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Multipath Video Transmission in Wireless Networks

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Each frame is divided into small blocks, each of the blocks is assumed to undergo a translation ... Can use unequal error protection for I B P frames ... – PowerPoint PPT presentation

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Title: Multipath Video Transmission in Wireless Networks


1
Multipath Video Transmission in Wireless Networks
  • Omesh Tickoo

2
Wireless Video - difficulties
  • High Error Rates
  • Link outages
  • High loss rates in multihop environments
  • Effects propagate if MC is used
  • Route switches can take unacceptable time
  • Challenge Effective streaming/conferencing under
    conditions of high bit rates, delay and loss
    sensitivity.

3
Video transmission
  • Source Coder efficiently convert the source
    output into sequence of binary digits (data
    compression)
  • Channel Coder introduce controlled redundancy
    which can be used at the receiver to overcome the
    effects of channel noise and interference.

4
Video transmission(contd..)
  • Shannonns Separation Principle
  • Source and Channel coding components of a source
    can be designed independently.
  • Assumes back channel for feedback
  • Under delay and complexity constraints the
    principle does not hold
  • In MPT random splitting can make bits received on
    a path useless
  • Use joint source/channel coding.

5
Perspectives
  • Source Coding
  • Layered (MPEG, H.263)
  • Error Resilient (MD, MDMC)
  • Channel Coding
  • FEC (RS codes)
  • Protocol
  • TCP Friendly
  • ARQ

6
Perspectives (contd..)
  • Network
  • Content Delivery (Akamai etc.)
  • Use edge architecture to achieve better load
    balancing, lower latency and higher throughput.
  • Ill behaved systematic properties in Internet
    Routing
  • 30-80 percent of cases the alternate paths are
    better than the default path chosen by the
    routing protocol.(savage-sigcomm99)

7
Wireless Video why multipath
  • Individual links lack adequate capacity
  • Multipath increases usable bandwidth
  • Routing protocols make multipath easy
  • DSR, AODV, TORA etc. can return multiple paths
  • between a source-destination pair.
  • Paths with uncorrelated loss/delay
    characteristics easy to find
  • Burst losses converted into isolated losses
  • Can use Error Protection (e.g. FEC) effectively
  • Multipath provides robustness

8
How to get multiple paths options
  • Frequency or spatial diversity
  • Use relays in the Internet
  • Use a source routed path with the default path.
  • Overlay networks
  • Multiple Connections
  • Pray that the connections follow uncorrelated
    paths

9
How to get multiple paths (contd)
  • Multiple servers
  • Receiver Driven
  • Rate Allocation minimize loss
  • Packet Partition Max. time between reception
    and playback
  • Problem Hotspots
  • Use load balancing methods
  • Multiple PHYs
  • Need multiple radios

10
Layered Coding
  • Non scalable coders require source coders to
    explicitly adapt to channel conditions
  • Not good for real time applications
  • Adapt to heterogeneous network by adapting the
    bit rate to available bandwidth
  • Different subsets of the same stream correspond
    to video sequence at different rates
  • Retransmission based low overhead

11
Layered Coding Drawbacks
  • Require reception of Base Layer for Enhancement
    Layer to be effective
  • Retransmission Time - O(RTT)
  • Interactive application tolerable delay (lt150ms)

12
Layered Coding example (MPEG)
  • Intraframe Separate coding for each frame using
    schemes like JPEG.
  • Exploit spatial redundancy
  • Interframe Use motion vectors
  • Exploit temporal redundancy
  • Translational block-motion model
  • Each frame is divided into small blocks, each of
    the blocks is assumed to undergo a translation
  • Find a single motion vector

13
MPEG (cont..)
  • Motion compensation performed on decompressed
    frame
  • Decoder is able to reconstruct the frame
  • MPEG Group of Pictures (GOP)
  • most important structure in MPEG coding
  • each sequence is divided into multiple GOPs
  • Each GOP consists of three different types of
    frames (I, P, B)

