synchronous transfer mode (STM)
Synchronous Transfer Mode (STM) is a standardized method for transmitting a group of different data streams synchronized over a clock.
This STM method is used by carriers to identify time slots or channels within a T1 or E1 line.
The synchronous transmission mode is divided into hierarchy levels. Synchronous Digital Hierarchy ( SDH) modulo 1 (STM-1) is the first hierarchy level and represents a 2,430 byte transmission frame transmitted at 155.520 Mbps over optical fiber with OC-3. The other hierarchy levels are defined by STM-n and range up to 40 Gbit/s with STM-256.