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

Complete Trace

0 0 1 1 2 2 3 3 4 4 5 5 66 7 7 …

Single Stream from AP1

0 1 2 3 4 5 6 7 …

0 1 2 3 4 5 6 7

Discarded

Single Stream from AP2

0 1 2 3 4 5 6 7 …

Split stream from AP1 & AP2

0 0 1 1 2 2 3 3 4 4 5 5 66 7 7 …

Split stream from AP1 & AP2

0 0

1 1

2 2

3 3

4 4

5 5

66

7 7

…

Discarded

Split stream from AP1 & AP2

0 0

1 1

2 2

3 3

4 4

5 5

66

7 7

…

Discarded

Split stream from AP1 & AP2

0 1

1 1

2 3

3 3

4 5

5 5

76

7 7

…

Discarded

Re-numbered

Split stream from AP1 & AP2

0 1 2 3 4 5 76 …

Fine-grained Site Selection

0 0 1 1 2 2 3 3 4 4 5 5 66 7 7 …

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

0 0 1 1 2 2 3 3 4 4 5 5 66 7 7 …

Equivalent to repetition coding

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

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