| With the development and popularization of Internet technologies, the simpleinformation such as texts, images and sounds can not satisfy users' requirements,and these will be replaced by a kind of new multimedia information which iscomposed by audio, video and data. It is easier than ever for users to get richermedia content nowadays, and these provide the shared resources through a wide range ofbroadband equipment. It is such a vast broadband network that stimulated theapplications of "Streaming media". Streaming media technologies make it possiblefor the users to enjoy files without the necessity to wait while downloading them.The aim of Streaming media application is to enable the users of streaming mediaexperience media information seamlessly and immediately. Streaming media hascharacters of real-time, high speed and wide bandwidth;however Internetresources cannot match its requirements. The conflict between limited Internetresources and extensive application of streaming media is becoming progressively;so it is an exigent subject to economize the internet resources to make sure theapplication of streaming media.Internet is an isomeric network, with its bandwidth fluctuating, transmissiondata more prone to lose, lacking of adequate QoS (quality of service) to ensurereal-time multimedia application. With the increase of multimedia traffic, networkcongestion phenomena such as packets delay and lost often occurs,which willcause multimedia deadlock and image blur, asynchronous audio and video data andother quality problems. The key question among applications of streaming mediain the network is how streaming media data can be adapt to different networkconditions. With the development of real-time multimedia technologies,the upperlevel application puts forward the QoS requirement of the network. The users hopeto use the VOD (Video-On-Demand) service with quality assurance on the network.Therefore, the research of flow control strategy is of great urgency. To design andimplement a network communication system with flow control mechanism is thetarget of this paper.In this paper Visual C++ platform is used to develop the networkcommunication system, and the system is running on windows 2000 professionaloperating system. This system adopts the CLIENT/SERVER structure, which hastwo subsystems--server and client,and utilizes corresponding network transmissionmedia format. On the foundation of the whole system framework, the correlatedtechnique: the RTP(Real-time Protocol), the DirectShow technique , the SOCKETprogramming technique and the multi-threading technique, are given a particularanalysis and discussion ,and the software design and implementation of detailedprocess of each mold of this system are presented. Firstly, the paper analyses of thenetwork communication system features, structures, and network communicationprotocol framework, especially analyses the RTP protocol, makes use of MicrosoftDirectShow SDK(software development kits) and WinSock to build the systemframework and design the network communication system. The system includesstreaming media server, storage system, player, transmission control, programs andmanagement parts. Secondly, with the deep work over the SERVER and CLIENTrequirements, SERVER, CLIENT and network transmission modules aredeveloped. Other discussions involving how to make up connecting, complete thecontrol of information transmission, achieve media stream transmission betweenclient and server, adopt flow control algorithms in the streaming mediatransmission, and achieve a flow control function are all within the research of thispaper.At present, there are many streaming media flow control methods, such asadmission control, buffer management and congestion control. On the basis ofdiscussing the characteristics of network transmission and real-time multimedia,the paper analyses the QoS control problems in real-time multimedia applications.For protecting the quality of the network communication system, this paper takesthe measure as follows: set up admission control at the SERVER, which willprevent performance from suddenly declining caused by a large number of visitors;At the same time, client adopts double-buffer model which uses the buffer zonewith the DataList and PoolList to manage transmission media data in the clientsystem. In addition, through the SR and RR reports in RTCP that communicatesbetween SERVER and CLIENT, the SERVER can adjust the data transmission ratedynamically to prevent the bandwidth fluctuation. By these methods, the systemdesigned in this paper protects the QoS for the network communication system to acertain degree.The admission control mechanism implements as follows: set a variable tolimit the largest access client numbers that the server can respond. And also set acounter, with its initial value set to zero when the server starts. While a client visitsthe server, the system first compares the current value of the counter with thelargest access client numbers. If the counter value is less than the largest number ofclients, authorize client to access, and increase the counter by 1. Otherwise, theprogram will refuse the client's request, and notice the client that the server is full.When a client leaves, the counter will minus 1. Under normal circumstances, thecounter should not be set to negative values. Through such methods, the effectivebandwidth of online clients is ensured and the aim of flow control is achieved.The transmission of video signals in networks is facing many problems suchas: delay, instable available bandwidth, high-bandwidth requirements and differentvideo applications. Buffer technology is one of methods to solve these problems.When client selects a program, it needs to wait for a buffer period for that themulti-media data will be transmitted to the client's buffer. In this process, if there isa decrease of bandwidth, it also needs buffering though the buffer still empty.Client can automatically change the size of the buffer. If the state of the network isbad, the buffer size will be larger, otherwise it will be smaller. This methodeffectively reduces the influence resulting from the instability of the networkbandwidth,and realizes the goal of flow control.The rate control algorithm base on TFRC (TCP Friendly Rate Control) in thispaper designs the following functions: before sending data, the server encapsulatesit to RTP packet, adds RTP header to the data, including time stamp, sequencenumber and so on, and transmits the RTP packet through UDP;then active theRTCP, make a statistics of total packet numbers that have been sent, and send theresult to the server;at client end, client handles the received RTP packets, thensends the results to the server. The server will analyze the RTCP packets back fromclients, according to the present situation of the network, and adjusts the sendingrate dynamically to fit the situation of the network.The paper improves the network transmission quality through the flow controlmechanism and attains a higher quality of service level. With the rapiddevelopment of computer network and multimedia technology, the networkcommunication technology is widely applied in hospital, education, public servicesand other areas. The network multimedia system with flow control mechanism hasa practical application value and a broad prospect. |
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