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Transmisión de Datos Multimedia – http://www.grc.upv.es/docencia/tdm – Master IC 2007/2008
Tema 0:Transmisión de Datos MultimediaTema 0:Transmisión de Datos Multimedia
�Clases de aplicaciones multimedia
�Redes basadas en IP y QoS
Computer Networking: A Top Down Approach Featuring the Internet,
3rd edition. Jim Kurose, Keith Ross
Addison-Wesley, July 2004.
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What is multimedia?
� Definition of multimedia
� Hard to find a clear-cut definition
� In general, multimedia is an integration of text, graphics, still and moving images, animation, sounds, and any other medium where every type of information can be represented, stored, transmitted and processed digitally
� Characteristics of multimedia
� Digital – key concept
� Integration of multiple media type, usually including video or/and audio
� May be interactive or non-interactive
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Various Media Types
� Text, Graphics, image, video, animation, sound, etc.
� Classifications of various media types
� Captured vs. synthesized media
� Captured media (natural) : information captured from the real world– Example: still image, video, audio
� Synthesized media (artificial) : information synthesize by the computer– Example: text, graphics, animation
� Discrete vs. continuous media
� Discrete media: space-based, media involve the space dimension only– Text, Image, Graphics
� Continuous media: time-based, media involves both the space and the time dimension– Video, Sound, Animation
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Classification of Media Type
SoundSound VideoVideo
ImageImage
AnimationAnimation
TextText GraphicsGraphics
Captured
From real world
Synthesized
By computer
Discrete Discrete
Continuous Continuous
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Text
� Plain text
� Unformatted
� Characters coded in binary form
� ASCII code
� All characters have the same style and font
� Rich text
� Formatted
� Contains format information besides codes for characters
� No predominant standards
� Characters of various size, shape and style, e.g. bold, colorful
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Plain Text vs. Rich Text
An example of Plain text Example of Rich text
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Graphics
� Revisable document that retains structural information
� Consists of objects such as lines, curves, circles, etc
� Usually generated by graphic editor of computer programs
-4
-2
0
2
4
-4
-2
0
2
4
-10
-5
0
5
10
Example of graphics (FIG file)
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Images
� 2D matrix consisting of pixels
� Pixel—smallest element of resolution of the image
� One pixel is represented by a number of bits
� Pixel depth– the number of bits available to code the pixel
� Have no structural information
� Two categories: scanned vs. synthesized still image
Computer
software
Computer
software
Capture and
A/D conversion
Capture and
A/D conversion
Digital still imageDigital still image
Synthesized
image
Scanned
image
Camera
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Images (cont.)
� Examples of images
� Binary image – pixel depth 1
� Gray-scale – pixel depth 8
� Color image – pixel depth 24
Binary image
Gray-scale imagecolor image
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Video vs. Animation
� Both images and graphics can be displayed as a succession of view which create an impression of movement
� Video – moving images or moving pictures
� Captured or Synthesized
� Consists of a series of bitmap images
� Each image is called a frame
� Frame rate: the speed to playback the video (frame per second)
� Animation – moving graphics
� Generated by computer program (animation authoring tools)
� Consists of a set of objects
� The movements of the objects are calculated and the view is updated at playback
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1
Sound
� 1-D time-based signal
� Speech vs. non-speech sound
� Speech – supports spoken language and has a semantic content
� Non-speech – does not convey semantics in general
� Natural vs. structured sound
� Natural sound – Recorded/generated sound wave represented as digital signal
� Example: Audio in CD, WAV files
� Structured sound – Synthesize sound in a symbolic way
� Example: MIDI file
0 1 0 0 2 0 0 3 0 0 4 0 0 5 0 0 6 0 0 7 0 0 8 0 0 9 0 0 1 0 0 0- 0 . 2
- 0 . 1 5
- 0 . 1
- 0 . 0 5
0
0 . 0 5
0 . 1
0 . 1 5
0 . 2
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Networked Multimedia
� Local vs. networked multimedia
� Local: storage and presentation of multimedia information in standalone computers
� Sample applications: DVD
� Networked: involve transmission and distribution of multimedia information on the network
� Sample applications: videoconferencing, web video broadcasting, multimedia Email, etc.
InternetInternetVideo server
Image serverA scenario of multimedia networking
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Consideration of Networked Multimedia
� Requirements of multimedia applications on the network
� Typically delay sensitive
� end-to-end delay
� delay jitter:– Jitter is the variability of packet delays within the same packet stream
� Quality requirement
� Satisfactory quality of media presentation
� Synchronization requirement
� Continuous requirement (no jerky video/audio)
� Can tolerant some degree of information loss
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Technologies of Multimedia Networking
� Challenges of multimedia networking
1. Conflict between media size and bandwidth limit of the network
2. Conflict between the user requirement of multimedia application and the best-effort network
3. How to meet different requirements of different users?
� Media compression – reduce the data volume
Address the 1st challenge
� Image compression
� Video compression
� Audio compression
� Multimedia transmission technology
Address the 2nd and 3rd challenges
� Protocols for real-time transmission
� Rate / congestion control
� Error control
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Multimedia Networking Systems
� Live media transmission system
� Capture, compress, and transmit the media on the fly (example?)
