Low Latency Wireless Video Over 802.11 Networks Using Path Diversity
John Apostolopolous
Wai-tian Tan
Mitchell TrottHewlett-Packard Laboratories
Allen MiuMIT Laboratory for Computer Science
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 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 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
AP1
Microwave(Interferer)
Sender MobileReceiver
Wired Wireless
AP2
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
SenderMobile
Receiver
Wired100MbpsEthernet
802.11b11MbpsWLAN
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
Diversity Scenarios
• Conventional single path case
SenderMobile
Receiver
Wired100MbpsEthernet
802.11b11MbpsWLAN
AP1
AP2
AP1 onlyAP2 only
Diversity Scenarios
• Conventional single path case
SenderMobile
Receiver
Wired100MbpsEthernet
802.11b11MbpsWLAN
AP1
AP2
AP1 onlyAP2 only
• Balanced split stream (non-adaptive)
Diversity Scenarios
• Conventional single path case
SenderMobile
Receiver
Wired100MbpsEthernet
802.11b11MbpsWLAN
AP1
AP2
AP1 onlyAP2 only
• Balanced split stream (non-adaptive)• Adaptive, fine-grain site selection (based on loss rate)
Diversity Scenarios
• Conventional single path case
SenderMobile
Receiver
Wired100MbpsEthernet
802.11b11MbpsWLAN
AP1
AP2
AP1 onlyAP2 only
• Balanced split stream (non-adaptive)• Adaptive, fine-grain site selection (based on loss rate)
• Oracle (optimal adaptive, can be realized by repetition coding)
Path Diversity Reduces Packet Loss
20 40 60 80 100 Infinite0
5
10
15
20
25
30A
vg P
acke
t L
oss
Rat
e (%
)
Delay cutoff (ms)
AP1AP2BalancedSite SelectionOracle
Path Diversity Reduces Burst Loss
20 40 60 80 100 Infinite
0
500
1000
1500
2000
Num
ber
of B
urst
Eve
nts
Burst event = 2 or more consecutive losses
Delay cutoff (ms)
AP1AP2BalancedSite SelectionOracle
H.263 Video Performance
20 40 60 80 100 Infinite24
26
28
30
32
34Mother and Daughter Sequence
Delay Cutoff (ms)
PS
NR
(d
B)
AP1AP2BalancedSite Selection
PSNR gain = 1.6 – 3.0 dB1/3 delay reduction
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
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
Existing Solutions
Robust video communication in lossy channel: Error resilient video compression ARQ
Delay can be intolerableHead of line blocking
FEC codingCan be inefficient
Receiver diversity antennaDoes not exploit path diversity
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
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
20 40 60 80 100 Infinite0
5
10
15
20
25
30Average Packet Loss Rate
Pe
rce
nt
20 40 60 80 100 Infinite0
500
1000
1500
2000Number of Burst Events
Co
un
t
AP1AP2BalancedSite SelectionOracle
20 40 60 80 100 Infinite18
20
22
24
26
28PSNR (Foreman)
Delay Threshold (msec)
PS
NR
(d
B)
20 40 60 80 100 Infinite24
26
28
30
32
34PSNR (Moth&Dthr)
Delay Threshold (msec)
PS
NR
(d
B)
AP1AP2BalancedSite Selection
0 5000 10000 15000 20000 25000-80
-60
-40
-20Received Signal Strength
Po
we
r (d
Bm
)AP1AP2
0 5000 10000 15000 20000 250000
10
20
30Average Packet Loss Rate (5 sec window)
Pe
rce
nt
0 5000 10000 15000 20000 250000
5
10Number of Burst Losses (5 sec window)
Co
un
t
0 10 20 30 40 50 60 70 80 90
20
25
30
35
Video Quality
Av
era
ge
PS
NR
(d
B)
Sequence Number
OracleAP1AP2
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