【 Weak current College 】 distributed video conference system key technologies and implementation---Power By 【 China power house network 】

Introduction
In the computer field has become the hot groups collaborative computing tools, video conference system is an important component. Circuit-switched network video conferencing system is a more mature model, such as the ITU standard for H.320, packet switching networks (including Ethernet, Internet, etc.) are increasingly popular, new solutions have focused on how to use this network to implement video system.
This proposal does not address a specific network, which is based on Internet multipoint video conference system needs. It takes full advantage of packet switching network multicast functionality and high-bandwidth characteristics, is based on the RTP distributed more conferencing systems, end-to-host supports IP multicast for Solaris 2.x system, has the following characteristics: ① each node's data through multicast reach other nodes. ② audio and video synthesis by end-host. ③ do not use the reference clock for send/receive codec good synchronization, jitter and packet loss better control. ④ dynamic flow control mechanism allows the video compressor according to network status adjustment to send rates. ⑤ uses an IP network and be able to travel across the firewall's directory services architecture.
Distributed video conference system for key technology, conference system control and data transfer, this is a centralized scheme MCU's main function, in a distributed system, MCU function can be used by network and/or the end node. In our scenario, the data transmission of the main advantage of distributed network of multicast functionality, many control function is represented by a common end-to-host and network.
Efficient use of bandwidth and QoS packet switching network of reuse mechanism can make efficient use of bandwidth, but may also result in packet jitter or even lost. Internet most also does not implement quality of service (QoS) guarantees, traditional application typically consists of a higher level of the TCP/IP protocol to ensure reliable transmission. TCP retransmission mechanism used for reliable transmission, its internal flow control mechanism under confirm package to send rates adjusted dynamically. For real-time meetings, retransmission caused by the delay is intolerable, and transport layer protocols use does not have reliable transmission and UDP internal flow control, and end-to-end synchronization and fluidic task then transferred onto the video conferencing system. Directory service features
Internet is not like a circuit switched network, it does not have a unified mechanism for addressing, there is a firewall and address issues that are not public, so the directory services is a distributed meeting system to resolve the key issues.
Distributed multipoint video conference system implementation programme overall structure
The system hardware: ① audio/video capture/playback card. Sounds, images and data as a separate stream delivered, the recipient may choose to receive only from a source, this is not a sound image handling features particularly useful end-node, use the silence detection to avoid speaking not send audio stream. ② C o d e c and DSP (digital signal processor) card. DSP based on the end-user's choice of synthetic video and audio feeds, it also has a shield clocks are not synchronized, voice/video are not synchronized and packet loss, etc. The card also has an Ethernet network card, conference system can be directly connected to the LAN, without CPU intervention. Audio/video capture/playback card and Codec/DSP card has a direct interface between, you can bypass the system bus, saving CPU time. Transport layer protocol selection
Because UDP does not provide end-to-end reliable transfer, based on UDP, designed for real-time communications to provide transport layer services of RTP. Although RTP itself does not implement the service quality guarantee, but it provides multiplexing, order number, the time scale, monitoring, and on IP multicast and flexibleinterface for our design of multicast, synchronization, session data encryption, dynamic flow control, directory services, security, travel across the firewall, etc. RTP is an open protocol, application for upper provides sufficient flexibility. But one of the components of the RTCP RTP (real-time Transport Control Protocol) provides loose management and monitoring capabilities to meet our required control and management functions (such as dynamically obtain and distribute multicast address, distribution of session keys, etc.), so we use H.323 for centralized management model.
(3) network for multicast
Multicast in an existing network implementation and, in this case, we believe that the implementation of multicast channels can have the following: ① use implements DVMRP of switched Ethernet Hub, Hub between Tunnel function on the Internet are constructed multicast network. ② in the traditional way on the Internet to be grouped by the copy-and-forward, end systems through to each end-node replication and forwards the packet to simulate multicast. ③ When data from the implementation of multicast LAN to LAN send is not implemented, use RTP multicast functionality of Translator simulation. We use a third, in order to achieve a more convenient address assignment, and security features, you need to have a dynamic, distributed and security characteristics of directory services. Synthesis of the compressed data stream
In a distributed system, the network of the multicast feature allows each end-node can also receive more source images and sound, and the synthesis by end-system implementation. In order to reduce overhead, our synthetic is a compressed video stream. Compressed video stream synthesis algorithm is the current hot topics, our algorithm takes advantage of the fact; almost all of the standard video compression data contains a series of separate from predefined delimiter separated coding group, by checking the separator can be compressed data stream intothe pixel area. The paragraphs compressed data and the pixel area map, you can according to user settings to reassemble these data. Sessions confidential
Receiver-initiated multicast allows the sender to control receiving the data of the user, the nature of broadcasting LAN allows any host on the LAN can potentially listening session, therefore it is necessary to encrypt the session data. You can use Session Initiation Protocol distributed session key, you can also use the RTP session configuration file holds the session key (it security). In order to prevent a known plaintext attack, each message should beIn one-time and unpredictable information. RTP header time scale field provides such a mechanism, which encrypts the RTCP packet before it should be encrypted message adds a random number. Clock synchronization, and voice/video synchronization
Point to point connection for receiver according to the data arrives at the rate of implementation and service side of the synchronization.
