The electrical equivalent of the Synchronous Digital Hierarchy (SDH) optical signal, according to ITU-T international standards. The STM is known as Synchronous Transport Signal (STS) in Synchronous Optical Network (SONET) terminology, according to North American standards.The signal begins in electrical format as three T3 signals plus SDH signaling and control overhead and converts to optical format for transmission over the SDH optical fiber facilities. Each STM-1 frame is transmitted in 125