IP network multimedia communication technology
Dongfang Communication Co., Ltd. Technology Center
1 Introduction
Today, in the century, humans have entered the information and network age, and the National Information Infrastructure (NII) has become the development strategic objective of national informationization in the world. Lan, intranet, and Internet have been exceeded with extraordinary development. It has entered the mid-1990s, and the Internet traffic is growing at a rate of 300% per year. According to AT & T forecast, the 2001 Internet's traffic will be flat with the telephone network. At the same time, the demand for multimedia communications is also inflated. Therefore, international standardized organizations, such as ITU, ETSI, IETF, IMTC, etc., have transferred multimedia communication standards to the standard of multimedia communication systems based on IP networks. Such as the H.323 standard of ITU-T. H.323 is a standard recommendation for the International Telecommunications Union (ITU), where H.323 V1 has passed the 15th study group of ITU in 1996. At present, SG-16 has just completed the development of the latest version H.323 V2. And get passed. H.323 Develop multimedia communication system standards on packet network PBN (Packet Based Networks) guaranteed by QoS (Quality). These grouping networks dominate today's desktop network system, including Ethernet, fast Ethernet, token, FDDI technology based on TCP / IP, IPX packet exchange. Therefore, the H.323 standard provides technical foundation and security for multimedia communication applications on LAN, Man, Intranet, and Internet. H.323 is part of the ITU-T multimedia communication series standard H.32X, which makes a video conference on existing communication networks. H.320 is a standard for multimedia communication on N-ISDN; H.321 is a standard for multimedia communication on B-ISDN; H.322 is a standard for multimedia communication on LANs with service quality assurance; H . 324 is a standard for multimedia communication on PSTN and wireless networks.
2 H.323 standard overview
The H.323 standard includes the technical requirements required for multimedia communication in groups without QoS guarantee. These packets include LAN, Man, Intranet / Internet, and packet protocols using PPP via PSTN or ISDN dial-up or point-to-point connections. H.323 defines four major components for network-based communication systems (Figure 1): Terminal, Gateway, Defold, Multi-Point Control Unit (MCU). 1) Terminal: PBN can provide real-time, two-way communication node devices. Figure 2 depicts its composition. All terminals must support voice communications, video and data communication options. H.323 specifies the operational modes required for different audio, video, and / or data terminals to work together. It will be the main criteria for next-generation Internet telephones, audio conferencing terminals, and video conferencing technology. All H.323 terminals must also support H.245, H.245 standards for controlling channel usage and channel performance. Optional components in the H.323 terminal are image codecs, T.120 data conferencing protocols, and MCU functions. 2) Gateway: an options for the H.323 conference system. The gateway provides a lot of services, including the conversion function between the H.323 conferencing node device compatible with other ITU standards. This feature includes conversion of transmission formats (such as H.225.0 to H.221) and communication procedures (such as H.245 to H.242). In addition, between the PBN end and the circuit switched network SCN end, the gateway also performs conversion of voice and image codecs, and call establishment and demolition. Figure 3 illustrates a H.323 / H.320 gateway. The terminal communicates with the gateway using the H.245 and H.225.0 protocol. With appropriate decoders, the H.323 gateway can support terminals that meet H.310, H.321, H.322, and V.70 standards. 3) Gatekeeper: Perform two important call control functions. The first is the address translation function, which is defined in RAS. For example, translate the PBN alias of the terminal and gateway into an IP or IPX address; the second is bandwidth management functions, and it is also defined in RAS. For example, a network administrator can define a threshold value for participating in the number of conference users on the PBN. Once the number of users reaches this setting, the watches can reject any connection requests that exceed the threshold. This will enable the bandwidth occupied by the entire meeting to a viable range of network total bandwidth, and the remainder leaves E-mail, file transfer, and other PBN protocols. Other functions of Gate may include access control, call verification, gateway positioning, and more. All terminals, gateways and multipoint control units managed by a single guard, we call H.323 district (Figure 4). Although logically, wandere and H.323 node devices are separated, producers can integrate attention to physical devices such as H.323 terminals, gateways and multi-point control units. 4) Multi-point Control Unit (MCU): MCU supports three conferences for more than three node devices. In H.323 systems, an MCU consists of a multipoint controller MC (required) and several multipoint processor MPs, but may not contain MP. The MC processing H.245 control information between terminals, which determines its usual processing capabilities for video and audio. In the case of the necessary case, MC can also control conference resources by judging which video streams and audio streams require multipoint broadcasts. The MC does not process any media information directly, and leaves it to MP. MP is mixed, handover, and processing audio, video, and / or data information. MC and MP may exist in a dedicated device or as part of other H.323 components. 5) Multi-Point Conference: In H.323 standard, multi-point implementation has a variety of different methods and configurations, divided into centralized, distributed and mixed meetings.
