1 TCP / IP protocol
TCP / IP is a complete set of data protocols, named after the two most important protocols: Transmission Control Protocol (Transmission Control Protocol. Abbreviated TCP) and Internet Protocol (Internet Protocol. Abbreviated IP). At present, the TCP / IP protocol has become the dominant communication protocol. It can enable various services to be interconnected on different networks, laying a foundation for network integration from a technical perspective. The reason why it can achieve such achievements is because it has the following characteristics: The open protocol standard can be independent of the open mode of specific computers and operating systems. Independent of specific physical network hardware, it can run on multiple network transmission media.
The common address planning scheme makes any TCP / IP device in the entire network have a unique address. Standardized high-level protocols provide a variety of reliable user services.
2 Development of IP technology
TCP / IP is an important technical foundation in the Internet. The emergence of WWW in 1989 has caused a fundamental change in the Internet. It has evolved from a pure data communication network to a distributed file system that can share and send information around the world. Its use value has increased rapidly, and the TCP / IP protocol has also Therefore, it is widely used. Especially after the commercialization of the Internet in 1993, the scale of the network has grown at an average rate of doubling in half a year, making people realize that different types of networks should be interconnected at the network layer using network interconnection protocols to form a platform independent of network infrastructure. This is the IP network. Ordinary IP networks can not guarantee the quality of service when transmitting real-time services. Therefore, broadband IP networks came into being. With the emergence of some key technologies such as IP Over DWDM and MPLS traffic engineering. It can provide a service quality comparable to that of an ATM network, and can achieve an integrated service network that seamlessly connects to many different networks. Broadband IP network provides a platform for developing new broadband data communications and broadband interactive media new services. The demand for multimedia communications in IP networks is also expanding day by day. For this reason, the ITU has established the H.323 standard, making it possible to transmit video over existing communication networks. The broadband IP network based on the TCP / IP protocol will surely integrate the advanced technologies and network resources of the three major networks, namely, cable television network, telecommunications network, and computer network, and become the mainstream of the future information highway.
3 IP video transmission
3.1 Application requirements of network multimedia
Because multimedia signals are interactive, it puts forward the following application requirements on the network:
(1) Throughput requirements refer to the requirements for high transmission bandwidth, large storage buffer bandwidth, and flow control.
(2) Reliability requirements: Reliability requirements are not the focus here. Appropriate data loss will not affect the actual effect of the video broadcast too much.
(3) Network delay requirements: Higher network delay and jitter requirements because multimedia video streams are more sensitive to network transmission delay and jitter. For example, the transmitted video signal and audio signal must be synchronized.
3.2 IP Multicast
IP multicast, the English original IP MulTIcast, can also be translated as "group communication". In 1999, IP multicast had three key conditions for development: â‘ routing protocol to support IP multicast; â‘¡ use of open standard testable management protocols; â‘¢ business opportunities for development So that it has entered a stage of rapid development. IPv4 defines three IP data communication methods: â‘ point-to-point communication; â‘¡ broadcast across the network; â‘¢ multicast. Point-to-point communication refers to data communication between two IP addresses; whole-network broadcast refers to sending packets to all IP addresses in the IP subnet in a broadcast manner, and all IP stations in the subnet can receive the whole network Broadcasting; multicasting refers to the transmission of data to a specific set of IP addresses on the IP network, and is the communication method between â‘ and â‘¡ above. Comparing the three data communication methods defined by IPv4, we find that IP multicast has the characteristics distinctly different from the methods of â‘ and â‘¡: when an IP site sends data to multiple sites on the IP network, it can reduce unnecessary overlapping transmission. Effective use of network bandwidth can reduce network delay and jitter. The reliability of IP group communication is worse than that of â‘ and â‘¡.
