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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.