Overview of access network

The so-called access network refers to all the equipment between the backbone network and the user terminal. Its length is usually several hundred meters to several kilometers, so it is vividly called "the last kilometer". Because the backbone network generally adopts the optical fiber structure and the transmission speed is fast, the access network becomes the bottleneck of the whole network system. The access modes of access network include copper wire (ordinary telephone line) access, optical fiber access, hybrid access of optical fiber coaxial cable (CATV cable), wireless access and Ethernet access. Overview of access network According to ITU's recommendation on access network framework (G.902), access network is a system consisting of service node interface (SNI) and related user network interface (UNI), which provides the carrying capacity required for transmitting telecommunication services and is configured and managed through Q interface. Therefore, the access network can be defined as three interfaces, that is, the network side is connected with the service node through SNI, the user side is connected with the user through UNI, and the management side is connected with the telecom management network (TMN) through Q interface. A service node is an entity that provides services. Service nodes that can provide specific services include local exchanges, private line service nodes or specially configured video-on-demand and radio and television service nodes. SNI is the interface between access network and service nodes, which can be divided into SNI supporting single access and SNI supporting integrated access. The standardized interfaces supporting single access mainly include V 1 interface providing ISDN basic rate (2B+D) and V3 interface providing main group rate (30B+D). At present, there are V5 interfaces supporting integrated service access, including V5. 1 and V5.2 interfaces. UNI excuse between access network and users can support all kinds of access types and services that the existing network can provide, and the development of access network should not limit the existing services and access types. The management of access network should be brought into the category of TMN, and different network elements should be managed in a unified and coordinated way. The management of access network should not only complete the management of each functional block of access network, but also complete the test and fault location of subscriber lines. The whole telecommunication network is divided into three parts according to network functions: transmission network, switching network and access network. Access network is responsible for transparent transmission of telecommunication services to users. Specifically, access is the connection part between local exchange and users, which usually includes subscriber line transmission system, multiplexing equipment, cross-connection equipment or user/network terminal equipment. In fact, the entity that receives the service is the service node. According to the requirements of access network framework and system, the important characteristics of access network can be summarized as: 1. Access network provides carrying capacity for accessed services and realizes transparent transmission of services. 2. Access network is transparent to user signaling, except for some user signaling format conversion, signaling and service processing functions are still in service nodes. 3. The introduction of access network should not limit the existing access types and services, and access network should be connected with service nodes through limited standardized interfaces. 4. Access network has a network management system independent of service nodes, which is connected with TMN through standardized interfaces, and TMN realizes the operation, maintenance and management of access network. Twisted-pair editing this paragraph structure (1) bus structure. Refers to the network structure that uses optical fiber as a common bus and each user terminal is directly connected to the bus through a coupler. Its characteristics are * * * sharing backbone optical fiber, saving line investment, convenient node addition and deletion, high dynamic range requirement and low interference effect. The disadvantage is loss accumulation, and users accept strong dependence on backbone optical fiber. (2) Ring structure. Refers to a closed-loop network structure in which all nodes * * * use an optical fiber link, which is connected end to end. Its characteristic is self-healing, that is, in the absence of external intervention, the network can automatically recover the transmitted service from the failure fault in a short time, with high reliability. The disadvantage is that the number of single-ring users is limited, and multi-ring interworking is more complicated, which is not suitable for distributed services such as CATV. (3) star structure. This structure is actually a point-to-point mode, and each user terminal exchanges information through a star coupler with control and exchange functions located at the central node. Its characteristics are simple structure, convenient use and maintenance, easy upgrade and expansion, relative independence among users, good confidentiality and strong business adaptability. The disadvantages are that the required optical fiber is expensive, the networking flexibility is poor, and the reliability of the central node is extremely high. (4) Tree structure. Similar to the shape of branches, it has a hierarchical structure, and multiple splitters are used at junction