Bài giảng Mạng máy tính - Chapter 9: Multimedia networking - Nguyễn Lê Duy Lai
Computer Networks
Lectured by:
Nguyen Le Duy Lai
(lai@hcmut.edu.vn)
Computer
Networking: A Top
Down Approach
7th Edition, Global Edition
Jim Kurose, Keith Ross
Pearson
April 2016
Multimedia Networking 9-1
Chapter 9
Multimedia
Networking
Computer
Networking: A Top
Down Approach
7th Edition, Global Edition
Jim Kurose, Keith Ross
Pearson
April 2016
Multimedia Networking
9-2
Multimedia networking: outline
9.1 multimedia networking applications
9.2 streaming stored video
9.3 voice-over-IP
9.4 protocols for real-time conversational
applications
9.5 network support for multimedia
9-3
Multimedia Networking
Multimedia: audio
§ analog audio signal
sampled at constant rate
quantization
error
quantized
value of
analog value
• telephone: 8,000
samples/sec
analog
signal
• CD music: 44,100
samples/sec
§ each sample quantized, i.e.,
rounded
• e.g., 28=256 possible
time
quantized values
sampling rate
(N sample/sec)
• each quantized value
represented by bits,
e.g., 8 bits for 256
values
9-4
Multimedia Networking
Multimedia: audio
§ example: 8,000 samples/sec,
256 quantized values: 64,000
bps
quantization
error
quantized
value of
analog value
§ receiver converts bits back to
analog signal:
analog
signal
• some quality reduction
example rates
§ CD: 1.411 Mbps
§ MP3: 96, 128, 160 kbps
time
sampling rate
(N sample/sec)
§ Internet telephony: 5.3 kbps
and up
9-5
Multimedia Networking
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)
Multimedia: video
§ video: sequence of images
……………………...…
……………………...…
displayed at constant rate
• e.g., 24 images/sec
§ digital image: array of pixels
• each pixel represented
by bits
§ coding: use redundancy
within and between images
to decrease # bits used to
encode image
frame i
temporal coding example:
instead of sending
complete frame at i+1,
send only differences from
frame i
• spatial (within image)
• temporal (from one
image to next)
frame i+1
9-6
Multimedia Networking
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)
Multimedia: video
§ CBR: (constant bit rate):
video encoding rate fixed
……………………...…
……………………...…
§ VBR: (variable bit rate):
video encoding rate changes
as amount of spatial,
temporal coding changes
§ examples:
• MPEG 1 (CD-ROM) 1.5
frame i
Mbps
• MPEG2 (DVD) 3-6 Mbps
• MPEG4 (often used in
temporal coding example:
instead of sending
complete frame at i+1,
send only differences from
frame i
Internet, < 1 Mbps)
frame i+1
9-7
Multimedia Networking
Multimedia networking: 3 application types
§ streaming, stored audio, video
• streaming: can begin playout before downloading entire
file
• stored (at server): can transmit faster than audio/video
will be rendered (implies storing/buffering at client)
• e.g., YouTube, Netflix, Hulu
§ conversational voice/video over IP
• interactive nature of human-to-human conversation
limits delay tolerance
• e.g., Skype
§ streaming live audio, video
• e.g., live sporting event (futbol)
9-8
Multimedia Networking
Multimedia networking: outline
9.1 multimedia networking applications
9.2 streaming stored video
9.3 voice-over-IP
9.4 protocols for real-time conversational
applications
9.5 network support for multimedia
9-9
Multimedia Networking
Streaming stored video:
2. video
sent
1. video
recorded
(e.g., 30
3. video received,
played out at client
(30 frames/sec)
network delay
(fixed in this
example)
time
frames/sec)
streaming: at this time, client
playing out early part of video,
while server still sending later
part of video
Multimedia Networking
9-10
Streaming stored video: challenges
§ continuous playout constraint: once client playout
begins, playback must match original timing
• … but 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
Multimedia Networking
9-11
Streaming stored video: revisited
constant bit
rate video
transmission
client video
reception
constant bit
rate video
playout at client
variable
network
delay
time
client playout
delay
§ client-side buffering and playout delay: compensate
for network-added delay, delay jitter
Multimedia Networking
9-12
Client-side buffering, playout
buffer fill level,
Q(t)
playout rate,
e.g., CBR r
variable fill
rate, x(t)
client application
buffer, size B
video server
client
Multimedia Networking
9-13
Client-side buffering, playout
buffer fill level,
Q(t)
playout rate,
e.g., CBR r
variable fill
rate, x(t)
client application
buffer, size B
video server
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
Multimedia Networking
9-14
Client-side buffering, playout
buffer fill level,
Q(t)
playout rate,
e.g., CBR r
variable fill
rate, x(t)
client application
buffer, size B
video server
playout buffering: average fill rate (x), playout rate (r):
§ x < r: buffer eventually empties (causing freezing of video
playout until buffer again fills)
§ x > r: buffer will not empty, provided initial playout delay is
large enough to absorb variability in x(t)
• initial playout delay tradeoff: buffer starvation less likely
with larger delay, but larger delay until user begins
watching
Multimedia Networking
9-15
Streaming multimedia: UDP
§ server sends at rate appropriate for client
• often: send rate = encoding rate = constant
rate
• transmission rate can be oblivious to
congestion levels
§ short playout delay (2-5 seconds) to remove
network jitter
§ error recovery: application-level, time permitting
§ RTP [RFC 2326]: multimedia payload types
§ UDP may not go through firewalls
Multimedia Networking
9-16
Streaming multimedia: HTTP
§ multimedia file retrieved via HTTP GET
§ send at maximum possible rate under TCP
variable
rate, x(t)
video
file
TCP send
buffer
TCP receive
buffer
application
playout buffer
server
client
§ fill rate fluctuates due to TCP congestion control,
retransmissions (in-order delivery)
§ larger playout delay: smooth TCP delivery rate
§ HTTP/TCP passes more easily through firewalls
Multimedia Networking
9-17
Multimedia networking: outline
9.1 multimedia networking applications
9.2 streaming stored video
9.3 voice-over-IP
9.4 protocols for real-time conversational
applications
9.5 network support for multimedia
9-18
Multimedia Networking
Voice-over-IP (VoIP)
§ VoIP end-end-delay requirement: needed to maintain
“conversational” aspect
• higher delays noticeable, impair interactivity
• < 150 msec: good
• > 400 msec bad
• includes application-level (packetization, playout),
network delays
§ session initialization: how does callee advertise IP
address, port number, encoding algorithms?
§ value-added services: call forwarding, screening,
recording
§ emergency services: 911
Multimedia Networking
9-19
VoIP characteristics
§ speaker’s audio: alternating talk spurts, silent
periods.
• 64 kbps during talk spurt
• pkts generated only during talk spurts
• 20 msec chunks at 8 Kbytes/sec: 160 bytes of data
§ application-layer header added to each chunk
§ chunk+header encapsulated into UDP or TCP
segment
§ application sends segment into socket every 20
msec during talkspurt
Multimedia Networking
9-20
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