Transcript Streaming Stored Video

Ack.: Slides are adapted from the slides of the book: “Computer Networking” – J. Kurose, K. Ross
Overview
2
Learning Outcomes
1
To describe the characteristics and
requirements of multimedia applications
To explain the problem of multimedia
transmission over the Internet
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3
4
5
6
To define the challenges of streaming
stored video/audio
To differentiate between the different schemes
and protocols for streaming multimedia
To understand and analyze the RTSP protocol
To understand the difference between streaming
stored and live multimedia
Overview
4
Multimedia & QoS
Multimedia and Quality of Service:
What Is It?
multimedia : network audio and video
(“continuous media”)
QoS
network provides application with
level of performance needed for
application to function.
Multimedia & QoS - Applications
MM Networking Applications
MM Application Types:
1. Streaming, stored audio, video
 Streaming: can begin playout before
downloading entire file
 Stored (at server): which allows pause,
rewind or fast-forward (implies
storing/buffering at client)
 E.G., Youtube, netflix, hulu
2. Streaming live audio, video
 E.G., Live sporting event (football)
3. Conversational voice/video over IP
 This type allows people to use
audio/video to communicate with each
other in real time
 Interactive nature of human-to-human
conversation limits delay tolerance
 E.G., Skype
Multimedia & QoS - Applications
MM Networking Applications
Fundamental characteristics:
 Delay sensitive
 End-to-end delay
 Delay jitter
 Loss tolerant:
infrequent losses cause minor glitches
 Different from data, which are loss intolerant but delay tolerant.
Jitter is the variability
of packet delays within
the same packet stream
Overview
8
Introduction to Audio and Video - Audio
Multimedia: Audio
Analog audio signal is converted to a digital
signal as follows:
 Each sample quantized, i.e., Rounded
 E.G., 28=256 possible quantized values
 Each quantized value represented by fixed
number of bits,
 e.g., 8 bits for 256 values
audio signal amplitude
 Analog audio signal sampled at constant rate
 Telephone: 8,000 samples/sec
 CD music: 44,100 samples/sec
quantization
error
quantized value
of
analog value
analog
signal
time
sampling rate
(N sample/sec)
Introduction to Audio and Video - Audio
Multimedia: Audio

Example: 8,000 samples/sec, 256 quantized
values, what is the bit rate?
Answer: 64,000 bps
Receiver converts bits back to analog signal:
 Some quality reduction
example rates
 CD: 1.411 Mbps
 MP3: 96, 128, 160 kbps
 Internet telephony: 5.3 kbps and up
quantization
error
audio signal amplitude

quantized value
of
analog value
analog
signal
time
sampling rate
(N sample/sec)
Introduction to Audio and Video - Video
Multimedia: Video
……………………...…
……………………...…
spatial coding example:
instead of sending N values of
same color (all purple), send
only two values: color value
(purple) and number of
repeated values (N)
 Video: sequence of images displayed at
constant rate
 E.G. 24 images/sec
 Digital image: consists of array of pixels
 Each pixel represented by bits
 Coding: use redundancy within and between
images to decrease # bits used to encode
image
 Spatial (within image)
 Temporal (from one image to next)
frame i
temporal coding example:
instead of sending complete
frame at i+1, send only
differences from frame i
frame i+1
Introduction to Audio and Video - Video
Multimedia: Video
 CBR: (constant bit rate): video encoding rate
fixed
……………………...…
……………………...…
spatial coding example:
instead of sending N values of
same color (all purple), send
only two values: color value
(purple) and number of
repeated values (N)
 VBR: (variable bit rate): video encoding
rate changes as amount of spatial, temporal
coding changes
 Examples:
 MPEG 1 (CD-ROM) 1.5 Mbps
 MPEG2 (DVD) 3-6 Mbps
 MPEG4 (often used in Internet, < 1 Mbps)
frame i
temporal coding example:
instead of sending complete
frame at i+1, send only
differences from frame i
frame i+1
Introduction to Audio and Video - Video
Multimedia Over Today’s Internet
TCP/UDP/IP: “best-effort service”
• no guarantees on delay, loss
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But you said multimedia apps requires
QoS and level of performance to be
? effective!
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?
Today’s Internet multimedia applications
use application-level techniques to mitigate
(as best possible) effects of delay, loss
Multimedia & QoS - Applications
How Should The Internet Evolve To
Better Support Multimedia?
Integrated services philosophy:
 Fundamental changes in internet so that
apps can reserve end-to-end bandwidth
 Requires new, complex software in hosts
& routers
Differentiated services philosophy:
 Fewer changes to internet infrastructure,
yet provide 1st and 2nd class service
Laissez-faire
 No major changes
 More bandwidth when needed
 Content distribution, application-layer
multicast
 Application layer
What’s your opinion?
Overview
15
Streaming Stored Video - Challenges
Streaming Stored Multimedia
Stored multimedia:
 Media stored at source
 Transmitted to client
Streaming:
 client playout begins before all data has arrived
 Timing constraint for still-to-be transmitted
data: on time for playout
Streaming Stored Video
Streaming Stored Video
1. video
recorded (e.g.,
30 frames/sec)
2. video
sent
network delay
(fixed in this
example)
3. video received,
played out at client
(30 frames/sec)
time
Streaming: at this time, client
playing out early part of video,
while server still sending later
part of video
Streaming Stored Multimedia - Interactivity
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
Timing constraint for still-to-be transmitted data: in
time for playout

