VoD_main2

A Video-on-Demand system has many elements that are necessary for the use of the complete service. This includes video servers, community network, switching office, set-top unit, and backbone network (on the figure 1). VoD system providers will offer services which select the right technology, features, performance, price, reliability, and ease of use. Equipment are developed so that they will allow to operate in different environments and in a variety of services. ** Figure 1 : ** Inside VoD The main VoD scenario consists of a local database and server connected to the user via a communications network. The data is stored on local distribution sites which are connected through high speed backbone network to information archives and video servers. This distribution scheme serves many purposes. First, it is possible to implement it in a distributed fashion, increasing availability and reliability. Second, a provider can tailor the information delivery to the specific tastes of a user community in a partic ular geographic area, reducing costs. Third, it is easier to manage, as each local system is responsible for its own billing and accounting. Fourth, the system can be constructed in a regional, piecewise fashion.  ** The user interacts with the services by the set-top unit in the subscriber premises. Along with the television and the remote control, it gives for the consumers opportunity to be connected to a video server and browse through a selection of movies or contents such as news or games. The key components of the set-top device are the line transceiver, demodulator, decompression unit, back-channel interface, remote control, and display driver. The cost of the set-top unit must limit to reasonable price (few hundred dollars) for the VoD technology to succeed. Open and interoperable systems that let the users to subscribe to several different services are preferred.
 * - Inside Video On Demand - **
 * - Element to use Video On Demand -

The switching office means both the telephone companys central office and the cable company s head-end. It is the place where services are fed and distributed to individual subscribers. It contains a head-end, video dial tone gateway, switches, and video servers.
 * a. Switching office**

In the head-end equipment, the video streams are formatted and organised to get th em into the community network. If ADSL is used, it switches the video streams onto the subscriber loops like it does today with telephone calls. When coaxial cable is used, the head-end is basically the same as CATV providers use today, except that the di gital channels need digital modulators.
 * b. Head-end**

The video dial tone (VDT) is an asymmetric switched video service in which the customer chooses among a wide selection of video material and receives real time response. Gateway is the entry point for an information provider to a carrier s VDT network. It should create and manage the connection between the information provider and the set-top device. The standardisation of the interfaces and functions of the VDT gateway is under way.
 * c. VDT gateway**

The video server is the network equipment providing the storage for video program material, which can be requested by the customers. It has to perform many functions, such as admission control, request handling, data retrieval, guaranteed stream transmiss ion, stream encryption, and support of functions found in VCRs including pause, rewind, and fast forward.  The video material can be stored on a combination of magnetic or magneto-optic disks, and magnetic tape devices. Different storage media offer different memory bandwidth for VoD services. The more popular movies are stored on RAM, the less popular ones on hard disk and the least popular on tertiary storage. Optical system like a CD-ROM and magnetic system like a tape drive will be possible media for inexpens ive tertiary storage for archival purposes. This kind of storage system reduces operating costs and can offer a wide selection of programmes to customers. In contrast, the downloading and caching of complete programme at user home has many disadvantages. First, communication bandwidth is often so limited that the user must wait for most of the information to arrive. This delay could be unacceptable. The customers equipment will cost too much, because caching an entire programme needs a large amount of mem ory. Finally, information providers do not accept that their programmes are available for data duplication and piracy. A metadata server is a database system which manages the metadata information. It contains abstract information about the location and characteristics of the data to be retrieved. The user can look for the information summary and select the programme to get it to the home.
 * d. Servers**

Outside of the local switching office, the backbone network connects it to the other video servers which are not in the local switching office and provide some national or specialised information. Currently, the high speed backbone network uses fiber cable and SDH-based transmission system. In the future ATM technology comes to the backbone network and then probably also to the community networks to simplify the interface requirements.
 * e. Backbone Network**

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