"I. architecture of H.323
In order to carry out multimedia conference on packet switched networks without QoS guarantee, the 15th research group sg-15 of ITU adopted the first version of H.323 recommendation in 1996 and proposed the second version of H.323 in 1998. H. 323 has formulated the multimedia communication system standard on packet based networks (PBN) without QoS guarantee. These packet networks dominate today's desktop network systems, including Ethernet based on TCP / IP and IPX packet switching, fast Ethernet, token network and FDDI technology. Therefore, H.323 standard provides technical basis and guarantee for multimedia communication applications on LAN, Wan, Internet and Internet.
H. 323 is a part of ITU multimedia communication series standard h.32x, which makes it possible to carry out video conference on the existing communication network, H. 320 is the standard for multimedia communication on N-ISDN; h.321 is the standard for multimedia communication on B-ISDN; H.322 is the standard for multimedia communication on LAN with quality of service assurance; H.324 is the standard for multimedia communication on GSTN and wireless network. H. 323 provides multimedia communication standard for existing packet network PBN (such as IP network). If combined with other IP technologies such as IETF Resource Reservation Protocol RSVP, multimedia communication in IP network can be realized. IP based LAN is becoming more and more powerful, such as IP over SDH / SONET, IP over ATM technology is developing rapidly, and LAN broadband is constantly improving. Because it can provide interoperability between devices, applications, and suppliers, H.323 can ensure the interoperability of all H.323 compatible devices. Higher speed processors, increasingly enhanced graphics devices and powerful multimedia acceleration chips make PC a more and more powerful multimedia platform. H. 323 can provide an interconnection standard for multimedia communication between PBN and other networks. Many computer and network communication companies, such as Intel, Microsoft and Netscape, support the H.323 standard. H. 323 standard includes the technical requirements for multimedia communication in packet networks without QoS guarantee. These packet networks include LAN, Wan, Internet / Internet and dial-up connection or point-to-point connection through GSTN or ISDN using packet protocols such as PPP.
On the whole, H.323 is a framework construction, which involves terminal equipment, video, audio and data transmission, communication control and network interface. It also includes multipoint control unit (MCU), multipoint controller (MC), multipoint processor (MP), gateway and gatekeeper, etc. Its basic component unit is "domain". In H.323 system, the so-called domain refers to a collection composed of gateways, multipoint control units (MCU), multipoint controllers (MC), multipoint processors (MP) and all terminals. A domain contains at least one terminal, and there must be only one gatekeeper. H. Each logical component in H.323 system is called H.323 entity. Its types include terminal, gateway, multipoint control unit (MCU), multipoint controller (MC) and multipoint processor (MP). Among them, terminal, gateway and multipoint control unit (MCU) are the terminal equipment in H.323 and the logical unit in the network. Terminal equipment is callable and callable, while some entities are not callable, such as gatekeeper. H. H.323 includes the end-to-end connection between h.323 terminal and other terminals through different networks.
2、 Composition of h.323 terminal
H. 323 defines four main components for network-based communication system: terminal, gateway, gatekeeper and multipoint control unit (MCU). Terminal is not only a node device that can provide real-time and two-way communication in packet network, but also an end-user device, which can communicate with gateway and multipoint access control unit. All terminals must support voice communication, and video and data communication are optional. H. 323 specifies the operation modes required for different audio, video or data terminals to work together. It will be the main standard of the next generation Internet telephone, audio conference terminal and video conference technology. Figure 6-2 shows the composition block diagram of h.323 terminal. At the transmitting end, the video and audio signals obtained from the input device are compressed by the encoder, packaged in a certain format and sent through the network. At the receiving end, the data packets from the network are first unpacked, and the obtained video and audio compressed data are decoded and sent to the output device, User data and control data have also been processed accordingly. It includes various functional units and their standard equipment or protocols:
Video codec (H.263 / H.261): complete redundant compression coding of video code stream.
