CS 414 – Multimedia Systems Design Lecture 30 – Buffer Management (Part 3)
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Transcript of CS 414 – Multimedia Systems Design Lecture 30 – Buffer Management (Part 3)
CS 414 - Spring 2012
CS 414 – Multimedia Systems Design Lecture 30 – Buffer Management (Part 3)
Klara Nahrstedt
Spring 2012
Administrative MP2 posted MP2 Deadline – April 7, Saturday, 5pm.
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Covered Aspects of Multimedia
Image/VideoCapture
MediaServerStorage
Transmission
CompressionProcessing
Audio/VideoPresentationPlaybackAudio/Video
Perception/ Playback
Audio InformationRepresentation
Transmission
AudioCapture
A/V Playback
Image/Video InformationRepresentation
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Outline
VoD Client Buffering TechniquesMinBufMaxBuf
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Client/Server Video-on-Demand System
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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 strategyMaxbuf buffers more than one unit of information
and eases QoS guarantees demands on networkBuffering only up to a limit (Bufmax)
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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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Client Buffer Constraints(Received amount of data Cr(t) should always be between ZOi (t) and ZOi (t) + Bufmax)
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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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1
minmin
ii
Oibuf
tt
ZTh
bufOi
x Th
Zt
minmin
Example (1) 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
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ZOiZOi-1
ti-2ti-1ti
Display ThreadReceiving Thread
ZOiZOi+1
ti+1titi-1
Display Thread Receiving Thread
Phase 1 Phase 2
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
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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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DurationDisplayStream
BufZTh
n
iOi
buf
__1
maxmaxmin
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
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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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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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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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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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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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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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Conclusion Will talk about media server in next
lectures
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