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VOD(VIDEO-ON-DEMAND)SYSTEMVideo on demand (VOD) or Audio video on demand (AVOD) systems allow users to select and watch/listen to video or audio content on systems either stream content through a set-top box, allowing viewing in real time, or download it to a device such as a computer, digital video recorder, personal video recorder or portable media player for viewing at any time. The majority of cable and telco based television providers offer both VOD streaming, such as pay-per-view, whereby a user buys or selects a movie or television program and it begins to play on the television set almost instantaneously, or downloading to a DVR rented from the provider, for viewing in the and streaming video on demand systems provide the user with a large subset of VCR functionality including pause, fast forward, fast rewind, slow forward, slow rewind, jump to previous/future frame etc. These functions are called trick modes. For disk-based streaming systems which store and stream programs from hard disk drive, trick modes require additional processing and storage on the part of the server, because separate files for fast forward and rewind must be stored. Memory-based VOD streaming systems have the advantage of being able to perform trick modes directly from RAM, which requires no additional storage or CPU cycles on the part of the is possible to put video servers on LANs, in which case they can provide very rapid response to users. Streaming video servers can also serve a wider community via a WAN, in which case the responsiveness may be reduced. Download VOD services are practical to homes equipped with cable modems or DSL connections. Servers for traditional cable and telco VOD services are usually placed at the cable head-end serving a particular market as well as cable hubs in larger markets. In the telco world, they are placed in either the central office, or a newly created location called a Video Head-End Office (VHO).[edit] In-flight entertainmentSome airlines offer AVOD as in-flight entertainment to passengers through individually-controlled video screens embedded in seatbacks or armrests or offered via portable media players. Airline AVOD systems offer passengers the opportunity to select specific stored video or audio content and play it on demand including pause, fast forward, and rewind.[edit] HistoryFrom September 1994, a VOD service formed a major part of the Cambridge Interactive TV trial[1]. This provided video and data to 250 homes and schools connected to the Cambridge Cable network (later part of NTL, now Virgin Media). The MPEG-1 encoded video was streamed over an ATM network from an ICL media server to set top boxes designed by Acorn Online Media. The trial commenced at a speed of 2 Mbit/s to the home, subsequently increased to 25 Mbit/s.[2] The content was provided by the BBC and Anglia Television. Although a technical success, difficulty in sourcing content was a major issue, and the project closed in 1998, Kingston Communications became the first UK company to launch a fully commercial VOD service and the first to integrate broadcast TV and Internet access through a single set-top box using IP delivery over ADSL. By 2001, Kingston Interactive TV had attracted 15,000 subscribers. After a number of trials, HomeChoice followed in 1999, but were restricted to London. After attracting 40,000 customers, they were bought by Tiscali in 2006. Cable TV providers Telewest and NTL (now Virgin Media) launched their VOD services in the United Kingdom in 2005 in attempt to snatch subscribers from the country's leading pay TV distributor BSkyB whose satellite-based network is unable to offer such a service. BSkyB responded by launching Sky by broadband, which was later renamed Sky Anytime on PC. The service went live on 2 January 2006. Sky Anytime on PC uses a legal peer-to-peer approach, based on Kontiki technology, to provide very high capacity multi-point downloads of the video content. Instead of the video content all being downloaded from Sky's servers, the content comes from multiple users of the system who have already downloaded the same content. Other UK TV broadcasters have implemented their own versions of the same technology, such as the BBC's iPlayer, which launches on 25 December 2007, and Channel 4's 4oD (4 On Demand) which launched in late 2006. The BBC, ITV and Channel 4 plan to launch a joint platform provisionally called Kangaroo in 2008.[3]VOD services were first offered in Hawaii by Oceanic Cable in January 2000[citation needed], but are now available in all parts of the United States. Streaming VOD systems are available from cable providers (in tandem with cable modem technology) who use the large downstream bandwidth present on cable systems to deliver movies and television shows to end users, who can typically pause, fast-forward, and rewind VOD movies due to the low latency and random-access nature of cable technology. The large distribution of a single signal makes streaming VOD impractical for most satellite TV systems; however, EchoStar recently announced a plan to offer video on demand programming to PVR-owning subscribers of its Dish Network satellite TV service. After the programs are automatically recorded on a user's PVR, he or she can watch, play, pause, and seek at their convenience. VOD is also quite common in more expensive hotels. VOD systems that store and provide a user interface for content downloaded directly from the Internet are widely to the European Audiovisual Observatory, 142 paying VoD services were operational in Europe at the end of 2006. Since the start of 2008 virgin media have significantly increased their ondemand content,[edit] Near video on demandNear video on demand (NVOD) is a pay-per-view consumer video technique used by multi-channel broadcasters using high-bandwidth distribution mechanisms such as satellite and cable television. Multiple copies of a program are broadcast at short time intervals (typically 10–20 minutes) providing convenience for viewers, who can watch the program without needing to tune in at a scheduled point in time. This form is bandwidth intensive and is generally provided only by large operators with a great deal of redundant capacity and has been reduced in popularity as video on demand is implemented. Pay-per-view provider In Demand provided up to 40 channels in 2002, with several films receiving up to four channels on the staggered schedule to provide the NVOD experience; however the service now provides only six channels of content.[edit] Push video on demandPush video on demand is a technique used by a number of broadcaster on systems that lack the interactivity to provide true video on demand, to simulate a true video on demand system. A push VOD system uses a Personal Video Recorder (PVR) to automatically record a selection of programming, often transmitted in spare capacity overnight. Users can then watch the downloaded programming at times of their choosing. As content occupies space on the PVR hard drive, downloaded content is usually deleted after a week to make way for new programmes. The limited space on a typical PVR hard drive means that the flexibility and selection of programmes available on such systems is more restricted than true VOD systems.[edit] List of video on demand servicesComcast Apple iTunes Store Google Video Rogers The Movie Network Xbox 360 Movielink CinemaNow Vongo ZML Hulu Joost 4oD BBC iPlayer Walmart Video Download (beta) -----------Much hope has been placed in the past on Educational Television for improving educational standards and distance education. However, in many countries the impact of this technology has been limited. Some of the reasons are discussed along with possible technical solutions. One solution that has been developed at HP Labs India is presented. This involves the use of satellite TV channels, using the MPEG format, as the distribution channel for educational content in digital form. Inexpensive TV equipment, including those that work with Direct To Home (DTH) TV transmissions are used to capture TV programs in digital form and store them on a server. The system is also capable of capturing text content being sent with the program in the Electronic Program Guide format as well as content in various other data formats including PDF. All captured content is stored in a digital library, offering students an on-demand video facility on campus. Easy search facilities are also offered to help find content items of interest.