Concept of SDH

SDH [1] (Synchronous Digital Hierarchy, synchronous digital hierarchy) optical transceiver has a large capacity, generally between 16E 1 and 4032E 1. SDH is an integrated information transmission network with functions of multiplexing, line transmission and switching, which is operated by a unified network management system. It is a synchronous optical network (SONET) proposed by Bell Institute of Communication Technology. CCITT (now ITU-T) accepted the concept of SONET in 1988, and renamed it SDH, making it a general technical system suitable not only for optical fiber, but also for microwave and satellite transmission. It can achieve a variety of functions, such as effective network management, real-time service monitoring, dynamic network maintenance, and interoperability between devices from different manufacturers. It can greatly improve the utilization rate of network resources, reduce management and maintenance costs, and realize flexible, reliable and efficient network operation and maintenance. Therefore, it is a hot spot in the development and application of transmission technology in the information field in the world today, and it has been widely valued by people.

Edit this paragraph to generate the background.

The birth of SDH technology is inevitable. With the development of communication, the information to be transmitted is not only voice, but also text, data and image SDH technology.

And videos. Coupled with the development of digital communication and computer technology, T1(DS1)/E1carrier system (1.544/2.048Mbps), X.25 frame relay, ISDN (integrated services digital network) and FDDI (optical fiber) appeared in the 1970s and 1980s. With the advent of the information society, people hope that modern information transmission networks can provide various lines and services quickly, economically and effectively. However, due to the monotony of services, the complexity of expansion and the limitation of bandwidth, it is useless to modify or improve the above network technology only within the original framework. SDH is developed under this background. Among all kinds of broadband optical fiber access network technologies, SDH access network system is the most widely used. The birth of SDH solved the "bottleneck" problem that users and core networks can't keep up with the development of backbone networks and the access requirements of users due to the bandwidth limitation of home media, and at the same time improved the utilization rate of a large amount of bandwidth on transmission networks. SDH technology has been a mature standard technology since it came out in 1990s. It has been widely used in backbone networks and its price is getting lower and lower. The application in access network can bring the huge bandwidth advantage and technical advantage of SDH technology in the core network into the field of access network, and make full use of SDH synchronous multiplexing, standardized optical interface, strong network management ability, flexible network topology ability and high reliability to bring benefits, which will benefit from the construction and development of access network for a long time.

Edit the basic transmission principle of this paragraph.

The information structure level adopted by SDH is called STM-N (synchronous transmission module STM-N (n = 1 4, 16 process).

, 64), the most basic module is STM- 1, four STM- 1 are synchronously multiplexed to form STM-4, 16 STM- 1 or four STM-4 are synchronously multiplexed to form STM-16; SDH adopts block frame structure to carry information, and each frame consists of 9 vertical rows and 270×N horizontal columns of bytes, and each byte contains 8 bits. The whole frame structure is divided into three areas: Section OverHead area, STM-N payload area and management unit pointer (AU PTR) area. Among them, section overhead District is mainly used for network operation, management, maintenance and distribution to ensure the normal and flexible transmission of information. It can be divided into Regenerative section overhead (RSOH) and Multiplex section overhead (MSOH). The payload area is used to store bits really used for information service and a small amount of channel overhead bytes used for channel maintenance and management; The management unit pointer is used to indicate the exact location of the first byte of information in the payload area of STM-N frame, so that the payload can be separated correctly when received. SDH frames are transmitted from left to right and from top to bottom in serial code stream. The transmission time of each frame is 125μs, and1125×1000000 frames are transmitted every second. For STM- 1, the byte of each frame is 8 bit× (the transmission rate of 9× STM-4 is 4×155.520 mbit/s = 622.080 mbit/s; The transmission rate of STM- 16 is 16× 155.520 (or 4× 622.080) = 2488.320mbit/s. When SDH transmits service signals, all kinds of service signals have to go through three steps: mapping, positioning and multiplexing. Mapping is a process of loading signals of various rates into corresponding standard containers (C) after code speed adjustment, and adding channel overhead (POH) to form virtual containers (VC). The frame phase deviation is called frame deviation. Positioning is the process of receiving the frame offset information into the branch unit (TU) or the management unit (AU), which is realized by the function of the branch unit pointer (TU PTR) or the management unit pointer (AU PTR). The concept of multiplexing is relatively simple. Multiplexing is a process of adapting signals of multiple low-order channel layers to high-order channel layers or adapting signals of multiple high-order channel layers to multiplexing layers. Multiplexing is a process of organizing TU into higher-order VC or organizing AU into STM-N, which is interleaved by bytes. Because the signals of VC branch processed by TU and AU pointers have been synchronized, the multiplexing process is synchronized, and the multiplexing principle is similar to the serial-parallel conversion of data.

