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Designing A Video On Demand Server Essay, Research Paper

Designing a Video on Demand Server

INTRODUCTION

Video-on-demand (VoD) service will allow customers to request that certain movies or television shows be played on their television sets or computers immediately, giving individuals direct access to the materials stored in a video server. The goal of this project is to design a Video-on-Demand Server capable of performing to the following specifications:

+ Capacity – 25 feature-length movies

+ Serve 4 different movies simultaneously

+ No loss of video/audio

+ No “stutter” or skip

+ 100% fault tolerance

As previously discussed in the cost estimate, the Video on Demand (VoD) Server will be based on a multi-processor x86 architecture. The VoD server with all elements described in the cost estimate (excluding RAID level 4 support) will be implemented on the Windows NT operating system running on a 500MHz dual-processor Pentium III Xeon system with 512MB of memory. The configuration will consist of two Ethernet interfaces connected to an Ethernet switch on a T3 backbone, providing a peak network bandwidth of 30 Mbits/sec. Storage will consist of a set of four 18GB ULTRAWIDE SCSI-2 drives configured as a striped logical volume using standard Widows NT system tools. To further increase transfer speed, a 64MB hardware disk cache will also be used in the system. Arrays of optical drives will also be employed as secondary storage. The VoD Server will be linked to an array of Uninteruptable Power Supplies (UPS) capable of supplying 10 minutes of backup power.

Similar servers were tested in this configuration with multiple client access and was able to saturate the network with 150 clients playing MPEG1 and 2 streams. The effective network usage was 25 Mbits/sec. However, the network is saturated at this level due to the overhead associated with the TCP protocol. I expect CPU utilization vs. number of clients to be nearly linear as the number of clients increase.

In the following pages, I will discuss the qualities of an effective video server. Furthermore, I will also discuss the qualities of the video server which I have designed for this project.

HARDWARE

ARCHITECTURE

Video servers are basically large repositories for data and information which are provided on demand to clients. The VoD server architecture can be defined by the way the data is stored within the video server. Data can be stored locally within a server or can be distributed across a large number of servers. Thus, two distinct methodologies exist for video server configuration:

Autonomous Servers: Video servers of this type can be viewed as stand-alone entities since they store encoded streams locally to themselves and. thus, do not require the cooperation of other servers to serve a VoD client. The actual physical implementation and the logical implementation of the video server are relatively the same. The server usually consists of a CPU, a storage medium (which can consist of a standard storage hierarchy), and a network interface.

Distributed Servers: Instead of storing entire video streams locally, encoded streams may be spread out among a larger number of video servers which are usually distributed over a local (but can be a wider) area network. Thus, in order to serve a VoD client, a set of servers need to cooperate. The client may need to be aware of the topology of the VoD servers. The encoded bit stream can be stored in any granularity throughout the system.

Autonomous servers suffer from a well-known problem associated with centralized processing, i.e., the common point of failure. In this case, when an autonomous server fails, all the clients being served by it are affected. In the case of the distributed paradigm, if we assume that some loss of data can be tolerated at the cost of reduced Quality of Service (QoS) to the clients then when a server fails all the clients being served by the distributed system are affected as they all have a part of the encoded stream stored on the faulty server.

One of the driving forces behind the distributed server paradigm is the concept borrowed from Redundant Arrays of Inexpensive Disks (RAID). In order to serve a large number of clients from a VoD server, the server needs to have a large data bandwidth from the storage system to feed the demand from the VoD clients. In the field of scientific computing this large demand for bandwidth was provided by the RAID paradigm. For a VoD server, however, it does not appear that providing a large data bandwidth will necessarily lead to a larger number of clients being serviced. This is due to the fact that RAID was designed for a small number of large bulk data transfers which achieved the higher bandwidth. A VoD system, however, must service a large number of relatively low bandwidth requests. In the case of MPEG1 & 2 encoded video, this is typically about 2.5 Mbits/sec. Thus we might achieve better results by replicating autonomous servers to provide service.

The distributed system suffers also from the overhead associated with coordinating resources which are distributed across to service a single request. This overhead could be in terms of network bandwidth (by the use of an extra messaging protocol) or CPU usage. Alternatively, if the client needs to be aware of the server topology, the design may be harder to port to other network topologies. Thus, the best approach appears to be the autonomous server paradigm combined with the optimizing of each server to provide the best service it can by itself. These servers can then be distributed around a wider area network which can then provide service to VoD clients more efficiently. The VoD client can find the closest server storing a specific stream and fetch it on demand from that server.

STORAGE SYSTEM

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