The STM-1 (Synchronous Transport Module level-1) is the SDH ITU-T fiber optic network transmission standard. It has a bit rate of 155.52 Mbit/s. Higher levels go up by a factor of 4 at a time: the other currently supported levels are STM-4, STM-16, STM-64 and STM-256. Above STM-256 wavelength-division multiplexing (WDM) is commonly used in submarine cabling.

Frame structure

The STM-1 frame is on the basic transmission format for SDH (Synchronous Digital Hierarchy). An STM-1 frame has a byte-oriented structure with 9 rows and 270 columns of bytes, for a total of 2,430 bytes (9 rows * 270 columns = 2430 bytes). Each byte corresponds to a 64 kbit/s channel. TOH: Transport Overhead (RSOH + AU4P + MSOH) VC4: Virtual Container-4 payload (POH + VC-4 Data)

Frame characteristic

The STM-1 base frame is structured with the following characteristics:

RSOH (regenerator section overhead)

The Regenerator Section OverHead uses the first three rows & nine columns in the STM-1 frame

MSOH (multiplex section overhead)

X = Bytes reserved for national use. The Multiplex Section OverHead uses the 5th through 9th rows, and first 9 columns in the STM-1 frame.