Technical requirements for video conferencing systems
Video conferencing system refers to people in two or more different places. With the support of multimedia computer technology and communication technology, audio, video and file data are transmitted to each other through transmission network and multimedia equipment to achieve real-time interactive communication. System equipment to accomplish the purpose of the meeting. Now that the video conferencing system has become an important means of communication and collaboration within the enterprise, what are the key technical requirements for the video conferencing system?
First, the bandwidth
In general, the higher the bandwidth, the more clear images and smooth video effects can be provided. For example, a high-definition video conferencing system that can provide 720P, assuming that the data volume of a single image photo is represented by 24 bits, then: 720P image photo The original data amount is 1280 × 720 × 24 / 8 / 1024 = 2700 Kbyte. If the moving image is 25 frames per second, the limit data amount per second of the 720P moving image is 2700 KByte × 25 = 67500 KByte, and the transmission bit stream is 67500 × 8 = 540 000 Kbit / S = 524 Mbps. In the actual application process, the amount of data will fluctuate between 10% and 40% due to the fact that the scene is not fixed. The amount of data is quite amazing.
Second, compression technology
Having a sufficiently high bandwidth cannot effectively solve the problem of excessive transmission, and blindly pursuing high bandwidth will only increase the technical difficulty. The compression algorithm with high compression ratio can effectively reduce the amount of data and make video and audio data. Successfully transmitted on the IP network. There are two characteristics of moving images: spatial correlation and temporal correlation, both of which will generate a large amount of redundant information in the image. Excellent compression algorithms can effectively remove these redundant information. For example, in a high-definition video conferencing system, digital image decoding mainly uses the H.264 standard, and the H.264 compression ratio is as high as 80:1 compared to the conventional standard. For example, in the above example, the stream of 720 moving image transmission is 524/80=6.55 Mbps.
Third, multicast technology
Multicast is a multi-address broadcast, and the transmission and reception are one-to-many relationships. During the transmission process, the sender only needs to send a data report once, and each user located in the multicast group can share the data packet. In the video conferencing system application, when a node signal is transmitted to each node, whether it is repeated point-to-point communication or broadcast, the network bandwidth is seriously wasted, and the data transmission of the multicast technology is distributed in the network node, which reduces The total amount of data in the network.
Fourth, the transmission protocol
The H.320 protocol (for group meetings on ISDN, proposed and adopted in 1990, is the first international standard protocol) mainly for N-ISDN (Narrowband Integrated Services Digital Network), capable of meeting and adapting to circuit switching characteristics, widely Used in circuit switched networks such as VSAT/DDN/ISDN. The H.320 standard includes the H.200 series standard and the T.120 series standard. The H.200 series of standards are aimed at audiovisual services, specifically communication services that mainly transmit moving images. The T.120 series of standards is a framework agreement for data conferencing in conference television systems.
H.323 protocol (a video conference implemented on an IP network, H.323, proposed in March 1997, provides multimedia communication standards for existing packet network PBNs (such as IP networks), and is currently the most widely used protocol. Components, protocols, and procedures for providing multimedia communications over packet-switched networks, including IP networks. H.323 defines four components: terminals, gateways, gatekeepers, and multipoint control units. Using them, H .323 can support point-to-point or point-to-multipoint communication of audio, video and data. The hardware video conferencing system basically adopts this technical standard, which ensures that all manufacturers' terminals and MCUs can be interconnected. A considerable portion of the manufacturer's equipment is compatible with two standards, while the latest equipment uses the H.323 standard.
The SIP protocol (text-based application layer control protocol, which is a multimedia communication protocol specified by IETE) is an application on the Internet for audio and video transmission, which increases signaling and QoS requirements, and draws on the design of other Internet standards and protocols. Thoughts, following the principles of conciseness, development, compatibility and scalability, are relatively simple and are the development trend of future video conferencing.
V. QoS service quality assurance
QoS is a security mechanism of the network and is a technology used to solve problems such as network delay and congestion. In order to get the real feeling of each venue, the delay of audio and video should be less than 0.25S, and the jitter should be less than 10ms at the maximum. At the same time, lip synchronization is required. As long as the synchronization is achieved in time, the packet loss rate is controlled within the range that the human can receive, and a certain transmission error is allowed, so that the complete information about the site can be naturally and effectively expressed.