Video surveillance technology plays an important role in the security of political, economic, military, and cultural facilities. With the development of multimedia and computer network technology, video surveillance systems have experienced the development stage of analog surveillance and digital surveillance. Video surveillance stage. The remote monitoring system introduced in this article can use the existing IP network to realize remote monitoring and control camera focusing, pan-tilt rotation, etc., which can meet the unattended requirements and realize a powerful and easy-to-operate monitoring solution.
1 Overall system structure
The network remote monitoring system consists of cameras, network video servers, and network remote clients. The front end directly connects all the video, audio, alarm and other signals of each monitoring point to the monitoring center through the network to realize the functions of network display front-end video images, video storage, playback, PTZ control and so on. This monitoring system transmits video images through the network no matter whether it is local or at the network control end. All operations and functions are implemented by installing monitoring software on the management computer, such as display, recording, and various controls. Any authorized network client can implement remote monitoring, video storage, PTZ control and other operations on the local end through the network platform, and can implement rights management. Adopting the leading H.264 high compression technology in front of the mouth, greatly improving the compression ratio and network transmission effect, each channel can achieve full real-time display / recording; automatically adapt to bandwidth and configure resources, optimize network transmission speed, improve playback picture Quality, reduce network delay; the system supports simultaneous remote access to multiple main control terminals by multiple sub-control terminals, or simultaneous remote access to multiple main control terminals by multiple sub-control terminals, each point is independent of each other, and does not interfere with each other; The sub-control authority can be set, so that the sub-controller can enjoy different operation authority through different passwords to distinguish the management level and strengthen the management security.
2 Network video server structure
The rigid network video server selected in this system uses H.264 hardware compression technology, which integrates 1-channel or 4-channel audio, video acquisition, real-time compression, network transmission, and linkage alarm functions. It is plug and play, and users can Use the client software or IE browser to directly view the images transmitted by the video server in real time through the Internet or LAN.
Each video server has an independent IP address, which can simultaneously access 4 channels of video signals and audio signals. After processing, it is converted into IP packets, and these data packets are sent to the video host of the monitoring center using broadband local area network and TCP / IP protocol. (PC) or online authorized users. The client-side monitoring software runs on the video host, which can realize real-time video monitoring of 1/4/9/16 screens, and can also drive the display array for large-screen display. Other authorized users can conduct general monitoring through IE browser.
3 System software structure design
This software adopts modular programming and client / server model design, and the data is transferred between the modules through interfaces (function calls).
3.1 Server-side software structure
The server-side software is mainly composed of a main control module, a video acquisition module, a monitoring control module, a system setting / user setting module, an information query module, a video information processing module, and a network transmission module. It mainly realizes the functions of multimedia data collection, compression, playback, storage, transmission, information processing and query.
The main control module mainly controls the normal operation of each module, effectively manages each module, and performs some necessary work such as initializing the system.
In order to reduce the burden on the CPU, we use hardware to achieve the collection and compression of video data. The video card captures moving images at 30 frames per second. The collected video data is divided into two channels, one channel is directly sent to the display buffer for local display, and the other channel is compressed by hardware and stored on the local hard disk, or transmitted to the client at the same time, thus real-time video image collection Transmission and display.
The main function of the monitoring control module is to encode the actions of various front-ends and then send them to the front-end, so as to realize the PTZ control and lens control of the front-end camera.
In the system setting / user setting module, the super user is allowed to modify the password of any user, and users can be added, deleted, and assigned permissions to other users. The system has three levels of authority to choose from: "System Administrator", "General Administrator", and "General Operator". Each level has been assigned default permissions.
The video information processing module includes sub-functions of video file playback, storage, deletion, and picture capture.
The information query module mainly includes the functions of querying video files, pictures and logs. When saving video files, the system takes the time of file creation as part of the file name, and the video files taken by different cameras are saved in different directories. In this way, video files can be easily queried by year, month, day, time period and camera in the file list.
The network transmission module can enable the client software to share the information of the main control server through the LAN or the Internet. It uses Socket to provide different socket system calls for the client and server programs to achieve information sharing.
3.2 Client software structure
Remote client computers distributed on the LAN or Internet through the client software can realize video information sharing and remote control operations. This part adopts C / S (client / server) mode design, and its realization function is completed by two parts of the client software and the rigid network transmission module in the server software.
The client software should include the main control module, login module, playback module, remote download module, real-time transmission module, remote control module, etc.
The upper control module mainly controls the normal operation of each module, effectively manages each module, and performs some necessary work such as initializing the system.
The login module mainly uses the user name and password to enable users of different levels to have different permissions, thereby effectively implementing security control and preventing illegal users from entering the system.
The playback module includes functions such as local playback and remote playback. Video file playback is achieved using DirectShow. The filter table manager automatically generates a filter table to play the video file. We can design to encapsulate a class CMediaPlay, encapsulate many functions in this class, such as P layFromFile () function to play video files, PausePlay (), ResumePlay (), STopPlay () and other functions to control playback.
The main function realized by the remote download module is to download the video files on the remote server (main control machine) to the local client.
The main function of the real-time transmission module is to transmit the video signal collected by the main control computer in real time to the client that issues the real-time transmission request in real time without distortion, and it is displayed by the playback module.
