The Background of SDH
The Background of SDH Technology
The birth of SDH technology was inevitable. With the development of communications, the information to be transmitted includes not only voice, but also text, data, images and video. With the development of digital communications and computer technology, in the 1970s and 1980s, various network technologies emerged successively, including T1(DS1)/E1 carrier systems (1.544/2.048Mbps), X.25 frame relay, ISDN (Integrated Services Digital Network) and FDDI (Fiber Distributed Data Interface). With the advent of the information society, people expect modern information transmission networks to provide various circuits and services quickly, economically and efficiently. However, due to the monotony of services, the complexity of expansion and the limitation of bandwidth, modifying or perfecting the above network technologies within the original framework is no longer effective. SDH was developed under this background. Among various broadband fiber access network technologies, the access network system adopting SDH technology is the most widely applied. The birth of SDH solved the access "bottleneck" between users and the core network, which was caused by the bandwidth limitation of the access medium failing to keep up with the development of backbone network and user service demands, while improving the utilization of substantial bandwidth on the transmission network. Since its introduction in the 1990s, SDH technology has become a mature and standardized technology, widely adopted in backbone networks, with increasingly lower prices. Its application in access networks can bring the enormous bandwidth advantages and technical advantages of SDH technology in the core network into the access network field, fully leveraging the benefits of SDH synchronous multiplexing, standardized optical interfaces, powerful network management capabilities, flexible network topology capabilities and high reliability, providing long-term benefits in the construction and development of access networks.
The Transmission Principle of SDH
The information structure level adopted by SDH is called the Synchronous Transport Module STM-N (Synchronous Transport, N=1, 4, 16, 64). The most basic module is STM-1; four STM-1s synchronously multiplexed form STM-4; 16 STM-1s or four STM-4s synchronously multiplexed form STM-16. SDH uses a block-shaped frame structure to carry information. Each frame consists of 9 rows vertically and 270×N columns of bytes horizontally, with each byte containing 8 bits. The entire frame structure is divided into three areas: the Section OverHead (SOH) area, the STM-N payload area and the Administration Unit Pointer (AU PTR) area. The section overhead area is mainly used for network operation, management, maintenance and provisioning to ensure the normal and flexible transmission of information. It is further divided into Regenerator Section OverHead (RSOH) and Multiplex Section OverHead (MSOH). The payload area is used to store bits that actually carry information services and a small number of channel overhead bytes used for channel maintenance management. The administration 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 the payload can be correctly separated upon reception. SDH frames are transmitted in a serial bit stream in the order of left to right and top to bottom. Each frame has a transmission time of 125μs, with 1/125×1000000 frames transmitted per second. For STM-1, each frame contains 8bit×(9×270×1)=19440 bits, so the transmission rate of STM-1 is 19440×8000=155.520Mbit/s; the transmission rate of STM-4 is 4×155.520Mbit/s=622.080Mbit/s; the transmission rate of STM-16 is 16×155.520 (or 4×622.080)=2488.320Mbit/s.
When SDH transmits service signals, all service signals must go through three steps—mapping, alignment and multiplexing—to enter the SDH frame. Mapping is the process of first adjusting the code rate of signals of various rates and loading them into the corresponding standard Container (C), then adding Path OverHead (POH) to form a Virtual Container (VC). A deviation in frame phase is called frame offset. Alignment is the process of incorporating the frame offset information into the Tributary Unit (TU) or Administration Unit (AU), which is implemented through the functions of the Tributary Unit Pointer (TU PTR) or Administration Unit Pointer (AU PTR). Multiplexing is the process of adjusting the code rate of multiple lower-order channel layer signals to make them enter the higher-order channel, or adjusting the code rate of multiple higher-order channel layer signals to make them enter the multiplex section layer.