� Send stored media across the network
� Media is pre-compressed and stored at the server. This system delivers the stored media to one or multiple receivers. (example?)
� Differences between the two systems
� For live media delivery:
� Real-time media capture, need hardware support
� Real-time compression– speed is important
� Compression procedure can be adjusted based on network conditions
� For stored media delivery
� Offline compression – better compression result is important
� Compression can not be adjusted during transmission
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Classes of multimedia applications
� Streaming stored audio and video
� Streaming live audio and video
� Real-time interactive audio and video
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Streaming Stored Multimedia: What is it?
1. videorecorded
2. videosent
3. video received,played out at client
Cum
ulat
ive d
ata
streaming: at this time, client playing out early part of video, while server still sending laterpart of video
networkdelay
time
t>0
100%
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Streaming vs. Download of Stored Multimedia Content
� Download: Receive entire content before playback begins
� High “start-up” delay as media file can be large
� ~ 4GB for a 2 hour MPEG II movie
� Streaming: Play the media file while it is being received
� Reasonable “start-up” delays
� Reception Rate >= playback rate. Why?
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Streaming Stored Multimedia: Interactivity
VCR-like functionality: client can pause, rewind, FF, push slider bar
• 10 sec initial delay OK• 1-2 sec until command effect OK• RTSP often used (more later)
timing constraint for still-to-be transmitted data: in time for playout
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constant bit rate video
transmission
Cum
ulat
ive d
ata
time
variablenetworkdelay
client videoreception
constant bit rate video
playout at client
client playoutdelay
buf
fere
dvi
deo
Streaming Multimedia: Client Buffering
� Client-side buffering, playout delay compensate for network-added delay, delay jitter
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Streaming Multimedia: Client Buffering
� Client-side buffering, playout delay compensate for network-added delay, delay jitter
bufferedvideo
variable fillrate, x(t)
constantdrain
rate, d
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Interactive, Real-Time Multimedia
applications: IP telephony, video conference, distributed interactive worlds
� end-end delay requirements:
� audio: < 150 msec good, < 400 msec OK
� includes application-level (packetization) and network delays
� higher delays noticeable, impair interactivity
� session initialization
� how does callee advertise its IP address, port number, encoding algorithms?
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Internet multimedia: simplest approach
audio, video not streamed:
� no, “pipelining,” long delays until playout!
� audio or video stored in file
� files transferred as HTTP object
� received in entirety at client
� then passed to player
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Progressive Download
� browser GETs metafile
� browser launches player, passing metafile
� player contacts server
� server downloads audio/video to player
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Streaming from a streaming server
� This architecture allows for non-HTTP protocol between server and media player
� Can also use UDP instead of TCP.
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Multimedia Over Today’s Internet
� TCP/UDP/IP: “best-effort service”
� no guarantees on delay, loss
� But multimedia apps requires QoS and level of performance to be effective!
� Today’s Internet multimedia applications use application-level techniques to mitigate (as best possible) effects of delay, loss
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Streaming Multimedia: UDP or TCP?
UDP
� server sends at rate appropriate for client (oblivious to network congestion!)
� often send rate = encoding rate = constant rate
� then, fill rate = constant rate - packet loss
� short playout delay (2-5 seconds) to compensate for network delay jitter
� error recover: time permitting
TCP
� send at maximum possible rate under TCP
� fill rate fluctuates due to TCP congestion control
� larger playout delay: smooth TCP delivery rate
� HTTP/TCP passes more easily through firewalls
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Multimedia, Quality of Service: What is it?
Multimedia applications: network audio and video(“continuous media”)
network provides application with level of performance needed for application to function.
QoS
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Improving QOS in IP Networks
� Thus far: “making the best of best effort”
� Future: next generation Internet with QoS guarantees
� RSVP: signaling for resource reservations
� Differentiated Services: differential guarantees
� Integrated Services: firm guarantees
� simple model for sharing and congestion studies:
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Principles for QOS Guarantees
� Example: 1Mbps IPphone, FTP share 1.5 Mbps link.
� bursts of FTP can congest router, cause audio loss
� want to give priority to audio over FTP
packet marking needed for router to distinguish
between different classes; and new router policy
to treat packets accordingly
Principle 1
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Principles for QOS Guarantees (more)
� what if applications misbehave (audio sends higher than declaredrate)
� policing: force source adherence to bandwidth allocations
� marking and policing at network edge:
� similar to ATM UNI (User Network Interface)
provide protection (isolation) for one class from
others
Principle 2
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Principles for QOS Guarantees (more)
� Allocating fixed (non-sharable) bandwidth to flow: inefficient use of bandwidth if flows doesn’t use its allocation
While providing isolation, it is desirable to use resources as efficiently as possible
Principle 3
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Principles for QOS Guarantees (more)
� Basic fact of life: can not support traffic demands beyond link capacity
Call Admission: flow declares its needs, network may block call (e.g., busy signal) if it cannot meet needs
Principle 4
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Summary of QoS Principles
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