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
Thanks