boxes and junction boxes to distribute signals step by step. The top office has strong control and coordination ability. It is characterized by being suitable for broadcasting service. Disadvantages are large power loss and difficult two-way communication. The development of this section of power communication network has undergone tremendous changes, from analog to digital, from cable to optical cable, from PDH to SDH, from STM to ATM, from ATM to IP/DWDM, and new technologies and systems emerge one after another. However, most new technologies and systems are used in backbone networks, and the user access network is still dominated by analog twisted pair technology. Due to the development of social economy and communication technology, pure voice service has been difficult to meet the needs of users and the market, especially the emergence of optical fiber technology, and users' demand for new services, especially broadband image and data services, is increasing day by day, which affects the structure of the whole network and brings opportunities for the transformation and update of user access networks. In a word, users' demand for broadband integrated services and the rapid development of communication technology have become the two driving forces for the development of access network technology. Edit this paragraph to define the definition of access network. In July, 1995, ITU-T13 adopted the new recommendation G.902 on the framework of access network, in which the definition of access network is as follows: Access network consists of a series of transmission entities (such as line equipment and transmission facilities) between service node interface (SNI) and user network interface (UNI). In principle, there is no limit to the types and quantities of UNI and SNI that can be realized by an access network. The access network does not interpret the signaling. The traditional access network mainly provides users with general voice services and a small amount of data services in the form of copper cables. With the development of social economy, people's demand for new services, especially broadband integrated services, is increasing day by day, and a series of new access network technologies have emerged, including new copper cable technology based on existing twisted pair, hybrid optical fiber/coaxial (FHC), network technology and hybrid optical fiber/wireless access technology, wireless local loop technology (WLL/DWLL) and Ethernet to the home technology (ETTH). New technology of copper cable based on (1) twisted pair. At present, the user access network technology is mainly copper cable composed of multiple pairs of twisted pairs. The cost is very high. How to give full play to its benefits and meet the needs of many new services as much as possible is the main subject of the development of user access networks, and it is also the main means for telecom operators to cope with competition, reduce costs and increase income. Developing new technology and making full use of twisted pair has always been the focus of telecom industry. The so-called copper wire access technology refers to the technology of using advanced digital processing technology to improve the transmission capacity of twisted pair and provide users with various services, mainly including digital pair gain (DPG), high bit rate digital subscriber line (HDSL), asymmetric digital subscriber line (ADSL) and very high data rate subscriber line (VDSL). (2) Hybrid optical fiber/coaxial (HFC) network. Fiber/coaxial hybrid network is a broadband access technology based on frequency division multiplexing technology. Its backbone network uses optical fiber and frequency division multiplexing to transmit a variety of information, while the distribution network uses tree topology and coaxial cable system to transmit and distribute user information. HFC is a new economic evolution strategy to gradually push optical fiber to users, which can realize multimedia communication and interactive video services. At present, many international organizations and forums, including ITU-T, are standardizing the next generation digital HFC system combining MPEG-2 and ATM, which will certainly further promote its development. (3)FTTx+ETTH .FTTH+ETrH is an access mode of optical fiber to the building, optical fiber to the roadside and Ethernet to users. It provides users with high and reliable broadband guarantee, and truly realizes gigabit to community, 100 megabits to building units and 100 megabits to households. With the further growth of broadband demand, it can be upgraded smoothly and realize 100 Mbps to the home without rewiring. Fully realize multimedia communication and interactive video services. For example, this technology is adopted in the residential broadband access system of Navy Lianbao Erli Life. (4) Wireless subscriber loop access network. Wireless subscriber loop, also known as "wireless subscriber access", is a user access system that uses microwave, satellite, wireless cellular and other wireless transmission technologies to realize the service access of multiple dispersed users or user groups in remote areas and islands with blind spots in subscriber lines. It has the characteristics of fast construction speed, quick and flexible equipment installation and convenient use. In the case of using wireless, the cost of user access is insensitive to transmission distance and user density. Therefore, it is very suitable for areas with long access distance and low user density. The necessity and importance of editing this paragraph access network construction 1. The needs of business development require users to demand new broadband services. The popularization and application of SDH and ATM technologies in trunk networks require access networks to provide broadband transmission channels. Copper subscriber network has small capacity and narrow frequency band, and it is not easy to expand and digitize. The development direction of telecommunication business is digitalization, integration, broadband, intelligence and personalization of telecommunication network, which has become the bottleneck of telecommunication network. The contradiction between digitalization and integration and the backward access means has become one of the main contradictions in the construction of telecommunication networks. 