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.

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

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

Classification of Media Type Sound Video Image Animation Text Graphics Captured From real world Synthesized By computer Discrete Continuous

Text Plain text Rich 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

Plain Text vs. Rich Text An example of Plain text Example of Rich text

Graphics Revisable document that retains structural information Consists of objects such as lines, curves, circles, etc Usually generated by graphic editor of computer programs Example of graphics (FIG file)

Images 2D matrix consisting of pixels Have no structural information 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 Capture and A/D conversion Digital still image Synthesized image Scanned Camera

Images (cont.) Examples of images Binary image – pixel depth 1 Gray-scale – pixel depth 8 Color image – pixel depth 24 Gray-scale image color image Binary image

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

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

Networked Multimedia Local vs. networked multimedia Image server 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. Image server A scenario of multimedia networking Internet Video server

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

Technologies of Multimedia Networking Challenges of multimedia networking Conflict between media size and bandwidth limit of the network Conflict between the user requirement of multimedia application and the best-effort network 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

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

Classes of multimedia applications Streaming stored audio and video Streaming live audio and video Real-time interactive audio and video

Streaming Stored Multimedia: What is it? 100% streaming: at this time, client playing out early part of video, while server still sending later part of video 3. video received, played out at client Cumulative data 2. video sent 1. video recorded network delay time

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?

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

Streaming Multimedia: Client Buffering constant bit rate video transmission variable network delay client video reception constant bit rate video playout at client client playout delay Cumulative data buffered video time Client-side buffering, playout delay compensate for network-added delay, delay jitter

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

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?

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!

Progressive Download browser GETs metafile browser launches player, passing metafile player contacts server server downloads audio/video to player

Streaming from a streaming server This architecture allows for non-HTTP protocol between server and media player Can also use UDP instead of TCP.

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

Streaming Multimedia: UDP or TCP? 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

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

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:

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

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

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

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

Summary of QoS Principles