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CS 414 – Multimedia Systems Design Lecture 30 – Buffer Management (Part 3)

CS 414 – Multimedia Systems Design Lecture 30 – Buffer Management (Part 3). Klara Nahrstedt Spring 2012. Administrative. MP2 posted MP2 Deadline – April 7, Saturday, 5pm. Covered Aspects of Multimedia. Audio/Video Presentation Playback. Image/Video Capture. Audio/Video

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CS 414 – Multimedia Systems Design Lecture 30 – Buffer Management (Part 3)

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  1. CS 414 – Multimedia Systems DesignLecture 30 – Buffer Management (Part 3) Klara Nahrstedt Spring 2012 CS 414 - Spring 2012

  2. Administrative MP2 posted MP2 Deadline – April 7, Saturday, 5pm. CS 414 - Spring 2012

  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 CS 414 - Spring 2012

  4. Outline • VoD Client Buffering Techniques • MinBuf • MaxBuf CS 414 - Spring 2012

  5. Client/Server Video-on-Demand System CS 414 - Spring 2012

  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) CS 414 - Spring 2012

  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 CS 414 - Spring 2012

  8. Client Buffer Constraints(Received amount of data Cr(t) should always be between ZOi (t) and ZOi (t) + Bufmax) CS 414 - Spring 2012

  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: CS 414 - Spring 2012

  10. Example (1) Receiving Thread Display Thread Display Thread Receiving Thread Phase 2 Phase 1 ZOi-1 ZOi ZOi+1 ZOi ti-2 ti ti-1 ti+1 ti-1 ti • Question: Given Video Stream with • 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 = 76800 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 CS 414 - Spring 2012

  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 = 76800 bytes, 20 fps (50 ms period) • ti – ti-1 = 50 ms = 50/1000 sec • Throughput = Z/ti – ti-1 = 76800 bytes/ (50/1000 sec) = 1536000 bytes/sec = 12288000 bits/sec = 12000 kbps = 11.71875 Mbps CS 414 - Spring 2012

  12. Implication of Maxbuf Strategy • Delivery schedule of VOD server and network may cause that Bufmaxbits 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: CS 414 - Spring 2012

  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 = 5.8599375 Mbits/8 Mbps = 0.83242 seconds • What is the Bufmax size if frames arrive on average every k = 1 second? • Answer:Bufmax= k*Throughput = 8 Mbits = 8388608 bits = 1048576 bytes = 13.653 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 CS 414 - Spring 2012

  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? CS 414 - Spring 2012

  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? CS 414 - Spring 2012

  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? CS 414 - Spring 2012

  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 CS 414 - Spring 2012

  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 CS 414 - Spring 2012

  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 CS 414 - Spring 2012

  20. Conclusion Will talk about media server in next lectures CS 414 - Spring 2012

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