SDH technology

The synchronous digital system SDH (synchronous digital hierarchy) uses a unique method to encapsulate data into frames and has a global unified interface. It uses the synchronous transmission module STM-N as the basic concept. Its modules are composed of information payload (payload), segment overhead (SOH), The management unit pointer (AU) is constituted, and its outstanding feature is the use of virtual containers to be compatible with various PDH systems. SDH transmission network has the advantages of intelligent routing configuration capability, convenient upper and lower circuits, strong maintenance, monitoring and management capabilities, and uniform optical interface standards.

  1. The basic transmission principle of SDH
    The information structure level adopted by SDH is called synchronous transport module STM-N (synchronous transport mode, N=1, 4, 16, 64), the most basic module is STM-1, and 4 STM-1 interleaved multiplexing constitute STM -4, 16 STM-1 or 4 STM-4 interleaved multiplexing constitute STM-16. SDH uses a block-shaped frame structure to carry information, as shown in Figure 1. Each frame consists of 9 vertical rows and horizontal 270×N columns of bytes. Each byte contains 8 bits. The entire frame structure is divided into section overhead areas, STM-N payload area (payload) and management unit pointer (AU PTR) area 3 areas. Among them, 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 regenerator section overhead (RSOH) and multiplex section overhead (multiplex section overhead). 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 to indicate that the first byte of information in the payload area is in the STM-N The accurate position within the frame so that the payload can be separated correctly when receiving. The SDH frame is transmitted in sequence from left to right and top to bottom in sequence. The transmission time of each frame is 125μs, and 1/125×1 000 000 frames are transmitted per second. For STM-1 Say that each frame byte is 8bit×(9×270×1)=19 440bit, then the transmission rate of STM-1 is 19440×8000=155.520Mbit/s; and 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.
SDH technology
Figure 1 STM-N frame structure

When SDH transmits service signals, the frames of various service signals to enter SDH must go through three steps of mapping, positioning and multiplexing. Mapping is a process in which signals of various rates are loaded into the corresponding standard container (C) after code rate adjustment, and then the channel overhead (POH) is added to form a virtual container (VC). The deviation of the frame phase is called the frame offset; positioning; That is, the process of receiving the frame offset information into the tributary unit (TU) or the management unit (AU), which is realized through the function of the tributary unit pointer (TU PTR) or the management unit pointer (AU PTR); It is a process in which multiple low-priced channel layer signals are adjusted to enter the high-priced channel through code rate adjustment, or multiple high-priced channel layer signals are adjusted to enter the multiplex layer through code rate adjustment.

The typical parameters of SDH equipment generally describe the backplane rate and cross-capacity. For example, the backplane rate of 2.5Gbit/s refers to the highest support 2.5Gbit/s optical port, and the matrix cross-capacity container is 96×96 VC4 or 4032×4032 VC12, among which , The rate of the virtual container VC12 is about 2Mbit/s, the rate of VC3 is about 34Mbit/s, and the rate of VC4 is about 140Mbit/s.

  1. Features of SDH
    SDH has a unified frame structure, digital transmission standard rate and standard optical interface in the world, compatible with PDH, and has formed a global unified digital transmission system standard. It uses a more advanced add/drop multiplexer (ADM) to directly add/drop out low-speed tributary signals. The network has self-healing and reorganization functions, powerful network management functions, very flexible network topology, transmission and switching performance, strict synchronization, stable and reliable network, less error, and easy reuse and adjustment.
  2. Multiplexing and mapping of SDH equipment
    As shown in Figure 2, the SDH multiplexing mapping structure recommended by ITU-TG.703, a 155M contains 63 2M, 2M is commonly known as a time slot of SDH. 2M access to STM-1 mode, 3-7-3 multiplexing structure is wider, that is, 3 2M multiplexed into TUG-2, 7 TUG-2 multiplexed into TUG-3, 3 TUG-3 multiplexed Used as 1 VC4, VC4 plus overhead becomes STM-1. The corresponding relationship between the virtual container YZW and the time slot N can be calculated by the formula N=Y+(Z-1)×3+(W-1)×21; Y≤3; Z≤7; W≤3, for example: 1- The 2-1 time slot N is 4.
SDH technology
Figure 2: SDH multiplexing mapping structure recommended by ITU-T G.703