Long Duration Continuous Media Retrieval
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responsible for sending/receiving signals to/from the VoD server. The buffer is used to store the video data before playback to maintain a continuous and jitters-free display.
Central Repository
Video Server
clients
Unicast client
Unicast clients
BroadcasV^ulticast Segment (LAN/Campus Network) Unicast clients Figure 1. An architecture of a typical large scale VoD system.
Caching To reduce the backbone network traffic, a proxy server can be installed between the central server and the clients as shown in Figure 1. In this hierarchical VoD system, video data can be temporarily stored in a proxy server. In general, the proxy server caches the most popular videos for users' repeating requests in order to minimize the backbone transmission cost. Upon the user's request received by the proxy server, it will send back the video request if the video have been already cached. Otherwise, it will bypass the request to the higher level and then retrieve the requested video from the central server. One of the challenges in this hierarchical architecture [4] is to decide which video and how many copies have to maintain at each proxy server. On the other hand, instead of storing the videos as entity, the proxy servers can just pre-cache the beginning portion of
Encyclopedia of Multimedia
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the video [5] to support the local customers in order to efficiently utilize the limited proxy server capacity. Apart from updating the video content in the proxy server periodically say 1 or 2 days, dynamic replacement algorithms may also be implemented to maximize the server's utilization. The least frequently used (LFU) algorithm [6] exploits the history of accesses to predict the probability of an object. However, it was originally designed for web caching purpose that does not work well for continuous media data like videos. A resource-based caching (RBC) algorithm [7] for constant bit-rate encoded videos was proposed to employ caching gain and resource requirement of videos to make a decision in a limited cache resource environment. In addition, some people suggested encoding the videos into a number of fixed-sized [8] or variable-sized [9] segments as an atomic unit for a caching policy. For a heterogeneous network environment such as Internet, the system may use layered encoded videos [10] [11] to flexibly provide different quality of the videos streams according to the clients' available bandwidth. In this case, some layers of the videos will be stored in the proxy server.
Broadcasting If the network supports the broadcast/multicast traffic, data broadcasting techniques [12] can further increase the system scalability. For example, if the video server is installed in the broadcast/multicast enabled network (see Figure 1) in which routers/switches support the multicast protocols such as Internet Group Management Protocol (IGMP) [13], all the users in this domain then enjoy the broadcast/multicast transmission technology and the video server can simply use the broadcast/multicast channels to serve a large group of
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