Transcript Chapter 6

Quality of Service in the
Internet
The slides of part 1-3 are adapted from the slides of chapter 7 published at the companion website of the book:
Computer Networking : A Top-Down Approach Featuring the Internet, James F. Kurose, Keith W. Ross, Addison-Wesley,
3rd edition, 2004.
Multimedia Applications
Multimedia applications: transmit and receive audio
and video over the Internet (e.g. streaming video, IP
telephony, Internet radio, teleconferencing)
Fundamental characteristics:
 delay sensitive
 loss tolerant: infrequent losses cause minor glitches
 Different from elastic applications (e.g., file
transfer, Web, email, telnet), which are loss
intolerant and delay tolerant
Classes of Multimedia Applications
 Streaming stored audio and video
 Streaming live audio and video
 Real-time interactive audio and video
Streaming Stored Audio and Video
 Prerecorded media stored on servers, media
transmitted to client on demand

Examples: audio of a lecture, archives of radio
broadcasts, movies, MTV clips
 Streaming: client playout begins
has arrived

before all data
buffer needed at client
 Delay constraint: data must be received in time
for playout at the client
 Interactivity: client can pause, rewind, fastforward
 1-2 sec until command effect OK
Streaming Live Audio and Video
 Examples: Internet radio talk show, live sporting
event
Streaming
 playback buffer at client
 playback can lag tens of seconds after request
 delay constraint: data must be received in time for
playout at the client
Interactivity
 Not stored  fast forward impossible
 rewind, pause possible with local storage of
received data
Real-Time Interactive Audio and Video
 Allow people to use audio/video to communicate
with each other in real time

Internet phone, video conferencing
 End-end delay requirements:
audio: < 150 msec good, < 400 msec OK, higher
delays impair interactivity
 Video: a few hundred msec acceptable
 Rigid constraint on delay jitter
 delay jitter: the variability of packet delays
within the same packet stream

Delay Jitter
variable
network
delay
(jitter)
client
reception
constant bit
rate playout
at client
buffered
data
constant bit
rate
transmission
client playout
delay
time
Multimedia Over Today’s Internet
 IP: best-effort service
 no guarantees on delay, loss
 But multimedia apps require QoS to be effective!

QoS: network provides application with level of performance
needed for application to function.
Today’s Internet multimedia applications
use application-level techniques to mitigate
(as best possible) effects of delay, loss
How Should the Internet Evolve to
Better Support Multimedia?
Integrated services philosophy:
 Fundamental changes in Internet so that apps can reserve endto-end bandwidth
 Requires new, complex software in hosts & routers
Laissez-faire approach
 no major changes in Internet
 ISPs add more bandwidth when needed
 content distribution networks, multicast overlay networks
Differentiated services philosophy:
 Make relatively small changes to Internet infrastructure
 Introduce a small number of traffic classes with different
levels of service