In a traditional telephone system, a call from the establishment of a system is connected to a demolition connection requires a certain signaling to cooperate with completion. Similarly, in the IP phone, how to find the called party, how to establish a response, how to send data according to each other's data processing capabilities, and also need the corresponding signaling system, generally referred to as a protocol. At present, the international agreement in the international IP phone includes ITU-T H.323 protocol and IETE SIP protocol, which mainly introduces the most widely used H.323 protocol. First, H.323 Architecture In order to expand multimedia meetings on group exchange networks that do not guarantee QoS, ITU's Chapter 15 SG-15 passed H.323 advice through H.323 in 1996, in 1998 The second edition of H.323 is proposed. H.323 develops multimedia communication system standards on packet network PBN (Packet Based Networks) guaranteed without QoS (Quality), which dominate today's desktop network system, including TCP / IP, IPX group exchange Net, fast Ethernet, token network, FDDI technology. Therefore, the H.323 standard provides technical foundation and security for multimedia communication applications on LAN, WAN, Internet, and Internet. H.323 is a part of the ITU Multimedia Communication Series Standard H.32X, which makes a video conference on existing communication networks, where 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 a LAN with service quality assurance: H.324 is a standard for multimedia communication on GSTN and wireless networks. H.323 provides multimedia communication standards for existing packet network PBNs (such as IP networks). For combination with other IP technologies such as IETF's resource reservation protocol RSVP, multimedia communication of IP networks can be implemented. IP-based LAN is becoming more powerful, such as IP over SDH / SONET, IP Over ATM technology is rapid development and LAN broadband is constantly improving. Since the equipment and equipment, application, and application, the interaction between suppliers and suppliers can be provided, and therefore, H.323 ensures interoperability of all H.323 compatible devices. Higher speed processors, increasingly enhanced graphics devices and powerful multimedia accelerator chips make PCs a more and more powerful multimedia platform. H.323 can provide interconnection interconnection standards for multimedia communication between PBN and other networks. Many computers, network communications companies, such as Inter, Microsoft and Netscape support H.323 standards. The H.323 standard includes the technical requirements required for multimedia communication in group networks without QoS assurance. These packet networks include LAN, WAN, Internet / Internet, and packet protocols using PPP via GSTN or ISDN dial-up or point-to-point connections. From a whole, H.323 is a framework building that involves termination devices, video, audio, and data transmission, communication control, network interface, and multi-point control units that make up multi-point conferences (MCU ), Multi-point controller (MC), multi-point processor (MP), gateway, and guardian and other devices. Its basic composition unit is "domain", in the H.323 system, the so-called domain refers to a gateway, multi-point control unit (MCU), multipoint controller (MC), multi-point processor (MC), multi-point processor MP) and a collection of all terminals. One domain contains minimal contains one terminal, and there must be only one guard. Each logical component part of the H.323 system is called H.323, the types are: terminal, gateway, multi-point control unit (MCU), multipoint controller (MC), multipoint processor (MP).
Among them, the terminal, gateway, and multi-point control unit (MCU) are terminal devices in H.323, which are logical units in the network. The terminal device is a call and called, and some entities are unlocked, such as wishes. H.323 includes a connection between H.323 terminals and other terminals, through different networks, end-to-end connections. Its architecture is shown in Figure 6-1. Figure 6-1 H.323 Architecture Second, H.323 The Composition of H.323 is a network-based communication system defines four main components: (Gageway), Gagekeeper, Multi-Point Control Unit (MCU). The terminal is a node device that provides real-time, two-way communication in the packet network, is also a terminal user equipment, can communicate with a gateway, multi-point access control unit. All terminals must support voice communications, video and data communication options. H.323 specifies the operating modes required to work together with different audio, video or data terminals. It will be the main criteria for next-generation Internet telephony, audio conferencing terminals and video conference technology. Figure 6-2 shows a block diagram of the H.323 terminal, in the hair transmission, the video and audio signal acquired from the input device, after the encoder is compressed, in accordance with a certain format, send it out through the network, in the closing, from the network The packet is first unpacking, the received video, the audio compression data is decoded, and the user data and control data are also processed. The various functional units and their standard preparations or protocols are: video codec (H.263 / H.261): Complete redundant compression encoding for video streams. Audio decoding (H.723.1, etc.): Complete the codec of the voice signal and selectively add buffer delay to ensure continuity of voice. The standard used is ITU-T H.723.1, which provides two types of code rates of 5.3kbit / s and 6.3kbit / s, using linear prediction integrated analysis coding methods, using algebraic motivation linear prediction and multi-pulse maximum However, it is quantified to obtain optimization of coding complexity and quality. Various data applications: including electronic whiteboard, still image transmission, file exchange, database coordination, data conference, operational equipment control, etc., available standards are T.120, T.84, T.434, etc. Control unit (H.245): Provides end-to-end signaling to ensure normal communication of the H.323 terminal. The protocol used