With the rapid development of China’s economy and the rapid growth of urban population, many large and medium-sized cities are facing the problem of traffic congestion caused by the growth of urban population and vehicles. In order to adapt to and promote the rapid development of the city, we must solve the traffic congestion and congestion of the main road sections and intersections in the city, and try to reduce accidents and violations. Facts have proved that the key and effective way to solve the problem of urban traffic congestion is to introduce advanced communication technology, monitoring concept and control technology while strengthening urban traffic planning and urban road infrastructure. The key to solve this problem is to establish a modern and intelligent traffic monitoring and command system. In urban traffic control system, optical terminal plays a very important role. This paper will introduce the application of optical terminal in urban road traffic monitoring.
Urban traffic monitoring system is generally composed of video acquisition system, control system and network transmission system. According to the functions, it is divided into video monitoring system, signal control system, electronic police system, video detection and other subsystems
Real time monitoring of the intersection screen, convenient for unified command and scheduling
Carry out the detection of traffic flow and traffic condition
Video the scene of intersection for a long time for processing and archiving
Through the control of traffic lights, traffic flow can be adjusted to effectively alleviate traffic congestion
According to the need, electronic police can be set up at some intersections
Effective detection of traffic behavior, to provide protection against vehicle violations
Discover and assist in handling public security incidents in time to maintain social stability and unity
From the perspective of the system composition of urban traffic monitoring system, to complete the above functions, each component of the system needs to be able to complete its own functions independently and cooperate closely: the video acquisition system needs to monitor the scene of each intersection in real time, including the analog video signal and the picture information collected by the electronic police, The collected video information is transmitted to the monitoring center through the network transmission system. The control system of the monitoring center can know the situation of the front end in time, and save and record the video information; At the same time, the control system of the monitoring center should also control the front-end cameras, signal lights, electronic police, etc. according to the needs. The control system of the monitoring center sends different control signals to the front-end devices, and then sends them to the front-end devices through the network transmission system, so as to complete the control.
In the above process, each part of the system plays a very important role, and considering the functions of each part, you will find that whether it is the video monitoring system, signal control system, or electronic police system, they all share the same part, that is, the network transmission system. With the development of optical fiber communication, optical transceiver has become the first choice of network transmission system. In the video acquisition and central control system, they all use different devices to complete different functions, the front end uses cameras and signal lights, the center uses matrix and hard disk video recorder to process video information, and uses signal light control software and electronic police software to control. The important task of communication and signal transmission between the front end and the monitoring center falls on the optical terminal
Video signal of surveillance camera
Control signal of control camera
Data signal of signal lamp network
Data signal of electronic police network
These signal types are different, including analog signal, data signal, Low-speed serial data and high-speed Ethernet data. Such complex and diverse signals bring us great trouble in network transmission. To transmit these signals, traditional methods need to use different equipment to transmit different types of signals, need to invest more equipment, lay more lines and so on, so as to increase the investment and complexity of system construction. Today, with the development of optical fiber system, optical transceiver can undertake this task independently.
Optical transceiver can be divided into analog optical transceiver and digital optical transceiver in terms of transmission mode. No matter what type of optical transceiver is, in essence, electro-optic and photoelectric conversion of transmitted image, voice or data signal. Optical transceiver is essentially a converter from electrical signal to optical signal and from optical signal to electrical signal. The purpose of conversion is to let all kinds of signals transmit in optical fiber. The advantages of optical fiber transmission are: first of all, it has small attenuation. At present, optical terminals can transmit image, data and voice signals over 100 kilometers without relay. Secondly, it has strong anti-interference ability, and it is not affected by lightning and electromagnetic radiation, which is not possessed by copper axis cable transmission and twisted pair transmission. Optical fiber also has the characteristics of large transmission capacity. Optical transceiver can not only realize the function of transmitting image, voice and data on a single optical fiber, but also realize the function of transmitting telephone and computer network signals, which can be said to cover almost all the functions needed in the monitoring field.
