| The author has been working on videoconference system research for about 1 and a half years, during which time has undertaken the planning and construction work of the remote education H.323 videoconference system for the Online Education Academy of Sichuan University, the construction work of the encrypted videoconference system for the Secrecy Bureau of Chengdu government, and the development work of the videoconference system for Beijing ShiXun Inc, and also some research works in JAVA programming and JMF areas. Based on the research of videoconference systems of H.323 protocol over IP networks and the author's experiences of implementing H.323 videoconference systems in remote education area, in this thesis the main factors that affect videoconference quality are analyzed, and a dynamic bit rate allocation model is proposed and partly implemented. This model is designed to dynamically allocate bit rate for multi-media data flow (including audio data and video data) in fixed bandwidth network environment. When continuous multi-media packet losses are detected in IP based H.323 videoconference system, the bit rate of video data is adjusted meanwhile the bit rate of audio data remains unchanged, and the bit rate allocation of multi-media data (including audio data and video data) is optimized as a whole effect.The topics in this thesis are analysing the drawbacks of current H.323 videoconference systems and popular solutions of them, discussing the advantages of dynamic bit rate allocation model and its features. 5 sub-modules implement the model. The dynamic bit rate allocation module receives relevant data from the Loss Rate of multi-media data packet, Jitter, Delay, and compares these data withpredefined values to make one of these 3 decisions that: decrease the video bit rate (Stat=DECREASE), or maintain current video bit rate (Stat=HOLD), or increase video bit rate (Stat=INCCEASE). The video bit rate will be adjusted according to the decision and the total bit rate is changed accordingly. In this way the impact of network environmental variation is settled, and consequently the adaptability of videoconference systems in remote education area is improved.The features of the dynamic bit rate allocation model are listed below:1. The existed dynamic bit rate allocation algorithms that only involved the situations of data Loss can't meets the requirement of the videoconference system. The model can take care of the situations of data Loss, Jitter, Delay in videoconference systems. But at the same time the complexity of the model has been increased.2. When the existed dynamic bit rate allocation algorithms is working, the video data bit rate and the audio data bit rate are changed. Comparing with the existed dynamic bit rate allocation algorithms, the model can provide better audio data quality by adjusting the video data bit rate. This meets the requirement in remote education system.3. The model is implemented in Java language. Therefore can benefit from the OO concept supported by Java with better program structure and class interfaces. And also the model is independent of operation system because Java has the "Once developed, use everywhere" transplant feature.Combined in the environment of the videoconference system provided by Beijing ShiXun Inc, preliminary experiment of the dynamic bit rate allocation module has been conducted on the ATM lines of the Online Education Academy of Sichuan University. The correctness and practicability of the dynamic bit rate allocation algorithm are verified by the analytical result of the experiment data.The structure of this thesis is as follows: the first chapter introduces the project background; the second chapter introduces related H.323 protocols and RTP/RTCP protocols; the third chapter details the components and the implementation of the bit rate adjust model; the forth chapter gives the experiment data of the model on ATMconnections, and the last part the summary of this thesis by analyzing the experiment data.
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