Key Technologies and Implementation of Distributed Video Conferencing Systems

With the rapid development of the Internet, people have begun to design distributed multipoint video conferencing systems suitable for this network. This article discusses the key issues to be addressed in implementing a distributed video conferencing system in the Internet environment, and then proposes a specific implementation solution.

Keywords: Video Conferencing, Directory Services, Group Switching Network

1. Introduction

In the currently hot field of group collaboration computing tools in the computer domain, video conferencing systems are an essential component. There are already mature models for video conferencing systems in circuit-switched networks, such as ITU's H.320 standard. However, the use of packet-switched networks (including Ethernet, the Internet, etc.) is becoming increasingly prevalent, and new solutions must focus on how to leverage such networks to implement video conferencing systems.

The proposed solution is not tailored to any specific network but designed to meet the requirements of multipoint video conferencing systems on the Internet. It leverages the multicast functionality and high bandwidth characteristics of packet-switched networks, and is a distributed multipoint conferencing system based on the RTP protocol. The end hosts support IP multicast and are running the Solaris 2.x operating system, featuring the following: 1. Data at each node is delivered to other nodes via multicast. 2. Audio and video synthesis is performed by the end hosts. 3. Good synchronization of send/receive codecs without using a reference clock, with good control over packet jitter and loss. 4. A dynamic flow control mechanism allows the video compressor to adjust the sending rate based on network conditions. 5. It employs a directory service system suitable for IP networks and capable of traversing firewalls.

Key Technology for Distributed Video Conferencing Systems

Control and data transmission of the conference system

This is the primary function of the MCU in the centralized solution. In a distributed system, the MCU's functions can be realized by the network and/or end nodes. In our solution, data transmission primarily utilizes the multicast functionality of the distributed network, with many control functions being jointly implemented by the end hosts and the network.

Effective Bandwidth Utilization and Service Quality Assurance

The multiplexing mechanism of packet-switched networks can effectively utilize bandwidth but may also lead to packet jitter and even loss. Most of the Internet has yet to implement Quality of Service (QoS) guarantees, with traditional applications typically relying on higher-layer TCP/IP protocols to ensure reliable transmission. TCP achieves reliable transmission through a retransmission mechanism, with its internal flow control adjusting the sending rate dynamically based on acknowledgment packets. For real-time conferences, the latency caused by retransmissions is intolerable, so the transport layer protocol uses UDP, which lacks reliable transmission and internal flow control. The tasks of end-to-end synchronization and flow control are then shifted to the video conferencing system.

Table of Service Features

The Internet, unlike circuit-switched networks, lacks a unified addressing mechanism and also faces issues with firewalls and non-disclosed addresses. Therefore, directory services are a key problem to be addressed in distributed conference systems.

Specific Implementation Plan for Distributed Multi-Point Video Conference System

Overall Structure

The main hardware components of the system include: ①Audio/Video Capture/Playback Card. Sound, image, and data are transmitted as separate streams, allowing recipients to choose to receive only audio from a specific source, which is particularly useful for end nodes without image processing capabilities. Silent detection is used to prevent the transmission of audio streams when no speech is present. ②Codec and DSP (Digital Signal Processor) Cards. The DSP synthesizes video and audio sources based on the end user's selection and also features functions to address clock desynchronization, audio/image desynchronization, and packet loss. An Ethernet network interface card is also included on the card.