Analysis on the Automatic Monitoring System of Optical Cable Lines

　　Optical fiber communication has gained popularity due to its advantages of large capacity, high quality of information transmission, long transmission distance, stable performance, strong anti-electromagnetic interference and corrosion resistance. Especially in the past decade, with the continuous increase in people's demand for broadband services, optical fiber communication has been vigorously developed.

　　At present, the total length of optical cables in the national communication industry has reached more than 2 million kilometers. Together with the optical cables used in cable television networks and various special networks, it is estimated that the total length of optical cables in the country has reached more than 3 million kilometers. On the other hand, with the rapid development of optical synchronous digital transmission network (SDH) and dense wavelength division multiplexing (DWDM) technology, the transmission capacity of optical fiber is also developing at an unprecedented speed. But at the same time, the maintenance and management of optical cables are becoming more and more prominent. With the increase in the number of optical cables and the aging of early laid optical cables, the number of optical cable line failures is increasing. The traditional optical cable line maintenance and management model is difficult to find faults, takes a long time to troubleshoot, affects the normal operation of the communication network, and causes huge economic losses every year due to communication optical cable failures. Therefore, it is very important to implement real-time monitoring and management of optical cable lines, dynamically observe the degradation of optical cable line transmission performance, and promptly discover and predict optical cable hidden dangers to reduce the incidence of optical cable blocking and shorten the duration of optical cable failures.

　　1 Introduction

　　The Optical Fiber Cable Line Automatic Monitoring System (OAMS) is a subnet of the transmission network management domain in the Telecommunication Management Network (TMN). It is an important technical means to effectively reduce the duration of total resistance and timely discover the hidden dangers of optical cable lines. It uses computer technology, optical fiber communication measurement and other technologies to automatically and real-time monitor and test the quality and operation of optical cable lines.

　　2 Necessity of establishing an OAMS system

　　In long-distance and intra-city relay optical cable transmission systems, the transmission equipment is equipped with bit error rate (BER) monitoring equipment or monitoring units. However, the traditional line maintenance department is not equipped with monitoring means. Usually, when a BER alarm occurs, the maintenance personnel first determine the cause of the alarm. After finding out that the cause is the transmission line - the optical cable, the maintenance personnel will notify the relevant line maintenance department and report to the relevant competent department. Then the line maintenance department will take corresponding maintenance measures based on the known degradation of the optical cable line transmission performance. If an optical fiber break occurs, personnel will be immediately sent with an instrument (OTDR) to find the location of the optical fiber break, and personnel, equipment, and other equipment will be organized for emergency repair, which is usually called obstacle repair; if the total attenuation of the optical fiber channel increases, it will be included in the line maintenance and transformation plan when its value is acceptable; if it is not acceptable, personnel will be organized to repair it in order to improve its transmission performance and provide a reliable circuit.

　　Obviously, if the maintenance department only uses traditional BER monitoring, and after the maintenance personnel determine that the BER alarm is caused by the transmission line, they will notify the line maintenance department to carry out emergency repairs, maintenance, and modification. In this way, the line maintenance department is overly dependent on the maintenance department for the grasp of the line situation and is in a passive position. It is difficult to ensure the smooth operation of the high-speed, broadband, and large-capacity optical cable transmission network. Therefore, it is very necessary to establish a real-time, automatic optical cable line automatic monitoring system. The optical cable line automatic monitoring system provides an advanced maintenance method for the optical cable line maintenance department, which enables the line department to change from passively accepting information from the maintenance department to actively mastering the optical cable transmission characteristics, providing reliable guarantees for the high-quality, efficient, safe, and stable operation of the network.

　　3. Working Principle of OAMS System

　　The Optical Fiber Line Automatic Monitoring System (OAMS) reports a large amount of basic data on optical fiber transmission performance, such as optical power, backscattering curve, etc., to monitoring centers and monitoring stations at all levels through optical devices distributed at a large number of data collection points in the optical cable line, and analyzes and processes the data, and promptly and accurately feeds back the operation status of the optical cable system to maintenance personnel, so that maintenance personnel can promptly discover potential faults and sudden faults; and guide fault repair.

