Summary: With the increasing maturity of video compression technology, digital video surveillance products have gradually become the mainstream of the market. It is urgently desirable that network technology can be successfully applied to the field of digital video surveillance to enable people to achieve off-site monitoring through the network. It is in this context, "network-based digital video transmission" research is important. This article comes around the real-time and transmission quality of network video transmission, starting from "Solving Transportation Protocol Issues", by comparing the TCP protocol and RTP protocol, the design of "RTP real-time video transmission based on UDP protocol" is proposed. Thoughts, better guarantee the real-time and quality of service of digital video transmission.
Key words: digital video; network; transport layer protocol
1 Introduction
Digital video transmission is a communication method that uses visual to obtain information. It has the characteristics of exact, intuitiveness, and high efficiency compared to other information transfer methods. Due to the large amount of information and limited transmission bandwidth of digital video transmission, the video's compression coding, transmission channel, and network protocol, IP multicast (IP multicast), and software-based software-based software-based numbers Key technologies in video transmission applications. Among them, the transmission channel and network protocols are critical, which will directly affect the real-time performance of digital video transmission and the quality of video image received after the client is transmitted.
Figure 1 shows the basic model of the network-based digital video transmission. This article will be specifically analyzed for the selection of network transport protocols based on network-based digital video transmission applications.
2 Selection of network transport protocols is concentrated in the transport layer
We know that ISI (Open System Interconnection) network reference model is divided into 7-storey structure, from bottom to: physical layer, data link layer, network layer, transport layer, session layer, representation Layers and application layers. The functions of each layer are independent of each other, and the functions implemented by each layer are transparent to the above layer, and each layer is only concerned about the services provided in the next layer.
The task of the physical layer is transparently transmitted bit stream. The task of the data link layer is through various protocols, and there is no error in the line between the two adjacent nodes, and their functions have been transmitted. Media and NIC are cured, and they cannot be changed by programming, so they do not need to consider these two layers when programming; the task of the network layer is to select the appropriate route, complete the data packaging and transfer, at this layer, NetWare Using the IPX protocol, UNIX and Windows use IP protocols; the task of the transport layer is to best utilize network resources according to the characteristics of the subnet, and establish a transmission connection for both ends of the host in a reliable and economical way, transmitting the report in transparent way Wen, in this layer NetWare uses the SPX protocol, and Windows uses TCP / UDP protocols, UNIX uses TCP protocols; the session layer task is to determine how to communicate between information between interconnected hosts, ie: full duplex, half Duplex and single work, generally do not consider in the LAN; the mission of the layer is to transmit data formatting and code conversion, generally used for communication between herself, there is no such problem between the microcomputer, so this layer Needless to consider; the application layer task is to determine the nature of communication between the processes to meet the needs of the user, it is the task we have programmed, but there is no need to make an agreement. From this point of view, in order to ensure the real-time and image quality of the network-based digital video transmission, the selection of the transport layer protocol is the key to the entire design and implementation.
The Internet uses two transport protocols on the IP layer: one is the transfer control protocol TCP (Transmission Control Protocol), which is a connected network protocol; the other is the user Data News Protocol UDP (User DataGram Protocol), it It is a connected network protocol. Figure 2 is a concept hierarchy of the network: We need to know "Connect" and "no connection" before analyzing the transport layer protocol. The connection service is an abstraction of the telephone system service mode, that is, each complete data transfer has to be established, using the connection, and terminating the connection. During the transmission of data, each data packet does not carry the destination address, but use the connection number (Connect ID). Essentially, the connection is a pipeline, and the transmission and reception data is not only the same, but also the same content. The TCP protocol provides a connected virtual circuit service.
The connectionless service is an abstraction of the postal system service, and each packet carries a complete destination address, and each packet is transmitted independently in the system. No connection service does not guarantee the order of the group, and the recovery and retransmission of grouping errors are not performed, and the reliability of the transmission is not guaranteed. The UDP protocol provides a connectionless datagram service.
3 TCP is not suitable for real-time transmission video and audio data
IP networks have been widely used in various occasions. Among them, TCP / IP protocols are industrial standards for interconnecting and communications in xenografts. The system is built on top of TCP / IP, which can broaden its application range. However, a simple TCP / IP protocol is difficult to adapt to the requirements of video and audio communications, especially continuous media streams (such as video streams). The TCP protocol is a connection-oriented protocol, which is used in a variety of virtual circuit services that provide orderly and reliable data transmission. Its retransmission mechanism and congestion control mechanism are not suitable for real-time viewing.
The TCP / IP protocol was originally designed to provide non-real-time data services. The IP protocol is responsible for data transmission between the host and does not perform error detection and error correction. Therefore, data loss or sequencing phenomenon often occurs. In order to ensure the reliable transmission of data, people use TCP protocols for the transmission of IP data to increase the error detection and error correction capability of the receiving end. When a packet loss or error is detected, the sender will be required to resend, so that the bandwidth of the transmission delay and the consumption network is inevitably caused. Therefore, traditional TCP / IP protocols transmit real-time audio, and video data is poor. Of course, when transmitting video and audio data for playback, the TCP protocol is also a choice. If there is a sufficiently large buffer, a sufficient network bandwidth, on the TCP protocol, close to real-time video and audio transmission is also possible. However, if the packet loss rate is high, the network is not good, video or audio communication is almost impossible to use the TCP protocol.
TCP and other reliable transport layer protocols such as XTP is not suitable for real-time view of real-time view, and the following areas:
3.1 retransmission mechanism of TCP
We know that in the TCP / IP protocol, it will require retransmission of lost packets when the sender discovers data loss. However, this will need one or even more cycles (according to the fast retransmission mechanism of TCP / IP, this will require three additional frame delays), which is almost almost in real time requirements. It is catastrophic because the receiver has to wait for retransmission data to cause delay and breakpoints (non-continuous or video coagulation of audio, etc.).
3.2 TCP congestion control mechanism
