With the advent of the information society, people hope that modern information transmission networks can provide various circuits and services quickly, economically and effectively. The above-mentioned network technologies are only in the original due to the monotony of their services, the complexity of expansion, and the limitations of bandwidth. Modification or improvement within the framework has been useless. At this time, the emergence of SDH and its many features have made it a great development in the field of WAN and private networks.
This paper discusses the working principle of SDH from the SDH frame, the common network topology of SDH, the network equipment and the protection mechanism of the network. Based on these, the functions of commonly used devices in the SDH network are introduced. Finally, an example is given to its application in reality and how to build an SDH network.
In recent years, as a new generation of ideal transmission system, SDH has the advantages of automatic route selection, convenient upper and lower circuits, strong maintenance, control and management functions, unified standards, and easy transmission of higher-speed services. The need for rapid development. The combination of SDH technology and some advanced technologies, such as optical wavelength division multiplexing (WDM), ATM technology, and Internet technology (IP over SDH), has made SDH networks more and more important. SDH has been listed in the 21st century high-speed communication network application projects by various countries. It is the development direction of the digital transmission network recognized by the telecom industry and has great commercial prospects.
The SDH (Synchronous Digital Hierarchy) optical transceiver has a large capacity, generally 16E1 to 4032E1. SDH is a comprehensive information transmission network that integrates multiplexing, line transmission and switching functions and is operated by a unified network management system. It is a synchronous optical network (SONET) proposed by the Bell Communications Technology Institute of the United States. The International Telephone and Telegraph Advisory Committee (CCITT) (now ITU-T) accepted the SONET concept in 1988 and renamed it SDH, making it a general-purpose technology not only for fiber but also for microwave and satellite transmission. It can realize network effective management, real-time service monitoring, dynamic network maintenance, interoperability between devices of different vendors, etc., which can greatly improve network resource utilization, reduce management and maintenance costs, and achieve flexibility.
The birth of SDH technology has its inevitability. With the development of communication, the information required to be transmitted is not only voice, but also SDH technology of text, data and image.
And videos, etc. In addition to the development of digital communication and computer technology, in the 1970s and 1980s, T1 (DS1)/E1 carrier systems (1.544/2.048 Mbps), X.25 frame relay, ISDN (Integrated Services Digital Network) and FDDI ( A variety of network technologies such as fiber-optic distributed data interfaces. With the advent of the information society, people hope that modern information transmission networks can provide various circuits and services quickly, economically and effectively. The above-mentioned network technologies are only in the original due to the monotony of their services, the complexity of expansion, and the limitations of bandwidth. It is useless to modify or improve within the framework. SDH developed in this context. Among various broadband fiber access network technologies, access network systems using SDH technology are the most common applications. The birth of SDH solved the problem of the backbone network and user service demand due to the bandwidth limitation of the home media, which caused the problem of access bottleneck between the user and the core network, and increased the bandwidth of the transmission network. Utilization. Since the introduction of SDH technology in the 1990s, it has been a mature and standard technology. It is widely used in the backbone network, and the price is getting lower and lower. The application in the access network can make the SDH technology huge in the core network. Bandwidth advantages and technical advantages are brought into the access network field, making full use of SDH synchronous multiplexing, standardized optical interfaces, powerful network management capabilities, flexible network topology capabilities and high reliability, and benefiting from the development of access networks. Benefit.
Basic transmission principleThe information structure level adopted by SDH is called synchronous transmission module STM-N (Synchronous Transport, N=1, 4, 16 flow, 64), the most basic module is STM-1, and four STM-1 synchronous multiplexing constitutes STM- 4, 16 STM-1 or four STM-4 synchronous multiplexing constitutes STM-16; SDH adopts a block-like frame structure to carry information, each frame consists of vertical 9 lines and horizontal 270 & TImes; N columns of bytes, each The byte contains 8 bits, and the whole frame structure is divided into three areas: SecTIon OverHead (SOH) area, STM-N payload area and management unit pointer (AU PTR) area. The section overhead area is mainly used for network operation and management. , maintenance and assignment to ensure that information can be transmitted normally and flexibly, it is divided into Rege nerator SecTIon OverHead (RSOH) and multiplex section overhead (MulTIplex Section OverHead, MSOH); the payload area is used for real use. The bits of the information service and a small number of channel overhead bytes for channel maintenance management; the management unit pointer is used to indicate the exact position of the first byte of information in the payload area within the STM-N frame so that it can be correctly separated when received. negative Lotus. SDH frame transmission is sequentially transmitted in a serial-type code stream from left to right and top to bottom. The transmission time per frame is 125μs, and 1/125×1000000 frames per second. For STM-1, The frame byte is 8bit×(9×270×1)=19440bit, then the transmission rate of STM-1 is 19440×8000=155.520 Mbit/s; and the transmission rate of STM-4 is 4×155.520 Mbit/s=622.080 Mbit. /s; The transmission rate of STM-16 is 16 x 155.520 (or 4 x 622.080) = 2488.320 Mbit/s.