　　4 OAMS system components

　　The OAMS system is mainly composed of the provincial monitoring center PMC, the regional monitoring center LMC and the field monitoring station MS.

Figure 1 OAMS system composition

　　The monitoring center is responsible for controlling each monitoring station and is the center for collecting and processing data. It consists of controllers (servers, clients, workstations), routers, hubs/switching hubs, network adapters, MODEMs, printers, and corresponding software. PMC generally adopts the active-standby mode.

　　The monitoring station is responsible for remote and automatic monitoring of the optical cable line and tracking changes in optical fiber transmission loss. It consists of an alarm monitoring module, OTDR module, control module, power module, programmable optical switch, WDM (wavelength division multiplexer), optical filter, MODEM, router, network adapter and corresponding software (including OTDR simulation software), etc. It is usually installed in a standard rack used in the transmission room.

   OAMS adopts a modular, distributed multi-level architecture to effectively prevent and reduce optical cable line obstacles and realize the interconnection of multi-level monitoring networks.

　　5 Main functions of the system

　　The acquisition unit (AIU) of the optical power monitoring module of the monitoring station (MS) monitors and collects the optical power of the optical fiber under test, and transmits the collected data to the optical power control unit (ACU). The optical power control unit analyzes and compares the monitored optical power data, and promptly transmits the optical power data exceeding the alarm threshold to the monitoring center (LMC). The monitoring center analyzes and counts the data reported by each optical power control unit, issues an alarm for the optical power change exceeding the threshold value, counts and determines the optical cable section where the fault occurs, and automatically and quickly starts the optical time domain reflectometer (OTDR) and programmable optical switch (OSW) of the monitoring station to test the faulty optical cable section. The curve data obtained after the test is uploaded to the monitoring center, which compares and analyzes the test curve with the reference curve to determine the location, type and alarm level of the fault point. When a fault does occur, the operation and maintenance personnel can be notified by sound and light alarm information, and relevant personnel can be notified remotely by SMS, telephone, fax, email and other methods. [page]

　　(1) National and Provincial Monitoring Centers (GMC, PMC)

　　Responsible for the technical management of LMC and MS of the entire network, and real-time grasp of the operation status of the entire monitoring network; can provide maintenance management reports, statistical analysis reports and comprehensive information query functions.

　　(2) Local Monitoring Center (LMC)

　　It can conduct roll call, regular and simulated alarm tests; conduct comprehensive analysis on the collected curves to timely discover the degradation of optical cables and accurately locate the optical cable obstacles; it has the alarm receiving function, and can automatically judge the nature of optical cable obstacles in combination with reference curve data, and notify the repair personnel of the obstacle information through SMS and other means; it can conduct two-way data transmission (such as curves, alarms, reports, status files, etc.) with PMC.

　　(3) Monitoring station (MS)

　　MS can complete the roll call, cycle, and obstacle alarm tests according to the center's instructions; local test function; simulated OTDR analysis function; and data reporting function.

　　According to different needs, the field monitoring station can implement the following test methods:

　　① Online monitoring. The OTDR in the monitoring station uses test light with a wavelength different from the working wavelength of the optical transmission equipment for testing. It uses wavelength division multiplexers (WDM), filters (FILTER), and programmable optical switches (OSW) through wavelength division multiplexing technology to monitor the operating status of the optical fiber in use in real time.

　　② Spare fiber monitoring: Monitor the operating status of spare optical fibers in the monitored optical cable line.

　　③ Span monitoring. By configuring active equipment and passive optical devices (mainly WDM and FILTER), remote online or spare fiber monitoring of more than one optical cable segment is performed. Span monitoring is generally used for optical cable lines with short relay segments and complex networks such as local networks and rural telephones.

