Rail Transit Video Surveillance System Requirements and Construction
In order to prevent the occurrence of urban rail transit emergencies, implement the scientific concept of development and the people-centered concept, ensure the safety of the people, and actively develop the urban rail transit train video surveillance system is one of the most urgent and important tasks.
Foreword
With the development and progress of technology, video surveillance technology has become an important part of social public safety prevention technology, and has been widely used in finance, culture and literature, hotels, traffic management, transportation, commerce, hospitals, factories, schools, Residential areas and other security applications. Urban rail transit system, as a public place with high concentration of personnel, is in particular need to use video surveillance technology to monitor the public safety within the system.
At present, CCTV CCTV monitoring systems have been installed in urban rail transit stations in major cities in China to achieve security monitoring within the stations; however, security monitoring in train compartments has just started. In order to prevent the occurrence of urban rail transit emergencies, implement the scientific concept of development and the people-centered concept, ensure the safety of the people, and actively develop the urban rail transit train video surveillance system is one of the most urgent and important tasks.
demand analysis
At present, domestic rail transit has installed a large number of cameras at station stations, stations, entrances, and substations, which can basically meet the needs of security. However, as the passenger's main activity venue - there are currently no cameras installed in subway cars. With the frequent occurrence of emergencies and the deepening of public security anti-terrorism, subway vehicles as a public dense area of ​​passengers pose a greater threat to security. . At the same time, because the driver is driving by one person, during the operation of the train, whether there is any operation in accordance with the standard or if there is any failure to comply with the rules and regulations, it is basically impossible to control and cannot verify. Therefore, it is very necessary and urgent for vehicles to install vehicle video surveillance systems. The driver and the ground control center dispatched to understand the status of passengers in the carriage, whether there is illegal activity. In the event of a failure to direct passengers to safely evacuate and so on. The status quo is that the driver and the ground control center do not know the condition of the compartments, and there are behaviors such as selling, begging, theft and so on, affecting the service quality.
Functionally speaking, monitoring can basically cover the whole car and focus on it. The purpose is to find and record the live image. The camera images on the train should all be displayed on the video controller of the driver's office. The image should have corresponding characters. The character content should have at least the camera number, date and time. Each image should be recorded. The recording time should be at least 7 days. The video should be easily searched and downloaded. The station can see all the video images on the car and download the videos in real time through the car-to-ground wireless network transmission.
Car video surveillance system should have stability, which is the primary consideration for system design. The design of monitoring points should be reasonable and have no blind spots. Due to the relatively small space in the car wiring, it is necessary to have reasonable wiring. Symmetrical design should be used to facilitate the re-allocation of marshalling needs in the future. The maintenance workload of the system should be small, and it is convenient for the maintenance personnel.
Operating System Management Agency
The video surveillance system shall be based on the administrative management level of the Jiaoji line operation system and be divided into two levels of monitoring structures: the route operation monitoring center shall be the first level; each station shall be provided with a second-level monitoring center, but each station does not need to monitor its The video images along the jurisdiction are only used as a platform for video distributed storage and forwarding. Control information, inspection information, alarm information, and video information can be transmitted at various levels of the network system according to a predetermined flow.
Video surveillance camera deployment
According to the situation of installation of on-board video monitoring systems and the installed position of the carriages in each domestic rail transit vehicle, each train carriage should be equipped with two or more cameras to monitor the on-site conditions in the carriages. Each driver's room at each end of the train should be equipped with a camera and pickup to record and record the driver's driving activities and discourse. Each cab is equipped with a video controller for the driver to observe and switch the camera image of the compartment.
Video surveillance system screen
Real-time video transmission, video recording, video image storage time of 1 month, to provide video evidence of the crime scene, for special places can be considered PTZ camera, the system can control the front-end head movement and lens zoom . The system can transmit the control commands sent by the network users to the front-end monitoring devices according to the authorization, and realize the call and control of the monitoring center and authorized users to any one-way monitoring images on the front-end. The image quality satisfies the requirements of the subjective evaluation index system and subjective evaluation index system.
Intelligent analysis needs video intelligent analysis alarm function, the purpose of automatic judgment of threats, early warning, because the schedule and the driver can not monitor the status of each car in real time. And can quickly help analyze and find the suspect after the fact. Drivers and ground station dispatches guide passenger behaviors through broadcasting or passenger information systems, through specific guidance of the monitoring images.
