With the development of EPON Technology, EPON networks, with their high bandwidth, long distance transmission and digital video surveillance systems, have taken over the functions of basic transmission networks. The transmission network of the EPON system uses passive devices and does not require power supply. Therefore, it has the characteristics of simple circuit, less failure, small maintenance, and suitable for harsh environments, and is particularly suitable for outdoor applications.
The EPON network is developed from the access network technology. The network structure characteristics and data transmission characteristics of the digital video surveillance system are different from those of the access network. How to use the EPON network in the video surveillance system is to design and implement a digital video surveillance system. Must pay attention to the problem.
In this article, Feng Runda combines the engineering application of EPON technology in the district video surveillance system. Through analyzing and solving the problems encountered in project implementation, some guiding principles to be followed in the system design and implementation are proposed to avoid detours in engineering practice. To give full play to the technological advantages of the EPON system.
1EPON technology introduction PON (passive optical network technology) network transmission is entirely composed of passive components, with the advantages of no maintenance, few failures, so it has been rapid development, has begun to scale applications in the telecommunications access network, Gradually replaced xDSL technology with copper conductors as the transmission medium.
An EPON network is an Ethernet technology on a PON network. It combines an Ethernet network with a PON optical network to form an Ethernet network that can be widely distributed. From a structural point of view, the EPON network is divided into three parts, one part is the OLT (line termination), installed in the center; the middle part is the optical cable and the passive optical splitter to form the transmission network; the end part is the ONU (optical line unit), installed At the user side, the user access interface is provided.
The optical network of the EPON system is a two-way network and uses wavelength division multiplexing to provide bi-directional access on one optical fiber. The downlink uses the broadcast mode. By modifying the preamble of the Ethernet, the address of a lower node can be specified in the broadcast, so that point-to-point communication with a lower node can be realized. The uplink adopts time division multiplexing technology, and the lower nodes share the bandwidth according to different time slots.
The optical network of the EPON system is a passive network. Optical fiber cables and passive optical splitters are used for transmission. No external power supply or special maintenance is required. Therefore, the EPON network is particularly suitable for use in outdoor environments, and the failure rate is very low. . Although the EPON network is passive, due to the use of optical communication technology, the EPON network also has a communication distance of 10 to 20 km without any relaying. Moreover, the EPON network uses a multiplexing technology, so it can support multiple users on a single core, reducing the wiring, which makes the غير مجاز مي باشدt of deploying EPON networks greatly reduced.
2 Application of EPON Network in Residential Area Projects 2.1 Project Overview In the video surveillance project of a living quarter in a city, EPON optical network is adopted as the transmission network of digital video surveillance system. The project includes 26 living quarters. Cameras must be installed at entrances and exits, walls, important pasغير مجاز مي باشدes, and public events in each community. Images of several communities are transmitted collectively to the sub-control centers of a neighboring community. For future completion. According to the geographical location of the cell, set the location of the monitoring point. The number of cell cameras is between 30 and 60. The entire project needs about 1000 cameras.
Considering the distribution range of the community and the future construction of the system, it is very suitable to use the EPON network in this project. Only one multi-core optical fiber cable is needed for each cell, and each terminal camera is connected to the cell through a splitter in the cell. The project غير مجاز مي باشدt is low. It is simple to implement, and it can be easily connected when building a general center in the future.
2.2 System Structure EPON technology has developed a 10Gb symmetric bandwidth device, but the غير مجاز مي باشدt is high and the bandwidth exceeds the requirements of the cell video surveillance system. Considering that the cell video surveillance system must take the control غير مجاز مي باشدt as an important goal, it is used in practical applications. The first generation of 1Gb symmetric bandwidth EPON equipment. This kind of equipment has mature technology and low price, and it has been mass-produced and is suitable for use in video surveillance systems.
Since there is no general center, a layer network is designed from the sub-control center to each cell. The OLT equipment is deployed in the sub-control center, providing 1 to 2 PON ports for each cell, and up to 32 splitters for each PON port. Two channels can support 60 cameras. A 4-core optical fiber cable is laid from the sub-control center to the lower-level sub-cells, and 1 to 2 cores are used to provide the trunk optical channels of the cell, and the other cores are used for backup. Set the splitter at the branch of the cell cable, set the ONU at the device end, and connect the network camera. In the arrangement of the optical splitters, a tree-like hierarchical configuration is adopted, starting from the main optical cable entering the cell and gradually branching to form a multi-level tree structure.
2.3 Problems in System Operation In order to accumulate experience and master the characteristics of the EPON network, the project is first piloted in a residential area, and the experience is fully explored after exploration. After the completion of the construction of the pilot community, the system was commissioned. During the process, problems with the optical path and system bandwidth allocation were encountered. Both of these problems were different from the theoretical calculations in the original design. Through analysis and resolution of the problems, Discovered some experience in using EPON networks.
3 Analysis and solution of optical path problems After the system is powered on, most of the ONUs cannot be registered in the OLT. Through the inspection of the optical path, the branch with no light path only occupies a small part. After the unqualified branches are corrected, the problem still remains. .
Through the analysis of the original design, it is found that the problem may be due to optical path loss. In an EPON network, a passive optical network is located between the OLT and the ONU, and the light received by the two optical fibers must meet the requirements of the optical power, with a minimum of -23 dB and a maximum of -3 dB, and the attenuation exceeds the range. The range ONU cannot be registered on the OLT. The optical loss from the OLT to the ONU is formed by the loss of the optical splitter at each stage, the splice loss of the joint, and the loss of the optical cable.
