| Real-time multimedia streaming will be the main transmission objective on the next generation network, which could have large effect on our lives. There exist many advantages such as reliable transmission, rapid reaction to network congestion, TCP friendliness, acceptableness to fire walls, convenient for implementation and maintenance and so on while using TCP to transmitting real-time multimedia streaming. However, on the other hand, the disadvantages are obvious as follows. The sending rate is unable to be adaptive to the available network bandwidth. The retransmission arisen from TCP would likely increase the delay jitter and confusion of multimedia packet. These disadvantages decrease notablely the quality of service of real-time multimedia streaming. Amied at the above problems, performance analysis of real-time multimedia streaming transmission, sending-delays prediction model, rate adaptive transmitting scheme and its application are researched in this thesis and the main creative achievements included are the following five aspects.(1) Requirement for transmitting real-time multimedia streaming via TCP with acceptable playing performanceThe process of transmitting real-time multimedia via TCP is deeply analyzed and then the requirement of transmitting real-time audio with acceptable playing performance via TCP is pointed as that the TCP MSS should be equal with the size of a audio frame. However, the transmitting efficiency of TCP will be very low under this condition so that it is infeasible in the Internet. The requirement of transmitting real-time video is also pointed as that the video frame sending-delays should be less than the results that play out buffer delays subtract half of the round trip time (RTT). The simulation results show that the requirement presended is reasonable.(2) The critical factor impacting on the end-to-end delays of real-time video transmitted via TCPThe end-to-end delays of real-time video are dissected by deriving and simulating, and it is discoveried that the waiting-delays are the main part of end-to-end delays of real-time video and the sending-delays are the critical factor impacting on the end-to-end delays. Two ways to decrease the end-to-end delays and jitters are presented, which are reducing the waiting-delays and cutting donw the sending-delays. Since the sending-delays are straightly and rigidly managed by the TCP stack, it is impossible for applications to control it. Therefore, a method to decrease the end-to-end delays by predicting and judging the sending-delays is proposed as follows. Predict the sending-delays of a video frame and if it can not achieve the requirement mentioned above, then the video frame will be selectively discarded. Utilizing the method of predicting and judging, the sending-delays of all the video frames deliveried are little. The simulation results show that the end-to-end delays of real-time video can be cut down notablely by using the method.(3) two real-time video sending-delays prediction modelsAimed to the two widely-used router loss strategies, DropTail and RED, two real-time video sending-delays prediction model are established in the thesis.1) The well-phased stochastic prediction model. The model microscopically considers the probability of video frame placed in diverse phases, especially the slow starting and RTO phases, which happen more easily while using DropTail strategy. The model's input parameters include frame size, loss ratio, congestion windows size, RTT, RTO time and so on. The mathematic expectation of sending-delays is looked as the prediction value of sending-delays.2) The recursive Markov prediction model. The model macroscopically treats the variation of TCP congestion windows during the whole TCP transmitting process as a Markov chain with discrete time and state. A recursive Markov prediction algrithm is presented, in which current congestion window size, RTT and loss ratio are input as the parameters of the state jumping matrix function, and congestion window size is input as the parameters of the reward matrix function. The recursive process of algrithm ends while the summations of prediction values are larger than the video frame sizes, and then outputs the prediction value of sending-delays.The simulation results show that the accuracy of both the prediction model are acceptable and the prediction values can be used in the method of predicting and judging mentioned above.(4) two real-time video rate adaptive transmission schemeConsidering the two way of decreasing the end-to-end delays pointed above, two real-time video rate adaptive transmission scheme are established.1) Rate adaptive transmission scheme based on multi-buffer scheduling. A sending buffer on application layer with delays level is set up to schedule real-time video frames between the sending buffer, TCP sender buffer, and play out buffer. Using the method of selectively discard video frames based on buffer delays level, the waiting-delays of each video frame is decreased to be only less than twice of encoding delays. Simultaneously, the video frame rate and sizes of play out buffer can be adjusted automaticly according the fluctuation of discarding ratio.2) Rate adaptive transmission scheme based on the sending-delays prediction model. Firstly, the sending-delays of video frame are predicted before it is sended using the sending-delays prediction model. Afterwards, the method of predicting and judging is ultilized to selectively discard the video frames without the acceptable playing performance. Therefore, the sending-delays of each video frame transmitted can be little and the waiting-delays of each video frame is cut down to be only less than encoding delays. At the same time, the video frame rate and sizes of play out buffer can be regulated automaticly according the fluctuation of discarding ratio.The simulation results show that compared with UDP and TCP, using either the adaptive schemes established in this thesis, the end-to-end delays and jitters can be reduced to a larger extent, the sending rate can be adaptive to the network congestion condition, and the key video frames can be avoided to be discarded. Moreover, the rate adaptive transmission scheme based on the sending-delays prediction model is better to decrease the end-to-end delays, howerver, it requires to get the loss strategy of bottleneck router, which brings forth some limitations.(5) Application and implementation of the rate adaptive transmission scheme in the real-time communication system based on 3G networkUsing the rate adaptive transmitting scheme based on TCP and utilizing the methods and techniques of object-oriented design, database optimization access, IO completion port, raw socket, and TCP NAT penetrating, a novel real-time communication system based on 3G network is designed and implemented. The system realizes the two-way real-time audio and video communication between PC in IP network and 3G phone in 3G network with the requirement of quality of service. The running results of system show that the visual quality of video can be improved obviously after using the rate adaptive transmission schemes proposed in the thesis.
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