Frame Structure of SDH
To add or drop low-speed tributary signals to or from high-speed signals, try to distribute tributary signals in the frame equably and regularly. The ITU-T regulates that STM-N frames are rectangular and expressed in bytes, as shown in Figure 16-3.
STM-N is the frame with the dimension of 9 rows x 270 x N columns. Here, N is consistent with that in STM-N, indicating how many STM-1 signals are multiplexed to this STM-N signal.
An STM-N frame consists of the following parts:
Section Overhead (SOH): includes Regenerator Section Overhead (RSOH) and Multiplex Section Overhead (MSOH).
Administration Unit Pointer (AU-PTR): is the pointer that specifies the first byte of the payload. The receiving end can correctly extract the payload according to the location of the pointer.
Payload