Application of the hottest access network technolo

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Application of access technology in railway communication

I Introduction communication technology at the turn of the century is an organic combination of advanced digital technology, computer technology, microelectronic technology and optoelectronic technology. It will develop in the direction of digitalization, broadband, intelligence, high speed and personalization. The future communication should completely overcome the limitations of time and space, and enable users to communicate with anyone at any time and at any place, including voice, data, video and other information. In this case, passengers also need to enjoy information exchange on the train as in the office environment, and access to the international Internet as compared with other people's voice, data, fax, image and other information exchange

in addition, with the rapid development of railway trains in the direction of high-speed and quasi high-speed, in order to ensure traffic safety, achieve effective man-machine control and improve transportation efficiency, it is required to establish a railway communication with more perfect functions and more advanced technical composition. With the breaking of the monopoly pattern of China's telecommunications industry, the Ministry of railways, which has a huge railway communication network second only to Chinatelecom, can actively participate in the competition by using the existing dedicated network facilities to provide high-quality telecommunications services to the whole society

in order to make the above ideas come true, we must break the conventional access mode of railway communication, adopt advanced and modern transmission and access modes of wired and wireless communication, realize the upgrading of railway communication, adapt to the development of information society, and give full play to the social and economic benefits of Railway Communication in the national economy

II Status quo of railway access technology

due to the characteristics of high-speed movement of railway trains, wireless (mobile communication) access occupies a considerable proportion in railway communication. Of course, for the communication mode between stations (yards), units and various fixed facilities at fixed locations, the preferred scheme is still to use SDH optical synchronous digital transmission equipment. At the same time, advanced technologies such as ATM switching and network IP communication should be considered to form the communication backbone and optical fiber user access. For example, the "double fiber unidirectional ring" access mode not only has the unique advantages of optical fiber communication such as high speed, safety, high transmission quality and reasonable price, but also has the characteristics of circuitous route and equipment standby. TPU and click Start button rubber constitute a composite material with good cost performance, which has the self-healing function and greatly improves the reliability of the system. In addition, remote user unit (RSU) and digital loop carrier (DLC) equipment are used to make the group more flexible and convenient. The investment and benefit should be comprehensively considered in the process of the group, so that the system can not only meet the needs of railway communication now and even within a few years, but also provide advanced telecommunication services for passengers and ground users, and also have the function of easy expansion

according to the idea that communication is divided into three parts: trunk, local area and access, railway communication can also be divided by the above division method. In terms of railway communication, access accounts for a considerable proportion, including wired access and wireless access. The situation of railway wired access is similar to that of telecommunication access. The Ministry of Railways will build a railway interconnection that can cover large and medium-sized cities nationwide in the next 1-2 years. It is a nationwide computer network with multimedia communication capability that can support many information services organized and built by the railway department relying on basic railway telecommunication. The Ministry of railways may become the sixth computer information interconnection unit for the public in China, Prepare for the railway communication going to the market. The wired access will not be described here. The following mainly discusses the wireless access of the railway. For this reason, we will first review the development process of mobile communication

1. development process of mobile communication

mobile communication technology has experienced the development process from analog to digital, from frequency division multiple access to frequency division + time division multiple access, and then to code division multiple access (CDMA), and is about to develop in the direction of broadband, intelligence and personalization. The mobile communication system can be roughly divided into two generations. The first generation is based on analog technology, frequency division multiple access, and operates in the frequency band of 400 ~ 800MHz. Due to the existence of the simulation system, there is only one domestic carbon fiber that can meet the use demand at present; China's aero-engine demand for superalloys will reach 20000 tons/year. Due to its inherent shortcomings such as low spectrum efficiency, small capacity, complex equipment, poor anti-interference performance, weak confidentiality, high price and narrow business scope, it can not meet the needs of the rapid development of the communication market. Therefore, the second generation mobile communication system was born. The second generation mobile communication system adopts digital, time division multiple access and other full digital technologies, which overcomes the shortcomings of the first generation mobile communication and has developed rapidly. The current mobile communication is digital analog compatible and mainly based on digital systems. With the exponential growth of users' demand for information access, telecom workers set out to establish the latest generation of mobile communication - the third generation mobile communication system

the third generation mobile communication system has the characteristics of globalization, intelligence, personalization and integration. It works in the 2000MHz band and adopts broadband CDMA technology, covering ground systems and satellite systems, including three-dimensional service surface of sea, land and air, and integrating voice, data, video, ISDN and multimedia services. The system can provide services to high-speed and slow-moving users through a variety of air interfaces and access modes