Edit the characteristics of this paragraph

The rapid development of SDH is inseparable from its own characteristics, and its specific characteristics are as follows: Features

superiority

(1)SDH transmission system has global unified frame structure digital transmission standard rate and standard optical interface, which enables network management systems to communicate with each other, so it has good lateral compatibility. It can be fully compatible with the existing PDH, accommodate various new service signals, form a global unified digital transmission system standard, and improve the reliability of the network. (2) The code streams of different levels of SDH access system are regularly arranged in the payload area of the frame structure, and the payload is synchronized with the network. Using software, high-speed signals can be directly demultiplexed into low-speed branch signals at one time to realize one-time multiplexing, which overcomes the process of decomposing all high-speed signals step by step and then regenerating multiplexing in PDH quasi-synchronous multiplexing mode. Because DXC is greatly simplified, back-to-back interface multiplexing equipment is reduced, and the transparency of network service transmission is improved. (3) With the advanced add-drop multiplexer (ADM) and digital cross-connect (DXC), the self-healing function and recombination function of the network are very powerful, and the survival rate is very strong. Due to the arrangement of 5% overhead bits in SDH frame structure, its network management function is particularly powerful, which can form a unified network management system and play a positive role in network automation, intelligence, channel utilization, reducing network blood vessel cost and survivability. (4) Because of the diverse network topologies of SDH, the network it forms is very flexible. It can enhance the functions of network monitoring, operation management and automatic configuration, optimize the network performance, make the network run flexibly, safely and reliably, and make the network functions very complete and diversified. (5)SDH has the performance of transmission and switching, and the composition of its series of equipment can realize networks of different levels and various topologies through the free combination of functional blocks, which is very flexible. (6)SDH does not belong to a certain transmission medium and can be used for twisted pair and coaxial cable, but SDH needs optical fiber to transmit high data rate. This feature shows that SDH is suitable for both trunk channel and branch channel. For example, both the national and provincial cable TV trunk networks in China adopt SDH, which is also easy to be compatible with HFC. (7) From the OSI model, SDH belongs to its lowest physical layer, and there are no strict restrictions on its upper layer, which facilitates the adoption of various network technologies on SDH and supports ATM or IP transmission. (8)SDH is strictly synchronized, which ensures the stability and reliability of the whole network, with few error codes and easy reuse and adjustment. (9) The standard open optical interface can be horizontally compatible on the basic optical cable segment, which reduces the networking cost.

disadvantaged

Validity and reliability are contradictory. The improvement of validity will inevitably reduce reliability, and the improvement of reliability will correspondingly reduce validity. One advantage of SDH is that the reliability of the system is greatly enhanced (high degree of automation in operation and maintenance). This is because a large number of overhead bytes of OAM function are added to the signal-STM-N frame of SDH, which will inevitably make the frequency band (transmission rate) occupied by PDH signal narrower than that occupied by SDH signal, that is, the usage rate of PDH signal is lower. For example, the STM- 1 signal of SDH can be multiplexed into 63 PDH signals of 2Mbit/s or 3 PDH signals of 34 Mbit/s (equivalent to 48×2Mbit/s) or 1 40 Mbit/s (equivalent to 64×2Mbit/s). Only when PDH signal is multiplexed into the frame of STM- 1 signal at 140Mbit/s, STM- 1 signal can accommodate 64×2Mbit/s, but at this time, its signal rate is 155Mbit/s, which is higher than that of E4 signal (66. 2. SDH system with complex pointer adjustment mechanism can directly connect low-speed signals from high-speed signals, thus eliminating the multi-stage multiplexing/demultiplexing process. This function is realized by pointer mechanism. The function of pointer is to indicate the position of low-speed signal at all times, so that the required low-speed signal can be separated correctly when unpacking, which ensures the realization of direct connection between low-speed signal and high-speed signal of SDH. It can be said that pointer is a major feature of SDH. But the realization of pointer function increases the complexity of the system. The most important thing is to make the system produce the only jitter of SDH-combined jitter caused by pointer adjustment. This kind of jitter often occurs at the boundary of the network, and its frequency is low and amplitude is large, which will lead to the performance deterioration after the low-speed signal is removed, and it will be quite difficult to filter this kind of jitter. 3. The influence of the widespread use of software on system security One of the characteristics of SDH is the high degree of automation of OAM, which also means that software occupies a considerable proportion in the system, which makes the system vulnerable to computer viruses, especially today when computer viruses are everywhere. In addition, human error operation and software failure in the network layer also have a fatal impact on the system. In this way, the security of the system becomes a very important aspect. Therefore, the maintenance personnel of equipment must be familiar with the software and choose a highly reliable network topology.

