【 Weak current College 】 video conference system system analysis: from technology to process---Power By 【 China power house network 】

Since the early 1980s, Tobacco Monopoly Bureau system communication enters the era of digital communication, video conference system provides smooth launch. 　　Take advantage of the tobacco industry itself has a wealth of transmission channel, at the same time, given the H.323 system of high flexibility and high scalability, high performance-cost ratio, in a TCP/IP network resources, uses the H.323 protocol for videoconference system is an ideal solution. From video conferencing technologies, current video conference system establishment can be based on two main frame ITU-T recommends to H.323 and H.320, forming two different building programmes. H.320 is ITU-T earlier video conferencing standard that completely built on a series of video conferencing proprietary technologies and standards, and H.323 standard built on a common, open computer network communication technology that has broad development prospects. Since 1996 the standard ITU-T, video conference technologies and markets have undergone revolutionary change. 　　More and more manufacturers are racing to input H.323 development of new products, an increasing number of users using H.323 technologies and products to construct their own video conference system. First, the video conferencing technology system 1.H.320 standard H.320 standard developed in 1990, is a video conference, one of the early is recommended primarily for narrowband ISDN network. Because of the narrowband ISDN network is a network based on circuit-switched, H.320 standard primary meet and adapt to circuit switching characteristics, thus H.320 standard video conference is widely used for VSAT, DDN, ISDN and other circuit-switched network. Circuit switching characteristic is connection-oriented, transfer rate and delay of stable, low-delay is small, the error rate, and therefore the quality of the video conference to be guaranteed. But the disadvantage is that in addition to the ISDN connection fixed is to remove online newsletter, other network application must be point-to-point, permanent connections, bandwidth utilization is low, open, and set the join is not convenient. 　　Since 1993, the construction of our introduction video conference network, the majority adopted the H.320 standard videoconferencing, and VSAT, DDN, E1 leased line, ISDN, network, greatly limits the video business for further expansion. 2.H.323 standard H.323 standard covers the audio, video and data over IP packet-based networks (LAN, EXTRANET and Internet), the establishment of communication on the H.323 standard is designed to allow different vendors of multimedia products and applications to interoperate. For a wide range of IP network-based multimedia communication applications, H.323 standard is very important for the widget. 　　In addition, the standard also allows ISDN and POTS and based on the network is directly connected to the PPP. In 1996, the ITU H.323 specification was approved. The standard range is wide, covering a variety of stand-alone equipment, personal computer technology, as well as point-to-point and point-to-multipoint video conferencing. 　　The standard resolution of the video conference calls and session control, multimedia and bandwidth management, and many other issues. Second, the network structure (1) single based on standard H.320 MCU star network and two grade MCU network (master-slave) single MCU Star Network refers to the use of an MCU devices with multiple video conferencing Terminal composition star network structure. When more than one conferencing Terminal MCU provides the port number, a single MCU cannot connect all videoconference terminals. At this point to cascade MCU ways of expanding the network to increase network capacity, constitute a slave. Each is from the MCU and main MCU constitute fixed connection, network stability is very poor. From a topology perspective, the single star structure will vary depending on the main problems result in MCU full network instability. 　　In principle, MCU cascade can not restricted, but the network when you reach level 3 will not be able to meet the Conference most hours delay, synchronization, therefore in the application generally only use a single MCU star network and two grade MCU network (master-slave). User level access generally use E1 leased lines or ISDN2B + D of the access connection. 　　Two methods has advantages and disadvantages, dedicated line can guarantee good stability and image quality, but the network utilization is low; ISDN line impact is large, but access flexibility. H.320 videoconferencing system on the network bandwidth requirements more stringent. For example, when a meeting has 10 meeting attended, and each site can see each other, we need 10 x 11/2 = 55 of the channel. 　　Therefore, when the network Terminal reaches a certain number, network control and circuit leveling appears difficult, therefore is not suitable for larger meetings. 