Administrative requirements
In order to ensure the normal operation of the network monitoring system, the system users should be classified, and various factors should be taken into account to formulate a reasonable authority management strategy to achieve the control of user access.
The surveillance station of the secondary station monitoring center, operation monitoring center, and other operating agencies must have a camera and provide superior leaders to check the status of their duty. The system can realize the intercom function between the upper and lower level monitoring room.
Construction and maintenance requirements
This system is in principle constructed and maintained by the Group's operating department and can guarantee long-term use. At the same time, the system has two modes of automatic automatic inspection and manual automatic inspection for remote equipment inspection. The artificial automatic inspection priority level is higher than the system timing automatic inspection. The regular automatic inspection function and the manual automatic inspection function are implemented by the monitoring and control system monitoring centers at all levels. Requires inspection results and content to be filed in the form of statistical reports in accordance with the management requirements of the organization. The system platform can understand the running status and fault conditions of the equipment in real time.
Information storage and backup requirements
Front-end monitoring equipment and monitoring centers at all levels should all store information such as inspections, alarms, video and audio, and system logs. The corresponding department of the organization shall store the alarm information and the video and audio information associated with the alarm in accordance with the prescribed format. Historical data information such as alarms and patrols stored and backed up can be accessed in the network system based on authorization.
Train vehicle video monitoring system
The most important function of the on-board part of the train video surveillance system is to transmit real-time video data acquired by a vehicle-mounted part (camera) of the vehicle video monitoring system of the front-end equipment of each car to the train driver's cab, displayed on the monitor and stored in the Storage media. If you are building such a video surveillance system in a building, you can use a digital video surveillance system solution, that is, connect all cameras to an image capture card and store and display them in a centralized manner; or use a more advanced network camera via wired or wireless The network is transmitted to the terminal display device.
The biggest difference between the train and the building is that the train is formed by connecting several cars. The electrical connection between the cars is achieved through a jumper cable connector (usually an aviation plug). The use of a connector actually creates a "breakpoint" in the transmission line, taking into account the loss caused by the "breakpoint" to the transmission and the limited space of the connector itself, the cable passing through the connector when building the communication and transmission system on the train. It must be as small as possible. In fact, the previously mentioned train bus trunk cable has only two twisted pairs passing through the connectors between the cars (one of which is spare). This is taken into account in the hierarchical and node structure of the train communication network. The nodes of each car are connected to the electronic equipment of the car through the vehicle bus to receive data and send instructions. After processing the received data, they are uploaded to the train bus and the interconnection between the nodes is realized through the bus. This is the train communication network. The stratification and node structure have important reference for the construction of train video surveillance system.
Wired transmission network
Based on the above analysis and discussion, when using cable to build a video surveillance system on trains, it is necessary to obey the principle of “as few as possible through the cable connectors between carsâ€, so the first kind of introduction introduced in the introduction can be basically eliminated. The mode (analog CCTV mode) and the second mode (digital video surveillance mode), because both of these methods require that all camera output signal cables pass through the inter-vehicle cable connector.
The camera transmission cable from the front end to the driver's cab at the end of the bus passes through three connectors, which means that three "breakpoints" are generated. The loss and noise caused by the breakpoint will affect the transmission; the previous connector of the end vehicle must pass through. Six cables take up a lot of space. Therefore, this scheme is not desirable in practical applications.
The method of reducing the number of transmission system cables can be used as a reference for the node and hierarchical structure of the train communication network (TCN). In each carriage, "nodes" are set to connect with the camera. The "nodes" are connected by a simple trunk cable to transmit video data to the driver's computer at the two ends of the train to achieve display and storage functions. The work done by the “nodes†here is to compress and package the video data collected by the cameras in the car in this section for transmission, and to be able to forward data of other “nodesâ€. According to this train of thought, taking the train composed of four sections of vehicles as an example, the model structure diagram of the onboard video surveillance system shown in the following figure can be obtained.
The function of the "node" in the model is to compress and encode the car video data in this section on one hand, and to forward other node data on the other hand. Therefore, the "node" should consist of two parts: the network video server and the switch. A network video server (including an encoder) with an embedded operating system can perform video compression encoding and packing, while the switch can forward data from other nodes.
In this way, the monitoring equipment of the entire train constitutes a small local area network, and both the host computer and the video server are connected to the local area network through the switch, forming a video monitoring system based on network transmission.
