represents a 1 bit
• A bit 0 is represented by no change
• A transition (low-to-high or high-to-low) at the
  beginning of a bit time denotes a binary 1 for
  that bit time no transition indicates a binary 0

15
Non Return to Zero (NRZ)

• Problem with NRZ-I
• NRZ-I is superior to NRZ-L due to synchronization
  provided by signal change each time a 1 bit is
  encountered
• The string of 0s can still cause problem but
  since 0s are not as likely, they are less of a
  problem

16
Non Return to Zero (NRZ)

• The NRZ codes are the easiest to engineer and, in
  addition, make efficient use of bandwidth
• The main limitations of NRZ signals are the
  presence of a dc component and the lack of
  synchronization capability
• Because of their simplicity and relatively low
  frequency response characteristics, NRZ codes are
  commonly used for digital magnetic recording
• However, their limitations make these codes
  unattractive for signal transmission applications

17
Return to Zero (RZ)

• Any time, data contains long strings of 1s or
  0s, receiver can loose its timing
• In unipolar, we have seen a good solution is to
  send a separate timing signal but this solution
  is expensive
• A better solution is to somehow include sync in
  encoded signal somewhat similar to what we did in
  NRZ-I but it should work for both strings of 0
  1
• One solution is RZ encoding which uses 3 values
  Positive, Negative and Zero
• Signal changes not between bits but during each
  bit

18
Return to Zero (RZ)

• Like NRZ-L, ve voltage means 1 and a ve voltage
  means 0, but unlike NRZ-L, half way through each
  bit interval, the signal returns to zero
• A 1 bit is represented by positive to zero and a
  0 is represented by negative to zero transition

19
Return to Zero (RZ)

• Problem with RZ
• The only problem with RZ encoding is that it
  requires two signal changes to encode one bit and
  therefore occupies more bandwidth
• But of the 3 alternatives we have discussed, it
  is most effective

20
Biphase

• Best existing solution to the problem of
  synchronization
• Signal changes at the middle of bit interval but
  does not stop at zero
• Instead it continues to the opposite pole
• There are two types of biphase encoding
• Manchester
• Differential Manchester

21
Manchester

• Uses inversion at the middle of each bit interval
  for both synchronization and bit representation
• Negative-to-Positive
  Transition 1
• Positive-to-Negative
  Transition 0
• By using a single transition for a dual purpose,
  Manchester achieves the same level of
  synchronization as RZ but with only two levels of
  amplitude

22
Differential Manchester

• Inversion at the middle of the bit interval is
  used for synchronization but presence or absence
  of an additional transition at the beginning of
  bit interval is used to identify a bit
• A transition means binary 0 no transition means
  binary 1
• Requires 2 signal changes to represent binary 0
  but only one to represent binary 1

23
3-Bipolar Encoding

• Although the biphase techniques have achieved
  widespread use in local-area-network applications
  at relatively high data rates, they have not been
  widely used in long-distance applications
• The principal reason for this is that they
  require a high signaling rate relative to the
  data rate
• This sort of inefficiency is more costly in a
  long-distance application

24
Bipolar Encoding

• An approach is to make use of some sort of
  scrambling scheme
• The idea behind this approach is simple
• Sequences that would result in a constant voltage
  level on the line are replaced by filling
  sequences that will provide sufficient
  transitions for the receiver's clock to maintain
  synchronization
• The filling sequence must be recognized by the
  receiver and replaced with the original data
  sequence

25
Bipolar Encoding

• Like RZ, it uses three voltage levels
• Unlike RZ, zero level is used to represent binary
  0
• Binary 1s are represented by alternate positive
  and negative voltages
• AMI
• Pseudoternary
• B8Zs
• HDB3

26
Alternate Mark Inversion(AMI)

• Simplest type of bipolar encoding
• A binary 0 is represented by no line signal, and
  a binary 1 is represented by a positive or
  negative pulse
• The binary 1 pulses must alternate in polarity
• Alternate Mark Inversion means alternate 1
  inversion

27
Alternate Mark Inversion(AMI)

• Pros and Cons
• There will be no loss of synchronization if a
  long string of is occurs
• Each 1 introduces a transition, and the receiver
  can resynchronize on that transition
• A long string of 0s would still be a problem
• Because the 1 signals alternate in voltage from
  positive to negative, there is no net dc component

28
Pseudoternary

• Inverse of AMI
• In this case, it is the binary 1 that is
  represented by the absence of a line signal, and
  the binary 0 by alternating positive and negative
  pulses

29
Pseudoternary

• Two variations are developed to solve the problem
  of synchronization of sequential 0s
• B8Zs (used in North America)
• HDB3 (used in Europe Japan)
• Both modify original pattern of AMI only on case
  of long stream of zeroes

30
B8Zs

• Bipolar with 8-zeros substitution
• Difference between AMI and B8Zs occurs only when
  8 or more consecutive zeros are encountered
• Forces artificial signal changes called
  violations
• Each time eight 0s occur, B8Zs introduces
  changes in pattern based on polarity of previous
  1 (the 1 occurring just before zeros)
• Same as bipolar AMI, except that any string of
  eight zeros is replaced by a string with two code
  violations

31
B8Zs
32
HDB3

• High-Density Bipolar-3 Zeros
• Alteration of AMI adopted in Europe and Japan
• Introduces changes into AMI, every time four
  consecutive zeros are encountered instead of
  waiting for eight zeros as in the case of B8Zs
• As in B8Zs, the pattern of violations is based on
  the polarity of the previous 1 bit
• HDB3 also looks at the number of 1s that have
  occurred since the last substitution
• Same as bipolar AMI, except that any string of
  four zeros is replaced by a string with one code
  violation

33
HDB3

• High-Density Bipolar-3 Zeros

34
HDB3

• High-Density Bipolar-3 Zeros
• If the last violation was positive, this
  violation must be negative, and vice versa
• The table shows that this condition is tested for
  by knowing whether the number of pulses since the
  last violation is even or odd and the polarity of
  the last pulse before the occurrence of the four
  zeros

35
HDB3

• High-Density Bipolar-3 Zeros