【 Weak current College 】 IP network of multimedia communication technologies---Power By 【 China power house network 】

1 preface
At the turn of the century, mankind into the information and network era, the national information infrastructure (NII) has become the world information technology development strategy objectives. LAN, Intranet and Internet have been extraordinary development. Which entered since the mid-1990s, Internet traffic by 300% per year in the growth rate, according to the forecast AT & T, 2001 Internet traffic will be balanced with the telephone network.
At the same time, the IP network for multimedia communication demand expansion. Therefore, the International Organization for Standardization, such as ITU, ETSI, IETF, IMTC etc are multimedia communication standards focus shifts to IP network-based multimedia communication system standards. For example, the H.323 standard ITU-T.
H.323 is an International Telecommunication Union (ITU), a standard recommendation H.323 family, where V1 in 1996 by ITU study group 15, currently SG-16 has just completed the latest version of H.323 V2 development work, and was adopted. H.323 developed no QoS (quality of service) guarantees packet network PBN (Packet Based Networks) on standard multimedia communication system. These packet networks dominate today's desktop network system, including TCP/IP, IPX packet switched Ethernet, fast Ethernet, token ring, FDDI technology. Therefore, the H.323 standard for LAN, MAN, Intranet, Internet, multimedia communications applications provides the technical infrastructure and security.
H.323 is a series of standards for multimedia communication ITU-T H.32X part of series of standards makes existing communication network for video conferencing. Which is in the H.320 N-ISDN on standard multimedia communication; H.321 is carried out on the B-ISDN multimedia communication standards; H.322 is QoS LAN on standard multimedia communication; H.324 is in PSTN and wireless networks for multimedia communication standards. 2 introduction to H.323 standard
H.323 standards including a non-QoS packet network for multimedia communication skills required. These packet networks including LAN, MAN, Intranet/Internet as well as the use of PPP, such as grouping agreement through the PSTN or ISDN dial-up connection or a point-to-point connection.
H.323 is a network-based communication system defines four main components: Terminal, gateway, gateway, multipoint control unit (MCU).
1) Terminal (Terminal): PBN can provide real-time, in a two-way communications device nodes. Figure 2 describes its composition. All terminal must support voice communications, video and data communication is optional. H.323 defines different audio, video and/or data terminal together the required mode of operation. It is the next generation Internet phone, audio conferencing Terminal and the main video conference technology.
All H.323 terminals must also support standard for H.245, H.245 control channel usage and channel performance. In H.323 Terminal optional component is image codec, T.120 data conferencing protocols and MCU function.
2) gateway (Gateway): H.323 conferencing system of an option. Gateway offers many services, including H.323 Conference node equipment and other ITU standard compatible terminal between the conversion function. Such features include transmission format (such as H.225.0 to H.221) and communication procedures of conversion (such as H.245 to H.242). In addition, the PBN-and circuit-switched network between the SCN, gateway also performs voice and image codec conversion, as well as the call setup and clearing function. Figure 3-Schematic of a H.323/H.320 gateway. Terminal use H.245 and H.225.0 protocol to communicate with the gateway.
The use of appropriate decoder, H.323 gateway can support H.310, H.321, H.322 and V.70 standard terminal.
3) gateway (Gatekeeper): perform two important call control features. The first is address translation function, defined in the RAS. For example, the Terminal and the gateway of PBN alias translation into IP or IPX address; the second is the bandwidth management function, RAS also defined. For example, a network administrator can define PBN simultaneously meeting the threshold of the number of users, once the user number of the set value, the gateway will reject any more than the threshold value of the connection request. This will make the entire Conference of bandwidth in the network to a total bandwidth to the extent feasible, the remaining portion is reserved for E-MAIL, file transfer and other PBN Protocol. Gateway's other features may include access control, call validation, gateway, location, etc.
Management by a single gateway to all Terminal, gateway and multipoint control unit of collection, we refer to as H.323.
While logically, gateway and H.323 nodes device is detached, but producers can gateway functionality into H.323 Terminal, gateway and multipoint control units, and other physical device.
4) multipoint control unit (MCU): MCU support three or more nodes in the device. In H.323 systems, an MCU consists of a multipoint controller MC (required) and a few more processor MP, but also can not contain MP. MC processing terminal control information between H.245 and decide it on the video and audio in General. If necessary, MC can also determine which video stream and an audio stream needs to be multicast to control conference resources.
MC does not deal directly with any media information flow, and it is left to the MP. MP on audio, video and/or numberAccording to the information to be mixed, switching and processing. MC and MP can exist within a special equipment or as part of other H.323 components.
5) multipoint meetings: in the standard, H.323 multipoint implementations there are different ways and configured into a centralized, distributed and mixed meeting.
* Centralized multipoint Conference (Centralized): requires an MCU to organize a multipoint session. All Terminal to an ad hoc manner to the MCU sends video streams, audio streams, data flow and control flow.
* Distributed multipoint Conference (Decentralized): use multicast (Multicast) technology. Participation in meetings of H.323 Terminal to the other participating Terminal multicast video and audio information without having to send to the MCU. Note: multiple points of data control is still carried out centrally by the MCU, H.245 control channel information also continues to point to point transmission to the MC.
* Hybrid multipoint Conference (Hybrid): centralized and distributed feature combinations. H.245 signaling and video streams (or audio stream) point-to-point transmission to MCU. The remaining signal (video or audio) to multicast transmission to attend the meeting of H.323 Terminal.
