error control in video streaming. introduction development of different types of n/ws such as...
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Introduction
Development of different types of n/ws such as internet, wireless and mobile networks has created new applications and communications.
Multimedia and video communication Eg) youtube(a video sharing website)
Compression is an essential component of digital video.
Uncompressed digital video requires - huge storage
- more bandwidth
The figure is about the packet based video transmission system.
Packet losses may happen in the lossy channel.
Compressed video is very sensitive to packet loss.
Video Conferencing over Packet-Switching Networks.
Packet Loss
•¡¡ Compressed video frame is packetized and transmitted.•¡¡ Packet loss can occur quite frequently due to congestion •and user mobility.
To solve the problem of packet errors or packet losses the application can use
Forward error correction (FEC)
Automatic repeat request (ARQ)
Conventional downloading application use ARQ for robust file transfer(FTP).
Two problems
* receivers may need to large buffers.
* users need to wait for a long time before starting playback.
Error control Techniques in video
Transport level error control– Error detection and correction through FEC– Retransmission of lost packets
Error resilient encoding– Adding redundancy in the bit stream to
help the decoder recover from transmission error Error concealment
– Recover lost/damaged regions based on image/video characteristics and human visual system properties at the decoder
Encoder-decoder-network interactive error control
– Joint source/channel coding Ex: layered coding with unequal error propagation
– Feedback-based adaptive encoding Ex. Reference picture selection, Selective intra update
– Path diversity Different bit streams sent through separate paths
Channel coding for video multicast
Providing a service such as video delivery for multiple users.
Four different approaches for this. Virtual multicast Multicast Broadcast Peer to peer
Virtual multicast
Group of users Server starts a separate unicast for
each client. Requires more resources Lack of efficiency So max amount of client is restricted.
Multicast
Clients can share the same channel. Server put the data or stream on
channel once , the clients can receive it. If feedback channel is available, the
users can send feedback, therefore the server can monitor the transmission quality.
Broadcast
In contrast to video multicasting, video broadcasting server is responsible for large no. of users, up to millions in tv stations.
So server cannot monitor the receivers quality or channel conditions
Peer to Peer
Difference between general peer to peer and latest peer to peer media streaming system is that in the data sharing mode among peers, the former uses the open-after-downloading mode while the later uses the play- while-downloading.
Conventional error control techniques for multicasting.
FEC ARQ Hybrid ARQ RPS Rate Distortion optimization and
Collision Distortion optimization
Forward error correction
. Forward Error Correction
send parity packets over a PTDD period.
Periodic frame Periodic frame
Parity Packet for periodic frame
FEC
FEC is a technique used for controlling errors in data transmission over unreliable or noisy communication channels.
A number of FEC algorithms Hamming code, Reed-Solomon code Bose-Chandhuri-Hocquenghem code.
Error detecting
assume the list of numbers that you wish to transmit might be 7, 3, 8, 10, 12, and 21.
6, 7, 3, 8, 10, 12, 21, and 61. The person at the receive end would look at the first
number and then ensure that the correct number of additional numbers is received and then verify that the number at the end of the sequence that is the sum of the transmitted numbers.
They allow you to determine if the transmission was accurate, but they don’t allow you to correct the errors.
Error correction
two terms binary –base 2 numbers Parity
Parity is a term that is used in binary communication systems to indicate whether the numbers of 1's in a transmission is even or odd. If the number of 1's is an even number, then parity is said to be even and conversely for odd parity.
Features
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@Sender MBs are grouped in data units and entropy
coding used Error Control before transmission over lossy
channel Forward Error Correction(FEC) Backward Error Correction (BEC) Prioritization Methods Combinations of above
Features
@Receiver Erroneous and missing data
detected and localized Decoder gets correct data units or
error indication Error concealment applied at
positions where no data received Report loss of data units to encoder
Design Principles
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error-resilience tools decrease compression efficiency Main goal:
Shannon’s separation principle: compression separated with transport
In low delay situations, error-free transport is impossible System Design Principles
1. Loss correction below codec layer 2. Error detection 3. Prioritization methods 4. Error recovery and concealment 5. Encoder-decoder mismatch avoidance
Error Concealment Basic Idea
Decoder should generate a representation for lost area
Match as close as possible to the lost info Within manageable complexity
Techniques Spatial Error Concealment Temporal Error Concealment Hybrid Concealment Other Techniques
Spatial Error Concealment
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Based on assumption of continuity of natural scene content in space
Use pixel values of surrounding
available MBs Estimate of lost pixel:
αβγ are weighing factors Determine relative impact of vertical,
Horizontal, upper, lower… Disadvantage
Blurred reconstruction
Temporal Error Concealment
Rely on the continuity of a video sequence in time
Use temporally neighboring areas to conceal lost regions
Previous Frame Concealment (PFC) Use previous corresponding data to copy to
current frame Only good when little motion Widely used due to simplicity
Hybrid Concealment
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When only apply spatial concealment Concealed regions are significantly blurred
When only use temporal error concealment Significantly discontinuities in the concealed
regions Hybrid temporal-spatial technique applied
MB mode info of reliable and concealed neighbors decide which concealment method to use
Hybrid (cont.)
For intra-coded images Only use spatial concealment
For inter-coded images Use temporal concealment when more than half
of the available neighbor MBs are inter-coded Otherwise, use spatial concealment
Referred to as Adaptive temporal and spatial Error Concealment (AEC)