Local area network (LAN) is organized in the form of Ethernet, token bus, token ring network, fiber distributed data interface (FDDI) and so on. When selecting the specific form of the LAN, the following factors must be considered to ensure that video surveillance images can be transmitted. The networking is simple and the communication protocol is mature and reliable, economical and practical.
Compared with the above-mentioned networks, Ethernet has penetrated almost all the LANs because of its simple and practical (CSMA/CD) protocol and networking convenience. After several decades of improvement and development, Ethernet has evolved from the initial 10M Ethernet. The network developed to 100M, 1000M Ethernet. Although Ethernet has some defects in the transmission of multimedia services, that is, when time-sensitive services and general data services are mixed on Ethernet, there is no mechanism for providing preferential treatment for time-sensitive services. However, the train video surveillance system we construct only transmits video surveillance images, and the video surveillance system transmits services that are basically in line with the third category of services, namely, video information transmission is used for real-time use. Therefore, there is no problem of mixing with other services. In view of the simplicity and convenience of Ethernet and the universality of its application, we determined that the specific form of the transmission network of the on-board part of the train video surveillance system is Ethernet (100Base-T).
Overall system structure and configuration
After determining that the vehicle part of the train video surveillance system adopts 100Base-T Ethernet as the transmission network, the overall structure and specific configuration of the system can be determined.
The overall structure
The on-board part of the train video surveillance system can be generally divided into three parts: the front-end camera, the encoding part, the network transmission part, the terminal display, and the storage part. The overall structural model diagram is shown in Figure 4.
Specific configuration
Each carriage is equipped with two color fixed cameras. The two cameras are installed at both ends of the carriage. The specific installation position should ensure that the camera's viewing angle can completely cover the entire carriage (including through passages) and there is no video blind spot. The horizontal resolution of the camera should be no less than the PAL system 520TV line, to ensure better monitoring results.
Each camera is connected to a single video server via a video coaxial cable (as described earlier, using video coaxial cable to transmit analog video signals at a short distance).
A single video server should include an encoder, controller, memory, and so on. The encoder first samples and quantizes the input analog video signal, converts it into a digital signal, and then encodes the digital video signal according to a certain video compression standard. The encoding standard can be MPEG-4 or H.264 because both compression standards can generate compressed video streams suitable for IP network transmission.
Network transmission section
An Ethernet switch (with 100Mbps RJ-45 ports) is installed in each car. Two single-channel video servers in the car are connected to the switch through Ethernet cables (twisted pair). The switches in adjacent cars are also connected through Ethernet cables. The exchanges with cabs at both ends are connected to the video surveillance system terminal host, which constitutes the Ethernet transmission network of the onboard part of the train video surveillance system. Taking into account the vibration and impact caused by the train operation, the switch should use industrial grade industrial Ethernet switches; in addition, taking into account the special power supply mode of the above-mentioned urban rail trains and the frequent start-stop of switching components, the complexities in the vehicle are caused. In the harsh electromagnetic environment, the Ethernet cable should be shielded twisted pair with a certain anti-jamming capability to reduce interference.
Terminal display and storage section
Metro trains have cabs at both ends. Therefore, each driver's room is equipped with a host and a monitor. Two hosts and one host are used as masters. The master and standby can be switched by switching programs. .
Both hosts access the in-vehicle Ethernet transmission network through the switch. The host computer receives video streams from video servers of various cars through the network. On the one hand, it is stored in the hard disk of the host computer in the form of video files. On the other hand, it is decompressed and displayed on the monitor in real time.
Considering the vibration and impact environment of the train during operation, the host should use a ruggedized industrial computer. The host consists of CPU, main board, electronic hard disk (host itself use), data hard disk (storage compressed video files), network card, power supply and other components, the motherboard should be configured VGA output interface, USB interface, RS232 or RS485 interface. In order to facilitate the train driver's control and operation of the train video surveillance system, the display uses an LCD display with a touch screen. The host computer communicates with the monitor through the VGA output interface to transmit video images. The operation commands from the touch screen are transmitted through the RS232 or RS485 interface. The monitor power is supplied from the host computer.
Train video surveillance system
To realize the real-time monitoring of the running train cars in the urban rail transit control center, the real-time transmission of the video surveillance images in the train cars during travel to the control center must be implemented by wireless transmission. The introduction of analog and digital video transmission methods in the previous chapter mentioned the use of radio frequency and microwave media, but it is clear that the analog transmission mode in which a single channel uses a wireless channel cannot satisfy the urban rail transit system with many trains running. Considering that the on-board part of the train video surveillance system adopts the network transmission method, the on-board host has collected all the monitoring images in the vehicle. In order to make better use of the on-board host and achieve the system's acceptance, it is necessary to build a vehicle-to-ground wireless network. Realize the transmission of video images.
