The module (see Figure 16-1) contains eight optical STM-1 interfaces that meets the S-1. The physical connector is a LC connector. Other signals include STM-4, STM-16, STM-64, and STM-256. The following sections detail the speed and capacity of these STM levels relative to E1 and E4: A comparison of STM levels (STM0, STM1, STM4, STM16, STM64, and STM256) based on their data capacity and. The STM-1 (Synchronous Transport Module level-1) is the SDH ITU-T fiber optic network transmission standard. 1 optical. STM-1 (Optical / Electrical), E1 and Ethernet Multi-Service SDH Transmission Unit is a modular platform unit with two 155.