Streaming Video
Gabriel Nell
UC Berkeley

Outline
 Scalable MPEG-4 video
– Layered coding method
– Integrated transport-decoder buffer model
 RAP streaming congestion control
 Quality adaptation over RAP
 Prefetching VBR prerecorded video

Streaming Video Design
Considerations
 Stream must adapt to widely varying
Internet link speeds and client processing power
 Server should be low complexity
 Client-driven flow control and lost packet recovery

Layered Video
 Prediction-based base layer
– DCT motion-compensated
– Carries minimally-acceptable quality video
 Fine-granular enhancement layer
– Can be decoded progressively
– Fully utilizes available bandwidth

Enhancement Layer Details
 Highly efficient wavelet-based compression
 Organize wavelet coefficients into spatial orientation trees
– Decaying spectrum hypothesis: energies of wavelet coefficients decay as they move from the root
– Allows pruning to select most important coefficients

Enhancement Layer Details
 Wavelet coefficient organization (cont’d)
– “Significant” coefficients are identified, most significant bit transmitted
– Second MSB for each significant coefficient transmitted, then third, etc.
– After complete information about significant coefficients transmitted, change threshold and repeat

Enhancement Layer
Performance
 Simulation shows PSNR improvement proportional to amount of enhancement layer received

PSNR at Different Rates

Integrated Transport-Decoder
 Receiver buffer model
– Transport delay parameters

Packet loss, jitter, etc.
– Video encoder buffer constraints
 Min and max buffer bounds

Integrated Transport-Decoder
 Buffer divided into temporal segments
 Buffer size optimized to accommodate
–
–
Start-up delay
Retransmission delay
 Flow control regulated to match bottleneck link
 Retransmitted packets have higher priority than enhancement layer packets

ITD Performance
 Tested 15kbps video over 33.6kbps modem locally and across the country
 Performance measured by number of retransmission requests, failures, successes
 Performance was good, proportional to start-up delay (3-7 seconds)

RAP Congestion Control
 RAP: Rate Adaptation Protocol
 Motivation: make realtime streaming applications behave properly; “TCPfriendly”

RAP Protocol
 Source sends data packets with sequence numbers
 Receiver acknowledges packets
 Congestion detection
– Indicated by lost packets
– Variable such as RTT calculated similar to the way TCP calculates them

RAP Protocol
 Sender rate-control decision function
– If no congestion detected, periodically increase transmission rate
– If congestion detected, immediately decrease transmission rate
 Additional Features
– Clustered loss detection
– Fine-grain rate adaptation

RAP Performance
 Simulation shows TCP-friendliness
 Performance is a little different from TCP
– TCP more sensitive to number of outstanding packets
– RAP more aggressive due to clustered loss detection and fine-grain rate adaptation

Quality Adaptation for
Congestion Control
 Motivation: single video server streaming on demand to heterogeneous clients
 Quality Adaptation: adjust the quality of the stream during playback to maximize quality given bandwidth
 Use hierarchical (layered) approach

Layered Quality Adaptation
 Coarse-grain mechanism for adding and removing layers
 Adding a layer
– When available bandwidth is greater than existing consumption plus the new layer
– When the receiver has sufficient buffer space available
 Layers dropped immediately on congestion

Layered Quality Adaptation
 Inter-layer buffer allocation
– Keep track of buffered data, send less to clients with data already buffered
– Provide more buffering for lower layers for increased protection
– Use smoothing

If there is bandwidth for 2.9 layers, send 3 layers 90% of the time
 Performance
– Depending on smoothing parameters, buffer allocation efficiency ranged from 96% to 99.99% in simulation

Prefetching VBR Prerecorded
Video
 Prerecorded video
– Size (in bits) for every frame is known in advance
– Frames can be prefetched into memory
– Nature of VBR means there will be periods of link underutilization
 Collaborative protocol manages buffers of all clients

JSQ Prefetch
 JSQ: join shortest queue
– Measure buffer length in terms of number of frames rather than bits
– Balance number of frames across all prefetch buffers
– At each time interval, the server iteratively adds a frame to the buffer with the smallest number of frames
– Allows near-100% link utilization, quick recovery from pauses and time jumps

Decentralized Prefetching
 Multiple video servers
 Window-based flow control
 Dynamic send window: increase send window to clients with depleted buffers

Performance
 Allows near-100% link utilization, with low probability of buffer starvation

Questions?