Distributed Multimedia

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• Distributed Multimedia
• An Introduction
• How to solve the many to many communication
  problem?
• Peter Parnes
• LTU-CDT
• 021003

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Who am I?

• Dr Peter Parnes Assistant Professor
• peppar_at_cdt.luth.se
• http//www.cdt.luth.se/peppar/
• Founder Marratech AB
• Research manager Media Technology
• Holds 5 patents in the area of distributed
  multimedia
• Skåning in exile

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Overview

• Many to Many data distribution
• Data transmission and IP-multicast
• Applications
• Protocols
• Reliable Multicast
• Scalable media distribution in heterogeneous
  environments

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Many to Many

• How to implement many-to-many traffic?
• 1. Central server Have a central server that
  duplicates packets to all other members.
• 2. (Fully) connected mesh Let every member have
  a connection to all/some other members.
• 3. Multicasting Let the network duplicate the
  packet when needed.
• 1 and 2 wastes bandwidth!!!!

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IP Addressing

• The TCP/IP family includes four types of
  distribution of a packet from a single host
• Unicast To one host
• Normal IP-traffic
• The packet is seen only by the receiving host
• Broadcast To all hosts on a network
• When trying to find another host
• The packet is seen by all hosts on the local
  network

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IP Addressing

• Anycast To one host of a group of hosts
• To access a resource that is served by several
  computers
• IP6
• The packet is seen by one off the receiving
  hosts
• Multicast To a group of hosts
• The packet is seen by all hosts in the group
• The packet is only duplicated when needed

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Multicast vs. Unicast

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Multicast Applications

• Broadcasting conferences, meetings, seminars,
  concerts and radio-stations are multicasted
  daily.
• Conferencing Multicast is used for traditional
  video-conferencing (but MUCH cheaper!!)
• News Distribution of Usenet-News
• M-FTP Multi-user File Transfer

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More Applications

• Software-distribution Forget the very costly
  procedure of new software CDs for each new
  release and bug-fix! Just supply the latest
  version in a known multicast-group.
• Mirroring Instead of letting each client fetch
  all new files from a server, send out the changed
  files using multicast!

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Even More Applications

• Real News All news is transmitted on the net.
  Indexed and ready. (Reuters have this since
  1996!)
• TV Why not watch your favourite TV-channel over
  the network?
• File-Caches If all file-requests are issued
  using multicasting its much easier to cache them
  locally!
• And much much much.......

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Example Desktop audio/video
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Handheld Audio/Video
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MPEG-4 Video
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Design Issues

• Scalable The environment should scale to a very
  large number of users - IP-Multicast is the
  solution!
• Robust The environment should survive network
  failures and not be dependent on any central
  services
• Accessible Users should be able to participate
  from their desktop
• Network based No need for any special ISDN
  connections, just the standard local network and
  the Internet.

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Protocols

• MANY different protocols involved with Media
  distribution
• UDP User Datagram Protocol
• Unreliable Packets can be lost
• The applications has to take care of reliability
• TCP Transport Control Protocol
• The OS helps out
• Only point-to-point

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RTP

• RTP - Real-Time Transfer Protocol
• Developed by the IETF (RFC1889/90) and later
  copied into ITU/H.225.
• End-to-End transport functionality for real-time
  data
• Designed for real-time media
• Completely network layer independent

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Reliable Multicasting

• Scalable Reliable Multicasting - SRM
• NACK based - every member participates in repairs
  and not only the original sender of a packet
• Used in the Marratech Pro
• I have designed a RTP-extension to include SRM.

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Summary

• IP multicast provides a scalable solution for
  many-to-many communication.
• A number of tools are being deployed on the
  Internet today to utilize the power of IP
  multicast.
• Native multicast is slowly being deployed around
  the world.

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• Media Scaling over
• Heterogeneous Networks

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Overview

• Background
• Current Problem
• Proposed Solutions
• Layered Multicast
• Current Status in the Internet
• Summary

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Challenges

• Broadcasts of real-time media on the Internet
  is becoming more and more important.
• QoS, Best effort networks
• Heterogeneity in terms of
• bandwidth availability
• CPU-power
• End-user equipment
• Scalability to large sessions and global networks

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Requirements and Restrictions

• Best-effort delivery
• Reliability not required
• Applications have to be adaptive, i.e. have to
  adapt to network congestion and be able to handle
  different configurations.

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Heterogeneous Environment
Narrowband Access
Broadband Access
Internet
Broadband Access
Wireless Access
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• Which bandwidth should be used when transmitting
  a real-time media stream over heterogeneous
  networks?