Wireless network technology
As with wired networks, wireless networks can also be divided into circuit switching and shared bandwidth. Typical circuit-switched wireless networks such as GMS cellular mobile communication systems, shared-bandwidth networks include wireless local area networks (WLANs), wireless broadband networks, Bluetooth technology, and the like. For long-term transmission of high-resolution video images, the bandwidth of circuit-switched mobile communication systems is insufficient, while Bluetooth technology transmits very short distances, which is suitable for terminal devices in environments such as rooms to be fixed via wireless access. The internet. In recent years, wireless LANs that have been continuously developed and improved are suitable for completing video transmission in such regional systems as urban rails regardless of bandwidth or coverage. Wireless networks must consider taking certain measures to avoid or reduce their impact on transmission performance when designing and selecting physical layer transmission methods.
Train video surveillance wireless transmission system
At this stage, relatively mature and commercially available high-speed wireless access systems mainly use wireless LAN technologies based on the IEEE 802.11 series of standards. By comparison, the vehicle-to-ground wireless transmission system proposes the adoption of a wireless LAN system based on the 802.119 standard, which can be used for secondary development and modification of its technology and equipment to meet the requirements of video transmission under the high-speed operation of urban rail trains and the comparison of urban rail lines. Poor use of the environment.
The urban rail transit line consists of stations and sections. There is usually 15 to 40 stations on one route, and the length of the route is between 15-50 Km. Obviously, it is not practical to configure a pure wireless LAN over such a long path. Because the current 802.11g standard wireless access equipment (seconds) has a signal coverage of only about 200rn (shorter distance when cornering), basically a few AP points need to be set in a range, which determines the wireless local area network. PCF mode is used during configuration (one AP corresponds to one terminal device in DCF mode). PCF mode is used to set a "base station" at each station. Each "base station" manages all AP devices in a range. The "base stations" are connected by a wired transmission network to form a wireless transmission system for the entire line. Therefore, the vehicle-to-ground wireless transmission system should include two parts: a wired backbone transmission network and a vehicle-to-ground wireless transmission platform.
Wired backbone transmission network
The wired backbone transmission network provides a video monitoring image transmission channel between the control center and the station, and serves as a backbone transmission route for the vehicle-to-ground wireless transmission system. The backbone transmission network provides access ports for the vehicle-to-ground wireless transmission platform at each station, and interconnects the switching matrix with the CCTV monitoring system of the control center and the station, so that the in-train video surveillance image is transmitted to the backbone network through the vehicle's wireless transmission platform. The monitoring equipment at the station and control center is displayed in real time.
When the urban rails are under construction, generally redundant optical fibers are laid for the subsequent communication system construction. These reserved optical fibers can be used when configuring the backbone transmission network, and LAN switches are installed in the control center and each station, and become LANs through optical fiber connections. The network form can be defined as 100M Ethernet, or it can be set as Gigabit Ethernet. Considering the need for future system functions to be added or improved, the backbone transmission network is recommended to adopt the form of Gigabit Ethernet. To enhance the fault tolerance and reliability of the network, single-ring redundancy or dual-ring redundancy can be used when connecting to the network.
Vehicle wireless transmission platform
The vehicle-to-ground wireless transmission platform adopts a wireless LAN technology product based on the IEEE 802.119 standard and is mainly composed of a central wireless network switch, a station wireless access switch, a photoelectric converter (or a wavelength division multiplexing device), a wireless access point (AP), and Car AP and other equipment components. The central switch is installed in the urban rail transit control center and works in dual-system mutual backup mode. When one of the two switches fails, the control and management of all APs can automatically switch to another switch. Wireless access points (APs) use single-mode optical fibers and optical-to-electrical converters (or wavelength division multiplexing equipment) to connect to the backbone transmission network at each station through the station's wireless network switch to form a wireless transmission system covering the entire line range and stations. The overall structure of the vehicle-to-ground wireless transmission platform is shown in Figure 5.
Conclusion
Security has become a service network. The industrialization and customization of video intelligent analysis management will bring new opportunities to manufacturers. From the perspective of development trends, the mainstream development direction of future video surveillance technologies is still networked, high-definition, and Intelligent and industry-oriented, in line with this development trend, we will be able to further develop more extensive video surveillance technologies to serve the safety of urban rail transit.