The main function of the remote control module is to realize partial control of the main control server on the client. For example, you can set the PTZ and monitoring effect on the client. The design idea is to classify and encode the control content. The client adds the control parameters after the encoding and then transmits it to the server; after the server process receives the data, it decomposes the encoding and parameters from it, and then calls the corresponding function function to achieve Control of the target. In this module, since control commands are transmitted, in order to ensure the reliability of transmission, we use a connection-oriented communication method based on TCP protocol. In order to avoid control conflicts, this module requires that only one user can control the PTZ and monitoring effects at any time. Therefore, on the server side, only one user's connection request is accepted for this port. Once the connection is established, the control functions of these two parts on the server side are set to invalid state; when the connection is disconnected, they are restored to Active state. In order to avoid that the client is in the control state for a long time and other users cannot enter the control state, a timer is set in this module. When the connection is established, the timer is started to start timing; when the user presses any key on the keyboard or mouse, the timer returns to zero and restarts timing; when the timer counts a certain time, it automatically disconnects and closes the control dialog box.
4 Key technologies of software design
4.1 Selection of communication protocol
The TCP protocol is responsible for data flow control, ensuring the correctness of the transmission, and has the function of data retransmission. The jitter caused by the transmission delay will seriously reduce the quality of real-time communication and is not suitable for real-time communication. Therefore, when transmitting multimedia information on the Internet, the UDP / IP protocol is adopted, and neither UDP nor IP provides link guarantee, and the reliability of data transmission is not guaranteed. Therefore, the widely used RTP / RTCP (real-time transmission protocol / real-time transmission control protocol) is adopted here.
RTP provides an end-to-end network transmission function, suitable for transmitting real-time data streams through multicast. The RTP message format includes a fixed RTP header, optional role identification (CSRC), and load data. RTP itself cannot provide a reliable transmission mechanism for sequentially transmitting data packets, nor does it provide flow control and congestion control. It relies on RTCP to provide these services. The number of RTCP packets accounts for 5% of all transmissions.
In the data plane, the server will load the compressed audio and video data into the data payload section of the RTP message according to the RTP message format, and configure the timestamp, synchronization information, sequence number and other important parameters of the RTP message header At this time, the data packet has a typical time characteristic, that is, it is "streamed". At the UDP / TCP layer, RTP messages are loaded into UDP / TCP messages as load data. Finally, the IP layer is responsible for the final message header configuration to achieve network transmission. On the client side, the implementation is reversed. Each network layer sequentially removes the packet header and reads the relevant control parameters and time parameters to finally obtain audio and video data that can be played in real time.
4.2 Multicast technology
If unicast transmission technology is used to achieve multipoint transmission of surveillance video, a separate data channel needs to be occupied between the sender and each recipient to transmit huge amounts of real-time video data, which will cause the sender to have a heavy burden, Long delays and network congestion. However, the use of broadcast is extremely easy to cause a large occupation of network bandwidth and affect the communication efficiency of the entire network. IP multicast technology is an effective method that can maximize the use of existing network bandwidth resources.
Multicast refers to sending data to a group of IP sites on the Internet, and all recipients who join the group can receive the information. This group of IP sites is formed dynamically, and each IP site can dynamically join or exit. Each host can be added to multiple groups at the same time. IGMP is an Internet group management protocol dedicated to IP multicast communication. It is used to manage group members joining or leaving a multicast group.
4.3 Implementation of Multicast Technology by Winsock
Winsock is a Windows API set based on the socket model. The Windows Sockets API is a network programming interface for Windows. The Windows Socket specification defines and records how to use the API to link with Internet protocol clusters. It supports stream sockets and data sockets. The application program calls the Windows Sockets API to communicate with each other, and Windows Sockets uses the underlying network communication protocol function and the operating system call to implement the actual communication work.
Winsock includes Winsock1 and Winsock2 two types, each type defines a group of application IP multicast interface functions, in actual programming applications. IP multicast can be realized through a series of application program interfaces provided by Winsock. This system uses Winsock1 to achieve IP multicast communication, the main process is as follows:
(1) Use the Socket function to create a datagram socket;
(2) If you receive data from a multicast group, use the Bind function to associate the socket to a local port; here we should pay attention to: do not use port numbers less than 1024, because all port numbers less than 1024 are The system is reserved. If it is not used by other programs, you can select a port from 1024 to 65535. If only sending data, this step is not required;
(3) Use IP MulTIcast-TIL, call the setsocketopt function to beat TIL;
(4) Set the IP Add Membership option and the multicast group address structure to join, and call the setsocketopt function to join the specified multicast group; if you plan to send data only, you do not need to join a multicast group;
(5) Call Sendto function to send multicast data, and call RecvFrom function to receive multicast data;
(6) Set the IP-Drop-Membership option and the address structure of the multicast group to be detached, and call the setsocketopt function to leave the specified multicast group.
With the development of Internet technology, people have put forward higher requirements for the image and sound quality of remote monitoring systems. This system can not only provide good picture quality, but also provide excellent sound quality. The features of perfect function and reliable operation can be widely used in security monitoring of banks, warehouses, residential areas and other unattended occasions such as electric power stations and telecommunications base stations.
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