| IP-phone conference system, also called video conference system, witch is a system equipment to transfer audio, video, and document data between two or more persons or groups in different areas through multimedia equipment. It can achieve on-line and interactive communication and obtain the purpose of the multimedia conference. The rapid developing Internet has covered more than 180 nations and areas, and it is the anticipant medium because of no ultimate limitation with the multimedia transmission, which makes the IP video conference system. It brings unparalleled concussion to conventional conference mode, and in addition, provides more convenient and high-quality communication method. IP video technique has obvious advantages, such as low cost and high-quality communication service, and various IP video schemes have been widely used in many fields of our national corporations. With the increasing global demand of video conference equipment, many corporations do business through video conference, remote phone conference, and network conference. With the distributed, real-time and synergic Internet, in the further period, the system will be more and more widespread. This paper studies IP-phone conference system based on ITU-H.323 protocol.The communication in H.323 multimedia communication system can be regarded as the admixture of video, audio and control information. According to the different function in system, they can be separated into control signaling protocol, media transfer protocol, audio and video code protocol, and data protocol. Each logic element in H.323 system is called the entity of the H.323 system, which includes Endpoint, Gateway, Gatekeeper and Multipoint control unit (MCU). And they are the terminal of the endpoint equipment in H.323 and the logic unit of the network. The terminal can transmit and receive calling, and media information stream will be generated or ended conclusion at the terminal. H.323 system includes the end-to-end connection between the H.323 terminal and other terminals through different networks. In order to achieve IP video conference system base on H.323, provide the system with stability and expandability and good interconnection with non-IP network, such as ISDN and PSTN, and use the IP network regularly when the conference system is off, it is necessary to design an appropriate IP network topology to carry the system. In this paper a RPR(Resilient Packet Ring) network module with flexible tree topology and star topology has been established. Open standard OSPF network protocol has been allocated, perfect linkage backup has been done with HSRP protocol, and finally, the technical index and performance of the network has been analyzed.MCU is the multi-point communication functional unit in H.323 system, and is used to support the network conference between three endpoints or more endpoints. In the H.323 system, a MCU is composed with one multi-point controller (MC) and one or several multi-point processors. The MC provides the conference control function between the endpoints. In multi-point conference, MC performed the capability exchange with respective endpoint. The MC transmits the capability set to the endpoint in the conference, which can show the accepted transmission mode. When an endpoint joins or leaves the conference, or because of other factors, MC can modify the capability set and transmit it to the endpoint. The MP can receive the audio, video and data stream from the endpoint, process the media stream and return to the endpoint.The integral structural design can be separated into centralized MCU design scheme, distributed MCU design scheme, and combined design scheme. In the centralized MCU design, every endpoint transmits audio, video and control data to MCU in the point-to-point form. In the distributed MCU design, a H.323 endpoint transmits the audio, video and data to other H.323 endpoint through multi-point transmission mode, not through MCU. But the control of multi-point conference is still processed by MCU. And in Combined MCU design, MCU will be placed at the center of the network as the server. In addition, there are distributed MCU at the endpoints. The relationship diagram of openH.323 class, MCU software module and engineering unit have been analyzed in this paper. The design of MCU has refer to many functional module and critical technique, the original functional achievement has been described in this paper, the design of connective signaling in the MC and the improvement of audio-mixture algorithm has been mainly discussed. The design of MC signaling is the precondition of MCU's working regularly, and is a technical difficulty of MCU. Because MCU is a special terminal in the network, according to the system demand and H.225 standard signaling message, it has been simplified, and using the idea of 7th signaling, through transmitting message and receiving message, the three situations can be combined, and call generating and releasing process of an endpoint has been designed. MCU control channel and logic channel generation process, conference control process, and conference conclusion process has been analyzed.Real-time multimedia communication operation in the packet network has obtained rapid development, multi-point communication products emerge as the times require. To simulate the situation of real conference place, enhance the comfort of the meeting, the system should make everyone anticipating the conference hear several speakers'voices at the same time, which need the system process the voices from several users. Voice admixture in the MCU network is one of critical techniques and has very important function. The main difficulties include choosing appropriate encoding/decoding method, multi-point voice synchronous control, voice admixture method and schedule scheme, and so on.With the incessant dissemination for the application of IP-phone conference, conventional voice admixture algorithm has emerged several shortcomings, which influences the quality of the conference. Therefore, adaptive voice admixture algorithm has been proposed in this paper, in which the power will be modulated with the variation of the voice amplitude in each branch. This algorithm avoids the noises, and performs good smoothing process when the number of endpoints changes, which overcomes the problem that the voice intensity will reduce after voice admixture. The simulation for adaptive algorithm has been made in this paper, which demonstrates the feasibility of the algorithm. And by comparison with conventional algorithm, it is easy to find the merits of the adaptive algorithm.In general, the IP-phone conference based on H.323 has been deeply analyzed in this paper. According the practical demand, RPR intelligent two-ring network that can carry IP-phone conference system has been designed using network layered technique. Certain improvement on the design of the call linkage signaling and the voice admixture algorithm of the important functional unit in the conference system has been made. And nowadays, there are a lot of faultiness in the system, and it will be improved in the further work. |
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