Joint control of MPEG VBR video over ATM networks

In video applications, digital encoding techniques are frequently associated to the use of compression algorithms. The rate of compression depends on the quality/bit rate requirements specific to each application. In TV broadcast/distribution applications, compression algorithms such as the MPEG2 standard allow economies in transmission bandwidth, while providing a service with a quality similar or better to the current analogue systems. The reduction in the number of bits necessary to represent a given sequence is achieved by exploiting spatial and temporal redundancy in the original video sequences. The number of bits necessary to achieve a given picture quality level will be variable because the redundancy in the original sequence is also variable. This means that compression algorithms such as the MPEG2 standard will generate Variable Bit Rate (VBR) encoded bit streams if constant quality is to be obtained. Likewise, a constant bit rate compressed bit stream will reproduce a video sequence presenting a variable picture quality. ATM networks allow the transmission of VBR traffic while providing quality of service guarantees to meet the real-time constraints of TV distribution services. Dynamic multiplexing schemes are able to instantaneously assign the available transmission resources among video sources according to their instantaneous needs. Considering that independent video sources are highly de-correlated, it is possible to re-use in other sources bandwidth made available by one source in periods of low activity. This means that n video sources, each requiring independently a CBR channel of bandwidth R bits/s, can be transmitted together over one single channel with bit rate less than R*n. This reduction in capacity can be obtained in the network through the use of the ATM technology and setting up n independent VBR channels for each video source. It can also be achieved directly at the user terminal equipment by requesting one single VBR channel with average bit rate less than n*R for all the n sources and dynamically multiplexing them before accessing the ATM network. Performing statistical multiplexing at both levels, benefits from the advantages of independent VBR encoding schemes already reported in previous experiments. It also allows a distribution of the negotiated bandwidth among different sources in order to maintain a certain level of quality for all of them while reducing the total required bandwidth.