Distributed multipoint Conference has multiple send/receive on demand synchronization, this programme will not fit. We design a simple and efficient way to solve the clocks are not synchronized and the same source voice/video are not synchronized. This method uses a time scale of RTP provides, you can simply summarized as: silence suppression audio packets sent. Sound in the receiver to the receiver's audio playback, audio clock clock synchronization were offset in silent period. Audio/video synchronization is at each moment of sudden start audio, delayed by discarding some video frames or reuse some of the video frame. This mechanism is not required to play back the clock with clock synchronization capture, it can achieve the desired performance is based on the following facts: ① burst mean duration of relatively short duration of silence; ② capture and play back-end clock synchronization is small. These two points make audio/video synchronization in a relatively short burst duration not many may drift. We have different source data flow between sequenced relationship did not take any control. With the RMP (reliable multipoint send Protocol) Protocol in the use of Group communications, we will be on the order of the control. IP network directory services
Directory services in centralized and distributed meetings are very important. Circuit switching in the node identified by the fixed number, packet switching network by IP address in the node to identify it. Heterogeneous network, ATM nodes identified by the E.164, POTS and ISDN node identity by phone numbers, Internet node identity by IP address, if the directory service can be the name of the Conference participants into its physical address, will have a great convenience. In mobile communication, Conference participants from different parts of the Internet, use dynamic addresses, directory services, it becomes all the more necessary. If the firewall user does not want to expose your own IP address, directory service functionality is more complex.
Internet domain name system (DNS) is a distributed directory service solutions, but ordinary DNS system does not support dynamically assigned IP address. Dynamic IP address query scenarios require a real-time registration mechanism for users to log on a dynamically assigned IP address. Current real-time registration agreement with SDP, LDAP and secure dynamic update DNS etc (distributed). Internet database provider for various application offers dedicated real-time registration agreement (centralized). Centralized programs easy to achieve, but the extensibility, and requires the Council to the same service provider registration is unlikely. Distribution system based on DNS, practice shows it running stability, scalability, good. Secure dynamic update DNS is an ideal choice.

Currently it's directory services are not taken into account through the firewall. Passing through the firewall to the most commonly used method is to use a proxy server. The generic proxy server can also be converted to IP address, and a full suite of powerful security features, but their versatility also has the following issues: ① while there are many applications use may result in the delay, cannot be guaranteed real-time; ② as hackers provides breakthrough vulnerability; ③ cannot provide different subnets domain name query service; ④ in IP address conversion level link will lead to unexpected situations. We use a dedicated agent can overcome the above shortcomings, RTP Mixer or Translator.
Suppose A and B are located at two different firewalls, we can in A and b in the same subnet as the firewall has a proxy PA and PB, together they have a connection Internet service provider in the public directory. Assuming that A is caller, b is the callee. The following is the process of communicating through the firewall:
· Users log on to A Web-based registration to PA. PA to A creation of a internal records, registered A IP address and E-mail address. Then, PA to the external directory service provider to register A user name (E-mail address) and your own IP address. User B logs on, the B and PB do the same.
· When A to communicate with B, A call to the send a PA request, give the call destination B e-mail address.
· PA to external directory service provider issues a request to resolve the name B. External directory services in step 1 will be returned to the address of the B registration (i.e. PB's IP). Under B domain name or the directory service provides special information, PA can know B is a firewall.
· PA to PB issues a connection request, the caller and the called party's name A and B. This PA and PB for A and b establish a virtual connection, subsequent traffic can pass through this link A-PA-PB-B. Closing remarks
Internet development led to a new distributed multipoint video conferencing solution, distributed solutions and circuit-switched network centralized scheme there is a big difference. As an important group computing applications, distributed multipoint video conferencing system will get new group communication technology further support, such as: better multicast routing algorithms and protocols; able to adapt to complex network environments of resource reservation and information filtering technologies; reliable ordered communication security; application of the system for conference support. However, how to most effectively use these support to adapt video conference complex, diverse needs will continue to be our research topics.