Research And Implement Of Video Communication System Over IP Network Based On DM642

With the development of network communications and multimediatechnologies, the application of delivering video over Internet is getting more andmore popular, such as video phone based on IP network, video conferencing anddistance learning. Moreover, with the rapid improvement of DSPs processingability, video processing platform quickly trends to miniaturization. The researchon video real-time communications based on embedded DSPs system becomes ahotspot. Transmission of real-time video has demands of bandwidth, delay, jitterand packet loss ratio. However, the current Internet does not offer any quality ofservice guarantees to video transmission over the Internet. Thus, designingprotocols and mechanisms for video transmission is facing many challenges. In thispaper, H.323 protocol and QoS (Quality of Service) control mechanism inapplication layer are researched, and an embedded real-time video communicationssystem is implemented, QoS control mechanism in application layer based on akind of feedback using RR (receiver report) of RTCP is realized.The whole system is based on Client/Server mode, and transmits real-timevideo stream from server to clients which have set up H.323 connection with thefront end server. System configuration includes three parts: front end server,communication devices and clients. Front end server is composed of analog camera,television and DM642 video processing platform running server programs;itcarries out system multipoint control and realizes video sample, local videorendering, video encoding, encoded video stream segmenting, and transmissionusing RTP;according to RTCP report packets come from clients, QoS controlmechanism in application layer analyzes and estimates network status to controltransmission rate. Communication devices are adopted transmission channels andrelated devices;communication network is LAN using Ethernet 10M switcher.Clients are composed of personal computers running client programs;they receivevideo stream from server, RTP headers of video stream are removed, and the restvideo data are assembled to video frame, then client programs decode video framesand display them;client transmits network status reports to server through RTCPchannels in order that server can control video transmission rate according tonetwork status reports.Server is built on TI DM642 video processing platform. TI DM642 is a kindof DSP chip designed for multimedia applications. Its core clock is up to 600MHz,and it has 8 parallel operation units, its processing ability is up to 4800MIPS. Formultimedia applications, DM642 video processing board integrates 3 configurablevideo ports, McASP for audio applications, 10/100Mb/s Ethernet MAC, and so on.In this paper, a simple H.323 video communication system using Client/Serverframework on DSP real-time operating system DSP/BIOS and Windows operatingsystem is designed respectively. This simple H.323 protocol consists of simpleH.225.0 call control protocol, simple H.245 media control protocol, embeddedRTP/RTCP protocol and video coder/decoder protocol, and supports applicationsby API functions.Selection of network transmission protocol has a significant effect onperformance of video communications system. Generally speaking, due to contentsof sampled video frames are different, sizes of encoded video frames are different,and the size of video frame may be longer than the data part of datagram, in thiscase, the encoded video frame must be handled. From the viewpoint oftransmission, according to maximum length of the data section of datagram, weshould segment video data to same length before transmission. UDP is adopted totransmit video data, but UDP does not guarantee delay, correct order of datagram,so related mechanism must be adopted at receiver to get correct sequence.RTP/RTCP is adopted to reduce disadvantages of using UDP. RTP/RTCP protocolcan support function of point-to-point transmission;it is suitable for real-time datatransmission. RTP is used to transmit real-time data, and RTCP is used to managequality of transmission and to supply related QoS information.Standard control system of H.323 protocol is very complex, this paper onlyresearch communication between endpoints. Server has the function of multipointcontrol to control the multipoint video communication system based on C/Sframework. Server manages multi-clients using three structures: Endpoint, CALLand LogicalChannel. Server uses these structures and stores data in messagestructures to exchange data, in this way, system control is realized.Communication procedures between H.323 endpoints can be divided intoseveral stages. First, H.225.0 protocol is carried out, call control channel is reliablechannel. After one call is set up, H.245 media control channel will be set upbetween endpoints. Second, H.245 protocol is carried out, media control channel isalso reliable channel, and it is used for terminal capability determination and modeenactment. Third, set up video logical channels between endpoints, and they areunreliable channels using UDP protocol. During communication procedure, H.245media control channel is used for parameter determination, and H.225.0 call controlchannel is used to end communication and to close logical channels and controlchannels.For QoS realization of video communications, control of video transmission isvery important. For video transmission, primary characteristic of channel inapplication layer is available bandwidth and packet loss. In the case of packet lossand variable bandwidth, QoS in application layer can promote the quality of videoin furthermost. Rate control attempts to minimize network congestion and theamount of packet loss by matching the rate of the video stream to the availablenetwork bandwidth. This paper mainly discusses realization of source-based ratecontrol. To reduce frequency of video sampling can reduce output video data rate.In this way, adjusted output data rate can be suitable for different networkbandwidth. Moreover, to reduce sending frame rate also can reduce sending rate.Video communication system can monitor QoS using sender report and receiverreport of RTCP. If RTP packet loss ratio is higher than some threshold, video datarate should be reduced. If interval of receiver report longer than some threshold,loss ratio section of receiver report should be checked. If the section shows networkcongestion, data rate should be reduced.Above all, in this paper, some key technologies of video communications areresearched, an embedded real-time video communication system is implemented,and QoS control mechanism in application layer based on a kind of feedbackcontrol using RR of RTCP is realized. The system effect is very well. Due tolimited time, this system is not perfect, and the author will improve it duringafterward works.