Researche On Network Video Adaptive Transmission Mechanism And Strategy

Emerging multimedia application businesses on the Internet are particularly sensitive to network congestion,the changes of network status have greatly impact on the QoS(Quality of Service) of the businesses.However, the Internet based on TCP/IP currently provides a Best-Effort service model,local network congestion often occurs,which will lead to the degradtion of the QoS of the businesses.So,in order to improve the QoS of multimedia services,researches on how to transmit video better on the internet have great significance.This paper focuses on video transmission rate control mechanism, and tries to find out a network video adaptive transmission mechanism and strategy, in order to reduce the video packet loss, transmission delay and improve network bandwidth utilization, and finally achieve the purpose of promoting network video transmission quality. Network video transmission is sensitive to transmission delay, and requires the smoothness of the transmission rate, but it can tolerable certain rate of the packet loss, and without requiring reliable transmission.The rate control mechanism fo TCP based on the congestion window,which may cause the transmission rate sawtooth waving,and its retransmission mechanism is also not necessary for video transmission.The congestion control mechanisms based on detection such as AIMD(Additive Increase Multiplicative Decrease) and MIMD(Multiplictive Increase Add Decrease) will lead to transmission rate volatility and the increasing of the transmission delay, and therefore are not suitable for the network video transmission. TFRC(TCP-Friendly Rate Control protocol) adjusts the sending rate based on the TCP throughout model,under the same conditions,TFRC flows can be "fair" to the TCP flows to share the network bandwidth,meanwhile,the sending rate of TFRC is more stable and the delay jitter is very small.So it is very suitable for the video transmisson with the demand of smoothness of the sending rate.Firstly,this paper have used TFRC as the congestion control algorithm in the video transmisson,and improved TFRC in consideration of the characteristics of video transmission.The slow start stage of TFRC uses the loss of the packet as the only signal to end this stage.Simulation experiments show that the sending rate is not very matched to the receiving rate in the end of the slow start stage, which may lead to lots of packets be lost.So, this paper have used the difference between the sending rate and the receiving rate as the other signal to end the slow stage.In addition,TFRC uses the loss of the packet as the only signal of network congestion.However,to the video transmission,the transmission delay of the packets have greatly impact on video transmisson quality,and it can also be used as a warning signal of network congestion.So,this paper proposes a novel TFRC with the transmission delay as a warning signal of network congestion in the congestion avoidance stage,so as to the sender can earlier forecast to network congestion, and make corresponding transmission rate adjusting to reduce packet loss and transmission delay. Secondly,this paper combines the TFRC with RTP/RTCP(Realtime Transport Protocol/Realtime Control Transport Protocol),and provides a novel packet format specifications and RTP packetization algorithm based on RTP/RTCP which are suitable for H.264video. Finally,the simulation experiments based on the EvalVid and NS-2are implemented,experiment results demonstrate that the video adaptive transmission strategy based on TFRC is correct and effective.