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Hierarchical VOD System

video layers proxy videos

Definition: A hierarchical Video-on-Demand system consists of the central server and several layers of proxy servers.

Hierarchical network architecture is one of the solutions for building a cost effective VoD system. Figure 1 shows the architecture of a hierarchical VoD system . The central server has a large storage to store all available videos for the customers. The central server is connected to several proxy servers, which are physically located closer to the customers. Also the proxy servers are able to store a small portion of videos compared with the capacity of the central server. In a heterogeneous environment like Internet, layered encoded videos can be used to provide different quality of video streams. To serve the low bandwidth environment, the system simply transmits the base layer of the videos to provide the basic video quality. The quality can be increased with the enhancement layers of the videos if the customers subscribe the broadband service.

In a layered caching strategy, it is assumed that the m th video is encoded into l m layers. Denote p m be the popularity of video m . Upon the arrival of the customer’s request, the proxy will serve the video request from its cache if the requested layers have been already cached. Otherwise, it will bypass the request to the higher level and the missing video layers will then be retrieved from the central server. By the layered video concept, the number of layers requested is determined by the available bandwidth between the server and the customer. The number of layers in the request is independent of the video being requested. Assume that r j is the probability of the customers requesting the j th quality of the videos and, hence we have . In order to consider the property of the layered video, i.e. all the lower quality layers must be stored before caching the enhancement layer, we define q mj as the fraction of the customers requesting the j th layer of video m such that The popularity of each layer can used to determine which layers of the videos should be stored in the proxy. The proxy stores the most popular layers to maximize the cache hits. Defining b mj is used to describe the subsets of the video layers in the proxy. b mj is set to 1 if layer j of video m is in the proxy. Otherwise, it is set as 0. The optimization problem is formally stated as follows.

where s mj is the size of layer j of video m, K is the proxy size and M is the total number of videos.

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