Sender
100Mbps
Internet
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Proposed solutions

• Max/Min client bandwidth
• Simulcast
• Network transcoders
• Bandwidth Guessing - TCP friendly
• Active Networks
• Active Services
• Layered Multicast

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Max/Min client bandwidth

• Just ignore some set of receivers
• Send the stream with high bandwidth
• Ignore low bandwidth receivers
• Send the stream with low bandwidth
• Force high bandwidth receivers to use low quality
• Does not take congestion into account

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Simulcast

• Send the same stream with different encodings
  from the sender and let the receivers choose what
  they want to receive.
• Can be very expensive CPU wise
• Wastes bandwidth on shared links
• Does not take congestion into account in the way
  it is being used today.

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Network Transcoders

• A common approach is to deploy transcoders on the
  boundaries between different networks.
• Transcoding
• Mixing
• Downscaling

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Transcoding Media Gateways

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Transcoding

• Transcoding
• transcode MJPEG to H.261 when the traffic leaves
  a campus (high bandwidth network). -gt high CPU
  requirement!
• Trancode from high bw to low bw
• Used in SIRAM media gateway

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Downscaling

• Downscaling
• Throw away parts of the mediadata
• Lower CPU than transcoding but worse result
• Used in mTunnel

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Mixing

• Mixing
• Generate one stream from several active streams.
• Mix audio
• Combine video

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Bandwidth Guessing

• In early 97 a proposal called TCP-Friendly was
  distributed.
• Describes a way of estimating the bandwidth
  between a sender and a receiver based on RTT and
  current packet drop.
• Takes TCP into account and will be a nice
  participant in the network

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BW Guessing Problems

• Hard to calculate RTT accurately
• Works only for broadcast situations
• Not very tested yet

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Active Networks

• Basic idea
• Allow injection of small programs into network
  nodes
• Network nodes perform computations on user data

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Active Networks...

• Two Different Approaches
• Code and control is handled out-of-band
• Each packet carries miniature programs (capsules)
• Allows networks to be modified on-demand
• Opens a completely new area for real-time media
  scaling

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Active Networks...

• Issues
• Safety, fairness, appropriate architecture,
  common programming model, robustness
• Status
• At the very beginning
• A very political problem

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Active Services

• Deploy user controllable programs-pads in the
  network.
• Users can deploy their own transcoding programs
  and can easily up-grade these when needed
• A system for this is currently being deployed and
  tested on Berkeley Campus

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Layered Multicast

• Layered (hierarchical) media encodings
• Multi resolution representations of media
• Incremental decoding / refinement
• Layered multicast transmission
• True leaf-initiated join mechanism
• Scalable to large groups
• Receiver-driven flow control algorithm
• Congestion avoidance

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Scenario
Low quality
Leased Line
Internet (multicast enabled)
Medium quality
Leased Line
Low quality
Dial-up access
Leased Line
R
Multicast Router
High-speed LAN
Video
Transmitter
High quality
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Layered Video Encoding

• Temporal layering
• Increased number of refinement layers correspond
  to higher frame-rate
• Spatial layering
• Increased number of refinement layers correspond
  to higher image resolution
• Layered quantization
• Increased number of refinement layers correspond
  to finer quantization

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Temporal Layering
Images of a video sequence
Transmission channels that can be received
independently
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Spatial Layering
Transform
Base layer signal refinement signals
Original image
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Layered Multicast

• The layers of the hierarchically encoded media
  are transmitted to a set of IP multicast group
  addresses
• Each receiver subscribe to an appropriate number
  of multicast groups depending on the network and
  CPU resources available

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Example - Wavelet based
224.3.4.5
Base layer
224.3.4.6
Refinement layers
224.3.4.7
224.3.4.8
Four subbands, four multicast groups
One iteration of the wavelet transform
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Algorithm for Flow Control

• loop
• if (no_congestion) then
• join next group to get higher layer
• else
• leave group to drop highest layer

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Layered Multicast Problems

• How to detect congestion caused by my tests or by
  others
• Shared learning proposed
• Does it scale?
• Today long timeout in mcast forwarding trees
• Might lead to false interpretation of the current
  situation
• Dynamic Layers
• Is not nice to TCP

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Dynamic Layers

• Lower the bandwidth for each layer over time
• The receiver has to join new layers to keep the
  same total bandwidth
• Based on congestion

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Status in Internet

• Almost all traffic is still sent using Unicast -
  transcoding at the server
• Network transcoders probably most common
• Simulcast less common than one could imagine
  (lack of good support in todays applications).

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Summary

• A number of more or less proposed solutions
• Max/Min client bandwidth
• Simulcast
• Network transcoders
• Bandwidth Guessing - TCP friendly
• Active Networks
• Active Services
• Layered Multicast
• Still a lot of research needed

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Questions?

• peppar_at_cdt.luth.se
• http//www.cdt.luth.se/peppar/