Transmission Principle of MSTP Optical Transmission Equipment
The information structure level adopted by MSTP optical transmission equipment is called Synchronous Transport Module STM-N (Synchronous Transport, N=1, 4, 16, 64). The most basic module of MSTP optical transmission equipment is STM-1; four STM-1s are synchronously multiplexed to form STM-4, and 16 STM-1s or four STM-4s are synchronously multiplexed to form STM-16. MSTP optical transmission equipment 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 Management Unit Pointer (AU PTR) area. The section overhead area is mainly used for network operation, management, maintenance, and assignment to ensure that information can be transmitted normally and flexibly. 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, as well as a small number of channel overhead bytes used for channel maintenance. 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 the payload can be correctly separated upon reception. During frame transmission of Raytrans SDH, frames are transmitted sequentially as serial code streams from 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 MSTPoptical transmission equipment transmits service signals, all service signals entering the SDH frame must go through three steps: mapping, alignment, and multiplexing. 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 phase deviation in the frame is called frame offset. Alignment is the process of incorporating the frame offset information into the Tributary Unit (TU) or Management Unit (AU), which is achieved through the functions of the Tributary Unit Pointer (TU PTR) or Management Unit Pointer (AU PTR). Multiplexing is the process of adjusting the code rate of multiple lower-order channel layer signals to make them enter a higher-order channel, or adjusting the code rate of multiple higher-order channel layer signals to make them enter the multiplex section layer.