Realization Of PC Based Visual Phone Under H.324 Standard

As the development of Information Technology, a single phonetic service supported by PSTN can no longer meet people's needs. How to provide people with multimedia services through PSTN has been a commonly concerned issue. For this reason, ITU has recommended H.324 Terminal for low bit-rate multimedia communication. H.324 is an international standard. It specifies the system structure, video and audio frequency compression, mixed signal transmission and phone line signal format of a terminal system, so that through the system the video communication can be undertaken on PSTN. The aim of this research is at the realization of video telephone system software satisfying the H324 standard.It is very difficult work in technology, because the video telephone system involves the tasks of media data collecting, compressing, multiplexing, controlling, transmitting and the interaction among the tasks. We have now realized the modules of sound and image acquisition and compression under H.324 protocol, multiplexing under H.223, controlling under H.245, modem communication and connecting into a complete system using multithreads. We have proposed in the research some new methods which deal with the key units of H.223, such as the head of multiplexing frameN package ending mark, resending mechanism. These new methods increased the efficiency of multiplexing and make the system more robust in weak channel. On unstable speed lines, we added a management of Quality of Services (QoS) to the controlling module to control in real time the bit-rate of transmission. In stead of V25ter AT indicator, we applied TAPI technology to realize the modem control and complete call connection.In the paper, Chapter 1 introduces the structure of H.324, the possibility of realization on PC and realization scheme. Chapter 2 introduces the methods for image and voice acquisition and play back. Chapter 3 discusses in detail the H.223 multiplexing protocol and their improvements in the realization. Chapter 4 emphasizes on the speed and system controlling of video telephone. Chapter 5 introduces each sub-module in a modem communication module. Chapter 6discusses in detail module assembling, synchronization and compatibility and an improvement suggestion for the system.