Streaming Stored Video - Challenges
Streaming Stored Video: Challenges
 Continuous playout constraint: once client playout begins, playback must match original
timing
 Network delays are variable (jitter), so will need client-side buffer to match playout
requirements
Other challenges:
 Client interactivity: pause, fast-forward, rewind, jump through video
 Video packets may be lost, retransmitted
Streaming Stored Video
Streaming Stored Video:
Client Buffering
constant bit
rate video
transmission
variable
network
delay
client playout
delay
constant bit
rate video
playout at client
buffered
video
client video
reception
time
 Client-side buffering and playout delay: compensate for network-added delay, delay jitter
Streaming Stored Video: Buffering
Client-Side Buffering, Playout
buffer fill level,
Q(t)
variable fill
rate, x(t)
video server
drain rate d
to decompression
and playout,
e.g., CBR r
client application
buffer, size B
client
 Client-side buffering and playout delay: compensate for network-added delay, delay jitter
Streaming Stored Video: Buffering
Client-Side Buffering, Playout
buffer fill level,
Q(t)
playout rate,
e.g., CBR r
variable fill
rate, x(t)
video server
client application
buffer, size B
client
1. Initial fill of buffer until playout begins at tp
2. Playout begins at tp,
3. Buffer fill level varies over time as fill rate x(t) varies and playout rate r is constant
Streaming Stored Video - Challenges
Streaming Stored Video
 Application-level streaming techniques for
making the best out of best effort service:
 Client-side buffering
 Use of UDP versus TCP
 Multiple encodings of multimedia
Media Player




Jitter removal
Decompression
Error concealment
Graphical user interface
w/ controls for interactivity
Streaming Stored Video - Challenges
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!
Streaming Stored Video - Challenges
Internet Multimedia: Streaming Approach




Browser gets metafile
Browser launches player, passing metafile
Player contacts server
Server streams audio/video to player
Streaming Stored Video - Challenges
Streaming from a Streaming Server
 HTTP is not optimized for streaming
 This scheme allows for non-HTTP protocol between server, media player
 It also allows for UDP or TCP for step (3)
Overview
27
Streaming Protocols
Streaming Multimedia: UDP or TCP?
UDP
 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 remove
network 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
Streaming Protocols
Streaming Multimedia: Client Rate(s)
1.5 Mbps encoding
28.8 Kbps encoding
Q: How to handle different client receive rate capabilities?
 28.8 Kbps dialup
 100 Mbps Ethernet
A: Server stores, transmits multiple copies of video, encoded at different rates
Streaming Protocols - RTSP
User Control of Streaming Media: RTSP
HTTP
 Does not target multimedia content
 No commands for fast forward, etc.
RTSP: RFC 2326
 Client-server application layer protocol
 User control: rewind, fast forward,
pause, resume, repositioning, etc…
 Out-of-band control

RTSP control messages use
different port numbers than media
stream: out-of-band - port 554
What it doesn’t do:
 Doesn’t define how audio/video is
encapsulated for streaming over network
 Doesn’t restrict how streamed media is
transported (UDP or TCP possible)
 Doesn’t specify how media player buffers
audio/video
Streaming Protocols - RTSP
RTSP Example
Scenario:
 Metafile communicated to web browser
 Browser launches player
 Player sets up an RTSP control connection, data connection to streaming server
Streaming Protocols - RTSP
Metafile Example
<title>Twister</title>
<session>
<group language=en lipsync>
<switch>
<track type=audio
e="PCMU/8000/1"
src = "rtsp://audio.example.com/twister/audio.en/lofi">
<track type=audio
e="DVI4/16000/2" pt="90 DVI4/8000/1"
src="rtsp://audio.example.com/twister/audio.en/hifi">
</switch>
<track type="video/jpeg"
src="rtsp://video.example.com/twister/video">
</group>
</session>
Streaming Protocols - RTSP
RTSP Operation
Overview
34
Multimedia Applications
Streaming Live Multimedia
Examples:

Internet radio talk show
Live sporting event
Streaming (as with streaming stored multimedia)


Playback buffer

Playback can lag tens of seconds after transmission
Still have timing constraint
Interactivity


Fast forward impossible

Rewind, pause possible!
Multimedia Applications
Real-time Interactive Applications
PC-2-PC phone
Skype
PC-2-phone
Dialpad
Net2phone
Skype
Video conference with webcams
Skype
Polycom
Going to look at Voice-over- IP in detail
Conclusion
Conclusion
 Internet provides best effort service only which is not suitable for multimedia
applications
 MM applications can be classified into three types:
 Streaming, stored audio, video
 Conversational voice/video over IP
 Streaming live audio, video
 Multimedia traffic is bursty in nature and requires stringent delay guarantees
 Streaming mulimedia can be improved using UDP protocol, buffering content, adapt
compression level and send redundant packets.
References
References

Computer Networking: A Top-Down Approach Featuring the Internet by James
Kurose and Keith Ross, Addison Wesley, 2012 (chapter 7 )
 Dynamic Adaptive Streaming over HTTP – Design Principles and Standard