When SDH transmits service signals, the frames of various service signals entering SDH must be mapped, located and multiplexed. The mapping is to load the signals of various speeds into the corresponding standard containers (C) through code rate adjustment. The process of adding a channel overhead (POH) to form a virtual container (VC), the frame phase deviation is called a frame offset; positioning is the process of collecting frame offset information into a branch unit (TU) or a management unit (AU). It is implemented by the function of the tributary unit pointer (TU PTR) or the management unit pointer (AU PTR); the concept of multiplexing is relatively simple, and multiplexing is a method of adapting signals of a plurality of low-order channel layers into high order. Channel layer, or the process of adapting multiple higher order channel layer signals into the multiplex layer. Multiplexing is the process of organizing TU into high-order VC or organizing AU into STM-N by byte interleaving. Since the VC tributary signals processed by TU and AU pointer are already phase-synchronized, the complex The process is synchronous multiplexing and multiplexing is similar to the serial-to-parallel conversion of data.
The reason why SDH can develop rapidly is inseparable from its own characteristics. Its specific characteristics are as follows.
advantage(1) SDH transmission system has a unified frame structure digital transmission standard rate and standard optical path interface in the world, which makes the network management system interoperable, so it has good horizontal compatibility, it can be fully compatible with the existing PDH, and can accommodate Various new business signals have formed a global unified digital transmission system standard, which has improved the reliability of the network.
(2) The code streams of different levels of SDH access system are arranged regularly in the payload area of ​​the frame structure, and the payload is synchronized with the network. It can use software to directly insert and insert high-speed signals into the low-speed branch. The signal realizes the characteristics of one-time multiplexing, overcomes the process of PDH quasi-synchronous multiplexing method for step-by-step decomposition and re-multiplexing of all high-speed signals, which greatly simplifies DXC, reduces back-to-back interface multiplexing equipment, and improves The transparency of the business delivery of the network.
(3) It is very powerful due to the adoption of advanced add/drop multiplexer (ADM), digital cross-connect (DXC), network self-healing function and recombination function, and has a strong survival rate. Because the 5% overhead bits of the signal are arranged in the SDH frame structure, its network management function is particularly powerful, and can form a network management system uniformly, which is the automation, intelligence, channel utilization, and network maintenance cost of the network. Survival has played a positive role.
(4) Due to various network topologies of SDH, the network formed by it is very flexible. It can enhance network monitoring, operation management and automatic configuration functions, optimize network performance, and also make the network run flexible, safe and reliable. The network is very functional and diverse.
(5) SDH has the performance of transmission and exchange. The structure of its series of devices can realize the network of different levels and various topologies through the free combination of function blocks, which is very flexible.
(6) SDH is not specific to a certain transmission medium, it can be used for twisted pair, coaxial cable, but SDH is used to transmit high data rate. This feature shows that SDH is suitable for both trunk and branch channels. For example, China's national and provincial cable TV trunk networks use SDH, and it is also compatible with fiber-optic cable hybrid networks (HFC).
(7) From the point of view of the OSI model, SDH belongs to its lowest physical layer and has no strict restrictions on its upper layers. It is convenient to adopt various network technologies on SDH to support ATM or IP transmission.
(8) SDH is strictly synchronized, thus ensuring the stability and reliability of the entire network, less error, and easy to reuse and adjust.
(9) The standard open optical interface can achieve horizontal compatibility on the basic cable segment, reducing the cost of networking.
Effectiveness and reliability are a pair of contradictions. Increasing the effectiveness will definitely reduce reliability, and increasing reliability will also reduce effectiveness. A big advantage of SDH is that the reliability of the system is greatly enhanced (the automation of operation and maintenance is high), because a large number of overhead words for OAM functions are added to the SDH signal-STM-N frame. Therefore, in the case of transmitting the same amount of valid information, the frequency band (transmission rate) occupied by the PDH signal is narrower than the frequency band (transmission rate) occupied by the SDH signal, that is, the rate used by the PDH signal is low. For example, the STM-1 signal of SDH can be multiplexed into 63 2Mbit/s or 3 34Mbit/s (equivalent to 48×2Mbit/s) or one 140Mbit/s (equivalent to 64×2Mbit/s) PDH signal. . Only when the PDH signal is multiplexed into the frame of the STM-1 signal with a signal of 140 Mbit/s, the STM-1 signal can accommodate 64×2 Mbit/s of information, but at this time its signal rate is 155 Mbit/s, the rate. The E4 signal (140 Mbit/s) is higher than the same information capacity of the PDH, that is, the transmission band occupied by the STM-1 is larger than the transmission band of the PDH E4 signal.
