1. Klara Nahrstedt Spring 2012
CS 414 – Multimedia Systems Design Lecture 30 – Buffer Management (Part 3)
Klara Nahrstedt
Spring 2012
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2. Administrative MP2 posted MP2 Deadline – April 7, Saturday, 5pm.
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3. Covered Aspects of Multimedia
Audio/Video
Presentation
Playback
Image/Video
Capture
Audio/Video
Perception/ Playback
Image/Video Information
Representation
Transmission
Transmission
Compression
Processing
Audio
Capture
Media
Server
Storage
Audio Information
Representation
A/V
Playback
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ORCHID Research Group
Department of Computer Science, University of Illinois at Urbana-Champaign

4. Outline VoD Client Buffering Techniques MinBuf MaxBuf
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5. Client/Server Video-on-Demand System
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6. Buffering Strategies at VOD Client
Minbuf - minimum buffering strategy
Minbuf minimizes buffering requirements at VOD client, but makes more demands on network (throughput and delay guarantees)
Maxbuf – maximum buffering strategy
Maxbuf buffers more than one unit of information and eases QoS guarantees demands on network
Buffering only up to a limit (Bufmax)
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7. Buffering Model
ZOi(t) – amount of bits in multimedia object (e.g., Video frame) Oi displayed at time t
Cr(t) – amount of bits received at receiver at time t
Two buffer states in each buffering strategy:
Starvation if Cr(t) ≤ ZOi(t)
If Cr(t) > ZOi(t), no starvation at VOD client
Overflow if Cr(t) ≥ ZOi(t) + Bufmax
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8. Client Buffer Constraints (Received amount of data Cr(t) should always be between ZOi (t) and ZOi (t) + Bufmax)
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9. Implications of Minbuf Strategy
With minbuf strategy – delivery time of a unit of information is the display time of previous unit
Minimum Throughput (Th) in bits per time unit required is
Delivery time instant of the first unit before start of display:
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10. Example (1) Question: Given Video Stream with Answer:
320x240 pixels, 20 fps, 8bits per pixel, uncompressed
What is the minimal buffer size if Minbuf technique is used?
Answer:
Buffer Size is of the application frame Z = bytes if receiving and display operations are done sequentially
One can also deploy a dual buffering scheme of 2* Z if receiving and display operations are done concurrently
ZOi
ZOi+1
ti+1 ti
ti-1
Display Thread
Receiving Thread
Receiving Thread
Display Thread
Phase 1
ZOi
ZOi-1
Phase 2 ti
ti-1
ti-2
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11. Example (2)
Question: What is the throughput under the above given video characteristics and minbuf strategy?
Answer: MinBuf Throughput needed in end-to-end fashion
Z = bytes, 20 fps (50 ms period)
ti – ti-1 = 50 ms = 50/1000 sec
Throughput = Z/ti – ti-1 = bytes/ (50/1000 sec) = bytes/sec = bits/sec = kbps = Mbps
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12. Implication of Maxbuf Strategy
Delivery schedule of VOD server and network may cause that Bufmax bits will be delivered every K seconds, where
K = Bufmax/Th, Th – throughput of network
Minimum Throughput required
∑ZOi – total size in bits of all objects that will be presented in stream:
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13. Example (1)
Question: Given Video frame 320x240 pixels, 8bits/pixel; Video frame rate 20 fps, uncompressed, and Network throughput 8 Mbps
What is the worst case arrival time of frames if Bufmax is 10 frames (holds ½ second of video frames)?
Answer: arrival time every k = Bufmax/Throughput = Mbits/8 Mbps = seconds
What is the Bufmax size if frames arrive on average every k = 1 second?
Answer: Bufmax = k*Throughput = 8 Mbits = bits = bytes = video frames ; round up to 14 frames; Bufmax should be 14 frames if throughput is 8 Mbps and arrival of frames is approximately every 1 second
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14. Example (2)
Question: Given video characteristics of 320x240 pixels, 8 bpp, 20 fps, uncompressed, length of the move is 1 minute, Bufmax holds 1 second of frames, what is the minimum throughput we would need if we use MaxBuf strategy?
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15. Example (3)
Given video characteristics of 320x240 pixels, 8bpp, 20fps, MJPEG compressed with I frames 50% compression (1:2) of each frame;
Question: what would be the arrival time (k) of frames if we have buffer size, holding 1 second frames, and network throughput of 10 Mbps?
Question: what would be the minimal throughput needed from the network under above conditions if the video clip would be 30 seconds long?
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16. Example (4)
Given MPEG-2 compressed video with GOP of IPBBPBBPBBI, average I frame size of 10KB, average P frame size of 6 KB, and average B frame size of 2 KBytes.
Question: What would be the buffer size (express it in bytes and time duration) if network throughput is 4 Mbps and arrival time of each compressed frame is on average every 500 ms?
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17. Standard Video Buffering
Video delivered over “deterministic” channel (satellite, cable, digital TV broadcasting) simple
Small buffer (minbuf technique)
Data arrives to the media player with deterministic delay and rate and infrequent data drops
Video delivered over IP networks problematic
Big buffers (maxbuf technique)
No guarantees in IP networks
Streaming servers manage simple buffering
Data sent as quickly as possible; when buffer full, (reached predefined threshold), video playback starts and server continues to send data
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18. Implementation Issues
Buffers for Uncompressed Periodic Streams
Use circular buffers as prefetch buffers with maxbuf strategy
Buffers for Compresses Periodic Streams
Use circular buffers, but carefully consider the size of buffer unit (depending on MPJEG or MPEG) with maxbuf strategy
For MPEG may consider dynamic buffer allocation
Buffers for Control Information
Use priority queues or FIFO
Buffers for Non-RT Data
Use maxbuf strategy with static buffer allocation
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19. Implementation Issues
Dual-Threshold Buffering (in Flash Media Server)
Buffer has two thresholds:
When buffer filled with data up to first threshold, start playback
After that continue buffering until second, higher threshold
Advantage:
assures fast start, and at the same time
provides fairly high resilience to bandwidth fluctuation
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20. Conclusion Will talk about media server in next lectures
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