Audio coding and decoding (h.723.1, etc.): complete the coding and decoding of speech signals, and optionally add buffer delay at the receiving end to ensure the continuity of speech. The adopted standard is ITU-T h.723.1, which provides two bit rates of 5.3kbit/s and 6.3kbit/s. The linear prediction comprehensive analysis coding method is adopted, and the algebraic codebook is used to stimulate linear prediction and multi pulse maximum likelihood quantization respectively, so as to obtain the optimization of coding complexity and quality respectively.
Various data applications: including electronic whiteboard, still image transmission, file exchange, database coexistence, data conference, transportation equipment control, etc. the available standards are T.120, t.84, t.434, etc.
Control unit (H.245): provide end-to-end signaling to ensure normal communication of h.323 terminal. The adopted protocol is H.245 (multimedia communication control protocol), which defines four kinds of information: request, response, signaling and indication. The communication control is completed through communication capability negotiation between various terminals, opening / closing logical channel, sending command or indication, etc.
H. Layer 225: format and send video, audio, control and other data, and receive data from the network at the same time. In addition, it is also responsible for processing some functions such as logical framing, adding serial number, error detection and so on.
3、 H.323 standard protocol cluster
H. 323 is a standard protocol stack of the International Telecommunication Union (ITU). The protocol stack is an organic whole. It can be divided into four types of protocols according to its functions, that is, the protocol has made detailed provisions from the overall framework of the system (H.323), video codec (H.263), audio codec (h.723.1), system control (H.245), multiplexing of data stream (H.225), etc. It provides good conditions for the further development of network telephone and videoconference system and the compatibility of the system. The system control protocols include 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 h.323 terminal. The whole system control is provided by H.245 control channel, H.225.0 call signaling channel and RAS (registration, permission and status) channel. The audio codec protocol includes G.711 protocol (required), g.722, G.723.1, G.728, G.729 and other protocols. The audio standard used by the encoder must be negotiated and determined by H.245 protocol. H. 323 terminal shall perform asymmetric operation on its own audio encoding and decoding capability. If it is sent by G.711, it is received by G.729. Video codec protocols mainly include H.261 protocol (required) and H.263 protocol. H. The video function in 323 system is optional. The data conference function is also optional. Its standard is multimedia conference data protocol T.120.
1. H.323 assembly
H. H.323 terminal is the most basic component defined by H.323. All H.323 terminals must also support H.245 standard, which is used to control channel usage and channel performance. Other optional components in h.323 terminal are image codec, T.120 data conference protocol and MCU function.
Gateway is also an optional component of H.323 conference system. The gateway provides many services, including the conversion function between H.323 conference node equipment and terminals compatible with other ITU standards. This function includes the conversion of transmission format (such as h.250.0 to H.221) and communication protocol (such as H.245 to h.242). In addition, between the packet network end and the circuit switching network end, the gateway also performs voice and image codec conversion, as well as call establishment and removal. The terminal uses H.245 and H.225.0 protocols to communicate with the gateway. With appropriate decoder, H.323 gateway can support terminals conforming to H.310, h.321, H.322 and v.70 standards.
Gatekeeper is an optional option of H.323 system. Its function is to provide call control services to H.323 nodes. When H.323 is closed in the system, it must provide the following four service addresses: address translation, bandwidth control, license control and area management. Bandwidth management, call authentication, call control signaling and call management are optional functions for gatekeeping. Although gatekeepers are logically separated from H.323 node devices, manufacturers can integrate gatekeeper functions into physical devices such as H.323 terminals, gateways and multipoint control units. The collection of all terminals, gateways and multipoint control units managed by a single gatekeeper is called H.323 domain. Multipoint control unit supports the conference of more than three node devices. In H.323 system, a multipoint control unit is composed of a multipoint controller MC and several multipoint processors MP, but it may not contain MP. MC processes the H.245 control information between endpoints to determine its normal processing capacity for video and audio. If necessary, MC can also control conference resources by judging which video streams and audio streams need multicast. MC does not directly process any media information stream, but leaves it to MP for processing. MP mixes, switches and processes audio, video or data information. MC and MP may exist in a special equipment or as part of other H.323 components.