3.3 IP video application requirements
Because of the interactivity and interactivity of online information, the amount of information transmission in the network is increasing rapidly, and the bottleneck problem of network transmission is outstanding! In multimedia applications, the problem of network bandwidth caused by video transmission is more prominent. When n IP addresses receive network multimedia video streams at the same time, set the required transmission bandwidth of each video stream to 1.5M. According to the current network structure, the required bandwidth is n * 1.5M, and it will bring intolerable network delay And jitter. Most of the existing networks use TCP / IP point-to-point protocol configuration, so the focus of our research is how to achieve video transmission without making too many changes under the existing network conditions, that is, IP multicast solution Be compatible with existing networks. As mentioned before: Multimedia video streaming does not require high data reliability, and proper data loss will not affect the actual effect of video broadcasting too much. Although multimedia video streams are sensitive to network transmission delay and jitter, IP multicast has very little delay and jitter in the network. Therefore, it is feasible to use IP group communication to transmit IP video signals.
4 A method for implementing video transmission using IP multicast and its characteristics
At present, there are two main ways to provide video services on the IP network:
1) The MulTIcast technology of the router is fully used, and no additional server forwarding is required, but it will increase the burden of the router, there is a risk of "broadcast storm", and the network routing protocol needs to be adjusted.
2) Using software and servers, an overlay network for processing streaming media is added to the entire IP broadband network, and the overlay network realizes functions such as point-to-multipoint multicast, media stream routing, and multipoint injection. The video service method we use is 2) solution. Specifically: the computer cooperates with special software to form a server to realize real-time control. The purpose of the control is: for the multimedia video server, it must have a maximum efficiency transmission mechanism, that is, the system can respond to and meet the video request sent from the multimedia video receiver in the shortest time to the maximum, and complete the request to the user Data transmission of all addresses, computer real-time control system monitors the quality of video transmission at any time, and automatically adjusts bandwidth, etc. Of course, the realization of the transmission method can be compatible with the current network facilities.
During the implementation of the above scheme, the computer (server) constantly monitors the system to achieve the best possible broadcast quality and high efficiency, and there is no need to cause dangers such as "broadcast storm". According to the above requirements, we have designed the following system, which consists of four parts: video sending, video forwarding, video receiving, and video control. The following specifically describes the functions of each part:
â‘ Video sending
The video is sent as pre-made video or real-time video. It can be an independent computer, or it can share a computer with the first-level "video forwarding" unit. Specifically, we first compress the video in real-time video according to the MPEG-1 encoding technology. The digital rate of this format is 1.5Mbit / s, and the image is in SIF format (352Ñ…288), 30 frames per second, with two stereo sounds. The reason for real-time video compression according to MPEG-1 encoding technology is that the quality of the video signal after compression is satisfactory, and the digital rate bandwidth is relatively narrow, which is beneficial to IP multicast. (Of course, other coding techniques can also be used). Then we send the compressed signal to the video forwarding end. The signal transmission from video transmission to video forwarding is point-to-point transmission (this unit belongs to IPv4 communication method â‘ ).
â‘¡Video forwarding
The main function of video forwarding is to forward the video signal sent from the video sending end to the video receiving end or the next level video forwarding end through the IP network. It is the core of IP multicast transmission of video signals. Video signals are forwarded by IP multicast, that is, data transmission to a group of specific IP addresses (users of the same type of request). Video forwarding is done by the forwarding computer (server).
â‘¢Video receiving
Video reception is the user's multimedia terminal. Of course, the user's multimedia terminal equipment must support IP multicast.
â‘£ Video control
Its main function is to control the forwarding site to establish and manage the transmission of IP multicast data groups on the forwarding site. The control system needs to be satisfied to the greatest extent, and complete the data transmission to the users who need it, while paying close attention to the quality of the video transmission. Specifically, it is necessary to send data for as many requesting users as possible, but within the allowed bandwidth. This bandwidth is controlled by the computer in real time. The computer real-time control system monitors the quality of video transmission at any time and automatically adjusts the bandwidth. At the same time, it also monitors other network parameters in real time. It can be seen that video control is essentially real-time control of the computer. The real-time control of computer real-time control determines the effect of IP multicast.
The key technology of this solution is reasonable video forwarding, seeking maximum benefits and satisfactory transmission quality. The realization of this program benefits from the real-time control technology of the computer. We use this method to implement a trial broadcast of real-time network IP multicast video transmission on a 100M Ethernet LAN network, transmitting MPEG-1 video signals (each video stream bandwidth is 1.5M), and the video signal transmission effect is good. Prove that the above scheme is feasible.
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