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Low Latency Wireless Video Over 802.11 Networks Using Path Diversity. John Apostolopolous Wai-tian Tan Mitchell Trott Hewlett-Packard Laboratories. Allen Miu MIT Laboratory for Computer Science. Motivation. Prevalent 802.11 infrastructure provides inexpensive connectivity

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low latency wireless video over 802 11 networks using path diversity

Low Latency Wireless Video Over 802.11 Networks Using Path Diversity

John Apostolopolous

Wai-tian Tan

Mitchell Trott

Hewlett-Packard Laboratories

Allen Miu

MIT Laboratory for

Computer Science

motivation
Motivation
  • Prevalent 802.11 infrastructure provides inexpensive connectivity
  • Emergent mobile devices integrated with cameras and 802.11 interfaces
  • High quality (low-latency) conversational communication over 802.11 networks (e.g. VoIP, Video conferencing)
challenges
Challenges
  • 802.11 operates in ISM band
    • Interference from other electronic devices (e.g. BT, microwaves)
  • Lossy Environment
    • Access point coverage can be spotty
    • Quality changes over time
      • Signal fading due to multi-path
      • Shadowing due to obstacles and human traffic
    • Contention among exposed and hidden nodes
  • Low Latency requirement
    • 802.11 ARQ error recovery can add large delays
our approach
Our Approach
  • Use error resilient video compression
    • H.264/MPEG-4 AVC
  • Best-effort error recovery (standard 802.11 ARQ)
  • Distributed AP infrastructure to stream video via multiple access points (exploit path diversity)
    • Use multiple paths simultaneously or switch between them (site selection) as a function of channel characteristics
preliminary investigation
Preliminary investigation

In a 802.11 path diversity network with mobile clients,

  • How does path diversity affect packet loss characteristics?
  • What is the resulting performance gain for conversational video communication?
test bed setup
Test-bed Setup

~40m (max)

AP1

Wired

100Mbps

Ethernet

802.11b

11Mbps

WLAN

Sender

MobileReceiver

~25m

AP2

  • Two 360kbps cbr streams
  • 1500 byte UDP packets
  • time-stamped
  • Ad hoc mode
  • ARQ up to 16 retries
  • Open cubicle area
  • Receiver moves @1m/s
diversity scenarios
Diversity Scenarios

AP1 only

  • Conventional single path case

AP2 only

AP1

Wired

100Mbps

Ethernet

802.11b

11Mbps

WLAN

Sender

MobileReceiver

AP2

diversity scenarios8
Diversity Scenarios

AP1 only

  • Conventional single path case

AP2 only

  • Balanced split stream (non-adaptive)

AP1

Wired

100Mbps

Ethernet

802.11b

11Mbps

WLAN

Sender

MobileReceiver

AP2

diversity scenarios9
Diversity Scenarios

AP1 only

  • Conventional single path case

AP2 only

  • Balanced split stream (non-adaptive)
  • Adaptive, fine-grain site selection (based on loss rate)

AP1

Wired

100Mbps

Ethernet

802.11b

11Mbps

WLAN

Sender

MobileReceiver

AP2

diversity scenarios10
Diversity Scenarios

AP1 only

  • Conventional single path case

AP2 only

  • Balanced split stream (non-adaptive)
  • Adaptive, fine-grain site selection (based on loss rate)
  • Oracle (optimal adaptive, can be realized by repetition coding)

AP1

Wired

100Mbps

Ethernet

802.11b

11Mbps

WLAN

Sender

MobileReceiver

AP2

path diversity reduces packet loss
Path Diversity Reduces Packet Loss

AP1

AP2

Balanced

Site Selection

Oracle

30

25

20

Avg Packet Loss Rate (%)

15

10

5

0

20

40

60

80

100

Infinite

Delay cutoff (ms)

path diversity reduces burst loss
Path Diversity Reduces Burst Loss

Burst event = 2 or more consecutive losses

2000

AP1

AP2

Balanced

Site Selection

1500

Oracle

Number of Burst Events

1000

500

0

20

40

60

80

100

Infinite

Delay cutoff (ms)

h 263 video performance
H.263 Video Performance

AP1

AP2

Balanced

Site Selection

Mother and Daughter Sequence

34

PSNR gain = 1.6 – 3.0 dB

1/3 delay reduction

32

30

PSNR (dB)

28

26

24

20

40

60

80

100

Infinite

Delay Cutoff (ms)

conclusion
Conclusion
  • All path diversity schemes help reduce burst losses
  • Optimal path diversity drastically reduce loss rate and improves video quality
  • A simple site selection algorithm can effectively increase video quality without increasing bandwidth usage
complete trace
Complete Trace

0

0

1

1

2

2

3

3

4

4

5

5

6

6

7

7

single stream from ap1
Single Stream from AP1

0

1

2

3

4

5

6

7

0

1

2

3

4

5

6

7

Discarded

split stream from ap1 ap2
Split stream from AP1 & AP2

0

0

1

1

2

2

3

3

4

4

5

5

6

6

7

7

split stream from ap1 ap219
Split stream from AP1 & AP2

0

0

2

2

4

4

6

6

1

1

3

3

5

5

7

7

Discarded

split stream from ap1 ap220
Split stream from AP1 & AP2

0

0

2

2

4

4

6

6

1

1

3

3

5

5

7

7

Discarded

split stream from ap1 ap221
Split stream from AP1 & AP2

Re-numbered

0

1

2

3

4

5

6

7

1

1

3

3

5

5

7

7

Discarded

fine grained site selection
Fine-grained Site Selection

0

0

1

1

2

2

3

3

4

4

5

5

6

6

7

7

fine grained site selection24
Fine-grained Site Selection

0

1

2

3

4

5

6

7

  • Selected site transmits 95% packet
  • Other site transmits 5% packet for probing
  • Site selection based on error rate of last 300 packets
oracle
Oracle

Equivalent to repetition coding

0

0

1

1

2

2

3

3

4

4

5

5

6

6

7

7

oracle26
Oracle

Equivalent to repetition coding

0

0

1

1

2

2

3

3

4

4

5

5

6

6

7

7

0

1

2

3

4

5

6

7

existing solutions
Existing Solutions
  • Robust video communication in lossy channel:
    • Error resilient video compression
    • ARQ
      • Delay can be intolerable
      • Head of line blocking
    • FEC coding
      • Can be inefficient
    • Receiver diversity antenna
      • Does not exploit path diversity
analysis
Analysis
  • Loss Characteristics
    • Varied delay threshold
    • Average Packet Loss Rate
    • Number of burst events
    • Burst Packet Loss Rate

(# Packets lost in burst ) / (Total # packets)

  • H.264/MPEG4 Video Performance for 4 different sequences
    • PSNR
    • Nthresh (# of times when PSNR drops below 30dB)
methodology
Methodology
  • Walking with receiver in open cubicle area
  • Analyzed packet loss from a 15 minute trace
    • Experiment repeated once after 3 months, showing similar performance results
  • Analyzed the performance of 5 different diversity scenarios from sampling the same trace
    • Difficulty in finding meaningful comparisons between different scenarios