spring 2001cs 5851 14. multimedia, qos multimedia (7.2, 9.3) compression rtp realtime applications...
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Spring 2001 CS 585 1
14. Multimedia, QoS
Multimedia (7.2, 9.3)CompressionRTP
Realtime Applications
Integrated Services
Differentiated Services
Quality of Service (6.5)
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Spring 2001 CS 585 2
Compression Overview
• Encoding and Compression– Huffman codes
• Lossless – data received = data sent
– used for executables, text files, numeric data
• Lossy– data received does not != data sent
– used for images, video, audio
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Spring 2001 CS 585 3
Lossless Algorithms
• Run Length Encoding (RLE)– example: AAABBCDDDD encoding as 3A2B1C4D
– good for scanned text (8-to-1 compression ratio possible,fax)
– can increase size for data with variation (e.g., some images)
• Differential Pulse Code Modulation (DPCM)– example AAABBCDDDD encoding as A0001123333
– change reference symbol if delta becomes too large
– works better than RLE for many digital images (1.5-to-1)
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Spring 2001 CS 585 4
Dictionary-Based Methods
• Build dictionary of common terms (LZ algorithm)– variable length strings
• Transmit index into dictionary for each term– Compression 11*7=77bits 4978th of 25000 world
dictionay(15bits)
• Static dictionary vs tailored dictionary• Variation of LZ used to compress GIF images
– first reduce 24-bit color to 8-bit color – treat common sequence of pixels as terms in dictionary– not uncommon to achieve 10-to-1 compression (x3)
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Spring 2001 CS 585 5
Image Compression • JPEG: Joint Photographic Expert Group (ISO/ITU)• Lossy still-image compression • Three phase process
– process in 8x8 block chunks (macroblock)– grayscale: each pixel is given by a 8 bit value– DCT: transforms signal from spatial domain into and
equivalent signal in the frequency domain (loss-less)– apply a quantization to the results (lossy) – RLE-like encoding (loss-less)
Sourceimage
JPEG compression
DCT Quantization EncodingCompressed
image
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Spring 2001 CS 585 6
Discrete Cosine Transform(DCT)
• 8*8 matrix spatial domain 8*8 matrix frequency domain
• DC coefficient located at (0,0) indicates average value of 64 pixels
• AC coefficients (rest), variations
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Spring 2001 CS 585 7
Quantization and Encoding• Quantization Table
3 5 7 9 11 13 15 17
5 7 9 11 13 15 17 19
7 9 11 13 15 17 19 21
9 11 13 15 17 19 21 23
11 13 15 17 19 21 23 25
13 15 17 19 21 23 25 27
15 17 19 21 23 25 27 29
17 19 21 23 25 27 29 31
• Encoding Pattern
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Spring 2001 CS 585 8
MPEG
• Motion Picture Experts Group• Lossy compression of video • First approximation: JPEG on each frame• Also remove inter-frame redundancy
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Spring 2001 CS 585 9
MPEG (cont)• Frame types
– I frames: intrapicture – P frames: predicted picture– B frames: bidirectional predicted picture
• Example sequence transmitted as I P B B I B B
Frame 1 Frame 2 Frame 3 Frame 4 Frame 5 Frame 6 Frame 7
I frame B frame B frame P frame B frame B frame I frame
MPEGcompression
Forwardprediction
Bidirectionalprediction
Compressedstream
Inputstream
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Spring 2001 CS 585 10
MPEG (cont)• B and P frames
– coordinate for the macroblock in the frame
– motion vector relative to previous reference frame (B, P)
– motion vector relative to subsequent reference frame (B)
– delta for each pixel in the macro block
• Effectiveness– typically 90-to-1
– as high as 150-to-1
– 30-to-1 for I frames
– P and B frames get another 3 to 5x
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Spring 2001 CS 585 11
RTP (9.3)Real-time Transport Protocol
• Application-Level Framing• Data Packets
– sequence number– timestamp (app defines “tick”)
• Smooth out jitter and synchronization of multiple streams
• Control Packets (send periodically, RTCP)– loss rate (fraction of packets received since last report)– measured jitter
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Spring 2001 CS 585 12
Quality of Service (6.5)
Realtime Applications
Integrated Services
Differentiated Services
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Spring 2001 CS 585 13
Realtime Applications• Require “deliver on time” assurances
– must come from inside the network
• Example application (audio)– sample voice once every 125us
– each sample has a playback time
– packets experience variable delay in network
– add constant factor to playback time: playback point
Microphone
Speaker
Sampler,A D
converter
Buffer,D A
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Spring 2001 CS 585 14
Playback BufferS
eque
nce
num
ber
Packetgeneration
Networkdelay
Buffer
Playback
Time
Packetarrival
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Spring 2001 CS 585 15
Example Distribution of Delays
1
2
3
Pa
cke
ts (
%)
90% 97% 98% 99%
150 20010050
Delay (milliseconds)
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Spring 2001 CS 585 16
TaxonomyApplications
Real time
Tolerant
Adaptive Nonadaptive
Delay-adaptive
Rate-adaptive
Intolerant
Rate-adaptive Nonadaptive
Interactive Interactivebulk
Asynchronous
Elastic
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Spring 2001 CS 585 17
Integrated Services
• Service Classes– guaranteed
– controlled-load
• Mechanisms– signaling protocol
– admission control
– policing
– packet scheduling
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Spring 2001 CS 585 18
Flowspec• Rspec: describes service requested from network
– controlled-load: none– guaranteed: delay target
• Tspec: describes flow’s traffic characteristics (leaky bucket)– average bandwidth + burstiness: token bucket filter– token rate r– bucket depth B– must have a token to send a byte– must have n tokens to send n bytes– start with no tokens– accumulate tokens at rate of r per second– can accumulate no more than B tokens
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Spring 2001 CS 585 19
Per-Router Mechanisms
• Admission Control– decide if a new flow can be supported– answer depends on service class– not the same as policing
• Packet Processing– classification: associate each packet with the
appropriate reservation– scheduling: manage queues so each packet receives the
requested service
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Spring 2001 CS 585 20
Reservation Protocol• Proposed Internet standard: RSVP• Consistent with robustness of today’s connectionless model• Uses soft state (refresh periodically)• Designed to support multicast• Receiver-oriented• Two messages: PATH and RESV
– Source transmits PATH messages every 30 seconds– Destination responds with RESV message
• In case of Multicast– Merge requirements in case of multicast– Can specify number of speakers
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Spring 2001 CS 585 21
RSVP Example
R
R
R
R
R
Sender 1
Sender 2
PATH
PATH
RESV(merged)
RESV
RESV
Receiver B
Receiver A
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Spring 2001 CS 585 22
RSVP versus ATM (Q.2931)• RSVP
– receiver generates reservation– soft state (refresh/timeout)– separate from route establishment– QoS can change dynamically– receiver heterogeneity
• ATM (Asynchronous Transfer Mode)– sender generates connection request– hard state (explicit delete)– concurrent with route establishment– QoS is static for life of connection– uniform QoS to all receivers
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Spring 2001 CS 585 23
Differentiated Services• Problem with IntServ: scalability
– Per flow state information
• Idea: support two classes of packets– premium– best-effort P(drop)
1.0
MaxP
Min in MaxinMaxoutMinout
AvgLen
• Mechanisms– packets: ‘in’ and ‘out’ bit– edge routers: tag packets– core routers: RIO (RED
with In and Out)