Foreword
With the development and progress of technology, video surveillance technology has become an important part of social public safety prevention technology, and has been widely used in finance, culture and literature, hotels, traffic management, transportation, commerce, hospitals, factories, schools, Residential areas and other security applications. Urban rail transit system, as a public place with high concentration of personnel, is in particular need to use video surveillance technology to monitor the public safety within the system.
At present, CCTV CCTV monitoring systems have been installed in urban rail transit stations in major cities in China to achieve security monitoring within the stations; however, security monitoring in train compartments has just started. In order to prevent the occurrence of urban rail transit emergencies, implement the scientific concept of development and the people-centered concept, ensure the safety of the people, and actively develop the urban rail transit train video surveillance system is one of the most urgent and important tasks.
demand analysis
At present, domestic rail transit has installed a large number of cameras at station stations, stations, entrances, and substations, which can basically meet the needs of security. However, as the passenger's main activity venue - there are currently no cameras installed in subway cars. With the frequent occurrence of emergencies and the deepening of public security anti-terrorism, subway vehicles as a public dense area of ​​passengers pose a greater threat to security. . At the same time, because the driver is driving by one person, during the operation of the train, whether there is any operation in accordance with the standard or if there is any failure to comply with the rules and regulations, it is basically impossible to control and cannot verify. Therefore, it is very necessary and urgent for vehicles to install vehicle video surveillance systems. The driver and the ground control center dispatched to understand the status of passengers in the carriage, whether there is illegal activity. In the event of a failure to direct passengers to safely evacuate and so on. The status quo is that the driver and the ground control center do not know the condition of the compartments, and there are behaviors such as selling, begging, theft and so on, affecting the service quality.
Functionally speaking, monitoring can basically cover the whole car and focus on it. The purpose is to find and record the live image. The camera images on the train should all be displayed on the video controller of the driver's office. The image should have corresponding characters. The character content should have at least the camera number, date and time. Each image should be recorded. The recording time should be at least 7 days. The video should be easily searched and downloaded. The station can see all the video images on the car and download the videos in real time through the car-to-ground wireless network transmission.
Car video surveillance system should have stability, which is the primary consideration for system design. The design of monitoring points should be reasonable and have no blind spots. Due to the relatively small space in the car wiring, it is necessary to have reasonable wiring. Symmetrical design should be used to facilitate the re-allocation of marshalling needs in the future. The maintenance workload of the system should be small, and it is convenient for the maintenance personnel.
Operating System Management Agency
The video surveillance system shall be based on the administrative management level of the Jiaoji line operation system and be divided into two levels of monitoring structures: the route operation monitoring center shall be the first level; each station shall be provided with a second-level monitoring center, but each station does not need to monitor its The video images along the jurisdiction are only used as a platform for video distributed storage and forwarding. Control information, inspection information, alarm information, and video information can be transmitted at various levels of the network system according to a predetermined flow.
Video surveillance camera deployment
According to the situation of installation of on-board video monitoring systems and the installed position of the carriages in each domestic rail transit vehicle, each train carriage should be equipped with two or more cameras to monitor the on-site conditions in the carriages. Each driver's room at each end of the train should be equipped with a camera and pickup to record and record the driver's driving activities and discourse. Each cab is equipped with a video controller for the driver to observe and switch the camera image of the compartment.
Video surveillance system screen
Real-time video transmission, video recording, video image storage time of 1 month, to provide video evidence of the crime scene, for special places can be considered PTZ camera, the system can control the front-end head movement and lens zoom . The system can transmit the control commands sent by the network users to the front-end monitoring devices according to the authorization, and realize the call and control of the monitoring center and authorized users to any one-way monitoring images on the front-end. The image quality satisfies the requirements of the subjective evaluation index system and subjective evaluation index system.
Intelligent analysis needs video intelligent analysis alarm function, the purpose of automatic judgment of threats, early warning, because the schedule and the driver can not monitor the status of each car in real time. And can quickly help analyze and find the suspect after the fact. Drivers and ground station dispatches guide passenger behaviors through broadcasting or passenger information systems, through specific guidance of the monitoring images.
Administrative requirements
In order to ensure the normal operation of the network monitoring system, the system users should be classified, and various factors should be taken into account to formulate a reasonable authority management strategy to achieve the control of user access.
