The STM-N frame structure consists of 9 rows and 270×N columns, using byte-interleaved multiplexing, resulting in 9×270×N=2430×N bytes. Each byte is 8 bits, and the data rate per byte is 64 kbit/s. Each frame has a period of 125 μs and a frame rate of 8 kHz (8000 frames per second). STM-1 is the most basic SDH structure. Each frame has a period of 125 μs, containing 19440 (9x270x8) bits, and a transmission rate of 19440bit/125μs=155520kbit/s. Because STM-N is formed by multiplexing N STM-1s through byte-interleaved synchronization, its data rate is N times that of STM-1.
An SDH frame consists of three parts: payload, management unit pointer (AU-PTR), and section overhead (SOH), as shown in the figure.
【Example 1-1】 Calculate the frame rate, frame length, and MSOH rate in STM-16.
(1) Since the frame period of STM-N is 125 μs, STM-16, as a rate within STM-N, should also have a frame period of 125 μs.
(2) Since the frame structure of STM-N is 9 rows and 270×N columns, the frame length of STM-16 is 9×270×16=44880 bytes or 9×270×16×8=359040 bits.
(3) Because the MSOH is located in the first 9×N columns of rows 5-9 in STM-N, there are 5×9×16=720 bytes of MSOH in an STM-16 frame. Since each byte has a rate of 64 kbit/s, the MSOH rate in an STM-16 frame is 720×64 kbit/s=46080 kbit/s.
The Segment Overhead (SOH) area is used to store bytes related to frame positioning, operation, maintenance, and management to ensure the correct and flexible transmission of the main information payload. The SOH is further divided into Regenerator Segment Overhead (RSOH) and Multiplex Segment Overhead (MSOH). The Regenerator Segment Overhead is located in rows 1-3 and columns 1-9×N in the STM-N frame and is used for frame positioning, monitoring, and maintenance management of the regenerator segment. The Regenerator Segment Overhead (RSOH) is generated at the beginning of the regenerator segment and added to the frame, and terminates at the end of the regenerator segment, meaning it is extracted from the frame for processing. Therefore, in an SDH network, the regenerator section overhead must terminate at each network element. The RSOH can be accessed and de-located at both the regenerator and the terminal equipment. The multiplexing section overhead is distributed in rows 5-9 and columns 1-9×N in the STM-N frame. It is used for monitoring, maintenance, and management of the multiplexing section. It is generated at the beginning of the multiplexing section and terminates at the end. Therefore, the multiplexing section overhead is transparently transmitted on the repeater and terminates at all other network elements except the repeater.
The physical entities between repeaters or between a repeater and a digital multiplexing device are called regenerator sections. All physical entities between two multiplexing devices constitute a multiplexing section. The regenerator section overhead in different regenerator sections is independent of each other, and the multiplexing section overhead in different multiplexing sections is also independent of each other. From a network layering perspective, SDH networks are divided into the channel layer and the transmission medium layer. The channel layer is further divided into low-order channel layers and high-order channel layers. The transmission medium layer can be divided into the segment layer and the physical layer. The segment layer can be further divided into multiplexed segment layers and regenerated segment layers. The physical layer is the transmission line, as shown in the figure.
Overhead provides granular monitoring and management functions for SDH signals at each layer. Monitoring can be divided into segment layer monitoring and channel layer monitoring. Segment layer monitoring is further divided into regenerated segment layer monitoring and multiplexed segment layer monitoring, while channel layer monitoring is further divided into high-order channel layer monitoring and low-order channel layer monitoring. This achieves granular monitoring of STM-N at each layer. For example, in monitoring a 2.5 Gbit/s system, the regenerator section overhead monitors the entire STM-16 signal, the multiplexing section overhead is refined to monitor any one of the 16 STM-1s within the STM-16, the high-order path overhead further refines this to monitor the VC-4 within each STM-1, and the low-order path overhead further refines the monitoring of VC-4 to monitor any one of the 63 VC-12s. This achieves multi-level monitoring from the 2.5 Gbit/s level to the 2 Mbit/s level. These monitoring functions are implemented using different overhead bytes.
The management unit pointer is stored in columns 1 to 9 × N of row 4 of the frame. It indicates the precise location of the first byte of the information payload within the STM-N frame, ensuring accurate identification of the required information. Rate adjustment is performed using this pointer to ensure compatibility with various services or connections to other networks.
The information payload area stores various telecommunications service information and a small number of channel overhead bytes used for channel performance monitoring. It is located in all areas of the STM-N frame structure except for the segment overhead and management unit pointer area.
