Real-World Multimedia Systems
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the data does not arrive in time, the playing back process will stop and the artifact can be easily picked up by human ears and eyes. Third, multimedia data stream is usually bursty due to the dynamics of different segments of the media. For most multimedia applications, the receiver has a limited buffer. The bursty data stream, if not smoothed, may overflow or underflow the application buffer. When data arrives too fast, the buffer will overflow and some data packets will be lost, resulting in poor quality. When data arrives too slowly, the buffer will underflow and the application will starve, causing the playing back process to freeze. Other characteristics of multimedia data include powerhungry, synchronous, loss-tolerant, having components of different importance, highly adaptable, etc. Some of the characteristics such as loss-tolerance, prioritized components and adaptability can in fact be exploited in a real-time multimedia communication system. Contrary to the high bandwidth, real-time and bursty natures of multimedia data, in reality, networks are typically shared by thousands and millions of users, and have limited bandwidth, unpredictable delay and availability. For example, the Internet provides only the best effort service, i.e., data packets can be lost, re-ordered, or delayed for a long time. As a result, advanced networking technologies have been designed specifically for the efficient delivery of multimedia data. There is typically a trade-off between delay and quality. Different applications may require different levels of quality of service (QoS).
Real-Time Networked Multimedia Real-time multimedia can be broadly classified into interactive multimedia and streaming media. Interactive multimedia applications include Internet telephony, Internet video-conferencing, Internet collaboration, Internet gaming, etc. In interactive multimedia applications, the delay constraint is very stringent in order to achieve interactivity. For example, in Internet telephony, human beings can only tolerate a latency of about 250 milliseconds. This imposes an extremely challenging problem for interactive multimedia applications over the Internet that provides only the best effort service. Over the years, great efforts have been made to facilitate the development of interactive multimedia applications over the Internet. For example, Microsoft Research's ConferenceXP Research Platform 1 supports the development of real-time collaboration and videoconferencing applications by delivering high-quality, low-latency audio and video over broadband connections that support multicast capability. The second class of networked multimedia technology is streaming media. Streaming media technology enables the real time or on demand distribution of audio, video and multimedia on the Internet. Streaming media is the simultaneous transfer of digital media so that it is received as a continuous real-time stream. Streamed data is transmitted by a server application and received and rendered in real-time by client applications.
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