Spring 2001CS 5851 14. Multimedia, QoS Multimedia (7.2, 9.3) Compression RTP Realtime Applications...
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Transcript of Spring 2001CS 5851 14. Multimedia, QoS Multimedia (7.2, 9.3) Compression RTP Realtime Applications...
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)
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
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)
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)
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
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
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
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
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
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
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
Spring 2001 CS 585 12
Quality of Service (6.5)
Realtime Applications
Integrated Services
Differentiated Services
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
Spring 2001 CS 585 14
Playback BufferS
eque
nce
num
ber
Packetgeneration
Networkdelay
Buffer
Playback
Time
Packetarrival
Spring 2001 CS 585 15
Example Distribution of Delays
1
2
3
Pa
cke
ts (
%)
90% 97% 98% 99%
150 20010050
Delay (milliseconds)
Spring 2001 CS 585 16
TaxonomyApplications
Real time
Tolerant
Adaptive Nonadaptive
Delay-adaptive
Rate-adaptive
Intolerant
Rate-adaptive Nonadaptive
Interactive Interactivebulk
Asynchronous
Elastic
Spring 2001 CS 585 17
Integrated Services
• Service Classes– guaranteed
– controlled-load
• Mechanisms– signaling protocol
– admission control
– policing
– packet scheduling
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
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
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
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
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
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)