STM1 STM1 Synchronous Transport Module level-1 Synchronous Transport Module (STM) STM1 STM1 STM-1 STM-1 STM1 STM1 STM1 STM-1 STM-1 STM-1 STM-1 STM-1 STM1 STM1 Synchronous Transport Module level-1 STM1 Synchronous Transport Module level-1 Synchronous Transport Module level-1 STM1 STM1