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Peer-to-Peer Streaming of Scalable Video
in Future Internet Application
Authors:
Naeem Ramzan, Emanuele Quacchio,
Toni Zgaljic, Stefano Asioli, Luca Celetto,
Ebroul Izquierdo, Fabrizio Rovati
Speaker :吳靖緯 MA0G0101
Communications Magazine, IEEE,
On page(s): 128 - 135, March 2011
2012.04.24
Outline
• Introduction
• Scalable video coding
• Streaming of scalable video over P2P networks
• The MMV platform
• The NextShare platform
• SEAcast platform
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• Conclusion
Introduction
• In conventional streaming architectures the client-server model
and the usage of content distribution networks (CDNs) along
with IP multicast were the most desirable approaches for many
years.
• However, severely limits the number of simultaneous users in
video streaming.
• The reason is the bandwidth bottleneck at the server side, since
usually many clients request the content from the server.
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Introduction
• A CDN overcomes the same bottleneck problem by
introducing dedicated servers at geographically different
locations, resulting in expensive deployment and maintenance.
• Compared to conventional approaches, a major advantage of
peer-to-peer (P2P) streaming protocols is that each peer
involved in content delivery contributes its own resources to
the streaming session.
• Administration, maintenance, and responsibility for operations
are therefore distributed among the users instead of handled by
a single entity.
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Introduction
• The main advantage of P2P systems is bandwidth scalability,
network path redundancy, and the ability to self organize.
• Nevertheless, several problems are still open and need to be
addressed in order to achieve high quality of service and user
experience.
• In particular, the bandwidth capacity of a P2P system is
extremely varying, as it relies on heterogeneous peer
connection speeds, and directly depends on the number of
connected peers.
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Introduction
• Moreover, displaying devices at the user side may range from
small handsets (e.g., mobile phones) to large HD displays (e.g.,
LCD televisions).
• Therefore, video streams need to be transmitted at a suitable
spatio-temporal (ST) resolution supported by the user’s display
device.
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Scalable video coding
• A scalable video sequence can be adapted in three dimensions:
• temporal
• spatial
• quality
• The complexity of adaptation is very low, in contrast to the
adaptation complexity of non-scalable bitstreams.
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Scalable video coding
• Figure 1 shows an example of video distribution through links
supporting different transmission speeds and display devices.
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Scalable video coding
• The SVC scheme gives flexibility and adaptability to video
transmission over resource-constrained networks in such a way
that.
• At each point where video quality/resolution needs to be
adjusted, an adaptation is performed.
• Since the adaptation complexity is very low, the video can be
efficiently streamed in such an environment.
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Streaming of scalable video
over P2P networks
• A generic P2P streaming architecture using SVC is depicted in
Fig. 2.
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Streaming of scalable video
over P2P networks
• A chunk represents the smallest unit of data that will be
transmitted over the P2P network.
• Sometimes, the term piece is used to denote a chunk.
• In BitTorrent, file chunks are downloaded in rarest-first
fashion.
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Streaming of scalable video
over P2P networks
• In video streaming this can result in an interruption of the
video playback since chunks are not received sequentially.
• Therefore, special care needs to be given to those chunks that
are close to the playback position.
• An example of an algorithm that takes into account these
considerations is Give-to-Get (G2G).
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Streaming of scalable video
over P2P networks
• In this algorithm chunks of compressed video are classified
into three priority categories: high, medium, and low.
• This classification depends on the current playback position.
• Chunks close to the playback positions are marked as highpriority chunks.
• Medium- and low-priority chunks are downloaded according
to the standard BitTorrent strategy: rarest-first.
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The MMV platform
A. Piece Picking Strategy
• At the beginning of the streaming session, information about
GOPs(groups of pictures) and layers is extracted from the
bitstream description file.
•
A sliding window is defined, made of several GOPs
(typically three to four).
•
Chunks are picked only from those inside the window unless
all of them have already been downloaded.
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The MMV platform
• In the latter case, the piece picking policy will be rarest-first.
• Inside the window, chunks have different priorities, following
the idea from the original G2G algorithm.
• First, a peer will try to download the base layer (BL), then the
first enhancement layer (EL1).
• Figure 3 shows the behavior of the system with a window three
GOPs wide.
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The MMV platform
• An early stage of the prebuffering phase is shown in Fig. 3,
first row.
• Second row, the first two layers have been downloaded, and
chunks are being picked from EL2 according to the rarest-first
policy.
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The MMV platform
• Third row, the window has shifted. The system will pick
chunks from GOP 3 until the quality of received layers is the
same.
• Fourth row, all GOPs within the window have the same
number of completed layers, and pieces are picked from EL3.
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The MMV platform
B. Peer Selection Strategy
• Good neighbors are those peers that own the piece with the
highest download rates.
•
Each time the window shifts, download rates of all the
neighbors are evaluated, and the peers are sorted in
descending order.
•
Pieces are then requested from peers providing download
rates above the threshold.
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The MMV platform
• The performance of this framework is shown in Fig. 4.
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The NextShare platform
• The procedure implemented in NextShare to download
scalable data chunks is an extension of the G2G algorithm.
• Priorities are defined as in G2G and extended to the multiple
files, as depicted in Fig. 5.
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The NextShare platform
• In the high-priority set pieces are downloaded sequentially,
while in the low-priority set pieces are downloaded in a rarestfirst fashion.
• Each block in the figure represents a time slot.
• In Fig. 5 at time instance t (playback position), the algorithm
has to decide which block to download for time point (t + x).
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The NextShare platform
• The controller implemented in NextShare tries to switch to a
higher quality as soon as there is enough saved buffer for the
current quality.
• Therefore, a safe buffer of chunks downloaded and not yet
delivered to the player is defined; the size of this buffer is a
function of the parameter x depicted in Fig. 5.
• The minimum value for x corresponds to five time slots, and
can vary depending on network performance.
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SEAcast platform
• In SEACast data packets are simply forwarded from parent to
children nodes.
• As shown in Fig. 6, the publisher is connected to the SEACast
root node by means of a different Real-Time Transport
Protocol (RTP) connection for each scalable layer.
• Each SEACast client keeps a buffer of a few seconds for each
tree in which it participates.
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SEAcast platform
• The structure of the P2P tree generated with the SEACast
application is depicted in Fig. 6.
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Conclusions
• In P2P networks video is streamed to the user in a fully
distributed fashion.
• Network resources are distributed among users instead of
handled by a single entity.
• However, due to the diversity of users’ displaying devices and
available bandwidth levels in the Internet, the underlying
coding and transmission technology needs to be highly flexible.
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Conclusions
• In this article we have presented several advanced P2P systems
supporting streaming of scalable video and designed to support
future Internet applications.
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