The surveillance station of the secondary station monitoring center, operation monitoring center, and other operating agencies must have a camera and provide superior leaders to check the status of their duty. The system can realize the intercom function between the upper and lower level monitoring room.
Construction and maintenance requirements
This system is in principle constructed and maintained by the Group's operating department and can guarantee long-term use. At the same time, the system has two modes of automatic automatic inspection and manual automatic inspection for remote equipment inspection. The artificial automatic inspection priority level is higher than the system timing automatic inspection. The regular automatic inspection function and the manual automatic inspection function are implemented by the monitoring and control system monitoring centers at all levels. Requires inspection results and content to be filed in the form of statistical reports in accordance with the management requirements of the organization. The system platform can understand the running status and fault conditions of the equipment in real time.
Information storage and backup requirements
Front-end monitoring equipment and monitoring centers at all levels should all store information such as inspections, alarms, video and audio, and system logs. The corresponding department of the organization shall store the alarm information and the video and audio information associated with the alarm in accordance with the prescribed format. Historical data information such as alarms and patrols stored and backed up can be accessed in the network system based on authorization.
Train vehicle video monitoring system
The most important function of the on-board part of the train video surveillance system is to transmit real-time video data acquired by a vehicle-mounted part (camera) of the vehicle video monitoring system of the front-end equipment of each car to the train driver's cab, displayed on the monitor and stored in the Storage media. If you are building such a video surveillance system in a building, you can use a digital video surveillance system solution, that is, connect all cameras to an image capture card and store and display them in a centralized manner; or use a more advanced network camera via wired or wireless The network is transmitted to the terminal display device.
The biggest difference between the train and the building is that the train is formed by connecting several cars. The electrical connection between the cars is achieved through a jumper cable connector (usually an aviation plug). The use of a connector actually creates a "breakpoint" in the transmission line, taking into account the loss caused by the "breakpoint" to the transmission and the limited space of the connector itself, the cable passing through the connector when building the communication and transmission system on the train. It must be as small as possible. In fact, the previously mentioned train bus trunk cable has only two twisted pairs passing through the connectors between the cars (one of which is spare). This is taken into account in the hierarchical and node structure of the train communication network. The nodes of each car are connected to the electronic equipment of the car through the vehicle bus to receive data and send instructions. After processing the received data, they are uploaded to the train bus and the interconnection between the nodes is realized through the bus. This is the train communication network. The stratification and node structure have important reference for the construction of train video surveillance system.
Wired transmission network
Based on the above analysis and discussion, when using cable to build a video surveillance system on trains, it is necessary to obey the principle of “as few as possible through the cable connectors between carsâ€, so the first kind of introduction introduced in the introduction can be basically eliminated. The mode (analog CCTV mode) and the second mode (digital video surveillance mode), because both of these methods require that all camera output signal cables pass through the inter-vehicle cable connector.
The camera transmission cable from the front end to the driver's cab at the end of the bus passes through three connectors, which means that three "breakpoints" are generated. The loss and noise caused by the breakpoint will affect the transmission; the previous connector of the end vehicle must pass through. Six cables take up a lot of space. Therefore, this scheme is not desirable in practical applications.
The method of reducing the number of transmission system cables can be used as a reference for the node and hierarchical structure of the train communication network (TCN). In each carriage, "nodes" are set to connect with the camera. The "nodes" are connected by a simple trunk cable to transmit video data to the driver's computer at the two ends of the train to achieve display and storage functions. The work done by the “nodes†here is to compress and package the video data collected by the cameras in the car in this section for transmission, and to be able to forward data of other “nodesâ€. According to this train of thought, taking the train composed of four sections of vehicles as an example, the model structure diagram of the onboard video surveillance system shown in the following figure can be obtained.
The function of the "node" in the model is to compress and encode the car video data in this section on one hand, and to forward other node data on the other hand. Therefore, the "node" should consist of two parts: the network video server and the switch. A network video server (including an encoder) with an embedded operating system can perform video compression encoding and packing, while the switch can forward data from other nodes.
In this way, the monitoring equipment of the entire train constitutes a small local area network, and both the host computer and the video server are connected to the local area network through the switch, forming a video monitoring system based on network transmission.
Local area network (LAN) is organized in the form of Ethernet, token bus, token ring network, fiber distributed data interface (FDDI) and so on. When selecting the specific form of the LAN, the following factors must be considered to ensure that video surveillance images can be transmitted. The networking is simple and the communication protocol is mature and reliable, economical and practical.
