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VBI (Vertical Blanking Interval)
Note: Many topics at this site are reduced versions of the text in "The Encyclopedia of Networking and Telecommunications." Search results will not be as extensive as a search of the book's CD-ROM.
Television signals include a nonviewable portion of signal that carries no visual information. It is called VBI (vertical blanking interval) and can be used to carry other information, usually close-captioned signals for the hearing impaired. Internet designers are now using it to transmit unidirectional digital information from Web sites to Web clients.
The model for sending information over VBI is often called "data broadcasting." The technique is a one-way transmission from the station to users, just like television. Stations broadcast popular information such as stock quotes, sports news, and so on. Users cannot interact with the Web server, but the technique is useful for delivering specific content and reduces the load on the actual Internet. Instead of sending this bulk news information to each subscriber individually over the Internet, the information is broadcast at periodic intervals to a special browser, which shows the latest updates. Anyone can choose to tune into the broadcast and select just the news items they want to view.
A company called WavePhore transmits data over PBS stations nationwide. All you need is a TV card in your computer to receive the transmissions, which appear in a special browser on your desktop. Since the service relies on over-the-air television signals, it is always on and a user's browser is constantly updated with the latest news.
RFC 2728 (The Transmission of IP Over the Vertical Blanking Interval of a Television Signal, November 1999) describes a method for broadcasting IP data using the VBI. It includes a description for compressing IP headers on unidirectional networks, a framing protocol identical to SLIP, a forward error-correction scheme, and the NABTS (North American Basic Teletext Standard) byte structures.
Copyright (c) 2001 Tom Sheldon and Big Sur Multimedia.