Error Control for Multimedia Communications

1
Chapter 4

• Error Control for Multimedia Communications

2
Contents

• 1. Introduction to Error Control
• 2. Forward Error Correction
• 2.1 Hamming Code
• 2.2 Multiple Hamming Codes with Interleaving
• 2.3 Unequal Error Protection
• 3. Packet Combining
• 3.1 Majority Packet Combining
• 3.2 Aggressive Packet Combining
• 4. Packet Recovery
• 4.1 XOR Packet Recovery
• 4.2 Packet Recovery Using Erasure Code
• 5. Concealment of Packet Loss
• 5.1 Audio
• 5.2 Image
• 5.3 Video

3
1. Introduction to Error Control
4
1. Introduction to Error Control
5
1. Introduction to Error Control
Parity Bit Check, also known as Vertical
Redundancy Check (VRC), parity check is a method
of adding a parity bit to a data stream to check
that data for any errors.  A test that determines
whether the number of ones or zeros in a data
stream is odd or even.
The CRC is a very powerful but easily implemented
technique to obtain data reliability Extreme
error detection capabilities. Little overhead.
Ease of implementation. The CRC algorithm treats
all bit streams as binary polynomials. Given the
original data stream, the transmitter generates
the Check Sequence. The CS is generated so that
the resulting data stream (the cascade of the
original data stream frame and the CS), is
exactly devisable by some pre-defined polynomial.
This pre-defined polynomial is called the devisor
or CRC Polynomial.
6
1. Introduction to Error Control
Stop and Wait transmission is the simplest
reliability technique and is adequate for a very
simple communications protocol.
7
1. Introduction to Error Control
Go-Back-N error recovery is a procedure which is
implemented in some communications protocols to
provide reliability.

• Features required for Go-Back-N ARQ
• To support Go-Back-N ARQ, a protocol must number
  each packet which is sent.
• The local node must also keep a buffer of all
  packets which have been sent, but have not yet
  been acknowledged.
• The receiver at the remote node keeps a record of
  the highest numbered PDU which has been correctly
  received.

8
1. Introduction to Error Control
Selective Repeat error recovery is a procedure
which is implemented in some communications
protocols to provide reliability. It is the most
complex of a set of procedures, however the most
efficient scheme. Selective repeat is employed by
the TCP transport protocol.
Features required for Selective Repeat ARQ is to
support Go-Back-N ARQ
9
1. Introduction to Error Control
10
1. Introduction to Error Control
11
1. Introduction to Error Control
12
2. Forward Error Correction
13
2. Forward Error Correction
14
2.1. Hamming Code
Hamming code can correct one erroneous bit in
each codeword.
Coding Example The data bits 100 1000 are coded
as 0011 0010 000.
15
2.1. Hamming Code
16
2.1. Hamming Code

• The key to the Hamming Code is the use of extra
  parity bits to allow the identification of a
  single error. Create the code word as follows
• Mark all bit positions that are powers of two as
  parity bits. (positions 1, 2, 4, 8, 16, 32, 64,
  etc.)
• All other bit positions are for the data to be
  encoded. (positions 3, 5, 6, 7, 9, 10, 11, 12,
  13, 14, 15, 17, etc.)
• Each parity bit calculates the parity for some of
  the bits in the code word. The position of the
  parity bit determines the sequence of bits that
  it alternately checks and skips. Position 1
  check 1 bit, skip 1 bit, check 1 bit, skip 1 bit,
  etc. (1,3,5,7,9,11,13,15,...)Position 2 check 2
  bits, skip 2 bits, check 2 bits, skip 2 bits,
  etc. (2,3,6,7,10,11,14,15,...)Position 4 check
  4 bits, skip 4 bits, check 4 bits, skip 4 bits,
  etc. (4,5,6,7,12,13,14,15,20,21,22,23,...)Positio
  n 8 check 8 bits, skip 8 bits, check 8 bits,
  skip 8 bits, etc. (8-15,24-31,40-47,...)Position
  16 check 16 bits, skip 16 bits, check 16 bits,
  skip 16 bits, etc. (16-31,48-63,80-95,...)Positio
  n 32 check 32 bits, skip 32 bits, check 32 bits,
  skip 32 bits, etc. (32-63,96-127,160-191,...)etc.
  
• Set a parity bit to 1 if the total number of ones
  in the positions it checks is odd. Set a parity
  bit to 0 if the total number of ones in the
  positions it checks is even.

17
2.1. Hamming Code
18
2.1. Hamming Code
19
2.1. Hamming Code
20
2.2. Multiple Hamming Codes with Interleaving
21
2.2. Multiple Hamming Codes with Interleaving
22
2.2. Multiple Hamming Codes with Interleaving
23
2.3. Unequal Error Protection
24
2.3. Unequal Error Protection
25
3. Packet Combining
26
3. Packet Combining
27
3.1. Majority Packet Combining
28
3.1. Majority Packet Combining
29
3.2. Aggressive Packet Combining
30
3.2. Aggressive Packet Combining
31
3.2. Aggressive Packet Combining
32
3.2. Aggressive Packet Combining
33
4. Packet Recovery
34
4.1. XOR Packet Recovery
35
4. Packet Recovery Using Erasure Code
36
4. Packet Recovery Using Erasure Code
37
4. Packet Recovery Using Erasure Code
38
5. Concealment of Packet Loss
Concealment of Packet Loss Objective When a
receiver cannot recover a lost/erroneous packet,
it tries to make this loss less perceptible by
humans.
39
5.1. Concealment of Packet Loss - Audio
40
5.1. Concealment of Packet Loss - Audio
41
5.1. Concealment of Packet Loss - Audio
42
5.1. Concealment of Packet Loss - Audio
43
5.1. Concealment of Packet Loss - Audio
44
5.1. Concealment of Packet Loss - Audio
45
5.2. Concealment of Packet Loss - Image
46
5.3. Concealment of Packet Loss - Video
47
5.3. Concealment of Packet Loss - Video
48
5.3. Concealment of Packet Loss - Video