Design of monitoring system for power fiber-to-the-home cable

0 Introduction
The power fiber-to-the-home (FTTH) construction project is an important part of the State Grid Corporation's smart grid. It uses optical fiber composite low-voltage cable (OPLC) in the low-voltage communication access network, lays optical fiber along the low-voltage power line, and realizes access to the meter and the household. In conjunction with the passive optical network (PON) technology, a power fiber-optic communication network is built to cover every power user, carrying power consumption information collection, intelligent power consumption two-way interaction, "three-network integration" and other services. After completion, residents will realize the mutual integration of the power grid with the Internet, telecommunications network, and radio and television network, realize the co-construction and sharing of network infrastructure, greatly reduce the implementation cost of "three-network integration", and improve the comprehensive operation efficiency of the network. With the continuous advancement of smart grid construction, the scale of fiber-to-the-home deployment will continue to expand, and the number of optical cables on the Internet will increase dramatically. In a large-scale, multi-user unit environment, how to maintain these optical cables is an urgent problem to be solved. The traditional optical cable line maintenance and management model requires a large number of well-trained professionals and test tools, and fault finding is difficult, troubleshooting time is long, and affects the normal operation of the communication network. Therefore, it is of great practical significance to implement real-time monitoring and management of optical cable lines, dynamically observe the degradation of optical cable line transmission performance, timely discover and predict optical cable hidden dangers, and achieve rapid and efficient response to customer service recovery.

1 System Monitoring Principle
The difference from traditional optical cable testing is that the PON optical cable network is a point-to-multipoint communication connection. Due to the introduction of a splitter with a large splitting ratio, there will be multiple optical cables behind the splitter, which brings complexity to the test. Since the PON network involves a splitter and a large number of optical cables behind it, it is not suitable to use spare fiber testing, and only wavelength division multiplexing technology can be used. Add wavelength division equipment (WDM) and use a 1650 nm wavelength different from the PON service wavelength for testing. Use a filter at the receiving end to filter out the test wavelength and eliminate the impact of the test light on the ONU (Optical Network Unit).
During the test, the 1650 nm test light and the service light are combined and then pass through the OLT (Optical Line Terminal) side optical cable to the splitter, and then divided into each ONU segment cable. The light reflection of the test light in the OLT to splitter section is the reflection signal of a single optical cable, while the splitter to ONU transmits the 1650 nm reflection light on all ONU optical cables back. The reflection signal after aggregation and superposition by the splitter is sent to the OTDR for analysis. The characteristic signal of each section of optical cable is the superimposed total signal, plus the test light attenuated by the splitter. The signal itself has a large loss, and the reflected signal is not strong. For this reason, a specially designed strong reflector unit is added to increase the reflected light energy of each section of ONU optical cable at the end. In addition to enhancing the strong reflection of the ONU end cable, the monitoring station (RTU) uses the OT-DR test signal analysis algorithm for PON and is equipped with a dedicated OTDR module, which can distinguish the characteristics of multiple ONU optical cables with a length difference of less than 2 m. Even if a 1:64 splitter is used, the end reflection signal of each section of ONU optical cable can be distinguished. When one of the ONU optical cables is interrupted, the corresponding strong reflection peak disappears. With the disappearance of this strong reflection peak, the system can accurately determine that the corresponding optical cable has an interruption fault. The online method makes full use of the existing optical splitters and fiber cores in the existing PON network, without the need to occupy additional fiber cores, install optical splitters, or perform engineering fiber jumpering. It can ensure that 100% of
the customer's fiber core conditions are tested without affecting existing user services. The test method and test results are shown in Figure 1.

