Transcript Transmisión de Datos Multimedia - GRC

Tema 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.
Transmisión de Datos Multimedia –
http://www.grc.upv.es/docencia/tdm
– Master IC 2007/2008
Transmisión de Datos Multimedia - Master IC 2007/2008
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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
Transmisión de Datos Multimedia - Master IC 2007/2008
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
Sound
Video
Continuous
Image
Discrete
Captured
From real world
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Animation
Continuous
Text
Graphics
Discrete
Synthesized
By computer
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Text
 Plain text
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Unformatted
Characters coded in binary form
ASCII code
All characters have the same style and font
 Rich text
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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
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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
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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
Digital still image
Camera
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Computer
software
Synthesized
image
Capture and
A/D conversion
Scanned
image
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Images (cont.)
 Examples of images
 Binary image – pixel depth 1
 Gray-scale – pixel depth 8
 Color image – pixel depth 24
Gray-scale
colorimage
image
Binary 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
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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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Sound
 1-D time-based signal
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 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
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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.
A scenario of multimedia networking
Video server
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Internet
Image server
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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
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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?
t>0
100%
1. video
recorded
2. video
sent
network
delay
3. video received,
played out at client
streaming: at this time, client
playing out early part of video,
while server still sending later
part of video
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time
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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?
Transmisión de Datos Multimedia - Master IC 2007/2008
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
variable
network
delay
client video
reception
constant bit
rate video
playout at client
buffered
video
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Streaming Multimedia: Client Buffering
client playout
delay
 Client-side buffering, playout delay compensate for networkadded delay, delay jitter
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time
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Streaming Multimedia: Client Buffering
 Client-side buffering, playout delay compensate for network-added
delay, delay jitter
constant
drain
rate, d
variable fill
rate, x(t)
buffered
video
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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 or video stored in file
 files transferred as HTTP object
 received in entirety at client
 then passed to player
audio, video not streamed:
 no, “pipelining,” long delays until playout!
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Progressive Download
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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”)
QoS
network provides
application with level of
performance needed for
application to function.
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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
Principle 1
packet marking needed for router to distinguish
between different classes; and new router policy
to treat packets accordingly
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Principles for QOS Guarantees (more)
 what if applications misbehave (audio sends higher than declared
rate)
 policing: force source adherence to bandwidth allocations
 marking and policing at network edge:
 similar to ATM UNI (User Network Interface)
Principle 2
provide protection (isolation) for one class from
others
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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
Principle 3
While providing isolation, it is desirable to use
resources as efficiently as possible
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Principles for QOS Guarantees (more)
 Basic fact of life: can not support traffic demands beyond link
capacity
Principle 4
Call Admission: flow declares its needs, network
may block call (e.g., busy signal) if it cannot meet
needs
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Summary of QoS Principles