Compared with the above-mentioned networks, Ethernet has penetrated almost all the LANs because of its simple and practical (CSMA/CD) protocol and networking convenience. After several decades of improvement and development, Ethernet has evolved from the initial 10M Ethernet. The network developed to 100M, 1000M Ethernet. Although Ethernet has some defects in the transmission of multimedia services, that is, when time-sensitive services and general data services are mixed on Ethernet, there is no mechanism for providing preferential treatment for time-sensitive services. However, the train video surveillance system we construct only transmits video surveillance images, and the video surveillance system transmits services that are basically in line with the third category of services, namely, video information transmission is used for real-time use. Therefore, there is no problem of mixing with other services. In view of the simplicity and convenience of Ethernet and the universality of its application, we determined that the specific form of the transmission network of the on-board part of the train video surveillance system is Ethernet (100Base-T).
Overall system structure and configuration
After determining that the vehicle part of the train video surveillance system adopts 100Base-T Ethernet as the transmission network, the overall structure and specific configuration of the system can be determined.
The overall structure
The on-board part of the train video surveillance system can be generally divided into three parts: the front-end camera, the encoding part, the network transmission part, the terminal display, and the storage part. The overall structural model diagram is shown in Figure 4.
Specific configuration
Each carriage is equipped with two color fixed cameras. The two cameras are installed at both ends of the carriage. The specific installation position should ensure that the camera's viewing angle can completely cover the entire carriage (including through passages) and there is no video blind spot. The horizontal resolution of the camera should be no less than the PAL system 520TV line, to ensure better monitoring results.
Each camera is connected to a single video server via a video coaxial cable (as described earlier, using video coaxial cable to transmit analog video signals at a short distance).
A single video server should include an encoder, controller, memory, and so on. The encoder first samples and quantizes the input analog video signal, converts it into a digital signal, and then encodes the digital video signal according to a certain video compression standard. The encoding standard can be MPEG-4 or H.264 because both compression standards can generate compressed video streams suitable for IP network transmission.
Network transmission section
An Ethernet switch (with 100Mbps RJ-45 ports) is installed in each car. Two single-channel video servers in the car are connected to the switch through Ethernet cables (twisted pair). The switches in adjacent cars are also connected through Ethernet cables. The exchanges with cabs at both ends are connected to the video surveillance system terminal host, which constitutes the Ethernet transmission network of the onboard part of the train video surveillance system. Taking into account the vibration and impact caused by the train operation, the switch should use industrial grade industrial Ethernet switches; in addition, taking into account the special power supply mode of the above-mentioned urban rail trains and the frequent start-stop of switching components, the complexities in the vehicle are caused. In the harsh electromagnetic environment, the Ethernet cable should be shielded twisted pair with a certain anti-jamming capability to reduce interference.
Terminal display and storage section
Metro trains have cabs at both ends. Therefore, each driver's room is equipped with a host and a monitor. Two hosts and one host are used as masters. The master and standby can be switched by switching programs. .
Both hosts access the in-vehicle Ethernet transmission network through the switch. The host computer receives video streams from video servers of various cars through the network. On the one hand, it is stored in the hard disk of the host computer in the form of video files. On the other hand, it is decompressed and displayed on the monitor in real time.
Considering the vibration and impact environment of the train during operation, the host should use a ruggedized industrial computer. The host consists of CPU, main board, electronic hard disk (host itself use), data hard disk (storage compressed video files), network card, power supply and other components, the motherboard should be configured VGA output interface, USB interface, RS232 or RS485 interface. In order to facilitate the train driver's control and operation of the train video surveillance system, the display uses an LCD display with a touch screen. The host computer communicates with the monitor through the VGA output interface to transmit video images. The operation commands from the touch screen are transmitted through the RS232 or RS485 interface. The monitor power is supplied from the host computer.
Train video surveillance system
To realize the real-time monitoring of the running train cars in the urban rail transit control center, the real-time transmission of the video surveillance images in the train cars during travel to the control center must be implemented by wireless transmission. The introduction of analog and digital video transmission methods in the previous chapter mentioned the use of radio frequency and microwave media, but it is clear that the analog transmission mode in which a single channel uses a wireless channel cannot satisfy the urban rail transit system with many trains running. Considering that the on-board part of the train video surveillance system adopts the network transmission method, the on-board host has collected all the monitoring images in the vehicle. In order to make better use of the on-board host and achieve the system's acceptance, it is necessary to build a vehicle-to-ground wireless network. Realize the transmission of video images.