Edit this application.

Due to the above characteristics, SDH has been greatly developed in the field of wide area network and private network. Telecom operators such as China Mobile, China Telecom, China Unicom and Radio and Television have built large-scale backbone optical transmission networks based on SDH. Use large-capacity SDH loop to carry IP services and ATM services, or lease them directly to enterprises and institutions by renting circuits. And some large-scale private networks also adopt SDH technology, and establish SDH optical loop inside the system to carry all kinds of services. For example, in power system, SDH loop is used to carry internal services such as data, remote control, video and voice. For the units that are in urgent need of networking and can't set up dedicated SDH loops, many of them use the way of renting telecom operators' circuits. Because SDH is based on the physical layer, companies can carry all kinds of services on leased circuits without transmission restrictions. There are many bearing modes, such as using integrated multiplexing equipment based on TDM technology to realize multi-service multiplexing, or using IP-based equipment to realize multi-service packet switching. SDH technology can really guarantee the bandwidth of leased circuits, and its security is better than VPN. SDH leased lines have been widely used in government agencies and enterprises that attach great importance to safety. Generally speaking, SDH can provide interfaces such as E 1, E3, STM- 1 or STM-4, which can fully meet various bandwidth requirements. At the same time, the price has been accepted by most units.

Edit the development trend of this paragraph

As a new generation of ideal transmission system, SDH has many advantages, such as automatic routing, convenient up and down circuits, strong maintenance, control and management functions, unified standards and convenient transmission of higher-speed services, which can well meet the needs of the rapid development of communication networks. So far, SDH has been applied and developed unprecedentedly. In terms of standardization, a series of suggestions that have been established and will be established have basically covered all aspects of SDH. It has been widely used in trunk networks and long-distance networks, relay networks and access networks. And actively carry out the research and application of optical fiber communication, microwave communication and satellite communication. In recent years, TV on demand, multimedia services and other broadband services have sprung up, providing a broad space for the application of SDH in access networks. The advantages of SDH technology applied to access network are: 1) For large enterprises and users who require high reliability and high quality service, SDH can provide ideal network performance and service reliability. 2) The network management scope can be extended to the client, which simplifies the maintenance work. 3) Using the inherent flexibility of SDH, network operators can provide users with long-term and short-term business needs more quickly and effectively. [2] From a technical point of view, the access layer needs less relative bandwidth and needs to provide integrated services such as IP, TDM and possibly ATM. A system based on SDH system that can provide IP and ATM transmission and processing (including TDM, IP and ATM interfaces, and even IP and ATM switching modules) will be the main method to solve the transmission at the access layer. It can provide high-quality private line, ATM, IP and other services access, transmission and protection at a lower cost in a service provider (POP). With the continuous increase of backbone transmission capacity, the access capacity of metropolitan transmission network is also diversified. However, the IP-based network service is still unpredictable, which requires the transmission network to have better adaptability, not only the adaptability of network interface or network capacity, but also the adaptability of network connection. Generally speaking, low-cost, flexible and fast service access and service integration from the operator's end office to the users are the main requirements of the future metropolitan area network access system. To put it simply, this way of using SDH to transmit Ethernet and other services is to map the services of different network levels to each time slot of SDH circuit through VC cascade. SDH network provides a completely transparent transmission channel and is an integrated whole from the perspective of physical layer equipment. This solution can greatly reduce the investment scale, equipment floor space and power consumption, thus reducing the operating costs of network operators. At the same time, the ability to provide a variety of services can also enable network operators to quickly deploy network services, increase business income and enhance market competitiveness. To sum up, SDH has become the mainstream of transmission network development with its obvious advantages. The combination of SDH technology and some advanced technologies, such as optical wavelength division multiplexing (WDM), ATM technology and Internet technology (IP over SDH), makes SDH network play an increasingly important role. SDH has been included in the application project of 2 1 century high-speed communication network by various countries, and it is recognized as the development direction of digital transmission network in telecom industry, with broad commercial prospects.