Research And Implementation Of Virtual Class Teaching System

The network and multimedia technology made great development, it makes the transmission of video, audio and picture realistic. At the same time,modern distance education comes into being. With the cost of virtual studio decreasing,it is popular.The application of virtual studio in education enriches the courseware and innovates the method of teaching.The virtual class teaching system combines the advantage of modern distance education and virtual studio.The practice shows that it attracts the student's attention and improves the efficiency of learning.Content-based encoding is the characteristic of MPEG-4.It takes the corresponding method according to the different content to achieve the high rate compressing.MPEG-4 is so popular that it can not only be used to high quality digital television, but also be used to extreme low rate application,i.e.mobile multimedia communication .Virutal class teaching system adopts the hardware encoder complying with MPEG-4 ASP profile.RTP/RTCP is a special protocol for multimedia communication.RTP is used to transmit the real-time data, but it does not control data flow. It needs RTCP's assistance.The application uses the RTCP to control congestion and communicate other control information.So, it needs to make deep research on the interval of RTCP report's transmission, packet lost rate and jitter.Corresponding to unicast and broadcast, multicast can definitely decrease the traffic load of bone network.Therefore, multicast is suitable for the transmission of multimedia. The control plane and data plane is implemented. The receiver module is embedded into a ActiveX control. Therefore, it made the installation of software convenient.Internet access maybe have different bandwidth.For example,some may be accessed through the telephone line,some may be accessed through ISDN ,some may be accessed through Ethernet.This is a big challenge.If the traffic rate is low,the client accessed through Ethernet wastes much of bandwidth,reducing the performance of playback;if the traffic rate is high,the client accessed through telephone line can't receive the data regularly.To solve this problem ,the scalable multicast is realized.The different kinds of frame is transmited through different multicast address.At the multicast receiver,the data received should be sorted for playback. With the SR report ,the synchronization of video and audio is implemented using the RTP timestamp and NTP timestamp.