Key Technologies and Implementation of Distributed Video Conferencing System

In the collaborative computing tools that have become the hot topics in the computer field, video conferencing systems are an important part of them. Videoconferencing systems in circuit-switched networks have matured models, such as ITU's H.320 standard, but the use of packet-switched networks (including Ethernet, Internet, etc.) is becoming increasingly popular, and new solutions must focus on how to use This kind of network realizes the video system.

The scheme proposed in this paper does not aim at a specific network, but is designed according to the needs of the multipoint video conference system on the Internet. It makes full use of the packet switching network multicast function and high bandwidth characteristics. It is a distributed multipoint conference system based on RTP protocol. The end host is a Solaris 2.x system that supports IP multicast. It has the following features: 1 Each node's Data reaches other nodes by multicasting. 2 Audio and video synthesis is done by the end host. 3 Does not use the reference clock to achieve good synchronization of the transmit/receive codec, and has better control over packet jitter and loss. 4 The dynamic flow control mechanism allows the video compressor to adjust the transmission rate according to the state of the network. 5 Adopt a directory service system that is suitable for IP networks and can traverse firewalls.

Key technologies for distributed video conferencing systems:

Conference System Control and Data Transfer

This is the main function of the MCU in the centralized solution. In the distributed system, the functions of the MCU can be implemented by the network and/or the end nodes. In our solution, the data transmission mainly uses the distributed network multicast function, and many control functions are implemented by the end host and the network.

Effective use of bandwidth and quality assurance

The packet-switched network reuse mechanism can effectively use the bandwidth, but it may also cause packet jitter or even loss. Most of the Internet has not yet achieved Quality of Service (QoS) guarantees. In traditional applications, higher-level TCP/IP protocols are usually used to ensure reliable transmission. TCP uses the retransmission mechanism to implement reliable transmission, and its internal flow control mechanism dynamically adjusts the transmission rate according to the acknowledgment packet. For real-time conferencing, the delay caused by retransmission is unbearable, so the transport layer protocol uses UDP that does not have reliable transmission and internal flow control, and the task of end-to-end synchronization and flow control is passed on to the video conferencing system.

Directory service function

The Internet is not like a circuit-switched network. It does not have a unified addressing mechanism. In addition, there are still problems with firewalls and addresses being unlisted. Therefore, the directory service is a key issue to be solved in a distributed conference system.

Distributed Multi-point Video Conference System Implementation Plan

the whole frame:

The system's main hardware is as follows: 1 audio/video capture/playback card. Sounds, images, and data are transmitted as separate streams. The receiver can choose to receive only the sound from a source. This is particularly useful for end nodes that do not have image processing capabilities. Silent detection avoids sending audio streams when not speaking. 2Codec and DSP (digital signal processor) cards. The DSP synthesizes the video and audio sources according to the end user's choice. It also has the ability to mask the clock from being out of synch, sound/image out of sync, and lose packets. There is also an Ethernet network card on the card. The conference system can be directly connected to the LAN without CPU involvement. There is a direct interface between the audio/video capture/playback card and the Codec/DSP card to bypass the system bus and save CPU time.

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Posted on February 24, 2023