Multimedia Networking - Computer Networks - Lecture Slides, Slides of Computer Networks

Some concept of Computer Networks are Aloha Ethernet, Application Layer Protocols, Basic Building Blocks, Computer Networking, Content Free Protocols, Medium Access Sub Layer, Packetised Filetransmission. Main points of this lecture are: Multimedia Networking, Computer Networking, Multimedia, Streaming Stored, Real-Time Interactive, Excludes, Entire File, Received, Multicasting, Tolerate

Typology: Slides

2012/2013

Uploaded on 04/27/2013

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Computer Networking Technology II
Multimedia Networking
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Computer Networking Technology II

Multimedia Networking

Multimedia Networking

  • Recent years have seen massive growth in

Internet audio and video apps

  • Streaming video, IP telephony, Internet radio, teleconferencing, interactive games, distance learning, etc.
  • Older Internet apps (email, WWW, FTP) were

very elastic in bandwidth needs, but

multimedia is much fussier – delay sensitive

  • But they’re also more loss-tolerant

Streaming stored AV

  • Here, clients request on-demand compressed AV files that are stored on servers - Content may include lectures, music, TV, etc. - CNN video, YouTube, etc.
  • Main features of this app type are:
    • Stored, prerecorded media; pause, ffwd, rew
    • Streaming , hence can play part of the media while downloading more of it
    • Continuous play out – should keep original timing

Streaming live AV

  • This is live broadcast of radio or TV over the Internet - Can’t fast forward, since it hasn’t happened yet, but local storage of what’s been received can allow pausing and rewinding in some cases - Often accomplished using IP multicasting or IPTV, but more often done via separate unicast streams - Also has continuous play out, can tolerate some startup delay

Real-time interactive AV

  • This class of apps allows interaction between

people at both ends (or many ends) of a

connection, such as Internet telephony or

teleconferencing

  • Apps can be integrated, e.g. Web-phone
  • Microsoft Live Messenger, Live Meeting (was NetMeeting), WebEx, GoToMeeting, Skype
  • Transmission delays under 150 ms are good, under 400 ms is okay, and over 400 ms is bad

Multimedia Challenges

  • The Internet provides best-effort service
    • No guarantees from IP on when stuff will get there, consistency in delay times, if it will get there, or getting there in order
    • All packets are equal in the Internet!
  • Streaming stored or live AV has been pretty successful, real-time interactive AV less so
  • To help, use tricks to make transmission smoother

Fix the Internet!

  • Some argue the Internet should allow for end-

to-end bandwidth guarantees, like virtual

circuit networks can provide

  • Would require massive changes to routers to establish fixed paths for some service types
  • Hard guarantees mean you’re certain to receive the QoS you paid for
  • Soft guarantees mean you’re likely to receive the QoS you paid for

Fix the Internet!

  • Others insist the Internet doesn’t need

massive changes

  • Let ISPs upgrade bandwidth as customers demand it
  • Increase ISP caching for common requested stored AV
  • Add content distribution networks (CDNs) for paid stored media, conveniently near network edges

AV Compression

  • Compression helps speed information

transmission rate by reducing its volume

  • Long done for static images (JPG, GIF)
    • Each pixel is 24 bits of color data (RGB),
      • A 1024x768 pixel image is 1024x768x24/8 = 2.36 MB
    • Compression can reduce image size a factor of 10 without severe quality loss - Key tradeoff: more compression = more losses
  • Huge field (e.g. ISBN 1565921615)

Audio Compression

  • Digital audio signals have three major

characteristics

  • The sampling rate (samples per second)
  • The number of channels sampled (mono, stereo, etc.)
  • The number of bits per sample
  • These all affect the raw amount of data being

sent, before compression is applied

Audio Compression

  • Each sample is quantized into some number of bits to describe its relative strength - 8 bits gives 256 values from silent to REALLY LOUD, which is typical for cheap built-in audio - CD quality uses 16 bits per sample - Pro audio typically uses 24 or 32 bits per sample
  • So one minute of absurd quality 7 channel surround would use 192k sample/sec x 7 channels x 32 bits/sample / 8 bit/byte * 60 sec/min = 322.56 MB/min!

Audio Compression

  • This approach for audio is formally called

Pulse Code Modulation (PCM)

  • Mono speech recording at 8kHz sample rate and 8 bits/sample equals 64 kbps of data
  • CD quality (44.1 kHz, 16 bit, stereo) = 1.411 Mbps
  • Modems can’t handle 64 kbps, and most

broadband users can’t consistently get 1.

Mbps, so compression is needed even for

just audio

Video Compression

  • Video is a series of images presented at 24 or

30 images per second

  • Hence the size of the images (X by 0.75*X pixels), and the color resolution (number of bits per pixel) also affect the amount of raw data
  • Widescreen image sizes have a 16x9 ratio instead of 4x3 ratio

Video Compression

  • Video compression standards are mostly the

MPEG family

  • MPEG 1 for CD quality video at 1.5 Mbps
  • MPEG 2 for DVD quality video, 3-6 Mbps
  • MPEG 4 for object oriented video
  • H.261 from ITU
  • Proprietary formats, such as QuickTime (includes MPEG-4 and H.264), Real networks, etc.