2. based on the H.323 standard system advantages of H.323 videoconference system and H.320 MCU networking is that of different H.323 MCU only logic in the sense of the mesh network structure, without a fixed connection between them. In forwarding the meeting time rendering tree structure, but at this point the transfer of MCU does not occupy port resources. 　　According to the Second Edition recommendation H.323, generally can be divided into GK networking and networking two MCU. H.323 GK is a component of the system, its function is to H.323 nodes provide call control services. This area of all H.323 nodes must be in this domain to the gatekeeper registration, GK on the provision of basic services are: address translation, bandwidth management, admission control, regional management. Multiple GK networking can form mesh network structure or a slave. Mesh structure equal status for each GK, manage the connected MCU for address resolution, regional management and other functions. Slave network consists of a top-level GK GK and multiple domain GK constitute tree structure for easy upgrades to network capacity. 　　Top-level GK is responsible for the resolution of the domain, domain GK GK is responsible for the connected MCU analysis, regional management, etc., but in this way, centralized management, poor stability. H.323 videoconference system bandwidth configuration consists of domain's bandwidth requirements and various regions of the bandwidth requirements. On a regional,An MCU's maximum number of users is 100 384kbit/s Terminal at the same time access, press this point IP networks computation of actual utilization to 80 percent, every MCU access bandwidth should be at least 100 × 384/0.8 = 48Mbit/s. 　　On different areas, can be cascaded between the MCU to save bandwidth, according to operators of empirical data, the average domain conferences at least three, by average minimum number of meetings, approximately two-thirds of the bandwidth savings, interregional minimum bandwidth 16Mbit/s. Third, H.320 and H.323 Conference organization processes in H.320 videoconferencing system, due to the MCU and MCU, Terminal and MCU is static fixed connection. 　　Therefore, the main MCU from MCU to treat as a Terminal, the terminal-by-case basis and the connected MCU to establish calls and in order to join the meeting. In H.323 videoconference system, due to the MCU and MCU, Terminal and MCU is not static fixed connection, thus meeting organizational processes are complex, there is a Terminal and choose between the MCU. In a meeting, the Terminal is the first to access the connected this domain to MCU, other domain terminal can be joined to this MCU on the Conference or the Terminal overflow to neighbouring MCU on the meeting. In the Select routing, if all the terminals scattered, you can select any participating location of Terminal Parties convene parties, MCU as other Terminal meeting received this MCU. For the meeting invitation and join the other Terminal, if the meeting uses port number in the MCU is full, the terminal will be links to neighbouring MCU, establish a connection. For all the terminal does not belong to a GK, should not cross-domain use MCU GK, will attend the MCU terminal access this domain. 　　H.323 videoconference system three Conference organization processes form an ad-hoc meetings respectively, multipoint session, and cross-regional video conference between. IV. network quality assurance technology H.320 videoconferencing system on the host network needs to have the clock synchronization and ensure that the requirements of the transport indexes. Clock synchronization using internal clock or an external clock in two ways to meet the requirements of the three-level clocks MCU. 　　Domestic transport indicators are based on the recommendations proposed, ITU-TN.90 ITU-TN.86 required channel performance limit value: domestic/International Conference on television circuit for 2048kbit/s transmission, bit error rate (BER) should be less than 10, and correct code seconds (EFS) should be greater than 92%. H.323 videoconference system is an IP packet network to transport, IP network transmission H.323 videoconference system of indicators should mainly be including packet loss, delay, jitter and BER, and other indicators. 　　Voice of maximum delay 0.25s, maximum delay variation should be less than 10ms, acceptable error rate of less than 10-1; active images of maximum delay 0.25s, maximum delay variation should be less than 10ms, acceptable error rate of less than 10-2, 1.H.323 conferencing system in practical use of QoS guarantee (1) the use of IP network or VPN technology to connect all service components of the network network, ensuring the MCU and MCU, MCU and gateway, gateway and communication between a gatekeeper. (2) accessible through H.323 proxy on H.323 data package, providing QoS mechanisms. Use the directional routing will H.323 data routed to a special network QoS assurance mechanism. 　　