14
MPEG (contd..)
  • Types of pictures
  • I (intra) frame
  • compressed using only intraframe coding
  • Moderate compression
  • P (predicted) frame
  • Coded with motion compression using past I frames
    or P frames
  • Can be used as reference pictures for additional
    motion compensation
  • B (bidirectional) frame
  • Coded by motion compensation by either past or
    future I or P frames

15
Channel coding Mechanisms
  • Forward Error Correction(FEC)
  • Add redundancy to data
  • Example Reed Solomon (RS) codes
  • RS(n,k) codes can correct (n-k)/2 symbol errors
  • efficient for random bit errors or burst errors
    of limited length
  • does not require any feedback
  • FEC reduces delay due to retransmissions (to a
    degree)

16
Channel coding Mechanisms (contd ..)
  • Automatic Repeat Request (ARQ)
  • Selective re-transmission of important frames
  • Maintain an ARQ buffer at the source
  • Lower overhead than FEC
  • ARQ retransmission delays can be unacceptable at
    times

17
Layered Coding Multipath
  • Need to use Unequal Error Protection(FEC and
    retransmissions)
  • LC coupled with FEC (Zakhor)
  • Using FEC the lost information on one path can be
    recovered from packets received through an
    alternate path.
  • Can use unequal error protection for I B P frames
  • FEC is ineffective if paths are correlated or the
    bottleneck link is shared.
  • LC coupled with ARQ (Panwar)
  • Paths (BLARQ) EL
  • Retransmit BL (on EL path) only once based on
    NACKs

18
Multiple Description Coding
  • Drawbacks of Layered Coding
  • if the base layer is lost retransmission is the
    only solution
  • real-time applications do not encourage
    retransmissions
  • Error resilient joint coding scheme
  • decoded video degrades more gracefully in lossy
    environments
  • better recovery since future as well as past
    frames are available
  • Prevents stalling due to packet loss
  • Creates multiple bit-streams of equal importance

19
MDC (contd..)
  • All descriptions are received - high fidelity
    estimate of the original data.
  • Only some descriptions - acceptable quality
  • both fail (very low probability) - can not
    receive any information

20
MDC with Motion Compensation (MDMC)
  • Coder makes use of two previous frames for motion
    compensated prediction for a current frame
  • Includes even frame errors in one description,
    and odd frame errors in another
  • Also codes the mismatch signal
  • which is the difference between the prediction
    obtained with the past two frames and that from
    the past even (or odd) frame only

21
MDC Difficulties
  • MDC approaches SDC in lossy channels.
  • MDC outperforms SDC in bursty slowly varying
    environments.
  • Currently developed schemes correspond to M2
    only (increasing M reduces usability)
  • M is tied to prediction complication, per stream
    resolution and action in the clip.
  • Complex coders
  • Needs to store 2 previously encoded frames to
    support prediction switching ? high bit rate
    encoder

22
Comparisons
  • FEC vs. MDC
  • FEC is a pure channel coding scheme - remain
    agnostic to the type of source codec used.
  • On an average lower computational complexity than
    MDC codecs
  • FEC based recovery adequate for most practical
    applications.
  • Coupled with error concealment techniques, FEC
    can be very effective in improving visual quality
  • MDC on average has smaller bandwidth requirements.

23
Multipath (Layered or Multiple Description)
  • LC
  • the base-layer packets should be delivered over
    more reliable paths
  • good alternative when limited retransmission of
    the base layer is acceptable
  • when it is feasible to apply unequal error
    protection over different paths
  • Loss rates on best path for efficient operation
  • medium to high (provided overhead due to UEP/ARQ
    is acceptable and occasional retransmission of
    base layer is acceptable)
  • good for wireless

24
Multipath (Layered or Multiple Description)
  • MDC
  • Packets from different descriptions evenly
    distributed over paths
  • effective under stringent delay constraint and
    relatively long round trip times on each path
  • Effective in peer-to-peer scenarios
  • Server sends one description directly, client
    gets other descriptions from the peers
  • Loss rates on best path (assuming unbalanced MD)
    for efficient operation
  • low to medium

25
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