STM-1

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. Beyond this we have wavelength-division multiplexing (WDM) commonly used in submarine cabling.

The STM-1 frame is the basic transmission format for SDH (Synchronous Digital Hierarchy). A 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 64kbit/s channel.

TOH: Transport Overhead (RSOH + AU4P + MSOH)

VC4: Virtual Container-4 payload (POH + VC-4 Data)

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

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

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