H.323 proxy server has two main subsystems: gateway and gateway agent subsystems. 　　Gateway agent subsystems: provide H.323 traffic control, bandwidth control and QoS mechanisms, leads to the external network of reliable communications joins etc. 　　Gateway agent subsystems: provides the user authorization, call detail records, LAN bandwidth management and meetings management, H.323 call routing and address directory. Redirect routing: for QoS, administrators configure a separate network, separated from the public network. 　　H.323 proxy via ASR (special application routing) to fully utilize the advantages of independent network. (3) the gatekeeper-registry mapping because the Terminal is hidden in H.323 proxy server, even behind a firewall, and through H.323 proxy registration to GK subsystem, the Terminal is called, the Terminal cannot be H.323 proxy directly outside the Terminal, you must pass through H.323 proxy forwarding. 　　If an H.323 proxy servers have multiple terminals in H.323 proxy server then came forward to who, this is an address resolution. 　　Here for H.323 through the firewall to handle transport there are two solutions. 　　Firewall support H.323 protocol (for example CheckPoint4.1 or above): we can add H.323 proxy server on the firewall. The firewall does not support the dynamic access protocol H.323: inside the firewall, you can use proxy servers to provide simple access control scheme. Because only H.323 proxy server is a node of the server, the nodes and devices outside of the firewall, so that interaction is easy on the firewall to establish access control list through H.323 proxy server. 　　If the firewall does not support the dynamic access protocol H.323, we can put the H.323 proxy servers and firewalls are placed side by side. 　　Through an H.323 proxy server sends the signal has three indirectly reasons: you can resolve the server's security mechanism; you can represent the H.323 Terminal to undertake QoS signaling; you can complete the special application of routing tasks to improve H.323 transmission quality. 2. use VPN-MPLS implementation flow control and QoS guarantee virtual private networks (VPNs) built on the public network or shared on a network, used to distinguish between the network traffic. Service providers can establish new applied MPLS VPN, MPLS VPN capability is connectionless IP network, and frame relay and multiple levels of IP businessConfidentiality, making MPLS VPN operations more effective, service providers can provide cheaper managed IP service. Established in a third layer of MPLS-based VPN is build-to-peer mode, as compared with traditional VPN essentially better scalability and more conducive to the development and management. 　　In addition, a number of value-added services, such as video application and data hosting, e-commerce, voice services, etc. are available through a dedicated MPLS VPN easily deployed implementation, the reason for this is the service provider's transmission network can be a VPN is recognized as a secure connectionless IP network. V. H.320 and H.323 systems interoperability for H.320 Protocol is based on narrow-band ISDN network multimedia communications standards, is based on circuit-switched network; H.323 protocol is a packet switching network based multimedia communication standards. 　　Both are Web-based multimedia communication standard, the audio and video compression algorithm is the same, the above agreement and H.323 protocol H.320 videoconferencing standards of comparison, through the Gateway (gateway), you can implement both types of conversions between different protocols. 　　1. two meetings of the interoperability between systems (1), H.320 Terminal and H.323 terminal point-to-point interoperability; (2), H.320 MCU on the H.323 Terminal accession; (3) H.323 terminal into H.320 MCU on; (4), H.320 MCU on the Conference and the Conference on the H.323 MCU for concatenated. 　　2. network management interoperability for H.320 videoconferencing system basically has no real significance of network management, just-to-end, MCU has remote dial-up remote control function, you can use modem remote dial-up configuration and maintenance of the Terminal, for convening and scheduled conferences, master meeting control functions. 　　H.323 videoconference system's network management capabilities than stronger H.320 videoconferencing system, H.323 videoconference system of network management is based on TCP/IP protocol SNMP (simple network management protocol), including on Terminal, MCU's management, configuration, and maintenance can be achieved through SNMP. In response to the above, the proposed full interworking way take a distributed structure for centralized management. In the area, you can interconnect with gateway, through a customized dedicated server by IP and H.320 MCU connected via IP MCU on the network, including scheduled meeting, convened group meetings and other functions, the realization of the network management center to the MCU control, so the entire network to achieve centralized management.