Error Recovery

1
Error Recovery
2
Problems with Internet

• Limited Bandwidth
• Varying Conditions
• Delay Jitter
• Packet Loss
• Delay
• Heterogeneity

3
You Are Here
Encoder
Decoder
Middlebox
Sender
Receiver
Network
4
Overview

• Characteristics of the Internet
• General techniques
• Error recovery for audio
• Effect of loss on MPEG
• Error recovery for MPEG

5
Loss Characteristics of The Internet
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Characteristics of Internet

• 60-70 of paths do not show any loss
• Those with loss have an average of 4.5 6
  packet loss
• Paxson97 End-to-end Internet packet dynamics

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Packet Loss Pattern
Number of Occurrences
1000
Burst Length
30
8
Characteristics of Internet

• Bursts of loss are typically short (2-3
  consecutively loss packets)
• Long burst do occur
• Burst may occur periodically

9
Wireless Link

• Loss rate measured in my office 10 20
• up to 50 reported!

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Gilbert Model
GOOD
BAD
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A Survey of Packet-Loss Recovery Techniques for
Streaming Audio

• Perkins, Hodson and Hardman
• IEEE Network Magazine 1998

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Retransmission
1
2
3
4
3
X
1
2
4
3
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Redundant Data
1
2
3
4
2
3
4
5
1
2
4
2
3
5
3
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Error Concealment
1
2
3
4
2
4
1
15
Retransmission

• for audio

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Pros/Cons of Retransmissions
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Scalable Retransmission

• On packet loss
• T random(0, RTT)
• wait for T
• multicast NACK
• On receiving NACK from others
• suppress own NACK

18
Retransmit when

• Group size is small
• Loss rate is low
• Large latency acceptable

19
Selective Retransmission

• Retransmission based on priority of packets
• Important/urgent packets are retransmitted first

20
Redundant Data

• for audio

21
Parity Forward Error Correction
1011001
XOR
1000010
0010010
0001001
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Parity Forward Error Correction
1011001
1000010
0001001
0010010
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Parity FEC Ordering
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Parity FEC Ordering
25
Parity FEC Ordering
26
Parity FEC Ordering
27
Parity FEC Ordering
28
Reed-Solomon Code

• RS(n,k)

RS






n
k
29
Reed-Solomon Code








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Media Specific FEC
1
2
3
4
2
3
4
5
1
2
4
2
3
5
3
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Pros/Cons of Redundant Data
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Interleaving
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Pros/Cons of Interleaving
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Error Concealment
35
Effect of Loss on Audio

• Speech
• Human ears can interpolate
• Loss up to length of phoneme can still be
  tolerable

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Insertion-based Repair

• Splice
• Silence Substitution
• Noise Substitution
• Repetition

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Other Repair Methods

• Interpolation
• Regeneration

38
Colins Recommendations
39
Non-Interactive Apps

• Interleaving
• FEC
• Retransmission for unicast only

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Interactive Applications

• Media Specific FEC

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Error Concealment

• Repeat

42
Packet Loss Effects on MPEG Video Sent over the
Public Internet

• Jill Boyce and Robert Gaglianello
• ACM Multimedia 1998

43
Measurement-based Study

• Need to understand the problem before proposing
  solution

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Data Gathering Method

• From NYC13 , Austin21, London18
• To Holmdel, NJ

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Data Gathering Method

• Video
• Two 5-mins MPEG
• 30 fps
• 384 kbps and 1 Mbps
• IBBPBBPBBPBBPBB
• QSIF 176x112 and SIF 352x240
• One row per slice

46
Average Packet Loss
47
Frame Effected by Errors
Frames in Error
Packet Loss Rate
48
Overview of Error Recovery for Video
49
Methods

• Retransmission
• Interleaving
• Error Concealment
• FEC
• Limiting Error Propagation
• Reference Frame Selection

50
Reference Frame Selection
I P B B P

I
X
51
Methods

• Retransmission
• Interleaving
• Error Concealment
• FEC
• Limiting Error Propagation
• Reference Frame Selection
• Changing Temporal Pattern

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Error Control Techniques for Interactive Low
Bitrate Video Transmission over The Internet

• Injong Rhee
• SIGCOMM 98

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Basic Idea

• Better Late Than Never!, or
• Late packet is still useful

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MPEG Frame Pattern
I
B
B
P
B
B
P
I
P
B
B
P
B
B
55
H.261 Error Propagation
I
P
P
P
P
P
P
X
X
X
X
X
X
loss
X
X
X
X
retransmission
56
H.261 Frame Pattern
I
P
P
P
P
P
P
X
loss
57
H.261 Frame Pattern
X
X
X
retransmission
loss
58
PTDD

• Periodic Temporal Dependency Distance
• Large PTDD
• Higher Chance of Repair
• Longer Error Propagations
• Less Temporal Redundancy

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QAL
I
P
P
P
Enhancement Layer
I
P
P
P
Base Layer
60
QAL
I
P
P
P
X
Enhancement Layer
I
P
P
P
Base Layer FEC
61
QAL PTDD
I
P
P
P
Enhancement Layer
I
P
P
P
Base Layer
62
Error Propagation
63
Recovery from Error Propagation
64
Frame quality

• PSNR

65
PSNR vs. Loss Rate
PSNR
Packet Loss Rate
66
Todays Summary
67
How to recover packet loss

• Retransmission
• FEC
• Error Concealment

68
Limit the damage of error

• Interleaving
• Key frame selection
• Change reference frame

69
Project Stage 2

• Packet Reassembly and MPEG Decoding

70
Decoding Pipeline
MntRTPMPEGPacket
MntRTPMPEGPacket
MntRTPFileReader
MntRTPDropper
MntRTPMPEGReassembler
MntMPEGRTPReassemblerBuffer
MntPPMWriter
MntYUVFrame
MntRTPMPEGFrameDecoder
MntPSNRCalculator
MntYUVFrame