2. An important link to reduce the operating costs of enterprises and improve economic benefits: optical cable exchange programming and relay transmission. The transmission from telephone office to user (access network) is still analog. Access network is the window of the whole telecommunication network and the "last mile" of the telecommunication network. The proportion of investment accounts for about 50% of the total investment in telecommunications networks. The analog transmission quality is not good, and the bearer service is limited. Consume a lot of non-ferrous metals-the price of copper cable is rising gradually, and the price of optical fiber transmission system is falling. With the provision and practicality of V5 interface, the cost of accessing the system will be further reduced and the maintenance cost will be saved. Developing access network in an all-round way is also an inevitable requirement of operating cost accounting. 3. Optimize the network structure, deepen the reform of telecom network operation and maintenance system, build a new generation of access network, lay optical fiber near (residential area, building) and connect to the power grid, build the overall situation, optimize the network structure, centralize maintenance personnel and centralized monitoring and maintenance. 4. The competition in the telecom market requires the network to be close to users and provide a variety of services quickly. The monopoly situation of post and telecommunications with market initiative has been broken by two advantages: one is talent advantage, the other is network advantage to expand and maintain network advantage. Vigorously developing access network is a strategic measure for the construction and development of telecommunications network. The development trend of this section is that with the disappearance of the monopoly market in the telecommunications industry and the opening of the telecommunications network service market, the telecommunications service function and access technology are constantly improving, and the access network is also developing, which is mainly manifested in the following points: (1) The complexity of the access network is increasing. The competition and comprehensive use of different access technologies, as well as the support demand for a large number of telecommunications services, increase the complexity of access networks. (2) The service scope of the access network is constantly expanding. With the development of communication technology and communication network, the capacity of local exchanges is expanding and the number of exchange offices is decreasing. The use of hubs and multiplexers in places with small capacity makes the service scope of access networks expand continuously. (3) The standardization of access network is getting higher and higher. After the local exchange gradually adopted the open interface based on V5. X standard, telecom operators can choose access network technology and system equipment more freely. (4) Access networks should support higher-level services. With the development of market economy, commercial and corporate customers put forward higher requirements for access lines for data applications, especially for local area network interconnection, which requires reliable short-term connections. With the extension of optical fiber technology to user network, the development of CATV brings opportunities to the development of user loop. (5) The technologies supporting access networks are more diversified. Although there are more and more optical transmission contents in the access network at present, how to make better use of the existing twisted pair is still valued, but for large-capacity access lines that need to be built quickly, wireless links can be chosen. (6) Optical fiber technology will be more applied to access networks. With the expansion of optical fiber coverage, optical fiber technology will be more and more applied to access networks. From the development point of view, SDH, ATM and IP/DWDM are only suitable for backbone optical fiber segments and digital local computer interfaces. With the development of business, the optical fiber interface will further extend to the roadside and eventually enter the home, truly realizing broadband optical fiber access and realizing a unified broadband all-optical network structure. Therefore, the telecommunication network will truly become the solid network foundation of the information superhighway in this century. Edit the business voice services supported by this service access network: new program-controlled telephone service, magnetic card telephone service and other data services: image communication services such as DDN service and packet switching service; multimedia services such as conference television service and videophone service: home office, shopping, VOD and telemedicine.