TITRE Adaptive Packet Video Streaming Over IP Networks - LaBRI

In a packet video application, the packet is either received correctly or lost entirely. This means
that the boundaries for lost information are exactly determined by the packet boundaries. To
combat the effect of packet loss, it is necessary to design a specific packet payload format for each
video standard. The video server knows best, it can apply a good packetization. For example, if the
network path-MTU is known, the video encoder can design video packets having the MTU size
and independently decodable. Many codec such as H.263+, MPEG-4, and H.264 support the
creation of different form of independently decodable video packets.
Since the reliable transmission mechanism offered by TCP incurs considerable overhead by
retransmission, RTP [110] does not rely on TCP. Instead, applications are put on top of UDP. How
to cope with the lost packets is up to the applications. Following the application-level framing
principle, RTP functionality is usually integrated into the application rather than being implemented
as a separate protocol entity. RTP provides basic packet format definitions to support real-time
communication but does not define control mechanisms or algorithms. The packet formats provide
information required for audio and video data transfer, such as the incoming video data packet
sequence. Continuous media such as non-compressed PCM audio can be synchronized by the use
of sequence numbers. Non-continuous data such as MPEG can be resynchronized by using the
time stamp fields. Many RTP payload format was proposed for many video codec such as for
H.261 [111], for MPEG1/MPEG2 Video [112], for H.263 [113], for MPEG-4 Audio/Visual
Streams [114], and in progress RTP payload for JVT/H.264 video [28].
In MPEG-2, at the start of a new slice, information called a slice header is placed within the
bitstream. The slice header provides information which allows the decoding process to be restarted
when errors have occurred. In MPEG-4 a periodic synchronization marker is inserted to allow the
decoder to resynchronize after synchronization loss.
In the data partitioning technique the video bitstream is organized and prioritized. Encoded
video data is organized into high priority data such as headers, motion vectors, low frequency DCT
coefficients, and addresses of blocks, and low priority data such as higher frequency DCT. Such
organization follows from this idea that bits which closely follow a synchronization marker are
more likely to be accurately decoded that those further away. This approach is referred to data
partitioning in MPEG-4. Thus, if there are two available channels, higher data priority is
transmitted in channel with better error performance and less critical data is transmitted in the
channel with poor error performance. The degradation to channel errors is minimized since the
critical parts of a bitstream are better protected. Data from neither channel may be decoded on a
decoder that is not intended for decoding data partitioned bitstreams.
3.2.2.5 Error Concealment
In order to conceal the fact that an error has occurred on decodable video, it is necessary that
the receiver apply error concealment techniques which can estimate the lost information or missing
pixels. Since the video compression removes redundant data, estimating lost information is not an
easy task. A trivial technique to conceal packet lost is to replace the corresponding block by a green
or black square. However, this approach can be highly disturbing for human eyes. Other techniques
more efficient are considered such as (1) interpolation (2) freeze frame, and (3) motion estimation
compensation. The interpolation is used by smoothly extrapolate the missing block from the
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