d events to develop a "best-guess" present frame. back porch The portion of a video signal that contains color burst information and which occurs between the end of the horizontal synch pulse and the start of active video. back projection When a projector is placed behind a screen (as it is in television and videoconferencing applications) it is described as a back projection system. The viewer sees the image via the transmission of light as opposed to reflection used in front projection systems. backbone network A transmission facility used to interconnect distribution networks of typically lower speed. A backbone network often connects major sites (hubs). From these sites, spoke-like tail circuits (spurs), emanate and, in turn, often terminate in minor hubs. bandwidth A term that defines the information carrying capacity of a channel-its throughput. In analog systems, it is Video Communication: the Whole Picture the difference between the highest frequency that a channel can carry and the lowest, measured in hertz. In digital systems the unit of measure of bandwidth is bits per second. bandwidth-on-demand The ability to vary the transmission speed in support of various applications, including videoconferencing. In videoconferencing applications, an inverse multiplexer or I-MUX takes a digital signal that comes from a codec and divides it into multiple 56- or 64 Kbps channels for transmission across a switched digital network. On the distant end, a compatible I- MUX recombines these channels for the receiving codec, and, therefore, ensures that, even if the data takes different transmission paths, it will be smoothly recombined at the receiving end. Bit-rate Allocation Signal. Used in Recommendations H.221 and T.120 to transmit control and indication signals, commands and capabilities. baseband In a Local Area Network (LAN) context, this means a single high-speed information channel available to and shared by all the terminals or nodes on the network. Because there is sharing of this resource, means have to be p