1. Streaming To Mobile Users In A Peer-to-Peer Network
Jeonghun Noh, Mina Makar, and Bernd Girod
September 8, 2009
Image, Video and Multimedia Systems Group
Information Systems Laboratory
Stanford University

2. Heterogeneous Mobile Devices
Supporting heterogeneity
Screen size: 160x120, 320x240, 480x320, …
Channel characteristics : GPRS, EDGE, 3G, WiFi, …
Video capabilities: H.263, H.264, …
Mobile phone: more than a communication tool.
Personal entertainment center
Real time video streaming -> we need to consider phones’ differences
List the different categories
Conclusion: transcoding is required.
EMPHASIZE: TRANSCODING. 
Transcoding adapts video to mobile devices

3. Streaming to Mobile Device
server
Client-server streaming
Peer-to-peer streaming
Client-server model: server is a bottlenet
P2P is getting popular. <- Explain it.
However, technical challenge: unreliable peer.
In this talk, we propose a new coding scheme suitable for unreliable p2p network.
Mention: mobile phones are leeches in P2P network ?
Transcoding
Distributed transcoding
mobile phone

4. Outline Transcoding at multiple parents
IDT: Interleaved Distributed Transcoding
Encoding
Decoding under lossy condition
Advantage of multiple parents
Experimental results for distributed transcoding

5. Transcoding at Multiple Parents
Regular transcoding
Parent 1
Parent 2
Parent 3
Multiple Description Coding (MDC)
Parent 1
Parent 2
Parent 3
A train of frames represents the raw content to transcode.
Blue frames: encoded AND transmitted
Orange frames: encoded BUT not transmitted => waste of CPU power
MDC: requires multiple decoders at a mobile
IDT: produces a single bitstream at a mobile => suitable for mobile phones (any typical H264 decoder will work)
Interleaved Distributed Transcoding (IDT)
Parent 1
Parent 2
Parent 3

6. Distributed Transcoding: Encoding
Original video 1 2 3 4 5 6 7 8 9 10 11 12 13 14 I1 B2 B3 P4 B5 B6 P7 B8 B9
P10
B11
B12
P13
B14 5 9 1 1 5 9 13 13 14
Parent 1
DETAILS: We use the multiple-reference frame functionality of H.264/AVC baseline profile. Note that it is available in most mobile phones.
Fixed-line peers receive original video. -> first downsample it.
I frames, P frames.. Encoded only at a peer.
POINT 1: GOP size can be made independent of that of the original video
POINT 2: P frames in a substream are not used as a reference in the other substreams.
To complete substreams, the remaining frames are encoded as P frames, but they contain control bits for copying the entire previous frame => independent of the original video content => thus, very small. 6 10 2 2 6 10
Parent 2 7 11 3 1 3 7 11 13
Parent 3 8 12 4 1 4 8 13
Parent 4 I1 P2 P3 P4 P5 P6 P7 P8 P9
P10
P11
P12 I1 P2

7. Distributed Transcoding: Decoding1 5 9
Substream 1 1 1 1 5 5 5 9 9 9 2 6 10 2 6 10
Substream 2 2 2 6 6 10
parent loss 1 3 7 11
Substream 3
I frames are contained in a substream only.
Decoder assembles substreams.
For missing frames, decoder takes collocating copy frames.
The assembled stream -> can be fed to any typical H.264 decoder. Copy frames will perform copy error concealment at the bit level. 4 12
Substream 4 1 1 4 4 4 8 8 8 8
packet loss
Stream assembled at decoder

8. Advantage of Multiple Parents
parent loss
time
Effective frame rate
4 parents
Mention: the average number of missing frames is independent of the number of parents
Emphasize how we address peer churn challenge and its implication on video quality
Note: No packet loss (due to network congestion) is assumed
Trade-off: Severity and frequency of video degradation
4 parents
time
Effective frame rate

9. Experimental Setup
Video: Foreman and Mthr & Dthr (H.264 main profile, CIF, 30 fps)
Distributed transcoding (H.264 baseline profile)
Downsampling (CIF to QCIF)
Number of parents: 1 ~ 4 peers
Lossy scenario
Peer lifetime ~ Exp(1/90)
Reconnection time ~ Exp(1/3)
Gilbert model-based channel
0.01
0.99
0.85
Good
Bad
No packet loss
50% packet loss
0.15
[Konrad , Zhao, Joseph, Ludwig 2003]

10. End-to-End Rate-Distortion Performance
Foreman sequence

11. Distribution of Transcoding Load
60% load reduction
Mention: the vertical axis shows the average time of encoding one frame
Encoder: x264-based software encoder
CPU: Pentium 4 with 2.8GHz clock speed

12. Demo Setup SPPM peers SPPM mobile SPPM server
SPPM: Stanford Peer-to-Peer Multicast streaming system
[Baccichet, Noh, Setton, Girod 2007]

13. Graceful Video Degradation

14. Conclusions Video transcoding to meet mobile device heterogeneity
Distributed transcoding in a P2P network
Graceful video degradation against peer/packet loss
Distribution of transcoding load
Extensions
Rate control based on wireless channel condition
Application to fixed-line peers with limited downlink speed

15. Thank you!
Jeong-hun Noh

16. Comparison: MDC and IDT