Video on demand (VOD)
is a technology for many important applications. A VOD system allows distributed
clients to play back any video from a large collection stored on one or
more servers. To accept a user request, the VOD server must allocate
enough resources to guarantee a jitter-free playback of the video.
Such resources include storage and network I/O bandwidth. Sufficient
storage bandwidth must be available for continuous transfer of data from
storage to the network interface card (NIC), which in turn needs enough
bandwidth to forward the stream to remote clients. To support a large number
of users, two types of multicast have been studied. In
Non-periodic
multicast environment, users make requests of videos to the server;
and it serves requests according to some scheduling policy. To conserve
server bandwidth, requests made by several clients for the same video within
a short period of time can be served as a group. In
Periodic Broadcast
environment, users do not make requests to the server. Rather, the
server broadcasts the video periodically. Although, this type of technique
does not guarantee true VOD, the worst service latency experienced by any
client is always less than threshold. A distinct advantage of this
approach is that it can serve a very large community of users using minimal
server bandwidth. In fact, the bandwidth requirement is independent
of the number of subscribers to the system.
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VCR-like
interactivity in a Periodic Broadcast Framework
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In video-on-demand (VOD) applications, it is desirable to provide the
user with the video-cassette-recorder-like (VCR) capabilities such as fast-forwarding
a video or jumping to a specific frame. In the broadcast framework,each
video is broadcast repeatedly on the network. Existing techniques
rely on data prefetching as the mechanism to provide this functionality.
This approach provides limited usability since the prefetching rate cannot
keep up with typical fast-forward speeds. Fast-forwarding a video
for several seconds would inevitably exhaust the prefetch buffer. Because
of the nature of the periodic broadcast paradigm, the server can periodically
broadcast interactive versions of videos. For instance, an interactive version
might contain only every fifth frame in the original video. The client
software can therefore leverages these “interactive” broadcasts to provide
better VCR services. Our work is to investigate this new approach and compare
it to data prefetching algorithms.
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Bandwidth
Heterogeneity in Periodic Broadcast Video Streaming.
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Essentially all existing periodic broadcast techniques require the VOD
service providers to tailor their system design to fit the capability of
a specific class of users. This approach has two disadvantages.
First, users with lower bandwidth will not be able to watch the video as
continuity cannot be guaranteed; and second users having more bandwidth
at their disposal will not be able to benefit from their surplus.
To address the above drawbacks, we can consider encoding the video stream
into a decomposition of layers. Depending on the bandwidth available
at the end user, more or less layers are received, resulting in a variation
in the display quality. This strategy can be adapted for periodic
broadcast to provide adaptability to different receiving bandwidths.
This simple solution, however, would present some drawbacks for a high quality
VOD system. Firstly, a user with lesser bandwidth is enforced to sacrifice
video quality; secondly, it is very demanding on user bandwidth to obtain
a satisfying image quality; and finally, it is difficult to implement. One
solution to this particular problem is to have a single broadcast strategy
and different reception strategies tailored to the bandwidth capacity of several
classes of clients.
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Database
Terrain Updates in Distributed Interactive Simulation.
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In real-time interactive training and simulation systems, changes of
the natural terrain surface as well as other non-moving objects are important
to simulators. In fact, entities moving to new regions need to know the
latest updates to the dynamic terrain. Two scalable schemes to communicate
this information to moving entities are discussed. In the first scheme, changes
of states of the dynamic terrain are multicast to entities, while the second
sheme communicate the changes using a streaming periodic broadcast technique.
Both techniques present good performance under different situations. Under
heavy entity traffic the streaming periodic broadcast technique outperforms
the multicast scheme, and if the for lower traffic the multicast-based technique
outperforms periodic broadcast.
In Overlay Networks, the infrastructure-based
approach lessens the bottleneck burden at the server side due to the fact
that clients can get services not only from the server, but also from overlay
nodes. However P2P video streaming presents important design issues. Firstly,
P2P streaming systems should offer a short access latency (quick joining
time), allowing new peers to receive the desired video quickly. Secondly,
A quick and graceful recovery procedure is needed to handle peer failures.
The failure recovery procedure should not only reconnect a disconnected peer
to another peer, but must also quickly localize the failure such that only
few peers are affected. Finally, Overhead for exchanging information among
peers must be kept small. Existing techniques in the P2P approach can be
categorized into techniques supporting live video streaming and those that
support pre-recorded video streaming. Some techniques can offer both services.
Live video streaming differs from pre-recorded video streaming in two important
aspects. First, the access latency is more crucial to live video streaming
than to pre-recorded video streaming. Second, a user joining a current streaming
session of live-streaming is only concerned about a stream starting from
his/her joining time. Third, degradation of video quality for live video
streaming is crucial since the option of watching the video for a second
time may not be available.