Wireless network technology
As with wired networks, wireless networks can also be divided into circuit switching and shared bandwidth. Typical circuit-switched wireless networks such as GMS cellular mobile communication systems, shared-bandwidth networks include wireless local area networks (WLANs), wireless broadband networks, Bluetooth technology, and the like. For long-term transmission of high-resolution video images, the bandwidth of circuit-switched mobile communication systems is insufficient, while Bluetooth technology transmits very short distances, which is suitable for terminal devices in environments such as rooms to be fixed via wireless access. The internet. In recent years, wireless LANs that have been continuously developed and improved are suitable for completing video transmission in such regional systems as urban rails regardless of bandwidth or coverage. Wireless networks must consider taking certain measures to avoid or reduce their impact on transmission performance when designing and selecting physical layer transmission methods.
Train video surveillance wireless transmission system
At this stage, relatively mature and commercially available high-speed wireless access systems mainly use wireless LAN technologies based on the IEEE 802.11 series of standards. By comparison, the vehicle-to-ground wireless transmission system proposes the adoption of a wireless LAN system based on the 802.119 standard, which can be used for secondary development and modification of its technology and equipment to meet the requirements of video transmission under the high-speed operation of urban rail trains and the comparison of urban rail lines. Poor use of the environment.
The urban rail transit line consists of stations and sections. There is usually 15 to 40 stations on one route, and the length of the route is between 15-50 Km. Obviously, it is not practical to configure a pure wireless LAN over such a long path. Because the current 802.11g standard wireless access equipment (seconds) has a signal coverage of only about 200rn (shorter distance when cornering), basically a few AP points need to be set in a range, which determines the wireless local area network. PCF mode is used during configuration (one AP corresponds to one terminal device in DCF mode). PCF mode is used to set a "base station" at each station. Each "base station" manages all AP devices in a range. The "base stations" are connected by a wired transmission network to form a wireless transmission system for the entire line. Therefore, the vehicle-to-ground wireless transmission system should include two parts: a wired backbone transmission network and a vehicle-to-ground wireless transmission platform.
Wired backbone transmission network
The wired backbone transmission network provides a video monitoring image transmission channel between the control center and the station, and serves as a backbone transmission route for the vehicle-to-ground wireless transmission system. The backbone transmission network provides access ports for the vehicle-to-ground wireless transmission platform at each station, and interconnects the switching matrix with the CCTV monitoring system of the control center and the station, so that the in-train video surveillance image is transmitted to the backbone network through the vehicle's wireless transmission platform. The monitoring equipment at the station and control center is displayed in real time.
When the urban rails are under construction, generally redundant optical fibers are laid for the subsequent communication system construction. These reserved optical fibers can be used when configuring the backbone transmission network, and LAN switches are installed in the control center and each station, and become LANs through optical fiber connections. The network form can be defined as 100M Ethernet, or it can be set as Gigabit Ethernet. Considering the need for future system functions to be added or improved, the backbone transmission network is recommended to adopt the form of Gigabit Ethernet. To enhance the fault tolerance and reliability of the network, single-ring redundancy or dual-ring redundancy can be used when connecting to the network.
Vehicle wireless transmission platform
The vehicle-to-ground wireless transmission platform adopts a wireless LAN technology product based on the IEEE 802.119 standard and is mainly composed of a central wireless network switch, a station wireless access switch, a photoelectric converter (or a wavelength division multiplexing device), a wireless access point (AP), and Car AP and other equipment components. The central switch is installed in the urban rail transit control center and works in dual-system mutual backup mode. When one of the two switches fails, the control and management of all APs can automatically switch to another switch. Wireless access points (APs) use single-mode optical fibers and optical-to-electrical converters (or wavelength division multiplexing equipment) to connect to the backbone transmission network at each station through the station's wireless network switch to form a wireless transmission system covering the entire line range and stations. The overall structure of the vehicle-to-ground wireless transmission platform is shown in Figure 5.
Conclusion
Security has become a service network. The industrialization and customization of video intelligent analysis management will bring new opportunities to manufacturers. From the perspective of development trends, the mainstream development direction of future video surveillance technologies is still networked, high-definition, and Intelligent and industry-oriented, in line with this development trend, we will be able to further develop more extensive video surveillance technologies to serve the safety of urban rail transit.
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