is H.245 (Multimedia Communication Control Protocol), which defines the request, response, signaling, and indication of four information, communicating communication capabilities, opening / closing logical channels, sending commands, or instructions between various terminals. Wait, complete control of communication. H.225: Format and transmit data, audio, control, etc., and receive data from the network. In addition, it is also responsible for handling some functions such as logical fragmentation, order column number, error detection. Third, the H.323 Standard Protocol H.32 is a standard protocol stack of the International Telecommunications Union (ITU), which is an organic overall. According to the function, it can be divided into four types of protocols, that is, the agreement The overall frame of the system (H.323), video codec (H.263), audio codec (H.723.1), system control (H.245), multiplexed data streams (H.225) and other parties
Figure 6-23 Terminal block diagram faces a more detailed provision. Provide good conditions for the further development of the Internet phone and the visual call conference system. The system control protocol includes H.323, H.245, and H.225.0, Q.931, and RTP / RTCP are the main components of H.225.0. System control is the core of the H.323 terminal. The entire system control is provided by the H.245 control channel, the H.225.0 call signaling channel and the RAS (registration, license, state) channel, and the audio decoding protocol includes G.711 protocol (must), G.722, G.723.1 , G.728, G.729 and other protocols. The audio standard used by the encoder must be determined by the H.245 protocol. The H.323 terminal should be asymmetric operation by the audio codec capability of the itself. Send as G.711, received in G.729. The video codec protocol mainly includes the H.261 protocol (must) and the H.263 protocol. The video function in the H.323 system is optional. The data conferencing function is also optional, and its standard is a multimedia conferencing data protocol T.120. The structure is shown in Figure 6-3.
Figure 6-3 H.323 Protocol Stack 1, H.323 Component H.323 Terminal is the most basic component defined by H.323. All H.323 terminals must also support H.245 standards, H.245 standards are used to control channel usage and channel performance. Other optional components in the H.323 terminal are image codecs, T.120 data conferencing protocols, and MCU functions. The gateway is also an optional component of 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 formats (such as H.250.0 to H.221) and communication procedures (such as H.245 to H.242). In addition, between the packet network end and the circuit switching network, the gateway also performs voice and image codec conversion work, as well as call establishment and demolition work. The terminal communicates with the gateway using the H.245 and H.225.0 protocol. With proper decoders, the H.323 gateway can support H.310, H.321, H.322, and V.70 standard terminals. Defense is a set of group options in the H.323 system, which features a call control service to the H.323 node. When there is H.323 wandering in the system, it must provide the following four service addresses: address translation, bandwidth control, license control and regional management functions. Bandwidth management, call authentication, call control signaling and call management, etc. are optional features. Although it is logically, the Guanhe and the H.323 node device separated, the manufacturer can integrate the task of the Gate into the physical device such as H.323 terminal, gateway, and multi-point control unit. The collection of all terminals, gateways and multi-point control units managed by a single-depends, called H.323. The multi-point control unit supports three conferences of the above node devices. In the H.323 system, a multipoint control unit consists of a multipoint controller MC and several multipoint processor MP, but may not include MP. The MC handles H.245 control information between endpoints to determine its usual processing capabilities for video and audio. In the case of necessary, MC can also control conference resources by judging which video streams and audio streams need to be multicast. The MC does not process any media information directly, and leaves it to MP. MP is mixed, switched, and processing audio, video or data information. MC and MP may exist in a dedicated device or as part of other H.323 components. The audio encoder encodes the audio information input from the microphone, decodes the receiving end to output to the speaker, the audio signal contains digitized and compressed speech. The compression algorithm supported by the H.323 is in line with the ITU standard. For speech compression, the H.323 terminal must support G.711 voice standard, transfer, and receive A laws and U laws. Other audio codec standards such as G.722, G.723.1, G.729.A, MPEG-1 audio can be selected. The audio algorithm used by the encoder must be determined by H.245. The H.323 terminal should be able to perform an asymmetric operation on the audio codec capability itself, as transmitted in G.711, received in G.728. The video codec decodes the video information at the video source and decoded in the receiving end. Although the video function is optional, any H.323 terminal with video functions must support H.261QCIF format; other formats that support H.261 and optional support H.263 standards. On the packet network, use H.261, H.263 codes without BCH error correction and error correction frames. Data conferencing T.120 is an optional feature. When supporting a data conference, data conferencing can work together, such as whiteboard, application sharing, file transfer, static image transfer, database access, audio image conference, etc. Other data applications and protocols can also be used after H.245. 2, H.225, H.245 and other protocols H.323 communication can be seen as mixing of video, audio, and control information. 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 functions include call control (establishment and demolition), flush switching, command, and instruction signaling and packets for open and describing 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.
