13 - multimedia
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Trng Th M Trang ttmtrang@fit.hcmus.edu.vn
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Introduction
Streaming stored multimedia
Real-time protocols
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digital
Integration of multiple media type (text,
graphics, still and moving images, animation,
sounds, and any other medium) can be represented, stored, transmitted and
processed digitally
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Classifications of various media typeso Captured vs. synthesized media
Captured media (natural) : information captured from the realworld
Example: still image, video, audio
Synthesized media (artificial) : information synthesize bythe computer
Example: text, graphics, animation
o Discrete vs. continuous media Discrete media: spaced-based, media involve the space
dimension only Continuous media: time-based, media involves both the
space and the time dimension
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Sound Video
Image
Animation
Text Graphics
Captured
From real world
Synthesized
By computer
Discrete Discrete
Continuous Continuous
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Local vs. networkedo Local: storage and presentation of multimedia
information in standalone computers
Sample applications: DVDoNetworked: involve transmission and distribution
of multimedia information on the network
Sample applications: videoconferencing, web video
broadcasting, multimedia Email, etc.
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network audio and video (continuous media)
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Fundamental characteristics:
typically delay sensitive
o end-to-end delay
o delay jitter
loss tolerant: infrequent
losses cause minor glitches
Classes of MM applications:
1) stored streaming
2) live streaming
3) interactive, real-time
Jitter is the variability of packet delays within the
same packet stream
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Example:
o Music/video onlineo You tube
o VOA
o CNN
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1. video
recorded
2. video
sent3. 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
network
delaytime
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VCR-like functionality: client can pause,rewind, fast forward, push slider bar
timing constraint for still-to-be transmitted
data: in time for playout
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Examples: Internet radio talk show
live sporting event
Streaming (as with streamingstoredmultimedia)
playback buffer
playback can lag tens of seconds after transmission
still have timing constraint
Interactivity fast forward impossible
rewind, pause possible!
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applications: IP telephony, video conference,distributed interactive worlds
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TCP/UDP/IP: best-effort service
no guarantees on delay, loss
Todays Internet multimedia applications
use application-level techniques to mitigate
(as best possible) effects of delay, loss
But you said multimedia apps requiresQoS and level of performance to be
effective!
?? ??
?
?
? ??
?
?
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1. How should the Internet evolve to
better support multimedia?2. Transport: TCP? UDP? Other
protocol?
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Introduction
Streaming stored multimedia
Real-time protocols
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application-levelstreaming techniquesfor making the best out
of best effort service:o client-side buffering
o use of UDP versus TCP
o multiple encodings of
multimedia
jitter removal
decompression error concealment
graphical user interfacew/ controls for interactivity
Media Player
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audio, video not streamed:
no, pipelining, long delays until playout!
audio or video stored in file
files transferred as HTTP objecto received in entirety at client
o then passed to player
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browser GETs metafile (contains information of URL, type of
encoding, etc about the audio/video file)
browser launches player, passing metafile
player contacts server
server streams audio/video to player
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Introduction
Streaming stored multimedia
Real-time protocols
o RTSP
o RTP
o RTCP
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the protocols and their application fieldo stream description: M3U, RAM, SMIL...
describe the session and content
o stream control: RTSPremote control the session
o media transport: RTPsend data and metadata
o resource reservation (if any!): RSVP, DiffServmake sure the communication path offers appropriate guaranties
otherwise Best-Effort transmissions!
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Application protocol RFC 2326
acts as a network remote control
allows a media player to control the transmissionof a media stream for exchanging controlinformation
supports the following operations:o retrieval of a media from a servero invitation of a media server to a conference
o recording of a conference
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video
server
audio
server
media descr.
web serverclient
step 1: get description (in SDP format)
step 2: open streams with RTSP
step 3: play
step 4: teardown
C
W A V
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Major methodso SETUP: server allocates resources for a
stream and starts an RTSP sessiono PLAY: starts data tx on a streamo PAUSE: temporarily halts a streamo TEARDOWN: free resources of the stream, no
RTSP session on server any more
Additional methodso OPTIONS: get available methodso ANNOUNCE: change description of media objecto DESCRIBE: get low level descr. of media objecto RECORD: server starts recording a streamo REDIRECT: redirect client to new servero SET_PARAMETER: device or encoding control
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client
C
web
server
W
media
servers
A & V
HTTP GET
presentation description (sdp)
SETUP
PLAY
RTP audio/video
RTCP
TEARDOWN
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Transport protocol RFC 3550
RTP specifies packet structure for packets
carrying audio, video datao RTP packets encapsulated in UDP segments
RTP packet provides
opayload type identification
opacket sequence numbering
o time stamping
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Payload Type: 7 bits, providing 128 possible different types of encoding;eg PCM, MPEG2 video, etc.
Sequence Number: 16 bits; used to detect packet loss
Timestamp: 32 bytes; gives the sampling instant of the first audio/videobyte in the packet; used to remove jitter introduced by the network
Synchronization Source identifier (SSRC): 32 bits; an id for the source ofa stream; assigned randomly by the sour
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works in conjunction with RTP. Used to exchange control information (report)
between the sender and the receiver
o report statistics useful to application: # packetssent, # packets lost, interarrival jitter, etc.
feedback can be used to control performance
o sender may modify its transmissions based onfeedback
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Bi ging gio trnh Computer Networking: ATop Down Approach, 5th edition, J.F Kurose
and K.W. Ross
Bi ging EE442Multimedia Networking,Jane Dong, California State University, Los
Angeles
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