Post on 30-May-2018
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Low Latency Wireless Video Over 802.11Networks Using Path Diversity
John Apostolopolous
Wai-tian Tan
Mitchell Trott
Hewlett-Packard Laboratories
Allen MiuMIT Laboratory for
Computer Science
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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, Videoconferencing)
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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
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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 channelcharacteristics
AP1
Microwave
(Interferer)
Sender MobileReceiver
Wired Wireless
AP2
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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 forconversational video communication?
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Test-bed Setup
Sender MobileReceiver
Wired
100Mbps
Ethernet
802.11b
11Mbps
WLAN
AP1
AP2
~25m
Ad hoc mode ARQ up to 16 retries Open cubicle area
Receiver moves @1m/s
~40m (max)
Two 360kbps cbr streams 1500 byte UDP packets time-stamped
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Diversity Scenarios
Conventional single path case
SenderMobile
Receiver
Wired
100Mbps
Ethernet
802.11b
11Mbps
WLAN
AP1
AP2
AP1 only
AP2 only
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Diversity Scenarios
Conventional single path case
SenderMobile
Receiver
Wired
100Mbps
Ethernet
802.11b
11Mbps
WLAN
AP1
AP2
AP1 only
AP2 only
Balanced split stream (non-adaptive)
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Diversity Scenarios
Conventional single path case
SenderMobile
Receiver
Wired
100Mbps
Ethernet
802.11b
11Mbps
WLAN
AP1
AP2
AP1 only
AP2 only
Balanced split stream (non-adaptive)
Adaptive, fine-grain site selection (based on loss rate)
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Diversity Scenarios
Conventional single path case
SenderMobile
Receiver
Wired
100Mbps
Ethernet
802.11b
11Mbps
WLAN
AP1
AP2
AP1 only
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)
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Path Diversity Reduces Packet Loss
20 40 60 80 100 Infinite0
5
10
15
20
25
30
Avg
PacketLos
sRa
te(%)
Delay cutoff (ms)
AP1AP2BalancedSite SelectionOracle
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Path Diversity Reduces Burst Loss
20 40 60 80 100 Infinite
0
500
1000
1500
2000
Numb
erof
BurstEven
ts
Burst event = 2 or more consecutive losses
Delay cutoff (ms)
AP1AP2BalancedSite Selection
Oracle
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H.263 Video Performance
20 40 60 80 100 Infinite24
26
28
30
32
34
Mother and Daughter Sequence
Delay Cutoff (ms)
PSNR
(dB
)
AP1
AP2Balanced
Site Selection
PSNR gain = 1.6 3.0 dB
1/3 delay reduction
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Conclusion
All path diversity schemes help reduce burstlosses
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
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Complete Trace
0 0 1 1 2 2 3 3 4 4 5 5 66 7 7
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Single Stream from AP1
0 1 2 3 4 5 6 7
0 1 2 3 4 5 6 7
Discarded
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Single Stream from AP2
0 1 2 3 4 5 6 7
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Split stream from AP1 & AP2
0 0 1 1 2 2 3 3 4 4 5 5 66 7 7
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Split stream from AP1 & AP2
0 0
1 1
2 2
3 3
4 4
5 5
66
7 7
Discarded
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Split stream from AP1 & AP2
0 0
1 1
2 2
3 3
4 4
5 5
66
7 7
Discarded
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Split stream from AP1 & AP2
0 1
1 1
2 3
3 3
4 5
5 5
76
7 7
Discarded
Re-numbered
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Split stream from AP1 & AP2
0 1 2 3 4 5 76
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Fine-grained Site Selection
0 0 1 1 2 2 3 3 4 4 5 5 66 7 7
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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
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Oracle
0 0 1 1 2 2 3 3 4 4 5 5 66 7 7
Equivalent to repetition coding
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Oracle
0 0 1 1 2 2 3 3 4 4 5 5 66 7 7
Equivalent to repetition coding
0 1 2 3 4 5 6 7
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Existing Solutions
Robust video communication in lossy channel: Error resilient video compression
ARQDelay can be intolerable
Head of line blocking
FEC codingCan be inefficient
Receiver diversity antennaDoes not exploit path diversity
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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 thesame trace Difficulty in finding meaningful comparisons
between different scenarios
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20 40 60 80 100 Infinite0
5
10
15
20
25
30Average Packet Loss Rate
Percent
20 40 60 80 100 Infinite0
500
1000
1500
2000Number of Burst Events
Count
AP1AP2BalancedSite SelectionOracle
20 40 60 80 100 Infinite18
20
22
24
26
28PSNR (Foreman)
Delay Threshold (msec)
PS
NR(
dB)
20 40 60 80 100 Infinite24
26
28
30
32
34PSNR (Moth&Dthr)
Delay Threshold (msec)
PS
NR(
dB)
AP1AP2BalancedSite Selection
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0 5000 10000 15000 20000 25000-80
-60
-40
-20Received Signal Strength
Power(dBm
)
AP1AP2
0 5000 10000 15000 20000 250000
10
20
30Average Packet Loss Rate (5 sec window)
Percent
0 5000 10000 15000 20000 250000
5
10Number of Burst Losses (5 sec window)
Count
0 10 20 30 40 50 60 70 80 90
20
25
30
35
Video Quality
AveragePSNR
(dB)
Sequence Number
Oracle
AP1
AP2