2 System Design
2.1 Central Station Design
The system adopts a three-tier architecture based on the intermediate application server. The three-tier architecture reasonably separates data storage, application processing and result display (including graphics and data display), and transfers part of data processing and application calculation from the database server to the application server, which can reduce the processing pressure of the database server, so that the database service can focus on data storage management, while the graphic data display and processing can make full use of the advanced and powerful graphic processing functions of the GIS platform. From the perspective of the entire system, the load distribution is relatively uniform, which improves the data and graphic processing capabilities of the entire system. The monitoring center consists of servers, network management terminal computers, network equipment, printers and corresponding software. The devices are connected to each other through 10M/100M Ethernet and support TCP/IP communication protocol. The monitoring center directly manages all monitoring stations in the area. The central station realizes the topology, configuration, testing, analysis, fault, performance, security and other management functions of PON optical cable network monitoring. Provide monitoring functions for the link between the monitoring station and the server. Once the monitoring station itself or the link between it and the server fails, the monitoring center should be able to remind the user in time and provide corresponding security and recovery functions. The monitoring center can continuously or intermittently test, observe and monitor the system and the monitored PON optical cable network to find faults or performance degradation. The RTU monitoring stations in the managed network are all managed by a management software platform, monitoring the entire authorized management area in a working window. The server of the monitoring center supports time synchronization and status monitoring for all RTUs.
2.2 Design of monitoring station
The FTTH monitoring station (RTU) is the core of this system. This monitoring unit has the same test function and accuracy as the OTDR instrument. It realizes centralized control and management of the OTDR module, control module, power module, optical switch (OSW), wavelength division multiplexer (WDM), reflection filter, network adapter and corresponding software installed in this system to achieve monitoring of FTTH optical cables. The same RTU can be integrated with long-distance and backbone network monitoring modules at the same time, covering the optical cable test from backbone to access layer. The monitoring station has local testing and remote testing functions of the monitoring center. It switches the optical switch channel locally according to instructions and starts the test. After the test sent by the monitoring center is completed, the monitoring station will transmit the measured curve data file back to the monitoring center. The test content includes the full transmission loss of the optical fiber channel and the optical length of the optical fiber, the loss of each connector on the optical fiber, and the optical fiber attenuation coefficient between the two connector points. The functional structure is shown in Figure 2.

2.3 Early Warning Design
The system starts the optical time domain reflectometer (OTDR) manually or automatically to test the specified optical fiber and obtain the optical cable test data, that is, the scattering and reflection power level values ​​of tens of thousands of evenly distributed points along the optical cable. The connection line of all sampling points constitutes the OTDR curve of the optical fiber link. The vertical axis represents the power level (dB) and the horizontal axis represents the distance (km). Optical fiber connectors, breaks, and end points will cause light reflection, forming an upward sudden reflection event; optical fiber bending and fusion will increase the attenuation of the optical fiber, causing a downward sudden change and forming a non-reflection event. Through data analysis, the sudden change point of the curve is found, and the optical fiber event points such as the optical fiber head end, tail end, joint, and fusion are determined. By comparing with the reference data, the changes in the attenuation data of the event point, optical cable segment, and optical fiber link are analyzed to determine the operating status of the optical cable. When the data change exceeds the early warning threshold, an early warning message is issued. It mainly includes the following steps:
(1) Determine the position of the splitter. Calculate the minimum value of the noise data and find the leftmost point leftpts_noisefloor that reaches this value. The point before this point where the reflection event and attenuation are greater than the threshold is the splitter position. Determine the test data value from back to front. If it is less than or equal to noisefloor, set noisefloor to the value of this point and set leftpts_noisefloor to this point. The loop ends and the position can be found accurately.

(2) Analyze reflection events. Use the least squares method to fit a straight line L from the starting point of the curve to the end point E. Determine all reflection events R from the starting point of the optical fiber to the end E, and find the point above the straight line L where the difference between the test data point and the projection of the point on L exceeds the reflection threshold. Each reflection event after the splitter is the end position of the ONU optical cable.

(3) Analyze non-reflection event points. Divide the measured optical fiber into several sections according to the connector R, and fit a straight line in each section of the divided optical cable to determine the non-reflection event point in each section of the optical fiber. At least 10 consecutive points are above L, and the maximum value of the difference with the projection is greater than the event calculation threshold. It is regarded as a suspected non-reflection event. Then, based on the last fitted straight line, determine whether the attenuation of the point is greater than the threshold and confirm it.

(4) Early warning judgment. Take the comparison of attenuation data at the event point as an example: 3 Conclusion This system is mainly used for the FTTH optical cable network of the power system. It can also be used for telecommunications operators such as China Telecom, China Unicom, China Netcom and China Mobile, as well as all companies that use optical cables as transmission lines, to provide network performance monitoring, maintenance, resource management and other services for optical cable networks. With the deepening of smart grid construction, the requirements for intelligent line monitoring and management will become higher and higher, which has made the power fiber-to-the-home optical cable monitoring system a new highlight in the power communication market and has achieved unprecedented development.