2. current situation of railway wireless access

railway communication is to provide timely and reliable communication for passengers and railway official, emergency rescue, train maintenance and other personnel, so as to improve service level and transportation efficiency. It is established to ensure the safety of trains and achieve efficient operation. It is a train mobile communication system integrating train official communication and section mobile operation communication. However, the characteristics of the railway structure determine the difference between the system and the public mobile communication and regional professional mobile communication. It is a kind of chain which is a combination of lines and planes and is dominated by lines

at present, the only wireless access part of railway communication is the 400MHz wireless train dispatching system, which completes the communication between the station attendant and the train conductor and the train driver entering its jurisdiction. These calls can only be made when the train is about to enter or leave the station. Otherwise, if there are no special circumstances, the calls will not be made when the train is running in the section. This is mainly from the perspective of saving frequency resources and reducing CO frequency interference. However, with the rapid progress of railway modernization, the former single wireless train dispatching system has been far from meeting the needs of railway wireless communication, so it is urgent to build an advanced wireless communication system suitable for the needs of railway modern operation and command. This system should adopt the small area system and complete the big triangle function. In other words, the system must be able to realize the communication function between the dispatching center and the station attendant, between the station attendant and the train driver, and between the train driver and the dispatching center. It must also be able to realize the official mobile communication function in the line management section. At the same time, it must also be able to realize the real-time two-way data communication function between the dispatching center and the train cab. Based on this idea, the existing cluster communication mode, GSM (global mobile communication system) mobile communication mode and CDMA mobile communication mode can be used to form the railway wireless communication access

trunking communication system is a special mobile communication system with powerful functions. It is the product of the close combination of communication with microprocessor technology, program-controlled switching technology and computer network technology. It integrates switching, control and communication, automatically and optimally assigns a group of channels to internal users through wireless dialing, makes maximum use of system resources and frequency resources, reduces call loss in the system, and improves service quality. Because it has the functions of group call, group call, forced insertion, forced disassembly, etc., it is especially suitable for dispatching command, emergency, rescue and other occasions, and better solves the problem of reasonable distribution of communication frequency. Therefore, it is favored by professional operation and management departments and is determined as the preferred type of current railway mobile communication. However, this system also has some disadvantages, mainly including the use of dynamic frequency allocation, without considering the effective integration with the surrounding public, without advanced routing reasonable selection function, and in the establishment of access and automatic obsolescence, there is information loss, weak confidentiality, easy to be disturbed, etc. these disadvantages have little impact on voice communication, However, it will cause large bit error for the real-time two-way data communication between the train and the dispatching command center, so it is not suitable for field merging which requires high bit error rate of data communication

the mobile communication system of Qinhuangdao Shenyang passenger dedicated line to be constructed soon mainly includes a 400MHz wireless train dispatching system and an 800MHz trunked mobile communication system. Considering that the trunked mobile communication system will have information damage in the handover process, the data communication part will be completed by the wireless train dispatching system, The trunked mobile communication system only carries out interval communication (such as voice communication of the big triangle function, official communication, emergency rescue communication, etc.), and leaves room for dispatching and access to public communication. This system is also the first example of China's railway using trunking communication as wireless access system, and it is a major change in the history of China's railway communication

III Future development trend of railway wireless access

with the further deepening of reform and the progress of social informatization, railway communication is not only required to have a stronger communication function to ensure the safe operation of the railway, so as to meet the needs of high-speed train communication, but also to fully develop telecom value-added services and operate Telecom services with the same business scope as Chinatelecom, and participate in the competition with Chinatelecom, So that passengers and users in the network coverage area can easily enjoy information services. For example, it can provide railway passenger and freight transportation information, order train tickets and other services anytime and anywhere, and enjoy voice, fax, data, video, mobile communication, Internet and other services on the train. However, the existing communication network facilities of the railway are huge and backward, which is the biggest obstacle to the development of the network at present

the trunked mobile communication system developed and applied in the 1980s has the advantages of high channel utilization and flexible group. It can ensure the high quality and priority of passenger calls, and can be used for business communication by train officials. It can also use the dispatching function to form temporary emergency communication and mobile operation communication along the receiving line, which can basically meet the needs of current railway communication, Qinhuangdao Shenyang passenger dedicated line uses this system to realize the function of railway mobile communication. But in terms of higher communication objectives, for example, in order to realize the real-time positioning and tracking of the train, enable the public servants on and off the train to obtain the whole road condition information anytime and anywhere, realize the automatic control of train operation and dispatching, provide passengers with fax, data, video, mobile communication, Internet and other services in addition to voice services, and provide telecommunications services to the residents along the railway, With the emergence of these services, the original communication system can not meet the requirements. We should apply advanced mobile communication technology to transform railway communication and establish new and necessary mobile communication systems, such as micro cellular mobile communication system, or

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