Download 6 Multimedia Communications Across Networks

Multimedia Communication Systems
Techniques, Standards, and Networks
Chapter 6
Multimedia Communication Across
Networks
Audio/Video
Nguyen Slides
 Digitized versions are bursty
 Can be statistically multiplexed
 Can be packetized
 Continuously connected circuits waste
resources
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Packet Voice
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Mostly silence
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Delay
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Acceptable range 100 – 600 mSec
Packet size
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Only active 35% - 45% of the time
200 – 700 speech bits
Packet headers 4 – 8 bytes
Speech content
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10 – 50 mSec
Packet Voice
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Additional information
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Possibly required
Time stamps
 Sequence number
 Flow control
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Not required
Error detection
 Acknowledgement
 Retransmission
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Timing Reconstruction
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NTI – Null Timing Information
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No time stamps
An arbitrary fixed transmission delay is assumed
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Processing delay
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Essentially a fixed buffer
All subsequent packets are assumed to arrive after a
shorter networking processing delay
CTI – Complete Timing Information
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Each packet delay is measured
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Can be relative or global timing
Suitable for highly variable delay networks
Packet Video
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ATM
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Circuit switched (fixed delay, jitter)
Small cells (48 octet payload and 5 octet header)
IP
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Packet switched (Route is not predetermined)
Variable length packets (Max 65,535 octets)
IPv4 No QoS
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Variable queuing delays, out of order sequencing
IPv6 QoS
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25 bit flow identifier
Video Source
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Highly variable bit rate & delay
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Compression algorithm
Scene complexity
Network traffic
Packet switched networks
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Difficult to create statistical models
Hard to negotiate QoS at setup
UPC & NPC (usage/network parameter control)
Policing Mechanisms
Rate Control – the amount of information is
regulated
 Rate Shaping – controls when the information
is sent
 Leaky Bucket
 Multiresolution encoding (layering)

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Coarse resolution – high priority stream
Fine detail – low priority stream (prone to discard)
Simulcast Layers
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Multiple independent layers
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Different bit rates
Dependent Layering
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Layers can be dropped to support a
constant bit rate

congestion control, etc.
Transmission Errors

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FEC – allows the receiver to correct errors
RVLC – reversible variable length coding
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Error resilient entropy coding
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Inserts resynchronization markers
Rearranges variable length blocks into fixed length slots
Attaches a prefix code
Temporal/spatial errors

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Periodically inert I pictures
Segment the data domain

i.e. even/odd frames
Error Concealment
Replace a damaged macroblock with
the previous correct one
 Interpolate the block from surrounding
pixels
 POCS Projection onto convex sets

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An iterative technique, not suitable for
real-time
Rate Control

How to cope with bandwidth
fluctuations

Traffic shaping
Leaky bucket
 Token bucket

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SRC – scaleable rate control
The amount of compression is adjusted
 Complex math
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Internet Video Transport
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Download mode
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The entire clip is downloaded before
viewing
Streaming mode
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The video clip may be viewed while
transmission is in progress
Video Streaming Architecture
Video compression
 Application layer QoS
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Continuous media distribution services
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Retrieves the streaming media from storage
Media synchronization
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An extranet or internet
Streaming servers
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Adapts the video bit rate to the network
Audio, video, data etc.
Media protocols
Video Streaming Architecture
Streaming media systems
Video Compression
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Scaleable
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Deals gracefully with bandwidth fluctuations
A compromise between efficiency, flexibility, &
complexity
Compresses data into multiple substreams
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Each DCT coefficient bit can be assigned to a different
stream (i.e. MSB = coarse)
FGS PFGS
Non Scalable
Streaming Constraints
Requires bounded end-to-end delay
 Rate control is needed to avoid
congestion
 Receiver buffering is required
 Must deal with packet loss
 Simple decoding is needed for low
power receivers

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PDAs, cell phones, etc.
Application Layer QoS
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Avoid Congestion (Rate Control)
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Source based
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Receiver based
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The receiver adds/drops channels
Hybrid
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The rate is determined by a Probe or Model
Both the source & receiver adjust the bit rate
Maximize video quality
Continuous Media Services
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Use network filters
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Frame dropping for congestion control
Content replication
Caching (local copies)
 Mirroring (duplicated servers)
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Streaming Servers
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Consists of

Communicator
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Applications and transport layer protocols
Operating system
 Storage system
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Support VCR-like controls
 Flash Media Server
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Media Synchronization
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Three levels
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Intrastream
Maintains continuity of a logical stream
 i.e. Voice or video
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Interstream
Maintains timing between streams
 i.e. Voice and video
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Interobject
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Layers within streams
Streaming Protocols
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Network
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Transport
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IP
UDP, TCP, RTP, RTCP
Session
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RTSP, SIP
Streaming over ATM
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Internet video streaming