* Centralized multi-point conference (Centralized): A MCU is required to organize a multi-point meeting. All terminals send video streams, audio streams, data streams, and control flows to the MCU in a point-to-point manner. * Distributed multi-point conference (DECENTRALIZED): Using multi-point broadcasting (Multicast) technology. Participate in the H.323 terminal of the meeting to broadcast video and audio information multi-point multi-point to the terminal of the conference without being sent to the MCU. Note: The control of multi-point data is still in the MCU concentration, and H.245 control channel information is still transmitted to MC in a point-to-point manner. * Hybrid multi-point conference (Hybrid): a combination of centralized and distributed functions. H.245 signaling and video stream (or audio stream) is transmitted to the MCU in a point-to-point manner. The remaining signals (video or audio) are transmitted to the H.323 terminal of the participating conference in a multipoint broadcast mode. * H.323 Supports Hybrid Multi-Point Conference (Mixed), some terminals participate in centralized conferences, other terminals participate in distributed conferences and use MCU bridges two conferences. There is no need to know the mixed properties of the meeting, just understand the meeting mode where it sends and receives information. Multipoint broadcasts can more effectively utilize network bandwidth, but increase the calculation load of the terminal. The terminal needs to be mixed and switches the video streams and audio streams they receive. In addition, network switches and routers must support multipoint broadcasts. H.323 is limited to the network structure of only one MC per multipoint meeting. Although the theoretical participants can be a lot, people will find that when the number of participants reaches or more than 10 ~ 20, the effect is not satisfactory. 3 H.323 Communication Protocol Stack
Communication in the H.323 system can be seen as a mix of video, audio, and control information (Fig. 5). 1) System Control: The system control function is the core of the H.323 terminal, which provides signaling for the correct operation of the H.323 terminal. These features include call control (establishment and demolition), capability switching, command, and instruction signaling and packets for open and description logical channel content. The control of the entire system is provided by the H.245 control channel, the H.225.0 call signaling channel, and the RAS channel. 2) Grouping and synchronization: H.225.0 The standard describes the packaging packet and synchronous transport mechanism of the media stream on the LAN without QoS guarantee. H.225.0 Format the transmitted video, audio, data, and control flow to output to the network interface, and enter the packet from the network interface to compensate for the received video, audio, data and control flow. In addition, it also completed logically frame, sequential number, error correction and error detection. 3) Audio: The audio signal contains digitized and compressed voice. The compression algorithms supported by H.323 are compliant with ITU standards. For speech compression, the H.323 terminal must support G.711 voice standards. Other ITU standards are supported, such as G.723.1, G.729.A is optional. 4) Video: Although the video function is optional, any H.323 terminal with video features must support H.261 codec standards; support H.263 standard is an optional feature. The H.261 standard is communicated with the P * 64kb / s (P = 1, 2, ... 30) channel. Mobile compensation for improving image quality is an optional feature of H.261. H.263 is compatible with the H.261 standard, due to the use of 1/2 pixel mobile estimation technology, predicting frame, and HUFFMAN coding table optimized low rate transmission, making H.263 picture quality has great improvement . 5) Data: Data Conference T.120 is an optional feature. When supporting a data conference, data conferencing can achieve collaborative work, such as whiteboard, application sharing, file transfer, static image transfer, database access, audio image conference, etc.
4 multimedia communication in the IP network
In the H.323 multimedia communication system, the transmission of signaling and data streams utilizes a connection-oriented transmission mechanism. In the IP protocol stack, IP is collaborated with TCP to jointly complete the connection-oriented transmission. Reliable transmission ensures traffic control, continuity, and correctness at the time of packet transfer, but this may cause transmission delay and occupying network bandwidth. H.323 uses reliable TCP for H.245 control channel, T.120 data channel, call signaling channel. The video and audio information use unreliable, non-connection-oriented transmission mode, using the User DataGram Protocol. It is unable to provide good QoS. UDP only provides minimal control information, so the TCP is delayed when transmitting. H.323 will use unreliable UDP for video, audio, and RAS channels. In a multimedia communication system with multiple video streams and audio streams, the UDP-based unreliable transmission utilizes IP multipoint broadcast and processing video and audio information by IETF real-time transmission protocol RTP (Real-Time Protocol). IP multipoint broadcast is a protocol that is unreliable multi-point broadcast transmission in UDP. RTP works on the top of IP multipoint broadcasts for processing video and audio streams on IP online. Each UDP plug is plus a header that contains timecags and serial numbers. If the receiving end is equipped with an appropriate buffer, it can utilize the timecaster and the serial number information "recovery, reproduction" packet, record the sequencing package, synchronous voice, image, and data, and improve continuous playback. Real-time control protocol RTCP (Real-Time Control Protocol) is used for RTP control. RTCP monitoring service quality and online transmission information, and regularly send control information packets containing service quality information to all communication nodes. Since H.323 is based on RTP / RTCP, it can run on Internet-Internet's multicast backbone, running on Internet virtual networks with multipoint broadcast capabilities and supports audio, video, and data multimedia communication. In large packet networks, such as Internet, intranet, it is important and difficult to keep enough bandwidth for a multimedia call. Another IETF protocol: Resource Reservation Protocol: RSVP) Allows the receiving end to apply a certain number of bandwidths for a particular data stream, and get a reply that the application is licensed. Although RSVP is not a formal part of the H.323 standard, most H.323 products must support it because the bandwidth reservation is critical to the success of multimedia communication on IP networks. RSVP needs to get the terminal, gateway, MCU, multi-point processor, and support for intermediate routers or switches. 5 Conclusion
