Based on H. 320 and H. 323 Conference Television System Technology Development Information Industry Telecom Transportation Research Institute China Tel Laboratory Liu Xiaohong Conference TV is a modern communication method, and is also one of the important signs of a national or regional communication level. In recent years, people are being In this way, "three networks", in this fusion process, video communications will be extremely important market drivers, people expect: via the network, anywhere, can viaway, watch and on-demand global TV programs in any place. From a simple voice transfer to remote video and data transfer transformation, it has created conditions for China's open conference TV communication. Recently, the "National First Vision Telephone and Remote Video Conference Equipment Exhibition" was held in Shenzhen, China Telecom also listed the conference TV business as one of the telecommunications services promoted this year, which will will promote the development of China's video business. The conference TV has been developed as a pioneer industry for digital video communications for more than 20 years, with the trend of three networks, mature based on H. 320 The conference television system will now enter the transition phase, that is, the ISDN and the private network transition to the packet-switched IP network; the market is desired to a small working group meeting room, personal work desktop until the family . Therefore, the multimedia communication standard H based on the packet switched network. 323 Video communication has become the focus of industry insiders and users. Based on H. 320 Conference TV system based on H. 320 Conference TV is a multimedia audio-visual conference business in which information such as transport activity images, voice, application data is a interactive telecommunications service. H. The 1st edition of the 320 agreement was launched in 1989. It is currently the second edition. After years of development and practice, whether it is functional, performance, or in terms of engineering implementation and systematic reliability, this system is very mature. In the field of professional conference TV system, especially in large, key systems, dominant. The conference television system is mainly composed of a multi-point control unit (MCU) of the terminal device, the transmission channel, and the network node. Among them, the terminal device includes a video input and output device, an audio input and output devices, a video codec, an audio codec, and multiplexing / splitting devices. The video codec is the core device of the conference television terminal, H. 261 It is recommended to specify an image quality to ensure the unified algorithm and public image format recognized by countries, so that different television systems recognized can be directly interoperable. At the same time, it uses the characteristics of the amount of activity in the conference TV, using the hybrid encoding method for predicting coding and transform encoding and video composite encoding methods to accommodate the transmission of narrowband digital channels. The video codec operates at a rate of 64 to 1920kbit / s, which can cover the PCM channel.
In a multi-point communication environment, the video codec should support the MCU multi-point switching. In all telecommunications services, meeting business is the highest in real-time requirements for conversations, so meeting services are usually implemented in circuit switched networks. In principle, as long as it is a network that provides circuit swaps such as: DDN circuitry, satellite circuits, and ISDN circuits, H. 320. The synchronization signal transmitted on the network is strictly in accordance with the timing. If the network duration is often in an uncertain change, the received television signal will be distorted, and the heavy image is completely destroyed, so the system strict requirements The network is transmitted and determined. The ISDN circuit is characterized by using a public switched telephone network to communicate by dialing; its basic rate interface can transmit audio and video signals of audio and video telephony; its base group rate interface can transmit home recorder quality and conference TV quality video and Audio signals, but is highly required to support channel quality, usually used to support point-to-point or small-scale TV conferences. DDN circuit, satellite circuit network is extended (usually within dozens of milliseconds), and is the determination of delay; communication bandwidth is usually 384 ~ 2048kbit / s; usually use 384kbit / s bandwidth to form a good quality Conference TV system; this system has matured, functionally, and has a complete performance of large systems. The conference television system is supported by a star or tree network structure, but it belongs to a private system, and the system of different manufacturers cannot communicate. H. 323 Conference TV system compares in the IP addressing multimedia communication business, meeting business has become an important business. H. The introduction of the 323 protocol complies with all IP unity, interconnecting interconnection in an open network platform and application platform, has been welcomed by users and equipment manufacturers. The development of system and equipment is very rapid, and ITU (ITU-T) launched a standard of multimedia conference system based on group exchange network in 1996, that is, H. The first version of the 323 Agreement, which will continue the second edition, and introduced the third edition this year. The particularity of the IP network requires that the image coding and the speech coding scale can be changed, so that H. 263 and G. 729 is successively launched. Based on H. 323 Conference System by H. 323 terminal, gatekeeper, gateway, multi-point control unit, multi-point controller, multipoint processor, etc.
Based on H. 323 Protocol The conference system allows you to run on the platform where the network is delayed. Uncertainty of network delay. The actual operational effect and quality of video communications on these networks is a problem that requires a lot of test verification, based on H. 323 The characteristics of the system of the protocol are as follows: Using the packet switched network composition system, such as IP network, frame relay / ATM, etc. is time delay, but there is a certain circuit with a wide quality guarantee; communication bandwidth can reach 2m bit / s, but Usually used below 384kbit / s (such as 128 kbit / s); generally, the received image quality requires 384 kbit / s transmission rate. A actual point-to-point video communication may occupy a bandwidth of about 2.5 times that of this bandwidth. The conference television system and other application systems are: communication on the group exchange network is more sensitive to delay and jitter, if the quality of the network is not guaranteed, the meeting is unable to hold. H. 323 Using UDP as a transport layer protocol, the RTP / RTCP protocol is used as the synchronization mechanism of audio and video stream, and the QoS of the network is measured, and the QoS measurement and the combination of service quality are used to ensure that the meeting is normal. The code stream of the conference system is packaged into a packet for statistical multiplexing. Different information code streams require different real-time requirements and reliability requirements. For example, for audio and video streams, the lower network is required to provide a real-time transmission mechanism, and provide reliability transmission on data and control information. H. 323 In the conference system, since the channel is based on statistical reuse, its service quality (QoS) problem is a very important issue. H. 323 Version 2 Adds service quality assurance mechanisms, such as resource reservation agreements (RSVP protocols), in order to open real-time services on IP online, require individual nodes in the conference system to support the RSVP protocol, and ask the terminal to allow the request signal to the gatekeeper When it is pointed out that it needs to reserve the transmission resource, the gatekeeper confirms the possibilities and channels of resource reservation to the terminal.
Based on H. 323 The video terminal typically communicates via the IP (router) network via the local area network, thus supporting H. 320 The network can be used, and the ATM and Frame Relay Network can also be used. China's X. 25 networks are generally not considered when there is a higher demand for image quality because of the time of 800 ms. The factors affecting the conference TV system are mainly bandwidth and delay. Use DDN to organize H. 323 Communication bandwidth of the conference television system is usually selected for the selected conference TV equipment (such as 384 kbit / s), plus the necessary IP overhead (typically 20%); the system needs to use the gatekeeper to create a system call creation Demolition, network bandwidth control, etc. The characteristics of the frame relay network are in addition to the DDN circuit characteristics, the communication basic bandwidth (CIR parameters) add 20% IP overhead for the conference television terminal device, and should also consider increasing a small amount of IP packets into a frame relay grouping. Overhead. Based on H. 323 The advantages of system all kinds have attracted wide interest in people. From the domestic situation, due to H. 323 The openness and interoperability of the system, the existence of a large number of intelligent buildings and the rapid spread, and the advantages of low price of their desktop conference TV system, it will be greatly welcomed. Domestic in the construction of the system is not more than abroad, and there is even probably possible. H. 323 Conference System Allows Participants of the meeting to join in a variety of ways, such as: LAN access (primary users are unit groups), PSTN access and ISDN access (mainly for dispersion users), etc., The PSTN access is low, only a lower quality conference service, and is divided into PPP access and H. 324 Access; ISDN Access can provide better quality conference services, divided into PPP access and H. 320 access.