The H.225.0 standard describes the packaging packet and synchronous transport mechanism of the media stream on the LAN without QoS guarantee. H.225.0 Format the control flow to the network interface, and retrieved the received control flow from the network interface input message. In addition, it also completed logical frames, sequential numbers, error correction and error detection. In the H.323 multimedia communication system, the transmission of signaling and data streams utilizes a connection-oriented transmission mechanism. In the IP gaming 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 data packet transfer, but may also cause transmission delay and occupying network broadband. 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 data protocol UDP (User DataGram Protocol). UDP is unable to provide good QoS, only provide minimum control information, so the TCP is delayed when transmitting. In a multimedia communication system with multiple video streams and audio streams, the IP multipoint broadcast and the ITF real-time transmission protocol RTP process video and audio information are processed based on UDP and unreliable transmission. IP multicast is a protocol that is unreliable multi-point broadcast transmission in UDP mode. RTP works on the top of the IP multicast, used to handle video and audio streams on IP online, each UDP plug, plus a header containing timestamps and serial numbers. If the receiving end is equipped with an appropriate buffer, it can use the timestamp and serial number information "recovery, reproduction" packet, record the sequencing package, synchronous speech, image, and data, and improve the edge-losing effect. Real-time control protocol RTCP is used for RTP control. RTCP monitors service quality and information transmitted online, and regularly distributes control information packages containing service quality information to all communication nodes. In large packet networks such as the Internet, it is very important for a multimedia call retention point, which is also very difficult. Another IETF protocol-Resource preflow protocol RSVP allows the receiving end to apply a certain number of broadband for a particular data stream, and get a reply to confirm that the application is licensed. Although RSVP is not a formal component of the H.323 standard, most H.323 products must support him, because the broadband pre-exposing is critical to the success of multimedia communications on IP networks, RSVP needs to get terminal, gateway, loading There are multi-point processors' MCUs and support for intermediate routers or switches.
H.225.0 Suitable for different types of networks, including Ethernet, token ring network, etc. H.225.0 is defined in TCP / IP, SPX / IPX transport layer. H.225.0 The range of communication is between the H.323 gateway and is on the same network, using the same transmission protocol. If the H.323 protocol is used throughout the Internet, communication performance will drop. H.323 Attempts to extend H.320 to a local area network without quality assurance, by using powerful recognition control conference control, a special meeting from several thousand people. H.225.0 establishes a call model. In this model, call establishment and performance negotiation does not use RTP transfer addresses, and then established several RTP / RTCP connections after the call is established. Before the call is established, the terminal can register a GateKeeper. If the terminal wants to register somewise, it must know the annual limit of this paid (VINTAGE). Because of this, discovery and registration structures contain a H.245 type object flag, which provides the H.323 application version of the date. These structures also contain optional non-standard messages that allow terminals to establish non-standard relationships. At the end of these structures, the non-standard state of the version number is also included. Where: the version number is required, non-standard information is optional. Non-standard information is used to notify their age and non-standard states between two terminals. Although all Q.931 messages have optional non-standard information in the user to user information, in all RAS channel information, there is an optional non-standard information. In addition, a non-standard RAS message can be sent at any time. Unreliable channels for registration, approve, and status communication are called RAS channels. Starting a call usually must first send an authentication request message, then send an initial establishment message, which is ended to receive the connection message. When the reliable H.245 control channel is established, the transmission channel of the audio, video, and the data can be established accordingly. The relevant settings of the multimedia conference can also be set here. When a reliable H.245 control channel is transmitted, the H.225 terminal can send audio, video data by unreliable channels. Error Hide and Other Information is used to process the case where the packet loss occurs. In general, the audio, and video packets will not be reissued because retranspasse will cause delays on the network network. It is assumed that the underlying has been processed with the detection of the bit error, and the wrong packet will not pass to H.225. Audio, video data, and call signals are not transmitted in the same channel and do not use the same message structure. H.225.0 Benefits Use different transmission addresses to send and receive audio, video data in different RTP instances to ensure serial numbers of different media frames and the quality of service of each media. Now ITU is studying how to mix audio, video packets in the same frame in the same transmission address, although audio, video data can share the same network address with the error-transport layer service access point identity, but the manufacturer still chooses to use Network address to transmit audio, video data, respectively. The dynamic transport layer service access point identifier can be used instead of the fixed transport layer service access point identifier in the gateway, multi-point control unit, and wanderer. A reliable transmission address is used for call establishment between terminals and terminals, and can be used between the derivation, and a reliable call signal connection must be performed in accordance with the following rules. In the call signal transmission of the terminal and the terminal, each terminal can turn the reliable call signal channel. For wandere call signal transmission, the terminal must guarantee that the reliable port is opened throughout the process. Although the gates can choose whether to turn off the signal channel, the gates must ensure that it is open for the call channel being used by the gateway. Q.931 information such as display information can be transmitted between end-to-end. If a reliable connection is disconnected due to a reason for the transport layer, this connection must be rebuilt, and the call does not think that it is failed. Unless the H.245 channel is turned off. The call status and call reference values are not affected by the closing reliable connection.