The function of optical transceiver is very powerful, and it can transmit many types of signals. Therefore, different types of optical transceiver will be selected in different situations. When choosing the model of optical transceiver, we should consider the following points:
1. What kind of system is the optical fiber network based on
If it is to expand the capacity on the original network, it is better to choose the same optical fiber, unless it can not achieve the expected function. If it is a brand new project, it is better to use single-mode fiber. In the traditional concept, we always think that in a short distance (within 5km), the cost of multi-mode system is lower than that of single-mode system. In fact, with the continuous popularization of optical fiber and the decline of equipment price, the price advantage of multimode optical transceiver has long been absent. In terms of transmission distance and effect, multi-mode devices are inferior to single-mode devices, so at present, the cost performance of single-mode optical transceiver is higher than that of multi-mode optical transceiver.
2. What kind of optical transceiver should be selected
From a technical point of view, digital optical transceiver is definitely better than analog optical transceiver. But in practical application, we should analyze the specific situation. The implementation of analog optical transceiver is simpler, and the system cost is relatively cheaper. At the same time, the analog optical transceiver can transmit one or two video signals to a long distance and get better signal quality, but if it transmits multiple video signals, the transmission distance and signal quality will be greatly reduced. Digital optical transceiver is relatively complex, high-tech and expensive, but it is easy to realize multi-channel and multi signal mixed transmission. Therefore, in the selection of one-way and two-way image monitoring optical transceiver, the cost performance of analog optical transceiver is better than that of digital optical transceiver. In the series of four-way and more than four-way image monitoring optical transceiver, the cost performance of digital optical transceiver is better than that of analog optical transceiver.
3. Is the optical fiber system to transmit video? Audio? Data? Or a combination of these signals
According to the transmitted optical signal, optical transceiver can be divided into data (RS232 / RS422 / RS485 / Manchester / TTL / biphase / contact) optical transceiver, video optical transceiver, audio optical transceiver, video / data optical transceiver, video / audio optical transceiver, video / data / audio optical transceiver and multiplexing optical transceiver. At the same time, we should also consider whether the data transmitted is unidirectional or bidirectional.
Next, through the actual case of a city road monitoring system in Zhejiang Province, we briefly introduce the use of optical terminal in the city traffic monitoring system.
This is a large-scale traffic monitoring system. The front end includes more than 270 fixed cameras, 60 fast integrated ball machines, 300 multi-channel video images, signal light control lines and electronic police data lines at intersections, which are connected to more than 70 signal boxes at urban traffic intersections, and one or more optical terminals are installed in each signal box, So that the video images of each intersection and the pictures taken by the electronic police can be transmitted to the monitoring center through optical fiber. The control center uses a large matrix to control the video, switches the image to the monitoring screen wall, and saves all the images with a hard disk video recorder. At the same time, the matrix host of the center sends the control signal to the ball machine, the data signal of the signal light control software and the electronic police software are also transmitted to the front-end intersections through the optical terminal to control the angle, distance, aperture of the ball machine, the time of the signal light and the setting of the electronic police, so as to better count and adjust the traffic flow and effectively alleviate the traffic congestion. Because the front-end equipment connected to each signal box of the front-end is not the same, and the signals to be transmitted are also different, so different types of optical terminals are needed（ As shown in Figure 1)
In this system, we use infinova brand digital optical transceiver, which is launched by infinova (formerly NTK) Company of the United States. Its excellent circuit design and high-quality components make it have a large enough range of optical chain loss and dynamic state. We can use different types of optical terminals according to the actual situation of the front end: some intersections only have monitoring images, no other equipment, we use pure video optical terminals; Some of them not only have fixed cameras but also integrated ball machines. We use video plus reverse 422 to control signal optical terminals; There are also front-end electronic police or signal lights, we use video and data 232 control signal optical terminal; There are also some computer networking, we need to use Ethernet data optical terminal; In addition, the optical transceiver can transmit all the above data types together. No matter what kind of data is transmitted, it has real-time status indication, which is convenient for us to find and solve problems. At the same time, we also reserve some video and data channels when we choose the optical terminal, so as to facilitate the overall expansion of the system and the networking between various urban systems in the future.
From the above cases, it is not difficult to see that the optical transceiver plays an indispensable role in the traffic monitoring system. At the same time, it is also crucial to select different optical transceiver according to different signal types, which can help us more effectively monitor and solve traffic problems in real time.
At the beginning of the century, with the rapid development of urban traffic, road construction and the maturity of optical fiber communication technology, we believe that the development prospect of optical terminal in urban traffic monitoring will be broader.
Editor in charge: GT