The STM-N segment overhead is composed of N STM-1 segment overheads multiplexed synchronously by byte interleaving. However, only the first STM-1 segment overhead is fully retained; the remaining N-1 STM-1 segment overheads only retain bytes A1, A2, and B2, omitting all other bytes.
The purpose of each byte of the segment overhead is explained using an STM-1 signal as an example. For an STM-1 signal, the segment overhead includes RSOH located in rows 1-3 x columns 1-9 and MSOH located in rows 5-9 x columns 1-9. The arrangement of the STM-1 segment overhead is shown in the figure.