Taking into account the factors of optical path loss, optical power meter and OTDR were used to test the loss of optical splitter, optical cable loss and joint loss, and the actual optical path loss was calculated. As a result, it was found that there was a big gap between the actual loss of multi-stage optical splitting and theoretical calculation. The device is the main source of attenuation, and the same distribution area of the optical splitter loss values is large and the device quality is general.
In the original design of the system, the network branches are designed according to the distribution of the cameras. Due to the irregular geographical position of the camera, in order to reduce the pipeline length and reduce the construction difficulty, the network has multiple stages of beam splitting and is calculated to meet the requirements of optical power attenuation. However, in the actual optical path, due to the general quality of the device, the welding quality is unsatisfactory, and the number of branch stages is large, the joints have a large cumulative loss, and the optical splitter has a large cumulative loss, making the actual loss larger than the calculated loss. The maximum loss we measured is as high as -37 dB. The ONUs that cannot be registered are caused by large optical loss.
In the actual engineering situation, it is inconvenient to solve this problem. It is necessary to repeatedly weld and replace the optical splitter. However, modifying the optical path design can improve the tolerance for increasing the loss at various levels. The method of changing the original branch to the split-by-group design is divided. The method; first according to the distribution of the camera grouping, each group to design a branch; then in the same group once split to the camera; this only requires two levels of light. After such changes, and with attention paid to controlling the quality of the welding, the system was successfully connected.
4 Analysis of Bandwidth Problems and Solution The cameras of the district video surveillance system have selected 1080P network HD cameras, which use H.264 encoding and have an average bandwidth of 4 Mb. After the light path was opened, from the point of view uploaded to the sub-control center, some pictures showed the phenomenon of Karton. The effective data bandwidth of the selected EPON equipment in the system is 1Gb. Subtracting system overhead, at least 700Mb of bandwidth is available for use. The maximum number of cell cameras is 60. Even if the total bandwidth added is only 240Mb, it is much smaller than the system bandwidth available. Caton phenomenon should not occur.
The picture is stuck indicating that there is a delay in the transmission of the video signal. By testing the bit stream of the camera, it is found that the bit stream of the camera is not fixed. When the picture changes, the bit stream will increase several times. The reason for this analysis is that the camera works in the picture priority mode and changes with the picture. In order to ensure the quality of the picture, it is necessary to increase the amount of data; further tests have found that many cameras often appear to increase the amount of data at the same time, which will cause data transmission delay.
In the EPON system, uplink data transmission adopts the time division multiplexing technology. Each ONU occupies different time slots when sending data to the OLT. In each time slot, only one ONU is communicating. In order to make full use of the uplink bandwidth, the OLT dynamically allocates bandwidth according to the ONU's data transmission request, and allocates longer time slots to ONUs that need to upload data, so as to obtain larger bandwidth. This function has a great role in the access network, because the user's uplink data transmission is very random, showing the characteristics of bursts, the use of dynamic bandwidth allocation can allow users who need bandwidth to get a larger bandwidth, while the user needs large The probability of upstream bandwidth is very low. However, in a video surveillance system, multiple cameras may need to increase bandwidth at the same time. As a result, one or more cameras requested first are satisfied and allowed to transmit as much data as possible, so that the time slot extension causes other cameras. The wait time was extended and there was a Kardon phenomenon. This phenomenon is like a crossroads. If there are a large number of vehicles in both directions, the best way is not to let one direction go as far as possible, and then allow another direction to pass, but to stagger the two directions before they can Ensure the smooth flow of traffic.
Understand this principle, you can re-set the system's bandwidth allocation strategy, the dynamic bandwidth allocation is changed to a fixed bandwidth allocation, each camera is assigned a fixed bandwidth, so even if the stream of multiple cameras are increasing, you can also get the opportunity For data transmission, due to the large upstream bandwidth of the EPON system, the fixed bandwidth allocation can meet the needs of real-time display and will not cause the picture to appear stuck.
5 Conclusion Because of the poor conditions of outdoor construction, the index of optical path loss will be larger than the theoretical index. Therefore, the optical path splitting design should be controlled at two levels. From the perspective of the needs of the district video surveillance, control can be achieved at two levels. First, the cell cameras are grouped and the first layer is split by group; within the group, the second layer is split.
Use a fixed bandwidth allocation method for bandwidth allocation. Although dynamic bandwidth allocation technology is one of the more advanced features of EPON networks, it can have adverse consequences in the event of improper settings.
In equipment selection, as far as possible to meet the غير مجاز مي باشدt requirements under the premise of using good quality equipment. It is not possible to use telecom-class, high-end EPON equipment in the residential video surveillance system. In order to control the غير مجاز مي باشدt, only mass production of ordinary products can be used, so we must pay attention to the choice of brand and try to select good quality. For example, the selection of optical splitters is an important part of the optical path design. If conditions are available, some tests must be conducted in advance and the best indicators should be selected.
The selection of OLT and ONU is also very important. Currently, there is no uniform bandwidth allocation standard for EPON technology, allowing manufacturers to choose their own algorithms. Therefore, the technical content should be understood as much as possible before the selection. If it is inconvenient to use power for construction, an ONU with PoE function, such as Feng Runda's EPS5081, can be used. The problem of bandwidth allocation via query data analysis [2] should be that the selected device will have a device occupying the channel for a long period of time in the bandwidth allocation algorithm. This algorithm may adapt to random access bursts in the access network. In the case of data, all devices in the video surveillance system are sensitive to delay, and do not adapt when multiple devices need to transmit large amounts of data at the same time.
In fact, when using the EPON network in the district video surveillance system, the design and configuration of the system cannot be simply based on theory, and the actual situation of the project must be fully considered.
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