Basic principles of SDH network
SDH (Synchronous Digital Hierarchy, Synchronous Digital Hierarchy), according to the recommended definition of ITU-T, is the transmission of digital signals at different rates to provide a corresponding level of information structure, including multiplexing methods and mapping methods, and related synchronization methods. Technical system.
In 1966, Dr. Gao Kun from the United Kingdom put forward the theory of optical transmission; however, it was only in 1976 that commercial transmission equipment began to appear; in the 1980s, Plesiochronous Digital Hierarchy (Plesiochronous Digital Hierarchy) products began to appear in scale Application; In the 1990s, SDH (Synchronous Digital Hierarchy Synchronous Digital Hierarchy) began to appear. After ITU-T specifications, it was rapidly popularized worldwide; in the late 1990s, higher-rate DWDM (Dense Wavelength Division Multiplexing (Dense Wavelength Division Multiplexing) technology has begun large-scale construction. The application of DWDM technology can simultaneously transmit information of multiple wavelengths in a single fiber, which improves the utilization of fiber resources and reduces construction investment costs;
At the beginning of the 21st century, in order to increase the transmission capacity to the order of Tbit/s or even more than ten Tbit/s, signals were processed at the optical layer (such as optical signal add/drop/multiplex, optical wavelength conversion/switching, etc.). Products based on OTN (Optical Transport Network) technology began to appear and applied; with the rapid development of data services and network requirements, a new and more powerful technology supporting data service transmission PTN (Packet Transport Network) Has also appeared and applied.
The current mainstream system is SDH/WDM, which incorporates new technologies such as MSTP (Multi-Service Transport Platform)/ASON (Automatic Switched Optical Network). The transmission network is the carrier of various business networks and an important network in the public telecommunication network level. The quality of the transmission network will also restrict the development of other business networks.
Technical background of SDH:
Optical fiber communication with cheap and excellent bandwidth characteristics has become the main transmission means of communication networks. The traditional PDH transmission system can no longer meet the requirements of the development of modern communication networks.
The information structure level adopted by SDH is called the synchronous transport module STM-N (Synchronous Transport Mode, N=1, 4, 16, 64). The most basic module is STM-1. Four STM-1 synchronous multiplexing constitute STM- 4. 16 STM-1 or four STM-4 synchronous multiplexing form STM-16, four STM-16 synchronous multiplexing form STM-64, and even four STM-64 synchronous multiplexing form STM-256; SDH adopts Block-shaped frame structure to carry information, each frame is composed of 9 vertical rows and horizontal 270×N columns of bytes, each byte contains 8 bits, the whole frame structure is divided into Section OverHead (SOH) area, STM-N net The load area and the management unit pointer (AU PTR) area are three areas. The section overhead area is mainly used for network operation, management, maintenance and assignment to ensure the normal and flexible transmission of information. It is divided into regeneration section overhead (Regenerator Section OverHead (RSOH) and Multiplex Section OverHead (MSOH); the payload area is used to store bits that are actually used for information services and a small amount of channel overhead bytes used for channel maintenance and management; the management unit pointer is used Indicate the exact position of the first byte of the information in the payload area in the STM-N frame so that the payload can be correctly separated when receiving.
Advantages and disadvantages of SDH:
- The SDH transmission system has a unified frame structure, digital transmission standard rate and standard optical interface in the world, so that the network management system can communicate, so it has good horizontal compatibility. It can be fully compatible with the existing PDH and can accommodate various This new service signal has formed a globally unified digital transmission system standard, which has improved the reliability of the network.
- Due to the use of more advanced add/drop multiplexers (ADM), digital cross-connect (DXC), network self-healing functions and reorganization functions, it is very powerful and has a strong survival rate. Due to the 5% overhead bits of the signal arranged in the SDH frame structure, its network management function is particularly powerful, and can uniformly form a network management system, which can contribute to the automation, intelligence, channel utilization, and reduction of network maintenance and management fees. Survivability played a positive role. To
- Due to the various network topologies of SDH, the network it constitutes is very flexible. It can enhance network monitoring, operation management and automatic configuration functions, optimize network performance, and make the network run flexible, safe, and reliable. The functions are very complete and diverse. To
- SDH has the performance of transmission and exchange. The composition of its series of equipment can be freely combined through functional blocks to realize networks of different levels and various topologies, which is very flexible.
- SDH is not exclusive to a certain transmission medium, it can be used for twisted pair, coaxial cable, but SDH is used to transmit high data rates and requires optical fiber. This feature shows that SDH is suitable for use as a trunk channel as well as a branch channel.
- From the point of view of the OSI model, SDH belongs to the physical layer of the bottom layer, and does not have strict restrictions on its upper layers. It is convenient to adopt various network technologies on SDH to support ATM or IP transmission. To
- SDH is strictly synchronized, which ensures that the entire network is stable and reliable, with few errors, and easy to reuse and adjust.
- Effectiveness and reliability are a pair of contradictions. Increasing effectiveness will definitely reduce reliability, and increasing reliability will also reduce effectiveness accordingly. A big advantage of SDH is that the reliability of the system is greatly enhanced, which will inevitably make the effectiveness very low when the same amount of effective information is transmitted.
- The pointer adjustment mechanism is complicated. The SDH system can directly download low-speed signals from high-speed signals, eliminating the need for multi-stage multiplexing/demultiplexing. The realization of this function is accomplished through the pointer mechanism, but the realization of the pointer function increases the complexity of the system. The most important thing is to make the system produce SDH combined jitter caused by pointer adjustment. This kind of jitter usually occurs at the boundary of the network, and its frequency is low, and the amplitude is large, which will cause the performance of low-speed signals to deteriorate after being removed.
- The impact of extensive use of software on system security. A major feature of SDH is the high degree of automation of OAM, which also means that the software occupies a large proportion of the system, which makes the system vulnerable to computer viruses. Especially when computer viruses are everywhere today. In addition, human error operations and software failures at the network layer have a fatal impact on the system.
As a new generation of ideal transmission system, SDH has the advantages of automatic routing selection, convenient upper and lower circuits, strong maintenance, control, and management functions, unified standards, and easy transmission of higher-speed services. It can well adapt to the rapid development of communication networks. need. 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, Internet technology (Packet over SDH), etc., make SDH networks more and more important.
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