Video Streaming Over Wireless: Video Streaming Over Wireless: Where TCP is Not Enough Where TCP is Not Enough

Xiaoqing Zhu, Jatinder Pal Singh and Bernd GirodXiaoqing Zhu, Jatinder Pal Singh and Bernd Girod

Information Systems LaboratoryInformation Systems Laboratory

Stanford UniversityStanford University

X. Zhu et al: Video Streaming over Wireless April 1, 2008 2

Wireless Home NetworkingWireless Home Networking

54 Mbps

6 Mbps

24 Mbps

12 Mbps

X. Zhu et al: Video Streaming over Wireless April 1, 2008 3

• TCP throughput over wireless• Demo: video streaming vs. file transfer• Comparison with ideal solution• What’s missing in TCP?

OutlineOutline

X. Zhu et al: Video Streaming over Wireless April 1, 2008 4

Heterogeneity in Wireless Link SpeedsHeterogeneity in Wireless Link Speeds

C1Cl

CN

Channel Time

…

X. Zhu et al: Video Streaming over Wireless April 1, 2008 5

UDP Throughput over WirelessUDP Throughput over Wireless

10 20 30 40 500

5

10

15

20

25

30

Nominal Speed of Second Link (Mbps)

Thr

ough

put

(Mbp

s)

Simulation in NS2, for 802.11a network

Stream 1, alone

Stream 2, alone

Stream 1, shared

Stream 2, shared

54Mbps

) ) ) ) )

) ) ) ) )

Stream 2

Stream 1

6 ~ 54 Mbps

X. Zhu et al: Video Streaming over Wireless April 1, 2008 6

TCP Throughput over WirelessTCP Throughput over Wireless

10 20 30 40 500

5

10

15

20

Nominal Speed of Second Link (Mbps)

Thr

ough

put (

Mbp

s)

54Mbps

) ) ) ) )

) ) ) ) )

Stream 2

Stream 1

6 ~ 54 Mbps

Simulation in NS2, for 802.11a network

Stream 1, alone

Stream 2, alone

Stream 1, shared

Stream 2, shared

X. Zhu et al: Video Streaming over Wireless April 1, 2008 7

Overhead of TCP ACKOverhead of TCP ACK

6 9 12 18 24 36 48 540

20

40

60

80

100

Nominal Link Speed (Mbps)

Pe

rce

nta

ge

of

Ch

an

ne

l Tim

e (

%)

Data ACK

X. Zhu et al: Video Streaming over Wireless April 1, 2008 8

Demo: Two NodesDemo: Two Nodes

Link Speed: 11 Mbps

Throughput : 4.4 MbpsShared : 1.0 Mbps (~ 20 % channel time)

Link Speed: 2 Mbps

Throughput : 1.4 Mbps

Shared : 1.0 Mbps (~ 70% channel time)

Video Source @ 2Mbps

File Transfer Source: 3.7MB

Scenario AScenario A

X. Zhu et al: Video Streaming over Wireless April 1, 2008 9

TCP PerformanceTCP Performance

Video Streaming @ 2 Mbps

Time

Ra

te

…

File Transfer @ 1.0 Mbps

Share of Channel Time

23%

71%

6%

Video Streaming

File Transfer

ContentionOverhead

~ 30 s

X. Zhu et al: Video Streaming over Wireless April 1, 2008 10

What Could Have Happened …What Could Have Happened …

Share of Channel Time

45%

50%

5%

Video Streaming

File Transfer

ContentionOverhead

Ra

te

Time

…

Video Streaming @ 2 Mbps

File Transfer @ 0.7 Mbps

~ 42 s

X. Zhu et al: Video Streaming over Wireless April 1, 2008 11

Scenario BScenario B

Link Speed: 54 Mbps

Throughput : 20 Mbps

Shared : 1.2 Mbps(~ 6% Channel Time)

Link Speed: 2 Mbps

Throughput : 1.4 Mbps

Shared : 1.2 Mbps(~ 85% Channel Time)

Video Source @ 3 Mbps

File Transfer Source: 3.7MB

X. Zhu et al: Video Streaming over Wireless April 1, 2008 12

TCP PerformanceTCP Performance

Time

Video Streaming @ 3 Mbps

File Transfer @ 1.2 Mbps

~ 25 s

Share of Channel Time6%

86%

8%

Video Streaming

File Transfer

ContentionOverhead

Ra

te …

X. Zhu et al: Video Streaming over Wireless April 1, 2008 13

What Could Have Happened …What Could Have Happened …

Ra

te

Time

Video Streaming @ 3 Mbps

File Transfer @ 1.2 Mbps

…

~ 27 s

Share of Channel Time

15%

79%

6%

Video Streaming

File Transfer

ContentionOverhead

X. Zhu et al: Video Streaming over Wireless April 1, 2008 14

What’s Missing in TCP?What’s Missing in TCP?

• Awareness of application’s utility function– For file transfer, aggregate rate matters– For video streaming, instantaneous rate matters– Video streams differ in their rate-quality tradeoffs

Utility function only needed at the source

• Knowledge of wireless link heterogeneity– Channel time shared among competing links– Congestion due to neighboring transmissions– High rate over a fast link vs. low rate over a slow link

End-to-end measurement no longer suffices

Notion of fairness should be revisited

Clean Slate Design Clean Slate Design or or

Evolution? Evolution?

X. Zhu et al: Video Streaming over Wireless April 1, 2008 16

TCP Throughput over WirelessTCP Throughput over Wireless

1.22 MTUR

RTT p

1.22 MTUR

RTT p

packet size

roundtrip time

packetloss rate

data rate

[Mahdavi, Floyd 1997]

[Floyd et al. 2000]

• Per-packet fairness at the MAC layer

• Similar end-to-end observations of p, and RTT for competing wireless links

• Approximately equal rate, regardless of link speed

[Heusse et al. 2003]

X. Zhu et al: Video Streaming over Wireless April 1, 2008 17

TCP Throughput over WirelessTCP Throughput over Wireless

54Mbps

) ) ) ) )

) ) ) ) )

Stream 2

Stream 1

6 ~ 54 Mbps

Simulation in NS2, for 802.11a network