　　6 Technical features of OAMS system

　　The system integrates modern network communication, computer communication, and optical fiber measurement technology. At the same time, it uses technologies such as Geographic Information System (GIS) and Global Positioning System (GPS) to provide reliable guarantees for line information and line positioning. It can realize remote distributed real-time, online automatic monitoring of changes in the transmission attenuation characteristics of optical fibers in optical cables and optical fiber blocking faults, without affecting the transmission performance of the optical transmission system in use, thus achieving both service (providing customers with high-quality circuits) and maintenance (providing advanced maintenance methods for the lines).

　　7 Problems

　　After a long period of development, application and continuous improvement, the optical fiber automatic monitoring system has become an important means of fault location in my country's trunk optical cable maintenance work, and plays a huge role in the maintenance of the national trunk network. However, due to technical and other reasons, the system itself still has certain limitations.

　　(1) Extraction of alarm signals

　　At present, there are three ways to extract alarm information from the optical fiber monitoring system: using a splitter to extract 3% of the in-use light and analyze it through AIU and ACU; using the alarm signal of the maintenance equipment; using the no-light alarm signal of the equipment relay optical disk. However, all three methods have certain problems:

　　When using the AIU method, 3% of the optical power of the system in use needs to be diverted, which is not feasible for the relay section with less optical power surplus;

　　When using the rack alarm signal, the monitoring system will respond urgently to all the alarm signals of the rack (including power alarm, equipment alarm, etc.), which may easily cause false alarms;

　　Since the relay optical disks of different manufacturers have no-light alarm signals in different data formats, this method is difficult to implement and has high cost.

　　(2) System intervention attenuation

　　Since the system needs to introduce passive optical devices such as WDM and FILTER, it will affect the receiving light power of the system in use.

　　(3) Lack of rapid optical path switching function

　　The current monitoring system only has the functions of testing, analysis and alarm. When an optical cable fails, it still needs to wait for maintenance personnel to arrive at the site for emergency repairs. It does not fundamentally solve the problem of instant optical path switching and communication restoration.

　　8 Development prospects of OAMS system

　　The optical fiber in the optical cable may have physical defects, such as impurities and bubbles. Coupled with factors such as changes in the filling ointment, the optical and mechanical properties of the optical fiber will be affected. Under the influence of the external environment and external forces, the strength of the optical fiber will be reduced, resulting in damage to the optical fiber and shortening the life of the optical cable. If we can grasp the optical fiber status of the optical cable line, we can change from passive to active and discover potential faults as early as possible. These problems are difficult to achieve only through OTDR testing. For this reason, B-OTDR (Brillouin Optical Time Domain Reflectmeter) testing equipment can be introduced into the system. B-OTDR uses the Brillouin scattering principle to accurately test the stress and strain of optical fiber cables, and even very small bends can be accurately tested. This product was previously only used to monitor special requirements such as small movements of the earth's crust and small vibrations of large bridges, and the current price is relatively expensive. If conditions permit, this device can be added to the system to monitor the stress and strain distribution of the optical fiber in the optical cable line and predict potential faults.

　　In addition, the existing OAMS system has formed a computer network. This network has the ability to carry more information processing work in terms of computer processing power and network structure. The existing system can be integrated with the various functions of the inspection system, reporting system, office automation system, optical cable geographic information system and other application systems, that is, to achieve the exchange and sharing of relevant information, greatly avoiding repeated investment in hardware. A comprehensive management network for long-distance optical cable lines can be built to put the daily maintenance of optical cables into the management track, so that long-line maintenance can move from manual management to computer management, and also provide reliable guarantees for the maintenance of optical cables in the national backbone network, local network and access network.
At the same time, the optical cable communication network and the automatic monitoring system of optical cable lines will continue to develop and advance in technology. In particular, with the realization of all-optical networks in the future, it will be possible to combine the monitoring and management of equipment with the monitoring and management of lines to form a unified monitoring system for optical fiber communication networks.

References:

    1 Fang Dong. Principle and application of optical cable line monitoring system. 2002
    2 Li Hui. Automatic monitoring system of optical cable line. 2002