The audio encoder encodes and transmits the audio information input from the microphone, decodes it at the receiving end for output to the speaker, and the audio signal contains digitized and compressed speech. H. The compression algorithm supported by 323 conforms to ITU standard. For speech compression, h.323 terminal must support G.711 speech standard, transmitting and receiving A-Law and u-law. Other audio codec standards such as g.722, G.723.1, G.729. A and MPEG-1 audio can be optionally supported. The audio algorithm used by the encoder must be determined by H.245. H. 323 terminal shall be able to perform asymmetric operation on its own audio encoding and decoding capability, such as sending with G.711 and receiving with G.728.
The video codec encodes and transmits the video information at the video source and decodes and displays it at the receiving end. Although the video function is optional, any h.323 terminal with video function must support h.261qcif format; Support other formats of H.261 and optional H.263 standard. On packet networks, BCH error correction and error correction frames are not required for encoding and decoding using H.261 and H.263. Data conference T.120 is an optional function. When supporting data conference, data conference can work together, such as whiteboard, application sharing, file transmission, still image transmission, database access, audio image conference, etc. Other data applications and protocols can also be used after H.245 processing.
2. H.225, H.245 and other agreements
H. The communication in 323 system can be regarded as the mixture of video, audio and control information. System control function is the core of h.323 terminal, which provides signaling for correct operation of h.323 terminal. These functions include call control (establishment and removal), force switching, command and indication signaling, and messages used to open and describe the content of logical channels. The control of the whole system is provided by H.245 control channel, H.225.0 call channel and RAS channel. H. 225.0 standard describes the packet packaging and synchronous transmission mechanism of media streams on LAN without QoS guarantee. H. 225.0 format the transmitted control flow for output to the network interface, and retrieve the received control flow from the network interface input message. In addition, it also completes the functions of logical frame, sequence number, error correction and error detection.
In H.323 multimedia communication system, connection oriented transmission mechanism is used for the transmission of control signaling and data flow. In the IP game stack, IP cooperates with TCP to complete connection oriented transmission. Reliable transmission ensures the flow control, continuity and correctness of data packet transmission, but it may also cause transmission delay and occupy network broadband. H. 323 uses reliable TCP for H.245 control channel, T.120 data channel and call signaling channel. The video and audio information adopts unreliable and non connection oriented transmission mode, that is, using user data protocol (UDP). UDP cannot provide good QoS and only provides the least control information, so the transmission delay is smaller than TCP. In a multimedia communication system with multiple video and audio streams, based on UDP and unreliable transmission, IP multicast and IETF Real-time Transmission Protocol RTP are used to process video and audio information. IP multicast is a protocol for unreliable multicast transmission by UDP. RTP works at the top level of IP multicast and is used to process video and audio streams on IP networks. Each UDP packet is added with a header containing time stamp and sequence number. If the receiver is equipped with an appropriate buffer, it can recover with time stamp and serial number information, regenerate "" data packets, record out of order packets, synchronize voice, images and data, and improve the effect of side connection playback. Real time control protocol RTCP is used for RTP control. RTCP monitors the quality of service and the information transmitted on the Internet, and regularly sends the control information packet containing the quality of service information to all communication nodes.
In large packet networks such as the Internet, it is important and difficult to reserve enough broadband for a multimedia call reservation point. Another IETF protocol - resource pre Streaming Protocol RSVP allows the receiver to apply for a certain amount of broadband for a special data stream and get a reply to confirm whether the application is licensed. Although RSVP is not a formal part of the H.323 standard, most H.323 products must support it, because broadband pre streaming is very important to the success of multimedia communication on IP network. RSVP needs terminals, gateways and multiple devices
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