The Design And Implementation Of Video Conferencing System Based On ALM

Real-time video stream technology, such as video phone, distance learning and video conference, plays an important role in all aspects of our life in the future. Besides, application layer multicast technology (ALM) is the very essential of video stream technology. The advantage of ALM exists in its convenience to implementation, promotion, optimization for special application by changing the end system.According to the functionality, real-time video multicast system can be divided into the following four modules:video capture module, encoding module, transmission module and server module. The hybrid P2P tree network architecture is taken use of to reduce the delay and the pressure of the central publisher, balance the load of the network and enhance the timeliness of video, and the server is responsible only for the management of the presence information of the member, which supplies support for the construction and maintaining of the application layer multicast tree. Contrarily, the transmission of the video is beyond its responsibility. We build the multicast tree based on a degree-constrained algorithm in which we consider not only the bandwidth but also the computation ability of the member. Moreover, in order to improve the efficiency of multicast, multi-threading technology and the completion port model is also used. The technology related to video capture module is already quite mature. Taking account of the convenience and simplicity of the development in Windows environment, we choose Video for Windows (VFW) platform. Because of the instability of ALM, we use H.263standard for coding, which is an ITU-T draft standard, designed for low speed stream communication. The coding algorithms in H.263is the same as that in H.261, but H.263makes some improvements and changes to improve the performance and the capability of error correction.According to identity, the entire system consists of three parts:server, publisher and client. Server is composed of only server module while publisher includes video capture module, encoding module and the transmission module. And client is mainly made up of the transmission module and encoding module. The system is a combination of the computer network technology and multimedia technology, which is quite practical.