2. The pointer adjustment mechanism is complicated
The SDH system can directly lower the low-speed signal from the high-speed signal, eliminating the multi-stage multiplexing/demultiplexing process. The realization of this function is accomplished by the pointer mechanism. The role of the pointer is to indicate the position of the low-speed signal at all times, so that the required low-speed signal can be correctly split during the "unpacking", and the SDH is guaranteed from the high-speed signal. The implementation of the function of direct low-speed signals. It can be said that the pointer is a major feature of SDH. But the implementation of the pointer function increases the complexity of the system. The most important thing is to make the system produce a unique jitter of SDH - the combined jitter caused by pointer adjustment. This kind of jitter occurs mostly at the boundary of the network. Its low frequency and large amplitude will cause the performance of the low-speed signal to deteriorate after being removed. This filtering of jitter can be quite difficult.
3. The impact of extensive use of software on system security
A major feature of SDH is the high degree of automation of OAM, which means that software occupies a considerable proportion of the system, which makes the system vulnerable to computer viruses, especially in today's computer viruses. In addition, human error operations and software failures at the network layer are also fatal to the system. The security of such a system becomes an important aspect. Therefore, the maintenance personnel of the equipment must be familiar with the software and choose a network topology with high reliability.
applicationDue to the numerous characteristics of SDH described above, it has been greatly developed in the field of wide area network and private network. China Mobile, Telecom, China Unicom, Radio and Television and other telecom operators have already built a large-scale SDH-based backbone optical transmission network. The large-capacity SDH loop is used to carry IP services, ATM services, or leased to enterprises and institutions directly by renting circuits. Some large private networks also use SDH technology to set up SDH optical loops inside the system to carry various services. For example, the power system uses the SDH loop to carry internal data, remote control, video, voice and other services.
For units that are more urgent in networking and have no possibility to set up dedicated SDH loops, many adopt the method of renting telecom operators' circuits. Since SDH is based on the characteristics of the physical layer, the unit can carry various services on the leased circuit without being restricted by transmission. There are many types of bearers, which can be achieved by using a multi-service multiplexing device based on TDM technology, or using IP-based devices to implement multi-service packet switching. SDH technology can truly realize the bandwidth guarantee of leased circuits, and the security is better than VPN. In government agencies and companies that are very concerned about security, SDH leased lines have been widely used. In general, SDH can provide interfaces such as E1, E3, STM-1 or STM-4, which can fully meet various bandwidth requirements. At the same time, in terms of price, it has also been accepted by most units.
development trendAs a new generation of ideal transmission system, SDH has the advantages of automatic route selection, convenient upper and lower circuits, strong maintenance, control and management functions, unified standards, and convenient transmission of higher-speed services. It can adapt well to the rapid development of communication networks. need. So far, SDH has achieved unprecedented application and development. In terms of standardization, a series of recommendations that have been established and are about to be established have basically covered all aspects of SDH. It has been widely used in trunk networks and long-haul networks, trunk networks, and access networks. Research and application are also actively carried out in optical fiber communication, microwave communication, and satellite communication.
In recent years, on-demand TV, multimedia services and other broadband services have sprung up, providing a wide space for SDH applications in the access network.
The benefits of applying SDH technology to access networks are:1) For large enterprises and enterprises that require high reliability and high quality services, SDH can provide ideal network performance and service reliability.
2) The network management range can be extended to the user side to simplify maintenance.
3) Utilizing the inherent flexibility of SDH, network operators can provide the long-term and short-term business needs that users need faster and more efficiently. From a technical point of view, the relative bandwidth requirement of the access layer is small, and it is necessary to provide IP, TDM, and possibly integrated service transmission such as ATM. Systems based on SDH systems that provide IP and ATM transport and processing (including TDM, IP and ATM interfaces, and even IP and ATM switch modules) will be the primary method for addressing access layer transport, which is inexpensive Provide access, transmission and protection of high-quality private lines, ATMs, IPs, etc. on a service delivery point (POP).
As the backbone transmission capacity continues to increase, the access capabilities of the metro transmission network are also diversified. However, IP-based network services are still unpredictable, which requires the transmission network to have better adaptive capabilities. This adaptive capability is not only the adaptability of the network interface or network capacity, but also requires the network connection. adaptability. In general, the low-cost, flexible and fast completion of service access and service convergence from the carrier to the client is a major requirement for the future metro access system.
Simply put, this way of using SDH to transmit Ethernet and other services is to map different network level services to each slot of the SDH circuit through VC cascade, and provide a completely transparent transmission channel by the SDH network. From the perspective of the physical layer device, it is an integrated whole. This solution can significantly reduce the investment scale, reduce equipment footprint, reduce power consumption, and thus reduce the operating costs of network operators. At the same time, the ability to provide multiple services can also enable network operators to rapidly deploy network services, increase business revenue, and enhance market competitiveness.
In summary, SDH has become the mainstream of transmission network development with its obvious superiority. The combination of SDH technology and some advanced technologies, such as optical wavelength division multiplexing (WDM), ATM technology, and Internet technology (IP over SDH), has made SDH networks more and more important. SDH has been listed in the 21st century high-speed communication network application projects by various countries. It is recognized as the development direction of digital transmission network in the telecom industry and has great commercial prospects.
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