* H.323 support mixed multipoint Conference (Mixed), some terminal in centralized meeting other Terminal in distributed meetings, and MCU bridging two meetings. Terminal do not need to know the meeting of the mixed attribute, just understand it to send and receive information is session mode.
Multicast allows more efficient use of network bandwidth, but increases the load on the terminal's calculation. Terminal switch needs to be mixed, they receive a video stream and an audio stream. In addition, network switches and routers must support multicast.
H.323 limited to each multipoint session only one MC's network structure. Although, in theory, the number of participants in the meeting can be a lot, but people will notice: when the number of participants met or exceeded the 10-20, the result is not satisfactory. 3 H.323 communication stack
H.323 system communication can be seen as video, audio, and control information.
1) system control: System control function is the core of H.323 Terminal, it provides H.323 Terminal to the proper operation of the signalling. These features include call control (construction and dismantling), the ability to switch, the command and direct the signalling as well as for opening up and description logic channel content of message, etc. The entire system of control by H.245 control channel, H.225.0 call signaling channel and RAS channel.
2) grouping and synchronization: H.225.0 standard describes no QoS guarantees LAN media streaming Pack grouping and synchronous transmission mechanism. H.225.0 on transmission of video, audio, data and control flow is formatted for output to a network interface, also from the network interface to enter text in the compensation received for video, audio, data and control flow. In addition, it also complete logic into frames, order number, error correction and error detection capabilities.
3) audio: audio signal contains digitized and compressed voice. H.323 supported compression algorithms are in line with the ITU standard. For speech compression, H.323 terminal must support the G.711 voice standards. Support other standards, such as the ITU G.723.1, G.729.A is optional.
4) video: Although the video feature is optional, but any H.323 terminal of a video feature must support the H.261 codec standards; supports H.263 standard is an optional feature. H.261 standard use P * 64Kb/s (P = 1, 2, ... 30) channel for communication. Used to improve the image quality by moving the compensation is an optional feature H.261. H.263 backward compatible with standard, H.261 because 1/2 pixel mobile estimation techniques, forecasting, and optimization of the low rate of frames transferred by Hoffmann (Huffman) encoding table, H.263 picture quality is greatly improved.
5) T.120 data: the data session is an optional feature. When supporting data, the data session for collaborative work, such as whiteboard, application sharing, file transfer, static image transmission, database access, audio, video conference, etc. 4 IP network for multimedia communication
In H.323 multi-media communication system, the signalling and control the flow of data is transferred using a connection-oriented transport mechanism. In IP protocol stack, IP and TCP collaborate to complete the connection-oriented transport. Reliable transmission guarantees that the data packets of a flow control, continuity and accuracy, but this may cause a delay and consume network bandwidth. H.323 is a reliable TCP for H.245 control channel, T.120 data channel, call signaling channel.
While the video and audio information using unreliable, object-oriented non-connected transmission mode that uses the User Datagram Protocol UDP (User Datagram Protocol). It cannot provide excellent QoS. UDP only provide minimal control information, therefore, relatively small TCP transmission delay. H.323 is an unreliable UDP is used for video, audio, and RAS channel.
There are multiple video and audio streaming of multimedia communication system, based on UDP not reliable transmission using IP multicast, and real-time Transport Protocol from the IETF RTP (Real-Time Protocol) processing of video and audio information. IP multicast is a UDP mode is not reliable Multicast Transport Protocol. RTP in IP multicast for the top level for IP network video and audio stream. Each UDP packets are coupled with a time-scale and number of the header. If the receiver is equipped to fit the buffer, it can take advantage of the time-scale and serial number information "recovery, recycling" packet, records of sequence package, synchronous voice, video and data, and to improve the effectiveness of continuous replay.
RTCP real time control protocol (Real-Time ControL Protocol) is used for the control of RTP. RTCP monitoring service quality as well as online shipping information, and will contain information about the service quality control information package circulated to all communications node.
Since H.323 based on RTP/RTCP, therefore it can be Internet multicast backbone (IMB-Internet\'s Multicast Backbone), has multicast capabilities of the Internet to run virtual online, as well as support for audio, video and data of multimedia communication.
In a large packet networks, such as Internet, Intranet, as a multimedia call retain sufficient bandwidth is very important and difficult. Another IETF protocols: RSVP (Resource Reservation Protocol: RSVP) allows the receiver to a particular data stream for a certain amount of bandwidth, and whether the application is permitted to reply. Although the RSVP is not a formal part of H.323 standard, but most of the H.323 products must support it because the bandwidth reserved for IP network is critical to the success of multimedia communication. RSVP required Terminal, gateway, equipped with more processor MCU and intermediate routers or switches support. 5. concluding remarks
On a world scale information network technology development trend, multimedia communications will be the "information superhighway" main communication business, but also the future direction of communication development. With LAN, Internet, Intranet in the world of rapid development, and growth, based on the IP network for multimedia communication is imperative, cheap and features increasingly powerful computer-based H.323 V2 terminal will eliminate expensive H.320 Terminal. In view of this situation, the world famous computer, network, communication equipment, video conference system manufacturers have multimedia communication terminal/system development focused on positioning in H.323-based IP networks V2 Terminal/system.
At present, China is building a large scale "169 China public multimedia communication network" and it is IP Over ATM-based packet network of the H.323 V2 multimedia communication terminal/system in China will be promising and will be China's "national information infrastructure (NII) play an extremely important role. Perhaps, our communication equipment manufacturing industry can "169 China public multimedia communication network" construction as an opportunity to intensify the design, development, production, sales, based on the H.323 V2 standard IP multimedia communications terminals and systems, enabling our emerging multimedia communication devices on the market.